■ 
 
 -I 
 
 • 
 
 i 
 
 \ 
 
 I 
 i 
 
 I 
 
 i 
 
 1 
 
 lii 
 
 Hlii^^iUyt^U^; 
 
Nortlj (Earnltna S>tatP 
 
 C86 
 1915 
 
 This book was presented by 
 
 Frederick L. Wellman 
 
 I.e. STATE UNIVERSITY D.H HII.l LIBRARY 
 
 S00273402 
 
This book is due on the date indicated 
 below and is subject to an overdue fine 
 as posted at the Circulation Desk. 
 
 DEC G 7 
 UEU 
 
 %i^ 
 
NATURE AND 
 DEVELOPMENT OF PLANTS 
 
 BY 
 
 CARLTON C. CURTIS, A.M., Ph.D. 
 
 Professor in Botany in Columbia University 
 
 FOURTH EDITION-REVISED 
 
 NEW YORK 
 
 HENRY HOLT AND COMPANY 
 
 1915 
 
Copyrighted 1907, 1910, 1914, 1915 by 
 Henry Holt axd Company 
 
 ,„^ PRESS OF 
 
 THE NEW ERA PRINTING COMPANY 
 LANCASTER, PA. 
 
PREFACE 
 
 In the present volume an attempt has been made to present 
 the more important aspects of botanical science, not only as 
 they are related to pure biological science, but also as they are 
 connected and associated with our daily life and interests. The 
 general reader will find here a discussion of the laws and prin- 
 ciples that control and direct plant life and of the importance 
 of this life. The author also has had in mind the difficulties 
 that the student experiences in entering a new field. To make 
 the subject more approachable, many of the technical terms have 
 been eliminated and an easily comprehended terminology has 
 been maintained throughout the book. The text is designed 
 especially for the benefit of the student who is beginning the 
 subject. We trust that a study of it will bring him to the class 
 room prepared for a discussion of the topics and we also trust 
 that this work of preparation will tax him to the full measure 
 of his intellectual capacity. The author is old fashioned in his 
 ideas of education. Work that simply entertains or imparts 
 information and that does not create the sufi^erings associated 
 with mental effort can be of little permanent value or make for 
 any considerable development. Finally the book will fall short 
 of its real purpose unless used in the laboratory in connection 
 with the plants themselves. Detailed directions are not given 
 for the laboratory work — these of necessity must vary with the 
 instructor and with the locality — but the text gives the student 
 sufficient guidance and understanding to enable him to find (nit 
 for himself the principles and facts in the material to \\ liich he 
 has been directed. 
 
 Acknowledgments are due to Professors C. B. Atwcll, J. H. 
 Schafifner, F. D. Kern, Ira D. Cardiff, Jean Broadiiurst, C. A. 
 Darling, A. H. Chivers, B. O. Dodge, R. C. Benedict, (i. W. 
 Martin and several others for valuable suggestions, in the prepa- 
 
VI 
 
 PREFACE 
 
 fV 
 
 ration of various portions of the text. The author desires to 
 express his especial indebtedness to his good friend, Mr. H. O. 
 Hanson, for the preparation of many of the illustrations; to Mr. 
 W. A. Andrews, for microscopical studies from which draw^ings 
 have been made, and to Miss Mary L. Williams, for the execu- 
 tion of many of the studies. Special mention is also made of the 
 author's indebtedness to the assistance and collaboration of his 
 former pupils and present associates, Drs. E. Altenburg and H. 
 
 J. MuUer. 
 
 Carlton C. Curtis. 
 Columbia University, 
 June, 1915. 
 
CONTENTS 
 
 PAGE 
 
 Introduction i 
 
 PART I 
 
 Nature of Plants 
 
 I. The Leaf 7 
 
 II. The Root 46 
 
 III. The Stem 71 
 
 IV. The Flower, Fruit and Seedling 115 
 
 PART II 
 
 The Development of Plants 
 
 V. Classification of Plants 146 
 
 VI. Thallophyta 149 
 
 VII. Bryophyta 269 
 
 VIII. Pteridophyta 311 
 
 IX. Spermatophyta 35^ 
 
 X. Angiospermae (Spermatophyta concluded) 378 
 
 
NATURE AND DEVELOPMENT OF PLANTS 
 
 INTRODUCTION 
 
 I. The Nature of the Plant.— It Is a familiar fact that most 
 plants have stems, leaves and roots and that these organs are 
 variously modified. Why have such organs been developed? 
 Why do they assume such varied forms and arrangements? How 
 does it come about that the stem reaches up in the air and that 
 the branches and leaves are arranged in a very definite order 
 while the root penetrates the soil and grows toward moisture and 
 other soil material? Is a plant, like an animal, conscious of its 
 surroundings, seeking suitable foods and avoiding unfavorable 
 conditions? It is evident to the casual observer that the plant 
 is sensitive to its surroundings; some plant organs bend toward 
 a light that we recognize with difficulty or react to a touch that 
 we cannot appreciate. Has the plant a nervous system and 
 faculties like ourselves that enable it to adjust itself to its sur- 
 roundings? To answer these questions it will be necessary to 
 
 f%-"' -J 
 
 Fig. I. Diagram of a cell showing its three dimensions; mesh- work of 
 granular cytoplasm which incloses colorless cell sap, the vacuoles, and a denser 
 dark body, the nucleus. 
 
 consider the nature of the living substance of the plant. The 
 plant body is composed of cells that are like boxes of living 
 matter having length, breadth, and thickness (Fig. i). While 
 the cells assume various forms and perform different work or 
 
2 THE LIVING SUBSTANCE OF PLANTS 
 
 functions, they all possess cell walls which contain during the 
 life of the cell a substance termed protoplasm. This substance 
 resembles somewhat the white of an egg, being viscid and rather 
 tenacious. The protoplasm is not of uniform consistency. The 
 larger portion of it is finely granular and is called the cytoplasm 
 while a denser rounded part is termed the nucleus. Other bodies, 
 plastids, also denser than the cytoplasm are of common occur- 
 rence. Such plastids as contain a green pigment, chlorophyll, are 
 called chloroplastids or chloroplasts, and these produce the green 
 
 
 Fig. 2. Structure of the cell: A, cell ditch-moss, Philotria; n, nucleus, 
 V, vacuole, ch, chloroplast. B, cell of carrot; ch, yellowish chromoplasts, n, 
 nucleus. C, reddish chromoplasts from rose hip. D, cells of Begonia; s, 
 starch grains, c, crystals of lime. E, leucoplast, /, of potato forming a starch 
 grain. 
 
 color of the vegetation (Fig. 2, A). Still other plastids, the 
 chromoplasts, contain red or yellow pigments that give the color 
 to many fruits and flowers, as the tomato, rose-hip, squash, nas- 
 turtium, etc. (Fig. 2, B, C). Colorless plastids, leucoplasts, often 
 
INTRODUCTION 3 
 
 occur in cells hidden from the Hght. One group of these leuco- 
 plasts forms the starch that appears in underground storage 
 organs as in the potato (Fig. 2, E). Usually spaces or vacuoles, 
 that appear as cavities also occur in the granular cytoplasm 
 (Fig. 2, A, v). In reality they are filled with watery solutions of 
 various substances, the so-called cell sap. Many other structures 
 (Fig. 2, D) appear in the protoplasm to which attention will be 
 called later. It is well to remember that these visible structures 
 do not represent the real composition of the protoplasm. A 
 variety of units beyond the range of visibility, grow, multiply, 
 and build up the various structures which we recognize as con- 
 stituting the protoplasm. The cell walls make up the body of 
 the plant and give stability to its various organs but the living 
 part of the cell is the protoplasm. This substance reaches 
 through delicate pores in the cell wall to the protoplasm of 
 adjacent cells so that all the living substance of the entire plant 
 body is in contact and forms one united mass. 
 
 2. The Nature of the Living Substance of the Plant. — The pro- 
 toplasm possesses most remarkable powers. It can absorb vari- 
 ous fluids and gases, decompose them into simple elements, re- 
 unite them into foods or discharge from the cell such substances 
 as are not required. Furthermore the protoplasm effects those 
 changes termed growth by transforming the foods into the sub- 
 stances that compose the cell walls and other parts of the cell, 
 as the protoplasm itself. How are these changes brought about? 
 Every substance is composed of elements. Water consists of 
 two elements, hydrogen and oxygen. The elements that compose 
 any substances are held together with great energy, owing to their 
 mutual attraction for one another. When two different sub- 
 stances are brought together, it may happen that the attraction 
 of certain elements of one substance is greater for one or more 
 of the elements of the other substance than for its own elements. 
 The result is, that the elements will be torn away from their 
 respective substances and united into new combinations. We 
 say that a decomposition and a re-combination has been effected, 
 or, that a chemical reaction has occurred. We see an illustration 
 of these chemical changes when iron-ore is heated with charcoal. 
 
4 NATURE OF THE LIVING SUBSTANCE 
 
 The ore is composed of the elements iron and oxygen and the 
 charcoal consists of carbon. In the presence of heat the carbon 
 has a stronger attraction for the oxygen than the iron has. Con- 
 sequently the oxygen is drawn away from the iron and unites 
 with the carbon, forming a new combination of carbon and oxy- 
 gen, and leaving the iron free. This decomposition and re-com- 
 bination is only a part of the chemical change that takes place, 
 but it will serve to illustrate the nature of many chemical re- 
 actions. The energy that was required to hold the oxygen and 
 iron together begins to be set free as soon as the decomposition 
 starts and it contributes to the rise of temperature during the 
 reaction. The living substance of the plant is composed of a 
 great variety of chemical substances that are very readily de- 
 composed by gases and other substances which it absorbs. Light, 
 heat, gravitation and moisture are even more powerful in pro- 
 ducing these changes. These substances and forces are often 
 referred to as stimuli (sing, stimulus) because they start the 
 chemical changes referred to above. So the various substances 
 and forces in nature stimulate or cause chemical changes in the 
 living substance of the plant and the energy thus set free is used 
 by the protoplasm in the performance of its work. The cells of 
 a seed have no power of their own to grow or perform any duty. 
 It is not until a certain amount of heat, moisture, gases or other 
 stimulating forces have acted upon the protoplasm or substances 
 contained in it and so aroused chemical changes that it is fur- 
 nished with the requisite energy to begin growth. Furthermore, 
 these forces exert a very definite influence upon the protoplasm 
 and cause it to accomplish very definite results. This is due to 
 the fact that not all portions of the protoplasm of the plant body 
 are equally influenced by, or we may say equally sensitive to 
 heat, light, and other forces and consequently the various parts 
 of the plants do not respond alike. The force of gravity, for 
 example, acts upon some stems and the living substance is in- 
 fluenced by this stimulus so that a growth is aroused that bends 
 the stem into an upright position. The protoplasm in the cells 
 of the root, however, is so constituted that the stimulus of gravity 
 causes a growth that bends the root down into the soil. Light 
 
NATURE OF PLANTS 5 
 
 stimulates the mustard plant in such a way that the stem grows 
 toward the light and the root away from it. So the living sub- 
 stance of the plant while forming one united whole varies in its 
 sensitiveness in various parts of the plant. In this respect the 
 various parts may be compared to the receiving stations of a 
 wireless telegraphy system. The receiving stations may have 
 instruments sensitive to certain intensities of electrical currents 
 and other currents do not affect them. So the various organs of 
 the plant are sensitive, each in its own peculiar way, to certain 
 stimuli. As a result of this adjustment the various parts of the 
 plant, being attuned or sensitive to certain forces, are led or 
 induced to grow so that they come into the most helpful and 
 beneficial relation to these forces. Thus the leaves, branches, 
 and roots are stimulated to grow and develop so that each part 
 becomes properly related to light, moisture, gravity, etc. The 
 broad blades of the majority of leaves are exposed to the direct 
 rays of light. This arrangement is of the greatest benefit be- 
 cause they are using the light in the manufacture of foods. This 
 position is assumed, however, because they are sensitive to light, 
 gravitation and other forces which direct and cause the arrange- 
 ment. This is the most noteworthy feature about the sensitive- 
 ness of the protoplasm. Forces stimulate to growth in such a 
 way that the results are helpful and the greatest good comes to 
 the plant. In other words the reactions are purposive. These 
 growths are so elaborate and beneficial that they often appear as 
 the result of reason and will. The bending of the root into the 
 soil brings it into contact with water and other foods; the tendril 
 coils about a branch and binds the plant firmly to the support. 
 The plant, however, does not direct these adjustments. These 
 reactions do not involve consciousness. These movements and 
 all others are absolutely directed and controlled by the various 
 forces which act upon the sensitive protoplasm. A definite re- 
 action follows a stimulation which the plant has no power to 
 alter or control. The root will bend down into a dish of mercury 
 with the same energy and directness as that with which it pene- 
 trates the soil, and the tendril will clasp your finger or a branch 
 of its own body as firmly as a serviceable support. 
 
AIM OF THE STUDY 
 
 We shall now proceed to study the plant from this standpoint, 
 Wiving attention to the fitness or adaptation of the leaf, root and 
 /stem, to the conditions under which the plant lives, and to the 
 ^work which it performs; and finally we will be interested to 
 ) learn how the seed is formed and how the plant lives from year 
 /to year. 
 
PART I 
 THE NATURE OF PLANTS 
 
 CHAPTER I 
 
 THE LEAF 
 
 3. The Work of the Leaf. — Each plant has a character or a 
 personality of its own. What is it that gives this individuality 
 to the plant? When the work that it performs is understood it 
 will be seen that this character is very largely due to the adapta- 
 tion of the leaves to the performance of certain duties. The 
 
 Fig. 3. Forms of leaves: A, leaf of white birch with netted veins — p, 
 petiole; b, blade. B, leaf of Solomon's seal with parallel venation and blade 
 clasping stem without petiole. 
 
 leaves bring the plant into harmony with its surroundings and 
 give to it a subtle individuality owing to the perfection of their 
 arrangements, structures and forms for the work in hand. The 
 
 7 
 
8 THE WORK OF THE LEAF 
 
 more important work performed by the leaves is the construction 
 of foods, the giving off of water or transpiration, and breathing 
 or respiration. The magnitude of this work far exceeds the 
 energy expended in all the industries of the world. The leaves, 
 however, accomplish this work so quietly and economically that 
 most people are scarcely conscious of it. 
 
 In order to understand the purpose of the petiole and green 
 blade (Fig. 3) or the significance of the various forms and 
 arrangements of leaves it will be necessary to examine the struc- 
 ture of the leaf and see the character of the apparatus that is 
 used in the performance of its work. The blades of the majority 
 
 Fig. 4. Blade of lilac leaf cut across showing the more compact arrange- 
 ment of cells upon the upper side of the leaf. 
 
 of leaves are flat. A section through such a blade, cut as in Fig. 
 4 so that we can look into the end of the blade, shows that the 
 leaf is composed of a complicated arrangement of cells. Fig. 5 
 is a greatly enlarged view of Fig. 4 taken at X. 
 
 4. The Epidermis. — It is now seen that a layer of compact cells 
 (Fig. 5, e) surrounds the leaf on all sides. In fact such a layer 
 covers all parts of the plant body. This layer of cells, the epi- 
 dermis, is provided with minute openings or stomata (sing, 
 stoma), Fig. 5, s. The stomata are especially abundant in the 
 epidermis on the under surface of the leaf and often quite lacking 
 from the upper surface. A better idea of these openings and of 
 the epidermal cells may be gained by stripping off the epidermis 
 
NATURE OF PLANTS 
 
 Fig. 5. Greatly enlarged view of region A' in Fig. 4: e, epidermis; s, stoma; 
 p, palisade mesophyll; ch, chloroplast; sp, spongy mesophyll; i, intercellular 
 spaces; v, small vein cut across; /, end of vein seen from the side, consisting 
 of elongated and banded cells (tracheids); col, collecting cells that transfer 
 material to and from the palisade cells. — H. O. Hanson. 
 
 Fig. 6. Surface view of epidermis of lilac leaf: g, guard cell; s, stoma; ch, 
 chloroplasts; «, nucleus. — H. O. Hanson. 
 
10 
 
 STRUCTURE OF THE LEAF 
 
 from such leaves as the live-for-ever, blue flag or hyacinth. The 
 cells of the epidermis are very compactly put together and may be 
 stripped off by means of a penknife as a thin white skin. Ex- 
 amined under a microscope, such preparations show the stomata 
 as minute openings guarded by two rather lens-shaped cells, 
 termed the guard cells (Fig. 6). Chloroplasts usually occur 
 only in the guard cells, the other epidermal cells having a watery, 
 colorless cell content. In addition to these features. Fig. 7 shows 
 that the outer portion of the walls of the epidermal cells is modi- 
 fied so as to form a delicate, skin-like layer over the outer sur- 
 
 FiG. 7. Fig. 8. 
 
 Fig. 7. Section across a stoma shown in Fig. 6: s, stoma; g, guard cell; 
 c, cuticle; a, air chamber. — H. O. Hanson. 
 
 Fig. 8. Hairs from the epidermis of squash leaf. 
 
 face of the epidermis. This part of the cell wall is called the 
 cuticle. It contains a fatty substance, cutin, that renders the 
 epidermis nearly impervious to gases and fluids, so that the leaf 
 is entirely surrounded by a water- and gas-proof coat. In many 
 leaves hairs of various forms grow out from_the epidermal cells 
 (Fig. 8). 
 
 5. The Mesophyll. — The cells within the epidermis have, for 
 the most parts, a greenish color due to numerous green bodies, 
 chloroplasts, contained in the cells (Fig. 5, ch). These green 
 cells of the leaf, termed the mesophyll, consist of one or more 
 layers of rather elongated and compact cells on the upper side 
 of the leaf (the palisade mesophyll, Fig. 5, p), and below they 
 
NATURE OF PLANTS n 
 
 are joined to rather irregular and loosely arranged cells (the 
 spongy mesophyll, Fig. 5, sp). Numerous air spaces extend in 
 all directions through this spongy mass of cells reaching up to 
 and between the palisade cells and down to the stomata (Fig. 
 5, i). Bundles of small cells also appear here and there between 
 the palisade and spongy mesophyll (Fig. 5, v, I). These struc- 
 tures are the so-called veins that appear as fine lines in the 
 majority of leaves (Fig. 3). These veins or vascular bundles 
 branch again and again and so extend to all parts of the leaf. 
 In Fig. 5, V one of the veins has been cut square across while 
 at / the end of a small vein is shown, having been cut in half 
 lengthwise. The walls of the mesophyll cells are smooth, thin, 
 and elastic and composed of a substance called cellulose. Thin 
 places or pores are often formed in the walls which appear as 
 openings owing to the extreme thinness of the wall at such places. 
 The term parenchyma is applied to all cells possessing the char- 
 acter of walls noted in the mesophyll cells. In the vascular 
 bundle, however, a portion of the cells have thicker walls which 
 are more rigid, hard and woody. Such cells are often referred to 
 as prosenchyma. 
 
 What is the significance of the peculiar and varied arrange- 
 ments of the cells in the leaf? We naturally anticipate that these 
 different structures are adapted to performance of special duties. 
 Attention will first be directed to the work performed by the 
 green portion of the leaf, the mesophyll. Similar cells containing 
 chloroplasts are of common occurrence in various parts of the 
 plant, being often found in stems and unripe fruits. In the leaf 
 such a tissue is termed mesophyll but a more general term, Chlor- 
 enchyma, includes all chlorophyll-bearing tissue wherever found. 
 
 6: The First Function of the Leaf, Photosynthesis.— Chlor- 
 enchyma is the most important tissue in the plant since it has 
 for its special function or work the construction of carbohydrates, 
 such as sugars and starches, which are among the most impor- 
 tant foods of the plant. The construction of these carbohydrates 
 by the chlorenchyma is called photosynthesis. This work easily 
 ranks as the leading wonder of the world because its processes 
 are so complicated and of such magnitude and importance in the 
 
12 NATURE OF CARBOHYDRATES 
 
 economy of this world of ours. It will be seen that this tissue 
 not only keeps the air in a wholesome condition for breathing 
 and so makes life possible upon the earth but that all animal life 
 is absolutely dependent, either directly or indirectly, for food 
 upon the products built up by the chlorenchyma. Let us see 
 how these foods are formed. Various substances are absorbed 
 by the roots, and the vascular bundles, which extend throughout 
 the plant body, transfer them to the chlorenchyma. This ab- 
 sorbed material is in large part water, but various mineral sub- 
 stances are dissolved in it. Various gases, as carbon dioxide, 
 also find their way through the stoma to the chlorenchyma. All 
 of these absorbed substances are composed of elements. Thus 
 water consists of two elements, hydrogen and oxygen, in the pro- 
 portion of two parts of hydrogen to one part oxygen. This com- 
 position is indicated by the abbreviation, H2O. So the gas, 
 carbon dioxide, contains one part of carbon and two parts of oxy- 
 gen, indicated thus CO2. Chloroplasts have the power under 
 suitable conditions of decomposing or tearing apart these com- 
 pounds and of reuniting their elements into new compounds that 
 are quite different from the original ones. In this way CO2 
 and H2O are decomposed and the elements reunited into complex 
 compounds or foods consisting of carbon, hydrogen and oxygen. 
 Such complex foods as cane sugar, consisting of twelve parts of 
 carbon, twenty-two of hydrogen, and eleven of oxygen are formed 
 in this way. This very common food is expressed by the formula 
 C12H22O11. So also other foods are formed, as grape sugar 
 C6H12O6 and starch usually indicated as some multiple of CeHioOs. 
 The cellulose walls of the parenchyma cells have the same com- 
 position as starch. It will be seen that all these foods have 
 carbon united to hydrogen and oxygen in the same proportion 
 as it exists in water. Cane sugar has carbon united to eleven 
 times H2O, and starch five times H2O. It was supposed that 
 these substances were a combination of carbon and H2O. For 
 this reason they were called carbohydrates. This name is still 
 applied to these foods although it is known that the base is not 
 H2O but the hydroxyl OH. The actual changes that are effected 
 in the formation of these various carbohydrates are not known. 
 
NATURE OF PLANTS 13 
 
 It has been suggested that CO2 and H.O enter into solution form- 
 ing carbonic acid (H2CO3). This change is represented by the 
 equation, CO2 + H2O = H2CO3. By giving off one part of 
 oxygen this acid is changed to formic acid (CH2O2), i. e., H2CO3 
 = CH2O2 +0. A similar decomposition is eiTected in the formic 
 acid which results in the formation of formaldehyde (CH2O), 
 i. e., CH2O2 = CH2O + O. The two parts of oxygen pass 
 from the cells and escape through the stomata as a gas. In 
 the presence of certain alkalies and acids, formaldehyde increases 
 the number of elements which compose it, thus six times CH.O 
 would give CeHnOe or grape sugar. This is but a theory based 
 upon the facts that CO2 and H2O do unite in nature to form 
 H2CO3 and that this acid may decompose as stated above, and 
 finally that formaldehyde can be detected in plant cells and it 
 is possible to produce sugar from this substance by treating it 
 with alkalies and acids. All that can be definitely stated about 
 the changes going on in the leaf, during the formation of the 
 carbohydrates relates solely to the beginning and end of the 
 process. Water and carbon dioxide enter the chlorenchyma 
 cells. A series of changes follows, the nature of which can only 
 be conjectured. Finally, as the end result, sugar appears in 
 the cells and oxygen is set free, escaping as a gas. 
 
 These complex changes are brought about in the leaf in so 
 subtle a way that we are not conscious of them or of the great 
 amount of energy that is required to effect them. It requires 
 the energy expressed by a temperature of 1300° C. to decompose 
 CO2 into its elements. Where does the plant obtain the energy 
 to bring about the decompositions and recomposition? This 
 work is accomplished by the energy- of the sunlight acting upon 
 the chloroplasts. The chloroplasts of the seed plants are minute, 
 rather lens-shaped grains, and increase in number by the division 
 of the plastid into two equal parts (Fig. 9). These bodies are 
 denser portions of the cytoplasm and like it are nearly colorless. 
 In the presence of light a green, oily substance, chlorophyll, is 
 formed in the plastids, thus producing their green color. Chloro- 
 phyll is rarely formed except in the presence of light. Plants 
 grown in cellars or in the dark are of a pale color owing to the 
 
14 NATURE OF CHLOROPHYLL 
 
 absence of chlorophyll in the plastids. Similarly green plants 
 lose their chlorophyll when placed in the dark. This is the 
 principle employed in the blanching of celery. The stalks are 
 surrounded by earth or the light is excluded from them by some 
 other means; chlorophyll disappears from the plastids and fails 
 to develop in the new cells that are formed in the dark. Plants 
 in which the chlorophyll is not developed are said to be etiolated. 
 The plastids are in the cells, however, for if an etiolated seed- 
 ling is removed from the dark to the light the chlorophyll will 
 begin to appear in a few hours. The greening of potatoes when 
 removed from the ground and left in a strong light is a familiar 
 example of this. It is evident that the formation of chlorophyll 
 is usually closely connected with the action of light. Another 
 important point regarding the influence of light is the fact that 
 such a food as starch is not usually found in the leaf in the 
 absence of light. This may be easily verified by cutting the 
 initials of your name in rather large letters in a strip of black 
 paper or smooth tin-foil and fastening the strip in the early 
 morning to the upper surface of a well-sunned leaf of a starch 
 forming plant. After a few hours remove the leaf from the plant 
 and place it in alcohol. This will dissolve the chlorophyll and 
 the leaf will become quite white in a few hours. Now place the 
 leaf in a tincture of iodine which turns starch to a blue or blue- 
 black color. Wherever the light was excluded from the leaf 
 there is only a pale yellow color but all portions of the sunned 
 leaf show an abundance of starch as revealed by the blue or blue- 
 black coloration. Consequently the letters appear black on a 
 yellow background which marks the limits of the strip of paper. 
 No starch can be detected as a rule in leaves taken from plants 
 that have been growing in the dark for twenty-four hours. 
 Plastids that do not contain chlorophyll, examples of which are 
 found in the cells forming the white bands or blotches in many 
 variegated plants, do not form starch in the light. It is evident 
 from these facts that the chlorophyll and light co-operate in 
 some way in the formation of starch. It is known that chloro- 
 phyll absorbs a certain portion of the sunlight. If a beam of 
 sunlight is caused to pass through a prism it is separated into 
 
NATURE OF PLANTS 15 
 
 seven colors, i. e., red, orange, yellow, green, blue, indigo and 
 violet. When, however, a beam first passes through an alcoholic 
 solution of chlorophyll or through a green leaf a portion of certain 
 colors and all of other colors do not appear. The chlorophyll has 
 taken up or absorbed certain of the light rays especially portions 
 of the red, blue and violet. It is reasonable to suppose that the 
 energy in these rays is utilized in part by the plastids in effecting 
 the changes noted above in the formation of foods. The sun 
 gives to the earth great quantities of energy in the form of light. 
 A portion of this light is taken up by the chlorophyll, and the 
 plastid uses the energy of the light to bring about the recombi- 
 nation of CO2 and H2O and the formation of sugars, starches, 
 and other carbohydrates. Because of the importance of the 
 sunlight in the formation of carbohydrates this process is called 
 photosynthesis. This term means the putting together by means 
 of light. 
 
 7. The Magnitude of the Work of Photosynthesis. — Carbon is 
 an important element in the composition of plant foods and also 
 in the walls of the cells. It forms one half of the dry weight of 
 the plant. All the carbon appearing in the plant is derived solely 
 from the CO2 in the air, of which it forms a very small part, only 
 about three parts in ten thousand. Furthermore the carbon 
 comprises only 3/11 by weight of the CO2, 8/1 1 being oxygen. 
 A square meter of leaf surface, however, can withdraw all the 
 CO2 from 2500 liters of air in one hour. Such a volume of air 
 would be a space one meter square and 2^-2 meters high. This 
 would furnish sufficient carbon for the construction of one gram 
 of starch. In this way such large quantities of CO2 are drawn 
 into the chlorenchyma cells as to make possible each >car the 
 harvest and the renewal of vegetation of the earth. In the 
 United States alone in this w^ay there was in 1914 built up over 
 734 millions of pounds of cotton, 891 million bushels of wheat. 
 1 139 million bushels of oats, etc., representing a value for all 
 crops of 7 billion dollars. Our leading crop is corn which amounted 
 to nearly 2^ billion bushels and valued at about i^i billion 
 dollars. The gold and silver coin and bullion of the country are 
 not of greater value. This corn came up and matured in 120 
 
i6 NATURE OF PROTEIDS 
 
 days, consequently these coin plants were manufactUi^ng food 
 at the rate of over 15 million a day. These figures represent only 
 a small portion of the work performed, since the grain is but a 
 part of the plant, and furthermore the larger portion of the land 
 is covered with forests and other forms of vegetation. The fact 
 must not be overlooked that this process of photosynthesis is of 
 vital importance to our welfare in another way. Owing to the 
 large volumes of CO2 that are constantly formed by fires and the 
 respiration of animals such an excess of this gas would accumu- 
 late in the atmosphere that all animal life would eventually cease 
 were it not for its absorption in photosynthesis. So balanced 
 are the rates of formation and absorption, however, that the 
 percentage existing in the air does not materially vary. 
 
 8. The Construction of Proteids. — A second group of foods 
 formed by plants are called proteids. These differ from the car- 
 bohydrates in that they contain nitrogen in addition to carbon, 
 hydrogen and oxygen. They are more complex compounds than 
 the carbohydrates and may contain sulphur and phosphorus in 
 addition to the four elements mentioned above. Less is known 
 about their formation than of the carbohydrates. It is probable 
 that they are largely formed in the leaves and by a process simi- 
 lar to that of photosynthesis. The sulphates, phosphates, and 
 nitrates absorbed from the soil are decomposed and the elements 
 of nitrogen, sulphur or phosphorus are united to simple carbon 
 compounds and complex proteids are the result. Light probably 
 does not co-operate directly in this construction although it 
 may do so indirectly. These foods are formed in much smaller 
 quantities than the carbohydrates but they are of the greatest 
 importance in the nourishment of the plant. This is especially 
 true as regards the living substance, protoplasm, which resembles 
 somewhat in composition some of the more complex proteids. 
 
 9. The Distribution of the Foods. — Let us now consider what 
 becomes of these foods. A small part is consumed on the spot 
 by the manufacturing cells themselves, a larger portion is trans- 
 ported through the vascular bundles to all the living and grow- 
 ing cells of the plant body, but as the plant approaches the com- 
 pletion of its annual growth a larger and larger part of the food 
 
NATURE OF PLANTS 17 
 
 is transferred to special parts of the plant, such as buds, roots, 
 seeds. Here it is stored as a reserve food to meet the needs of 
 the plant at such times as it is not able to manufacture food. 
 This transfer of foods is slow and consequently the rapidly con- 
 structed sugars gradually accumulate in the chlorenchyma during 
 the day. This would result in the saturation of the cells with 
 sugar and so stop the work of photosynthesis were it not for the 
 fact that the chloroplasts quickly change the sugar to insoluble 
 starch, thus leaving the cells free to receive more sugar. If 
 chloroplasts of well-sunned, starch-forming leaves are examined 
 in the afternoon, they will be found to contain minute glistening 
 bodies, the starch grains (Fig. 9, s). The chloroplasts have the 
 power to absorb sugar and secrete starch. The construction of 
 
 Fig. 9. Greatly enlarged chloroplasts from leaf of moss: a, plastid with 
 three starch grains, s; b, plastid elongating preparatory to division; c, d, 
 later stages in division. 
 
 sugars and their transformation into starch is effected with sur- 
 prising rapidity. If some plants of pond scum are placed in the 
 dark for twenty-four hours so that all sugars and starches may 
 be consumed, starch will re-appear in the cells in these plants in 
 from three to five minutes after their return to the light. Con- 
 sider the absorptions, decompositions, recompositions, and trans- 
 formations that have been effected in this short time. Is it 
 surprising that the exact nature of the changes effected in the 
 manufacture of foods is not known? It is evident as a result of 
 the rapid formation of sugar that starch must accumulate in 
 the plastids during the day. At night sugar is no longer formed 
 but the transfer of food continues as in the day. The insoluble 
 starch is now dissolved bv means of a ferment or enz\me into 
 
i8 
 
 STORAGE OF FOODS 
 
 sugar so that by morning the cells are quite emptied of the sugar 
 and ready for another day's work. This accumulation and trans- 
 fer of foods can easily be demonstrated by cutting off in the 
 afternoon leaves from clover, bean or other starch-forming plant 
 and placing them in alcohol. When blanched and tested with 
 iodine the blue or blue-black color shows that they have accumu- 
 lated a large amount of starch. Test in the same way leaves 
 taken from these plants early in the morning. No starch re- 
 action will be seen because the cells have been freed from their 
 starch during the night. The formation of starch is often used 
 as a test for photosynthesis, but it must be borne in mind that 
 photosynthesis first results in the formation of various sugars 
 and that starch only appears when the sugars have reached a 
 certain percentage in the cells and indeed that some plants do 
 not form starch at all. So when starch appears in a leaf it is 
 simply the measure of the excess of carbohydrate manufactor 
 over transportation. Furthermore starch will form in the cells 
 in the dark if a plant is supplied with an excess of carbohydrate. 
 10. The Storage of Foods. — It has been stated that the foods 
 not required for the nourishment and growth of the plant are 
 stored in the region of buds, the growing portions of stems, 
 
 Fig. 10. Storage food: A, starch grains in cell of potato — n, nucleus. 
 B, cells of bean, the smaller particles being proteid grains. 
 
 in bulbs, in roots, in seeds, and in fruits. These are the so- 
 called storage foods and they remain in these regions for con- 
 siderable periods or until the conditions are favorable for a re- 
 newal of growth. They generally assume much more definite 
 
NATURE OF PLANTS 
 
 19 
 
 forms than the products constructed in the chlorencliyma which 
 were designed for immediate use or translocation to other parts. 
 If we examine a potato the cells will seem to be filled with 
 very regularly constructed starch grains (Fig. 10, A). In the 
 pea seed and in wheat and other grains proteid granules are 
 associated with the starch grains (Fig. 10, B). These starch 
 grains are formed by rather colorless plastids called leucoplasts 
 (Fig. 3, E) that closely resemble the chloroplasts. The starch 
 grain first appears in the leucoplast as a minute point. Its strati- 
 fied structure is due to the successive deposits of starch by the 
 leucoplasts. If the grain is built up equally on all sides by the 
 leucoplasts it will possess regular and concentric strata as in the 
 bean (Fig. 11, ^). If the bulk of the leucoplast lies on one 
 side of the grain, this side of the grain will receive more material 
 and consequently the starch grain will become more or less one- 
 
 FiG. II. Fig. 12. 
 
 Fig. II. Starch grains: A, from bean. B, from potato. C, compound 
 grain from potato. 
 
 Fig. 12. Section of the outer portion of a grain of wheat : p, cells containing 
 proteid grains. The larger cells are filled with starch. — After Strasburgcr. 
 
 sided as in the potato (Fig. 11, B). Frequently two or more 
 grains originate in one leucoplast and compound grains result 
 (Fig. II, C). Proteids and other foods are likewise transported 
 in solution to the storage organs where they may or may not 
 be deposited in solid form. This is usually efTected directly by 
 the protoplasm of the cell without the aid of any plastid. Ani- 
 mals have learned to use the foods stored in these organs just 
 as does the plant. It is interesting, however, to note that we 
 
20 NATURE OF ENZYMES 
 
 cannot use them as economically as the plant does. In the 
 wheat, for example, the proteid material is largely confined to 
 the outer portion of the grain (Fig. 12). This layer is of a 
 darker color and in the "best flour" it is excluded in the milling 
 process, leaving the flour very white. So the most nutritious 
 part of the wheat does not appear in white bread, but it should be 
 remembered that white bread is better than the much exploited 
 "whole wheat" products because the proteid material is digest- 
 ible to only a slight degree. These resei-ve foods are made 
 available to the plant in the same way as to the animal. They 
 are first put into solutions by ferments or enzymes. Ptyalin in 
 the saliva of animals and diastase in the plant are common ex- 
 amples of enzymes which change starch into sugar. So there are 
 enzymes which transform each reserve food and render it ca- 
 pable of transport and incorporation into the substance of the 
 plant body. These enzymes are formed by the living substance 
 of certain cells and resemble in many ways the living matter 
 itself. They are most extraordinary in their action and behavior. 
 The decomposition of the substances upon which they react is 
 effected by the mere presence of the ferment — at least there is 
 no permanent union between ferment and the substance. Conse- 
 quently a very small amount of a ferment may effect a decom- 
 position of an almost unlimited amount of a given substance. 
 More commonly water enters into the composition of the sub- 
 stance as a result of the presence of the enzyme. As a result its 
 stability is upset and it breaks down into simpler compounds. 
 This decomposition due to the incorporation of water is termed 
 for this reason hydrolysis. We say that diastase hydrolyzes 
 starch into simpler compounds, as maltose and dextrines. 
 Some enzymes appear to effect decompositions without the co- 
 operation of other substances. Still more remarkable is the 
 action of these ferments in that some of them appear to have a 
 reverse action, i. e., the power of building up again complex 
 material from the parts into which they have separated it. In 
 many ways enzymes behave like living matter, being quickened 
 in their activities by suitable temperatures, the concentration 
 of the fluid or by the presence of certain acids and alkalies; or 
 
NATURE OF PLANTS 21 
 
 retarded by solutions containing copper or mercury which also 
 poison living matter. The distribution of foods made possible 
 by enzymes has been compared to the currency of a country; 
 the storage foods are the bank reserve while the sugars and other 
 solutions are the circulating currency. 
 
 11. Significance of the Leaf Structure. — We now begin to 
 understand the significance of the structure and form of the leaf. 
 The broad blade is a device to catch the energy of the sunlight. 
 The stomata afford access of CO2 and escape of oxygen. The 
 intercellular spaces in the spongy mesophyll all lead to the 
 stomata, thus bringing about a quick distribution of the gases 
 and they also increase the area for the absorption of CO2 and 
 excretion of oxygen to such an extent that the internal surface 
 of the leaf exceeds the external many times. It will be noticed 
 that the cells of the spongy mesophyll vary in character. This is 
 because of the different functions that they perform. The col- 
 lecting cells (Fig. 5, col) are closely applied to the palisade cells 
 and collect the sugars and other foods from them while the elon- 
 gated conducting cells transport this food to the cells of the 
 vascular bundles. 
 
 12. The Second Function of the Leaf. — Respiration or breath- 
 ing is a second function of leaves. While work of this kind is 
 performed by all living cells it may be considered at this point 
 because in the leaves the nature of the work is \cr\ well illus- 
 trated. The w^ork of respiration is usually associated with the 
 absorption of oxygen and the giving off of CO2. Animals and 
 plants are constantly taking in oxygen and giving off CO2. This 
 interchange of gases is the reverse of that occurring in photo- 
 synthesis. Furthermore respiration goes on all the time when 
 conditions are suitable for life while photosynthesis only is pos- 
 sible in the light. Let us consider the significance of this inter- 
 change of gases. Why do animals and plants breathe.-* The 
 construction of the foods to which attention has been called may 
 well be termed the storing up of the energy of the sunlight. 
 When the chlorophyll absorbs certain rays of light it docs not 
 destroy any of the energy in those rays. The energy is changed 
 to another form and it still exists in the sugars and other com- 
 
22 NATURE OF RESPIRATION 
 
 pounds that are built up through its power. The energy is 
 locked up, so to speak, in the foods and finally it is lodged in the 
 compounds that are constructed from the foods, i. e., in the 
 living substance and in the tissues of the plant. If now any of 
 these compounds are decomposed into their constituent parts 
 then the energy will be set free unimpaired. Oxygen is the 
 principal agent in carrying on this decomposition. It has a 
 great attraction for nearly all the elements occurring in the plant 
 and so it is able to detach one element from another and so 
 effect decomposition and the liberation of the locked up energy. 
 You are not to think of the oxygen entering directly into combi- 
 nation with the foods or with the substances formed from the 
 foods but rather with the decomposition products that are 
 brought about by the living matter. It is probable that respira- 
 tion starts in the living substance. Some agent, other than 
 oxygen and possibly enzymic in nature, inaugurates these 
 decompositions and then oxygen comes in at some stage in this 
 breaking down process and makes possible a further reduction. 
 As evidence that the oxygen does not combine directly with the 
 more complex substances mention may be made of the fact that 
 the absorption of oxygen is quite independent of the amount of 
 oxygen present. The plant respires at the same rate in an 
 atmosphere of pure oxygen as in an atmosphere containing one 
 twentieth part of the oxygen in the air. Furthermore the 
 amount of oxygen absorbed is independent of the amount of 
 foods stored in the plant. So respiration is not a simple and 
 direct process like combustion where the percentage of oxygen 
 and the amount of material present determine the reaction. 
 
 In the majority of cases oxygen causes a continuation of the 
 decomposition processes until such simple substances as CO2 
 and HO compounds, such as water, are formed — in a word 
 until the very substances utilized in photosynthesis are formed. 
 Some of these simple products, such as H2O and CO2, escape 
 from the stomata as gas and water vapor. Accordingly respira- 
 tion is usually indicated by the absorption of oxygen and the giv- 
 ing off of CO2 and H2O. Some plants, however, have sufficient 
 oxygen in their tissues to enable them to breathe for considerable 
 
NATURE OF PLANTS 23 
 
 periods without absorbing oxygen from the air and on the other 
 hand in some plants CO2 is not always given oil because the 
 decomposition is not carried so far as to result in the formation 
 of so simple a product. We eat in order to gain possession of 
 the energy locked up in the foods. We breathe in order that the 
 oxygen may assist in the decomposition of these compounds and 
 set free this energy which gives us power to work and move 
 and keep our bodies warm. The plant lives in the same way. 
 It only differs from the animal in that it has the added power to 
 build up the complex food compounds from crude material. 
 By the decomposition of the products formed from these foods it 
 gains energy to grow and carry on its other vital functions. 
 The work of respiration is carried on more economically by green 
 plants than by animals since in the animal the CO2 escapes in the 
 breath as a waste product, while the plant uses this CO2 during 
 the day time for the construction of foods. Consequently the 
 escape of CO2 can only be observed during the night and cannot be 
 detected in the light unless an examination of very rapidly grow- 
 ing organs be made. If a jar be nearly filled with opening buds 
 of dandelions or rapidly growing shoots and then closed air tight, 
 sufficient CO2 will be respired in a few hours to extinguish a light 
 that is lowered into the jar. In such instances as these very 
 rapid respiration is necessar}- to furnish the required encrg\' for 
 growth and the volumes of CO2 expired exceed many times the 
 volume of CO2 utilized in photosynthesis. A handful of ger- 
 minating peas or beans placed in a closed jar for a few hours better 
 illustrates the giving off of CO2 because here there is no green 
 tissue to absorb any of the CO2. Plants are often considered 
 unhealthful in sleeping rooms at night because of their cxlialation 
 of CO2. It is well to remember that the amount of CO2 expired 
 by a plant is small and that a gas jet would furnish more CO2 to 
 the air than a window full of plants. A square meter of leaf 
 surface gives off about .12 gm. (60 c.c.) of carbon dioxide per 
 hour at 20° C. 
 
 We are now in a position to understand ilie imi->ortance of 
 photosynthesis and respiration. Photosynthesis keeps tlie air 
 pure for breathing, decomposes the simple inorganic compounds 
 
24 AMOUNT OF TRANSPIRATION 
 
 and recombines them into foods which represent a certain amount 
 of stored-up energy. Respiration breaks down the products 
 formed from these foods and Hberates the energy necessary for 
 the growth and activities of the plant. 
 
 13. The Third Function of the Leaf, Transpiration. — This 
 function refers to the giving off of water by the plant. While 
 other parts of the plant assist in transpiration, the leaf is the 
 principal organ upon which this very considerable work devolves. 
 Water is given off from the plant as a vapor and for this reason 
 transpiration is a more familiar phenomenon than photosynthesis 
 and respiration, where we are dealing with an interchange of 
 invisible gases. We see the vapor from plants growing in a 
 window precipitated on the cool window panes in the form of 
 drops. On hot summer days the leaves of plants droop. This 
 is because they have transpired so much water that their cells 
 are no longer distended by the water, consequently the cells shrink 
 and the leaves contract or wilt. When plants are covered by a 
 bell jar or placed in a tight glass jar the water given off soon 
 saturates the air and collects in drops on the sides of the jar. 
 The amount of water transpired by a plant is surprisingly large 
 and it is probably safe to state that usually it amounts daily 
 during the hot summer months to more than the plant's weight. 
 An oak with seven hundred thousand leaves was estimated by 
 Ward to transpire from June to October 244,695 pounds of water. 
 A birch with 200,000 leaves transpired 700 to 900 gallons on hot 
 summer days. This means that an acre of such trees would give 
 off in the course of the season 3,168,000 pounds of water. From 
 careful measurements of the amount of water given off by grass 
 plants it has been calculated that six and one-half tons per 
 acre may be transpired daily during the summer. It is estimated 
 that from 200 to 500 lbs. of water is transpired in the construction 
 of one pound of dry substance and that a square meter of leaf 
 surface evaporates about 50 gms. per hour. 
 
 The question naturally arises why are such large volumes of 
 water transpired? This depends in part upon the size of the loaf 
 and external conditions, the size of the leaf being determined by 
 the relation of transpiration to photosynthesis. Conditions mak- 
 
NATURE OF PLANTS 25 
 
 ing possible vigorous photosyntheses would result in extensive 
 growth and leaf development. An extensive leaf area, however, 
 may not necessitate a heavy transpiration; the amount of water 
 given off will depend upon external conditions. In warm moist 
 climates conditions are favorable for photosynthesis but not for 
 transpiration which is reduced in amount by tlie moist air. 
 So growth is vigorous, the largest leaves are developed and 
 transpiration is feeble. In warm but dry climates, on the other 
 hand, while conditions are favorable for photosynthesis the dry 
 air promotes excessive transpiration. This interferes with 
 photosynthesis, consequently very small leaves develop and the 
 volume of water transpired is correspondingly reduced. Under 
 conditions of moderate temperature and moisture as in most 
 temperate climates, the leaf assumes a size intermediate between 
 these two extremes because these conditions favor vigorous 
 photos^^nthesis and the resulting growth is not materially checked 
 by transpiration. So we see that the extensive transpiration in 
 temperate regions is in part the result of the large leaf develop- 
 ment and that this leaf area is dependent upon a vigorous photo- 
 synthesis. 
 
 Only a small part of the water absorbed by the plant is re- 
 quired to furnish the necessary mineral substances and the water 
 utilized in its growth. The surplus is forced out of the cells and 
 finds its way in the form of vapor along the intercellular spaces 
 in the spongy mesophyll through the stomata to the air. The 
 transpiration of these large volumes of water must be of impor- 
 tance in keeping down the temperature of the plant during the 
 burning summer heat. Transpiration is often compared to the 
 evaporation of water from a dish. While it is controlled to a 
 limited extent in the same manner as evaporation, it should he 
 borne in mind that the giving off of water is intimately associated 
 with the vital activities of the cells and that the loss of water is 
 to a degree under the control of the plant. The cuticle, which is 
 practically impervious to water and gases, extends as a coat over 
 all parts of the plant body, and materially assists in controlling 
 the amount of transpiration. The thickness of this protective 
 coating depends upon external conditions, being developed in 
 
26 THE STOMATA 
 
 proportion to the ratio of transpiration to absorption. So we 
 find that it becomes thicker as the volume of water given ofT 
 comes nearer and nearer to the volume absorbed. In this way it 
 serves as a very effective check in preventing a fatal loss of water. 
 As a result of this water-proof coat the vapor can only escape from 
 the leaf through the stomata. These minute openings are one 
 of the most interesting features of the leaf. When there is an 
 adequate supply of water the elliptical guard cells are drawn apart 
 and give free exit to vapors and gases. However in cases of 
 drought when a continued loss of water would prove harmful to 
 the plant, or when an interchange of gases is no longer required 
 then the guard cells close and so offer such a barrier against 
 further loss of water that only the severest conditions, such as 
 prolonged heat and drought, can overcome. It has been claimed 
 that the stomata do not regulate transpiration because it has 
 been observed in some cases that the stomata are not closed when 
 wilting begins or fully opened at the time of maximum transpira- 
 tion. Further observation is required to settle this question and 
 for the present we must believe that so elaborate a mechanism 
 as the guard cells is of significance. The stomata in the majority 
 of cases range in size from .0002 sq. mm. to .0008 sq. mm. so that 
 in comparison a needle prick would appear as a huge hole, but 
 they are so numerous, 40 to 300 to the sq. mm., as to comprise 
 about I per cent, to 3 per cent, of the area of the leaf surface. 
 It might be questioned if this extent of opening is sufficient to 
 permit the entrance and exit of the large volumes of gases and 
 water handled by the plant. It has been shown that a mem- 
 brane pierced by sufficiently small openings which are separated 
 from one another by a distance of at least ten times their diam- 
 eter, permits diffusion of gases as readily as though there were 
 no membrane at all. The structure of the epidermis is an 
 admirable example of such a perforated membrane and it is so 
 perfectly adjusted to the work in hand that it could accomplish 
 much more than the necessities of the plant demand. 
 
 14. Noteworthy Features of Leaves. — Now that some idea has 
 been gained of the nature and extent of the work performed by 
 the leaves Ave are prepared to comprehend the meaning of the 
 
NATURE OF PLANTS 27 
 
 large extent of the leaf surface and of the arrangement, siruciure 
 and modification of the leaves. The broad blades are devices for 
 gathering or absorbing as much of the sunlight as possible and 
 they increase the surface of the plant many hundreds of times. 
 Contrast the extent of surface of a tree in full foliage with the 
 bare branches of the winter. A mature maple develops annually 
 from one to two thousand square yards of leaf surface. 
 
 One of the most noteworthy features about the leaves is their 
 arrangement or "hang" on the branches. In some plants they 
 are arranged in two vertical rows, in other instances three, four, 
 five, eight or more rows. By fastening a thread to a leaf and 
 winding it about the stem so as to touch the petiole of each suc- 
 ceeding leaf the arrangement of the leaves becomes more obvious. 
 Some plants have their leaves opposite in two rows or ranks, 
 others opposite in four ranks, each succeeding set of leaves being 
 at right angles to the lower set. This latter arrangement is called 
 decussate (Fig. 13). More commonly the leaves are spirally 
 arranged and the thread passed once, twice, thrice, five, eight, 
 etc., times around the stem before a leaf is reached that is exactly 
 over the one from which we started (Fig. 14). The number of 
 leaves passed before reaching one that stands over the first leaf 
 indicates the number of rows or ranks of leaves on the stem. This 
 variation in the arrangement of the leaves is simply a device to 
 bring the leaves into the light and prevent the shading of one leaf 
 by another. In opposite two ranked leaves each succeeding leaf 
 is over the one below it but the cutting off of the light is pre- 
 vented by lengthening the intemodes and thus separating the 
 leaves. The decussate arrangement of the leaves is a more eco- 
 nomical arrangement because more leaves can be developed on 
 a given length of stem without danger of shading. The spiral 
 distribution of leaves is still a better device since each succeeding 
 leaf is placed a little above and to one side of the next lower leaf. 
 By this means the maximum of leaf surface can be exposed to 
 the light in a given length of stem. No one can look at the leaves 
 of the maples, dogwoods, ailanthus, creeping and climbing vines, 
 etc., and not be impressed with the fact that the leaves are 
 arranged so as to catch the most favorable light without shading 
 
28 
 
 LEAF ARRANGEMENT 
 
 or interfering with one another. Furthermore the arrangement 
 is such as to occupy practically all the space about the stem that 
 
 Fig. 13. Fig. 14. 
 
 Fig. 13. Branch of Forsythia, leaves decussate, in four rows. 
 Fig. 14. Branch of poplar with leaves spirally arranged in five rows. 
 
 receives light. This becomes very noticeable if we look directly 
 down upon a shoot or better if we stand under a tree and look 
 up into the branches. It will be seen that very little direct sun- 
 light finds its w^ay through the branch, so nicely are the leaves 
 adjusted to each other. Some plants like the hickories, catalpa, 
 etc., occupy all the available space with a few large leaves at the 
 tips of the branches. In other cases, as in the willows, some 
 lilies, etc., the same result is accomplished by many small leaves 
 that can be arranged along the stem for considerable distances 
 without shading. The nicety of leaf arrangements is especially 
 noticeable in many horizontal and creeping stems. In such cases 
 the leaves can only be exposed on the sides of the stem and con- 
 sequently the stem may become twisted or more usually the 
 
NATURE OF PLANTS 
 
 2Q 
 
 petioles are variously elongated and curved in order to bring the 
 leaves into proper relation to the light (Fig. 15). Notice the 
 
 Fig. 15. Horizontal branch of Forsythia with leaves in two rows owing to 
 the alternate twisting of each intcrnode, assisted by the curving of the petioles. 
 Compare Fig. 13. 
 
 change in leaf arrangement in horizontal and erect stems of maple. 
 In an example like the maple if the leaves could not change their 
 position they would all be standing edgewise to the light on hori- 
 zontal branches and therefore receive little of it. They not only 
 place their blades at right angles to the light but owing to the 
 greater elongation of each succeeding petiole from the apex 
 toward the base of the stem, all the leaves are arranged one be- 
 yond another so as to overlap very little. This same device is 
 noticed in many plants that produce their leaves close to the 
 ground in rosettes, as mullein, wood betony, plantain, etc. (Fig. 
 
 f^ 
 
 Fig. 16. Leaf rosette of evening primrose. Each leaf blade has a longer 
 petiole than the one above it, so that the leaves are not shaded ^^• ""- •"•.''!■' 
 
 16). Compare the erect and horizontal branches of a variety i>l 
 plants noting by what devices the leaves are brought into the 
 
30 
 
 FORM OF LEAVES 
 
 light. Especially instructive are the leaf arrangements of plants 
 growing in windows or creeping over trellises, etc., where compli- 
 cated twisting and elongation of stems and petioles are necessary 
 to adjust the leaves to the one-sided illumination. The angular 
 leaves of some begonias furnish excellent illustrations of this. 
 Perhaps so many plants bear angular leaves because they can be 
 better adjusted and fitted together without loss of space than 
 would be the case in rounded leaves, and smaller leaves can also 
 be more advantageously introduced between the larger ones 
 (Fig. 17). The lobing and branching of leaves has become a 
 characteristic of many plants because such variations permit the 
 
 Fig. 17. Leaf of Aralia. Note the angular shape of the leaflets and the 
 smaller ones filling the space between the larger ones. 
 
 illumination of a larger leaf surface. This is particularly notice- 
 able in our oaks where the outer leaves are often deeply lobed, 
 thus permitting considerable light to pass through to the under- 
 lying leaves. Observe also that these lower leaves are larger and 
 less lobed, thus catching as much as possible of this rather feeble 
 light (Fig. 18). In many plants the lobing extends quite to the 
 middle of the leaf and the lobes are often attached to the midvein 
 or midrib by a petiole. In this latter case the leaf is said to be 
 compound (Fig. 19). All such modifications permit the develop- 
 ment of numerous leaves upon the branches without the danger 
 of shading. If there is still any doubt as to the perfection of this 
 light catching arrangement of the leaves, try to substitute the 
 somewhat similar leaves of two different trees as the birch and elm 
 
NATURE OF PLANTS 
 
 31 
 
 or the water beech (Carpimis) and the beech, noting how the 
 leaves of one fit when placed upon the branch of the other. This 
 subject of leaf form and arrangement may be summed up by the 
 statement that these features are devices for exposing the 
 maximum leaf surface without one leaf interfering with another. 
 This is in accord with the fact that light is practically of no value 
 in photosynthesis after passing through two leaves. 
 
 Another interesting feature about the leaf is its relation to the 
 
 Fig. 18. Fig. 19. 
 
 Fig. 18. Leaves of red oak: A, sunned leaf. B, shaded leaf. 
 Fig. 19. Compound leaves: A, red ash. B, horse chestnut. 
 
 intensities of light. Some plants demand tlie full intensities of 
 sunlight while others can tolerate only a small fraction of it. 
 Some lichens will grow in a light only 1/156 part of the full in- 
 tensity while many plants, as the grasses, will endure the strongest 
 illumination. Beech, maple and spruce are tolerant of shade and 
 oak, hickory and chestnut are intolerant. This relation of plants 
 to light is an important factor in controlling their associations 
 and we frequently speak of plants as sun loving, partially so, 
 shade, and deep shade plants. 
 
 The coloration of leaves is often of significance. In addition 
 to the green color, of which we now have some iindcrst uuluig. 
 
32 LEAF COLORATION 
 
 a variety of other colors often appears. One of the most common 
 is red. Young shoots of the rose, grape vine and so much of the 
 early spring vegetation are tinged with red. So in the fall our 
 vegetation takes on a greater wealth of coloration than is seen 
 in any other land. And finally there are leaves variegated with 
 yellow, white and red, colored fruits and other organs, and the 
 endless hues of the flowers. These colors are caused by pigments 
 or by chromoplasts (p. 2) that are developed in certain cells and 
 which either transmit or reflect to the eye their particular colors. 
 The white blotches of variegated leaves are caused by the absence 
 of chlorophyll, thus allowing the light to pass practically un- 
 changed, or in some cases these areas are characterized by large 
 intercellular spaces filled with air. Such a structure would tend 
 to reflect the light and so contribute to the white appearance of 
 the area (p. 39). In some cases these colors may be of service to 
 the plant. The red probably functions as a screen to young or- 
 gans, shielding them from the intense light that would otherwise 
 decompose the forming chlorophyll and otherwise interfere with 
 the vital processes. Red is also a strong absorber of the heat 
 rays in light, and in some cases it may be of service in ensuring a 
 higher temperature in the organs and so expedite their work. 
 Many plants, some tradescantias, hawkweeds and spatter dock, 
 have in their mature leaves layers of cells, frequently the low^er 
 epidermis, filled with a red pigment. So many evergreens assume 
 a brown or reddish hue in the winter owing to the formation of a 
 reddish pigment. However, too much emphasis should not be 
 given to this matter of coloration. The colors of flowers, to be 
 sure, are of service in guiding insects when close at hand, to the 
 proper approach to the flower, and there is probably a significance 
 in the coloration of the algae (p. 173) but in general it is not 
 possible at present to offer an explanation for the variety of colors 
 that characterize so many leaves, especially noticeable in tropical 
 plants, and fruits and other organs, as the radish, and beet. 
 The copper beech grows slower than our green-leaved beech 
 and so doubtless in the case of many plants that have survived 
 not because of their coloration but in spite of it. In many 
 instances the colors are due to the acidity or alkalinity of the 
 
NATURE OF PLANTS 33 
 
 cell sap. Red cells often Indicate acidity and they become blue 
 in weak potash solutions. So violet and blue indicate an al- 
 kaline condition and such cells change to red in weak acid solu- 
 tions. This is essentially the explanation of the changing tints 
 in the autumn. The changing character of the cell sap is at- 
 tended with a gradual decomposition of the complex chloroi)liyll 
 and other materials that results in the formation of a series of 
 substances that are characterized by a variet>' of colors. Here 
 again the matter of coloration has been over emphasized. While 
 nitrogen, phosphorus and potassium are transported from the 
 leaves before they fall, the coloration does not effect so complete 
 an emptying of the leaves as was previously believed, if indeed it 
 is of any service. 
 
 15. The Cause of Leaf Arrangement. — How is the perfection 
 in the arrangement of the leaves accomplished? The leaf or 
 any other organ does not remain in a fixed position during its 
 growth and development but owing to the fact that first one side 
 and then an adjoining side of the organ is growing faster than 
 any other part it comes about that the organ is bent from side to 
 side by the more rapidly growing cells and the apex of the organ 
 is often caused to travel through a rather irregular circle. These 
 growing organs are sensitive to light, gravity, moisture and other 
 stimuli. As a result of these movements the organ is brought 
 into relations with various intensities of light, heat, etc. In cer- 
 tain positions the stimuli are not favorable and they cause it to 
 grow away from this position while in other positions the stimuli 
 act in the most favorable way and the organ is stimulated to its 
 best growth. So during the development of the orv^^an it is 
 brought into varied relations with the forces which affect it and 
 as a result of this experience the leaf or other organ is directed 
 by the stimuli and at maturity finally comes to rest in a fi.xed 
 position that is the most advantageous. The leaf ordinarily so 
 reacts to these stimuli that its blade is exposed at right angles 
 to the rays of light, but in the iris, many grasses and rushes 
 the blades are nearly erect. Quite a large numluT of plants l)ear 
 their leaves edgewise. The giant trees of Australia (Eucalyptus) 
 and the so-called compass plants are famili.ir txanipK's (Fig. 
 3 
 
34 
 
 LEAF ADJUSTMENT 
 
 20). Possibly these blades are driven into this position because 
 they are more sensitive and the direct sunlight upon the broad 
 surface of the blade would be injurious. This certainly appears 
 to be the case in young leaves when emerging from the bud. 
 Note the character of the foldings and the erect positions of such 
 leaves (Fig. 21). The leaves of the horse chestnut assume at 
 
 Fig. 20. 
 
 Fig. 21. 
 
 Fig. 20. Shoot of wild lettuce, Lactuca, with leaves turned edgewise to 
 the light. 
 
 Fig. 21. Position of young leaves of hickory: A, scales of bud curving 
 back showing the tightly folded leaves. B, later stage, leaves unfolding but 
 still erect. 
 
 least three different positions during their growth. The folding 
 of such leaves and their erect positions expose less surface. They 
 consequently lose less heat and moisture, and the delicate grow- 
 ing cells are better protected against the intense sunlight. You 
 must notice as the cells mature and become better protectet that 
 their irritability changes and they respond in a different way 
 to light and gravity, etc. , They begin to unfold and turn away 
 from their erect positions and at maturity assume a fixed posi- 
 tion to light, etc., that is quite different from the original posi- 
 
NATURE OF PLANTS 35 
 
 tion. The position of the leaves of our pines and several other 
 evergreens is not controlled by light and as a consequence they 
 assume a variety of angles to the incident rays. 
 
 The leaves of many plants do not have a fixed position but 
 show more or less motility during their entire life. Interesting 
 examples are seen in the sensitive plants and in nirmbers of 
 the bean and oxalis families where marked changes result in 
 the position of the leaves from alterations in the intensity of 
 light, temperature, or moisture. Many of these plants fold their 
 leaves when the temperature falls at sundown. Many flowers 
 close in the same way. These changes are popularly termed sleep 
 movements. They are caused not only by the changes in the 
 intensity of the light but also by changes in temperature and 
 moisture. In the case of these leaves the cells in tin- sw( lien 
 organ (the pulvinus) at the base of each leaf are kept full of 
 water so long as the temperature and external conditions exert a 
 suitable stimulus upon the plant, but when the conditions are 
 unfavorable certain of the cells on one side of the pulvinus lose 
 water and contract while the cells on the opposite side of the 
 pulvinus remain rigid; consequently, the leaves droop and fold 
 in various ways. This is due in part to the fact that the con- 
 tracted cells of the pulvinus are no longer capable of assisting in 
 the support of the leaf and also to the fact that the rigid cells con- 
 tinue to exert a pressure which bends the pulvinus towards the 
 side where the contracted cells are situated (Fig. 22). This fold- 
 ing of the leaves materially reduces the area exix)sed to the 
 atmosphere and consequently will lessen transpiration. Many of 
 our sensitive clovers {Meibomia, Lespedeza), sensitive peas (Cas- 
 sia), sensitive plant (Mimosa), are able to check the excessive 
 loss of water on hot dry days by the folding of their leaves. As 
 soon as the loss of water becomes detrimental some of the leaves 
 begin to fold and the reduction of leaf surfuc e continues until the 
 loss of water by transpiration is met b>' root absori)tion. \'ery 
 commonly leaves that do not have this power of adjustment are 
 able to accomplish the same results by a rolling of the leaf. This 
 is particularly noticeable among the grasses, which, on hot, dry 
 days, roll up their leaves so tightly as to rhange the appear- 
 
36 
 
 NATURE OF THE EPIDERMIS 
 
 ance of the plants. Observe a corn field during a dry period. 
 The interesting feature of all this is that these reactions and 
 adjustments are purposive but not intelligent. The movements 
 are not called forth by consciousness but by stimuli to which the 
 irritable living substance is attuned or sensitive. 
 
 1 6. The Significance of Certain External Leaf Structures, — 
 Many features connected with the structure of the leaf furnish 
 admirable illustrations of the fitness of the leaf for the perform- 
 
 FiG. 22. Leaf position of the sensitive plant, Mimosa: A, in light. B, in 
 darkness. The same movements occur as a result of unfavorable temperatures 
 and humidity. 
 
 ance of its work. The stimulus of light and moisture have a 
 marked influence upon the external structure and form of leaves. 
 The epidermis is strikingly modified by such forces. In shade 
 plants the epidermis consists of a rather delicate layer of cells 
 with very thin cuticle. This gives sufficient protection to such 
 plants, but in leaves exposed to intense sunlight and a hot and 
 dry air, the epidermal cells become greatly thickened and often 
 of two or more rows, while the cuticle may often form the larger 
 part of the outer cell wall or even extend in between the cells 
 (Fig. 23, c). Cell walls that are filled with cutin in this manner 
 are said to be cutinized. This development of cutin gives the 
 tough leathery aspect to many leaves. Such features are partic- 
 ularly noticeable in desert plants, in long-lived leaves of many 
 
NATURE OF PLANTS 37 
 
 evergreens, as laurels and rhododendrons, in the needles of cone- 
 bearing trees, and in all plants that are exposed to the hot, dry air 
 of summer or the drying winds of winter. Such leaves may be 
 further protected and strengthened by thick-wailed elon^^ated 
 cells, sterome (Fig. 23, st). Coatings of wax, mucilage and 
 lime are also frequently developed upon the cuticle to further 
 reinforce the impermeability of the epidermis. 
 
 The stomata are also so developed as to meet the conditions 
 under which the plant grows. In dorsiventral leaves they are 
 more numerous on the under side of the leaves because they are 
 less liable to be filled with water by rains and with dust which 
 would prevent the interchange of gases. The plugging of the 
 
 Fig. 23, Cross-section of the outer cells of a leaf of pine, showing the firm 
 character of the outer cells of the tough leaf: s, stoma; e, epidermis; c, cuticle; 
 st, stereome; m, mesophyll cells, 
 
 stomata by dust is one of the causes of the sickliness of plants in 
 homes. The arrangement of the stomata on the under side of the 
 leaf is also of especial advantage because the direct light does not 
 fall upon them and cause an excessive loss of water. The stomata 
 of floating leaves, however, are upon the upper surface and their 
 stoppage with water is prevented by waxy coatings, as ran l)e 
 easily demonstrated by dipping a leaf of a water lily or si^iittT 
 dock in the water. When the leaf is remo\ed the watt-r runs 
 off of the waxed surface without wetting it. Some leaves are 
 more or less erect, as the cattails, rushes and grasses, and these 
 have the stomata developed more or less evenly on Ixjtli surfaces. 
 Doubtless the intense light of midday is not beneficial to these 
 leaves and the blade of the leaf is consequently placed parallel 
 
38 
 
 LOCATION OF STOMATA 
 
 with the sun's rays. This arrangement of the leaf permits a 
 direct illumination of the leaf only in the morning and afternoon 
 when the intensity of the light is feebler. 
 
 The position and character of the stomata in relation to exter- 
 nal conditions show many interesting relations. Plants living in 
 the shade or in the presence of an abundant soil-moisture develop 
 
 Fig. 24. Cross-section of a leaf of the inch plant, Tradescantia, showing 
 the delicate character of the cells and the raised stoma of this shade plant. 
 
 the stomata on a level with the leaf surface (Fig. 24), because 
 there is no necessity of conserving the water supply. For the 
 same reason some aquatics have lost altogether the power of 
 closing their stomata. On the other hand plants that are exposed 
 to arid conditions or drying winds develop the stomata well 
 below the surface of the leaf, as in the cactus and in the needles 
 of conifers (Fig. 23, s), or in furrows, as in certain grasses, or 
 at the bottom of minute pores, as in the oleander. These de- 
 pressions remove the stomata from the dry winds and prevent 
 the direct contact of the moist air in the leaf with the dry atmos- 
 phere. The pores in the oleander contain hairs which would 
 check transpiration just as a plug of cotton in the neck of a 
 flask would lessen the evaporation of the fluid in the flask. The 
 chief purpose of this arrangement, however, is to prevent the 
 stopping of the stomata with water. This plant grows naturally 
 along the banks of streams where it is subject almost nightly to 
 heavy dews. When the stomata become filled with dew it re- 
 
NATURE OF PLANTS 39 
 
 quires several hours of sunshine to drive the moisture from these 
 capillary openings. Consequently during this time there could 
 be no interchange of gases and the vital functions of the leaves 
 would be largely stopped. Many devices appear that prevent 
 water entering the stomata. Attention has been called to the 
 development of stomata on the under surface of leaves and to 
 waxy coatings as protective features of this nature. Every one 
 is familiar with the waxy appearance or bloom of many fruits, 
 as the plum; or of leaves, as the cabbage; and of stems, as the 
 raspberry. Water is unable to spread over the wax and enter 
 the stomata because a thin layer of air clings to it. The silvery 
 appearance of many grass leaves and of the jewel-weed when 
 immersed in water is due to this thin film of air which reflects 
 the light. Accordingly the water does not really touch the leaf 
 and you notice that it is quite dry when it is removed from the 
 water save for a few drops which readily run off. Protection 
 against wetting is also obtained by the development of coatings 
 of hairs. This is noticeable in many Alpine and polar plants 
 where they are constantly exposed to hea\y fogs, dews, and rains. 
 Plant hairs do not always serve this purpose. Desert plants 
 and those living in dry localities are frequently characterized by 
 hairy coatings. Such plants are frequently termed xerophytes 
 in contradistinction to aquatics or hydrophytes and to plants 
 living in moderately moist soils, which are called mesophytes. 
 The hairy coatings of xerophytes give them their familiar silvery 
 or hoary appearance as seen in the sage-brush, dusty miller, and 
 mullein. The hairs are usually empty tubes or cells which re- 
 flect a large portion of the intense light and heat and consequently 
 materially lessen the loss of water, just as straw sprinkled upon 
 the ground keeps the earth below moist and cool. In many in- 
 stances plant hairs develop mucilage or resinous secretions which 
 protect as with a varnish growing regions as buds or young shoots 
 and leaves of birches, alders, poplars, and peach. In this con- 
 nection it should be noted that dense hairy coatings reduce the 
 loss of water only when the plant is exposed to air currents — 
 not In still air. Cutin, wax and resinous deposits retard trans- 
 piration under all conditions. Oil and poisonous substances arc 
 
40 ARRANGEMENT OF THE MESOPHYLL 
 
 also formed which repel by disagreeable odors or tastes the 
 attacks of animals. The nettles are shunned because of the 
 burning sting produced by the plant hairs. The upper portion 
 of these hairs (Fig. 25) is practically a delicate glass tube since 
 the walls are filled with silica. The basal portion of the hair is 
 composed of soft, yielding cellulose so that the fluid collecting 
 in the hair distends this portion of it. The knob at the end is 
 fastened to the hair by so thin a ring of wall, as seen in Fig. 25, 
 B, that the least touch breaks it off. In this way the hair is 
 
 Fig. 25. Stinging hair of nettle: A, portion of epidermis bearing hair. 
 B, tip of hair enlarged, showing easily detachable knob. — I. D. Cardiff. 
 
 transformed into a veritable hypodermic needle which easily 
 punctures the skin while the distended basal portion contracts and 
 forces out some of the fluid. The burning sensation or sting 
 that immediately follows the puncture of the skin is caused by 
 the injection of formic acid while a variety of other poisons pro- 
 duce the subsequent irritations. These poisons are so powerful 
 in some of the East Indian nettles as to produce serious results, 
 even a tetanus. 
 
 17. Significance of Certain Internal Leaf Structures. — The 
 changes or modifications produced in the chlorenchyma by vari- 
 ous stimuli are quite as striking as in the case of the epidermis. 
 The palisade cells are an excellent illustration of this point. 
 These cells are developed as a result of the stimulating influence 
 of light (Fig. 26). The photosynthetic activity caused by the 
 light increases the pressure of the fluids in the cells. This 
 
NATURE OF PLANTS 
 
 41 
 
 tends to round them out. They are unable to expand laterally 
 owing to the adjacent cells but are free to extend towards the 
 looser cells below, and so they become lon^^t-r and palisaded. 
 Leaves growing in the deep shade show little indication of palisade 
 
 Fig. 26. Fig. 27. 
 
 Fig. 26. Section of a leaf of Rhododendron. Note the compact palisade 
 tissue which results from intense hght. 
 
 Fig. 27. Section of a leaf of skunk cabbage, Spathyema. Note the poorly 
 developed palisade tissue and the loose arrangement of the cells of this plant 
 which lives in moist, shaded places. 
 
 structure. Shade and moisture plants are not obliged to conserve 
 the amount of water received and these two forces, feeble light 
 and moisture, produce a larger and looser arrangement of tissues 
 which is favorable to an interchange of gases (Fig. 27V Par- 
 ticularly is this noticeable in aquatics as in the water lilies and 
 many rushes, etc., where the tissue is loose and six)ngy and 
 permits a ready circulation of gases from the leaves to the roots 
 and to all parts of the plant body even when submerged. This 
 loose arrangement of tissues also renders water j^lants very 
 buoyant and consequently they are less liable to injury from 
 the currents of the water. Aquatic plants have little need of 
 strengthening and conducting tissues because they arc supfx^rted 
 
42 LEAF FALL 
 
 by the water. Neither do they require elaborate bundles for the 
 conduction of the crude foods which may be absorbed by nearly 
 all parts of the plant body. We now understand why shade leaves 
 are thin and broad and soft whereas leaves of desert plants and 
 those exposed to severe drying winds of summer or winter are 
 thick, compact, firmer and often leathery and hairy. 
 
 One of the most interesting adaptive features of leaves is seen 
 in the leaf fall of our deciduous trees and shrubs. In the tropics 
 the leaves remain on the perennial plants often for long periods, 
 but in temperate climates the severe winters necessitate the 
 annual dropping of the leaves except in a comparatively few 
 evergreens where the thick leathery leaves are able to endure 
 such conditions. In our deciduous plants, when the conditions 
 are no longer favorable for the performance of leaf work, the 
 cells at the base of the petiole begin to divide and a delicate 
 layer of cells, the separating layer, is formed across the petiole 
 (Fig. 28). Various conditions induce this growth for there is 
 more or less leaf fall at all seasons of the year. Doubtless lack 
 of nutriment has much to do with it. If a leafy branch of horse 
 chestnut is placed between moist paper in a few days the separa- 
 ting layer will have formed and the leaf will have dropped from 
 the branch. The innermost delicate cells of the separating layer 
 may become changed into cork cells just before or directly after 
 leaf fall while the outer cells of this layer may break down 
 through mucilaginous modification or they may become rounded 
 off so that water collects in the intercellular spaces. Thus the 
 leaf is attached to the stem chiefly by the veins since the delicate 
 separating layer offers little support. If now the water in the 
 intercellular spaces of the separating layer should freeze, the 
 expansion of the water as it freezes would so rupture the remain- 
 ing tissue that we would have the familiar sight of the leaves 
 falling in a shower in the morning after a frost — either of their 
 own weight or with the slightest breeze. It will also be noticed 
 that owing to the formation of the cork layer or the drying up 
 of the delicate cells of the separating layer, the scar formed by 
 the fall of the leaf is nicely healed and closed against any loss of 
 fluids or the entrance of any organism (Fig. 28, C). 
 
NATURE OF PLANTS 
 
 43 
 
 The casting off of the leaves reduces the area of the plant 
 that is exposed to the unfavorable conditions to a minimum. 
 This reduction of the surface is effected in one way or another 
 by a great variety of plants. Much of our spring vegetation 
 is possible, by reason of it. The spring beauties, anemones, fawn 
 lilies, jack-in-the-pulpits, etc., practically complete their growth 
 
 
 Fig. 28. Leaf fall: A, branch of horse-chestnut showing scar formed by 
 the fall of the leaf. The dots on the scar show the position of the vascular 
 bundles that are finally broken by the weight of the leaf. At the left the 
 base of the petiole is shown. B, diagram of a section through a twig of hickory 
 — s, separating layer at base of petiole; v, vascular bundles. C, enlarged 
 view of the separating layer — c, cork cells that heal the wound caused by the 
 fall of the leaf. The granular cells are the outer region of the separating 
 layer, and they are beginning to break down as seen in the upper jxirt of the 
 figure, at x, thus causing the fall of the loaf; st, cells of the stem containing 
 starch. 
 
 before the larger summer forms appear. I )uring this short pcricxl 
 sufficient food is manufactured by the leaves to mature the seeds 
 and fill the storage organs in the underground l)un)s and stems. 
 
44 NATURE OF DESERT PLANTS 
 
 When the larger forms appear later In the season these plants 
 are overshadowed, their leaves wither, and they are reduced to 
 the small bulbs and stems hidden in the earth. In this condition 
 they remain until the next spring when the abundance of stored 
 food enables them to complete their growth before other com- 
 peting forms appear. 
 
 This ability to reduce the surface of the plant body makes 
 possible the existence of much of the plant life of arid and desert 
 regions. During the short rainy seasons the stored foods enable 
 the plant to quickly put forth the leaves which in turn manu- 
 facture the foods which are stored in seeds, stem and roots. 
 Then practically all trace of these plants is destroyed by the 
 drought. Only the seeds or a greatly reduced portion of the 
 plant remain alive and this is often further protected by being 
 hidden in the ground. A return of favorable conditions quickly 
 awakens these plants to growth. Mr. Dan Beard relates an 
 interesting experience in the desert regions of Texas that illus- 
 trates this feature of plant life. At the close of the dry season 
 this territory was a bare, sun-cracked plain swept by hot, dry 
 winds. Not a green leaf was to be seen. Heavy rains trans- 
 formed this level tract of land into an inland sea. In a few days 
 the water disappeared and one morning he was surprised to see 
 green blades appearing everyw^here and in a few days the barren 
 earth was covered with an almost tropical profusion of vegeta- 
 tion of bright flowers. Many desert plants to be sure develop 
 permanent aerial parts but here, too, the most striking feature 
 is the extreme reduction of the organs. Compare the areas of 
 the Spanish bayonet, aloes, grease wood and sage with that of 
 our leafy plants. The cactus represents one of the extreme forms 
 of reduction. The leaves have been dispensed with entirely and 
 are not represented save possibly by the spines. Consequently 
 the work of the leaves devolves upon the stems. While desert 
 plants receive very meager amounts of water, this is so effectually 
 conserved that the plants are usually rich in water. Drinking 
 water is frequently obtained in desert regions from cacti by 
 pounding up the pulpy interior and squeezing out the water. 
 Animals are well aware of this rich storehouse and will eagerly 
 
NATURE OF PLANTS 45 
 
 devour the cacti after the spines are burned ofT. The develop- 
 ment of these storage cells accounts for tlie fleshy character of 
 many alkaHne, saline, and desert plants, many of which depend 
 upon organic compounds lodged in their cells for the retention of 
 water rather than upon the development of cutin. Curiously 
 enough there are many examples of xerophytic plants living in 
 bogs and marshes, as, for example, the rushes and sedges, the 
 horse tail ferns (Eqiiisetum), the lamb kill {Kalmia) and leather 
 leaf {Chaynccdaplme), etc. The cause of the association of these 
 plants with aquatic forms is not known. In exposed moors and 
 heaths these reduced forms would have decided ad\'antage 
 because of their protection against drying w^inds. Our sedges and 
 rushes are exposed to very intense heat and light which may 
 possibly cause so heavy a transpiration that these plants are not 
 able to meet the loss by root absorption. This is the more 
 probable because in many instances the absorption of water by 
 plants living under such conditions is slow owing to lack of 
 oxygen and to the concentration of the water in which the roots 
 grow^ 
 
CHAPTER II 
 
 THE ROOT 
 
 1 8. Character of Primitive Plants. — The first forms of life 
 upon the earth were doubtless unattached unicellular plants that 
 lived in the water or moist places. All the varied functions of 
 the plant body, as the absorption of gases and material from 
 the soil and the manufacture of organic compounds, etc., were 
 performed alike by each cell. This was easily accomplished 
 owing to the simplicity of the organism and also because the plant 
 was surrounded by water which contained all the substances re- 
 quired in the construction of foods. Perhaps the first change 
 produced in these plants by their surroundings was a modification 
 of a part of a cell so that it served as an organ of attachment, or 
 root, anchoring the plant to the substratum. As the plant con- 
 tinued to change and become more complex, and especially as it 
 became more and more subject to drier conditions, special organs 
 for the absorption of materials became a necessity. This change 
 from an aquatic to a terrestrial life finally left the root alone in 
 touch with the crude materials. Thus the root which at first was 
 only an anchoring organ, came later to function also as the prin- 
 cipal absorbing organ of the plant. Next to the leaf the root 
 commands our attention because of its fitness for the accomplish- 
 ment of this work. Practically all the water required by land 
 plants, and all the elements, save carbon and oxygen^ utilized in 
 the construction of foods are absorbed by the roots. 
 
 19. Root Hairs. — We will first be interested to examine the 
 structure and nature of the absorbing apparatus of the root. If 
 seeds of radish, mustard, or other plants are germinated upon 
 moist sand or moist blotting paper it will be seen when the roots 
 have attained a length of several cm. that a portion of each root 
 is covered with delicate hairs (Fig. 29, A), There are several 
 interesting features about these root hairs. In the first place they 
 are extremely delicate tubular outgrowths of the epidermal cells, 
 
 46 
 
NATURE OF PLANTS 
 
 47 
 
 scarcely more than one millimeter in length (Fi^. 30). Note 
 that they always begin to grow a few mm. back from the root 
 tip and at the opposite end of the zone of root hairs the lubes are 
 withering or dying off, leaving the older portion of the root 
 covered only with the epidermis. If a root is marked into 2 mm. 
 spaces with India ink it will be found after 12 to 15 hours that 
 only the portion below the root hairs is elongating. The first 
 space from the^ip has grown very little, the second and third 
 
 Fig. 29. Fig. 30. 
 
 Fig. 29. The absorbing surface of roots: A, seedling of mustard showing 
 extent of surface covered by root hairs. B, an older branching root, the 
 shaded areas near the tip are due to particles of earth clinging to the root hairs. 
 The remaining portions of the root are free hairs and take little or no part in 
 absorption. 
 
 Fig. 30. Origin and character of root hairs: A, first appearance of a root 
 hair due to the pushing out or growth of a definite portion of the surface of an 
 epidermal cell. B, older hairs that have become irregular through coming 
 in contact with soil particles. 
 
 much more, and in the other spaces there is a gradual retarding 
 of the growth until 8 to 12 mm. from the tip elongation has 
 ceased. It is at the point where elongation has ceased that the 
 root hairs begin to appear, but the total number existing on the 
 root at any one time does not materially vary owing to the fact 
 that they are dying off farther back on the root with the same 
 rapidity that characterizes their formation near the tip (Pig. 
 29, B). Before the significance of these peculiar features can be 
 
48 NATURE OF SOILS 
 
 understood it will be necessary to consider the nature of soils 
 and the relation of the substances absorbed by the plant to the 
 soils. 
 
 20. The Nature of Soils. — The soil is composed of minute 
 particles derived from decaying rocks and of organic particles 
 or humus derived from decaying plants and animals. The 
 mineral particles furnish several of the Important crude plant 
 foods, as calcium, magnesium, potassium, pho^horus, sulphur, 
 iron and small amounts of other elements. These substances 
 occur in great abundance, comprising over thirteen per cent, of 
 the earth's crust. It is a singular fact that two of the most 
 abundant substances, silicon and aluminium, are utilized by 
 only comparatively few plants, although it will be seen later on 
 that the latter element is indirectly of great service to plant life. 
 The remaining elements used by the plant, oxygen, nitrogen, 
 carbon, hydrogen, are equally abundant. The first three may be 
 obtained from the air while the hydrogen is a constituent of water 
 and other compounds. It is an interesting fact, however, if 
 rocks containing all these mineral elements should be ground up 
 into the form of a good soil and the other necessary substances be 
 added thereto that It would not support plant life at all. The 
 reason Is that these elements exist In the rocks In a form that the 
 plants can not utilize. They are largely combined with an ele- 
 ment silicon, of which substance pure sand or quartz is largely 
 composed. These compounds are termed silicates. The plant 
 can only utilize these elements when they are combined with some 
 element, other than silicon, and to which oxygen has been added. 
 The potassium, magnesium, etc., must be combined with nitro- 
 gen, phosphorus, sulphur or carbon to which oxygen is also added 
 In order to make a substance that the plant can utilize. These 
 crude foods of the plant are termed nitrates, phosphates, sul- 
 phates and carbonates. It is only In this form that the necessary 
 elements can be absorbed, silicates are rarely of service. The 
 question naturally arises: how do the silicates become trans- 
 formed into suitable compounds and how do the rocks become 
 changed into the form of a soil. Several factors are concerned 
 in the transformation and the reactions are complicated and 
 very slowly effected. 
 
NATURE OF PLANTS 49 
 
 Water is the most important force in starlin^r tlicsc changes, 
 tt dissolves, that is removes, several of the elements that api)ear 
 in the silicates. Note also when water has taken up certain of 
 these elements and compounds that its dissolving power is greatly 
 increased. For example when water has absorbed carbon dioxide 
 its power of dissolving calcium is increased 25 times. Further- 
 more the removal of an element by the water means that new 
 chemical compounds are formed and these are frequently of such 
 a nature as to lead to further chemical reactions. We say that 
 the dissolving action of water upsets the chemical stability of 
 the substances composing the rocks. Certain gases of the air, 
 particularly oxygen, also enter into union with some of the rock 
 elements, making them more soluable. So you can think of a 
 series of chemical struggles going on between the silicon and 
 other elements and compounds for the possession of the potas- 
 sium, calcium, etc., and that in this way these latter elements 
 are sloAvly removed from the silicon and united into new com- 
 pounds suitable for plants.- 
 
 The removal of these elements from the rock minerals forms 
 cavities and as a consequence the rock tends to become honey- 
 combed and finally begins to crumble and so approaches a soil 
 condition. Two forces are therefore involved in soil making: 
 the one chemical which is termed decomposition and the other 
 mechanical, termed disintegration. Disintegration or the crumb- 
 ling of the rocks is hastened by a number of factors, esp>ecially 
 the force of running water. The power of water to break up the 
 rock substance and distribute the particles thus formed is one 
 of the most important factors in soil formation. Changes in 
 temperature also constantly cause the contraction and expansion 
 of the rock material and of the water occupying any of the sjxices 
 in the rock and so assist in the disintegration. Finally the force 
 of the wind and the grinding action of moving ice are important 
 factors in wearing away the rock surface and reducing it to a soil 
 condition. By these slow and intricate processes comjxiuntls 
 for plant uses are formed and the rocks are reduced to the condi- 
 tion of a soil. Such a soil, however, does not contain suflicient 
 amounts of the crude food material to support tho -.r.lm.rv 
 
50 PROPERTIES OF SOILS 
 
 plant. So we find very simple plants, as the lichens and certain 
 mosses, occupying such primitive soils. They are able to live 
 upon such soils because they are small and slow growing and 
 therefore demand meager amounts of the crude foods. Note 
 the character of the first vegetation that appears on the slopes 
 of mountains below the solid rock. These simple plants, how- 
 ever, are very important factors in effecting further decomposition 
 of the minerals in the soil. Through the chemical activity of 
 their absorbing organs and especially through the decay of the 
 dead parts of these plants new compounds are formed so that 
 more elaborate plants, as certain ferns, are now able to live upon 
 the soil. These latter plants in turn carry on the work until 
 the rock material is sufficiently changed to make a good soil. 
 Do not imagine that the work of soil formation stops here. You 
 are to think of every growing plant performing the same work 
 as noted in the lichens and mosses. 
 
 The soil is one of the most remarkable things in this world of 
 ours and we must consider two of its more important features. 
 The first of its important characteristics is its power to retain 
 moisture. Water exists in the soil in three forms, — as hygro- 
 scopic, as capillary and as gravitational water. The first form 
 is the water that condenses from the atmosphere as extremely 
 delicate films about the air-dry soil particles. This water is not 
 available to the plant because it is held so firmly by the soil 
 particles. It comprises only a small per cent, of the total water 
 in the soil. The capillary water also exists as films about the 
 soil particles but it is not retained so tenaciously and it is this 
 water that furnishes the chief supply to land plants. Capillary 
 water may amount to lo per cent, of the dry weight of coarse 
 sand up to 25 per cent, in clay soils. Gravitational water is not 
 retained by the soil particles and it is therefore free to run down 
 through the pores and spaces in the soil under the control of 
 gravity. This water in the upper portions of the soil is not of 
 value to the majority of plants because it fills the pore spaces in 
 the soil and so deprives the roots of the air which is necessary 
 to their existence. When gravitational water exists a few feet 
 below the surface it often becomes of great value as a source of 
 supply for the capillary water. 
 
NATURE OF PLANTS 
 
 51 
 
 Let us now consider how the root is related to the soil water. 
 You have seen that the soil particles have an attraction for water. 
 This attraction is due to a force termed surface tension. Each 
 soil particle has a definite surface tension or capacity for retaining 
 water that is dependent upon the extent of its surface. The 
 retention of the hygroscopic water uses up a small portion of the 
 particle's capacity and then it adds capillary water until no more 
 can be retained. We say that the particle is in a saturated state. 
 It is evident that the last water added to the particle will not be 
 held very firmly and it is this water that the root hair can readily 
 absorb. But now follows a very remarkable movement of 
 water. No sooner has a particle lost some of its water than it is 
 able to replenish its supply from adjacent particles that are in a 
 saturated state. This transfer of water from particle to particle 
 goes on for surprising distances, depending upon the amount of 
 the surface tension. So we find particles four to six feet away 
 from the root hair contributing a portion of their capillary- water 
 to replace the water thus absorbed. So you can think of the 
 capillary water moving as films from particle to particle towards 
 the points of absorption. It is for this reason that the gravita- 
 tional water may be of great value. If it is only a few feet below 
 the absorbing roots It will furnish a constant supply of capillary 
 water to the overlying soil particles. So one of the most im- 
 portant features about a soil is the size of its particles. The 
 term texture refers to this condition of a soil and we speak of fine 
 or coarse textured soils. The finer the particles the greater the 
 surface exposed in a given unit of space; and consequentK' the 
 greater the surface tension and water-holding capacit\. A 
 cubic foot of coarse sand can not retain as much water as a cubic 
 foot of clay because the clay particles are 200 times smaller than 
 the sand particles. A fine textured soil may lose some of its 
 capacity to retain water owing to the compacting of Its particles 
 into larger ones. It is evident that two separate ixartlcles possess 
 more surface area than when combined into one particle. This 
 relation of the particles to one another is termed structure and 
 we see that soil structure is of equal Importance with soil texture. 
 Now we can undesrtand the chief advantage of tillage. Plowing 
 
52 SOIL ABSORPTION 
 
 and harrowing break up the compact structure and tend to keep 
 the soil in a fine textural condition. It is surprising to note the 
 amount of surface exposed by the particles of various types of 
 soils. A coarse sand where the particles range from .i to .5 mm. 
 in diameter has no less than 40,000 sq. ft. of surface exposed in 
 each cu. ft. of soil and a clay soil where the particles are .005 mm. 
 in diameter or smaller has over 140 thousand sq. ft. of surface 
 per cu. ft. 
 
 A second noteworthy feature of soil particles is their power to 
 retain certain elements that are brought in contact with them 
 in solutions. These elements are retained in some instances 
 much after the manner of film water but in other cases retention 
 is effected through chemical reactions. You see an illustration 
 of this property of vsoils when colored solutions, such as the 
 drainage w^ater from stables, after passing through a soil comes 
 out quite colorless. The old type of charcoal filter purified 
 drinking w^ater in this way. It is an important fact that many 
 of the important elements and compounds utilized by the plant, 
 such as potassium, ammonium, magnesium, calcium, phos- 
 phorus, etc., are retained in this way; and they are retained so 
 firmly that only small percentages of them are leached out and 
 lost through the action of running water. Certain aluminum 
 compounds (see above), also iron, calcium and the humus com- 
 pounds derived from dead plants and animals are very important 
 factors in retaining the above mentioned elements. 
 
 21. Relation of the Root Hairs to the Soil Particles. — We are 
 now prepared to understand the significance of the structures 
 noted in the root and their relation to the physical properties of 
 the soil. In the first place we see that these microscopic root 
 hairs can readily penetrate any cavity and they are so delicate 
 and soft that they become moulded about any soil particle in 
 their line of growth (Fig. 30, B). So firmly are they attached 
 to these particles that it is impossible to detach them without 
 injuring the root hairs (Fig. 29, B). This arrangement admir- 
 ably adapts them to gathering up the water and other crude 
 material in the soil. The tubes are filled with organic acids and 
 other substances which have an affinity for w^ater and such earth 
 
NATURE OF PLANTS 53 
 
 substances as the plant requires. In this way the materials in 
 the soils are drawn through the delicate walls of the root hairs, 
 a process termed osmosis. Some of the acids in the root hairs 
 may also reach through the cell walls and so come in contact with 
 the insoluble substances, dissolving them and thus rendering 
 them capable of absorption by osmosis. Certain it is that CO2 
 is given off by the root hairs and as we have seen, this gas enter- 
 ing into the film water would enable it to dissolve calcium — a 
 fact easily demonstrated by allowing the roots to grow over the 
 clean surface of oyster shells or pieces of polished marble. Deli- 
 cate grooves in the marble show where the roots have dissolved 
 the calcium in the course of their growth. Some roots show an 
 acid reaction, as is indicated by their ability to turn blue litmus 
 paper red ; but all roots do not react in this way and it is impossible 
 to state whether acids, other than carbonic acid which would 
 result from the elimination of CO2, are excreted. 
 
 We are now prepared to understand why the root hairs are 
 developed just back of the zone of elongation.. Any disturbance 
 of these delicate absorbing organs, such as the extension of that 
 part of the root to which they are attached, would result in 
 tearing and killing them. 
 
 22. Familiar Facts About Roots. — The character of the root 
 and its relation to the physical properties of the soil explain many 
 familiar phenomena of plant life. We see why plants must be 
 repotted. The root hairs, which onh- li\-e for a few da\'s or a 
 few weeks, are constantly being formed back of the growing 
 zone and dying off behind. Tliis would result in not onK* placing 
 the new root hairs in new soil but they would also \)v removed 
 in a measure from any harmful substances that may have arisen 
 through the activity of the older root hairs. In this connection 
 note should be made of the suggestive work iliat is being ac- 
 complished today tending to show tliat roots gi\e olT substances, 
 probably of organic nature, which are toxic in character and so 
 render the soils unfit for plant growth. These substances are 
 readily oxidized and rendered harmless. The presence of oxy- 
 gen and certain fertilizers increases the oxidizing power of the 
 roots. While this theory is far from being established it is jws- 
 
54 RELATION OF ROOT TO SOIL 
 
 sible that we may have a new explanation for the sterile condition 
 of some soils; also plowing and the use of fertilizers may have a 
 new meaning. Consequently when a potted plant shows signs 
 of starvation it is removed from the jar, the tangled mass of roots 
 next to the jar is cut off together with a portion of the earth and 
 then repotted with fresh earth. In this way room is given for 
 the rootlets to grow out into the earth and develop a new ab- 
 sorbing surface and additional materials are placed at the disposal 
 of the root. This relation of the root hairs to the soil particles 
 explains to us why transplanted seedlings usually wilt. Many 
 of the root hairs have been killed and the seedling does not revive 
 until new root hairs have been developed. So also it is clear 
 why most shrubs and trees should be transplanted in September 
 or October or in the early spring in order to permit the rootlets 
 which renew their growth very early in the spring to establish 
 in the soil new absorbing surfaces before the leaves develop and 
 bring about an excessive loss of water through transpiration. 
 Why are plants often pruned or cut back in transplanting? 
 Why is nursery stock frequently transplanted, generally a year 
 before it is to be permanently planted? 
 
 You have often noticed that fields of grain and other crops turn 
 yellow and die during a long rainy season. This is due to the 
 fact that the air spaces between the earth particles become filled 
 with water and there is no longer possible an interchange of gases. 
 At least two-fifths of the space in these capillary pores should be 
 filled with air to insure healthy plants. This is particularly 
 noticeable in clay soils which are referred to as cold, wet, and 
 sour soils. The reason of this is that the earth particles of such 
 soils are very fine and, owing to their enormous surface, they 
 hold the water with great tenacity. In this way acids from 
 decaying vegetation as well as other substances accumulate in 
 excess and render the soil unwholesome. Therefore heavy clay 
 soils, which are richer in plant foods than other soils, require an 
 admixture of sand and an adequate drainage to render them 
 sufficiently porous for a proper circulation of air. Sandy soils 
 are made of coarser particles and therefore looser. For this 
 reason the capillary water is not retained for any considerable 
 
NATURE OF PLANTS 55 
 
 time and the soil is properly aerated. Injudicious sprinkling of 
 lawns, gardens, or potted plants will result in the formal ion of a 
 compact surface crust. This acts exactly like a clay soil and 
 draws the water away from the region of the roots to the surface 
 of the soil where it is lost by evaporation. Potted plants are 
 best watered by standing the jar in the water until the soil is 
 wet. Further watering is not required until the jar sounds 
 hollow when tapped. So gardens and lawns should Ije thor- 
 oughly soaked during the night. The ordinary sprinkling only 
 moistens the surface and leaves the soil in a worse condition for 
 holding the moisture than before, because on drying it bakes 
 and the hard crust draws up the water to the surface where the 
 dry air readily evaporates it. A light shower in the summer time 
 may produce the same injurious efTect. On the other hand when 
 the surface of the soil is loose and air dry, so that the capillary 
 water does not extend from the surface to the deeper strata, 
 then a shower may do harm in quite another way. By supplying 
 the upper particles with films of water it reestablishes capillary 
 connection with the lower lying particles so that the film water 
 now begins to move again towards the surface of the soil. This 
 explains why the top soil should be harrowed and hoed during 
 periods of inadequate rainfall. The loosely arranged particles 
 quickly become dry and the capillary films leading to the water 
 supply below are thus broken at the surface and further loss of 
 water through evaporation is in part checked. A mulch of straw 
 and leaves upon the surface of a soil has the same result as 
 harrowing. Sometimes it is desirable to have a compact surface 
 layer of soil, as for example in sowing a field to grain or grass. 
 This is accomplished by rolling the field, which not only gives a 
 smooth surface to facilitate the cutting of the crop but also makes 
 a compact stratum at the surface which draws the moisture up 
 to the grains, causing them to grow. Otherwise the surface of 
 the soil containing the grains would be so dried out by the winds 
 often in a single day, as to prevent their sprouting. 
 
 23. Extent of the Root Surface.— It may seem surprising that 
 the roots are able to take up the large volumes of water that are 
 lost daily through transpiration. The spread of the root, how- 
 
56 ROOT GROWTH 
 
 ever is much more extensive than we think, usually quite equal 
 to that of the branches, so that the absorbing portion of the root 
 is beneath the drip of the branches or exposed to the rains and 
 dews. Furthermore, when we come to add together the lengths 
 of the numerous rootlets, the extent of the total root system is 
 still more surprising. The total length of the roots of an oat 
 plant has been estimated at 154 feet; of a corn plant, at 1320 feet; 
 and of a squash plant at fifteen miles. These roots in ordinary 
 soil reach a depth of from three to five feet, but in dry regions 
 sometimes much greater depths are attained; for example, alfalfa 
 31 feet and mesquite 60 feet. The most important increase in 
 the extent of the root system is due to the root hairs, 480 having 
 been estimated on .01 in. of a root of corn 1/17 in. in diameter, 
 and 230 on a square mm. of a pea root. By this means the ab- 
 sorbing surface of roots may be increased from five to ten times. 
 24. The Structure of the Root. — You have noticed that only 
 the apical portion of the root elongates and it may be well to first 
 examine the structure of this region in order to see how grow^th 
 is brought about and what changes are effected by it. The tip 
 of the root is composed of very delicate cells. Certain of these 
 cells are rapidly dividing by forming new walls through the 
 middle of the cells, thus dividing each cell into two new cells. 
 The division of a cell is effected in a very elaborate manner. The 
 first indication of this growth appears in the nucleus. This organ 
 of the cell enlarges (Fig. 31, ^) and w^e now see that it has a very 
 complex structure, consisting of a protoplasmic mass surrounded 
 by a delicate membrane and traversed by a network of delicate 
 threads (linin) and containing one or more large granules (nu- 
 cleoli). The most important portions of the nucleus are the 
 minute particles (chromatin) which are associated with the linin 
 threads. It has been supposed that this substance controls to a 
 considerable extent the activity of the cell and determines the 
 kind of cell that shall be formed and the work that it shall per- 
 form. As the nucleus enlarges the chromatin increases in amount 
 and often forms a ribbon-like structure (Fig. 31, B) which finally 
 contracts and divides, thus forming numerous small masses of 
 chromatin, called chromosomes (Fig. 31, C). At this stage in the 
 
NATURE OF PLANTS 
 
 ^/ 
 
 division delicate colorless strands begin to appear and ultinuitely 
 grow into a spindle as shown in Fig. 31, D. There are two sets of 
 these strands or fibrillse; an inner and an outer series. The outer 
 fibrillar, in some way not understood, arrange the chromosomes in 
 the center or equator of the spindle (Fig. 31, D) where each 
 
 Fig. 31. Cell division in root of corn: A, cell with nucleus enlarging pre- 
 liminary to division — /, linin; ch, chromatin. The central dark body is the 
 nucleolus. B, later stage, the chromatin has increased and appears as a ribbon- 
 like skein. C, formation of the chromosomes, cr. D, formation of the spindle 
 and the arrangement of the chromosomes in the center of the spindle — 
 I. D. Cardiff. 
 
 chromosome divides by a longitudinal division into two ecjual 
 parts. The fibrillae now separate the two halves of each chromo- 
 some and pull them to the opposite poles of the spindle so that 
 each pole receives a half of every chromosome (Fig. 32, E). 
 The chromosomes now become rearranged at the poles and two 
 new nuclei are gradually formed like the original nucleus (Fig. 
 32, F, G). In the meantime the inner fihrillx of the spindle have 
 shortened and become thicker at the equator (Fig. 32, G). Tliis 
 thickening goes on assisted by the addition of new fibrilhe at 
 either side of the spindle until a delicate line (the cell wall) reaches 
 across the old cell (Fig. 32, H), and the division of the mother 
 cell into two daughter cells is completed. In tiiis way new cells 
 are being constantly added to the root. It will be noticed, if a 
 longitudinal section through the middle of the root is examined, 
 that various regions of the elongating root may be recognized 
 owing to the difference in the character of their growth. In such 
 a section (Fig. 33) we see that the tip of the root is covered with 
 
58 
 
 STRUCTURE OF THE ROOT 
 
 a mantle of cells, the root cap. This cap protects the delicate 
 cells within like a thimble so that they are not exposed or injured 
 as the root extends through the soil. At the tip of the root, just 
 within the root cap, the cells are actively dividing and adding 
 new cells to the end of the root and some cells are also added 
 
 Fig. 32. Later stages in the division of the cell: E, the fibrillae pulling 
 the separated halves of the chromosomes to the opposite poles of the spindle. 
 The inner series of fibrillae are now seen at the equator between the two 
 groups of chromosomes. F, the chromosomes arranged at the poles and the 
 inner fibrillae increasing in size and number. G, the fibrillae have increased 
 in number until they nearly reach the opposite walls of the mother cell. Their 
 thickening at the equator is the first indication of the wall separating the two 
 new cells. H, position of the new wall clearly indicated. — I. D. Cardiff. 
 
 to the inner side of the root cap (Fig. 34). Owing to this unique 
 arrangement it does not matter if the outer cells of the cap are 
 injured or destroyed as the root pushes through the soil, because 
 the cap is constantly being renewed from within, and so always 
 furnishes adequate protection to the delicate cells within. The 
 cells that are added to the tip of the root divide several times 
 after their formation, so we find that the tip of the root for a 
 distance of one or two mm. is composed of small cells that are 
 in a process of division but that are enlarging to only a slight 
 degree (Fig. 34). This region is called for this reason the for- 
 mative region of the root. Back of the formative region for a 
 distance of tw^o or four mm. the cells are dividing to a less degree 
 but are elongating very rapidly and changing in form (Fig. 35). 
 This is the region of rapid elongation. Still further back elonga- 
 tion has ceased, but the walls of the cells are becoming thicker 
 
NATURE OF PLANTS 
 
 59 
 
 and the cells are changing in character, so that they perform 
 different duties. This is illustrated in the cross section of the 
 root (Fig. 36), where we see that the outer cells have become 
 modified, forming an epidermis with root hairs 
 for absorption. Within the epidermis is a broad 
 zone of cells, the cortex, often used for the stor- 
 age of manufactured foods, while in the center 
 of the stem are the vascular bundles. The woody 
 portion of the bundle, or xylem, radiates out- 
 ward from the center and the soft portion, or 
 phloem, alternates with it (Fig. 36, x, p). The 
 materials absorbed from the soil are largely 
 transported up to the stem and leaves through 
 the xylem, and the foods manufactured by the 
 leaves reach the root by means of the phloem 
 cells. The branches of the root originate in a 
 very curious way from the cells just outside the 
 xylem. These cells by repeated divisions form 
 lateral roots which gradually destroy the tissues 
 in their way and finally grow out to the surface 
 of the root (Fig. 37). By this arrangement they 
 are provided with a root cap and fully prepared 
 to enter the soil on emerging from the old 
 root. 
 
 25. The Transport of Water in the Root. — The inner la>er of 
 the cortex, the endodermis (Fig. 36, end), consists of a layer of 
 cells which, in the older part of the roots, forms a very compact 
 and more or less cutinized ring of cells. This tissue is supposed 
 to function in preventing an undue loss of water from the con- 
 ducting strands of xylem to the cortex. It should be stated here 
 that the root hairs not only absorb fluids but also force out the 
 absorbed substances into the adjoining cells of the cortex. All 
 living cells have this power of absorbing and expressing fluids. 
 So it comes about that the absorbed fluids are forcecj by the root 
 hairs into the cortical cells and by them they are passed on to the 
 cells of the xylem \\ hen by some force not known they are drawn 
 up the stem into the leaves. The force exerted by these nnriad 
 
 Fig. 33. Dia- 
 gram of a section 
 taken through 
 the center of a 
 corn root: r, root 
 cap; e, epider- 
 mis; c, cortex; p, 
 central region. 
 
60 
 
 ROOT STRUCTURE 
 
 Fig. 34. Portion of the formative region of the root taken at a in Fig. 33: 
 r, root cap; v, cells which by repeated division add new cells to end of root 
 and to inside of root cap. Note the numerous dividing cells and their uni- 
 formity in size. — I. D. Cardiff. 
 
 '.T 
 
 w 
 
 \ 
 
 Fig. 35. Portion of the region of elongation of the root taken at b in Fig. 
 33. Note the larger size of the cells, increase in cell sap as indicated by the 
 larger vacuoles and absence of cell division; e, epidermis; c, cortical region; 
 p, central region. — I. D. Cardiff. 
 
NATURE OF PLANTS 6i 
 
 number of cells in the root exerts a pressure that often amounts 
 to considerably more than one atmosphere, i. e., 15 lbs. to the 
 square inch. It is the steady expressing of fluids by these cells 
 that causes the familiar phenomena of the "bleeding," or flow 
 of water from stumps in the spring; likewise the "bleeding," of 
 injured branches, or the flow of sap, for in the stem the living 
 
 Fig. 36. Cross-section of root taken above section shown in Fig. 33: e, 
 epidermis with root hairs; c, cortex bounded on inner side by endodermis. 
 end. Within is the central region containing vascular bundles; x, xylem; p, 
 phloem. — I. D. Cardiff. 
 
 cells are constantly absorbing and giving oft* water as in tlie root. 
 In the summer, stumps and stems do not "bleed" as a rule be- 
 cause the water lost by transpiration nearly empties the cells, 
 whereas in the spring before the leaves ai^pear tlu'\- become filled 
 with fluid. 
 
 26. The Sensitiveness of the Root.- AW nia\ now ask how 
 does this elaborate root mechanism become so perfectly adjusted 
 to the soil. If the root of a pea or bean seedling is ]ilaced hori- 
 zontally in sawdust, after one or two hours it will begin to 
 curve down toward the earth center. No matter in what position 
 it is placed the result is always the same. We are so familiar 
 with the downward e:rowth of roots into tlie soil that we ne\iT 
 
62 
 
 SENSITIVENESS OF THE ROOT 
 
 stop to consider how they gain their sense of direction. Gravity 
 is the stimulus that acts upon the irritable protoplasm of the cells 
 and so brings about a growth reaction that sends the root in the 
 right direction. When roots are placed in a horizontal position 
 and slowly revolved on their longitudinal axes, no curvature re- 
 sults, since all parts of the cells are stimulated alike; but when 
 allowed to rest a curvature results because possibly heavier par- 
 ticles in the cells fall to the lower sides of the cells and so pro- 
 
 end 
 
 Fig. 37. Cross-section of a root of lupine showing the origin of the lateral 
 rootlets. Lettering as in Fig. 36. — H. O. Hanson. 
 
 duce an irritation through their unusual position. The cells in 
 the first millimeter and a half of the root tip, or possibly in the 
 root cap, are sensitive principally to the stimulus of gravity — 
 the other cells much less so. If this region of the root is care- 
 fully removed with a very sharp razor the root is no longer 
 capable of responding to gravity although it may curve in 
 various directions owing to the irritation produced by cutting. 
 Furthermore, while the tip alone perceives the stimulus of 
 gravity, the curvature occurs two or three mm. back of the 
 tip, i. e., in the region of rapid elongation, so that we have the 
 transmission of the impulse somewhat after the manner of our 
 own nervous system. To be sure there are no specially con- 
 
NATURE OF PLANTS 63 
 
 structed cells for conveying this impulse comparable to the nerves, 
 but it has been shown that chemical changes are set up that ex- 
 tend from the tip to the region of curvature where a more ex- 
 tended growth of the cells on the upper side of the root is induced 
 than on the lower side. This results in bending the root down 
 into the soil. These reactions apply to the main or tap root. 
 The lateral branches, or secondary roots, tend to grow more or 
 less at right angles to the stimulus of gravity, while the short 
 tertiary branches radiate out in all directions. Why the stimulus 
 of gravity causes a more extended growth on one side of the tap 
 root than on the other or why the lateral roots are so differ- 
 ently affected can not yet be answered, but the nicety of these 
 reactions in enabling the roots to reach in all directions and 
 thoroughly explore the soil in their quest for food, is very ap- 
 parent. The cells of the root tip are also sensitive to touch or 
 contact. This enables the root to avoid obstacles and grow 
 around rocks and stones. In this case the irritation of the root 
 tip produces a more considerable growth in the zone of curva- 
 ture on the side where the root is irritated with the result that 
 the tip of the root is bent from the object. In this way roots 
 work their way through the soil and avoid obstacles and grow 
 around rocks, for as soon as the irritation of the obstruction is 
 removed by the curvature, gravity will cause the root to grow 
 down again. 
 
 The cells of the root tip also react to light intensities and to 
 definite percentages of moisture and mineral foods. Roots avoid 
 light and as a consequence they grow down into the earth where 
 they are directed in their growth, owing to the peculiar sensitive- 
 ness of their cells, so that they come to lie in soils containing 
 suitable moisture and crude food materials. Every one is fa- 
 miliar with the fact that roots are caused to curve towards and 
 follow the course of decaying tree trunks, being stimulated by 
 the moisture and food contained in them. Numerous instances 
 might be cited where roots are attracted to wells, drains, etc., 
 even surmounting considerable obstructions in order to reach the 
 water. This localization of the sensitiveness to all external 
 conditions that affect the root at the very tip is altogether ad- 
 
64 ROOT FUNCTIONS 
 
 mirable, because it enables it at the very start to come into the 
 most helpful relations to its surroundings. It was a very clever 
 fancy of Darwin to compare this localization to a brain center 
 as found in certain low orders of animals. 
 
 27. Other Root Relations. — While 70 per cent, of our plants 
 develop roots adapted to absorbing substances from the soil 
 there are several interesting modifications of the structures noted 
 above. The roots of aquatics may not develop root hairs be- 
 cause the roots are so finely divided and delicate as to enable 
 them to absorb their crude materials directly from the water. 
 Many plants develop roots in the air. These are sometimes 
 simply clinging devices, as in the poison ivy; or they may be true 
 absorbing organs, in some cases reaching down to the ground 
 or to cup-shaped leaves that hold water, as in many tropical 
 climbing plants. One of the most interesting of these modifi- 
 cations is found in the orchids (Fig. 291) that live on the damp 
 tree trunks or on dripping rocks of tropical countries. Here the 
 root is covered by a thick mantle of cells that are capable of hold- 
 ing the water that conies to them either in the form of dew or 
 drainage or rain. Such plants, called epiphytes, are able to 
 flourish in the air without the assistance of soil roots. Quite a 
 large number of plants, parasites, depend entirely, or in part, upon 
 other plants for their foods. The mistletoe is an example of this. 
 The sticky seeds are carried by birds to the branches of oaks and 
 other trees where they germinate, the roots penetrating the 
 branches and withdrawing the necessary foods. The mistletoe 
 withdraws largely crude materials from the branch because it 
 has pale green leaves and is therefore capable of manufacturing 
 some food on its own account. Our common dodder {Cuscuta) 
 is entirely dependent upon other plants and illustrates the results 
 that follow from the disuse of any function. It has lost its chloro- 
 plasts, the leaves are reduced to inconspicuous scales and the stem 
 resembles a coarse yellow string while the root has disappeared 
 entirely. The seeds produce a small thread-like shoot which soon 
 perishes unless it comes in contact with a plant upon which it 
 can live. These thread-like plants are sensitive to touch and so 
 coil about the branches of any plant that they may chance to hit 
 
NATURE OF PLANTS 65 
 
 in their growth. If this plant happens to contain fcxxls suiialjle 
 to the dodder then a second stimukis is aroused which causes root- 
 like branches to form. These organs penetrate the brandies upon 
 which the dodder is growing and absorb foods from it. Many 
 plants live as parasites on the roots of a variety of herbaceous and 
 woody plants, as the beech drop, orobranches, broom rape, etc. 
 
 One of the most remarkable features of roots and one of the 
 most important in the economy of the earth is seen in that large 
 family of bean or leguminous plants that are characterized by 
 flowers and fruit like those of the pea. Minute plants, bacteria, 
 gain access to the roots of these plants through the root hairs 
 and cause wart-like enlargements, tubercles, on the roots (Fig. 
 38). These bacteria have the power of combining the nitrogen 
 of the air with other elements, termed the fixation of nitrogen, 
 and so forming a nitrogen-bearing compound that can be ab- 
 sorbed by the plant. To give an idea of the importance and 
 work of these bacteria, it is estimated that an acre of clover 
 yielding two tons contains 100 lbs. of nitrogen — 74 lbs. of which 
 has been derived from the air. The fixation of this amount of 
 nitrogen by the bacteria in the roots of the clover would nearly 
 suffice for the production of 50 bushels of corn and 30 busliels 
 of wheat. Nitrogen is one of the very essential elements rc- 
 5 
 
66 NITROGEN FIXATION 
 
 quired in the construction of the living substance of the plant. 
 Strangely enough, though the atmosphere contains over 70 per 
 cent, of this gas, the plant has no way of utilizing it directly but 
 only in compounds, such as ammonia and nitrates, etc. Com- 
 pounds of this kind exist in very meager quantities upon the 
 earth. The only sources of supply of any consequence are the 
 decaying animal and vegetable life; manures; small amounts 
 that are formed in the air and that are then carried to the earth 
 in rains; the guano deposits, now practically consumed; and the 
 saltpeter beds of Peru and Chile, which will be exhausted by 1925. 
 Consequently widespread alarm has arisen lest famines ultimately 
 result through lack of these nitrogen compounds, and the situ- 
 ation becomes the more serious since from one half to two 
 thirds of the nitrogen compounds placed on the soils are lost an- 
 nually in various ways, especially by the leaching out of these 
 substances by rains and drainage waters. Naturally of late 
 years much attention has been directed to devices for uniting 
 the nitrogen of the air with other elements in order to find a sub- 
 stitute for the rapidly disappearing nitrogenous compounds. 
 Working upon the fact that nitrogen compounds are formed in 
 the atmosphere by electrical discharges, several countries are 
 now manufacturing by electrical processes large quantities of 
 nitric compounds. One of the valuable substances thus formed 
 is calcium nitrate. Nitrogen is first combined with oxygen by 
 utilizing the high temperature of the electric arc (2500 to 3000° 
 C.) and the nitric oxide gas thus formed is passed through milk 
 of lime, thus forming calcium nitrate. Another nitrogen com- 
 pound, valuable for certain soils, is calcium cyanimid. It is 
 formed by passing nitrogen into closed retorts containing pow- 
 dered calcium carbide heated to a temperature of 1100° C. 
 Under this temperature the calcium carbide (this substance 
 forms acetylene gas when placed in water) unites with the 
 nitrogen, forming calcium cyanimid and carbon. The cyani- 
 mid slowly decomposes in the soil, yielding ammonia which can 
 be either directly or indirectly utilized by the plant. While 
 the value and general utility of these artificially formed nitrogen 
 compounds has not been thoroughly tested, it is not to be ques- 
 
NATURE OF PLANTS 67 
 
 tioned that products and processes will be improved so as to meet 
 the rapidly increasing demands for nitrate. 
 
 Much study has also been given to the nitrogen-fixing bacteria 
 as a further source of supply for nitrogen compounds. Failure 
 attended the numerous attempts to cultivate these important 
 plants with practical results, although our own department of 
 agriculture has met Avith a limited success in making cultures of 
 bacteria that are sold to farmers for mixing with their clover or 
 other leguminous seeds. See Bulletin 71, 1905, U. S. Dept. of 
 Agriculture. There are many difficulties in the way of intro- 
 ducing these forms, such as keeping them alive, supplying the 
 proper conditions for their growth and the variety of forms that 
 must be dealt with. Although there is possibly but one species 
 of these bacteria, they have become so changed through associ- 
 ation with the various kinds of leguminous plants that there are 
 numerous varieties, each adapted to one or a few species of plants. 
 Soils in one section may have the forms suitable for one kind of 
 plant and be totally deficient in forms required for other plants. 
 At present the only sure way of inoculating sterile soils is by 
 mixing with them soils containing the desired bacteria — a labori- 
 ous and expensive method. Quite recently ver>' promising 
 results have been obtained by growing the desired bacteria in 
 soils until they have so multiplied and permeated it that a small 
 amount of this soil will suffice to inoculate a large area. With- 
 out doubt means will be devised for cuftivating the various forms 
 of these bacteria so that they may be sold for mixing with the 
 soil or seeds. Thus the farmer will be able to raise a croj^ that 
 contains a large amount of nitrogen derived from the air and by 
 plowing under this crop or better still, if conditions permit, by 
 feeding it to stock and utilizing the manure, he will be able to 
 add a valuable nitrogenous fertilizer to the soil. In this way the 
 soil is prepared for another crop, as wheat, which is (Ki)eiukMit 
 for nitrogen upon nitrogenous compouiuls. This alternation of 
 crops is of prime importance in successful farming. I'^arh kind of 
 plant takes from the soil certain elements in large amounts and 
 for this reason the alternating or succeeding crop should Ik? of 
 such a kind as to require other elements. As these elements are 
 
68 NATURE OF MYCORRHIZA 
 
 removed from the soil suitable fertilizers containing nitrogen, 
 phosphorus, lime, potash, etc., must be added to the soil to 
 replace the absorbed crude foods. 
 
 The majority of plants do not absorb materials from the soil 
 in so direct a way as is illustrated in Fig. 29. They are de- 
 pendent for certain of their materials at least upon a low group 
 of plants termed fungi. These fungi more commonly consist of 
 delicate cobwebby threads (termed hyphse), such as are seen on 
 mouldy bread. This fungal group, known collectively as 
 mycorrhiza, comprises a number of forms or species, only a few 
 of which have as yet been identified. These delicate plants 
 spread through the soil and their hyphse enter either the outer 
 cells of the roots or form a mantle of more or less closely inter- 
 woven threads about them or the hyphae may sustain both of 
 these relations to the root. It has been demonstrated in many 
 instances that they transfer to the plant various substances 
 derived from the humus. Certain forms, probably a much larger 
 number than is now know^n, have the power to fix free nitrogen 
 much after the manner of the bacteria. It is also probable that 
 they sometimes take over the normal absorbing function of the 
 root hairs for frequently roots associated with Mycorrhiza develop 
 root hairs sparingly or not at all. This relationship is not 
 always one sided in its benefits for the fungus receives carbo- 
 hydrate material from the green plant. This state where two 
 or more plants live together is termed symbiosis. All stages in 
 the symbiotic relationship of the fungi and the green plants may 
 be seen. The green plant may not be benefited or even injured 
 by the fungus. This symbotic relationship is termed para- 
 sitism. The symbionts may be equally benefited and not all 
 injurious the one to the other. This relationship is termed 
 commensalism. The majority of our poplars, willows, beeches, 
 heaths, orchids and evergreens have attained this state and do 
 not flourish in soils Avhere suitable fungi do not abound. Per- 
 haps this explains why it is so difficult to transplant certain 
 shrubs and trees. The fungi are easily injured and do not be- 
 come established in the new soil soon enough to keep the plant 
 alive. This mutual relationship has gone so far in some species 
 
NATURE OF PLANTS 69 
 
 that the seeds are unable to grow unless they come in contact 
 with the hyph^e of the fungus. Finally some plants have become 
 so adapted to these fungi as to receive all necessary foods from 
 them. Accordingly, their green leaves and roots have largely 
 disappeared since they are no longer of service. Examples of 
 this are seen in the white Indian pipe and pine-sap and in the 
 coral root orchid. In these latter examples the relationship of 
 the two plants is completely turned about for we see that the 
 plant has become a parasite upon the fungus. 
 
 28. Roots as Store-houses for Foods. — In all the cases hereto- 
 fore considered the root has functioned in one way or another as 
 an organ for the temporary reception and transmission of materi- 
 als from the soil. It may also serve in other capacities, one of 
 the more important of which is as a storage organ. Such roots 
 become fleshy and filled with foods and are of great economic 
 importance, furnishing a variety of nutritious vegetables, as the 
 sweet potato, beet, turnip, etc. Many of these valuable plants 
 are biennials. During the first season the plant develops only 
 leaves and stores up food in its fleshy roots which is utilized in 
 the following season in the production of flowers and seeds, after 
 which the plant perishes. It is noteworthy that the biennial 
 habit has been induced in many of these plants by cultivation. If 
 they are left to themselves in a wild condition they will soon 
 revert to an annual growth, i. e., producing seed and perishing 
 during the season. Perennial plants li\e on from >c'ar to \ear. 
 As to whether some plants behave as annuals, biennials, or 
 perennials depends in some cases upon the place or time of 
 planting. The castor bean with us is an annual but in warm 
 countries it becomes a perennial. Winter wheat on the other 
 hand has acquired a biennial habit owing to the late planting of 
 summer wheat types. 
 
 29. Anchoring and Supporting Roots. — Roots also play a very 
 important role in anchoring and supporting the plant. Excellent 
 examples of the supporting roots are seen in the Indian corn 
 where numerous roots spring from the stem a short distance from 
 the ground and reach out on all sides like gu>' ropes steadying 
 the plant in the ground. Similar devices appear in our elms. 
 
70 BINDING ACTION OF ROOTS 
 
 maples, and beeches where enlargements of the roots at the base 
 of the trunk rise up like girders, bracing the tree against winds. 
 More marked illustrations appear in many tropical plants, as the 
 stilt roots of the mangrove and screw pines, and the buttressing 
 roots of one of the Indian rubber plants (Ficus), etc. In the 
 ban^^an tree of India the branches have an almost unlimited 
 lateral growth owing to the fact that they are supported by a 
 succession of roots that reach from the branches to the ground. 
 Kerner cites an example of one of these trees wdth 300 large and 
 3,000 small prop roots. This tree sheltered a village of 100 native 
 huts and an army of 5,000 men. 
 
 30. Binding Action of Roots. — The landscape would be con- 
 stantly subject to great change either by erosion or through the 
 action of winds w^ere it not for the solidifying and binding action 
 of roots upon the soil. The difficulty of breaking up the prairies 
 and the turf of abandoned fields or meadows indicates the extent 
 and completeness of the ramifications and interweaving of the 
 roots. Miles of sandy reaches are Held from shifting through 
 the restraining action of roots and stems of grasses and other 
 plants. The little town at the end of Cape Cod would have been 
 submerged long ago by the shifting sand dunes had not suitable 
 plants been planted to hold in check the loose sands. 
 
 Roots not only bind the soil together but the older portion of 
 the root usually possesses the power of contraction. This pro- 
 perty results in the pulling dow^n and fixing of the stem in the 
 ground. You must have often wondered how stems and bulbs 
 become so deeply buried in the soil although the seeds are 
 scattered on the surface of the soil. This is also w^ell illustrated 
 in the tips of raspberry bushes which come in contact with the 
 soil through the bending of the stalk. Roots strike out from the 
 tip and when thoroughly established in the earth the olde^ 
 portions contract and bury the tip in the soil. In the same 
 way seedlings of various plants are slowly pulled down into the 
 earth so that they finally become planted at considerable depths. 
 It is this contraction of the roots that keep the slowly elongating 
 stems of the dandelion, dock and many other plants buried in 
 the soil and that bind many creeping plants such as certain 
 clovers and knot weeds firmly to the earth. 
 
CHAPTER III 
 
 THE STEM 
 
 31. The Function of the Stem.— The stem has for its chief 
 function the production and display of the leaves and roots and 
 the conduction of the materials which these organs are especially 
 concerned in handling. It serves as a connection between them, 
 carrying up the material absorbed by the roots and distributing 
 the various substances received from the leaf. Like the traffic 
 of a city this material is received at many stations and trans- 
 ported along various channels to many points. At one place 
 some of it is used in the manufacture of food, at another point 
 material is required for the nourishment and construction of the 
 cells. Here a portion is stored, or again, there is the useless 
 or waste material to be carried away. 
 
 32. Character of the Stem. — In order to comprehend the nature 
 of the work performed by the stem it will be necessary to ex- 
 amine its external form and its internal structure. While stems 
 vary greatly in character we may gain a general understanding 
 of their more important features by studying some wood\' twigs 
 or branches as they appear in winter (Fig. 39). The most strik- 
 ing features in such twigs are the buds, the leaf scars (see page 43), 
 lines of fine scars forming rings about the stems at varying inter- 
 val?, minute roundish or lense-like elevations (the lenticcls), and 
 finally the character of the branching. The buds of the hickor>', 
 horse chestnut, Norway maple, lilac, cherry, etc., are excellent 
 for study. In these buds, as in most examples, the outer part 
 consists of a series of leathery, closely overlapping scales or 
 modified leaves. These function to protect the delicate parts 
 within against injury and especially against loss of water. By 
 carefully removing the scales with a sharp pen knife we find 
 within the ordinary green leaves, stem and also in some buds 
 the flowers that will appear next year. These organs are so 
 small and crowded that they appear to arise from a single point. 
 
72 CHARACTER OF BUDS 
 
 They have, however, the same arrangement as in the mature 
 shoot of the summer time and you should observe for a few days 
 in the spring the opening of a variety of buds, noting how this 
 crowded arrangement of the organs of the bud gives place to 
 that of the mature shoot. The formation of buds possibly came 
 about owing to climatic changes. The earlier vegetation of the 
 earth was subject to a uniform tropical climate but the formation 
 of high mountain ranges and other factors caused cold currents 
 of air to sweep over the earth and also produced variations in the 
 humidity of the atmosphere. These factors resulted in producing 
 a season favorable for growth and one in which growth would 
 be stunted or checked. The latter condition induced many 
 changes in the plant, of which the bud is an important example. 
 The great majority of woody plants are characterized by these 
 two phases: a period of rapid growth up to about the middle 
 of July, during which time all the organs of the season as well as 
 the buds are formed, after which they continue to manufacture 
 food and store it up in the buds and other organs until September 
 or October; and a period of dormancy in which the plant is in a 
 resting condition until the spring. Such a method of growth is 
 termed definite and should be distinguished from the indefinite 
 growth of a few plants such as the raspberries, locusts, some 
 honeysuckles, etc., where the growth goes on until the fall frosts, 
 with the result that the more recently formed, delicate parts are 
 killed off each year. But even in these cases we have essentially 
 the same rhythm of growth as in the first case because the older 
 portions of the shoot have perfected their tissues and developed 
 their buds and so prepared for the dormant period. 
 
 Whatever may be the causes that have resulted in the forma- 
 tion of the bud, certainly its closely united scales which are often 
 reinforced with resinous, mucilaginous or hairy coatings, are an 
 admirable device for protecting the delicate parts within against 
 winds which would rob them of water at a time when none could 
 be obtained from the soil. It is quite natural to think of these 
 devices as protection against cold, but strangely there is no adap- 
 tation known among plants that serves primarily as a protection 
 against low temperatures, and there is no cold upon the earth 
 
NATURE OF PLANTS 
 
 /o 
 
 ^ 
 
 
 Fig. 39. Types of stems: A, branch of hickory showing a large terminal 
 bud and several small lateral buds that were developed in the axils of the 
 leaves of the past season. The branch is three years old, as shown by the 
 three rings, r, r' , r" , of the bud scars which mark the successive positions of 
 the terminal bud during the past three seasons; /, leaf scars. B, branch of 
 pear — after Bailey. The ring at i shows the position of a bud which produced 
 the next year a pear, as is indicated by the large scar and swollen branch at a. 
 A short lateral branch was also developed the same season with its terminal 
 bud at 2. The next year this bud produced a branch that extended to 3. 
 bearing several leaves (note leaf scars) and of course axillary buds — one of 
 which, h, we see grew in the following seasons. No fruit was developed this 
 season. The bud formed at 3 behaved the next season very much as the one 
 previously noted at i, forming a pear at a' and a short lateral branch that 
 reached to 4. Note also that the axillary bud, h, of the previous sc-ason grew 
 a little and its subsequent history can be followed by the annual bud scars. 
 In the following season bud 4 developed a vigorous shoot reaching to 5. No 
 fruit was formed but three buds survived, h', b", b'", thus nearly duplicating 
 the growth of bud 2. Bud 5 develops the next season fruit at a" and also 
 two lateral shoots that extend to 6 and 7 respectively. The three buds below 
 5 had only a feeble growth during this and the succeeding seasons and bud b'" 
 evidently perished during its first period of growth. Bud 6 produces the next 
 year a pear at a'" and a lateral shoot reaching to 8 and bud 7 at the same 
 
74 NATURE OF BUDS 
 
 so intense that it may -not be endured by many plants, if this 
 were the only unfavorable factor. 
 
 Buds are usually found singly in the axils of the leaves and as 
 a rule only a few that are favorably situated as regards the light 
 ever develop. The development of the buds brings about the 
 characteristic appearance or habit of the plant. This growth is 
 not controlled alone by the favorable position of the buds but 
 also by the interaction or correlation of all the growing parts. 
 As a result in some trees we find a single terminal bud that is 
 larger and better developed than all others. This bud will pro- 
 duce the longest shoots and consequently such trees will have 
 spire-like, stems as in the spruces, larches, etc. In other cases 
 this same type of growth may continue for a time but eventually 
 terminal buds of equal vigor will be developed upon several 
 branches and consequently equal growths or diffuse types of 
 branching will result as in the maples, elms, etc. Buds that are 
 not favorably located generally perish after a few years but not 
 infrequently they remain alive and become overgrown by the 
 increase of the stem. In such cases the bud grows slowly and 
 maintains itself near the surface of the wood. These buds grow- 
 ing and often branching in the wood produce those curlings and 
 twistings in the grain that are commonly known as bird's-eye 
 wood. Somewhat similar markings are also produced by minute 
 outgrowths on the surface of the wood. These do not have, 
 however, the darkened center characteristic of the curl caused 
 by the bud growth. Dormant buds sometimes develop into 
 normal branches, as when a portion of a tree is removed and 
 
 time only develops a leafy shoot reaching to 9. Buds 8 and 9 both produce 
 fruit the next season (a*'", a^^) and short lateral branches reaching to 10 and 11. 
 These buds during the present season only formed leafy shoots. 
 
 Buds are formed in the axils of all the leaves that appear on the shoots of a 
 season's growth, but notice that but few or none at all develop. Some have 
 a thrifty growth, others remain small and perish after one or more years. 
 This is true of the fruit. Some of the pears mature, as is indicated by size 
 of the scars, while others drop off after one or more months. Note also the 
 almost regular alternation in the production of shoots bearing fruit and leaf 
 shoots. Bud 6 is the only exception to this succession. Observe that the 
 branches assume different positions and that the extent of the elongation of 
 the shoot from year to year varies. Can you explain these facts? 
 
NATURE OF PLANTS 75 
 
 consequently an additional supply of food stimulates them to 
 growth. Buds may also arise upon any part of the stem, root 
 or leaf. These are the so-called adventitious buds and their 
 formation upon roots often accounts for the colonial habit of 
 many trees, plants and shrubs, as in the poplars, Ailanthus, 
 sumacs, etc. They appear to rise naturally in some cases, but 
 in other instances their formation is due to the stimulus of a 
 wound or some other cause and like the dormant bud they serve 
 to prolong the life of the plant. Richards has shown when 
 living cells are exposed to the oxygen of the air, owing to a 
 wound, that these cells are stimulated as a result to renewed 
 activity. This doubtless explains the formation of callus and 
 the healing of wounds, as well as the formation of adventitious 
 buds in many cases. Common examples of buds due to wounds 
 are seen in the vigorous shoots that spring up from the stumps 
 of hard wood trees, pollarded willows, etc. Some buds become 
 fleshy owing to the storage of the food and dropping from the 
 plants serve to propagate new individuals. Examples of this 
 are seen in the fleshy buds on the tips of the branches or in the 
 axils of the leaves of the stone-crop and some lilies and in the 
 flower clusters of some onions. Many aquatic plants have the 
 habit of forming similar buds on the approach of winter. These 
 being compact and heavy with food sink to the bottom of the 
 ponds in the fall and renew their growth in the spring. 
 
 The rapid unfolding of the bud in the spring is a constant 
 source of surprise but when we recall that it contains usually 
 an abundant supply of food and practically all the organs that 
 will appear on the stem during the season we can understand 
 how the bud opens and elongates into a shoot bearing leaves 
 and flowers during the first few weeks of spring. The part of 
 the stem bearing the scale leaves of the bud does not elongate 
 materially since these leaves arc of service onh- during the winter 
 or dry season and soon fall ofl" after the o|X'ning of the bud. 
 Consequently the scars formed by the fall of these protective 
 scales form a ring (Fig. 39, r) each year about the stem which 
 marks the position of each successive bud. These scars are 
 therefore known as annual rings and by counting the number of 
 
76 REGIONS OF THE STEM 
 
 these rings on a twig you can ascertain its age and observe the 
 extent of elongation during each season. The minute dots or 
 lenticels consist of loosely arranged cells that permit an inter- 
 change of gases between the living cells within the stem and the 
 atmosphere, functioning in much the same way as do the stomata 
 of the leaves. 
 
 We are noAv interested to learn something of the Internal char- 
 acter of the stem. By cutting with a sharp knife across the stem 
 between the terminal bud and the first annual ring we see that 
 there are several regions in the stem. On the outside is a very 
 thin brown layer, largely composed of cork cells, which prevents 
 loss of water and which in old stems becomes quite thick and 
 variously split. Within is a zone of rather delicate tissue. The 
 outer part of this region, termed the cortex contains more or less 
 chlorenchyma— at least this is true of small stems — and functions 
 as in the leaf; while the inner rather colorless portion of this 
 region serves to conduct foods manufactured by the leaves and 
 it also frequently serves as a storehouse. The third zone is made 
 up of thick-walled, compact cells, regularly arranged. This is 
 the wood or xylem portion of the stem. It gives stability, con- 
 ducts the water and crude materials absorbed from the soil and 
 also serves as a storage organ for the reserve foods of the plant. 
 The center of the stem, known as the pith, consists of delicate 
 cells which conduct water for a short time and soon die. 
 
 33. The Anatomy of the Stem. — We are now desirous of seeing 
 how this complex structure of the stem comes about and of 
 studying in more detail the character and operations of the 
 apparatus. 
 
 If thin sections are made across the stems of seedlings, as of 
 the castor bean, the tissues will resemble the arrangement shown 
 In Fig. 40. It should be stated that the vascular bundles (Fig. 
 40, v) do not appear as separate strands in the earliest stages of 
 the development of the stem. This vascular tissue appears 
 rather as a more or less continuous circle of tissue surrounding 
 the pith. Very early in the development of the stem this circle 
 of vascular tissue is broken into segments or vascular bundles. 
 This separation of the ring of vascular tissue into segments is 
 
NATURE OF PLANTS 77 
 
 caused by strands of it here and there extending out into the 
 leaves, thus forming gaps in the circle. All seedling stems show 
 this arrangement of tissues and consecjuentK- there appears 
 more or less clearly three regions; an epidermis, a cortex, and a 
 central region surrounded by the cortex, an arrangement already 
 noticed in the roots. The epidermis does not matcrialh- differ in 
 structure, modification, or function from that already noted in the 
 leaf, see page 8. The cortex is largely composed of parenchyma 
 and extends from the epidermis to the endodermis which it in- 
 cludes. The endodermis is not so well marked as in the nxjt 
 and owing to the rapid growth of the tissues in this region it is 
 
 Fig. 40. Cross-section of young stem of castor bean: c, cpiderniis; c, corlt-x; 
 p, pith or inner portion of central cylinder; i\ vascular huucilc, arranged in 
 outer part of central cylinder. — H. O. Hanson. 
 
 often impossible to detect it. The cortex assists the leaves in 
 photosynthesis, the outer portion as a rule being well supplied 
 with chloroplasts. In old stems that become co\-ercd with a 
 thick layer of bark the chlorophyll quite disappears, owing 
 doubtless to its exclusion from air and light. The cortex also 
 serves as a storehouse for foods. Particularly is this true of the 
 endodermis and adjoining cells wliidi .uc often temporary 
 
78 
 
 STEM STRUCTURE 
 
 receiving stations for the carbohydrates during their transport 
 through the stem. The cells in the outer portion of the cortex 
 frequently become thickened and more or less elongated to give 
 strength to the stem. One of the most common modifications of 
 this kind is shown in Fig. 41, ^. This tissue, collenchyma, is 
 characterized by the thickening of the cells at the angles or on 
 all sides and by the silvery luster of the walls. The walls, 
 though very elastic and tough, are capable of growth and so 
 they are especially adapted to the support of the young elongating 
 stems. In older stems, where elongation has ceased, other 
 greatly elongated cells, called stereome fibers or sclerenchyma 
 
 B 
 
 (r ' ^^' 
 
 ! ^ 
 
 ilil 
 
 
 Fig. 41. The stereome or strengthening cells of the cortex: A , collenchyma 
 cells in growing stem of Begonia, showing above the cells in cross-section and 
 below in longitudinal section. B, a group of elongated thick-walled cells, 
 called sclerenchyma fibers or stereome fibers, from the stereome of mature 
 flax stem. 
 
 fibers, are often formed. These cells have thick walls and taper- 
 ing ends which interlock and bind the cells very firmly together 
 (Fig. 41, B). These strengthening cells form a compact zone 
 about the stem or they may be arranged in separate bundles. 
 In this latter case they often appear to the eye as light colored 
 bands extending along the surface of the stems of many herba- 
 ceous plants. The central region of the stem is characterized 
 by a mass of parenchyma with several vascular bundles arranged 
 in a circle (Fig. 40). The central mass of parenchyma is called 
 the pith. 
 
 34. The Vascular Bundle. — The vascular bundle contains three 
 distinct regions; an inner thick- walled portion (the wood or xy- 
 
NATURE OF PLANTS 
 
 79 
 
 lem), an outer thin-walled portion (phloem), and between these 
 two regions a delicate layer of cells, the cambium (Fig. 42). 
 The transport of all substances is largely confined to the vas- 
 cular bundles, the xylem conducting principally the crude ma- 
 terials while the bulk of the organic substances passes through 
 the phloem. We are now interested to study the character of 
 these cells and note their adaptation to the work in liand. In 
 the xylem occur various large spaces, the vessels or ducts (Fig. 
 42, v), and smaller spaces, wood cells of different kinds. The 
 
 Fig. 42. One of the vascular bundles shown in Fig. 40 cniargcHl: .v, xylem; 
 V, vessels or ducts; p, phloem; s, sieve tube; ac, accompanying cell; c, cam- 
 bium; st, stereome. — H. O. Hanson. 
 
 cells of the phloem are much smaller, thinner walled and less 
 numerous than those of the xylem. Therefore very thin sec- 
 tions are necessary in order to make clear all the different tissues. 
 Sections from a squash stem may well be studied for this purpose 
 because the various cells of the phloem are comparatively large 
 
8o 
 
 STRUCTURE OF BUNDLE 
 
 and easily distinguishable. It should be stated that the bundle 
 of the squash is peculiar in that a phloem region is developed on 
 both sides of the xylem. The phloem, like the xylem, is also 
 characterized by large cells, which are here known as sieve tubes, 
 because these cells are tube-like structures with the cross walls 
 perforated like a sieve (Fig. 4.2, s). A small cell, the accom- 
 panying cell, is associated with the sieve tube. Usually a vary- 
 ing amount of parenchyma also occurs in the phloem and often 
 thick walled stereome fibers (Fig. 42, st). Between the xylem 
 and phloem is a region of very delicate and regularly constructed 
 cells, the cambium (Fig. 42, c). The growth of the bundle in 
 diameter and, in fact, of the entire stem is brought about very 
 largely by the formation of new cells through the division of the 
 
 Fig. 43. Longitudinal section of the bundle shown in Fig. 42: x, xylem; 
 ph, phloem; p, pith; v-v'", annular, spiral, scalariform and pitted vessels; 
 c, cambium; s, sieve tubes; ac, accompanying cells; st, stereome. — H. O. 
 Hanson. 
 
 cells in the cambium. The cross section of the bundle reveals 
 the arrangement and distribution of the various tissues but it 
 will be necessary to examine a section taken parallel with the 
 length of the stem, i. e., a longitudinal section, in order to arrive 
 at an understanding of the structure and character of the cells 
 themselves. Fig. 43 shows such a section of a bundle. The 
 
NATURE OF PLANTS 8i 
 
 vessels are now seen to be tubular structures with peculiar 
 thickenings of their inner walls that assume the form of rings 
 (annular vessels), spirals (spiral vessels), or the spirals may 
 branch more or less (reticulate vessels), and often to such an 
 extent that they cover the entire surface of the wall with the 
 exception of numerous small spaces, pores, thus forming the 
 pitted vessels (Fig. 43, v'"). These peculiar sculpturings prevent 
 the crushing in and closing of these tubes and the annular and 
 spiral thickenings also permit considerable elongation of such 
 ducts which would not be possible if the thickening were more 
 uniform over the wall. It will be noticed that the later formed 
 ducts are characterized by reticulate or pitted walls since this type 
 of thickening must occur after the elongation has taken place. 
 The ducts are composed of elongated cells but owing to the 
 absorption of the majority of the cross walls the ducts finally 
 come to resemble hollow tubes that often run for considerable 
 distances through the stem without any cross walls at all. The 
 smaller cells of the xylem assume various shapes. Some of 
 them, the tracheids, have pointed ends and are characterized by 
 markings similar to the ducts. Strengthening fibers, similar to 
 those noted in the cortex, are of common occurrence and also 
 short cells with blunt ends, wood parenchyma. The ducts and 
 tracheids soon lose their cell contents but continue to function 
 in the conduction of water. The wood parenchyma and other 
 living cells retain their vitality for several years and generally 
 function as storage cells. It is not improbable that they may 
 furnish some of the energy required to force the water through 
 the ducts and tracheids. It is significant that living tissue is 
 always associated with these water-conducting cells of the stem. 
 Passing now to the phloem we see that the sieve tubes are made 
 up of elongated cells with perforated or sieve-like cross walls (I''ig. 
 43, s). These minute openings adapt these cells to the transport 
 of albuminous substances which do not readily diffuse through 
 cell walls. The accompanying cells (Fig. 43, ac) which are cut 
 off from the sieve tubes probably assist them in this work. At 
 any rate they have abundant cell contents and contain nuclei, 
 whereas, singularly enough, the nuclei of thc^ sit'\r lubes soon dis- 
 6 
 
S2 STRUCTURE OF BUNDLE 
 
 appear although the rest of the protoplasm of the cells remains 
 active for one or more seasons. Elongated parenchyma cells are 
 associated with the sieve tubes and accompanying cells, serving 
 chiefly for the transport and the temporary storage of the more 
 readily diffusible carbohydrates. Fibrous cells, stereome, are 
 also of common occurrence in the phloem (Fig. 43, st). The 
 cambium is composed of very delicate cells, owing to the fact 
 that in this region cell division is taking place and the cells 
 consequently have not yet attained their characteristic form (Fig. 
 43, c). We may best gain an idea of the growth of the cambium 
 if we consider it as a single layer of cells between the xylem and 
 phloem. Each of these cells divides into two daughter cells 
 (Fig. 44), usually the inner cell {i. e., the cell next to the xylem) 
 develops into a duct, tracheid, or other element of the xylem, 
 while the outer cell, after increasing to the original size of the 
 cambium cells, divides into two cells as at first. This process 
 is repeated again and again, usually the inner cell growing into 
 one of the cells of the xylem while the outer cell retains the 
 
 X 3^^ ^Qr 
 
 Fig. 44. Diagram showing the mode of division of the cambium cells. 
 The cambium cell is shaded to distinguish it from the cells derived from it. 
 Note in the last division at the right that the inner daughter cell becomes 
 the cambium cell while the outer develops into a phloem cell. 
 
 power of further division. Less commonly the reverse method 
 of growth takes place and we have the outer cell enlarging and 
 forming one of the cells of the phloem while the inner cell acts 
 as a cambium cell. In this way new cells are added to the xylem 
 and phloem and the enlargement of the vascular bundle is 
 effected. 
 
 35. The Conducting System of the Plant. — These vascular 
 bundles extend from the root up through the stem and branches 
 to the leaf where they divide again and again, reaching all parts 
 of it. In this way the bundles become much reduced in size, 
 the free end of the vein often consisting of a single tracheid to 
 
NATURE OF PLANTS 83 
 
 conduct the crude materials to the surrounding cells and elon- 
 gated parenchyma cells to collect the manufactured foods (Fig, 
 5, /). We now see how admirably the vascular bundles are 
 adapted to the transport of fluids. The xylem carries the sub- 
 stances received from the root hairs to all parts of the plant 
 body quickly and with a minimum expenditure of energy because 
 of the elongated character of its cells and the absorption of many 
 of the cross walls of its cells. So the manufactured foods lo- 
 cated at any point in stem or leaf are gathered up and distributed 
 by the phloem, the more soluble and dilTusible substances being 
 handled in part by the thin-walled parenchyma, while non- 
 difTusible, albuminous substances can readily be transported 
 from cell to cell through the perforations of the sieve tubes. 
 
 36. The Strengthening Tissues of the Stem. — Attention may 
 be directed at this point to the perfection of the arrangements 
 that give rigidity to the stem. The delicate cells of very young 
 stems or of any other part of the plant are distended by fluids 
 which they absorb. In this way a very considerable force, known 
 as the turgor of the cell, is exerted that may amount to over 200 
 pounds to the square inch. This pressure at first gives sufficient 
 rigidity to the rapidly elongating stem, but as it increases in 
 
 Fig. 45. Common form of girder. 
 
 size and the strain upon it becomes more considerable, thick- 
 walled collenchyma cells are formed that keep pace in their 
 growth with the elongation of the stem. When clongatit)!! finally 
 ceases, tough fibers of stereome appear which are able to meet 
 the increasing strain upon the stem due to the formation and 
 development of its various organs. These strengthening tissues, 
 with which must also be included the tissues of the xylem, are 
 arranged with the same mechanical effects as are employed in 
 the construction of buildings, bridges, etc. In bending a beam 
 
84 
 
 STRENGTHENING DEVICES 
 
 the strain falls especially on the convex and concave surfaces. 
 The cells will be compressed on the concave side and stretched 
 on the convex side, while the tissues in the center will be subject 
 to the least disturbance. For this reason girders are strength- 
 ened on the surfaces receiving the strain and weakened in the 
 center (Fig. 45). So also given a certain amount of material 
 
 Fig. 46. Cross-section of stems showing arrangement of strengthening 
 tissues: A, burdock with strands of collenchyma — st, at periphery; v, vas- 
 cular bundles. B, sweet clover, showing a similar arrangement. C, moon- 
 seed showing cylinder of stereome fibers outside of vascular bundles. D, 
 rush with strands of stereome fibers at periphery of stem and also on inner 
 and outer sides of the bundles; v, vascular bundles; st, stereome. 
 
 this is more effectively distributed in the form of a hollow column 
 than in a solid column since less strain falls upon the center of 
 the column. Therefore in stems we find the strengthening tissue 
 
NATURE OF PLANTS 85 
 
 very economically distributed at the periphery either as bands 
 or strands of coUenchyma and stereome fibers and these struc- 
 tures are frequently reinforced by additional mechanical tissue 
 in the phloem or just outside of it (Fig. 46). The vascular 
 bundles themselves frequently furnish excellent examples (jf the 
 girder, masses of stereome being found on the outside of the 
 phloem and on the inside of the xylem (Fig. 46, D). The resis- 
 tance of these tissues to cutting and breaking affords abundant 
 evidence of their hardness and rigidity. It may be surprising 
 to note that the sustaining strength of the stereome fibers is 
 equal to that of the best wrought iron or hammered steel while 
 their ductility is ten to fifteen times that of iron. The superior 
 quality of these tissues and the perfection of their arrangement 
 result in structures that can not be approximated in any of our 
 buildings. The height of the tallest chimneys scarcely exceeds 
 the diameter of their bases more than fifteen times but many 
 stems of rushes and grasses exceed the diameter of their bases 
 from 200 to 500 times. It would be impossible with aii\- metal 
 to construct a column of the same length and weight and ha\'ing 
 the same rigidity, elasticity, and resisting power as these stems. 
 We gain an idea of the toughness and durability of these fibers 
 when we consider that linen, rope, matting, etc., are manufac- 
 tured from them. In stems that increase greatly in diameter, 
 as our trees, the mechanical tissues are confined to the wood or 
 xylem. These are to be sure nearh' solid colunms, but e\cn 
 here there is sufficient strengthening tissue in the xylem at the 
 periphery of the stem to support the trunk, as is often attested 
 by the sturdy character of trees that have become hollow through 
 decay. 
 
 37. The Secondary Growth of the Stem. — The arrangement 
 of the tissues as outlined above, termed the i:)rimary growth, 
 remains practically unchanged in some animal plants but in such 
 forms as live on from year to year or that increase matrrially 
 in diameter there results extensive alterations in the structure 
 of the stems owing to the formation of new tissues, especially 
 in the region of the cortex and in the vascular bundles. The 
 epidermis furnishes sufficient protection to such stems as do not 
 
86 NATURE OF CORK TISSUE 
 
 increase materially in size, such as the majority of our annual 
 plants; but in long lived stems, as shrubs and trees, where growth 
 goes on from 3'ear to year the epidermis is not able to keep pace 
 with the growth. 
 
 38. Cork Tissue. — To meet this condition a new tissue, the 
 cork, is developed from certain cells called the cork cambium, 
 usually situated near the epidermis (Fig. 47). The cells of the 
 cork cambium divide much after the manner noted in the cam- 
 bium of the vascular bundle, but there is this difference, the outer 
 cell of the two daughter cells becomes a cork cell while the inner 
 cell remains capable of further division. Only rarely does the 
 outer daughter cell act as the cambium cell while the inner cell 
 develops into one of the cells of the cortex. As soon as the 
 cork cells have reached their growth, a substance called suber 
 
 Fig. 47. Cross-section of the outer part of the stem of geranium. The 
 cork cambium, c, originating in the cells immediately below the epidermis. 
 
 begins to be deposited in their walls. This completely changes 
 the properties of the cellulose walls and renders them impervious 
 to fluids and gases. So the cork cells have the same physical 
 properties as the cuticle of the epidermis and owing to the 
 continued activity of the cork cambium they are able to keep 
 pace with the growth of the stem. It is evident that the cork 
 cells must die as soon as they become impervious to fluids, and 
 it must also follow that all cells lying outside of these cork cells 
 will die since no fluids can reach them from the vascular bundles. 
 These dead cork cells give the characteristic aspect to the outer 
 bark of trees and we would naturally come to think of the coarse, 
 dark bark as composed of rather thick cells. As a rule, however, 
 the cork cells are quite delicate and compactly put together (Fig. 
 48) and the dark color is more usually due to a discoloration of 
 their walls or to the dried remains of the cell contents. The 
 
NATURE OF PLANTS 
 
 87 
 
 furrows and seams that occur in the bark of most trees are caused 
 by the continued activity of the cork cambium and the cambium 
 of the vascular bundles which adds each season new cells to the 
 stem and so pushes out the cork cells. In this way it comes about 
 
 Fig. 48. Fig. 49. 
 
 Fig. 48. Cross-section of cells of bottle cork showing the delicate character 
 of cork cells. 
 
 Fig. 49. Scale bark of pitch pine. The crescent-like lines in the bark 
 show the successive positions of the cork cambium. 
 
 that the cork cells are pushed further and further from the center 
 of the stem and since they are not capable of dividing they are 
 pulled apart, forming the characteristic furrows in the bark. 
 It will often be noticed that the bark of some trees, see the cone- 
 bearing trees, sycamore, etc., cleaves off in shell-like scales. 
 This is due to the formation of a new cambium that joins on to 
 
 Fig. 50. Cross-section of the outer part of a stem showing the early 
 development of a lenticel. Note the irregular character and loose arrange- 
 ment of the cells below the stoma and the cork cambium, c, extending out on 
 either side of the lenticel. 
 
 the old cambium in the form of crescents (Fig. 49). Conse- 
 quently as this bark is pushed out it breaks along these successive 
 crescent shaped cambiums and finally cleavts off in scales. In 
 
NATURE OF LENTICELS 
 
 some trees the cork cells break off about as fast as they are formed, 
 so that a comparatively thin layer of cork cells remains attached 
 to the trunk. Such trees have a smooth bark, as in the beech. 
 In other cases the cork cells are formed in great abundance, 
 and owing to the adhesion of the cells, thick layers of cork are 
 formed, as in the oaks, and giant trees of the Pacific. 
 
 It is evident that this mantle of impervious cork cells would 
 tend to prevent the access of the atmosphere to the stem. We 
 have seen that all living cells respire. In many instances the air 
 spaces extending from the leaves to all regions of the stem are 
 
 Fig. 51. Fig. 52. 
 
 Fig. 51. Later development of a lenticel: c, cork cambium; i, intercel- 
 lular space. 
 
 Fig. 52. Surface view of lenticels: A, lenticels on branch of horse-chestnut 
 appearing as minute brownish swellings. B, old lenticels on white birch 
 appearing as dark lens-shaped streaks. 
 
 sufficient to bring about an adequate interchange of gases but in 
 young stems where growth is vigorous and where therefore 
 respiration is considerable, special devices are required to bring 
 the living cells of the stem into more direct communication with 
 the air. This work is effectively accomplished in herbaceous 
 stems by the stomata, but in stems characterized by the forma- 
 tion of cork it is noticed that the cells just below the stomata 
 begin to divide and form a rather loose mass of cells that lift up 
 and finally rupture the tissues about the stomata, thus forming a 
 small, lens-shaped outgrowth on the surface of the young stem, 
 called a lenticel (Figs. 50; 52, A). Soon this growth becomes 
 
NATURE OF PLANTS 8q 
 
 localized in a layer of cells situated on the inside of the Icniicel 
 (Fig. 51). This layer of cells is a part of the cork cambium, 
 but strangely enough only loose cells are added to the lenticel 
 during each year's growth and consequently a passage way is 
 kept open to the living cells within the stem. The lenticels 
 appear as minute points upon the surface of young stems but 
 upon old trunks they often become greatly elongated, fcjrniing 
 the characteristic bands on the bark of the birch and cherry, etc. 
 (Fig. 52, B). In bottle cork, derived from the cork oak of the 
 Mediterranean, the lenticels appear as minute lines often errone- 
 ously referred to as worm holes. Usually as the cork layer 
 increases in thickness the cortical cells become less active, lose 
 their chlorophyll and the cork cambium closes the lenticels b>' 
 forming compact cork cells instead of the loose cells of the 
 lenticels. This same closure of the lentiels is frequently scene 
 in the fall but the renewal of growth in the spring results in the 
 formation of new loosely related cells which rupture the over- 
 lying cork layer and so open the lenticels. So it seems probable 
 that the lenticels arise owing to the active respiration and 
 transpiration of the cortical cells and, when these functions di- 
 minish, that the lenticels become closed and fmallv- fail to develop 
 further. 
 
 39. The Cambium Cylinder. — Let us now consider the changes 
 that are effected in the vascular bundles through the activity of 
 the cambium. The increase in the diameter of the stems of 
 many annual plants and especially of our trees and shrubs is 
 largely brought about by the formation of new cells derived 
 from the cambium. While the vascular i)undles are very small 
 it will be noticed that the parenchyma cells separating the vascu- 
 lar bundles begin to divide so as to form a line of cells connecting 
 the cambium of each bundle (Fig. 53). In some cases these 
 divisions are at first somewhat irregular but soon the growth 
 results in the formation of cells with parallel walls and in this 
 way a line of regular cells is formed which in cross section apix\ir 
 as a band or ring of cells but in longitudinal extent they constitute 
 a cylinder. These cells are therefore termed the cambium cylin- 
 der and they continue to divide as already not.d in ihc r.imbium 
 
90 
 
 THE ANNUAL GROWTH 
 
 of the bundles (Fig. 54). Consequently new cells are now added 
 not alone to the vascular bundles but also along the entire extent 
 
 Fig. 53. Cross-section of a stem of castor bean showing the formation of 
 the cambium between two vascular bundles: x, xylem; ph, phloem; c, cam- 
 bium of the bundle. The faint lines, ic, are the first divisions of the parenchyma 
 cells between the bundles that result in the formation of the cambium cylinder. 
 
 of the cambium cylinder. This growth results in the formation 
 of a layer of xylem on the inside of the cambium cylinder and a 
 layer of phloem on the outside, and so brings about the principal 
 
 cam 
 
 Fig. 54. Cross-section of a stem pf castor bean in which the formation of 
 the cambium cylinder, cam, as a ring of regular cells, has been completed; 
 p, pith; I', vascular bundles; st, stereome; c, cortex. Compare Fig. 40. — 
 H. O. Hanson. 
 
NATURE OF PLANTS 
 
 91 
 
 enlargement of the stem (Fig. 55). This growth is repeated 
 each spring in such plants as continue to enlarge from year to 
 year, as in the shrubs and trees. The majority of all the cells 
 that are to be formed in a year's growth are cut off from the 
 cambium cylinder early in the spring, usually by the last of April. 
 The majority of these cells become xylem cells while comparatixily 
 only a few of the cells are added to the phloem. Consequently the 
 
 Fig. 55. Cross-section of stem of castor bean three weeks okli-r than one 
 shown in Fig. 54. Note the changes that have occurred in the stem and 
 especially the numerous cells that have been added to the xylem. ck, cork; 
 ph, phloem; cam, cambium cylinder. — U. O. Hanson. 
 
 xylem increases faster than the phloem and forms tlu' bulk of the 
 tissues of the stem. The amount of the xylem added to the stem 
 each year is generally indicated by bands or annual rings (Fig. 
 56). This is due to the fact that the first cells formed in the 
 spring have thinner walls and often contain a great many ducts, 
 whereas the later formed cells are for the most part small and pro- 
 
92 
 
 ANNUAL RINGS 
 
 vided with thicker walls. So there is a sharp contrast between 
 the small thick-walled cells of the summer wood and the thinner 
 walls and larger cells of the spring wood (Fig. 57). This ap- 
 pears to the eye as a band, the annual ring (Fig. 56). The 
 difference in the structure of the cells of the spring and summer 
 wood is doubtless due to the large volumes of water that are 
 transported to the rapidly growing shoots, flowers, and leaves 
 each spring. Later in the season a lessened volume of water 
 
 Fig. 56. Fig. 57. 
 
 Fig. 56. Diagram of a cross-section of a stem of black oak four years old; 
 p, pith; I, 2, 3, 4, annual rings of xylem; c, cambium cylinder; ph, phloem; 
 cr, cortex; ck, cork; m, medullary rays. 
 
 Fig. 57. Magnified view of a portion of one of the bands of black oak in 
 Fig. 56, showing the thick-walled summer wood succeeded by the thinner- 
 walled cells and vessels. This association of cells causes the banded appear- 
 ance of the annual rings of growth, m, medullary rays; v, vessels in the spring 
 wood. 
 
 produces firmer and denser tissues. The age of a tree can gener- 
 ally be ascertained by counting the annual rings. However, two 
 rings may be formed in one season owing to the checking of 
 the growth by fire, severe drouth, depredations of insects and 
 the subsequent recovery and renewal of growth. These annual 
 rings also reveal the life history of the tree, broad bands indi- 
 cating favorable seasons and narrow bands telling of fires, 
 
NATURE OF PLANTS 
 
 93 
 
 drought, transplanting, and other factors that hiiiit the annual 
 growth. They also show that the development of the plant is 
 subject to the same rhythm of growth as is seen in"the animal. 
 
 
 
 Fig. 58. Diagram of a three-year-old stem of basswoocl cut so as to show 
 the structure in cross-section, C, in radial section, R, and in tangential stHTiion, 
 T; c, c ortcjc : cy^^^jork; ph, phloem with darker bands of stcreomc; ntc, medullary 
 ray in cross-section; mr, ray m radial section; vit, ray in tangential stxnion; 
 /, lenticels; cam, cambium cylinder; />. i^U]i»--Il. O. Hanson. 
 
 Namely];the thickness of the rings increases yearl>- uj) to a certain 
 age, after which time there is a gradual ri'tardation. So the 
 tree has its youth, maturity and old age. 
 
94 
 
 MEDULLARY RAY 
 
 40. The Medullary Ray. — Another characteristic of most 
 woody stems as seen in cross section is the series of delicate 
 lines, the medullary rays, that radiate from the center of the 
 stem (Fig. 58, mc). Some extend from near the pith through the 
 phloem and others originate in the various annual zones of the 
 xylem and extend partially through the phloem. When magni- 
 fied these medullary rays are seen to consist of rather thin- 
 walled, oblong cells and varying in width from one to a few cells 
 (Figs. 57, m\ 58, mc). In a longitudinal section cut parallel 
 to the rays, called a radial section, they appear as bars of oblong 
 cells running at right angles to the elongated cells of the xylem 
 (Figs. 58, mr] 59, A), while in longitudinal sections cut at right 
 
 Fig. 59. Relation of the medullary rays to the xylem cells in pitch pine: 
 A , radial section showing the elongated cells or tracheids, t, marked by circular, 
 thin places on bordered pores, p; m, medullary ray of two cells accompanied 
 by tracheids, mt. B, tangential section showing medullary rays one cell 
 broad and three to nine high — p, bordered pores. 
 
 angles to the rays, called a tangential section, we see that the 
 cells of the rays are arranged in a lens-shaped group (Figs. 58, mt; 
 59, B). The medullary rays are of great importance in the dis- 
 tribution and often also in the storage of foods. Their structure 
 and arrangement in the stem are admirably adapted for this work. 
 Their thin walls can readily withdraw from the ascending current 
 in the xylem as much water as is needed for the phloem and cro- 
 
NATURE OF PLANTS 95 
 
 tex. Trachelds are often associated with the medullar) rays to 
 further increase their absorbing power (Fig. 59, A, vit). Par- 
 ticularly is this true in the spring wood where the ray cells also 
 often become greatly enlarged in order to absorb the large volume 
 of water required at this time of year. In the same way the 
 carbohydrates and albuminous substances transported through 
 the phloem are withdrawn through the medullary cells and con- 
 veyed to the active cambium and the growing cells of xylem. 
 
 The value of oak and other woods for interior decoration is 
 materiall}^ increased by the medullary rays. The trees are sawed 
 into timber in such a way as to expose the medullary rays to the 
 best advantage, a process called quartering (Fig. 60). Conse- 
 
 Fig. 60. Diagram showing a common method of sawing in producing 
 quartered oak. The log is first squared to remove the sap wood and then a 
 number of boards, depending upon the size and grain of the tree, are removed 
 at a and b. These are the best boards obtained from the log, being broad and 
 approximately radial and therefore showing the medullary rays to good ad- 
 vantage. The remaining portions of the log are cut radially, as shown at c. 
 One of the ends of quartered lumber is oblique and must be squared, as shown 
 in the diagram. 
 
 quently the rays appear as flecks and bands upon the surface of 
 the board and vary greatly in character owing to the angle of 
 cutting and to the irregularities of the trunk. The medullary 
 rays can always be detected in the grain of wood because they 
 run across or at right angles to the xylem cells (Fig. 58, mr). 
 As a rule the wood cells of the stem remain alive for only a few 
 years. This fact is usually apparent in any log or stump where it 
 will be seen that the outer annular rings are of a lighter color, the 
 sap wood, and the older parts are of a darker color, the heart 
 wood. The sap wood is active in the transport of water and 
 contains many living cells. For this reason it is not so valuable 
 
96 OTHER TYPES OF STEMS 
 
 for lumber. The heart wood Is composed of dead cells and 
 while the most serviceable for building purposes, it is of little 
 use to the tree as we often see a vigorous tree whose heart wood 
 has been largely destroyed by decay. We ought not to leave this 
 vascular system of the plant without again emphasizing the per- 
 fection of it. First there is the completeness of the system, ex- 
 tending from the root to all parts of the leaf. As soon as the root 
 hairs begin to form and function there is already at that point the 
 beginnings of one of the terminals of the system while the other 
 terminal is also appearing in the leaves while they are forming and 
 still concealed in the buds. Then the elongated cells, those of 
 the xylem being in addition largely without living cell contents, 
 ensures a rapid transport of material. The weak feature of this 
 system is the terminals for here the materials must not only pass 
 through the numerous walls of the short cells but also through 
 the still more impermable membranes of the living matter of 
 these cells. But once the material is landed in the elongated 
 cells, then follows a rapid transport. Therefore it is the develop- 
 ment of this system that makes possible the stature of plants. 
 Without it, plants must remain small and in close reach of their 
 crude supply. 
 
 41. Other Types of Stems. — The cone bearing trees, such as 
 the pines and spruces, have essentially the same arrangement 
 of tissues and mode of growth as noted above in the dicotyledons. 
 The xylem, however, consists entirely of tracheids (Fig. 59) 
 with the exception of a few small spiral vessels that are formed 
 as the first cells of the vascular bundles. 
 
 The monocotyledons, plants distinguished usually by their 
 parallel veined leaves and single seed leaf, like the palms, lilies, 
 grasses, etc., are characterized by stems that do not increase 
 materially in diameter. Growth is largely confined to the top 
 of the stem and consequently it can only elongate, forming a 
 very regular, columnar trunk. The reason of this is apparent 
 when cross sections of such stems are examined (Fig. 46, D). 
 The vascular bundles are more or less scattered throughout the 
 stem. The cause of this arrangement is seen in the seedling 
 stage of the plant. Here we see the vascular bundles arising 
 
NATURE OF PLANTS 
 
 97 
 
 through the opening up of the vascular cyHnder just as in the 
 case of the dicotyledones, page 76. The vascular bundles thus 
 formed, however, do not long pursue a uniform course through 
 the stem, some soon turning into the pith and others bending 
 into the cortex. It is also especially noteworthy that there is 
 usually no cambium separating the xylem and phloem. It will 
 be seen by examining Fig. 61, that the tissues of the bundles are 
 
 Fig. 61. Cross-section of a single vascular bundle of corn stem: ph, 
 phloem; x, small cells of the xylem; v, vessels of xylem; st, stereome that 
 forms a sheath about the bundle; p, parenchyma of the stem. 
 
 of a similar character and have the same arrangement as in the 
 dicotyledons (Fig. 42) save for the absence of the cambium which 
 prevents the addition of new cells to the xylem and phloem. 
 Consequently there can be no considerable increase in the dia- 
 meter of the stem and no necessity therefore for the protective 
 layer of cork cells. 
 
98 
 
 ELONGATION OF THE STEM 
 
 42. Apical Growth of the Stem.— All stems are characterized 
 by a more or less extended apical growth. The changes pro- 
 duced in stems by the growth of the cambium only result in 
 increasing their diameters but the elongation of all stems is 
 effected by the formation of new cells at the apex of the stem. 
 Fig. 62 shows the general features to be noticed in the tip of 
 a stem as seen in longitudinal section. New cells are being 
 formed at the apex by division just as in the case of the root. 
 Below this formative region is the zone of elongation in which 
 the cells gradually change in character so that the cortical and 
 central regions are apparent and in certain elongated cells we 
 note the first indication of the vascular bundles (Fig. 62, x). 
 
 Fig. 62. Longitudinal section of the tip of a growing stem: e, epidermis 
 extending over surface of the entire tip; a, formative region; b, upper portion 
 of the zone of elongation; c, cortex; x, cells of the central region that by 
 further growth form the vascular bundles; /, first appearance of the leaves. 
 
 The leaves also originate in this region through the active divi- 
 sion of the superficial cells of the stem. The branches of the 
 stem develop somewhat later in the axils of the leaves, and more 
 deeply located cells in the cortex co-operate in their formation. 
 This relationship of the leaves and branches to the stem is shown 
 in Fig. 63, which is a diagram of an elongating stem, showing the 
 relation of the apical region of the stem (a in Fig. 62), to the 
 lower and older portion. The cells in these young leaves and 
 branches by rapid division and growth soon form the character- 
 
NATURE OF PLANTS 
 
 99 
 
 istic tissues already noted in the leaves and su-nis; while a cor- 
 responding growth in the cortex and adjacent regions results in 
 the formation of vascular bundles that connect the vascular 
 bundles of the stem with those of the leaves and branches. When 
 a leaf falls off, the ends of these vascular bundles can be easily 
 seen in the leaf scar (Fig. 28, A) but owing to their minuteness 
 
 Fig. 63. Diagram of the tip of a stem as seen in longitudinal section: 
 a, formative region corresponding to the part shown in Fig. 61; /, b, leaves 
 and branches in various stages of development; v, vascular bundles; c, cortex; 
 p, pith. 
 
 it is not an easy matter to trace them through the stem to the 
 point where they join on to the bundles of the stem. In the 
 branch, however, owing to its size and woody character, the 
 union with the stem is very manifest. Fig. 64 shows several 
 very small branches that continued to keep pace for one or more 
 
100 
 
 RELATION OF BRANCH TO STEM 
 
 years with the growth of the stem, but eventually they were 
 killed by the overhanging branches and in time became over- 
 grown with the annual layers of xylem. If the branch is favor- 
 ably situated so that it continues to live then a cone-like struc- 
 ture is developed (Fig. 64, a) since the branch increases in girth 
 each year in the same manner as the stem. These branches run- 
 ning through the xylem are the cause of knots that appear in 
 lumber. If the branch is living the tissues of stem and branch 
 
 Fig. 64. Section through the trunk of basswood showing relation of 
 branches to main stem. In the upper portion of the figure are three branches 
 that were killed after a few years growth' by shading and that have been 
 overgrown by the annual rings of the wood. The branch shown at a has 
 remained alive and increased in size after the manner of the main stem. 
 
 are closely bound together and we have a solid knot, but it not 
 infrequently happens that branches after living several years die 
 and begin to decay before they are overgrown. Such a limb will 
 produce in the lumber a knot that is often black and shaky or it 
 may fall out, forming a knot hole. The lower branches of trees 
 growing in a forest are quickly pruned off by the shade of the 
 upper branches. This causes straight, clean trunks to develop 
 and the lumber is free of knots. 
 
 It follows from what has been said above that in pruning trees 
 the limbs should be cut off close to the trunk. Painting the 
 the cut the cut surface does not promote the healing of the wound 
 though it may exclude organisms causing decay. When a wound 
 
NATURE OF PLANTS 
 
 lOI 
 
 is made that extends across the cambium as in the cutting off of a 
 Hmb or by a cut into the trunk that removes the tissues as far as 
 the xylem it will be noticed that the exposed cambium is stimula- 
 ted to from a mass of delicate cells, the callus, that gradually ex- 
 tend over the wound if this is not too large. While the callus is still 
 forming its outer cells become changed to cork cells which join 
 onto the old cork while a new cambium layer is also developed on 
 the inner side of the callus, which unites with the old cambium, 
 thus continuing the cambium of the stem over the surface of the 
 wound (Fig. 65, ^). In this way wounds are covered or healed 
 
 Fig. 65. Section of a portion of a stem showing the heaHng of a wound 
 caused by the cutting off of a lateral branch: A, formation of the callus, cl, 
 owing to the renewed activity of the living cells exposed by the wound; c, 
 cortex; cm, cambium; x, region of the xylem. B, a similar stem three years 
 later — c, cortex; cm, cambium; x', xylem added since the wound was made; 
 0, position of the cambium at the time the branch was cut off; .v, original 
 xylem of the stem. C, trunk from which three branches have been removed, 
 showing the gradual covering of the wounds by new tissue. 
 
 and the tissue exposed by the cut is gradually buried deeper by 
 the annual addition of new tissue derived from the cambium (Fig. 
 65, B, C). Grafting is made possible owing to the formation of 
 callus that unites the tissues of the scion and stock (Fig. 66). 
 While grafting is performed in a variety of wa>s the process 
 consists essentially in all cases of bringing the cambium and 
 cortex of the cutting or scion in contact wllh ibc ((.rresiHMKling 
 
102 PRUNING AND GRAFTING 
 
 region of the stock. The junction of the scion with the stock is 
 sealed with grafting wax to prevent decay and the drying out 
 of the exposed cells while the callus is joining together the corre- 
 sponding tissues in the two parts. When the wound is healed 
 only a slight ring or swelling indicates the point where the scion 
 was inserted — so complete is the union of the vascular and cor- 
 tical regions. It is evident that pruning and grafting should be 
 done at a time when the cambium is most active, i. e., in the 
 spring. In the case of fruit trees it is now the practice to defer 
 the pruning until some time in June, because the cambium is 
 
 Fig. 66. A common method of grafting: A, insertion of two scions into 
 cleft of stock at cambium region. B, wound protected with wax to prevent 
 drying out of tissues. — After L. H. Bailey. 
 
 still active at this season and especially since the buds are form- 
 ing. By removing that portion of the branch which would only 
 produce leaf buds a better development of the flower buds is 
 ensured. 
 
 Although the organic union between scion and stock is com- 
 plete, each part continues a separate and distinct existence. For 
 example, a scion from a plum grafted upon a branch of a peach 
 tree will develop a shoot bearing plums and the peach tree will 
 also continue its normal growth. Ordinarily only closely related 
 forms may be grafted, as tomatoes and potatoes, apples and 
 quinces, etc. It must be inferred from the above statement 
 that there is no interaction between stock and scion. Usually 
 this is not sufficient to change materially the character of either 
 but there are numerous instances showing that the chemical com- 
 position of the cells or even the form, coloration, and productive- 
 
NATURE OF PLANTS 103 
 
 ness of scion may be modified by the stock. Fruit j^owers take 
 advantage of this in grafting the cuttings from sccdhngs that 
 would not bear for several years upon fruiting trees, with the 
 result that the scion comes into fruitfulncss the next season. 
 In the same way it is claimed that seedlings may be made to 
 bear earlier than they normally would by grafting them with buds 
 from fruiting trees. 
 
 43. Comparison of Stem and Root. — It will be noticed that the 
 structure of the growing tip of the stem presents several striking 
 departures from that of the root. In the first place a protective 
 structure such as the root cap is not required (Fig. 62). The 
 leaves, and consequently the branches are formed with great 
 regularity from the superficial tissues of the stem, whereas the 
 rootlets are deep seated in origin and irregularly distributed. 
 Furthermore, the elongation of the stem is distributed over 
 several centimeters, while in the root it is confined to a few milli- 
 meters. The leaves and branches are distributed over the entire 
 zone of elongation, while lateral roots do not appear until elonga- 
 tion has ceased. This prolonged growth of the tip of stems is of 
 great advantage in bringing about the proper adjustment of 
 leaves and branches. These organs come into sharp competition 
 with each other as well as with other plants, and so long as growth 
 continues there is a possibility for them to become adjusted to the 
 light without interfering with one another. The mode of elonga- 
 tion of stems and roots is essentially alike. Each has a formative 
 region at the apex characterized by active cell formation and 
 slight elongation, back of this is the zone of rapid elongation and 
 following this is the zone in which elongation is gradually ceas- 
 ing and the tissues are assuming their characteristic forms and 
 functions. It is also important to note that the growth of the 
 cells on one side of the stem or root is faster at a certain time 
 than that of the other cells. This will cause the stem to be bent 
 towards the opposite side. This condition of rapid elongation 
 does not continue on one side for any considerable time but 
 slowly moves from point to point around tlie stem so that the 
 stem is successively bent from side to side, thus causing the apex 
 to travel in an irregular circle. By these constant movements, 
 
104 SENSITIVENESS OF STEMS 
 
 nutation, the stems and roots are more fully exposed to the vari- 
 ous forces that influence their development and consequently 
 they adjust themselves to the best advantage (see page 33). 
 
 As a rule the irritability or sensitiveness of the stem is not so 
 localized as in the case of the root. It is evident that it is an 
 advantage to have the entire growing surface of the stem sensi- 
 tive to all those stimuli that influence its growth, because by this 
 means it will be so influenced as to bring its leaves and branches 
 into proper relations with the surroundings. In some plants to 
 be sure the sensitiveness to stimuli is quite as localized as in the 
 root; as, for example, the tip of the cotyledons of many grasses 
 which have an appreciation of light almost the equal of our eyes. 
 These cotyledons will turn toward a light too feeble to enable us 
 to read the time of day on a watch. The significant fact to be 
 noted here is that this keenness and localization directs the plant 
 into a suitable place as soon as it emerges from the soil. Grass 
 plants grow in thick colonies and therefore the stems come into 
 sharp competition. Consequently it is a necessity that the young 
 shoot on emerging from the soil should be able to appreciate the 
 feeblest light and so be able to direct its growth through the 
 smallest openings. 
 
 The reactions of the stem to various stimuli, such as light, 
 gravity, etc., are quite as purposive as those noted in the root. 
 Lateral light, as in the case of plants growing in windows, acts 
 as a stimulus and causes a more considerable growth of the cells 
 on the darker sides of the stem. This results in bending the 
 stems towards the light and consequently in the exposure of the 
 broad blades of the leaves to the light. So, also, when erect stems 
 are placed in a horizontal position the stimulus of gravity awakens 
 a more active growth on the underside of the stem, thus bending 
 the tip into an erect position for securing light. Not all stems 
 and, indeed, not all parts of the same plant body are similarly 
 influenced by these forces. Many plants tend to arrange their 
 stems at right angles to the action of gravity and light, as in the 
 case of creeping and underground stems. So also the lateral 
 branches are inclined at various angles to the stem. These ad- 
 justments are not to be looked upon solely as the direct response 
 
NATURE OF PLANTS 105 
 
 to one or more stimuli but as the resultant of numerous interac- 
 tions aroused in the plant by various stimuli. The entire ])lant 
 responds as a unit to these forces. Stimulation and the conse- 
 quent growth in one part has its effects upon all other parts. As 
 a result of this interaction there comes about a correlation in the 
 growth of all the parts that is expressed in the symmetry of the 
 plant body and in the perfect adjustment of roots, stem and 
 branches, and leaves to its needs. Any window garden, forest, 
 or field furnishes abundant evidence of the complicated and 
 varied nature of these reactions. An illustration of this is 
 afforded in the removal of the terminal axis of a tree. An ad- 
 joining lateral branch that heretofore has been stimulated to a 
 more or less horizontal growth, now is so directed in its develop- 
 ment that it ultimately becomes the terminal axis of the tree. 
 So some parts are constantly being overshadowed and choked 
 out while others are directed into favorable positions and perform 
 the work of the destroyed parts. In this way there is brought 
 about that weaving and twisting of stems and branches and the 
 adjustment of leaves that so often suggest a conscious effort to 
 reach favorable positions. 
 
 44. Modifications of the Stem. — In the above discussion at- 
 tention has been called to the more common characteristics of 
 the stem. It is well to remember, however, that variation is 
 the law in the plant world. No two plants are alike. The 
 environment of the plant is constantly stimulating it and caus- 
 ing it to vary. Frequently the variations are so minute as to 
 escape our attention or totally distinct forms may arise. These 
 departures from the parent t}'pe ma^' be of no ad wantage to the 
 plant and it may therefore perish. On the other hand the varia- 
 tions may be of such a character as to enable the plant to more 
 successfully complete with other plants and as a consequence the 
 plant with its helpful variations will survive. So it hiis come 
 about that these changes going on during the past ages have 
 resulted in many remarkable modifications. A few of the modi- 
 fications of the stem that are of esj^ecial achantage to the plant 
 and therefore spoken of as adaptive variations will be considered 
 in the following paragraphs. 
 
io6 
 
 NATURE OF CLIMBING STEMS 
 
 45. The Climbing Type of Stems. — Climbing stems are char- 
 acterized by being rather small and having greatly elongated 
 internodes. Stems grown in the dark show a similar develop- 
 ment. Perhaps this variation that we call the climbing type 
 has been brought about by competition with larger plants. The 
 feeble light has stimulated these stems so that they attain a very 
 extended growth and finally are able to reach the light and 
 display their leaves. Many variations in the structure and sensi- 
 tiveness of the stem are associated with this elongation, all of 
 which are designed in one way or another to enable the stem 
 to reach the light. One type of these varia- 
 tions is seen in twining stems, as the morn- 
 ing glory, bean, hop, etc. Young twining 
 plants behave quite like the ordinary plant. 
 The stems are erect and actively nutating, 
 the apex traveling through a rather large 
 circle in one to three hours. When a cer- 
 tain height has been reached the stems are 
 stimulated by gravity so that their upper 
 portions grow more or less horizontally 
 (Fig. 67). This position is a decided ad- 
 vantage since the stem is now revolved 
 through a larger circle and has a greater 
 chance of coming into contact with an ob- 
 ject about which it can twine. As soon as 
 
 the stem comes into contact with any sup- 
 
 FiG. 67. Twining , •, ... .,, . ^ . 1 
 
 habit of wild bean. Note P^^^ '^^ nutation Will cause It to wind or 
 
 the horizontal position of twine about it. The contact also acts as 
 
 the upper portion of the a Stimulus, causing the stem to bend more 
 
 stem which results in the 
 apex nutating through a 
 wider circle. 
 
 energetically. Certainly in many plants the 
 size and roughness of the support as well 
 as other features, are important factors in 
 inducing the twining. At first the coils are merely horizontal 
 but owing to the elongation of the stem these coils are gradually 
 pushed upwards and become steep and very firmly bound around 
 the support. Twisting of the stems and reflexed bristles often 
 assist in anchoring the plant to its support. It is interesting to 
 
NATURE OF PLANTS u>-; 
 
 note that In the majority of cases the stem twines about the sup- 
 port In left hand spirals or clockwise; less frequently right hand 
 spirals are formed, as In the hop, some honeysuckles and knot 
 weeds. Why the growth of the stem results in left or right hand 
 twining is not known but the fact remains that the plants can not 
 be induced to change their method of twining. 
 
 46. The Tendril Type of Stem. — One of the most Interesting 
 variations of climbing plants Is seen in those stems that climb 
 by means of tendrils. In this type almost any part of the plant 
 may become modified so as to act as a tendril and bind the plant 
 to a support; e. g., the petioles of nasturtium and clematis, 
 stipule-like outgrowths In smilax, midrib of leaf in many members 
 of the bean family, or modified branches as In the grape vine, etc. 
 The ordinary tendrils as we see them In the gourd, passion flowiT, 
 etc., are highly specialized stems that are very sensitive to touch. 
 These organs are rapidly nutating, frequently completing tvvo 
 or more circles In an hour, and as they approach maturity they 
 often become slightly curved and hooked (Fig. 68, A). Their 
 sensitiveness to contact, which is often localized on the concave 
 upper third of the tendril, also becomes so keen In some forms 
 that a thread weighing one-fiftieth of a gram will cause a cur\a- 
 ture of the tendril when placed upon it. In fact a much smaller 
 weight If caused to vibrate avIU act as a stimulus, so that we 
 have here an Illustration of sensitiveness to weight that is keener 
 than our own. When the sensitive part of a tendril is stroked 
 by a pencil or comes In contact with any hard object, this acts 
 as a stimulus and causes a very rapid growth upon the con\ex 
 side of the tendril. As a result of this growth tlie tip of the 
 tendril curves, forming a complete loop in from a few seconds 
 to a few minutes. The contact of the tip of a tendril with any 
 object acts as a continued stimulus and the tendril soon becomes 
 tightly wound around It. As soon as tlie tip is firmly bound to 
 the support a new impulse is frequently transmitted to the free 
 portions of the tendril which In a day or so begin to assume the 
 form of coils that are often reversed in the middle (Fig. 68, B). 
 By this unique device tendril bearing stems are firmly fastened to 
 their supports but at the same time the coiling of the tendrils 
 
io8 
 
 NATURE OF TENDRILS 
 
 acts like a spring permitting considerable disturbance of the 
 vine without danger of breaking. This same device is copied 
 in making the attachment of the wires to the telephones and 
 other pieces of apparatus. From many standpoints the tendril 
 bearing stem is an admirable one. It is evident that such stems 
 can reach the light more directly and economically than twiners. 
 This doubtless accounts for their greater abundance and common 
 
 Fig. 68. Fig. 69. 
 
 Fig. 68. Tendrils of the bur cucumber: A, hooked tendrils in receptive 
 state. B, apical portion of tendril coiled about a branch and the remaining 
 portion of the tendril forming reversed coils, thus drawing the vine to the 
 supporting branch. — H. O. Hanson. 
 
 Fig. 69. Branch of Japanese ivy attached to wall by means of tendrils 
 with adhesive discs. — J. O. Hanson. 
 
 occurrence. It is also important to note that the tendrils are 
 especially abundant at the end of the branches where they reach 
 out considerably beyond the young leaves. Owing to the sway- 
 ing of the long free shoot of the climber in the wind and the 
 nutations of the tendril and shoot these sensitive organs are 
 
NATURE OF PLANTS 
 
 {')•> 
 
 brought into contact with branch after branch and llius the \ inc 
 is led up to the crown of the tree or to the top of the vegeta- 
 tion. The Virginia creeper, Boston or Japanese ivy, etc., have 
 pecuHar tendrils that grow away from the light and form ad- 
 hesive discs, instead of coils, that bind the vine to the support 
 (Fig. 69). These discs are caused by the growth of the cells at 
 the tip of the tendril. As soon as the tendril hits the support 
 upon which the plant is growing, the cells at the tip are stimulated 
 to grow out and fit into every little irregularity and groove in 
 the substratum. Indeed in some vines these discs become in this 
 way so thoroughly cemented to the crevices and irregularities of 
 the support as to resemble bits of melted wax. It is very in- 
 teresting to watch the results of the stimulation induced by one 
 of these tendrils striking a wall or other support. The formation 
 and cementing of the disc are accomplished in about four days 
 in the case of the Virginia creeper. Some plants like the poison 
 and English ivy and many tropical plants climb by means of 
 roots. These organs, like the disc forming tendril, avoid the 
 light and seek the dark nooks and comers in the bark of trees or 
 rocks over which the plants grow. 
 
 47. Prostrate and Creeping Stems. — It is interesting to note 
 that a large number of plants having rather small and weak 
 stems like the climbers, have never acquired the habit of climb- 
 ing. The young shoots of these plants are often erect at first 
 but soon the weak stems bend over and become prostrate. While 
 these procumbent and creeping stems are at a disadvantage in 
 that they are obliged to arrange their leaves in one i)lane, on 
 the other hand they are not obliged to build up the elaborate 
 structures that are necessary for the support of the erect stems. 
 Consequently plants of this class are of common occurrence in 
 old fields and unfertile soils since they can subsist on a meager 
 supply of food. They are also ])rotected against injury and 
 therefore adapted to wind swept sandy plains, to rocky hills and 
 mountains, and to districts visited by hea\y snows. The ma- 
 jority of these plants also have a ver>' decided achantage owing 
 to the fact that certain nodes form roots and buds that develop 
 into new^ plants. In some cases, stolons, only the terminal l.iid 
 
no RHIZOME TYPE OF STEMS 
 
 develops in this way and with the dying off of the old stem a 
 new plant is established, as in the houseleek (Fig. 70). In other 
 cases, runners, almost any node may form buds and roots which 
 become separate plants by the decay of the old stem, as in 
 the strawberry, gill-over-the-ground, cinquefoil, etc. These new 
 plants repeat the mode of growth of the parent plant and in 
 
 7%: 
 
 ''^^Ip 
 
 Fig. 70. Houseleek forming buds at the end of short branches or stolons. 
 
 this way many prostrate stems spread out over the soil, estab- 
 lishing new plants in wider and wider circles. If you will ob- 
 serve the number of new plants established and the distances 
 raversed by some of these creeping stems each year you willt 
 see the reason for the common occurrence of the large mats and 
 colonies of plants with prostrate stems. 
 
 48. The Rhizome Type of Stems. — In a third type, the stems 
 have become so modified that they respond to the various stimuli 
 in quite a different way from the forms noted above. They 
 creep along in the soil and frequently resemble roots more than 
 stems. For this reason they are called rootstocks or rhizomes. 
 They are however, real stems, as is attested by the numerous 
 leaves and often erect branches that spring from their nodes 
 as seen in the ferns, sweet flag, cat tail, grasses, etc. Plants of 
 this type are well protected against drought and fires, and like 
 prostrate stems they are adapted to establishing new plants as 
 is apparent in grassy meadows, colonies of golden-rod and daisies 
 or reedy banks of cat tails, sedges, etc. These stems are also 
 well adapted to propagating new plants because they generally 
 serve as storage organs for food and are therefore often of a 
 fleshy character (Fig. 71). For these reasons it is sometimes 
 very difficult to eradicate plants of this type. This is very well 
 illustrated in the quack grass, often a troublesome pest in cul- 
 
NATURE OF PLANTS 
 
 III 
 
 tivated land. Plowing and hoeing only serves to break up the 
 rhizome into numerous parts each of which may develop buds 
 and roots from their nodes and so establish new phints. 
 
 49. The Condensed Type of Stems. — In man>- cases we find 
 that the food is localized in special regions of the rhizome which 
 consequently become enlarged and rather fleshy, such enlarge- 
 ments appearing at the end of the rhizome or scattered along its 
 entire length. Such modified parts of a rhizome are called tubers, 
 e. g., the potato and Jerusalem artichoke (Fig. 72). The potato 
 
 Fig. 71. Fig. 72. 
 
 Fig. 71. Rhizome of Solomon's seal with aerial shoot just cmor^iiiK from 
 the ground. The seal-like scars mark the successive shoots produced during 
 the past three years. 
 
 Fig. 72. Formation of tubers: A, old potato or tuber with two shoots 
 reaching up into the air and from the base of these shoots, rhizomes have been 
 formed that are developing new tubers. B, mature tuber with spirally ar- 
 ranged buds, the so-called "eyes" of the potato. 
 
 is formed by the storage of foods in certain parts ol the rather 
 small rhizomes that branch out from the stem of the plant. 
 The "eyes" are the buds that develop at each node of the rhizome 
 and each is capable of forming a shooc, although but a few of 
 them so function, as is the case in an ordinary branch of a tree. 
 By passing a thread around a potato so that it touches each 
 successive bud you will see that there is the same arrangement of 
 the buds as appears in the leafy stem. There is considerable 
 evidence to show that the tuber form of rhizome originated 
 through association with fungi. Thus in the potato the rhizomes 
 appear to have a vigorous normal growth through the soil until 
 
112 CONDENSED TYPE OF STEMS 
 
 they are infested with fungi. This checks their elongation and 
 possibly accounts for the change in their mode of growth, which 
 is now characterized by the enlargement of the infested region 
 and the accumulation of starch. It is w^ell to remember that the 
 organs of many of these plants are exceedingly plastic and con- 
 trolled to a remarkable degree by conditions. As a single instance 
 of this fact note the leafy stems originating from the "eyes" of 
 the potato are inhibited in their elongation by strong light and 
 dry air. A potato grown on a moist surface in intense light and 
 dry air develops short cactus like stems; but grown in the dark 
 and in moist air they elongate vigorously after which their 
 nature changes, for if now exposed to light they develop as the 
 ordinary leafy stem. This is one reason for planting tubers 
 several inches in the soil. 
 
 A great variety of plants develop only very short stems that 
 are more or less buried in the ground. This type of stem modi- 
 fication has many of the advantages of the rhizome but not its 
 power to spread through the soil and so spread the plant. The 
 vital parts of such plants are only slightly exposed and conse- 
 quently suffer little from grazing animals or other sources of 
 injury, as in plantains, dandelions, etc. Frequently these short 
 stems are associated with an abundance of foods that are stored 
 in roots or other organs. Such plants can quickly send up and 
 mature their flower stalks or leafy stems and thus avoid un- 
 favorable conditions, as drought, competition with larger vege- 
 tation that will appear later, etc. These facts account for the 
 common occurrence of this type of stem in arid regions where 
 there are short rainy seasons. This habit has been made very 
 conspicuous by cultivation in many plants, as the carrot, turnip, 
 radish, and beet. But in nature the very short, almost flat stems 
 and fleshy roots are also of common occurrence, e. g., the wild 
 carrot, wood betony, dandelion, etc. In many cases the short 
 stem itself is the storage organ and consequently it becomes en- 
 larged and fleshy, as in the Jack-in-the-pulpit, garden crocus, 
 spring beauty, etc. (Fig. 73, A). These short erect stems are 
 termed corms and are suggestive of the tuber. In other plants 
 the bases of leaves attached to the short stems function for the 
 
NATURE OF PLANTS 
 
 II 
 
 storage of food, as in the bulb type of stems of lilies, onions, etc. 
 (Fig. 73, B). 
 
 It is noteworthy that these special types of stems, the runner, 
 rhizome, bulb, etc., are the most common. They represent the 
 more recently evolved forms of stems and because of their ad- 
 vantages they have become the dominant forms. The geological 
 
 Fig. 73. Shortened types of stems: A, conn of jack-in-the-pulpit. At 
 left surface view showing lateral buds, roots and sheathing leaf arising from 
 top of shortened stem. At right sectional view with folded leaf, /, in bud at 
 apex of stem. B, bulb'type of shortened stems. At left bulb of onion showing 
 the ensheathing leaves which are swollen at their bases with food, thus forming 
 the bulb. At right, section of a bulb of hyacinth showing the fleshy loaves 
 attached to the very short stem and in the center of the bulb a flower cluster. 
 
 ancestors of our seed plants were woody and tree like. The 
 uniform climatic conditions of these past periods favored the 
 tree type of stem with its admirable devices for loaf display. 
 With the appearance of climatic changes upon the earth there 
 resulted modifications in the stem: the medullary rays tended to 
 
114 EVOLUTION OF STEM TYPES 
 
 increase in size and so serve as a storehouse, thus meeting the 
 greater need of reserve material; the cambium also tended to 
 form more and more parenchymatous tissue so that the stem 
 became less woody and more of a storage organ. So you can 
 think of the stem becoming reduced in size and finally prostrate 
 through lack of supporting material. The contact of the stem 
 with the soil exposed it to a variety of new factors which lead to 
 further modifications, some of which have survived as seen in 
 the runner, rhizome, corm as well as in the annual and biennial 
 habit of plants. It is a striking fact that the majority of our 
 primitive plants are woody and tree like (pines, willows and oaks) 
 while the more recently evolved and higher types (daisies, 
 dandelions and golden-rods) are characterized by various forms 
 of these modified stems. 
 
CHAPTER IV 
 
 THE FLOWER, FRUIT AND SEEDLING 
 
 50. The Structure of the Flower. — We now come to the most 
 interesting feature in the nature and life of the plant. All the 
 energy of the plant is directed towards the perpetuation of its 
 kind. Numerous examples have been noted where this is ac- 
 complished by bulbs, rootstocks, buds, etc. Among our common 
 plants, however, new individuals are usually produced from seeds 
 that are formed in the flowers. We will first examine the struc- 
 ture of a flower and see how it is adapted to the performance of 
 this important work. A flower is a highly modified stem of the 
 bud type. This is very noticeable before the flower opens or is 
 in bloom (Fig. 74, ^). A series of leaves usually of a greenish 
 color envelop the delicate parts within and protect them, as in the 
 case of the bud, against drying winds. These green leaves are 
 known as the calyx and each individual leaf is called a sepal. As 
 the bud opens a number of organs are disclosed, particularly 
 noticeable are a set of more delicate and variously colored leaves, 
 the corolla, each leaf of which is called a petal (Fig. 74, B). The 
 corolla is of service to the flower in many ways. Like the calyx 
 it may assist in protecting the other organs. Especialh' is it of 
 importance in guarding them against dews and rains. For this 
 reason the calyx and corolla, collectively called the floral envelop 
 or perianth, in many flowers often close at night or on rainy 
 days or the perianth may be so developed or inclined as to pre- 
 vent the rain from falling into it. It will be seen later in the 
 work that the perianth is of the greatest importance in assisting 
 insects to visit the flowers in such a wa>' that seeds ma>- be pro- 
 duced. Within the perianth are two kinds of peculiar organs, 
 the central flask-like ones are called pistils or, if united, carj^cls 
 and surrounding them are the stamens (Fig. 74, B). These 
 two kinds of organs are collectively known as sporophylls since 
 their special work is to produce certain cells called spores. The 
 
 1 15 
 
Ii6 
 
 NATURE OF THE SPOROPHYLLS 
 
 sporophylls are the important organs of the flower. The calyx 
 and corolla may be lacking but no seed can be formed without 
 the sporophylls. 
 
 51. The Structure of the Sporophylls. — Let us examine the 
 structure of these organs and see how they co-operate in the 
 formation of the seed. The stamens consist of a lobed sack or 
 anther which is usually supported upon a stalk or filament (Fig. 
 
 1^ 
 
 Fig. 74. Flower of the stonecrop, Sedum: A, bud stage of flower — ca, 
 calyx showing three sepals. B, open flower — p, petals of corolla; s, stamens; 
 c, pistils or carpels. C, stamen, consisting of a four-lobed anther supported 
 on delicate stalk or filament. D, pistil or carpel — 0, ovary; s, style which 
 terminates in a small knob or stigma. E, longitudinal section of ovary showing 
 row of ovules attached to wall of ovary. F, cross-section of ovary, ovules in 
 two rows. 
 
 74, C). If a young anther is cut across four cavities, contain- 
 ing minute or dust-like grains, the microspores, will be seen 
 (Fig. 75, A). The microspores, also called pollen spores or 
 grains, are minute cells, provided with a cell wall often variously 
 sculptured and contain a nucleus and protoplasm like an ordinary 
 cell (Fig. 79). At maturity the anthers break open in a manner 
 
NATURE OF PLANTS 117 
 
 that varies in different plants, exposing the microspores to the 
 air (Fig. 75, B). The pistil or carpel is quite different in char- 
 acter. At or near the top is a more or less modified part, the 
 stigma, below which is an elongated stalk, the style, that broadens 
 out into a base or ovary. By longitudinal and transverse 
 sections of a pistil we gain a better idea of its real character 
 (Fig. 74, D-F). The ovary is now seen to contain a cavity 
 from the walls of which one or more rather spherical bodies, 
 the ovules, are developed. The structure of the ovule is very 
 complicated. If a thin section is made through the center of 
 one of them it will be seen to contain usually but one large spore, 
 therefore called the megaspore (Fig. 76, mg). This spore is 
 
 Fig. 75. Structure of the anther: A, diagram of the anther cut across, 
 showing the four cavities, sporangia (sing, sporangium), filled with spores. 
 B, cross-section of a mature anther. The tissue about the spores has broken 
 down, thus forming two cavities and at the left the breaking open of the wall 
 of the anther is shown. 
 
 developed in a mass of tissue, the nucellus, and the whole is 
 surrounded by one or two coats, the integument, which grows 
 up from the base of the ovule but never quite surrounds it, 
 thus leaving a small opening or micropyle (Fig. 76, mi). The 
 megaspore, unlike the microspore, is never set free but is retained 
 permanently in the ovule. 
 
 It is not known how the sporophylls have originated but there 
 are indications that they may have arisen through the modifica- 
 tion of leaves for the purpose of forming spores instead of con- 
 structing foods. It is for this reasion that these organs are 
 called sporophylls, or spore-forming leaves, from phylon, a leaf. 
 Those forming small spores are sometimes called microsporo- 
 phylls instead of stamens and those forming large spores mega- 
 sporophylls instead of pistils or carpels. Very man>- modifica- 
 tions of the sporophylls will be noted later in the work. 
 
Ii8 
 
 GERMINATION OF THE SPORES 
 
 52. The Germination of the Microspore and Megaspore.— The 
 
 question now naturally arises as to the meaning of these peculiar 
 structures. The important part of the sporophyll is the spore. 
 A spore is a cell that is capable of germinating or growing under 
 favorable conditions and thus producing some kind of a new 
 organism, or we may say a new plant. You are always to think 
 of a spore as having this power. In case of the spores under 
 consideration very rudimentary plants are produced that con- 
 sist of onh' a few cells. These minute plants are very important, 
 
 Fig. 76. Fig. 77. 
 
 Fig. 76. Section of the ovule of the Canada lily (greatly magnified): 
 mi, micropyle; i, integuments; n, nucellus, here consisting of a single layer 
 of cells, but it often forms a large mass of cells; mg, megaspore with large 
 nucleus in center. This spore is about to divide;/, stalk or funiculus which 
 attaches the ovule to the wall of the ovary. 
 
 Fig. 77. Germination of the megaspore: A, first division of the nucleus 
 of the megaspore. B, second stage in the germination, four nuclei being 
 formed. C, final stage in the division of the nuclei. 
 
 however, because they contain the male and female cells, or 
 gametes, so called because they unite, thus forming a new spore, 
 the gametospore, quite different from the micro- and mega- 
 spores. So let us not lose sight of the fact that these apparently 
 insignificant micro- and mega-spores germinate and produce 
 minute plants that are quite distinct from the plants that bore 
 
NATURE OF PLANTS 119 
 
 them. We shall first follow the germination of the mcgaspore 
 and note the character of the plant that develops from it. As 
 has been stated this spore is never discharged from the ovule 
 within which its entire growth occurs. The first indications of 
 germination are seen in the enlargement of the spore and the 
 division of its nucleus into two nuclei, which are pushed to either 
 end of the spore owing to the accumulation of water in the 
 central region of the spore (Fig. 77, A). The food for the nour- 
 ishment of the growing megaspore is derived from the cells of the 
 nucellus which are dissolved and absorbed as the megaspore 
 increases in size, and food is also conducted to it from the parent 
 plant by means of the funiculus. Each of the two nuclei at the 
 ends of the spore divides again, thus forming two nuclei at either 
 end. This process is repeated once more so that we now find four 
 nuclei at the ends of the spore which has become somewhat elon- 
 gated (Fig. 77, B, C). The so-called polar nuclei, a nucleus 
 from each end, now move toward the center of the spore and 
 fuse, forming one nucleus. This completes the germination of 
 the megaspore and we see that it has grown into a sac-like plant 
 consisting of seven cells, or nuclei imbedded in a mass of proto- 
 plasm (Fig. 78). The cells of this minute plant differ in char- 
 acter and perform very different duties. For example, the larger 
 cell toward the micropylar end of the plant is the female cell or 
 Qgg cell. It will be called the female gamete because it unites 
 with another cell, the male gamete. The two small cells near tlie 
 female gamete are the synergidae or cells that assist in the 
 nourishment and later development of the female gamete. The 
 two cells fusing in the center of the plant form the endosperm 
 nucleus which later by repeated divisions produces a mass of 
 nourishing cells that fill the space within the sac. The throe 
 cells at the bottom of the sac, known as the antipodal cells, may 
 increase in number and take part in the later growth, but as a 
 rule they are disorganized and used for food. They are often 
 provided with cell walls, the other cells consisting only of a 
 nucleus and a varying amount of cytoplasm. This plant formed 
 by the germination and growth of the megaspore is called the 
 female gametophyte, because it contains the female gamete, and 
 
120 GERMINATION OF THE MICROSPORE. 
 
 you will see that a corresponding plant appears in the ferns and 
 other groups of plants. 
 
 Returning now to the microspore, we find that its germination 
 also often begins while in its sporangium but in other cases 
 growth begins after it has escaped from the sporangium and has 
 
 Fig. 78. The germination of the megaspore complete. The plant thus 
 formed consists of a female gamete or egg cell, 9 , below which are two syner- 
 gidae and in the center are the two uniting polar nuclei, p, while at the opposite 
 end of the sac are three antipodal cells, a; mi, micropyle; i, integuments; /, 
 stalk or funiculus in which a vascular bundle, v, has been developed to trans- 
 port food from the plant to the ovule. 
 
 been carried by the wind or some insect visiting the flower for 
 food, to the stigma of the pistil. The stigma is admirably 
 adapted to hold the microspores, being provided with minute out- 
 growths, and frequently sugary solutions are exuded which fasten 
 the spores to the stigma. The real importance of these sugary 
 
NATURE OF PLANTS 121 
 
 solutions is to nourish the microspores. It is cvick-iii that tliese 
 minute dust-Hke cells can not contain sufficient f(xxl to bring 
 about any considerable growth. It is noteworthy tliat the 
 strength and nature of the sugar solutions varies in different 
 stigmas and that the microspores are often able to grow only 
 in solutions of such strength or in the presence of such substances 
 as are found on the stigmas of their own kind of plant. So it 
 results that the microspores are often unable to grow when 
 carried to the stigmas of a different kind of plant from them- 
 selves. The germination of the microspore results in the division 
 of its nucleus and the formation of a large and a small cell known 
 as the tube and antheridial cell respectively (Fig. 79, A, B). 
 The latter cell divides once, forming two cells, called the male 
 gametes (Fig. 79, C). This stage in the germination of the 
 microspore is reached either in its sporangium or after being 
 transferred to the stigma. Nourished by the foods of the stigma 
 the microspore continues its growth, the tube cell ruptures 
 the outer wall of the spore and forms a tube-like growth. This 
 tube cell grows down between the cells of the style into the 
 cavity of the ovary where it usually curves out and enters the 
 ovule by way of the micropyle. It now works its wa\- throut;h 
 the tissues of the nucellus to the female gametophyte, into the 
 cavity of which it enters by dissolving the wall. In the mean- 
 time the two male gametes have been carried down by the 
 streaming movement of the cytoplasm to the end of the tube 
 cell as it enters the female gametophyte (Fig. 79, D). The 
 question will naturally be asked, what directs the peculiar growth 
 of this minute plant? In the first place the tube cell is repelled 
 by the oxygen of the atmosphere, so as soon as it appears, it is 
 directed away from the atmosphere into the stigma. Its course 
 down the style to the ovary is very largely controlled !)>■ lines of 
 loose tissue through which it can easily work its way and also 
 by foods which are deposited in the cells just ahead of the tube 
 cell so that it is led along rather straight pathways to the ovary. 
 The curving out of the tube to the micropyle and its subsequent 
 growth down into the female gametophyte can only be accounted 
 for by some chemical stimulus that is located in the ovule and 
 
122 FECUNDATION OR FERTILIZATION 
 
 female gametophyte. A similar attraction brings about a move- 
 ment of the cytoplasm which carries the male gametes to the 
 end of the tube cell. It will be seen that this peculiar plant 
 (Fig. 79, D) consisting of a long tubular growth with three 
 naked cells is more rudimentary than the plant derived from the 
 megaspore. Because this plant produces the male cells or male 
 
 I 
 
 Fig. 79. Germination of the microspore: A, mature microspore. B, first 
 stage in its germination — t, tube cell; a, antheridial cell. C, end of cell divi- 
 sion — t, tube cell; a, antheridial cell forming two male gametes. D, diagram 
 showing the formation of the tube which grows through the style and finally 
 reaches the female gametophyte. The two male gametes, g, are shown passing 
 down the tube; t, tube nucleus. 
 
 gametes we call it the male gametophyte. So we see that the 
 flower forms two kinds of spores and that these germinate and 
 form two kinds of plants, a male and a female. 
 
 53. Fecundation or Fertilization.— As soon as the develop- 
 ment of these two plants is completed a very remarkable change 
 takes place which starts an entirely new growth that is quite 
 independent of the development of the male and female gameto- 
 phyte. The end of the tube cell becomes distended through the 
 accumulation of material and finally ruptures, discharging the 
 male gametes into the female gametophyte. The chemical com- 
 position of the female gamete is such that one of the male cells 
 is attracted to it (Fig. 80) and finally the two gametes unite 
 forming a single cell. This body is a sexually formed spore or 
 gametospore and like the ordinary spore has the power to germi- 
 
NATURE OF PLANTS 
 
 123 
 
 nate and produce a plant. The microspores and mcgaspores, 
 however, were not formed by the union of sexual cells or gametes. 
 Therefore we may distinguish them as asexual spores or simply 
 spores. It has frequently been observed that the endosperm 
 nucleus also attracts the other male gamete and causes a similar 
 fusion with it (Fig. 80). The union of the male anrl female 
 
 Fig. 80. The micropylar end of an ovule of Canada lily (sectional view), 
 showing the process of fertilization or fecundation. The tube, /, has grown 
 into the female gametophyte and ruptured, discharging the two male gametes. 
 One, cf , is seen fusing with the female gamete, 9 , and the other one, c", is 
 uniting with the two polar cells, thus forming the endosperm nucleus; s, one 
 of the synergids; i, integuments. 
 
 gametes is called fertilization or fecundation. Unless fertilization 
 is effected the growth outlined above usually ends the history of 
 the flower. But if fecundation is effected then a gametosiK)re 
 is formed that is capable of germinating and producing a plant 
 whose growth and development are attained with most remark- 
 able changes of the ovule and often of the surrounding parts. 
 54. The Germination of the Gametospore and the Formation 
 of Seed. — The first indication of the germination and growth of 
 the gametospore is seen directly after fertilization. It becomes 
 
124 GERMINATION OF THE GAMETOSPORE 
 
 surrounded by a cell wall and attached to one side of the female 
 gametophyte, which is henceforth called the embryo sac (Fig. 
 8i, ^). By a series of divisions the gametospore now forms a 
 
 Fig, 8i. Germination of the gametospore of peppergrass, Lepidium: A: 
 micropylar end of the embryo sac showing that the gametospore, g (fertilized 
 female gamete), has developed a cell wall and become attached to the wall 
 of the embryo sac. Compare Fig. 80. B, later stage in the germination — e, 
 embryo cell; 5, suspensory cells; en, endosperm cells. C, the embryo cell 
 has formed two cells. D, later stage. The embryo cell by further division 
 has formed a spherical mass of cells, here shown in section. Note the appear- 
 ance of an outer layer of cells, the epidermis, and a central or stem region, 
 s, a few of the suspensory cells. E, still later growth. Two growing regions, 
 the cotyledons are appearing on the side of the stem. F, the micropylar end of 
 the embryo sac, showing the stage of development where the parts of a small 
 plant can be clearly recognized — c, cotyledons; st, stem ending in root, r, 
 to which is still attached the supensory cells; en, endosperm cells, now pro- 
 vided with walls, are being absorbed as the plant or embryo enlarges. Note 
 that the embryo and embryo sac are slightly bent to the left. This curvature 
 is due to the fact that [the ovule in a great many plants becomes curved in 
 its development with the result that the embryo sac assumes a curved or 
 U-shaped form. This form of the ovule results in a complete bending over 
 of the cotyledons against the stem, as shown in the next figure. 
 
NATURE OF PLANTS 125 
 
 number of more or less elongated cells, the so-called suspcnsor, 
 and a terminal cell or embryo cell (Fig. 81, B, V). In the 
 meantime the endosperm cell has divided again and again and 
 the resulting naked nuclei have become arranged around the 
 sides of the embryo sac (Fig. 81, B, en). This formation of 
 endosperm cells continues until they more or less fill the embryo 
 sac and they also become surrounded with cell walls. These 
 endosperm cells are filled with food and serve to nourish the 
 germinating gametospore just as the nucellus nourished the 
 female gametophyte. While the suspensory cells usually soon 
 cease to grow, the embryo cell divides very actively, as shown 
 in Fig. 81, C~E, and soon there is a clear indication of a minute 
 plant with root, stem and leaves (Fig. 81, F). This minute 
 plant developed by the germination of the gametospore, is called 
 the embryo. In the case of the plant under consideration the 
 embryo, when fully formed consists of a stem with two laterally 
 placed leaves, the cotyledons, and a root. The region of the 
 stem above the attachment of the cotyledons is known as the 
 plumule and frequently assumes the form of a minute bud. The 
 region of the stem below the cotyledons is termed the hypocotyl 
 and the root and root cap appear at its lower end (Fig. 82, A). 
 The ovule containing the mature embryo is called a seed. The 
 interesting feature about the seed is the fact that it is a structure 
 in which growth has ceased and that it is capable, owing to a 
 remarkable series of devices that will be noted directly, of re- 
 maining in this condition, i. e., dormant often for one or more 
 years. 
 
 Great variation characterizes the germination of the gameto- 
 spore in the different groups of plants, but the de^•olopmcnt out- 
 lined above is fairly characteristic of those plants that form an 
 embryo with two cotyledons. Such plants are called for this 
 reason dicotyledons. Other plants develop an embryo with but 
 one cotyledon, as our grasses, lilies, etc., and for this reason they 
 are called monocotyledons. Further variations will be seen in 
 cone-bearing trees, ferns, etc. 
 
 It must be borne in mind that the formation ot the seed is 
 also attended with profound changes in the stnuture of the 
 
126 
 
 STRUCTURE OF THE SEED 
 
 ovule, and often in the pistil and surrounding parts. Frequently 
 the endosperm grows so extensively as to absorb and replace the 
 cells of the nucellus and thus comes to occupy all the space within 
 the coats of the integument, as in the castor bean, morning glory, 
 onion, etc. (Fig. 85, B). The embryo mayjemain comparatively 
 
 B 
 
 Fig. 82. Structure of the seed: A, section of an ovule of peppergrass in 
 which the growth of the embryo is nearly complete — -/, stalk or funiculus 
 attaching ovule to wall of ovary; mi, micropyle; i, integument; en, remains 
 of endosperm. The embryo consists of a stem which is differentiated into 
 a hypocotyl, hy, that ends below in a root, r, and root cap, and above in 
 the plumule, pi, and two cotyledons, c, which curve over and lie one upon the 
 other. V, vascular bundles which extend up through the stem into the coty- 
 ledons where they subdivide, forming a network of veins. B, section of seed 
 of water lily, after Conard — e, embryo surrounded by a layer of endosperm 
 cells; mg, cells of the nucellus; i, integument. 
 
 small as in the cases just cited or it may in turn absorb and 
 replace all the cells of the endosperm and so come to occupy 
 the space within the integument, as in the bean, pea, etc. (Figs. 
 82, A; 83, D). Various other arrangements may result; as for 
 example, there may be remains of the nucellus and endosperm 
 with the embryo (Fig. 82, B). The integument also undergoes 
 various changes during these growths, often becoming hard or pa- 
 
NATURE OF PLANTS 127 
 
 pery or provided with hairs, hooks, spines, and smooth or pitted. 
 Compare the bean, catalpa, cotton, milkweed, etc. Attention 
 must also be directed to the growths that occur outside of the 
 seed that are induced by fertiHzation and the germination of the 
 gametospore. Very frequently the pistil becomes tough and 
 hard and invests the seed like a second integument, as in wheat, 
 corn, acorn, etc.; again it forms a sac al)out the seed as in the 
 sedges and buttercups; or it may enlarge, forming the pods of 
 peas and beans or the capsules of lilies. In other cases it be- 
 comes fleshy throughout as in the tomato and currant, or only 
 the outer walls are fleshy while the inner part forms the pit or 
 stone as in the peach and plum. Frequently wings and hooks 
 grow out from the pistil as in the mapel, Ailanthus, etc. Even 
 the end of the stem, the receptacle, which bears the various 
 organs of the flower, may be induced to grow, as in the apple, 
 where it surrounds the pistils (the core) with a fleshy coat, or 
 the receptacle may enlarge, as in the strawberry, forming the 
 fleshy part of the fruit. In other cases outgrowths below the 
 flower develop, forming the burr of the chestnut, clotbur, beech- 
 nut, and the cup of the acorn. The entire growth that is asso- 
 ciated with the formation of the seed is termed the fruit and we 
 see that it may include a variety of modified organs. The term 
 seed, however, refers only to the modified ovule and its embr\o. 
 Later in the work it will be noted that these growths are often 
 devices to bring about a distribution of the seed or to protect it 
 during its dormant period. 
 
 55. The Seed and its Growth.— Spores of all kinds germinate 
 and produce new plants. Seeds do not germinate. Tliey simply 
 continue the growth of the embryo or plant which h.is been tem- 
 porarily stopped. The falsely called germination of the seed is 
 but the awakening or renewal of the growth of this plant. The 
 seed is essentially a young plant that is supplied with a certain 
 amount of food and that has temporarily stopped growing. The 
 seed may be looked upon as one of the many adaptations found 
 among plants to ensure their distribution and perj^et nation. 
 From a seed a single plant is developed that may produce many, 
 often thousands of seeds, i. e., plantlets. We haw .iln -.kK- noted 
 
128 STRUCTURE OF THE SEED 
 
 that these may be widely distributed and so spread and multiply 
 the number of plants. Some seeds are so fine that they float away 
 as a dust, as in the orchids. Other seeds and fruits are provided 
 with hairs and wings that serve to buoy them up or promote 
 their transport by winds, as in the case of pine seeds, milk- 
 weed, willow, and the fruits of maple, dandelion, etc. In other 
 cases spines and hooks or mucilage glands are developed that 
 attach them to any passing animal and so bring about a con- 
 siderable distribution. Note the common devices of this nature, 
 as in the burdock, stick tight, beggar lice, agrimony or the glands 
 on the nightshade (Circaea) and twin flower. So also seeds 
 are scattered by the spring of elastic stems and explosive fruits, 
 as in many lilies, witch hazel, violet, touch-me-not, etc. But 
 especially may seeds be looked upon as an adaptation to tide 
 the plant over seasons unfavorable for growth. In this respect 
 they may be compared to buds, which they also resemble in their 
 renewal of growth when conditions are again favorable. There 
 are some noteworthy exceptions to this statement, as in the man- 
 grove, certain oaks, and many grasses. The seeds of the former 
 plant begin their growth while still attached to the tree and a 
 similar growth has also been observed in the acorns of the 
 white oak. Grain often sprouts while in the sheaves and the 
 seeds from green tomatoes produce earlier and larger fruits. 
 However, the majority of seeds and fruits, as well as buds, do 
 not renew their growth immediately after their formation. A 
 longer or shorter resting or dormant period is required during 
 which time chemical changes occur that render the foods soluble 
 and therefore available to the young plant or embryo. For ex- 
 ample, in many seeds and buds the storage foods are in the form 
 of starch. This food is insoluble and can not be used by the 
 plant until it has been acted upon by an enzyme which changes 
 it into a soluble and readily diffusible sugar. These enzymes are 
 slowly formed during the resting period and a renewal of growth 
 is not possible until they have begun the transformation of the 
 insoluble foods into soluble forms. The changes thus effected 
 are so considerable that growing tubers, root stocks and seeds 
 often have a sweet taste owing to the considerable amount of 
 
NATURE OF PLANTS 129 
 
 sugar present. So the seed may be looked upon as a variation 
 in the growth of the plant that enables it to multiply and dis- 
 tribute its kind and to meet the various problems in connection 
 with its existence, such as drought and sometimes even fires. 
 
 56. Conditions Necessary for Growth. — The external condi- 
 tions necessary for starting the growth of the embryo are a 
 suitable temperature, moisture, and the oxygen of the atmos- 
 phere. Light only in rare instances appears to be a contributing 
 factor in the renewal of growth. Some seeds will sprout at near 
 the freezing point, while others require a temperature between 
 10° and 17° C. It is interesting to note that the rising tempera- 
 ture together with other factors as the season advances from 
 early spring bring out a regular succession in the awakening of 
 various kinds of seeds. A suitable amount of moisture must 
 also be supplied to the seed to assist in the solution and dilTusion 
 of the foods to the embryo. This is very necessary since the 
 seed contains very little water. In fact the seed owes its vitality 
 and power to resist long drought and severe cold to its dry con- 
 dition. Seeds soaked in water cannot endure such extremes as in 
 a dry state. Excess of water, however, is unfavorable to the 
 renewal of growth, because it excludes the oxygen of the air. 
 This is the cause of the failure to grow and of the decay of the 
 seeds planted in the spring when the season is rainy. The air 
 spaces in the soil become filled with water and as a consequence 
 oxygen can not gain access to the seed and co-operate in those 
 chemical changes that not only supply some of the foods to the 
 embryo but also provide it with the energy for growth. An 
 examination of a few seeds will show some of the more important 
 structural features that adapt them to the conditions that they 
 have to meet. 
 
 In the case of the bean the integument is tough and wouUl 
 appear to offer a rather effectual barrier to the entrance of water 
 and air. You have noted that a minute opening, the micro- 
 pyle, exists in the integument. This opening is seen near the 
 scar or hilum that marks the point where the seed was attached 
 by a minute stem to the walls of the ovary (Fig. 83, B). This 
 opening permits the entrance of the air and water as does the 
 9 
 
I30 
 
 NATURE OF THE SEED 
 
 looser structure of the hilum. This can easily be demonstrated 
 by placing beans in water, when the spread of the water from the 
 region of the micropyle will be indicated by the swelling and 
 wrinkling of the integument (Fig. 83, C). After a time the 
 entire integument becomes quite soft and water is readily drawn 
 through it owing to the fact that the storage foods in the embryo 
 are in close contact with it and draw the water through by 
 Carefully removing the integument from a soaked bean 
 
 osmosis. 
 
 
 ^^^ Cs^ 
 
 Fig. 83. Fruit and seed of the bean: A, mature fruit or pod: 5, style; 
 c, remains of calyx; 0, ovary. At left the pod has been opened, showing four 
 seeds attached to the walls of the ovary by funiculus. B, two views of a 
 seed — h, hilum or scar marking attachment of funiculus to seed; w, micropyle. 
 C, wrinkling of seed coat caused by the entrance of water through the micro- 
 pyle and hilum. D, embryo of seed after removal of integument — c, coty- 
 ledons. At right one of the cotyledons has been removed to show the plumule, 
 ep, and the hypocotyl, hy. E, half of the membranous integument. 
 
 we note that the embryo is well developed, consisting of two 
 fleshy cotyledons and a short stem (Fig. 83, D, E). The plumule 
 is well developed, bearing two leaves, and the short hypocotyl 
 and root are enclosed in a sheath formed from the integument. 
 Note that the hypocotyl and root are situated near the micropyle. 
 How does this arrangement work to the advantage of the em- 
 bryo? Allow the seed to germinate and it will be noted that the 
 absorption of water first causes the embryo to expand and rupture 
 the integument. This is immediately followed by the growth and 
 elongation of the lower part of the hypocotyl which results in the 
 pushing out of the root (Fig. 84, A). Thus it happens that the 
 very part of the embryo that shows the first signs of growth is in 
 
NATURE OF PLANTS 
 
 131 
 
 close proximity to the water as it enters the seed and doubtless 
 the sheath also assists in retaining water in this region. We are 
 already familiar with the properties of the root which direct its 
 growth in such a way that no matter how the seed may be placed 
 the root will ultimately grow into the soil and toward water and 
 the soil foods, page 61. It is noteworthy that many seeds are so 
 fashioned that they naturally assume such a position on the 
 ground as to bring the root as it emerges into direct contact with 
 the soil. Such an arrangement is equally advantageous for the 
 absorption of water. How many seeds can you find that will 
 naturally lie upon the soil in such a way as to cause the root on 
 
 Fig. 84. Renewal of growth of the bean seed: A, basal region of h\ ptxrotyl 
 elongating and pushing the root into the soil. B, upper region of the hypo- 
 cotyl elongating, lifting the cotyledons and epicotyl above the soil. C, hypo- 
 cotyl erect and epicotyl expanding, forming the first normal leaves. 
 
 emerging to be directed away from the earth? Thus at the \i r> 
 start the growth of the seed is of such a character that the young 
 plant is anchored to the soil by the root and brought into projx^r 
 relation with the water and other materials necessary for the 
 construction of foods. As soon as this work is well under way 
 the upper end of the hypocotyl begins to elongate, pushing up 
 into the air cotyledons and i^dunuile (Fig. 84, B). Here we 
 
132 GROWTH OF THE SEED 
 
 see that the upper end of the hypocotyl reacts to gravity like a 
 stem and consequently grows away from the earth. This growth 
 is attended with a curving and constant nutation of the hypo- 
 cotyl which enables it to work its way through the ground to 
 better advantage than would be the case if it were pushed pas- 
 sively up. Danvin compares this work of the seedling in emerg- 
 ing from the soil to the struggles of a man who is prostrate and 
 attempting to arise with a heavy load on his back. The ability 
 of the hypocotyl to overcome obstacles in its way and crowd 
 through small openings is due to the power generated in the 
 enlarging cells. In this way a force is often created that may 
 exceed 80 pounds to the square inch, over five times the atmos- 
 pheric pressure. In many seeds only the region above the coty- 
 ledons elongates, leaving the cotyledons in the ground, as in the 
 pea, acorns, walnuts, etc. The elongating region is called the 
 epicotyl. This behavior is doubtless associated with the storage 
 of food. In some seeds the cotyledons serve only as storage or- 
 gans and in such cases the position of advantage is in the soil 
 where they are better protected and in contact with moisture 
 which will assist in the solution and diffusion of the foods. In 
 many cases the cotyledons perform a dual function, first as storage 
 organs and later expanding and developing into the first green 
 leaves of the plant, as in the squash and many of our common 
 plants. In any of these cases it is evident that the young plant 
 requires a certain amount of food to enable it to get established 
 in the soil. It is not until considerable growth has taken place 
 that it develops its first green leaves and so begins to be self 
 supporting. In the case of the bean and in many other seeds and 
 fruits this need is amply met by the abundance of food that is 
 stored in the fleshy cotyledons (Fig. 84, C). These organs re- 
 main attached to the plant even after it is well established, and it 
 is only after the food has been withdrawn that the shriveled 
 cotyledons drop off. As the young plant becomes established 
 the plumule unfolds like a bud and develops the stem and leaves 
 and finally the flowers and fruit with which you are familiar. 
 
 The castor bean shows quite a variation from the structure 
 of the bean. The hilum and micropyle are more or less con- 
 
NATURE OF PLANTS 
 
 133 
 
 cealed by a fleshy outgrowth, the caruncle (Fig. 85, .4). Remov- 
 ing the hard integument we find within a while, oily mass of 
 cells which are attached to the horn-like coat only at the hilum. 
 Cutting across this mass of cells it will be seen that two thin 
 leaves are imbedded in the center. Taking two other seeds and 
 cutting them longitudinally, one at right angles and the other par- 
 allel to the leaves we see that the embryo consists of a very small 
 stem and root and two delicate leaves imbedded in an oily tissue, 
 the endosperm (Fig. 85, B). In the bean the seed consisted of 
 
 Fig. 85. Seed and seedling of the castor bean: A, the seed with fleshy 
 outgrowth, the caruncle, c, at base. B, section of seed. At right showing 
 the two cotyledons, c, and short hypocotyl in center of the endosperm, en. 
 At left the seed is split parallel to the surface of the delicate white cotyledon. 
 C, early stage in the growth of the seedling. The hard intei^unicnt is being 
 thrown off but the cotyledons are still entirely buried in the endosix-rm. D, 
 later stage. The cotyledons, c, still absorbing food from the endosptTm. en, 
 are beginning to appear and to develop chlorophyll. E, the cotyledons and 
 plumule are expanding, the endosperm being entirely abs..rl.<il. 
 
 an embryo and integument. In the castor bean, uw ciulosix^rm 
 forms the bulk of the seed, since it has been only partially con- 
 sumed by the embryo. It would naturally be inferred that the 
 hard coat would prevent the access of water and gases. Observe, 
 
134 GROWTH OF THE CASTOR BEAN 
 
 however, that the caruncle readily absorbs water and that the 
 embryo and endosperm are in direct contact with this region 
 of the seed. Many devices will be observed among seeds serv- 
 ing to convey water to the growing parts. Note the mucilag- 
 inous coats on flax, quince, many mustard seeds, etc., or the 
 spongy rinds of the fruits of the walnut, cocoanut, etc. It is 
 well worth your time to submerge in water colored a deep red 
 with eosin widely different seeds and examine them from time 
 to time, noting the period required for the penetration of water 
 and the lines along which the water enters and is distributed. 
 It will be found that many seeds and fruits exclude water and 
 oxygen very effectively for one or more years with the result 
 that the renewal of growth is delayed until the decay of the 
 parts permits their entrance. The breaking of the integument 
 would appear as a serious obstacle to the growth of the embryo. 
 However, the power generated by the cells owing to their ex- 
 pansion through the absorption of water is quite sufficient to rup- 
 ture the seed coat. The early growth of the embryo of the castor 
 bean is similar to that of the bean. The elongation of the lower 
 end of the hypocotyl pushes the root into the soil, while the 
 upper end of the hypocotyl, arching and twisting as it elongates, 
 works the cotyledons and endosperm through the soil into the 
 air (Fig. 85, C). In this seed the food is without the cotyle- 
 dons. Consequently, although the cotyledons gradually enlarge 
 and become green, they remain in contact with the endosperm 
 until all its food is absorbed and but a papery skin remains 
 (Fig. 85, D). 
 
 A grain of corn will illustrate another modification of the 
 seed. In this case the pistil remains in such close contact with 
 the seed that it appears to form an additional integument. Such 
 a fruit is called a grain and may be compared to a bean pod with 
 a single seed. We can still see upon the grain (Fig. 86, A) 
 at 5 the position of the style while at p appears the remains of 
 the stem that fastened the sporophyll to the cob. The long 
 threads, often called the silk, that project from the ears of corn 
 are greatly enlongated stigmas for catching the microspores. 
 While the outer parts of the grain of corn are very hard, if it 
 
NATURE OF PLANTS 
 
 35 
 
 is treated with the eosin solution referred to above it will be 
 seen that at first the water enters by way of the little stalk just 
 as did the food that was supplied to the growing grain. In 
 this way water is supplied directly to the root of the embryo 
 which lies near the stalk as shown in Fig. 86, B. If two seeds 
 are soaked until soft and then sectioned so as to cut one across 
 and the other longitudinally througli the embryo it will be ^rcn 
 
 Fig. 86. Grain and seedling of corn: A, two views of a grain — s, point of 
 attachment of style; p, stalk attaching the grain to cob; emb, location of 
 embryo. B, longitudinal section of a grain. The embryo consists of a 
 plumule, pi, root, r, and scutellum, sc. eji, endosperm. C, cross-section of 
 a grain showing endosperm, en, surrounding the embryo, emb. D, diagram 
 of the outer cells of the scutellum, showing the elongated cells that bring 
 about a solution and absorption of the foods stored in the endosperm. K-G, 
 stages in the growth of the seedling. 
 
 that the endosperm forms the larger part of the grain and mai 
 the embryo is relatively small, consisting of a stem and root, a 
 plumule of several closely wound leaves, and a shiekl-like organ, 
 the scutellum (Fig. 86, B, C). The entire embryo may easily be 
 removed from the endosperm with a knife if the seed is thoroughly 
 soaked. Unlike the two preceding examples the corn is a mono- 
 cotyledon and many regard the scutellum as a modified coty- 
 ledon. This organ forms the enzymes which put into soluble 
 
136 ALTERNATION OF GENERATIONS 
 
 form the food in the endosperm and it also functions as an 
 absorbing organ of the embryo (Fig. 86, D). Similar devices 
 for absorbing the storage foods and transferring them to the 
 embryo are seen in the wheat, onion, date, and cocoanut. The 
 early growth of the embryo goes on as in the preceding examples. 
 The root first emerges and anchors the grain in the soil. The 
 plumxule of closely rolled leaves forms a sharp bodkin that reaches 
 up through the soil with ease. This modification of the plumule 
 would appear to be of a decided advantage and you will notice 
 that this method of emerging from the soil is followed by a great 
 many plants with underground stems; see the large leaves of the 
 skunk cabbage, may apple, Solomon's seal, etc. The rest of 
 the grain remains in the ground as in the case of the pea. 
 
 57. The Two Phases in the Life of a Plant. — As has been 
 stated the plumule unfolds in all these cases like a bud and 
 gradually develops into the mature plant. This may be effected 
 in one year or several years may be required to complete the 
 work. At some time in this growth flowers will appear bearing 
 sporophylls and spores and thus we arrive again at the starting 
 point in the life history of a plant. There are two distinct phases 
 or generations in the life of a plant. The formation of the 
 megaspores and microspores marks the beginning of the sexual 
 generation or gametophyte which ends when the male and female 
 gametes have been developed. The formation of the gameto- 
 spore marks the beginning of the asexual generation or sporo- 
 phyte which ends in the sporophylls when those cells, called the 
 spore mother cells, appear which produce the micro- and mega- 
 spores. These two generations are also sharply distinguished 
 from each other by the number of chromosomes, page 56, which 
 their cells contain. The cells of the gametophyte generation 
 contain only one-half the number of chromosomes found in the 
 cells of the sporophyte generation. If the nuclei of male and 
 female gametes contain twelve chromosomes (the number varies 
 in different plants) the gametospore which marks the beginning 
 of he atsexual generation, will contain twenty-four chromosomes. 
 This number will be found in all the cells of the embryo and 
 continue to characterize the subsequent development of the sporo- 
 
NATURE OF PLANTS 137 
 
 phyte. When, however, the spore mother cells appear in the 
 anthers and ovules it is to be noted that these cells in dividing 
 to form the micro- and mega-spores reduce the number of 
 chromosomes by one-half so that the nuclei of these spores and of 
 all cells derived from them contain but twelve chromosomes. 
 For this reason the division of the spore mother cell is referred 
 to as the reducing division. The significance of this difference 
 in the number of chromosomes is not known but it furnishes a 
 basis for the sharp separation of the two generations. 
 
 A sexual and an asexual phase or generation characterize 
 the life history of nearly all the higher plants and the succes- 
 sion of these two phases is called the alternation of generation. 
 In the case of the bean we have as the most important features 
 of the sporophyte or asexual generation, (i) the gametospore, 
 (2) the seed, and (3) the flowering bean plant. The micro- 
 scopic plants that comprise the gametoph\'te or sexual genera- 
 tion are characterized by (i) the micro- and mega-spores which 
 develop respectively into (2) a tubular growth of three cells 
 and (3) a sac-like growth of seven cells, each of which contains 
 one or more (4) sexual cells or gametes. These two phases are 
 called respectively the sporophyte or asexual generation, and the 
 gametophyte or sexual generation, since in the first phase mother 
 cells are developed which form spores, i. e., this is the spore 
 forming or asexual generation and in the second phase gametes 
 or sexual cells are produced. 
 
 58. Significance of Fertilization. — Let us now stop to con- 
 sider the meaning of the complicated process that we have termed 
 fertilization. No satisfying explanation has been offered. Some 
 see in these changes only a process of nutrition and chemical 
 stimulation. The gametes are lacking in certain materials that 
 are essential to their further growth. According to this view 
 the male gamete is the complement of the female and by the 
 union of the two all the substances are supplied that arc neces- 
 sary for growth. 
 
 It has also been suggested that the growth culminating in 
 fertilization is attended with the removal of impurities from the 
 sexual cells. Every cell has its growth, maturity, and senescence 
 
138 PRINCIPLES OF HEREDITY 
 
 when it loses its power for further growth and division. The 
 reduction division and the subsequent formation of the gametes 
 results in the removal of substances that prevent their continued 
 activity and as a consequence they are restored to a youthful 
 condition which appears in the growth of the gametospore. 
 
 Much attention has been directed in recent years to the process 
 of fertilization as a method of controlling the character of off- 
 spring. You have already noted, page 56, that granular bodies 
 or chromosomes are constant features of every nucleus. These 
 complex bodies determine the type of plant that shall be de- 
 veloped. In other words the chromosomes contain the hereditary 
 substances, termed factors, that cause each plant to develop in a 
 certain definite way or to follow a certain pattern in its growth. 
 The chromosomes derived from the parent plants cause the off- 
 spring to resemble them in growth. The micro- and mega-spores 
 are formed in the sporophylls, and these spores must therefore 
 contain the same kind of chromosomes or factors as the plants 
 bearing the sporophylls, i. e., the parent plant, since they have 
 been derived directly from the parent plant by cell division. 
 Therefore gametes derived from the germinating spores must 
 also contain the same factors as the parent plants. In fertiliza- 
 tion we have the fusion of the gametes. This means that the 
 hereditary substances in the gametospore wull be of a dual 
 character since it has been derived from the male and female 
 gametes (Fig. 80). Consequently there is now lodged in one 
 body, the gametospore, all the possibilities of growth that each 
 gamete inherited from the parent plant. 
 
 Let us now examine some of the evidence indicating that the 
 hereditary substances or factors are distributed in a very definite 
 way from parent to offspring and that these factors are associ- 
 ated with the chromosomes. In the common garden plant, four- 
 o'clock, we find some plants bearing only w^hite flowers ; others 
 only red flowers. Here we have evidence of two factors; the one 
 causing the white color, the other causing the red color. If these 
 two plants be crossed the resulting offspring is pink or inter- 
 mediate between the two parents. This offspring is a hybrid, 
 meaning thereby an individual derived from parents that differ 
 
 I 
 
NATURE OF PLANTS 
 
 I .V > 
 
 from one another in one or more factors. If now one of ilie 
 hybrid four-o'clocks is crossed with the pure wliite, one half of 
 the resulting offspring will be pure white and one half pink 
 like the original hybrid. The reason for this variation in the 
 offspring will be apparent by studying the diagram, Fig. SO A. 
 Here we see that each parent {RR and WW) produced gametes 
 having the same color factor as itself and when two .i^amctes 
 
 /^e^ 
 
 T^cirerffs 
 
 (W W J 
 
 r)o^/>(vv) 
 
 \^ 7 ^r^^W- w w) 
 
 Fig. 86^1. Diagram showing distribution of the fiictors for red \R) and 
 for white ( PF) in the cross of the red and white four-o'clock; and the subsequent 
 cross of the pink hybrid with the white form. 
 
 derived from these parents unite the resulting hybrid (AMO 
 must contain the color factors of both parents. Note, howc\er, 
 that the hybrid produces gametes like those of the parents, u c, 
 gametes with the red factor or with the white factor. This is 
 due to the fact that in the formation of the gametes there is a 
 separating out of the factors so that the gametes are never 
 hybrid. They only contain the factors of one or the other of 
 the original parents. The lower part of the diagram show< ' 
 
I40 TRANSMISSION OF FACTORS 
 
 the results of crossing the pink hybrid (RW) with the pure 
 white (WW) come about. The white form can only produce 
 gametes with the white factor but we have seen that the hybrid 
 produces gametes, one half of which contain the white factor and 
 one half the red factor. Therefore in the cross between the pink 
 hybrid and the pure white there are only two possible combina- 
 tions of gametes. A gamete from the hybrid bearing the white 
 factor may fuse with a gamete from the white four o'clock, giving 
 a pure white offspring or a gamete from the hybrid bearing the 
 red factor may fuse with a gamete from the white four o'clock, 
 giving a pink hybrid. 
 
 It is now known that the hereditary substances or factors are 
 associated with the chromosomes and that they are separated 
 in a very definite way during the cell divisions that result in the 
 formation of the gametes. These chromosomes are associated 
 in pairs and in the division of the cell one member of a pair goes 
 to one daughter cell and the other member of this pair goes to 
 the other daughter cell. If now the factor for red is in one member 
 of a pair of chromosomes and the factor for white is in the other 
 member of this pair, it must follow in the above cell division that 
 one daughter cell w^Ill contain the red factor and the other 
 daughter cell will contain the white factor. Therefore the 
 gametes derived from these two daughter cells must contain 
 respectively the red factor and the white factor. This distri- 
 bution of the factors will be more manifest by examining the 
 accompanying diagram, Fig. S6 B. In figure i is shown a 
 nucleus with three pairs of chromosomes, one pair very small, 
 another somewhat larger and a third pair in which the factors for 
 red and white are indicated by the letters R and W. Of course 
 it is not to be supposed that any factor is visible to the eye. In 
 the second figure the two members of each pair of chromosomes 
 is seen drawing together preparatory to cell division. In the 
 third figure cell division is under w^ay and the chromosomes still 
 in pairs have been drawn to the center of the spindle, and in the 
 fourth figure we see a member of each pair being pulled to the 
 opposite pole of the spindle, where they will be organized into 
 two daughter nuclei. The later appearance of these two daughter 
 
NATURE OF PLANTS 
 
 141 
 
 nuclei is shown at the right and left of figure four. In tins way 
 the red factor comes to be lodged in one of the daughter cells 
 and the white factor in the other daughter cell. This separation 
 of the factors that appear in the two members of a pair of chromo- 
 somes is termed the segregation of the factors. It is one of the 
 most important features in the cell divisions that result in the 
 
 Fig. 865. Diagram of a nucleus showing mode of division in gamete 
 formation. See text. 
 
 formation of the gametes. The members of a pair of chromo- 
 somes are termed homologous chromosomes and the contrasting 
 factors which they bear, in this case red and white, are named 
 allelomorphs. We see that it is the behavior of the homologous 
 chromosomes and their allelomorphic factors that determine tlio 
 nature of the gametes. 
 
 The fundamental features to note in the above discussion is 
 first that the factors remain distinct and show no indication of 
 fusing with one another. The red and white factors apfxvar 
 unchanged in the gametes no matter how man\- crossings have 
 been effected. The second feature is that these factors, though 
 associated in pairs in the homologous chromosomes, undergo at a 
 
142 PLANT BREEDING 
 
 certain stage of cell division a separation or segregation. These 
 two principles of non-mixing and of segregation of factors were 
 discovered by Mendel and they are among the most important 
 and fundamental facts in biological science. As a single illustra- 
 tion of this note that the success in breeding is solely based upon 
 Mendel's principles of heredity. Desirable characters of one 
 individual may by crossing be associated with the desirable 
 characters of another individual and in the same way undesirable 
 traits may be eliminated. These new combinations of char- 
 acters may be effected in some cases with such certainty that 
 horticulturists have advertised new forms, describing their 
 characters and qualities before they have been bred. Late fruit- 
 ing grains have been made to mature earlier, species susceptible 
 to cold have become hardy, as in the case of wheat, which may 
 now be cultivated in Canada many miles north of its former range, 
 and non-resistant forms have become immune to disease. The 
 annual loss from a single plant disease, one of the rusts, is esti- 
 mated at 500 million dollars. In the same way the percentage 
 composition of the reserve food in grains has been changed and 
 the resulting yield greatly increased. 
 
 Another important feature in the transmission of the hereditary 
 factors is illustrated in the crossing of a blue-flowered tall sweet 
 pea with a red-flowered dwarf sweet pea. In this case we have 
 two pairs of allelomorphic factors instead of one as in the four- 
 o'clock. One member of a pair of chromosomes contains the 
 factor for blue and the other homologous member of this pair 
 contains the factor for red. In another pair of chromosomes 
 there is lodged in the same way the factors for tallness and 
 dwarfness. When these two plants are crossed we find that the 
 factors segregate as in the first example but the hybrids resulting 
 from the cross are never intermediate between the two parents. 
 It will be noticed in a hybrid from this cross containing the factors 
 for tallness and dwarfness that the plant is always tall like the 
 tall parent; so in a hybrid where blue and red are associated the 
 flower is always blue. It would look as though the factors for 
 dwarfness and redness in these cases had been obliterated. 
 Such, however, is not the case. They have been inhibited in 
 
NATURE OF PLANTS 
 
 14 
 
 their action in some way by the opposite allcloniorpliic lacLor 
 and reappear in the offspring when hyljrid tall hlue sweet peas 
 are crossed among themselves. In cases of this nature where 
 certain factors are suppressed and others are active, the inactive 
 factors are termed recessive and the active ones are termed 
 dominant. Tallness and blue are dominant factors, dwarfness 
 and red are recessive factors. 
 
 In the above examples we see that the hereditary factors are 
 segregated and distributed to the gametes in a ver\' defmite 
 way. There is one important exception to this rule. A large 
 number of instances is known where one factor is usually or 
 always associated with another factor. That is, these two 
 factors are not separated from one another in the formation of 
 the gamete and consequently both are transmitted to the 
 gamete. Factors that behave in this way are said to be linked 
 and linkage is to be contrasted with the segregation mentioned 
 above. One of the most striking illustrations of linkage is seen 
 in color blindness. The factor for this characteristic is always 
 linked with the factor for sex. Therefore this factor is distributed 
 differently to the two sexes. The offspring of a color-blind male 
 and a normal female w^ould not show this characteristic at all 
 because the sons are absolutely normal and the daughters, though 
 having the factor for color blindness, do not show it, since this 
 factor is recessive to the normal sight factor. The daughters, 
 therefore, can alone transmit this factor, and one half of their sons 
 by a normal male will be color blind. 
 
 In the above examples very simple cases have been taken in 
 order to bring out more clearly the principles involved. We 
 have spoken of only one factor being lodged in a chromosome. I r 
 is now known that many factors may be associated with each 
 chromosome and the evidence at present makes ver>' probable 
 the belief that these factors are arranged in a line in the chromo- 
 some and are separated from each other by definite distances. 
 The factors have also been spoken of as though the\' were the 
 sole cause of the characters. The factor for redyi'ss is merely 
 the deciding cause of this color. A great many factors are 
 really necessary to cause the production of a '.'i\'"^ .liiu- uf«r, 
 
144 ORIGIN OF PLANT VARIATIONS 
 
 some being absolutely necessary, others having only slight effect 
 upon it. The character is merely the end product of a series of 
 reactions. Therefore you are not to think of these characters 
 as definite chemical substances that are all lodged in the indi- 
 vidual at the start of its life and that maintain their identity 
 unchanged throughout its development. At the start of an 
 organism a definite number of factors are involved — just enough 
 to give direction to its growth and to cause it to follow a 
 pattern of development that is peculiar to its kind. As growth 
 goes on owing to the interaction of these factors and also owing to 
 the action of the environment characters are constantly created 
 and here and there in the organism characters appear that give 
 us ocular evidence of the presence of the factors which are the 
 deciding cause of a color, of a form, of a size, etc. 
 
 It will be seen from the above discussion that no cross can 
 result in a new creation. All the work in breeding shows that 
 the crosses result merely in new combinations of the old factors 
 that already existed in the parent stock. How then does vari- 
 ation come about and new characters arise? de Vries has 
 shown that one or more new characters may suddenly appear in 
 the offspring through the influence of unknown causes. These 
 sudden changes are termed mutations. It appears reasonable 
 that the gradual accumulations of these mutations going on 
 over a long period of time may have been the principal cause of 
 the enormous number of variations that are seen in plant and 
 animal life. 
 
 What then is the purpose of fertilization? We see that it is a 
 process for effecting crossing and so bringing about a new combi- 
 nation of the characters and the variations that appear in the 
 parent plants. Some of these new combinations will be of no 
 value, others will give their possessors an advantage because 
 they adapt them to the conditions under which they live. 
 These latter forms will therefore tend to survive and to crowd 
 out the less favored. This is natural selection, the environment 
 bringing about the survival of the fit. As long as the environ- 
 ment remains constant and natural selection has picked out forms 
 
NATURE OF PLANTS ,, 
 
 that are adapted to it. there is no need of farther crossine 
 Indeed we see many plants that flourish without crossing Owing 
 to the changes, however, that have occurred on the earth since 
 plants first appeared there has been a necessity for fertili/alion 
 in order that new combinations might arise and adapt the 
 ofTsprmg to the new conditions. 
 
 10 
 
PART II 
 DEVELOPMENT OF PLANTS 
 
 CHAPTER V 
 
 CLASSIFICATION OF PLANTS 
 
 59. The Method of Classifying Plants. — We have now arrived 
 at an understanding of the nature of the plant and the work 
 which it performs. In this study the most complex forms have 
 been largely considered. There are, however, a great variety of 
 very simple forms ranging from single-celled plants that often 
 form the green coatings on trees up to the complex types. How 
 have these different forms come about and what relation do they 
 sustain to one another? We will also be interested to compare 
 the life histories of these different forms and their modes of life. 
 In approaching this subject it is first necessary to gain a know- 
 ledge of the system employed in grouping or classifying plants. 
 Obviously many plants are closely related as the red, white, 
 scarlet oaks. Such a group of closely related plants is called a 
 genus (plural, genera) and the different kinds of individuals 
 which compose the group are known as species. The red oak is 
 one of the species of the oak genus. In scientific work the Latin 
 or Greek names are used to designate the genera and species. 
 In the case of the oak the genus is known as Quercus, an old 
 Latin name for oak, and the red oak species as rubra, meaning 
 red. Both the name of the species and the genus are employed 
 in naming a plant and consequently the scientific name of the 
 red oak is Quercus rubra. You will frequently see a letter or one 
 or more abbreviations after the scientific name, indicating the 
 person or persons who are responsible for giving the name now 
 in use. Thus Quercus rubra L. indicates that this name was 
 
 146 
 
DEVELOPMENT OF PLANTS 147 
 
 given by Linnaeus. So in other cases we can say white oak 
 or Quercus alba, scarlet oak or Qiiercus coccinea, etc. The oak 
 has several characters in common with the chestnut and beech- 
 nut genera, as for instance the fruit in each case is associated 
 with outgrowths that appear as the burr in the chestnut and 
 beech and as a cup in the oak. On account of tliesc and other 
 common characteristics these genera are supposed to be related 
 and are therefore grouped together in one family. A family 
 is composed of allied or related genera. The name of the family 
 is derived from some characteristic genus in the group. In 
 this case it happens to be the beech or Fagns. To distinguish 
 a family from a genus the termination aceae is added to the base 
 of the generic word, in this example making the family name 
 Fagaceae. So we have the three allied genera, oak, beech, and 
 chestnut forming the beech family or Fagaceae. In the same 
 way it will be found that families are related and joined together 
 into a still larger group known as the order. The flowers of 
 the birch, alder, water beech, and hazel genera are very similar 
 and consequently form a family known as the birch family or 
 Betulaceae, Betula being the name of the birch genus. It will 
 also be noticed that there is a similarity between the flowers of 
 the Betulaceae and Fagaceae. This relationship is expressed by 
 placing them in the same order known as the beech or Fagalcs, 
 the termination ales being employed to distinguish the order just 
 as aceae characterized the family. So also orders are related 
 and grouped into classes. For example, the Fagales and many 
 other orders are characterized by having seeds with two cot\le- 
 dons. Several other orders have seeds with but one cotyledon. 
 This relationship is expressed by grouping the orders into two 
 classes, Dicotyledones and Monocotyledones. In both of tlu-se 
 classes the seeds are inclosed within the pistil, but in the cone- 
 bearing trees the seeds are exposed on a flat scale-like organ. All 
 seeds producing plants belong to one or the other of these two 
 groups. So we have two subdivisions, the Angiosix^niae, mean- 
 ing seeds inclosed, and the Gymnospermae, meaning naked seeds. 
 If now we stop to consider still larger groups than subdivisions, 
 it will be noted that many plants do not produce seeds, as in the 
 
148 RELATIONSHIP OF PLANTS 
 
 case of the ferns and mosses, etc. So a final grouping of plants 
 into divisions or subkingdoms may be made. All seed-bearing 
 plants form the division Spermatophyta, meaning seed plants; 
 the ferns comprise the division Pteridophyta, meaning fern plants, 
 etc. The termination of phyta, from phyton a plant, distin- 
 guishes the division from the other groups. To repeat, the 
 division is composed of subdivisions which in turn are made 
 up of classes. Frequently the division contains only classes. 
 Classes consist of related orders. The orders are divided into 
 families which include allied genera and these latter groups com- 
 prise closely related individuals or species. The vegetation of the 
 earth is separable into four widely differing divisions: I., The 
 Thallophyta, including among others the fungi and algae; II., 
 The Bryophyta or moss plants; III., Pteridophyta or fern plants; 
 IV., The Spermatophyta or seed plants. The character and 
 relationship of these groups will be considered in the following 
 pages. 
 
CHAPTER VI 
 DIVISION I. THALLOPHYTA 
 
 60. Classification of the Thallophyta. — The Thallophyta com- 
 prise a multitude of plants that include the most primitive and 
 simple forms of vegetation upon the earth. They range in size 
 from single microscopic cells to forms that arc comparable in 
 bulk to some of our shrubs and trees. They are all of simple 
 structure and do not possess roots, stems and leaves in the sense 
 of the seed plants. Such a type of plant body is called a thallus 
 and it assumes a variety of forms. The absorption and manu- 
 facture of foods is carried on, as a rule, by any and all of the 
 cells and this is equally true of the reproductive process. This 
 Division is not a natural one in that some of the subdivisions are 
 not related so far as we know. Several of the groups represent 
 distinct lines of development and they are placed together 
 simply because they are all simple forms of plant life. So it is 
 impossible to give in a short space a general idea of the nature 
 and character of this division which includes many groups of 
 widely different forms. For the purpose of stud\' the Thallo- 
 phyta may be divided into five subdivisions: — (i) Myxonncetcs 
 or Slime Moulds; (2) Schizophyta or Bacteria and Blue green 
 Algae; (3) Diatomeae or Diatoms; (4) Euphyceae or Algae; 
 (5) Eumycetes or Fungi. 
 
 Subdivision i. Myxomycetes or Shme Moulds 
 
 61. The Life History of a Slime Mould. — In one stage of their 
 life the slime moulds have a motility and mode of feeding sug- 
 gestive of some of the lower animals, wliile on the otlur liand 
 the final stage of their existence is more suggestive oi \hv fungi. 
 For this reason it has often been suggested that lhe\ are inter- 
 mediate between plant and animal life. These jilants are widely 
 distributed over the earth and may frequently be found on de- 
 caying logs and rotting twigs and leaves in forests. A common 
 
 149 
 
150 
 
 LIFE OF A SLIME MOULD 
 
 form, resembling a miniature, brownish puffball, is often seen on 
 stumps and fallen logs (Fig. 87). Other kinds are illustrated in 
 Fig. 88. They range in size from scarcely a pin head to nearly 
 a foot in diameter and from spherical to cylindrical and cake-like 
 masses. Not infrequently they are of great beauty owing to their 
 coloration and lace-like structures. These small sacs or spor- 
 angia (sing, sporangium) as they are commonly called, repre- 
 sent but one stage in the life of the slime moulds. If we begin 
 with an examination of the dust that floats away from these 
 
 -^-M^ 
 
 'Mj 
 
 Fig. 87. Fig. 88. 
 
 Fig. 87. The sporangial stage of two common slime moulds: A, Ly co- 
 gala. B, Arcyria. C, sporangia rupturing, hair-like structures (the capil- 
 litium) and ppores protruding. D, sporangia emptied. 
 
 Fig 88. Open types of sporangia: A, Stemonith, at left a single spo- 
 rangium enlarged, showing net-like structure formed by capillitium radiating 
 from the central stalk of the sporangium. B, Cribaria. 
 
 sacs every time they are tapped, the life history will be found to 
 be about as follows: Under the microscope the particles of dust 
 are seen to be minute cells or spores (Fig. 89, ^). You may think 
 of a cell as a cube, a sphere or as assuming almost any form, 
 
DEVELOPMENT OF PLANTS 
 
 151 
 
 frequently greatly elongated. It is bounded by a wall and within 
 is the living substance or protoplasm. The more inipcjrtant parts 
 of the living substance are a viscid rather watery material termed 
 the cytoplasm and a minute denser body, the nucleus, which is 
 immersed in the cytoplasm. A spore is a special kliul nf k-W 
 
 
 Fig. 89. 
 
 Fig. 90. 
 
 Fig. 89. Mobile phase in life of slime mould: .4, group of spores. B, 
 germination of a spore. C, two forms of zoospores greatly enlarged. The 
 right-hand one mobile owing to a streaming movement of its protoplasm 
 and the left-hand one moves about owing to the rapid movements of the 
 cilium, c. D, association of the zoospores preliminary to the formation of 
 a Plasmodium. E, Plasmodium. The arrow indicates the direction of the 
 streaming movement. 
 
 Fig. 90. Character of the capillitium: A, in Arcyria. B, in Trichia, por- 
 tion of thread enlarged on the left hand. 
 
 that is capable of germinating under favorable conditions and 
 producing a plant. In this case it is advisal)le when licrniiiiating 
 the spores to place them in a nutrient solution made by boiling 
 in water bits of wood upon which the sporangia were growing. 
 Germination will begin in about one day. As the spore wall 
 ruptures the contents escapes as a naked bit of protoplasm 
 (Fig. 89, B) which possesses a singular power of motion. This 
 motility is due either to the slow crcci>ing or streaming motion 
 
152 MOVEMENTS OF THE SLIME MOULDS 
 
 of the soft plastic body or more commonly the body may possess 
 a delicate thread or cilium (plu. cilia) (Fig. 89, C). These 
 ciliated bodies are called zoospores. The cilium is a highly 
 sensitive organ which by rapid and rhythmical movements, 
 somewhat like the movements of the arms in swimming, beats the 
 water and thus drives the body along. While in this condition 
 the zoospores are rapidly multiplied owing to the repeated split- 
 ting of the little bodies into two similar parts. Finally this 
 greatly increased number of individuals begins to come together 
 in small groups (Fig. 89, D) which in turn merge and so form 
 large slimy masses termed plasmodia (sing, plasmodium) which 
 are quite destitute of all walls and consist of a great number of 
 nuclei surrounded by a watery cytoplasm (Fig. 89, E). You will 
 often find this plasmodial stage of the slime mould on the under 
 side of rotting limbs or bits of wood in the forest as a sticky mass 
 resembling in consistency the white of an egg. The mucilaginous 
 character of the plasmodium accounts for the name slime mould 
 popularly applied to these plants, also for the term myxomycetes, 
 from myxos, slime, and myces, mould or fungus. 
 
 The Plasmodium possesses a sensitiveness or capability of 
 responding to external stimuli to a degree that is remarkable 
 when we consider the extreme simplicity of these plants. For 
 example, it avoids too strong a light and slowly moves toward 
 moisture and food. This accounts for your failure ordinarily 
 to notice it since it seeks the darkness and food in decaying 
 logs or the under surface of sticks and leaves. The motion is 
 brought about by little arm-like branches that flow from the 
 jelly-like plasmodium and finally the entire plasmodium will 
 slowly follow along these lines with a complicated streaming 
 movement (Fig. 89, E). If bits of wood containing some of 
 the Plasmodium are placed in a damp chamber in the dark after 
 a time the plasmodium will be seen creeping up the sides of the 
 moist dish in a complex skein-like mass or if a glass slide, down 
 which a very meager amount of water is slowly allowed to trickle, 
 is brought in contact with the plasmodium it may be observed 
 creeping up the slide toward the source of the water supply. 
 The slime moulds do not contain chlorophyll and are therefore 
 
DEVELOPMENT OF PLANTS i^:^ 
 
 dependent upon organic food. Plants that feed in this way are 
 called saprophytes. Since the Plasmodium is not surrounded by 
 a cell wall feeding becomes a simple matter. The jelly-like 
 mass engulfs the decaying particles or other foods and after 
 digesting the nourishing portions leaves behind the worthless 
 parts as it creeps along. As the plasmodium approaches the 
 final stage of its life, its nature appears to change comi)letely 
 for now it avoids moisture and seeks the light. It crc'e[)s to 
 the surface of the wood or leaves in which it has been growing and 
 forms the characteristic bodies seen in Figs. 87, 88. These 
 sporangia are formed by the outer part of the plasmodium 
 hardening into a wall, while each of the inclosed nuclei, which 
 have greatly increased in number by division, becomes sur- 
 rounded by a wall, thus forming the spores. UsualK- a portion 
 of the substance of the young sporangium is transformed into 
 simple or branching threads or tubes, collectively called the capil- 
 litium (Fig. 90). In several of the genera the capillitium forms 
 a network within the delicate walls of the sporangia and owing 
 to the early breaking down of the wall, the feathery frame alone 
 remains (Fig. 88). These threads are hygroscopic and their con- 
 stant motion assists in stirring up the spores and exposing them 
 gradually to the wind as soon as the wall of the sporangium 
 ruptures. A somewhat simpler type of slime mould lives as a 
 parasite in the roots of turnip, cabbage and cauliflower, producing 
 a destructive disease known as clubroot. 
 
 There are several groups of low types of plant and animal life 
 that are suggestive of relationship with the slime moulds. The 
 most important among these, the myxobacteriales, have Inrn 
 made known by Thaxter. They are minute plants, ver>' sugges- 
 tive of the next group, the bacteria, but associated in definite 
 structures that resemble the sporangia of the slime moulds and 
 also of certain fungi. On the other hand, certain aquatic forms, 
 as Protomyxa, with a life history very similar to that of the 
 slime moulds, intergrade almost perfectly towards simple ani- 
 mal types, as the protozoans, of which tlu' common amoeba is 
 an example. 
 
 Thus we see that the life history of these plaii'< '< ' ^'■'■\ Mmnlr 
 
154 NATURE OF BACTERIA 
 
 one. By the formation of sporangia numerous dust-like spores 
 are produced that are capable of germinating and forming 
 zoospores that Increase rapidly by division. The aggregation of 
 the zoospores results In the formation of the plasmodlum which 
 after a time completes the life history by creeping upon suitable 
 dry objects and forming sporangia. The mingling of the zoo- 
 spores In the formation of the plasmodlum is not a sexual process, 
 since the nuclei do not fuse, but Olive has shown that the spores 
 are formed In the same manner as In plants characterized by 
 sexual reproduction and that therefore there must be a sexual 
 fusion of the nuclei at some time In the life history of the plas- 
 modlum. The structure and life history of these plants Is so 
 simple that It Is possible that they may have been derived from 
 forms allied to the first forms of life upon the earth. The 
 common occurrence among simple animals and plants of motile 
 stages or zoospores In their life history certainly suggests that 
 possibly such forms may represent the first appearance of living 
 matter. 
 
 Subdivision 2. Schizophyta or Bacteria and Blue Green Algae 
 
 As In the preceding group these forms are extremely simple 
 and possess some characters suggestive of the lower forms of 
 animal life. They have received the name of Schizophyta, mean- 
 ing splitting plants, owing to their common method of repro- 
 duction by division of the cell into two equal parts. There are 
 two Important classes of Schizophyta: A, Bacteria; B, The Blue 
 Green Algae. 
 
 Class A. Bacteria 
 
 62. The Structure and Nature of Bacteria. — Bacteria are com- 
 monly known by such vague terms as microbes and germs. They 
 are, however, unicellular plants that are of almost universal dis- 
 tribution, though more abundant about dwellings and less com- 
 mon In cold countries, at high altitudes and on the sea. They 
 include the smallest and simplest forms of plants, ranging from 
 scarcely 1/50,000 in. to 1/10,000 in. in diameter (Fig. 91). Such 
 forms would have many times more room in a drop of water 
 than a whale would find in New York harbor. So minute and 
 
DEVELOPMENT OF PLANTS 
 
 00 
 
 simple in structure are the bacteria that the real nature of the 
 plant body is somewhat a matter of dispute. The i)lants are 
 unicellular and surrounded by a delicate tliln wall which in- 
 closes a colorless and slightly granular protoplasm (Pig. 91, 
 A, i). There is no nucleus comparable to that of the higher 
 
 
 Fig. 91. Forms of Bacteria: A, Bacillus suhtilis, a form common in hay 
 infusions, i, motile state; 2, cells with spores; 3, slimy mass of bacteria, 
 the zooglea condition, that appears on the surface of infusions, cooked ^ cg- 
 etables, etc. B, Spirillum. C, a coccus form that appears in pus. D, mobile 
 and spore stage of lock-jaw bacillus. 
 
 plants, although indications of it are seen in a few scattered chro- 
 matin grains. The cell content is very simple and totally hicking 
 in plastids and other differentiations with which you are familiar. 
 The slimy appearance of bacteria not liveable where tlu-N t^row 
 together in colonies is due to the mucilaginous excretion from 
 their bodies which is often brightly colored. Bacteria range 
 from globular to rod-like and curved forms (Fig. 9O. Many 
 are motile by means of cilia, as in the case of the zo. spores of 
 the Myxomycetes, which project singly or in tufts from the ends 
 of the cells or in varying numbers from all sides. 
 
 (a) Reproduction of Bacteria. — This is a simpler ])rocess than 
 in the slime moulds and much more rapidly elTected when the 
 conditions are favorable for growth. This process consists of 
 
156 EXCLUSION OF BACTERIA 
 
 forming a partition through the middle of a cell when the two 
 daughter cells thus formed separate at once or they may remain 
 attached, forming ultimately a chain of cells, since these two new 
 cells continue to grow and repeat the dividing process. This 
 method of multiplication goes on with great rapidity, frequently 
 within a half hour, so that many millions of plants may be formed 
 in a day from a single bacterium. This accounts for the rapidity 
 with which solutions, as beef tea, in which bacteria thrivel 
 become turbid and also for the gelatinous masses and wrinkled 
 scum that appears upon cooked foods and soups, if allow^ed to 
 stand undisturbed for a day or more (Fig. 91, A, 3). Conn 
 estimates that a single bacterium, if allowed to develop under 
 perfect conditions, would produce in three days a mass of bac- 
 teria weighing 4,700 tons. When the conditions are unfavorable 
 for growth many species of bacteria have another form of repro- 
 duction and form small bodies, called spores, within the cell 
 (Fig. 91, A, D). These spores are exceedingly resistant to un- 
 favorable conditions but germinate readily, producing bacteria 
 when conditions are again suitable for growth. Some spores will 
 stand boiling for more than an hour. This peculiarity of the 
 spore, while of great advantage to the bacteria, renders them very 
 difficult to exterminate. 
 
 (6) Exclusion of Bacteria. — ^The various devices for preserving 
 fruits and meats and sterilizing utensils and instruments in 
 surgery is based upon some method of killing these organisms 
 and preventing the entrance of others. This is the purpose of 
 boiling and canning foods and of salting or drying fruits and 
 meats. The boiling, if thorough, effectually sterilizes, while the 
 canning excludes all organisms and the material will remain un- 
 changed indefinitely. Smoking or drying, with addition of salt 
 in the case of meat and of sugar in fruits, is effectual since 
 bacteria flourish only in the presence of moisture. This is the 
 reason that seeds and other structures are not speedily destroyed 
 by bacteria. Seeds are naturally dry and protected by coats that 
 tend to exclude the moisture for some time. Sugar is used as a 
 preservative because bacteria cannot endure over 40 or 50 per 
 cent, of it. Salt for the same reason is useful in preserving 
 
DEVELOPMENT OF PLANTS 157 
 
 meat because it is injurious to bacterial life and smr^king also 
 introduces volatile products that prevent their growtli. So too 
 acids are generally harmful to bacteria. Vou have heard much 
 regarding the use of salicylic acid, benzoate of soda, etc., in pre- 
 serving foods and certain beverages. Whether these substances 
 are harmful to us or not, certain it is that they are fatal to the 
 existence of bacteria. So also meats, fruits and vegetables are 
 kept in a wholesome condition for considerable periods in ice 
 boxes and for months in cold storage since the bacteria grow 
 more slowly as the temperature is lowered and their activity 
 ceases at the freezing point. It should be remembered, however, 
 that no amount of cold will kill all of the bacteria and that fcx^d 
 spoils very quickly when removed from cold storage. 
 
 (c) Economic Importance of Bacteria. — Insignificant and simple 
 as are the forms and structures of bacteria it is safe to say that 
 no plants are so vitally related to our welfare. Numerous forms 
 are instrumental in effecting changes in various substances termed 
 fermentation; others assist in the decay of dead plants and 
 animals; a comparatively small number of species build up or 
 construct substances that are of vital importance to plant life; 
 finally many diseases of plants and animals are due to bacteria. 
 The work performed in all the above mentioned processes, save 
 the third, is of the same nature. These plants have the power 
 of breaking down into simpler compounds the various sub- 
 stances with which they may be brought in ct)ntact. Thus 
 vinegar is formed through the decomposition of certain alcohols 
 into simpler substances, as acetic acid and a gas, carbon dioxide. 
 These changes are effected by substances, termed ferments or 
 enzymes, that are excreted by the bacteria. These enzymes are 
 very remarkable compounds. They do not enter into permanent 
 union with any substance but by their presence cause it to break 
 down into some of its component parts. When this decompo- 
 sition is attended with the liberation of gas, as in certain sugar 
 solutions, w^e term the process fermentation but when in an 
 exactly similar way bacteria break down the substances compos- 
 ing the bodies of plants and animals we term the process decay. 
 So also in disease bacteria simply cause the <?.' ..mi^.^miK.n of 
 
^s 
 
 158 BACTERIA AND FERMENTATION 
 
 certain of the compounds that make up the body. In some cases 
 these simpler products that result from the activity of bacteria 
 are very poisonous and in disease it is these substances, toxins, 
 that often cause the principal danger. Let us now consider the 
 more important of these operations. 
 
 1. Fermentation. — A very large group of bacteria, in common 
 with some fungi, live upon sugars and other carbohydrates, 
 causing a decomposition known as fermentation. Vinegar is 
 due to the decomposition of the alcohol in cider and other weak 
 alcoholic solution by numerous species of bacteria which appear 
 as slimy masses and are popularly known as mother of vinegar. 
 Milk sours and coagulates through the agency of bacteria which 
 reduce the sugar to lactic acid, which in turn coagulates casein. 
 The beverages known as zoolack, kum.iss, etc., are milk prepara- 
 tions soured by special kinds of bacteria or yeast. The Metch- 
 nikoff tablets contain bacteria that are especially active in this 
 way and they are also supposed to live in the lower alimentary 
 tract where they form decomposition products that are injurious 
 to the harmful bacteria that live in this same region. So also 
 butyric acid, necessary in the manufacture of cheese, is produced 
 by bacteria. It will be seen from this that the products of de- 
 composition are not necessarily harmful. It is interesting to 
 note that some of the flavors of cheese and of high grade butter 
 are due to the products of decomposition and the excretions from 
 bacteria. It is evident that this might be true in the case of 
 limburger cheese and rancid butter but also remember that species 
 of bacteria are cultivated and used to impart certain flavors to 
 cheese and also to a less extent to butter. It is altogether 
 probable that this will become a common practice in our dairies. 
 
 2. Decay. — The great majority of bacteria live as saprophytes 
 upon dead plants and animals. They begin their work of 
 
 y^ decomposition immediately upon the death of the organism. 
 >>- In the case of plants they are generally assisted at first by 
 ^ various fungi. There are numerous classes in this group, each 
 
 kind doing a special work in bringing about the decay. The 
 first steps in the decomposition are quickly performed and then 
 the simpler compounds, though mostly still quite complex, are 
 
DEVELOPMENT OF PLANTS 159 
 
 acted upon by an entirely different class of bacteria that carry 
 on the work to a further stage of decay. So you can think of 
 many different kinds of bacteria each taking up the work where 
 it was left by its predecessor and carrying it a step fartluT. In 
 this way the complex compounds that make up the Hvinj^^ organ- 
 ism are broken down into very simple ones, such as water (H2O) 
 or gases as carbon dioxide (CO2), ammonia (NH3), methane 
 (CH4), sulphureted hydrogen (HoS), nitrogen (N), etc. It is 
 not to be understood that all of these simpler substances arise 
 only in the final stage of decay. Some of them may arise very 
 early in the process, as ammonia and carbon dioxide, or they 
 may be set free at later stages in the decay. Some of these 
 gases are the cause of the foul odors associated with decay and it 
 should be added that certain of the rather simpler products of 
 decomposition, termed ptomaines, are exceedingly poisonous. 
 The ptomaines found in fish, cheese, ice cream, are decomposition 
 products formed by the bacteria of decay. Unfortunately these 
 substances are without taste or odor. 
 
 This process of decay is of importance from two standpoints. 
 First, it prevents the accumulation of organic matter upon the 
 earth. Were it not for these changes every plant and animal 
 would remain unchanged after death and so prevent further 
 continuation of life. Second, we see that the successive steps 
 in decay result in the formation of simple compounds that are 
 either directly or indirectly utilized by plants in the formation 
 of their tissues. Thus H2O and CO2 are used by the plant in 
 forming sugars and starches which furnish the materials that 
 go to make up the cell walls and also assist in building up the 
 living substance. So also the N and the S that we have seen 
 set free as elements or simple compounds are utilized in the 
 formation of the living matter of the plant, though not directly, 
 as we will see below. 
 
 3. Construction or Synthesis. — In sharp contrast to tiie alK)ve 
 mentioned forms stands a rather small group of bacteria that 
 possess the remarkable power of taking certain of the simpler 
 products of decomposition and adding other elements to them. 
 These forms, therefore, are quite different in thi-ir work from other 
 
i6o NITRIFYING BACTERIA 
 
 bacteria. They build up substances instead of breaking them 
 down. This work of combining two or more elements is termed 
 synthesis and we may term these forms the synthetic bacteria. 
 These bacteria are of great economic importance and those kinds 
 that have the power of combining nitrogen into a form in which 
 it can be used by plants are of first importance. Nitrogen, as it 
 appears in the process of decay is in a form that the plant can uti- 
 lize either not at all or only to a limited extent. There are several 
 bacteria that have the power of combining nitrogen with other 
 elements and so constructing a nitrogen compound, termed a 
 nitrate, that is the most valuable crude food product of plants. 
 Among these forms may first be mentioned the nitrifying bacteria. 
 They have the power of adding another element to nitrogen 
 compounds. They are able to add oxygen (O) to ammonia 
 (NH3), that is to oxidize it, and thus transform it into a nitrate. 
 This synthesis is accomplished in a truly remarkable manner; 
 no less than two forms working together are required to accom- 
 plish the work. The first form adds two atoms of oxygen to 
 the NH compound, thus changing it to a nitrous or nitrite state 
 while the second form adds to the nitrite an additional atom 
 of oxygen, thus completing the formation of the nitric or nitrate 
 condition. The reactions caused by these two forms may be 
 expressed as follows: 
 
 2NH3 + 3O2 = 2H2O -t- 2HNO2 (nitrous form of nitrogen), 
 2HNO2 +02= 2HNO3 (nitric form of nitrogen). 
 These two organisms work together, each doing its special work, 
 and they are so efiftcient that a solution of ammonia poured upon 
 soil will show no trace of the ammonia in the solution after it has 
 percolated through the soil. Fully 65 per cent, of the nitri- 
 fication is effected in the upper twelve inches of the soil and very 
 little below a few feet. It is probable that the Chilian saltpeter 
 beds, previously referred to, are the remains of inland seas 
 where great growths of seaweeds accumulated. Owing to the 
 drying out of these bodies of water and the decay of the vegeta- 
 tion ammonia was formed which became changed to nitrates as 
 outlined above. Were it not for these organisms the ammonia 
 compounds would quickly escape from the soil as a gas and thus 
 
 { 
 
DEVELOPMENT OF PLANTS l6i 
 
 the principal source of nitrogen in nature for tlie plant would be 
 lost. 
 
 You have noticed that free nitrogen (N) may also appear in 
 decomposition. A few forms of bacteria ha\e been discovered 
 that are able to synthesize this gaseous element into a compound 
 that is of use to plants in a manner quite as remarkable as in the 
 case of the nitrifying bacteria. Because they are able to fix the 
 free gaseous element nitrogen in a stable compound these bacteria 
 are referred to as the nitrogen-fixing bacteria — in contradis- 
 tinction to the nitrifying bacteria that only act upon nitrogen 
 when it is already in combination, as in ammonia. The most 
 important of the nitrogen-fixing bacteria are those that live in 
 the roots of plants belonging to the bean family. They cause 
 minute gall-like swellings, termed tubercles, upon the roots of 
 beans, clovers, alfalfa, etc., and are able to combine the free 
 nitrogen of the air into a nitrogenous compound suitable for the 
 nourishment of the plant. These bacteria are of inestimable 
 value in maintaining the fertility of the soil (see page 65). 
 Less important economically than the tubercle-forming bacteria 
 but more wonderful biologically are those forms thdt live in the 
 soil quite independent of higher plants and effect a somewhat 
 similar fixation of free nitrogen. In this work they are associated 
 with two other forms that take no part in the fixation of nitrogen 
 but simply remove the oxygen and probably the organic matter, 
 for only in the absence of these substances can these remarkable 
 bacteria live. In this manner they are able to fix the nitrogen 
 and build up complex compounds, probably of a i^roteid nature, 
 which are subsequently decomposed and e\entually con\'ert«.>d 
 into available nitrates by other bacteria. 
 
 As a final example of these synthesizing bacteria mention 
 should be made of the iron and especially of the sulphur barti-ria. 
 They are able to oxidize simple compounds of iron and sulphur 
 so that deposits of these minerals may often be seen as slimy 
 white, red or yellow masses in springs and streams. The sulphur 
 bacteria are of especial interest because they are able to oxidize 
 one of the products of decomposition, H2S, into a sulphate — one 
 of the valuable crude plant foods. These plants are practically 
 II 
 
 I 
 
i62 SIGNIFICANCE OF BACTERIA 
 
 independent of organic matter for their support, deriving the 
 necessary energy for their growth from the decomposition of 
 inorganic matter. This also is absolutely the case with the 
 nitrifying bacteria which are unable to live upon organic matter 
 and it should be added that these latter forms effect the same 
 decomposition of carbon dioxide as noted in photosynthesis 
 (see page 12). This is an important fact, for we see that there 
 are two methods for effecting the storing up of energy. Green 
 plants are the principal agents in this work. They build up, 
 synthesize, such substances as sugars by utilizing the energy of 
 the sunlight — for this reason the process is termed photosynthesis. 
 Certain bacteria also build up compounds by utilizing the energy 
 of chemical reactions. This process is therefore termed chemo- 
 synthesis. 
 
 We should now stop for a moment and consider what the real 
 significance of these bacterial forms is. Green plants build up 
 complex substances from certain elements and simple com- 
 pounds. In the first steps of this work they may be assisted by 
 certain bacteria and fungi. These complex compounds are now 
 reduced by other bacteria to their original state so that they may 
 be used again by the green plant. This in a word is the story 
 of the organic world. Certain substances in the air and in the 
 soil are going through an endless rotation or cycle of chemical 
 changes — ever changing from simple to complex, from a com- 
 plex state back to their simple form. So then the green plant is a 
 temporary carrier of certain elements that, owing to their combi- 
 nations, represent a certain amount of stored up energy. We 
 and the plant and the bacteria utilize these substances in the 
 same way. We eat them, i, e., decompose them. This is 
 practically the only way that life can be maintained and the end 
 result of this eating, this decomposition, is that the elements 
 making up the compounds are returned unchanged to the earth, 
 to go again through the cycle of changes. 
 
 4. Disease. — In contrast to the kinds of bacteria mentioned 
 above there is another group that live as parasites on plants 
 and animals producing disease either by destroying the tissue 
 and sapping the vitality or by the production of poisonous 
 
DEVELOPMENT OF PLANTS 163 
 
 compounds, toxins. Among the more terrible of these infec- 
 tious bacteria may be mentioned'those producing consumption. 
 These affect especially the lungs of animals and cause, according 
 to data now available, from 150 to 200 thousand deaths annually 
 in the United States. It is altogether probable with complete 
 returns that the death rate will exceed by 50 thousand the higlier 
 figure mentioned above. Consumption is no longer considered 
 so fatal a disease as formerly. A person of ordinary constitution, 
 if suitably nourished, can be cured by living in the open air. 
 This pest could probably be wiped out witliin ten years, if the 
 habit of spitting could be stopped. The spores and plants are 
 readily carried away in the air from the dried sputum to Ix; 
 inhaled again and so spread the disease. The bacteria causing 
 lockjaw, Of tetanus, are especially abundant in the soil in certain 
 localities. Through wounds they are carried into the s\stem. 
 These forms, like others previously noted, are peculiar in that 
 they can not grow in the presence of oxygen. Conse(juentl\- the 
 dangers of infection are great in deep wounds. An innnediatc 
 cauterization of the entire surface of the wound will kill the or- 
 ganisms. Other well-known bacterial diseases are j^neumonia, 
 diphtheria, erysipelas, Asiatic cholera, t>phoid, and sj)K"nic 
 fevers and grippe. It should be noted that many diseases, 
 as smallpox, malaria, hydrophobia, yellow fever and probably 
 scarlet fever, etc., are due to a low order of microscopic animal 
 life. Disease-producing bacteria are either localized, as in the 
 lungs, throat, intestines, or are generally distributed throughout 
 the system. The symptoms of the disease are largely due to 
 the products of decomposition caused by tlie growth of tlu- bac- 
 teria or by secretions from the bacteria themselves, all of whicli 
 act as poisons and are commonly referred to as toxins. In 
 other cases the toxins are formed by the infected animal owing 
 to the disturbance of its growth and nutrition b\ the bacteria in 
 its tissues. In the same way bacteria are the cause of many 
 diseases among the plants, as in corn, melons, many truits, etc. 
 See Bacteria in Relation to Plant Disease, by Hrwin V. Smith. 
 
 These bacteria are constantly finding their wa\- into our iuKlies 
 by means of the breath but so long as the system is in a ht-althv 
 
i64 BACTERIA OF DISEASE 
 
 condition there is only slight danger of the organism gaining a 
 foothold. Just how the system is able to combat the growth of 
 these plants is now the subject of earnest investigation. Certain 
 diseases, as mumps, measles, whooping-cough, smallpox, etc., 
 usually occur but once and it is probable that substances are 
 formed in the body as a result of the presence of the bacteria or 
 animal organisms that prevent a second growth of the germs and 
 the individual thus becomes immune to further attacks. The 
 formation of such a substance is known to occur in the case of 
 diphtheria. The bacteria of this disease occur in the upper air 
 passages of the throat and secrete a poison or toxine that affects 
 the entire body. Gradually the system forms a substance called 
 an antitoxine that stops the growth and eventually kills the bac- 
 teria. The vigor of the system and the virulence of the bacteria 
 decide whether the disease shall prove fatal before the anti- 
 toxines are formed in sufficient quantity to destroy the bacteria. 
 Note that these disease-producing bacteria vary in their power 
 to produce toxine. Sometimes, as in epidemics, they are so 
 virulent as to attack with fatal results the strongest individuals. 
 In other cases the bacteria appear to be degenerate and only pro- 
 duce mild symptoms of the disease. This is the principle of 
 vaccination. The little animals causing smallpox are cultivated 
 under conditions that weaken them and render them less virile. 
 Consequently when they are introduced into the system they pro- 
 duce only mild symptoms of the disease. The history of the 
 study of bacteria constitutes one of the most interesting and 
 fascinating pages in science. Pasteur was the first to demon- 
 strate that fermentations and putrefactions, and later that 
 certain contagious diseases, were due to bacteria and animal 
 organisms. His work is now commemorated by a tablet on the 
 rue Pasteur in Paris with the inscription: Here stood Pasteur's 
 laboratory. 1857 Fermentations; i860 Spontaneous Genera- 
 tion; 1865 Diseases of Wines and Beers; i§8i Virus and Vaccini; 
 1888 Silk-worm Distempers; 1 864-1 888 Hydrophobic Remedies. 
 To-day every state and government has it corps of workers and 
 every city its expert bacteriologists who are studying the nature 
 of bacteria and their relation to plants and animals. 
 
DEVELOPMENT OF PLANTS 165 
 
 Class B. Blue Green Algae or Cyanoi'iivc eae 
 63. The Structure and Nature of the Cyanophyceae.— These 
 plants are very simple organisms that have main features in 
 common with the bacteria. They are unicellular plants, al- 
 though the cells are more commonly joined into rows, forming a 
 thread or filament (Fig. 92, A, B). In some cases these filaments 
 may be regarded as multicellular plants since they branch and 
 because the various cells perform different functions. They form 
 slimy, blue-green, black, yellow or violet masses and, like the 
 bacteria, appear to be dependent to a degree on organic material. 
 At least they are especially abundant in the presence of decaying 
 organic matter, as in the drainage from stables and watering 
 troughs, or in pools, puddles and damp places where there is an 
 abundance of filth and decay. The enormous increase of these 
 plants often produces a discoloration of the water, as in the Red 
 Sea, and they are often the cause of the foul odors of muddy 
 ponds and streams and reservoirs and of the pollution commonly 
 know^n as water-bloom or working of ponds. Like the bacteria 
 also they are associated with varying amounts of gelatinous sub- 
 stances derived from their walls or excreted from the cells and in 
 both cases the walls may become colored with various pigments. 
 Doubtless the mucilaginous character of these plants enables 
 them to retain moisture and so adapts them to dr>er conditions 
 than would otherwise be possible. The Cyanophyceae differ 
 radically from the bacteria in possessing chlorophyll which is dis- 
 tributed in the outer portion of the protoplasm, and in the |X)s- 
 session of a more definite nucleus. A blue pigment, ph>coc\anine, 
 is associated with the chlorophyll and for this reason they are 
 called the blue-green algae or cyanophyceae. Cell division fol- 
 lows the ordinary method noted in the higher plants, at least this 
 appears to be true in those cases that have been accurately 
 studied. These plants accordingly show a decided advance over 
 the preceding forms not only in the dilTerentiation of the cells 
 but especially because they are capable, to a degree at legist , of 
 manufacturing food from inorganic substances by reason of their 
 chlorophyll. 
 
 In addition to the multiplication of indi\icluals by cell di\i- 
 
166 REPRODUCTION OF THE CYANOPHYCEAE 
 
 sion, as in the case of the bacteria, many of the thread-like forms 
 have another interesting method of increasing their numbers. 
 A few of the cells, termed a hormogonium, become separated 
 
 Fig. 92. Fig. 93. 
 
 Fig. 92. Forms of the Cyanophyceae : A, Gleocapsa. At the right a cell 
 has divided, but the two daughter cells are held together in a gelatinous 
 mass. Above numerous divisions have occurred, but all the cells are sur- 
 rounded by a mucilaginous envelope. B, Nostoc. Below appear the gelat- 
 inous, spherical masses of the plants as they appear floating upon the sur- 
 face or resting on the bottom of ponds. At the right a plant enlarged — 
 h, heterocyst; s, thick- walled resting cells or spores. At the left a spore has 
 germinated, producing five cells. C, Rividaria. At left gelatinous mass of 
 plants attached to stem of water plant. At right view of a few of the plants 
 • — hr, hormogonium; li, heterocyst. 
 
 Fig. 93. One of the most common forms of the Cyanophyceae, Oscillatoria. 
 The different sizes of the cells show that cell division may occur in any of 
 the cells of the filament — e, a decaying cell which will ultimately free the 
 cells below it as a hormogonium. 
 
DEVELOPMENT OF PLANTS 167 
 
 from the other cells and frequently possess for a lime a slight 
 motility that enables them to move away from the i)arint fila- 
 ment (Fig. 92, hr). In some species one or several raihcr larj^e, 
 colorless cells, heterocysts, appear in the filament and ma>- serve 
 to separate it into hormogonia (Pig. 92, //). In Oscillatoria 
 (Fig. 93) the filaments into which these hormogonia grow per- 
 manently retain their power of motion. No cilia have been 
 observed for a certainty in any of these forms and the cause of 
 the motion has been ascribed to the interchange of fluids attend- 
 ant upon the absorption and digestion of foods. A simple type 
 of spore production is also to be found in the majorit>' of forms. 
 This consists merely of a slight enlargement and thickening of 
 the walls of the ordinary cells (Fig. 92, s). These simple spores 
 are able to tide the plant over unfavorable conditions and germi- 
 nate, forming new plants when conditions are again suitable. 
 
 The Bacteria and Cyanophyceae are evidently very primitive 
 and ancient forms of life. The fact that they frequently occur 
 in hot springs and that they can endure greater extremes of 
 heat than higher plants may indicate that they are related to 
 forms that appeared upon the earth when just such conditions 
 existed and at a time when the environment was not suitable 
 for the development of higher types. It has been stated on 
 page 162 that certain bacteria are independent of organic foods 
 and that they affect the decomposition of CO2 as in photosyn- 
 thesis while in the purple and in the red sulphur bacteria we have 
 pigmented forms that are perhaps in a transition state to chloro- 
 phyll-bearing plants. It seems altogether probable that the 
 earliest forms of life could build up organic compounds without 
 chlorophyll and in such forms as these bacteria we Ikuc perhaps 
 an illustration of a tendency towards the acquisition of chloro 
 phyll as seen in the higher plants. 
 
 Subdivision 3. Diatomaceae or Diatoms 
 64. The Nature and Structure of Diatoms. The diatoms arc 
 
 among the most common and widely distributed plants (Fig. 
 94). While microscopic, they exist in such large mnnbers as 
 to form the familiar brown coatings on the bottom of ponds and 
 
i68 
 
 FORMS OF DIATOMS 
 
 sluggish streams and often render sticks and stones in the water 
 slippery with slimy deposits. They are widely distributed on 
 damp soil and in fresh and salt waters. Note should also be 
 
 FiG. 94; Common forms of diatoms: A, fan-shaped colonies of diatoms, 
 Licmophora, attached by gelatinous stalks to seaweed. The repeated divi- 
 sion of a solitary diatom gradually builds up the fan-like colonies. B, Tahel- 
 laria, forming colonies grouped in zig-zag blocks. C, Melosira, cylindrical 
 pill-box-like diatoms, arranged in chains. Below end view of the diatom. 
 D, a species of Navicula arranged in gelatinous branches. At right enlarged 
 view of a branch in which the diatoms glide back and forth. — After Wm. 
 Smith. 
 
 made of the peculiar species that float upon the surface of the 
 ocean and form the larger part of that organic life known as 
 
DEVELOPMENT OF PLANTS 
 
 [69 
 
 the plankton (Fig. 95). This is especially ahiuulant in northern 
 waters and has been compared to great pasture lands, since it 
 furnishes the principal food of surface feeding fishes and cjiher 
 marine life. The Diatoms are unicellular plants and exhibit 
 
 Fig. 95. Fig. 96. 
 
 Fig. 95. Plankton forms of diatoms: A, Coscinodiscus, B, Planktottitlla. 
 Below seen from side. — After Gran. 
 
 Fig. 96. Structure of the diatom, Pinnularia: .1, valve vit-w. B, girdle 
 view. C, cross-section, the two dark bands showing the position of the chro- 
 moplasts in the diatom — p, pore in wall which appears as a line in .1, running 
 from the ends towards the center of the valve. — After Lauterborn. 
 
 a great variety of forms as circular, elllplical. rod or wedge- 
 shaped, curved or straight. They are equally variable as regards 
 their association. Some are solitary and free swinmiing, others 
 are attached by stalks; some form bands or ribbons or zigzag 
 
170 
 
 STRUCTURE OF DIATOMS 
 
 chains, Avhile others are imbedded in a gelatinous mass of a 
 more or less regular form (Fig. 94, D). The cells are covered 
 by two valves, one of which overlaps the other like the cover 
 of a box. Therefore a diatom presents two quite distinct ap- 
 pearances — the top or valve view and the side or girdle view 
 (Figs. 96, 97). This difference is further intensified by the 
 sculpturing of fine lines that appear upon the walls. An inter- 
 esting feature about these valves is the fact that they are com- 
 
 FiG. 97. Structure of Cymbella: A, valve view. B, cross-section show- 
 ing the difference of the two sides of the diatom. C, the two girdle views. 
 —After Pfitzer. 
 
 pletely infiltrated with silica, a substance resembling glass. If 
 a diatom is burned in a flame on a strip of platinum or placed 
 in acid to remove the organic substance the appearance of the 
 valves remains unchanged. 
 
 The glass-like valves are quite transparent and it can be readily 
 observed that the cell contents is much more highly differenti- 
 ated than in the preceding group. The chlorophyll is deposited 
 in plastids of definite form, although this color is often masked 
 by a brown pigment which causes the characteristic appearance 
 of these plants when associated in masses. The oil drops seen 
 in the cells are the product of photosynthesis — starch not being 
 formed. Some species, however, can live upon decaying or- 
 ganic matter and in consequence contain colorless plastids. In 
 the free swimming forms the motion consists of an irregular 
 
DEVELOPMENT OF PLANTS 
 
 171 
 
 gliding movement and this is supposed to be due to ilie expan- 
 sion and contraction of minute strands of protoplasm that pro- 
 ject through the pores of the valves (Fig. 96, C, p). 
 
 (a) Reproduction of the Diatoms. — It will naturally l)e asked 
 how can these plants living in glass houses, grow? As the valves 
 become changed to silica naturally any increase in size must 
 cease. Nevertheless the cells reproduce with great rapidity and 
 in a very interesting manner. Through the growth of the living 
 substance the valves are pushed apart and the cell contents di- 
 vides, forming two diatoms with but one \alve each. A new 
 valve that fits into the old valve is soon developed on the un- 
 covered side of each diatom and two complete diatoms are thus 
 formed (Fig. 98) which may become free at once or remain at- 
 
 ] 
 
 Fig. 98. Diagram illustrating the divisions of a diatom and the resulting 
 reduction in size. The brackets connect the two diatoms that were formed 
 by each successive division. — After Pfitzer. 
 
 tached and by further division form the odd groupings or colo- 
 nies shown in Fig. 94. Since the valves are of unequal size, i. e., 
 one fitting into the other, the two diatoms formed !)>• the division 
 must vary in size and it will be seen that the majority of the off- 
 spring will become greatly reduced in size as a result of tlie 
 rapid and repeated divisions. That this reduction nuiy not go 
 on too far, when a certain minimum size has been reached the 
 cell contents, with or without dividing, throws ofT the \alves 
 entirely and grows to the full size of the diatom, wlun new 
 valves are formed and the diatom is ready to repeat the dividing 
 process. In some cases we find quite a difterent method of re- 
 production. Two diatoms become enclosed in a jelK-like mass, 
 into which the cell contents, with or without dixiding, is dis- 
 charged. These naked cells now unite in pairs and the bodies 
 thus formed finally grow into diatoms (Fig. 99)- This latter 
 
172 
 
 REPRODUCTION OF DIATOMS 
 
 method of reproduction is termed sexual reproduction. The two 
 fusing cells are looked upon as male and female cells or gametes, 
 although in these simple plants there is no indication of sex that 
 would enable us to recognize them as such. In the preceding 
 groups, multiplication in numbers or reproduction has been 
 effected by the division of the cells or by the formation of more 
 or less modified cells, termed spores. All such methods of repro- 
 duction are called asexual, since but one cell is utilized in the 
 process. In sexual reproduction a cell is formed by the union 
 of the contents of two cells. The nuclei and protoplasmic con- 
 tents of each unite so completely that a cell with but one nucleus 
 and protoplasmic contents results. These sexually produced 
 cells are called gametospores because they are formed by the 
 
 '%: A 
 
 B 
 
 Fig. 99. Sexual reproduction of Pinnularia: A two diatoms, enveloped 
 in a mass of jelly. The valves have been thrown off and the content of 
 each diatom has divided into two sexual cells or gametes. B, the fusion of 
 the gametes. — After Karsten. 
 
 union of two gametes and are capable of forming a new plant 
 in the same way as do spores. We may distinguish the spores 
 derived from single cells as asexually formed spores or we may 
 call them simply spores, while the spores formed by the fusion 
 of the two cells or gametes may be termed sexually formed spores 
 or gametospores. Both kinds of spores are devices to enable 
 the plant to increase in number or to meet some other problem 
 
DEVELOPMENT OF PLANTS 173 
 
 in life, such as conditions unfavorable for growth. The spores 
 are especially to be looked upon as devices to bring about a rapid 
 increase in the number of individuals while the gamctospores 
 more usually serve to carry the plant over periods of drought or 
 extremes of temperature which w^ould prove fatal to the plant. 
 For this reason the gametospores are often provided with thick 
 walls and dense cell contents which enable them to remain in a 
 dormant state until conditions are favorable for growth. The 
 gametospore is often called a resting spore for this reason. 
 
 So rapidly do the diatoms multiply that the larger part of the 
 soil and rocks in many localities is composed of their remains. 
 The constant casting off of the valves through reproduction or 
 death results in vast deposits, knowm as silicious earth, on the 
 bottom of ponds and in the sea. Beds of silicious earth formed in 
 this way are often seen in districts where ponds and lakes have 
 dried up. Sometimes these beds reach enormous proportions, 
 the city of Richmond, Va., being built upon such a deposit. 
 Some of the w-estern beds are quite 300 feet in thickness and >et 
 it takes about 40 million of these plants to make a cubic inch. 
 Silicious earth is used as polishing powders and as absorbents in 
 the manufacture of some explosives. 
 
 Subdivision 4. Euphyceae or Algae 
 65. General Features. — This subdivision includes a large num- 
 ber of plants that live chiefly in fresh or salt water. These plants, 
 popularly known as Algae, vary greatly in form and structure 
 and range from microscopic unicellular forms to some of the larg- 
 est and most highly constructed plants found among the Thallo- 
 phyta. Their advance over preceding groups appears especially 
 in their well-marked walls and distinct plastids and nucki 
 (Fig. 100, 104). Chlorophyll is always present in the cells, 
 although in certain groups it is masked by brown or red j)igmenls. 
 These latter pigments are supposed to adapt the plant to var>'- 
 ing intensities of light. This is supported b\' the fact that the 
 marine Algae exhibit a zonal distribution in the water. Forms 
 living near the surface of the water are predominantly green, at 
 depths where they are alternately exposed and covered by the 
 
174 ORDERS OF GREEN ALGAE 
 
 tide, they are more commonly brown, while in the shade of the 
 brown Algae, or below tidal limits, red forms occur. The 
 Algae appear to have developed along three lines, which are 
 indicated by the green, brown and red colors, although the basis 
 for this classification rests upon structural and reproductive 
 characters. These three classes are: A, Green Algae or Chloro- 
 phyceae; B, Brown Algae or Phaeophyceae ; C, Red Algae or 
 Rhodophyceae. 
 
 Class A. Green Algae or Chlorophyceae 
 
 66. The More Important Orders of the Chlorophyceae. — The 
 
 Chlorophyceae are one of the most interesting groups of the 
 Algae, because they contain primitive forms that are at least 
 suggestive of low animal types and they also exhibit a gradual 
 modification of the plant body and reproductive processes that 
 help us to understand how variations arose and how complex 
 types have been evolved from very simple forms. These plants 
 are doubtless the most primitive survivors of a line from which 
 the higher land plants have been derived. Starting with uni- 
 cellular motile forms, the green algae appear to have diverged 
 along several lines. Among the more important of these may be 
 mentioned the following orders: (a) Volvocales, largely uni- 
 cellular green Algae; (b) The Zygnematales or conjugating green 
 Algae; (c) Chaetophorales or filamentous green Algae; (d) 
 Siphonales or tubular green Algae. 
 
 67. Order a. Volvocales or Unicellular Green Algae. — The 
 lower members of this order represent a very primitive type of 
 plant. Their motility, certain features of their life history and 
 delicate cell walls are suggestive of some of the preceding groups, 
 as well as of low forms of animal life. Sphaerella (Fig. 100) is 
 a familiar example of this group, often appearing in rock hollows 
 as blood-red stains. If some of this red material is examined 
 in w^ater, the plants appear as spherical cells with dense red pro- 
 toplasmic contents and thick walls (Fig. 100, A). This con- 
 dition represents the resting state of the plant. If the plants, 
 after being dried, are allowed to stand in water for a few hours, 
 the nucleus will divide, forming usually from 4 to 16 daughter 
 
DEVELOPMENT OF PLANTS 
 
 /o 
 
 cells (Fig. 100, B) which escape by the rupturliii; of the old 
 mother wall and swim actively about h\ means of two cilia. 
 These cells, or zoospores, remain motile for var>in^ periods, 
 during which time they increase in size, but finally tlie rilia arc 
 
 Fig. 100. Stages in the life history of Sphaerella: A, resting state of the 
 plant. B, first division. C, second division, the four cells are about to 
 escape from the mother plant. D, one of the cells of C after escaping. This 
 is a zoospore of the first generation. E, zoospore at rest. F, forming four 
 new zoospores. G, one of these zoospores of the second generation. Note 
 that the red material, represented by the shaded area in the center of the 
 cell, has become greatly reduced and that the wall is becoming distended and 
 separated from the granular cytoplasm. H, third resting stage. /, cell 
 dividing. K, zoospore of third generation with greatly distended cell wall 
 and small red area. Delicate strands of cytoplasm connect the cell wall 
 and the central protoplasmic body. L, a resting cell dividing into a larije 
 number of zoospores which arc consequently smaller. — After Hazen. 
 
 retracted and the plants remain in a (luiescent state for a short 
 period (Fig. lOO, E). A division of the nucleus now results 
 again in the formation of two or more zoospores 0*'.^'- ^^x^. ^0 
 which repeat the life history noted above. This metluxi of 
 reproduction may occur again and again and thus rapidly bring 
 about a great increase in the number of plants. During tho 
 
176 LIFE HISTORY OF SPHAERELLA 
 
 divisions, the red coloring matter is usually rapidly replaced by 
 chlorophyll until but a red speck remains and the zoospores soon 
 appear as rather ovate green cells surrounded by delicate walls 
 which become widely separated from the chlorophyll owing to 
 the accumulation of water (Fig. lOO, G). Nearly all zoospores 
 that occur among the algae are characterized by a small red body, 
 known as the eye spot, which is located near the ciliated end of 
 the zoospore. It has been supposed that this body is sensitive 
 to light and gives the zoospore a sense of direction. While this 
 is questionable, since the eye spot may be lacking (as in Sphae- 
 rella), it is certain that these beautiful bodies swim about, cili- 
 ated end foremost, with a rotary motion from right to left and 
 adjust themselves to a suitable illumination. It can easily be 
 demonstrated that the zoospores of Sphaerella are keenly sensi- 
 tive to different intensities of light by placing them in a glass 
 dish by a window when the zoospores will congregate on the 
 illuminated side unless the light is too intense, when a reverse 
 action takes place. 
 
 Thus we see that the life of the plants is largely a motile one, 
 each generation being characterized by a short resting stage 
 during which division occurs and a longer motile zoospore stage. 
 In fact, the zoospore may be looked upon as the original state 
 of these plants, while the resting condition is a departure due 
 to changes in the environment or the condition of the organism. 
 This common occurrence of motility in the lower types of life 
 indicates that possibly such was the condition of the first life upon 
 the earth. 
 
 (a) Conditions Affecting the Life of Sphaerella. — Changes in 
 the surroundings sometimes produce remarkable variations in 
 the life of the plant. This fact is well illustrated in Sphaerella. 
 If the water dries up, a thick wall is formed about the central 
 protoplasmic body which becomes dense and of a deep red color 
 while the delicace distended wall and cilia disappear. This varia- 
 tion adapts the plant to conditions unfavorable to growth, such 
 as drought and severe temperature. The so-called red snow is 
 due to a certain species of these plants that are swept off from 
 the rocks in this resting condition by the winter winds and falling 
 
DEVELOPMENT OF PLANTS 177 
 
 to the earth produces red streaks in the snow. Hazen has 
 been able to show that a most suggestive change in the mode of 
 life of the plants may also be induced by low temperature or by 
 a reduction of the volume of water in which they live. Under 
 such conditions, bodies are formed that resemble zoospores, but 
 that are not capable of motion owing to the lack of cilia. This 
 helps us to understand how the motionless or stationary- forms 
 of plant life came about and how the motile stage became less 
 and less conspicuous. Owing to such causes as these and many 
 others, plants become stationary. At first, perhaps, simply an 
 aggregate of motionless cells, but finally there resulted chains or 
 filaments of cells owing to the repeated division of the plants in 
 one plane. By division in tw^o planes, membranous expansions 
 resulted and the more complex types arose by the division of 
 the cells in three planes and by modification of the cells. 
 
 One other feature appears in the life of the Sphaerclla that 
 indicates clearly how sexual reproduction came about. When 
 the conditions for growth are unfavorable, as for example, through 
 lack of moisture or low temperatures, there result plants that 
 are not so well nourished, or, at least they appear to lack some 
 of the qualifications that characterize the plants growing under 
 favorable conditions. , The same condition often arises in these 
 plants after several generations have been formed. Such plants, 
 which may be characterized as weaker, or, at least, lacking in 
 certain material, do not give rise to the ordinary zor)six)rcs, 
 but to much smaller though similar bodies that are lacking in 
 cell w^alls (Fig. 100, L). These small zoiisporcs usualh' perish 
 or produce small plants unless two of them meet, when a fusion 
 may occur and thus a plant is formed that is capable of reiH\Uing 
 the life history of Sphaerella. By suitable nourishment, how- 
 ever, these small zoospores may sometimes be made to develop 
 into the normal plant. It is very evident that these small zoo- 
 spores are lacking in some substance that is essential to their 
 growth. This is rarely furnished to them in nature exce[)t when 
 two of the bodies fuse. From numerous examples appearing 
 among different groups of Algae, it appears that si'xuality arose 
 in this way. Owing to certeiin conditi(ms of liglit, temixTUunv 
 
178 ORIGIN OF SEX 
 
 food, etc., zoospores were formed that were Incapable of further 
 growth. But, by the union of two of these zoospores, a cell was 
 formed possessed of renewed vigor and capabilities of growth. 
 So we can think of the sexual cells or gametes as zoospores 
 that are lacking In the materials essential to growth and of the 
 sexual process as a union of the two bodies for the purpose of 
 bringing together the missing material and supplying the neces- 
 sary energy for growth. 
 
 A closely allied genus, Chlamydomonas (Fig. loi) has a life 
 history very similar to that of Sphaerella but the formation of 
 the gametes reveals a variation that gives us an understanding 
 of how these bodies came to differ and finally became distin- 
 guishable as male and female gametes. In some of the species 
 of Chlamydomonas, to be sure, the gametes are similar and can 
 not be distinguished as male or female (Fig. loi, B). This 
 stage of sexuality is called isogamous, meaning similar gametes. 
 In other species, there is a decided variation in the size of the 
 gametes, certain cells or plants producing large and less active 
 cells that are termed female gametes, while other plants produce 
 numerous minute male gametes that swim actively about (Fig. 
 loi, C, D). This stage of sexuality, where the gametes differ 
 in size is called heterogamous, meaning unequal gametes. This 
 variation In the size of the gametes which we call the differentia- 
 tion of sex Is due to the amount and nature of the material stored 
 in them, the male gametes being formed In a cell in larger num- 
 bers than in the case of the females. These heterogamous 
 gametes do not differ in nature from the Isogamous gametes. 
 They are also lacking In some substance that renders them In- 
 capable of growth unless fusion of two gametes is effected. 
 
 (6) Colonial Unicellular Algae. — The various stages In the 
 evolution of sex are also well illustrated in several genera of 
 unicellular green algae that live together in colonies. These 
 colonial forms are also of Interest in that they help us to under- 
 stand how unicellular plants became associated — at first quite 
 independent of one another — and this led to the dependence of 
 one plant upon another and finally to a distribution of work 
 among the plants of the colony, so that certain plants or cells 
 
DEVELOPMENT OF PLANTS 
 
 179 
 
 would be largely concerned in reproduction and others in the 
 manufacture of foods, etc. This is essentially the state reached 
 by our higher plants which may be spoken of as a colony of cells. 
 Pandorina shows the earlier phases of the differentiation of the 
 gamete?. This plant is really a spherical colony of al)OUt 16, prac- 
 
 FiG. loi. Fig. 102. 
 
 Fig. ioi. Features in the life history of Chlamydomonas: A, character 
 of the motile plant. B, conjugation of isogamous gametes. C, a plant 
 dividing to form numerous small male gametes. D, a plant forming two large 
 female gametes. E, male and female gametes about to conjugate. 
 
 Fig. 102. Features in the life history of Pandorina: A, a colony of plants. 
 B, each plant of the colony dividing to form a new colon>'. C, the plants of 
 a colony escaping as gametes. D, the conjugation of two gametes of un- 
 equal size. £, later stage in the conjugation. F, gamctospore or resting 
 spore. G, large zoospore formed from the gametospore. //, a colonx- fi>rmed 
 by the division of the zoospore, G. 
 
 tically independent, motile plants, each one similar to ^piuicrclla 
 but held together in a mucilaginous mass (F'ig. 102. .1). These 
 beautiful colonies multiply by each plant or cell (ii\ i(hiiv; into lO 
 daughter cells which finally become an independent colony 
 (Fig. 102, B). The gametes are formed in essentially the same 
 way as the colonies, save that the 16 daughter cells finally escape 
 as free swimming bodies (Fig. 102, C). The modur plants or 
 cells producing these gametes difi'er in size, owinu, (loiibtless, to 
 
i8o DIFFERENTIATION OF SEX 
 
 the amount of nutriment which they receive. Consequently, 
 the gametes formed from them must also differ in size. It is 
 also possible that the mother plants may produce gametes of 
 different sizes because some divide a larger number of times than 
 others. The larger the number of divisions in a cell, the smaller 
 will be the gametes. It is interesting to note that these gametes 
 fuse quite regardless of their size, although there may be a slight 
 tendency for the small and more active gametes to fuse with 
 the larger and slower moving ones. Thus we have an illustra- 
 tion of the first appearance of the differentiation of sex although 
 the sexuality of the gametes is not thoroughly established. The 
 gametospore, resulting from the union of the gametes, develops 
 a thick cell wall, the contents assumes a reddish color and it 
 passes into a resting stage (Fig. 102, F). When conditions are 
 favorable, it germinates as shown in Fig. 102, G-H. in the 
 beautiful spherical colonies of Eudorina and Volvox (Fig. 103) 
 the differentiation of the gametes is complete. Certain cells 
 produce but one large female gamete that remains motionless 
 in the mother cell while other cells form numerous small motile 
 male gametes which are yellowish in color. In Volvox, over 
 200 male gametes may be formed from a single cell (Fig. 103, 
 D, E). Volvox also shows that the association of plants in 
 colonies resulted in an interdependence among the plants and 
 also in a specialization in the work performed by these plants. 
 Fig. 103, C, shows that the thousands of plants that make up the 
 colony are in communication with one another and they doubt- 
 less give and receive material in accordance with the needs of 
 the colony. Furthermore a few of the cells of the colony receive 
 more nourishment than the others and these cells increase in size, 
 divide and form new colonies as in Pandorina. Finally there is 
 to be noticed that certain cells function in the production of 
 gametes. So we see here in a simple way some of those speci- 
 alizations and distributions of labor that will appear as the most 
 characteristic features in higher forms. 
 
 The tendency among the algae to form gametes of different 
 sizes is manifestly of great advantage. The great number of 
 male cells increases the chances of fusion of the gametes and their 
 
DEVELOPMENT OF PLANTS 
 
 i8i 
 
 smallness promotes their movements through the water. Note 
 also that these results are obtained without an>' additional 
 expenditure of material — the 200 male gametes being j)n)duced 
 from a cell no larger than SphacreUa where only a few are found. 
 The increase in the size of the female gamete renders her move- 
 ments more difficult, but the storage of food in this gamete pro- 
 vides for the better nourishment of the next generation. Finally 
 the nourishing function becomes so strongly dcvelojxd that the 
 female ceases to move at all and remains protected in the mother 
 
 Fig. 103. Features in the life history of Volvox: A, a coloin- which may 
 contain as many as 20,000 plants. The three central groups arc young col- 
 onies which may arise by the repeated division of any of the plants. B, side 
 view of one of the plants showing the canal-like connections with the neigh- 
 boring plants. C, surface view of a plant with canals radiating out to the 
 adjoining plants. D, a plant enlarging and forming a single motionless female 
 gamete. E, a plant forming numerous small male gametes. F, male gametes 
 enlarged. ' 
 
 cell. Thus we see that sexuality may have arisen among plants 
 as the result of definite stimuli as heat, moisture, light, food and 
 other conditions that occur in the environment. These stimuli 
 produce enfeebled zoospores that are incapable ordinarily of 
 growth unless a fusion of two of them is effected. So sexuality 
 
1 82 
 
 MOTIONLESS UNICELLULAR ALGAE 
 
 arose from the asexual state of the plant as a result of the influence 
 of the environment and differentiation of sex appeared owing to 
 the variation in the amount and kind of material in the gamete 
 producing cells. 
 
 (c) Motio7iless Unicellular Algae. — In contrast to these motile 
 plants, there is a large group of the Volvocales that have lost 
 their motility and spend a large part of their life as stationary 
 plants. We have already noticed how this condition may have 
 
 Fig. 104. Common forms of stationary unicellular green algae: A, Pleu- 
 rococcus showing stages in the division of the plants, the nucleus and the 
 irregular chloroplnst. B, Scenedesmus. Below three of the plants are shown 
 dividing. C, Ankistrodesmus. At right three plants in process of division. 
 
 come about, owing to the temperature and moisture conditions 
 that were unfavorable for the motile existence. Pleurococcus 
 (Fig. 104, A) is one of the most common of the green algae 
 and illustrates many of the features in the life history of the 
 stationary forms. These plants often cause the green coating 
 that appears upon the shaded or moist side of tree trunks, 
 fences and buildings. If a bit of this material is examined under 
 
DEVELOPMENT OP^ PLANTS 183 
 
 the microscope, it will be seen that the phmts are rather si)herical 
 cells provided with nucleus, cytoplasm, chloroplasts and well 
 marked walls. This latter feature is characteristic of stationary 
 algae and forms a sharp contrast to the delicate walls of the 
 motile forms. These plants increase in numbers by the forma- 
 tion of a wall through the middle of the (x-11. Tlie two daughter 
 cells or plants grow to the size of tlie mother plant and repeat the 
 process in division by forming a wall at right angles to the old 
 wall. It will also be noticed that cell division may occur in 
 three planes. The plants adhere together for a considerable 
 period, or become detached shortly after the division of the 
 mother cell. In this way they multiply with extreme rapidity 
 and so come to form extensive green coatings. It has been 
 noted in allied genera and reported in certain species of Plcuro- 
 cccais also, that changes of temperature and amount of water, 
 etc., will cause the contents of the cell to divide and form zoo- 
 spores which are of the same nature and behave in the same way 
 as in Sphaerella. Thus we see how changes of the environment 
 may cause these stationary plants to return to the motile condi- 
 tion of simpler types. It is interesting to note in this connection 
 that Livingston was able to change the stationary- condition of 
 certain algae to the motile condition at will by diluting the 
 solutions in which they were living. Fig. 104, B, C, illustrates 
 several other forms of stationary green algae that are common 
 in the drinking water and in ponds and streams. Interesting 
 forms for study may be obtained by tying a woolen sack to 
 a water faucet and allowing the water to run into it lor several 
 hours. Invert the sack into a glass, washing olT the material that 
 collects on the inside of the sack in the water of the glass. By 
 means of a pipette, a drop of the material that collects at the 
 bottom or sides of the glass or forms on the surface of the water 
 may be transferred to a slide and examined under a microscoi^e. 
 Some of these forms are free, some attached and others aijgrc- 
 gated in a mucilaginous mass. 
 
 There are a great many of the stationar>' genera tiiat siiow the 
 same habit of uniting into colonies that we noticed among the 
 motile genera. These colonies may be ver>' simple and com- 
 
i84 
 
 HYDRODICTYON 
 
 posed of a few loosely aggregated plants forming flat discs or 
 star-shaped plates (Fig. 105, ^). In other cases, we find spher- 
 ical and net-like colonies. The water net, Hydrodictyon, is a 
 common illustration of this latter arrangement (Fig. 105, D, E). 
 These plants occur in ponds and sluggish streams, forming sac- 
 like nets from a few inches to a foot in length, that are constructed 
 
 Fig. 105, Colonial forms of unicellular green algae: A, Pediastrum, the 
 plants of the colony being arranged in a flat plate. B, a view of the outer 
 cells of the colony showing the formation of a new colony. C, one of these 
 new colonies. D, a plant of the water net containing a young colony. E, 
 enlarged view of one of the meshes of a net showing the geometrical arrange- 
 ment of the plants. 
 
 in a very regular manner. The reproduction is quite character- 
 istic of many of the Volvocales. Zoospores are formed in large 
 numbers in any of the plants and after a period of motility, they 
 come to rest and arrange themselves into a new net inside of the 
 mother plant (Fig. 105, D). The walls of the mother plant soon 
 break down and the new colony is set free. Under different 
 conditions, smaller but similar bodies are formed which escape 
 from the mother plant. These bodies are gametes and incapable 
 of growth unless a fusion is effected between two of them. 
 
DEVELOPMENT OF PLANTS 
 
 IH; 
 
 Especial interest attaches to the study of this i)hint because 
 Klebs was able to change the reproductive character of the plant 
 by subjecting it to certain definite conditions. By exposing the 
 plants to bright light, slight increase of temperature and in- 
 organic culture solutions of definite composition, zoospores are 
 produced. On the other liand, plants growing in solution of 
 cane sugar in subdued light or darkness formed gametes. 
 
 These reactions are of great importance in helping us to under- 
 stand the nature of plant life. They demonstrate that defuiite 
 factors may change motile plants to a stationary- condition and 
 in Ilydrodictyon we see that measurable factors, as light, heat 
 and concentration of the solution may bring about the motile 
 condition of the plant and may also cause a variation in the 
 motile condition which we call sexualit}-. 
 
 68. Order b. Zygnematales or Conjugating Green Algae. — 
 This order includes the most common and attnictixc forms of 
 green algae (Figs. lo6, io8). They are of very general occur- 
 
 FiG. 1 06. Two common forms of the Zygncmatalc' 
 71, nucleus; p, pyrenoid. B, Zygncvia. 
 
 A , Spirogyra- 
 
 rence in the still waters of ponds and streams, whcTt' tlu>- oftt-ii 
 form the large floating and frothy green masses pc)j)ularl>- known 
 as pond scum or frog spittle. They can usually be recognized 
 by their slimy nature which is due to the copious exudation of a 
 gelatinous substance from the cell walls. In one group of this 
 order the plants are multi-cellular (Fig. 106), the cells being 
 attached end to end and forming a filament— though it should be 
 stated that each cell is practically independent in its life process 
 
i86 REPRODUCTION OF SPIROGYRA 
 
 and the organism may therefore be regarded as a colony. Such 
 a type of plant would have originated in the case of Pleurococcus 
 if the two daughter cells had remained attached and continued 
 to divide in but one plane and parallel to the first division wall. 
 This is exactly the mode of growth in these filamentous plants. 
 Any of the cells may divide into two daughter cells which grow 
 to the original size of the mother cells. In this way the length 
 of the plant is increased. The most characteristic feature about 
 the Zygnematales is the remarkably large and attractive chloro- 
 plasts. In Spirogyra (Fig. io6, A) these appear as bands, from 
 one to several in each cell, spirally arranged just within the cell 
 wall. In Zygnema (Fig. io6, B) they assume the form of stars 
 and in other genera they appear as plates, bars and variously 
 modified bodies. Imbedded in the plastids are denser proto- 
 plasmic bodies, the pyrenoids, that appear to be connected with 
 the secretion of starch ; at least a layer of starch can be detected 
 about the pyrenoids by testing with iodine (Fig. io6, pr). 
 
 The filaments are fragile and readily become dissociated into 
 smaller portions. In this way, the plants may multiply, since the 
 fragments continue to grow after the manner of the original 
 filament. Singularly, there is no indication of a return to the 
 motile condition in the life history of these plants. No zoospores 
 or motile gametes are formed. Sometimes the walls of a cell 
 become thickened and after a dormant period, as through the 
 winter, may germinate and grow into a new plant. With this 
 rather rare exception, the asexual reproduction of the plant by 
 spores does not occur. The sexual reproduction is of a peculiar 
 character. This is very well illustrated in Spirogyra. When 
 filaments of this plant chance to lie side by side, under certain 
 conditions, tube-like processes from opposite cells begin to grow 
 out towards each other (Fig. 107, A). These tubes finally meet, 
 when the walls at the point of contact are absorbed, forming an 
 open passage between the two cells. While this growth has been 
 going on, the contents of the two cells have contracted some- 
 what and finally the contents of one cell passes through the tube 
 and fuses with the contents of the other cell. Several modi- 
 fications of the reproduction process outlined above will occa- 
 
DEVELOPMENT OF 1>LAMS 187 
 
 sionally be noted, such as the development of ganicio^jxjres in 
 either filament or the fusion of the adjacent cells of a filament by 
 means of lateral tubular outgrowth. In otlicr genera, there is a 
 considerable modification in the process, as in some sfK-cies of 
 Zygnema, where the fusion is effected in the middle of the tul)e. 
 
 Fig. 107. Sexual reproduction of Spirogyra: A, in lower portion of figure 
 the formation of the tubes between the opposite cells of the filaments is shown. 
 Above the contents of two cells have united with two cells of another filament* 
 forming two gametospores. B, germination of a gami-tosporr. 
 
 This is only a different form of the sexual process wiili wliich 
 you are familiar, the difference being, that but one gamete is 
 formed in each cell and these bodies are motionless or at least 
 not provided with cilia and do not escape from the mother plant. 
 When the fusion of the gametes has been etTected the resulting 
 
i88 GERMINATION OF THE GAMETOSPORE 
 
 gametospore becomes surrounded by a thick wall and in this 
 resting condition it tides the plant over seasons, such as the 
 winter or drought, unfavorable for the growth of the plant. 
 When conditions return that permit the growth of the plant, 
 the gametospore germinates by rupturing the outer wall and pro- 
 truding the inner wall of the spore as a delicate tube (Fig. 107, B). 
 Early in this germination the nucleus divides, forming four cells, 
 the outermost of which only continues to grow and so ultimately 
 forms the new plant. Note that the gametospore, therefore, 
 
 Fig. 108. Common forms of Desmids: A, Closterium. B, Micrasterias, 
 C, Xanthidium. D, Staurastrum. At left top view, at right side view. E. 
 Desmidium forming a chain of plants. At right, end view of chain. 
 
 does not grow directly into the parent type of plant. This be- 
 havior of the gametospore is one of the most fundamental 
 features in plant life. It appears in the preceding groups but 
 it is mentioned now for the first time because we see here a clear 
 demonstration of its character. The gametospore is different 
 in its nature and possibilities of growth from any one of the cells 
 of the parent plant. It can not produce the spirogyra plant. 
 It can only produce four cells and from one of these cells there 
 will develop the spirogyra plant. Further consideration of this 
 feature in the life history of the plant will be deferred to later 
 
DEVELOPMENT OF PLANTS 
 
 189 
 
 examples, where it will become still more obvious. The loss of 
 motility in the reproductive process in this order may possibly 
 be associated with their exposure to terrestrial conditions. 
 They are often exposed to the soil, owing to the drying up of the 
 water and it has been suggested that they have consequently lost 
 their motility. Their mucilaginous coatings would be of great 
 service in enabling them to meet such conditions by retaining 
 moisture, and indeed, they are often able to fl(jurish in many 
 damp places without the aid of surface water. 
 
 The Desmids. — A second group of the Zygnematales are strictly 
 unicellular plants known as Desmids (Fig. 108). They are the 
 most attractive of unicellular plants and are of conmion occur- 
 rence associated with coarse algae. The desmids are elaborately 
 and variously fashioned but can readily be recognized by the 
 
 j^:^^^^^ //' 
 
 T K ^,:T 
 
 Wm 
 
 ^n ^ 
 
 ^^•r-jii 
 
 Fig. 109. Reproduction of Cosmarium: A, a sexual rcproducti(jn, sIkav- 
 ing the elongation of the isthmus, i, and its gradual enlargement and divi- 
 sion to form the new lobes of the desmids. B, sexual reproduction. At 
 the left the cell contents of two desmids fusing to form a gametosjK)re. On 
 the right the mature gamctospore covered with a spiny coat. 
 
 fact that they consist of two similar hah'es (Fig. 109, .1). The 
 structure of the cells and the sexual method of reproduction 
 is essentially the same as in the filamentous forms which have 
 doubtless given rise to these unicellular j^lants. The asexual 
 
190 SPECIALIZATION IN ULOTHRIX 
 
 method of reproduction, however, is rather peculiar. The region 
 connecting the two halves (Fig. 109, A, i), called the isthmus, 
 becomes somewhat elongated and swollen. Soon a constriction 
 appears midway between these two halves which deepens until 
 the desmid is cut in half. Each desmid now consists of one of 
 the original halves and one half of the isthmus. This latter 
 part gradual!}' enlarges and forms the second half of the desmid. 
 Owing to the gelatinous character of the cell walls, the desmids 
 often become aggregated in masses and form chains of cells 
 (Fig. 108, E). 
 
 69. Order c. Chaetophorales or Filamentous Green Algae. — 
 Here belong a great array of green algae that grow attached to 
 various objects and appear as simple or more usually as branched 
 filaments which frequently end in hair-like tips. They are of 
 common occurrence at the bottom or margins of springs, wells, 
 streams and ponds. Ulothrix may be taken as an example of 
 one of the simple forms of the Chaetophorales. Fig. no shows 
 that it consists of a simple chain of cells that divide and so bring 
 about the growth of the plant as in Spirogyra. However, it is 
 important to note that we have here a higher type of vegetative 
 structure than in any of the algae heretofore studied. This 
 advance appears in the one or more rather colorless cells at the 
 base of the plant which serve to anchor it to the substratum. In 
 other words, there is apparent a distribution of labor or the per- 
 formance of definite duties by special parts or cells of the plant 
 body. The evolution of higher forms comes about in much this 
 way. So long as each cell of the plant performs the same func- 
 tion, there can be little variation in the form and structure. But 
 as these cells are differently stimulated by their environment, 
 they tend to vary in character and so become adapted to the per- 
 formance of different functions. Perhaps in Ulothrix the con- 
 tact of the substratum acted as a stimulus, as Pierce found to 
 be the case in many algae. Thus the plant comes to consist of 
 different kinds of cells, each of which performs a different kind 
 of work. The complexity of the plant is due to the variations of 
 its cells which are thereby adapted to the performance of special 
 duties. In Ulothrix, we have one of the simplest modifications 
 
DEVELOPMENT OF PLANTS 
 
 191 
 
 of this nature, one or more of the basal cells hecomin.i,^ sli^^htly 
 changed in form and contents and so adapted in an( horin^ the 
 plant. The other cells of the plant are praclicalh- alike, each con- 
 taining a single nucleus and chloroplast and therefore capable of 
 manufacturing food and forming the reproductive bodies which 
 are of the same simple type as noted in Sphaerella . 
 
 (a) Reproduction of Ulothrix. — The contents of an\- of the 
 green cells may divide into two or more cells which esca[)e as 
 
 10. 
 
 Fi(.. III. 
 10. Lower portion of Ulothrix, the basal cells are somewhat nuHli- 
 
 Fig. 
 
 Fig. 
 
 fied and the lowest one acts as an anchoring organ. Each of the up|K>r crils 
 contains a girdle-like chloroplast. 
 
 Fig. III. Asexual reproduction of Ulothrix: A, a few cells of a fila- 
 ment in the upper cells of which the formation and escape of the larRe zcxi- 
 spores are shown, while in the lowest cells a large number of small Z(x>s|K>rc» 
 appear. B, a large zoospore. C, a young plant formed by /i. P, a small 
 zoospore. E, a young plant formed from D. — After West. 
 
 zoospores through an opening formed in the cell wall (I'ig. m, 
 A). The cells in which the zotJspores are f<'r"i<'l .«r.- .allrd 
 
192 ASEXUAL REPRODUCTION OF ULOTHRIX 
 
 sporangia. The zoospores resemble the motile plants noted 
 above, but after a short motile period they return to the station- 
 ary condition and grow into new plants (Fig. iii, C). In this 
 way, the number of plants is rapidly multiplied. The point 
 must not be overlooked that this phase of the life history is the 
 same as that of Sphaerella, the difference appearing in the 
 shortening of the motile period and in the lengthening of the 
 non -motile period. During the resting stage the cells of Ulothrix 
 to be sure may divide in one plane forming a filament but re- 
 member also that non-motile cells of Ulothrix msiy divide, forming 
 a mass of cells that become detached or loosely associated — the 
 so-called palmelloid state — which behavior we have seen sug- 
 gested in the non-motile cells of Sphaerella. It should be stated 
 that the asexual method of reproduction may be looked upon 
 as a means of bringing about a rapid increase in the number 
 of individuals of a species while the conditions for growth are 
 favorable. When new conditions arise, such as changes of tem- 
 perature, food, etc., that would interfere with the normal growth 
 of the individuals then a modification of the contents of the cells 
 and its mode of division results. The cells of Ulothrix as a con- 
 sequence of such alterations divide into a larger number of motile 
 bodies which are consequently smaller than in the case of zoo- 
 spores (Fig. 112, A). These bodies may behave as gametes and 
 unite, forming gametospores. The cells in which gametes are 
 formed are called gametangia, sing, gametangium. Thus, Ulo- 
 thrix is controlled in the same manner as Chlamydomonas and 
 Hydrodictyon. It is interesting to note, however, that these 
 small bodies frequently behave as zoospores and develop into 
 small and weak plants, which fact accounts for the common asso- 
 ciation in this genus of puny and vigorous plants (Fig. iii, C, £). 
 These small bodies of Ulothrix represent a curious intermediate 
 condition between a zoospore and a gamete where the sexual 
 character is not strongly enough developed to overcome com- 
 pletely the zoospore character. Thus, we see again in Ulothrix 
 that definite environmental conditions caused motile bodies to be 
 produced that are lacking in some essential substance and conse- 
 quently do not have the energy for growth, or at least can only 
 
DEVELOPMENT OF PLANTS 
 
 193 
 
 develop into feeble plants. This weakness is overcome by the 
 fusion of two gametes which results in the formation of a cell 
 or gametospore with renewed energy for growth. \\r also note 
 how these departures in the behavior of the organism are o{ 
 advantage to it. The zoospores effect a distribution of the 
 species while the gametospore ensures a continuity of the species 
 by tiding it over unfavorable conditions. 
 
 The gametospore germinates after a period of rest somewhat 
 as in the case of Spirogyra. The contents of the spore, instead 
 of giving rise directly to a new plant, divides, f(jrming several 
 zotispores (Fig. 112, E, F) which after a period of motility 
 
 Fig. 112. Sexual reproduction of Ulothrix: A, a few cells of a filament 
 showing the formation and escape of the gametes. B, gamete. C, D, stages 
 in the union of the gametes. E, gametospore. F, gametospore germin- 
 ating and forming the sporophyte or asexual generation, which is a sai -like 
 like plant containing several zoospores. 
 
 become attached to some object and grow into a new i)lani. 
 The cause of this peculiar behavior of the germinating gameto- 
 spore is not known. Some change has occurred in it that 
 renders it incapable of developing directly into a mw plant. 
 This variation in the growth of the gametospore marks the 
 beginning of one of the most important departures in the evo- 
 lution of plant life and it will become more and more significant 
 as the work proceeds. It is evident that the formation of 
 
 13 
 
194 NATURE OF THE GAMETOSPORE 
 
 several zoospores, each of which produces a new plant, is a de- 
 cided advance over those types in which the gametospore develops 
 directly into but one new plant. Note also that in Ulothrix there 
 are two kinds of zoospores, one formed from the Ulothrix plant 
 and the other from the gametospore, the former being frequently 
 called zoogonidia to distinguish them from the latter. This dis- 
 tinction is of great importance, because most plants have two 
 distinct stages or generations in their life history. One is repre- 
 sented by the plant that bears the sexual cells or gametes and 
 therefore called the sexual plant or sexual generation, or, in short, 
 the gametophyte. The real nature of this plant is to produce 
 gametes, although it may for a time produce zoospores. The 
 other stage is derived from the gametospore and is called the 
 asexual plant, asexual generation or sporophyte, since this plant 
 can only produce zoospores. Among the green algae this dis- 
 tinction is not very manifest, the Ulothrix plant for example being 
 the sexual plant or gametophyte and the gametospore or the 
 little plant derived from it (Fig. 112, F) is the asexual plant or 
 sporophyte, in this case being merely a single cell. It is very 
 manifest, however, that this single cell or plant is decidedly 
 different from the Ulothrix plant because it cannot develop into 
 the Ulothrix plant, but can only produce zoospores. So we see 
 that there are two generations in the life history and that they 
 differ radically in their natures and possibilities of growth. In 
 higher types of plant life it will be seen that the gametospore 
 tends to develop into a more and more complex body or plant 
 and it will be one of the most intresting features of the work to 
 watch the evolution of this asexual plant. 
 
 The higher members of the Chaetophorales illustrate the same 
 gradual differentiation of the gametes, as has been noted in the 
 motile forms of the Volvocales. The female gametes become 
 distinguishable because of their larger size and shorter period of 
 motility, while the male gametes are small, owing to the large 
 numbers that are found in a cell or because of the small size of 
 the cells in which they are developed. Finally, in several of the 
 genera we find but a single female gamete which remains motion- 
 less in the cell. This condition is very well illustrated in one of 
 the genera of the next order. 
 
J\^Y^6ff\U^\io^r y j^;^0lO(/a^ 
 
 DEVELOPMENT OF PLANTS 
 
 195 
 
 70. Order d. Siphonales or Tubular Green Algae.-— This 
 
 order Includes a lari^^e number of odd forms that are Idamentous 
 in character and they differ from al^ae previously noterl in that 
 the filaments contain numerous nuclei, but with rare exceptions 
 no cell partitions. Such plants are called coenocytes. They 
 assume various forms and often resemble a small plant with stem, 
 root and leaf, but in all these cases the plant is essentially a huge 
 cell or tube without partitions and containing numerous nuclei. 
 Most of the Siphonales are marine, but one genus, Vauchcria, is 
 well represented in shallow streams, damp places and on the 
 earth of flower pots in greenhouses, where it forms rather coarse 
 green felt-like masses. The plant body consists of long tubular 
 threads, often branching and anchored to the ground by colorless 
 
 Fig. 113. Structure and asexual reproduction of Vaucheria: A, portion 
 of a plant showing the branching tubular filament and colorless root-like 
 outgrowth, r, B, end of a filament enlarging to form a zoospore. C, zoospore. 
 D, germination of a zoospore. 
 
 outgrowths (Fig. 113, A). The protoplasm forms a thick lining 
 layer on the inner wall of the filament and embedded in it are 
 numerous minute chloroplasts, nuclei and oil drops. A water\- 
 cell sap fills the center of the tube. Partitions onl>- appear when 
 reproductive bodies are formed or to close 1 wdimd in (.isc of 
 
196 SEXUAL REPRODUCTION OF VAUCHERIA 
 
 injury to the filaments. Asexual reproduction is effected by very 
 large zoospores which are developed in the enlarged extremities of 
 the filaments (Fig. ii^, B). These tips are cut off by a transverse 
 wall and a single zoospore escapes through an opening in the 
 tip of the sporangium thus formed. The entire surface of the 
 zoospore is clothed with cilia arranged in pairs, each pair being 
 associated with a nucleus so that the zoospore resembles the 
 motile colonies previously noted (Fig. 113, C). After a very 
 short motile period, the zoospore comes to rest and grows into 
 the characteristic tubular plant (Fig. 113, D). When the plants 
 are exposed to too dry conditions, the tips of the filaments often 
 
 Fig. 114. Sexual reproduction of Vaucheria: A, portion of a filament that 
 has formed two branches which have grown into a male, an, and female, og, 
 gametangia. B, later stage, the gametangia have opened, permitting the 
 escape of the male gametes and the fertilization of female gamete. C, gam- 
 etospore detached from the filament. 
 
 enlarge and finally become detached as motionless spores that 
 germinate when conditions are again favorable. 
 
 The male and female gametes are produced in gametangia that 
 are formed from short branches. The males are developed in 
 large numbers in curved branches that become cut off from the 
 filament by a cross wall (Fig. 114, A) and the gametes finally 
 escape through an opening that forms at the apex of the branch. 
 A gametangium that produces clearly differentiated male gametes 
 is called an antheridium (plu. antheridia) and the gametes are 
 frequently called antherozoids or sperms. A single female 
 
DEVELOPMENT OF PLANTS 
 
 197 
 
 gamete is found in a similar branch which becomes rather egg- 
 shaped and at maturity opens at the beaked end (Pig. 114, B). 
 A single-celled female gametangium is called an oo.uonluin and 
 the female gamete is frequently referred to as the o()si)here or 
 egg. The male gametes are discharged a few minutes after the 
 oogonium opens, when they swarm al)oul tlie open end of the 
 female gametangium and readily enter it. As soon as fusion has 
 been effected the gametospore becomes invested with a thick wall 
 and in this condition can endure a limited drought. In germi- 
 nating it develops directly into a new plant. This feature of the 
 
 Fig. 115. Growth and asexual reproduction of Oedogoyiium: A, young 
 plant showing basal cell modified as a hold-fast. B, two cells of a filanu-nt, 
 in one of which a zoospore is forming and the other cell has opened, jht- 
 mitting the escape of the zoospore shown at C. D, zoospore at rest. It 
 becomes attached to an object at its colorless, ciliated ind. K, later stage 
 in the germination of the zoospore. — After West. 
 
 life history of Vaucher'ia, therefore, appears as of a mori: i)rimiti\e 
 nature than in the case of Ulothrix. In tlie following studies nou 
 will repeatedly notice variations of this nature. In ilu- (k'Ntloj)- 
 ment of plants advances are often confined to one or another 
 feature of their organism which may become highly develofx^d 
 and specialized; at the same time one or more features may be 
 retained that have been subject to little or no \ariation and 
 which, therefore, remain in a iM"iniitive state. 
 
 71. Other Members of the Green Algae.- Tlure are several 
 
198 
 
 HIGHER TYPES OF REPRODUCTION 
 
 orders of the Chlorophyceae, some of which are marine, that 
 cannot be considered at this time. Two genera, however, de- 
 serve attention because they show significant advances in the 
 evolution of plant life. In Oedogonium, a member of a small 
 order, the Oedogoniales, we find a still higher form of the sexual 
 reproductive process. These plants are of very common occur- 
 rence in ditches, streams and springs where their filaments form 
 for a time greenish masses and coatings upon various objects, 
 but finally become detached and free-floating (Fig. 115, A). 
 Any of the cells of a filament, save the basal one, may form a 
 single zoospore, which are large pear-shaped bodies with a narrow 
 
 Fig. 116. Sexual reproduction of Oedogonium: A, portion of a filament 
 in which a female gamete has been formed — 0, opening in cell wall for en- 
 trance of male gamete. B, portion of filament showing formation and es- 
 cape of male gametes. C, gametospore free from mother cell. The germi- 
 nation of this spore results in the rupture of its outer wall and the protrusion 
 of the contents of the spore as four cells which are for a time retained by the 
 delicate inner wall of the spore, as shown in D. E, the four cells have become 
 mobile zoospores and the delicate inner wall of the gametospore is greatly dis- 
 tended and about to rupture. F, G, stages in the germination of the zoospore. 
 — ^After Hirn. 
 
 colorless end around the base of which arise a circle of numerous 
 cilia (Fig. 115, B, C). These zoospores develop into new plants 
 (Fig. 115, D, E) as in previous cases. The sexual reproduction 
 presents a high degree of specialization. The male gametes are 
 
DEVELOPMENT OF PLANTS 
 
 199 
 
 usually formed in pairs in small cells and arc similar to but 
 smaller than the zoospores, while the female gametes are de- 
 veloped singly in the ordinary cells of a filament which become 
 greatly enlarged and spherical or void in form (Fig. 116, A, B). 
 The female gamete is motionless but the colorless region on one 
 side of the gamete certainly suggests the idea that this body is 
 
 Fig. 117. Fig. 118. 
 
 Fig. 117. Coleochaete, showing the radiating filaments and hair-Hlcc out- 
 growths of the cells. 
 
 Fig. 118. Sexual reproduction of Coleochaete: A, end of a filament bearing 
 the male, an, and female, og, reproductive organs. B, the gametosix^re is 
 being enveloped by the adjacent filaments of the plant. C, gametospore 
 germinating. Eight cells have been formed, rupturing the walls of the gameto- 
 spore. — After Oltmann. 
 
 related to the zoospores and male gametes, but tiuu 11 lia> K'>i 
 its cilia. At maturity, openings of various kinds appear in the 
 oogonium through which the male gametes enter and one fus^^s 
 with the female (Fig. 116, A, 0). The thick-walled gametosiK>re 
 that results from this union is set free by the decay of the oogonial 
 walls and germinates after a resting period ver>' much as in the 
 
200 ADVANCED TYPE OF REPRODUCTION 
 
 case of Ulothrix. The contents of the gametospore is extruded 
 through the ruptured wall as a delicate sac and then divides, 
 forming four zoospores which produce new plants (Fig. Ii6, C-G) 
 — thus we see again an illustration of the peculiar nature of the 
 gametospore as in Spirogyra and Ulothrix. The question will 
 naturally arise as to the force that brings these and other gametes 
 together. The frequently observed aggregation or sw^arming of 
 the male gametes about the females renders probable the view 
 that some substances, as organic acids, etc., are developed in the 
 gametes that serve to draw them together whenever they come 
 within a certain distance of each other. 
 
 The consideration of Coleochaete, a plant of the same order as 
 Ulothrix, has been deferred to this point because it presents 
 several interesting departures from previous types which indi- 
 cate that a higher point has been attained in some respects by 
 this plant than by any other of the green algae. The filaments of 
 Coleochaete have a pronounced apical growth and are usually 
 associated together in a radiate manner, forming small discs or 
 cushion-like masses on the stems and leaves of water plants (Fig. 
 117). The advance of this type is indicated not only by the 
 localization of growth at definite points, but also by the formation 
 of the zoospores and gametes in definite regions, i. e., in special 
 cells which are usually located at the end of the filaments. The 
 zoospores are produced singly from such cells, and from smaller 
 pear-shaped cells single male gametes are formed. The female 
 gametes are developed singly in large flask-shaped cells, access 
 to which is afforded by an opening that appears at the end of 
 the long neck of the flask (Fig. 118, A). The gametospore de- 
 velops a cell wall and becomes enveloped by the adjoining cells 
 of the filaments (Fig. 118, 5). In this condition the winter is 
 passed and in the spring it germinates, forming neither a plant 
 like the parent type nor zoospores as in Ulothrix and Oedogonium, 
 but instead, the gametospore forms a number of cells. This 
 growth ruptures the coat of the spore and finally from each cell a 
 rather irregular zoospore (Fig. 118, c) is derived that develops 
 into a small plant. The plants thus formed multiply solely by 
 zoospores until finally, after several generations, larger plants 
 
DEVELOPMENT OF PLANTS 201 
 
 are produced that bear gametes, tlius coniplcliiiK the Hfe 
 history. 
 
 72. Noteworthy Features of the Chlorophyceae. Two features 
 in the study of green algae should he kept clearly in mind, 
 because they are closely connected with the tendencies that will 
 appear in the development of the mosses. First : The ganicto- 
 spore is essentially a dormant or resting cell that tides the life 
 of the plant over the conditions unfavorable for growth. St»cond : 
 In passing from lower to higher types, the gametosporc tends 
 to vary in its nature and possibilities of growth. At first it 
 forms directly a sexual or gamete-bearing plant as in SphaercUa 
 but in higher types, it develops a generation of zoospores from 
 which the sexual plant is derived as in Ulothrix and Oedogonium. 
 Finally in Coleochaete, we find a further advance in that^ several 
 non-motile cells are formed by the gametospore. These cells, 
 to be sure, are essentially like the zoospore because they develop 
 directly into zoospores, but this condition emphasizes the fact 
 that the nature of the gametospore is steadily departing from 
 that of the gamete-bearing plant. 
 
 This development could be summarized by sa\ing that there 
 is a tendency to separate the life history of the plant into two 
 stages ; a sexual or gametophyte generation and an asexual sporo- 
 phyte generation. These two generations follow each other in 
 the life history of all the higher plants. This peculiar relation- 
 ship is called the alternation of generations and the reason for 
 it will be more apparent when we come to study the mosses. 
 
 Class B. The Brown Algae or Piiaeopiivceak 
 
 73. General Features. — With few exceptions the Phaeoph> v v ..v 
 are marine plants. They are commonh' known as brown algae, 
 owing to the brownish pigments which conceal in a measure 
 the chlorophyll, thus producing their characteristic brown or 
 yellow color. They are of common occurrence along rocky 
 shores and attain enormous dimensions in the northern and 
 southern seas and on the Pacific Coast. As a rule, the plant 
 body is better differentiated than in the green algae and shows 
 a higher type of specialization than yet seen. It would appear 
 
202 SIMPLER BROWN ALGAE 
 
 probable that the brown algae have been derived from green 
 algae but there is no satisfactory evidence of a series of unicellu- 
 lar forms leading up to the rather complex types that are repre- 
 sentative of the simplest of the Phaeophyceae. The majority 
 of these lower forms have already reached the filamentous stage 
 and many of the higher genera exhibit a differentiation of the 
 plant body suggestive of the higher plants, as for example, a 
 definite axis with branches and leaf-like outgrowths and root- 
 like organs that anchor the plants to the substratum. Likewise, 
 the tissues of these higher types often reveal many of the features 
 already noted in the epidermis, cortex and central region of 
 terrestrial plants. The life history of the brown algae indicates 
 that they have undergone the same evolution as the Chloro- 
 ph3^ceae. For purposes of comparison we will consider only 
 two groups of the brown algae. 
 
 (a) The Simpler Brown Algae. — Ectocarpiis may be taken as 
 an example of this group. It is a filamentous branching form 
 that is almost universally distributed along the sea shore (Fig. 
 119). The zoospores are developed in certain cells of the upper 
 branches. It is important to note in the brown algae, that 
 the reproductive cells are generally confined to special parts 
 of the plant and not promiscuously developed as in the green 
 algae. This marks a decided advance in the evolution of the 
 plant. The tissues are becoming more specialized and the work 
 performed by the plant is apportioned to special cells and organs. 
 The zoospores have essentially the same structure and mode of 
 growing into new plants as noted in Ulothrix. The cilia, how- 
 ever, are laterally attached and this is a characteristic feature 
 of the zoospores of the Phaeophyceae generally (Fig. 119, A, g). 
 
 The sexual reproduction of Ectocarpus exhibits some of the 
 most instructive variations to be found in all the algae. The 
 gametes are formed in gametangia that become divided into 
 great numbers of very small cubical cells, each one of which 
 produces a single gamete (Fig. 119, B). Thus the origin of 
 the gametes is more specialized than in the case of the green 
 algae where numerous g metes were formed in a single cell. 
 You are to remember this feature of the gametangiu be ause 
 
DEVELOPMENT OF PLANTS 
 
 203 
 
 nn,e of its-Characteristics wil reappear in liryophyta Tl>e 
 tr "n£ ro. tl: ;l:e JL. ./..erentiati.. as 
 
 rangia, .f; «, zoospore <="'^;S^<'- .^^^^'^^^d by s'^' ral still .notilc g..mc.« 
 
 i. Ulo^nri. since they --^-^ ^'r^rcrildtt 
 forming the plant, or they may behave as gan 
 
204 SEXUAL REPRODUCTION OF ECTOCARPUS 
 
 form a gametospore. The nature and character of these bodies is 
 so imperfectly estabHshed that their behavior is entirely deter- 
 mined by external conditions. Low temperatures and bright 
 light tend to develop these bodies as zoospores, whereas, high 
 temperatures cause them to behave as gametes. (See Pandorina 
 and Hydrodictyon.) Other species of Ectocarpus show an ad- 
 vance over this stage. The gametes are alike, but it has been 
 observed that certain ones, the female gametes, have a shorter 
 period of motility and after coming to rest they attract the still 
 motile gametes, the males, and cause one to fuse with them 
 (Fig. 119, C, D). This is the simplest distinction that can be 
 pointed to as indicating a difference in the nature of the gametes 
 which we call sex. This variation in the period of motility of 
 the gametes must be due to an essential difference in the material 
 or substance of which they are composed, although there is no 
 external evidence of this. Certain species of this same genus 
 also reveal a variation in the size of the cells from which the 
 gametes are derived. In these species, the sex is clearly indi- 
 cated by the larger size of the female cell and it is noticeable 
 that there is a decided tendency for the smaller gametes to fuse 
 with the larger and more slowly moving females. It is also 
 worthy of note as these gametes become differentiated and their 
 sex more evident that they are less able to behave as zoospores 
 and grow directly into new plants. When the gametes produced 
 by the plant are all alike they may readily be made to grow into 
 new plants, but this is rarely the case where they can be dis- 
 tinguished as male and female. Thus, in Ectocarpus, we have 
 a most remarkable series of variations that indicate how sexuality 
 has arisen from the asexual condition and also how sex finally 
 became characterized by a shorter motile period in the female 
 gamete. The character of the female finally became more pro- 
 nounced, owing to its better nourishment and consequent increase 
 in size and slower movements. A more perfect illustration of 
 the evolution of sex is not found in nature. 
 
 The differentiation of reproductive parts, as will be frequently 
 noted, does not keep pace necessarily with the evolution of the 
 plant body. It is evident that sexuality has arisen independenlty 
 
DEVELOPMENT OF PLANTS 
 
 205 
 
 no different levels in the plant world and in groiii»> in no wise 
 cantected. We have noted the same state of sexuality in some 
 of the motile green algae, i. c, in Chlamydomonas and in non- 
 motile forms as Ilydrodidyon and Ulolhrix, and now a^ain in 
 Ectocarpiis although these plants arc widel>- sei)araled as far as 
 relationship is concerned and exhibit a marked \ariati()n in the 
 development of the plant body. It is also frecjuently to be 
 noted that plants may get along very well indeed with onl\- a 
 sexual or an asexual method of reproduction. 
 
 {h) The Coarser Brown Algae, the Kelps. — Under this head 
 we may consider two groups, illustrated by the keli)s and rock- 
 weeds. The kelps are related to Ectocarpiis and include tlie 
 largest and most highly organized forms of all the algae. Indeed 
 some of these forms are quite comparable in size with our shrubs 
 and trees. The Laminarias (Fig. 120, A) of our Atlantic coast 
 
 Fig. 120. Three of the larger brown algae: A, Laviiiiaria. B. Lcs^otm 
 
 C, Macrocystis. 
 
 have stalked blades ten to twenty feet long. The i^reat bladder 
 kelps of the Pacific, Nereocystis and Macrocystis, attain ^nwt 
 dimensions, the latter genus reaching a length of 5ck) to i><h) feet 
 and Lessonia with trunk-like stems and leaf-like segments forms 
 veritable submerged forests in the Antarctic Ocean (I-'ig. 120, 
 B, C). These highly organized plants, howe\er. do not show 
 any advance in sexual reproduction over the simplest fc^rm noted 
 in Ectocarpiis. Much attention has been directed to the kelps 
 
206 
 
 COARSER BROWN ALGAE 
 
 as a source of potassium (see page 48), from 23 to 48 per cent, 
 of their dry weight being potassium chloride. It is estimated 
 that there is between 600 and 800 sq. miles of these plants on our 
 Pacific coast and that they should yield annually one million 
 tons of potassium chloride, valued at 40 million dollars. 
 
 The rock weed or bladder wrack {Fucus) and the gulf weed 
 (Sargassum) are representatives of the second group, Fucaceae, 
 that contain the most specialized of the brown algae (Fig. 121). 
 The Sargassum with its stem and leaf-like organs which may 
 become modified into air sacs and reproductive organs bears the 
 closest external resemblance to the higher plants of any of the 
 algae (Fig. 121, A). It forms the major portion of that floating 
 
 r»^ . 
 
 Fig. 121. Two common forms of the Fucaceae: A, Sargassum, the stem- 
 like axis bearing air sacs, s, and leaf-Hke organs; g, reproductive branches. B, 
 Fucus — s, air sacs; g, reproductive branch. C, young plant. 
 
 vegetation in the Atlantic known as the Sargasso Sea. The 
 bladder wracks may be found firmly attached to the rocks by 
 disc-like holdfasts in almost all colder, temperate and northern 
 seas (Fig. 121, B). The elongation of the flat leathery stems 
 is largely localized in a terminal cell and results in a regular 
 
I 
 
 DEVELOPMENT OF PLANTS 2(>7 
 
 forking of the stem Into two equal i)arts, a niethocJ of branching 
 called dichotomy in contradistinction to the axial branching 
 characteristic of the majority of our llowcrin^^ plants. Fucus, 
 like many of the gross brown algae, contains air cavities or 
 bladders which buoy it up in the water; this feature accounts 
 for its popular name of bladder wrack. A cross secli(m of the 
 stem shows that the tissues of these plants have attained a con- 
 siderable differentiation (Fig. 122, A) as is attested bv a rudi- 
 
 FiG. 122. Structural features of Fucus: A, cross-section of a i>ortion of 
 the central stem-like part of the plant, showing an epidermal, e, cortical, 
 cr, and central region, c. B, section of one of the cavities that ai^KMrs to 
 the eye as a dot. See Fig. 121, B, g. This cavity contains only male game- 
 tangia. C, section of a cavity from another plant contains only female game- 
 tangia. — After Oltmann. 
 
 mentary epidermal, cortical and central region, the latter oltcn 
 containing well-marked sieve tubes. These elongated cells of 
 the central region promote the rapid distril)ution of materials 
 and doubtless account in part for the size obtained b>- the kelps 
 and rockweeds. 
 
208 
 
 SEXUAL REPRODUCTION OF FUCUS 
 
 The most characteristic feature of these plants, however, and 
 the one separating them sharply from the Laminarias appears 
 in their method of reproduction. None of the Fucaceae develop 
 zoospores, although an asexual reproduction may be effected by 
 the detachment of small branches from the plants. Sexual re- 
 production is a step in advance of any of the preceding types 
 of the brown algae, in that the female gamete becomes still 
 larger and loses entirely the power of motion. The reproductive 
 organs, or gametangia, are developed in specialized branches or 
 enlarged tips of the thallus. In Fucus (Fig. 121, g), these organs 
 are contained in small pits or cavities that appear as minute 
 points, or as dots when the enlarged tip of a branch is held up 
 to the light. A magnified section taken through such a branch 
 shows the nature of the cavities (Fig. 122, B, C). In some 
 species, the male and female gametangia are found in the same 
 cavity, or they may occur separately and on different plants. 
 The male gametes are developed in enormous numbers in numer- 
 ous little sacs, or antheridia, borne on branching filaments of cells 
 
 Fig. 123. One of the branching filaments from Fig. 122, B, greatly en- 
 larged. Some of the antheridia, an, have discharged the male gametes, which 
 are still retained in the inner wall of the antheridium, as at h. At a this wall 
 is ruptured, freeing the gametes. 
 
 which grow out from the sides of the cavities (Fig. 123). The 
 female gametes are produced in larger sacs or oogonia, which 
 are each supported on a single cell and associated with hair-like 
 chains of cells, paraphyses (Fig. 124, A). Usually eight gametes 
 are developed in each oogonium. The gametes when first dis- 
 charged from the antheridia and oogonia are enclosed in a delicate 
 
DEVELOPMENT OF PLANTS 
 
 209 
 
 cell wall which later ruptures and sets them free (Fig. 124, B). 
 The female gametes are without cilia and as they float away in 
 the water they appear to attract the male gametes which swarm 
 about them and finally fertilization is effected by one of the male 
 gametes working down to the nucleus of the female and fusing 
 with it (Fig. 124, C). In an allied form not only is an attractive 
 substance formed in the female gamete, but probaljh- a repellent 
 
 Fig. 124. Female gametangia: A, greatly enlarged view of one of the 
 GOgonia shown in Fig. 122, C. The oogonium is dividing and forming the 
 female gametes; p, paraphyses. B, the female gametes discharged but still 
 retained in the inner wall of the oogonium. C, greatly enlarged view of a 
 gamete which is surrounded by male gametes, some of which are seen as 
 dark bodies penetrating the cytoplasm of the female gamete. D, early stage 
 in the germination of the gametospore. See later stage. Fig. 121, C. — After 
 Thuret. 
 
 one after fertilization is effected, for Farmer observed thai the 
 male cells swarm about the female cells for a time and then sud- 
 denly swim away "like a flock of frightened birds." As soon as 
 fertilization has been effected, the gametospores become invested 
 with a cell wall and attached to the rocks. (\11 diNi-icn now 
 proceeds rapidly and soon establishes the characteristic thallus 
 of the plant (Figs. 124, D\ 121, C). Fiicus is a very jm-.U^^'- tilitir 
 14 
 
2IO THE RED ALGAE 
 
 and the reproduction process can be observed at almost any time. 
 After the plants have been exposed for several hours by the low- 
 tide the male and female gametes will often be found forming 
 orange yellow and olive green drops at the mouth of the cavities. 
 In this condition the entire process of fecundation and early stages 
 of germination can be studied by transferring a bit of these two 
 fluids to a drop of sea w^ater on a slide and studying them under a 
 microscope. The differentiation of the gametes of Fucus is an 
 interesting one because it represents the stage where the female 
 gamete has become motionless, but is not retained in the mother 
 cell as in Vaucheria and Oedogonium. 
 
 No resting spores are found among the brown algae. This 
 may be connected with the more uniform conditions that ob- 
 tain in the sea where also they are not exposed to the dangers 
 of desiccation as in fresh water forms. It is notew-orthy, that 
 although the reproductive organs are, on the w'hole, more com- 
 plex, fertilization is of a more primitive character than among 
 many of the green algae, in that the fusion of the gametes is 
 effected outside of the gametangia. 
 
 Class C. Red Algae or Rhodophyceae 
 74. General Features. — The Red Algae are largely marine 
 plants. Unlike the brown algae, they reach their greatest de- 
 velopment and abundance in the warmer waters of the temperate 
 and tropical seas and are usually found attached to various ob- 
 jects below tidal marks. Their red pigments probably adapt 
 them to the feeble illumination of the deep w-aters in w-hich they 
 generally occur. They range through a great variety of forms, 
 from delicate filaments or flattened ribbon-like bodies to struc- 
 tures wdth cylindrical axes and leaf-like branches (Fig. 125). 
 The elegant symmetry of their branching together with the deli- 
 cacy of structure and richness of coloration has always attracted 
 attention and made them the most familiar of all the algae. 
 They are popularly though inaccurately known as sea mosses. 
 
 Reproduction of the Red Algae. — The reproduction of the 
 Rhodophyceae, particularly the sexual method, presents so many 
 modifications and specializations that only the general features 
 
DEVELOPMENT OF PLANTS 211 
 
 can be pointed out. Singularly both the spores and ^^mctes 
 have lost their motility. The sexual method of rej)rodueiion is 
 the most complicated among all the algae, and e\'en in the 
 simplest form is far in advance of any nutiiod heretofore noted. 
 
 §||^>•/fe^||i.. 
 
 
 Fig. 125. Common forms of the Red Algae: A, a leaf-like form, DeUs- 
 seria. B, a branching thread-like form, Ceramium. C, a branching form 
 covered with delicate hairs, Dasya. 
 
 The female gametangium resembles a flask with a long neck and 
 is usually developed at the end of a branch (Fig. 126, .4), see 
 Coleochaete. The single female gamete is found at the base of 
 the flask. The male gametes are produced singl>' in small cells 
 also at the ends of branches and they are often closely aggregated 
 in dense clusters (Figs. 126, an\ 127, A, B). After the discharge 
 of the male gamete, they are carried b>- the currents of the 
 water to the long tube of the female organ. The wall of the tube 
 at the point of contact is now absorbed and the male gamete 
 passes down the tube and fuses with the female. The gameto- 
 spore does not germinate and produce a new plant similar to the 
 one that bore the gametes. On the other hand, it produces a 
 number of branches (Fig. 126, B, C), the terminal cells of which 
 develop as spores (Fig. 126, D). The mode of sexual repro- 
 
212 SEXUAL REPRODUCTION OF RED ALGAE 
 
 ductlon outlined above is complicated in the majority of the 
 red algae, owing to the fusion of the germinating gametospore 
 with adjacent cells that contain storage foods and from certain 
 of these storage cells the characteristic spore producing branches 
 are developed. In this way, the formation of a larger number 
 of spores is made possible by a single fecundation. Frequently 
 
 Fig. 126. Sexual reproduction in Nemalion: A, tip of branch bearing 
 female gametangium, c, and cluster of male gametangia, an, from some of 
 which the motionless male gametes are escaping. B, first division of the 
 germinating gametospore, g. C, later stage showing the early formation 
 of the branches from which the spores will be developed. D, spores forming 
 at the ends of the numerous branches. 
 
 a sac-like structure, the cystocarp, is developed about the spores 
 owing to the outgrowths of the adjacent cells (Fig. 127, C). Atten- 
 tion should be fixed upon the form of the female gametangium and 
 the sac-like structure of the cystocarp because you will see these 
 features reappear in certain groups of the fungi. 
 
 The spores formed from the growth of the gametospore are 
 finally set free and grow into plants resembling the gamete 
 bearing red algae but singularly these plants only produce 
 spores — never gametangia. These spores are formed in fours 
 in a single cell (Fig. 128) and therefore called tetraspores. These 
 spore-forming cells arise singly on the surface or in the tissues 
 
DEVELOPMENT OF PLANTS 
 
 213 
 
 Fig. 127. Features in the reproduction of Polysiphonia: A, branch of 
 the plant bearing clusters of male gametangia, an. B, one of the clusters 
 enlarged, showing the numerous small gametangia. C, branch on which the 
 spores derived from the gametospores are enveloped by a sac. From one 
 of these bodies, called cystocarps, the spores are escaping. 
 
 of the plant, or they may be associated in conspicuous groups. 
 The spores escape from the mother cell as naked bodies and 
 
 Fig. 128. A branching filament of Callithamniou, showing the spore 
 mother cells forming tctraspores. 
 
214 LIFE HISTORY OF ALGAE 
 
 finally secrete cell walls and develop into new plants. These 
 plants, however, normally bear only gametangia and not tetra- 
 spores. So in this life history we have two similar plants appear- 
 ing but they are manifestly different in nature as one produces 
 oprastetres and the other gametangia. 
 
 The red algae are of considerable economic importance. 
 Irish moss, CJiondrus, is used in the manufacture of jellies, and 
 agar-agar is obtained from several species of algae. Many tons 
 of various kinds of red algae are annually dried and consumed 
 for food in the East. The sw^allow's nest, of which you have 
 heard so much as an article of food in the Orient, is constructed 
 of algae. 
 
 75. Significant Features in Life History of the Algae. — The 
 germination of the gametospore in the Rhodophyceae is a note- 
 worthy departure in two respects. Unlike previous cases, it is 
 retained on the plant, where it is nourished during its germination 
 and grows practically as a parasite. This relation of the gameto- 
 spore to the mother plant will become more noticeable among 
 the fungi and the mosses and lead to pronounced changes in the 
 life history of the plant. In the second place, we notice that the 
 gametospore in the red algae produces a number of cells before 
 the spores are formed. In the green and brown algae it either 
 developed directly into a plant similar to the mother plant, as in 
 Vaucheria and Fucus, or zoospores w^ere first formed w^hich de- 
 veloped into a plant similar to the parent, as in Ulothrix and 
 Oedogonium. What is the significance of this variation in the 
 germination of the gametospore? Attention has been directed 
 in the study of Spirogyra and Ulothrix, p. 193, to the fact that 
 there are two phases in the life history of a plant, a spore-bearing 
 phase or generation, and a sexual phase or generation. This is 
 doubtless true of all forms characterized by a sexual reproduction. 
 On page 136 you have noticed that these two phases or genera- 
 tions are also sharply distinguishable by the number of chromo- 
 somes appearing in the nuclei of the cells, the sexual generation 
 having only half the number found in the asexual. This change 
 in the number of chromosomes occurs at two points in the life 
 history, first, when the gametes unite to form the gametospore, 
 
DEVELOPMENT OF PLANTS 
 
 215 
 
 at which time the number of chromosomes is (loul^lfd, and, 
 secondly, when the spore mother cell of the asexual Kt-neration 
 divides to form the four spores, tlie number of cliromosomes is 
 reduced by one-half. The two generations arc generally not 
 apparent in the Green and Brown Algae, because the asexual 
 generation is greatly reduced and often does not produce spores, 
 as in Vaticheria, etc., where the asexual generation consists of a 
 single cell, the gametospore. This gamctospore, however, must 
 have twice the number of chromosomes as the mother j^lant, since 
 it is formed by the union of the chromatic substance of two 
 gametes and it will doubtless be found that its first divisions in 
 germination are attended with a reduction of the chromosomes 
 to the original number found in the mother plant. This remark 
 applies also to Ulothrix and Oedogonium, where it is reasonable 
 to suppose that the formation of the four zoospores by the germi- 
 nation of the gametospore results in the reduction of the chromo- 
 somes to the number found in the mother plant. This has been 
 found to be the case by Allen in the division of the germinating 
 gametospore of Coleochaete, and Yamanouchi reports that the 
 reduction of the chromosomes in the Red Algae does not occur 
 until the mother Ciell divides to form the tetraspores (Fig. 128) 
 and the same reduction has been reported in Spirogyra in the 
 germination of the gametospore. The red algae give us the 
 best illustration of this relationship yet seen. Here the gameto- 
 spore germinates, producing a small spore-bearing plant, enclosed 
 in the cystocarp, which is parasitic upon the sexual plant. This 
 plant is a part of the asexual generation, as all its cells contain 
 the double number of chromosomes. The spores from tlie 
 cystocarp form plants externally similar to the sexual plants, 
 but these plants are really different, being characterized b\- cells 
 with the double number of chromosomes and also l)>- tiie fact 
 that they bear tetraspores. The formation of the telrasiwrcs, 
 however, is attended with a reduction of the chromosomes by 
 one-half, and these spores produce plants characterized by the 
 reduced number of chromosomes and also by ilu- tact ili.it they 
 bear the sexual organs. So in the Red Algae we have very 
 clearly brought before us the two generations in ilu- life of the 
 

 2i6 THE TRUE FUNGI 
 
 plant, the sexual generation, characterized by the reduced 
 number of chromosomes and the production of the sexual organs, 
 and the asexual generation, characterized by the double number 
 of chromosomes which appear in two spore-bearing plants, i. e., 
 the minute plant in the cystocarp and the tetraspore-bearing 
 plant. The distinction between the sexual and asexual genera- 
 tion is emphasized at this point because we see in the algae how 
 it gradually became more and more conspicuous, and in the 
 mosses and succeeding groups it will be noticed that the relation 
 of these two generations is intimately associated with the evolu- 
 tion of the higher types of plant life. 
 
 ^^rj Subdivision 5. Eumycetes or True Fungi 
 
 76. The Nature of Fungi. — The Fungi are the largest group 
 of the Thallophyta and include such familiar forms as moulds, 
 mildews, toadstools and mushrooms. The absence of chloro- 
 phyll is the most striking feature of these plants. They are 
 unable therefore to form sugar, starch and other foods from the 
 elements of the soil and air and must obtain them already manu- 
 factured. Consequently, they are either saprophytes living upon 
 decaying organic matter, or parasites preying* upon living organ- 
 isms. It is quite possible that this mode of life is responsible 
 for the disappearance of the chlorophyll. You have seen that 
 the accumulation of sugar in the chlorophyll bearing cells stops 
 their activity. So here the absorption of organic material from 
 plants and animals may have had the same result and so led to 
 the disappearance of chlorophyll. The majority of fungi procure 
 their food from decaying plant and animal matter, and in this 
 relation many of them are of the same economic importance as 
 the saprophytic bacteria. Other forms exist as parasites upon 
 living plants and animals. In this relation fungi cause almost 
 incalculable loss annually to the country and frequently crops 
 over large areas are ruined b}^ their depredations. The parasitic 
 forms gain access to the host through a wound, stoma or the 
 delicate tissues of seedlings or young parts of the plant, and in 
 other cases the fungus forms a ferment that dissolves the cell 
 walls and thus opens the way for the entrance of the pest. The 
 
DEVELOPMENT OF PLAM S 217 
 
 cultivation of plants has doubtless weakened their resistance in 
 many cases to the attack of the parasites, and like ourselves the 
 plant may inherit a weaker constitution that is more subject to 
 disease. Nearly every state now employs experts to study the 
 diseases caused by these fungi and to devise means {ot killing 
 them and to develop more resistant plants. 
 
 The fungi are of very simple structure because they li\c ujkui 
 foods already manufactured for them. There is no longer a 
 necessity for a plant body that is complex, because each part is 
 adapted to the performance of one or another of the many func- 
 tions that cooperate in the construction and distribution of the 
 organic substances. Consequently, the fungi do not exhibit 
 many of the characteristics of chlorophyll-bearing plants. Their 
 cell walls are thin and inclose a watery, colorless protoplasm in 
 which are usually dispersed many small nuclei (Fig. I2()). W'hat- 
 
 FlG. 129. A few cells from a branching filament of green mouUi, Pent- 
 cillium, showing the granular character of the cytoplasms and the absence 
 of plastids. The colorless areas in the cells, vacuoles, contain principally 
 water. 
 
 ever form the plant body may assume, it will usually be found to 
 consist of filaments of delicate cells or tubular growths, called 
 hyphae (sing, hypha) . These fine filaments or tubes are the essen- 
 tial portion of any fungus and as they spread over the substance 
 upon which they feed they form a branching and intenvovcn 
 mass of threads collectively known as the m> r(>liuni. This struc- 
 ture is well illustrated in the hyphae that sj^read over bread and 
 fruits, forming a cobwebby mass or mycelium that is commonly 
 known as mould or mildew. Bodies of ^'arious forms arise from 
 the mycelium which are often mistaken for the fungus itself, as 
 the mushroom, but this mushroom sustains the same relation to 
 
2i8 CLASSIFICATION OF FUNGI 
 
 the fungus proper (the mycelium) as does an apple to the apple 
 tree. The mushroom is simply a rather compactly interwoven 
 mass of hyphae that arise from a mycelium in the ground. 
 We shall expect to find remarkable departures in the fungi 
 from previous types because of their peculiar manner of ob- 
 taining food and also because they are largely terrestrial plants 
 and therefore exposed to a much wider series of conditions than 
 in the case of the algae. This will tend to cause variations and 
 so bring about new structures that are in harmony with the con- 
 ditions under w^hich the plants live. As a rule, these variations 
 have been so extensive and have so completely changed the char- 
 acter of the plffht as to render it impossible to state whence many 
 of the groups have been derived or what relationship they sus- 
 tain to 'one another. It may be stated, however, that the lower 
 fungi show unmistakable evidence of relationship with certain 
 of the green algae, while an intermediate class have evidently 
 branched off from the red algae. In nearly all cases it w411 be 
 seen that their parasitic and saprophytic mode of life has led to 
 reduction and degeneration. The fungi may be divided into 
 three classes: A, Phycomycetes or Alga-like Fungi; B, Ascomy- 
 cetes or Sac Fungi; C, Basidiomycetes or Basidia-bearing Fungi. 
 
 Class A. Phycomycetes 
 
 77. Alga-like Fungi. — Some of these plants show such a strik- 
 ing resemblance to certain algae, both in the structure of the 
 plant body and in their reproductive processes, that they are 
 called the Phycomycetes from phycos, alga, and myces, mould. 
 Attention will be called to three orders of this group. 
 
 78. Order a. Saprolegniales or Water Moulds. — These fungi 
 are either saprophytes, living upon dead animals and plants, 
 or parasites. In this latter relation one form causes a destructive 
 disease in fish while another genus produces the damping off and 
 decay of seedlings. They are all microscopic plants and many 
 are aquatic. Common examples of this order are seen in the 
 whitish masses that form around decaying insects in the water 
 and in fluffy or mould-like outgrowths that often appear upon 
 the bodies of fish in aquaria. The plant body consists of branch- 
 
DEVELOPMENT OF PLANTS 
 
 219 
 
 ing tubular threads Avithout partitions but conlaininj^ numerous 
 nuclei and thus resembling Vaucheria save for the absence of 
 chloroplasts (Fig. 130). The sporangia are also formed by the 
 cutting off of the tip of one of the l^ranches by a transverse walL 
 The contents of a sporangium, however, generally breaks up 
 into a very large number of biciliate zoospores (Fig. 130, C). 
 In the species that cause so much damage to fish, the spores 
 come to rest upon the fish and form tubular outgrowths that 
 readily penetrate the tissues of the fish, especially where a scale 
 has been rubbed off. Some of the hyphae also extend outward 
 
 Fig. 130. Features in the life history of the water moulds: .1, fly in- 
 fected with Saprolegnia, showing the tubular threads of the fungus radi- 
 ating out from its body. B, tip of a tube magnified, showing an early stage 
 in the formation of the sporangium. C, sporangium discharging zoosjK>res. 
 D, Reproductive organs of a related fungus, Achlya — 0, oogonium contain, 
 ing four female gametes; an, antheridium with tubes penetrating iM'igdnium 
 for transport of male gametes. 
 
 from the body, causing the mould-like blotches on the iiilroti-d 
 fish, and from the tips of these branches the sporangia mentioned 
 above are formed. Most of the species of Saprolc^ma also form 
 sexual organs as in Vaucheria, from one to sewral gametes Ix'ing 
 formed in an oogonium (Fig. 130, D). Ihe antheridia, however, 
 develop several non-motile male gametes. This peculiarit\- of 
 
220 REPRODUCTION OF A WATER MOULD 
 
 the gametes is perhaps due to their exposure to atmospheric 
 conditions, as would be the case when growing upon terrestrial 
 or floating organic matter. The lack of water for the transport 
 of the male gametes is nicely met by a tubular outgrowth of 
 the antheridium which penetrates the oogonium when it ruptures, 
 discharging the male close to the female gametes (Fig. 130, D, 
 an) . In the majority of the species, singularly enough, the female 
 gametes germinate without being fertilized. The gametospore 
 germinates as in Vaucheria. 
 
 It should be stated that, in related forms, large multicillate 
 zoospores, like those of Vaucheria, are formed and also motile 
 male gametes. It is also important to note that there are several 
 parasitic algae closely related to these alga-like fungi and this 
 remark applies to the Sac Fungi as well. Their parasitic habit 
 and partial loss of chlorophyll clearly indicate that they are in a 
 transition state from the algae to the fungi. It is therefore 
 safe to state that in this group we have evidence of the derivation 
 
 Fig. 131. Hyphae of Peronospora extending through the tissues of a plant 
 and absorbing food from the cells by means of haustoria, h. 
 
 of the fungi from the algae. This helps us to understand how 
 fungi arose. Plants and animals live upon organic materials. 
 These substances through chemical and physical reactions cause 
 the absorbing organs of the plant to grow towards them. Conse- 
 quently the green plant though capable of forming organic sub- 
 stances may through chemical stimulation be brought into con- 
 tact with these materials in other plants and animals and as a 
 result it becomes a fungus. 
 
 79. Order b. Peronosporales. Downy Mildews and White 
 Rusts. — These forms are parasites upon the higher plants espe- 
 
DEVELOPMENT OF PLANTS 221 
 
 daily and include two of llic most destructive fun^q, the ix>tato 
 and the grape vine bliglit. PJiytophthora infestans causes the 
 potato rot. This disease was widespread in the eastern Lnited 
 States in 1901, causing the entire loss of the crop in some sections. 
 It would be difficult to imagine a pest more perfectly adai)ted to a 
 destructive career. Let us begin the life history with the j^'crmi- 
 nation of a spore which has fallen upon a leaf. This spore ff)rms 
 a hypha which enters the leaf and cjuickh- spreads through its 
 tissues, sending into the cells short lateral branches called haus- 
 toria (Fig. 131), which absorb the cell contents and thus supply 
 the fungus with food. In severe cases this produces a withering 
 and decay of the leaf. The hyphae continue their growth into 
 the stems and all parts of the plant, thus causing the black dis- 
 coloration of the potato and its early decay in bad cases of in- 
 fection. This habit of many parasitic fungi of establishing them- 
 selves in those organs of the plant which live on from >ear to 
 year, is one of the most serious difficulties in combating these 
 pests. For with the renewal of growth of these organs the h>phae 
 spread and reestablish the disease. Thus the planting of infected 
 potatoes is sure to result in the appearance of the potato blight if 
 conditions are favorable. As soon as the mycelium has become 
 well established in the leaf, numerous branching hyphae extend 
 out through the stomata and form at their tips little sacs or 
 sporangia (Fig. 132, A). The formation of the sporangia is 
 effected in a few hours, when they drop off and are carried b>- the 
 wind to other plants, where they germinate at once, forming a 
 tube that penetrates the leaf and rapidly si)reads the disease. 
 If the sporangia chance to fall upon leaves that are wet b\' dew 
 or rain, the contents breaks up into several zoospores (Fig. 132, 
 B-E) which finally come to rest and develop the characteristic 
 tubular hyphae of the fungi. This behavior of the sporangium is 
 doubtless a survival of the zoospore stage seen in the algae. It 
 is equally suggestive that definite changes in tlie environment, 
 as the dry air, causes the sporangium to germinate as a non- 
 motile spore, whereas the presence of water causes the sp(»rangium 
 to produce several zoospores. The potato blight is largely 
 confined in our country to the northeastern states and usually it 
 
222 REPRODUCTION OF THE DOWNY MILDEWS 
 
 does not appear until the latter part of July. It only becomes of 
 serious importance in sultry weather, when a short period of 
 rain will result in the formation of the sporangia and whole 
 fields will be devastated in a few days. 
 
 m^.% i 
 
 Fig. 132. Fig. 133. 
 
 Fig. 132. Asexual reproduction of the mildew: A, hyphae of Plasmo- 
 para emerging from a stoma and bearing numerous sporangia. B, enlarged 
 view of sporangium of Peronospora germinating on a dry leaf. In this case 
 the sporangium behaves as a spore sending out a hypha that will penetrate 
 the tissues of the leaf. C, sporangium of Phytophthora germinating in the 
 water and forming zoospores. D, zoospore enlarged. E, zoospore has come 
 to rest and is forming a tube that will penetrate the tissues of the leaf as 
 in the case of B. 
 
 Fig. 133. Reproduction of the white rust, Albugo: A, asexual stage, 
 showing several erect hyphae forming spores. B, sexual stage which is 
 characteristic of the Peronosporales in general — o, gametangium contain- 
 ing a single female gamete which is being penetrated by tube from male 
 gametangium, an. — After Wager. 
 
 Very similar in character is the grape blight, Plasmopara viti- 
 cola. This disease causes a browning of the leaves and the stunt- 
 ing of the fruit, which become brown or .gray, and, finally, the 
 
DEVELOPMEx\T OF PLANTS 22;, 
 
 death of the infected parts. This pest caiisifl the ab.indonnKnt 
 of entire vineyards before the method of kiUin^r the fundus by 
 spraying the phmts with copper salts was chsroNCTrd. In Ixjth 
 of these pests and in aUied genera the sporangia-bearing h>phae 
 are produced in such numbers as to cause a downy, mould-Hke 
 appearance on the leaves, thus accounting for their popular 
 name. Downy Mildews. 
 
 In a related genus. Albugo, the sporangia are formed in' ilie 
 repeated cutting off of the tips of the hyphae, as shown in Eig. 
 133, A. In this way, not one, but a chain of sporangia, are 
 formed from the ends of the hyphae which do not project from 
 the leaf, but grow up in dense masses just under the epidermis, 
 producing glistening white blotches or blisters on the leaves. 
 This growth finally ruptures the epidermis wlien the spores are 
 scattered by the wind and germinate as in the preceding cases. 
 This fungus, known as white rust, is very common on mustards, 
 pigweed and other plants. 
 
 The sexual reproduction of the Peronosporales is suggestive of 
 Vaucheria. Gametangia are cut ofif from the ends of the hyphae, 
 as shown in Fig. 133, B. The male gamete gains access to the 
 female gamete, which is usually formed singly, by means of a tulx; 
 as in Saprolegnia (Fig. 133, B, an). The thick-walled gameto- 
 spore, as in many of the algae, tides the plant over the winter, and 
 being set free by the decay of the surrounding tissues it germi- 
 nates in the spring, starting anew the life of the pest. It may 
 germinate directly (see Vaucheria) into the fungus, or zoospores 
 are first produced, as in Oedogonium. 
 
 These two orders are more suggestive of the algae than an\- 
 others that we shall study and it is well to note the modifications 
 that have been induced in these plants as a result of their change 
 from aquatic to terrestrial conditions. Removed from the water, 
 special root-like organs and haustoria are evoUed for the absorp- 
 tion of foods. The absence of water brings about a lack of motil- 
 ity in the male gametes and the formation of a tube to conduct 
 them to the female. For the same reason, the zoospores arc re- 
 duced to light motionless spores that are developed ui>on elon- 
 gated hyphae that expose them to the air currents f«»r disinbuf ion. 
 
224 
 
 REPRODUCTION OF BLACKMOULDS 
 
 The ability of these spores, or sporangia, to produce zoospores is 
 doubtless the survival of a trait inherited from their algal an- 
 cestors. 
 
 80. Order c. Mucorales or Black Moulds. — These are among 
 the most common of the fungi and they are almost sure to appear 
 upon any cooked food or decaying matter that is exposed to the 
 air even for a very short time (Fig. 134). The mycelium has 
 practically the same structure as noted in the preceding groups, 
 but the black moulds have lost all motile reproductive bodies and 
 their relationship to any group of the algae is not known. Some 
 of the hyphae of the mycelium creep over the food supply and 
 send into it short branches which serve as organs of absorption 
 while other rather thicker hyphae grow away from the mycelium 
 and reach up into the air (Fig. 134). The tips of these erect 
 
 Fig. 134. Habit of growth of the black mould, Rhizopus: s, sporangia; 
 r, absorbing branches of the mycelium. 
 
 hyphae enlarge owing to the accumulation of protoplasm and 
 numerous nuclei in them, and finally become spherical, forming 
 the sporangia. The contents of the young sporangia differentiate 
 into a denser peripheral portion containing the bulk of the nuclei 
 and a more watery central and basal region. A layer of large, 
 round vacuoles now appears between these two regions and, 
 owing to the flattening out and fusion of these vacuoles, a cleft 
 is formed that ultimately separates the peripheral from the central 
 protoplasm. Next furrows advancing inward from the sporangial 
 wall and outward from the cleft divide the peripheral protoplasm 
 into numerous small portions, each part containing several nuclei. 
 These bodies round off, secrete a rather thick smoky black wall 
 
DEVELOPMENT OF PLANTS 225 
 
 and so become spores (Fig. 135). The numerous sporan^^a filled 
 with dark spores are the principal cause of the black color of these 
 fungi. 
 
 During the development of the spores a wall is constructed 
 over the central protoplasm, thus often forming a dome-like 
 structure in the sporangium known as the columella (Fig. 135, E). 
 The walls of the sporangia (save at the region in contact with 
 the stalk) readily dissolve in the presence of moisture, owing 
 to their mucilaginous character, and thus allow the spores to 
 float off in the air as an invisible dust. The spores will germi- 
 nate at once and produce a new plant under suitable conditions 
 of moisture and food, or they will retain their vitality for months 
 if kept dry. As the spores are disseminated, the dome-like struc- 
 
 FlG. 135. A, B, early stages in the development of the sporangium of 
 Rhizopus. C, the formation of the vacuoles which separate the denser peri- 
 pheral protoplasm from the more watery central region. D, the vacuoles 
 are flattening out and the denser protoplasm is becoming separated into 
 multinucleate segment, E, the multinucleate segments are rounding off to 
 form the spores ai^d a wall has formed over the columella, f, the sporan- 
 gium has ruptured, permitting the scattering of the spores and the columella 
 has formed an umbrella-like structure owing to the loss of its water)- con- 
 tents. — After Swingle. 
 
 ture in the sporangium relaxes, owing to the loss of its waier>' 
 contents, and assumes an umbrella shape, as seen in h'ig. 135. F. 
 You can readily understand why these black moulds are so com- 
 mon by counting the number of sporangia on a small bit of myce- 
 lium and then estimating the number of spores in a sporangium. 
 So numerous and light are the spores that they are carried every- 
 where. It is only necessary to expose a bit of moist bread for a 
 few moments to the air and then enclose it in a damj) chamlxr to 
 secure a luxuriant crop of these plants. An interesting variation 
 
226 
 
 SPORANGIUM OF PHILOBOLUS 
 
 in this mode of scattering the spores is seen in a related form, 
 Piloholus, which is of common occurrence upon horse dung. 
 Here the wall of the sporangium, unlike Rhizopus, is quite firm 
 and becomes mucilaginous only at the point of contact with the 
 stalk that bears it. Owing to the accumulation of water in the 
 stalk such a pressure is finally set up as to rupture it at the 
 mucilaginous point and so the sporangium with its contained 
 spores is hurled considerable distance — often quite one meter. 
 The sporangia bearing stalks are also sensitive to light. If 
 a glass jar covered with black paper so as to exclude all light is 
 
 Fig. 136. Sexual reproduction in the black mould: A, the meeting of 
 the tips of two hyphae. B, later stage, the lower part of the figure shows 
 the cutting off of the tips by transverse walls and in the upper part of the 
 figure the fusion of the contents of the two gametangia thus formed has be- 
 gun. C, mature gametospore. 
 
 placed over a colony of these plants and a minute opening is now 
 made in the paper, you will be surprised to note with what 
 accuracy each sporangium is shot off and hits this opening. 
 The results are the same no matter where you make the opening. 
 Under certain conditions the sexual method of reproduction is 
 effected by the union of two club-shaped hyphae, as shown in 
 Fig. 136, A. As these hyphae meet, the tip of each branch is 
 cut off by a wall and the contents of the two tips fuse, forming a 
 
DEVELOPMENT OF PLAN'IS 
 
 227 
 
 thick-walled gamctospore (Fig. 136, B, C). After a resting 
 period, this gametospore grows directly into a new mycelium. 
 In some of the black moulds Blakeslee has made known that 
 reproduction is only effected by the union of ]i\j)h;i(* frf)m 
 difTerent plants which must difTcr therefore in their nature, just 
 as you found to be the case in some species of Spirogyra. 
 
 81. A Fly Fungus. — An interesting form allied to the nmcors 
 is seen in the parasitic fungus Empiisa, that produrcs an epi- 
 
 FiG. 137. A fungus, Empusa, parasitic upon Hies: .1, lly surrounded hy 
 a mass of discharged sporangia. B, enlarged view of several h>phae, sh«>wing 
 the discharge of the sporangia which are surrounded by a mucilaginous sub- 
 stance. — After Brefeld. 
 
 demic among flies at certain seasons when tlie\- may be m-cu dead 
 and clinging to the woodwork and window panes surroundeii by 
 a white halo (Fig. 137, A). This appearance is caused by the 
 discharge of numerous white sporangia that are found at the 
 ends of the hyphac that project from all sides of the fly's Ixxiy 
 (Fig. 137, B). If, In- chance, a i\y sliould rome within r.uigc 
 
228 THE SAC FUNGI 
 
 of these discharging sporangia, it would become infested and 
 later, branches of the hyphae would project from its body and 
 repeat the process of sporangia formation. 
 
 Class B. Ascomycetes or Sac Fungi 
 82. General Characters. — The Ascomycetes are the largest and 
 most variable group of the fungi. They are illustrated by the 
 powdery mildews which affect the leaves of a variety of plants; 
 the brown and blue moulds that occur on preserves, decaying 
 fruit, old leather, etc. (Fig. 138); the cup fungi and morels 
 (Figs. 139, 141); the black knot of the cherry and plum trees 
 (Fig. 149), and the ergot of rye, etc. 
 
 The plant body or mycelium resembles in structure that of 
 preceding orders, but its hyphae are composed of numerous cells 
 instead of being tubular or with few cross walls as in the Phyco- 
 mycetes (Fig. 138). The sporangia (Fig. 152) are reduced in 
 
 Fig. 138. The green mould, Penicillium, one of the most common of the 
 Sac Fungi. The hyphae of the branching mycelium is composed of cells 
 and the spores or conidia are formed in chains that are arranged in brush- 
 like clusters at the ends of the erect hyphae. 
 
 size and contain but a single spore each, which is never dis- 
 charged from the sporangium. This body will be referred to 
 in the future as a spore or conidium (plu. conidia). 
 
 The gametangia are suggestive of those noted in the red algae 
 (Fig. 140), though in many forms they appear to be greatly re- 
 duced or even lacking. The fusion of the gametes results in the 
 formation of a gametospore which germinates at once, forming in 
 
DEVELOPMENT OF PLANTS 
 
 229 
 
 the simplest cases one or more sacs. These bodies arc known as 
 asci (sing, ascus) and they contain more frequently eight spores, 
 called ascospores (Fig. 160), wliich must he distinguished from 
 the spores or conidia mentioned above. The gametospore germi- 
 nates as a parasite on the mother plant as in the red algae. 
 The sexual process results, in the majority of forms, not only 
 in the formation of asci, but numerous ]iyi:)hae from the my- 
 celium are also stimulated to growth and become associated 
 with the asci in various ways, recalling the cystocarps of the red 
 algae. As a result fruit bodies of various forms, called ascocarps 
 or perithecia, are developed which often become the conspicuous 
 part of the fungus (Figs. 155-158). The ascospores germinate 
 immediately or after a period of rest and form a new mycelium. 
 It should be stated that the ascocarps have not been connected, 
 in many cases, with sexual processes, and it is inferred that sexu- 
 ality has been lost along wuth other characters as a consequence 
 of the degeneracy resulting from parasitic and saprophytic 
 habits. Only a few of the more important orders will be dis- 
 cussed. 
 
 83. Order a. Pezizales or the Cup Fungi. — These plants are 
 characterized by the formation of flesh}-, leathery or gelatinous 
 
 Fig. 139. One of the common cup fungi, order Pezizales, with broadly 
 opened ascocarps. Common upon rich hunuis soil and decaying wood. 
 
 cup-like ascocarps that range in size from mere specks to forms 
 four or five inches in diameter (Fig. 139). The mycelium lives 
 upon the humus in the ground or on decaying j)lants and ap- 
 parently in this and the next order frcc|uently develops the asco- 
 carps directly without a reproductive process. In Pyronema 
 and many other forms, however, Dodge has shown that reprodur- 
 
230 
 
 REPRODUCTION OF THE PEZIZALES 
 
 tive organs are formed that are very suggestive of those of the 
 red algae. The female gametangium in the simpler forms has 
 an enlarged basal region with a long tubular outgrowth sug- 
 gestive of Nemalion (Fig. 140). In the majority of forms this 
 organ consists of several cells and often becomes coiled and 
 greatly extended. The male gametangium is a somewhat 
 enlarged hypha. These organs are developed here and there on 
 the mycelium or several may be associated in a compact group. 
 
 Fig. 140. Sexual reproduction of one of the cup fungi, Pyronema: A, 
 the flask-shaped female gametangium, 0, is seen in various stages of fusion 
 with the male gametangium, an. This is effected by the curvature of the tu- 
 bular outgrowth. B, the gametospores, g, germinating and forming branching 
 hyphae which bear the asci, as. These asci are associated with hyphae or 
 paraphyses, p, that arise from the mycelium. The asci and paraphyses 
 constitute the hymenium. — After Harper. 
 
 Singularly the union between these two sex organs is effected 
 by the female — its tubular part curves towards the male game- 
 tangium and effects a fusion with it. As to whether the male 
 gametes pass over and unite with the females as a result of the 
 fusion of the gametangia no general statement can be made 
 at present, though in some forms there is evidence to indicate 
 that such in the case. The basal portion of the female game- 
 tangium, or one of its cells in cases where the gametangium con- 
 sists of several cells, behaves as though fertilization had been 
 
DEVELOPMENT OF PLANTS 2^1 
 
 effected and a gametospore had been fornicd. ii ^aTininaies by 
 developing a number of erect sac-like hyphae or asci (Fi^^ 140, B). 
 Adjoining hyphae grow up among the asci and around them, 
 forming a cup-like structure resembling that shown in Fig. 1.^9. 
 A section through one of these cups reveals the asci intermingled 
 with hyphae, also called paraphyses, in the form of a layer or 
 stratum. Such an association of spore-bearing organs and para- 
 physes is called a hymenium. The spores are discharged to con- 
 siderable distances, owing to the accumulation oi fluids in the 
 asci which finally rupture at their aj^ices. Thcx ;ire fornu-d 
 in such enormous numbers that under favorable conditions they 
 may be seen passing off as faint puffs of smoke. The bright 
 scarlet cups of Sarcoscypha, common in the spring on decaying 
 sticks and the gray cups of Peziza growing upon the ground 
 and decaying wood, arc familiar examples of the order. 
 
 84. Relationship of the Cup Fungi. — The Pezizales may be 
 regarded as the typical representatives of the Ascomycetes. 
 The sexual reproductive process as outlined above is fairly 
 representative of the other groups of ascomycetes and will not 
 be referred to again except in orders where noteworthy departures 
 occur. The formation of spores or conidia are characteristic 
 features of many of the groups but owing to the exceedingly 
 diverse character and origin of these bodies attention will only 
 be directed to some of the more familiar and common examples 
 as illustrated in the two last orders. Diverging from these typi- 
 cal ascomycetes are several lines of fungi that show varying de- 
 grees of relationship to them. As an example of this divergence 
 attention may first be directed to the Helvellales. 
 
 85. Order b. Helvellales. — These are fleshy forms like the 
 cup fungi and grow upon the ground, but the reproductive proce*^ 
 results in the development of oddh' shaped ascoccu-jis witii the 
 hymenium developed upon the upper exposed surface. Several 
 common forms of these ascocarps are illustrated in Fig. 141. 
 
 The morel or Morchella, in which the hymenium is spread over 
 the pitted or honey-combed surface of tlie ascocarp, is one of 
 the most highly prized of the edible fungi. The meml)ers of 
 this order are largely saprophytic and often attain considerable 
 
232 THE ASCO-LICHENS 
 
 size, forms of Morchella occasionally reaching the height of a 
 foot and some species of Gyromitra weigh over a pound. 
 
 86. The Asco-lichens. — A second line of departure from the 
 Pezizales includes a large group of plants known as the lichen. 
 The great majority of these forms show strong evidence of rela- 
 
 
 Fig. 141. Common forms of the Helvellales: A, the morel, Morchella, 
 surface view at left and in section at right. The asci and paraphyses form 
 a hymenium over the honeycomb surface. B, Leotia, a small gelatinous 
 form of a light, bluish-green color. C, Geoglossum, a black tongue-like form. 
 In B and C the hymenium is confined to the upper enlarged portion of the 
 fungus. Both are common in boggy ground. 
 
 tionship with the cup fungi in their reproductive processes and 
 it should be added that the sex organs are more suggestive of 
 the red algae than in any other group. A few species belong to 
 the third class of fungi, the basidiomycetes, but these plants show 
 the same general features as the asco-lichens and therefore a 
 consideration of the entire group may be taken up at this point. 
 These remarkable plants are of almost universal distribution upon 
 tree trunks, rocks, old fences and buildings, and upon the bare 
 earth, where they form variously colored incrustations or leaf- 
 like branching bodies (Fig. 142). 
 
 The lichen is one of the most extraordinary plants in the 
 vegetable kingdom, since it is a union of two separate plants, a 
 fungus and an alga. Naturally the relationship of the lichens 
 to other groups of plants has been a matter of dispute, some 
 
DEVELOPMENT OI- PLANTS 
 
 233 
 
 regarding them as constituting an independcni division, and by 
 others they are looked upon as fungi. The fart that ilic fungus 
 forms the bulk of the lichen and lives ui)on ilie alga somewhat 
 after the manner of a parasite and is usually alone capable of 
 forming reproductive spore bodies would lead to the latter 
 
 Fig. 142. Common species of Lichens: A, an erect branching form, 
 Cladonia — as, ascocarps. B, a foliaccous lichen, Sticta. C, Parmelia spread- 
 ing over the bark of tree. The centrally-placed ascocarps are surrounded 
 by smaller pycnidia. 
 
 position. However, all degrees of relationship appear between 
 the alga and fungus — from stages where their association is of 
 little or no consequence to stages where their existance is abso- 
 lutely dependent upon their association. The members of this 
 curious co-partnership are largely ascomycetes and blue-green 
 algae. Numerous species in each of these two groups of plants 
 have become accustomed to living together. The formation of 
 a lichen comes about through the attachment of a hypha of some 
 fungus to one or more algal cells, as shown in Fig. 143, B. In 
 this way, the food manufactured by the algae is absorbed os- 
 motically by the hyphae. In some cases, the fungus obtains 
 its food by means of haustoria which penetrate the algal cells. 
 By the continued growth of the fungus and algae there finally 
 results an interwoven mass of hyphae about the algae. The 
 walls of the fungus are sufficiently transparent to permit the 
 entrance of light and the color of the green algae can readily 
 be seen when the lichen is moistened. 
 
 (o) Structure of the Lichen.— The bulk of the lichen is more 
 usually composed of hyphae which show <-Mn<ul.r .l.h- ni-nlanf v 
 
234 STRUCTURE OF LICHENS 
 
 in their growth and the majority of the algae also are usually 
 confined to a definite zone near the sunned surface of the lichen. 
 Thus, in Fig. 143, A, which represents a cross-section of a lichen, 
 it will be seen that the fungus forms a rather firm layer at the 
 top and bottom of the thallus, while the algae are distributed near 
 the upper surface, where they are exposed to the light, and can 
 therefore carry on photosynthesis. Numerous hyphae projecting 
 from the under surface of the lichen serve to anchor it to the 
 substratum and also assist, doubtless, in the absorption of the 
 earth substances. In some of the gelatinous lichens the fungi 
 and algae are more promiscuously arranged. 
 
 While the fungus is dependent lipon the foods manufactured 
 by the algae, the latter are also benefited to an extent by this 
 arrangement since they are protected by the strata of hyphae, and 
 they are also provided with water and crude material which the 
 fungus readily takes up and holds. This mutually helpful rela- 
 tionship of two organisms is a form of symbiosis, termed com- 
 mensalism. The strange feature about this co-partnership is the 
 marked change produced in the nature of the two symbionts. As 
 long as they are independent of each other, very special conditions 
 are necessary for their welfare, but associated, they form the most 
 resistant plants known. This accounts for their distribution from 
 the equator to the pole and their association upon crystalline 
 rocks, baked earth, bark of trees and other places where no other 
 plant life is possible. Under unfavorable conditions the lichen 
 becomes dry and brittle, in which resting condition it is able to 
 meet any extreme temperature and drought. With the return of 
 suitable moisture and heat they become leathery or gelatinous 
 and renew their growth with considerable rapidity. Thus they 
 live on from year to year, but owing to the exposed places in which 
 they are usually found, their growing periods are frequently very 
 short and their total annual growth may not exceed a few milli- 
 meters — see soils, p. 50. 
 
 {b) Reproduction of Lichens. — Reproduction of the lichens is 
 brought about by means of fragments that are readily detached 
 when the lichens are dry and brittle, or by means of soredia. 
 These latter bodies consist of a few algal cells intertwined with 
 
DEVELOPMENT OF PLANTS 
 
 235 
 
 hyphae, in fact, a miniature lichen (Fig. 143, C). In some sfK-cies 
 the soredia form a rather powdery or granular coaling on the 
 upper surface of the lichen, and in other cases they are develo|)ed 
 within the lichen. These bodies are easil>' scattered hy the 
 wind when the lichens are dr>' and under favorable conditions 
 grow into new lichens. Reproduction is also affected by means of 
 ascospores that are developed as in tlie cup fungi. The frinaie 
 
 Fig. 143. 
 
 Fig. 144. 
 
 Fig. 143. Structure of the Lichen: A, section of a lichen, showing the 
 compact arrangement of the hyphae at the top and bottom, also the anchor- 
 ing fungal threads on the underside and the dark algal cells, a, near the top. 
 B, enlarged view of the algae to show their relation to the hyphae. C, dia- 
 gram of one of the powdery particles, sorodium, appearing upon certain 
 lichens, showing the hyphae and algae. These bodies are scattered by the 
 wind and form new lichens. 
 
 Fig. 144. Sexual reproduction of the lichen: A, section of an ast-ocarp, 
 the hymenium appearing as a dark band in the mouth of the cup. B, en- 
 larged view of the asci, a, and paraphyses, p, of the hymenium. 
 
 gametangium is a coiled organ consisting of many cells as in 
 some Pezizales but the male gametes are developed in ascnearp- 
 like bodies where they are formed at the ends of numerous 
 minute hyphae— in their origin, therefore, being strikingly sug- 
 gestive of the red algae. In one form, ic should be added that a 
 fusion between male and female gametangia has been n-tw.n.-d 
 
236 IMPORTANCE OF LICHENS 
 
 The ascocarps derived from this reproductive process assume a 
 great variety of shapes (Fig. 142) but they all show the hymenial 
 layer of asci and paraphyses (Fig. 144) or of basidia and para- 
 physes in the few forms belonging to basidio-lichens. 
 
 The ascospores germinate readily, producing hyphae which, 
 however, soon perish unless they chance to meet an alga with 
 which they can live. It should be noted that these fungi appear- 
 ing in lichens cannot associate indiscriminately with any species 
 of algae. Each species of fungus Is adapted to one or more species 
 of algae and is unable to live with any other form. As may be 
 imagined, the real nature of the lichen was for a long time mis- 
 understood, the algae even being looked upon as spore bodies. 
 The bitter dispute over the question was finally settled when a 
 lichen was produced artificially by bringing together a suitable 
 fungus and alga. 
 
 (c) Economic Value. — Lichens are of considerable economic 
 importance in the world. Their service in hastening the decay 
 and transformation of rock material has already been referred to. 
 In many sections they form the characteristic vegetation of the 
 country, as In certain alpine and desert regions and in the north- 
 ern barrens, where they ajfford rich pasturage to reindeer and 
 caribou. Their abundant growth in mountainous districts and 
 subsequent distribution by winds and rains accounts for the 
 showers of manna in biblical history. Certain species are still 
 used in northern regions to lengthen out a meager food supply. 
 Litmus, employed in acid testing, and various pigments are 
 derived from lichens, as were also the famous purple and blue 
 dyes of the East. 
 
 87. A Third Line Possibly Related to the Pezizales. — The 
 following orders may not represent a single line of descent but 
 there are certain features in their life history indicating that 
 they may represent a line of departure from the Pezizales. 
 These forms are mostly minute; parasitic, or saprophytic upon 
 dead plants. The more epiphytic position of these forms may 
 have been a factor in the reduction in size. Certainly the 
 lar.ger fleshy forms of the preceding orders are terrestrial In 
 habitat. There are two quite distinct groups In this alliance, 
 
DEVELOPMEXT OF PLAN TS 
 
 237 
 
 the one characterized by ascocarps that are only shj^hily ojK'ned 
 so that the asci are not as freely exposed to the air as in the 
 preceding orders and the other with closed ascocarps so that the 
 asci are exposed only on the decay of the fruit IxkIn. The 
 
 D ^ 
 
 Fig. 145. ii<'- u*'- 
 
 Fig. 145. Head of rye infested by the parasite, Clax'iccps, which has 
 transformed several of the grains into black masses of myci-lium known as 
 sclerotia. 
 
 Fig. 146. Various phases in the Hfe history of Claviceps: A, a young 
 grain or pistil infested with the parasite. B, enlarged view of the mycelium 
 as it appears on the surface of the pistil, showing the formation of numerous 
 spores. C, the hyphae of a sclerotium growing out and forming several 
 purplish stalks, each capped with knob-like clusters of ascocarps. D, en- 
 larged sectional view of one of these knobs, showing numerous ascocarps 
 on the periphery. E, one of the ascocarps enlarged, as aaci. P an ascus 
 containing eight thread-like ascospores. 
 
238 LIFE HISTORY OF CLAVICEPS 
 
 first group is further characterized by the fact that the ascocarps 
 are frequently associated with a more or less conspicuous growth 
 of the mycelium in which the ascocarps are imbedded in varying 
 degrees. This growth is known as the stroma and it may form 
 a rather compact mass of hyphae. Only two orders can be con- 
 sidered in each of these two groups. 
 
 88. Order c. Hypocreales. — These fungi are distinguished by 
 their rather fleshy or membranous ascocarps and stroma, which 
 range in color from white to yellow, purple, scarlet and brown. 
 Numerous species are saprophytic while others are parasitic 
 upon higher plants, fungi and insects. Claviceps is a common 
 example of this group, causing the disease known as ergot in the 
 flowers of rye and other grasses (Fig. 145). The mycelium at 
 first spreads over the outer part of the pistil, rapidly forming 
 spores (Fig. 146, A, B) and exuding a sweet slimy juice, honey 
 dew, which is eagerly eaten by flies. In this way the spores are 
 carried away to infest other plants. The mycelium finally 
 completely absorbs the substance of the grain and grows into a 
 hard blue-black body several times larger than the grain (Fig. 
 145). This body, known as the sclerotium, remains dormant 
 during the winter and in the spring groups of hyphae at various 
 points in the sclerotium grow out into rose-colored stalks that 
 terminate in globular heads (Fig. 146, C). These stalked bodies 
 represent the stromata as it appears in this genus. Numerous 
 ascocarps with minute openings are developed in the outer part 
 of globular head of the stroma (Fig. 146, D). The ascospores 
 germinate in the spring and infest the flow^ers of the grain. The 
 Chinese wonder, Cordyceps, is a related parasite that attacks 
 caterpillars, larvae of beetles and truffles. In the case of the 
 forms living upon insects, the parasite does not usually appear 
 until the spring, when they are in the pupa or cocoon stage. 
 At this time, the mycelium which flourishes in the tissues of the 
 host, sends up club-like bodies (Fig. 147), that bear the ascocarps 
 as in the case of the ergot. Among the important pests included 
 in this order are those causing the wilt disease of cotton, cowpea 
 and watermelon; the disease on currant, apple and pear trees; 
 nad the claviceps of grain. 
 
DEVELOPMENT OF PLANTS 239 
 
 89. Order d. Sphaeriales or Black Fungi.— This is ilic largest 
 group of the ascomycetes, over 2,000 sjK'cies being known in the 
 United States alone. Scarcely a fallen twig or bit of old wcxxi 
 can be examined without rcvealiniL; llu- luinule ascocarps wiiieh 
 
 ■"««=>, 
 
 Fig. 147. The stroma of Cordyceps emerging from the pupa of a moth 
 and forming a club-like organ with numerous ascocarps, as, in its apical 
 region. 
 
 more commonly are hard and black in contradistinction to the 
 Hypocreales (Figs. 148; 151, A). Many of these fungi are quite 
 conspicuous since the ascocarps are formed in large compact 
 masses and also because they are often associated with a more or 
 less conspicuous stroma (Figs. 149; 151, D). The majority of 
 the genera are saprophytic upon dead and decaying vegetation, 
 though some of them are destructive parasites. The black knot, 
 Plowrightia, the cause of a serious disease to plum and cherry 
 trees, illustrates very w^ell the characteristics of this order. The 
 mycelium grows in the cambium and cortical regions of the 
 branches, causing the bark to split open in the spring when spore 
 bearing hyphae extend up into the air forming a velvety coating 
 (Fig. 149, c). By the approach of winter, this mycelium has 
 grown into the familiar black knotty mass in which are de- 
 veloped numerous ascocarp-like bodies (Fig. 150). Tlu" six>rcs 
 from these ascocarps are carried by the wind in the early spring 
 to other branches and probably infest the budding trees. Other 
 conspicuous forms are Xylaria and Daldinia, which de\elop an 
 extensive stroma on stumps and trees that contains numerous 
 ascocarps (Fig. 151). In Hypoxylofi, the stroma containing the 
 
240 
 
 FORMS OF THE BLACK FUNGI 
 
 ascocarps breaks through the bark of a large variety of trees 
 and shrubs in the form of spherical or cake-like masses (Fig. 
 148). Among the more serious pests may be mentioned the very 
 destructive black rot of grapes, apple and pear scab; bitter rot 
 of apples; the s3xamore blight, and the chestnut bark disease. 
 
 ^U 
 
 ^. 
 
 Fig. 149. 
 
 A 
 
 Fig. 150. 
 
 Fig. 148. A common black fungus, Hypoxylon: A, habit of the fungus 
 as it appears on dead branches and logs. The round black bodies are an 
 association of the mycelium, stroma, and numerous ascocarps. B, a single 
 ascus enlarged, showing character of the ascospores. 
 
 Fig. 149. The black knot. Plowrightia, infecting a branch of cherry. At 
 the bottom of the branch is shown the early summer or spore-bearing stage, 
 c, and above a black warty mass of ascocarps, as, produced the previous 
 season. 
 
 Fig. 150. A, several ascocarps enlarged, taken from region, as, in Fig. 
 149, B, diagram of an ascocarp as seen in section, showing the asci and the 
 opening for the escape of the ascospores. 
 
 This and the following order represent possibly the consumma- 
 tion of those tendencies that we saw appearing in the third line 
 of departure from the Pezizales. As stated they are characterized 
 by their closed ascocarps. 
 
 90. Order e. Aspergillales or Blue-green and Brown Fungi. — 
 The Aspergillales includes perhaps the most widely distributed 
 and familiar examples of the fungi. They occur as blue-green or 
 brown moulds upon almost any organic matter, forming a deli- 
 cate mycelium from which are developed numerous erect hyphae. 
 
DEVELOPMENT OF PLANTS 
 
 241 
 
 In the common blue mould, Penicillium, these erect In i)hae are 
 broom like (Fig. 152, B) and the spores are formed from the 
 tips of the branches vary much after the manner of the budding 
 of the yeast cells. The tip of a branch buds out into a spherical 
 cell that is finally cut off from the stalk, thus forming the siKjre 
 (Fig. 152, C). This process is repeated again and again just 
 
 Fig. 151. Other common forms of the Spheriales: .1, habit of Ilysterio- 
 graphium, on a dead twig. B, ascocarps enlarged. C, ascus enlarged, show- 
 ing character of ascosporos. D, Daldinia. E, section of the same, showing 
 that the stroma forms a concentric stratum of ascocarps, as, each year. F, 
 Xylaria. G, the same with branch cut off to show the layer of ascocarps 
 on the periphery of the stroma. 
 
 below each successive spore and in this way chains of spores 
 are formed. It will be noticed that many single spores are found 
 instead of a large sporangium which contains many spores as 
 in the case of Mucor. These spores are sometimes regarded 
 as sporangia which have become reduced in size and contain but 
 a single spore. In Aspergillus, a fungus common ujjon preserves 
 and upon herbarium plants that have not been sutTiciently dried, 
 the spores are developed from very short branches that arise 
 from a bulbous swelling at the apex of the eroci h> phae (F*ig. 
 152, A). The color of the spores, as in Miicor, is the cause of 
 the characteristic blue or brown color of the funei. The^e ex- 
 16 
 
242 SEXUAL REPRODUCTION OF ASPERGILLALES 
 
 tremely small and numerous spores float off in the air as an 
 invisible dust and quickly germinate under favorable conditions, 
 forming new mycelia. 
 
 The male and female gametangia have departed considerably 
 from the form shown in the cup fungi. They now appear as 
 short, slightly modified branches of the hyphae and have lost 
 the tubular outgrowth so characteristic of the Pezizales and red 
 
 Fig. 152. Formation of spores or conidia in the Aspergillales: A, spore- 
 bearing hypha of Aspergillus. B, hypha of green mould, Penicillium. C, 
 one of the terminal branches of B enlarged, showing manner of spore formation. 
 
 algae. However they become closely intertwined (Fig. 153) 
 and the ends of the branches meet, thus the mingling of the 
 gametes is made possible through a dissolution of the separating 
 walls at the tips of the gametangia. The resulting gametospore 
 remains attached to the parent plant and germinates at once, 
 forming an irregular hyphal outgrowth (Fig. 153, B, s) which 
 becomes completely overgrown by the hyphae of the mycelium 
 (Fig. 153, B, C). Thus is formed a solid ascocarp or perithecium 
 (plu. perithecia), that appears to the eye as a minute grain of 
 sand. During this growth numerous lateral branches arise on 
 the hyphae derived from the gametospore and become trans- 
 
DEVELOPMENT OF PLANTS 
 
 'M3 
 
 formed into asci as shown in Fig. 154, A, B. The ascocarps 
 finally decay and set free the ascospores which develop a new 
 plant or mycelium. Thus the entire life history in these forms 
 as in other fungi is suggestive of the algae. While conditions are 
 
 Fig. 153. Sexual reproduction of the Aspergillalcs and the formation of 
 the ascocarp or perithecium: A, meeting of the male, an, and female, 0, 
 gametangia. B, early stage in the development of the ascocarp. The gamet- 
 ospore has formed a series of branches, s, which are being surrounded by hyphae 
 (unshaded in the figure) from the mycelium. C, later stage seen in section. 
 The gametospore has formed a much-branched body, s, which is surrounded 
 by a closely interwoven mass of hyphae that appear in section as cells. — 
 After Brefeld. 
 
 favorable for growth these plants are rapidh' nuihiplied .ind dis- 
 seminated by conidia. Changed conditions cause the develop- 
 ment of sexual organs and the consequent formation of the game- 
 tospore. The gametospore germinates at once, living on the 
 parent plant like a parasite just as in the case of the red algae. 
 Note also another resemblance, the gametospore first develops a 
 mass of cells, in this instance of a hyphal character, on which 
 later the spore-containing asci arise as lateral branches. 
 
 The truffles are a curious group of related fun.ui th.it li\e for 
 the most part entirely under ground. The niNcelium of many 
 forms is supposed to live in contact with the roots of oaks and 
 other trees as a mycorhiza. The fleshy tuber-like ascocarp, often 
 as large as a walnut, is a higliK- prized delicacy in EurojX' where 
 dogs and pigs are trained to locate the truffles by smell. This 
 industry amounts to more than f 1,000,000 annually in France 
 and Italy. 
 
244 
 
 THE POWDERY MILDEWS 
 
 91. Order f. Perisporiales or Powdery Mildews. — This name 
 is given to a common and widely distributed group of largely 
 parasitic fungi that form cobwebby mycelia on the under surface 
 of the leaves of the elm, maple, willow, lilac, rhododendron 
 
 Fig. 154. Further development of the ascocarp: A, sectional view, show- 
 ing the branches, s, derived from the germinating gametospore, that are 
 forming numerous lateral branchlets. B, one of the branchlets enlarged, 
 showing how it divides into cells which round off, forming the asci, as. C, 
 ascospore. D, germinating ascospore. — After Brefeld. 
 
 Virginia creeper, etc. (Fig. 155), and a great variety of herba- 
 ceous plants, as the dandelion and cocklebur. The majority of 
 them do not seriously interfere with the health of the plant, but 
 others produce serious diseases in young cherry and plum trees, 
 hop vines, gooseberry, etc. These parasites are external and 
 obtain their food by means of short branches, haustoria, which 
 dissolve the cell wall and absorb the cell contents as shown in 
 Fig. 156, h. The members of this order are of special interest 
 in that they throw considerable light upon the nature of para- 
 sitism. Frequently one and the same form will grow upon a 
 wide variety of plants, just as we will see to be the case in another 
 group, i. e., the rusts. Erysiphe graminis, for example, is found 
 upon barley, wheat, rye, oats and several genera of grasses. 
 But the form growing upon any one of these kinds of plants will 
 
DEVELOPMENT OF PLANTS 
 
 -M5 
 
 not infest any of the others. It would seem iluit tlie jiarasite 
 becomes changed and specialized, although there is no visible evi- 
 dence of this, and so after a time is able to live upon but one 
 kind of plant. These specialized forms are termed biological 
 species, in contradistinction to the general or morphological 
 species which include them all. Evidently tlu-n- is something 
 within the plant that not only attracts these parasites, but also 
 changes them, so that after a time they ran nnl\ live upon the 
 
 yC 
 
 Fig. 155. Fig. 156. 
 
 Fig. 155. Appearance of one of the powdery mildews, Uncinula, on leaf 
 or elm. 
 
 Fig. 156. Enlarged view of the myceUum, ascocarp, etc., of one of the 
 mildews, Erysiphe: c, erect hyphae forming spores or conidia; /;, hausloria 
 penetrating epidermis of leaf; a, ascocarp or perithecium. 
 
 plant having these substances. If this material is absent from 
 the plant then it is immune and it has been shown in a few cases 
 where individual plants were not subject to a plant disease that 
 this was due to the lack of a substance which the infest(xl jilants 
 had or to the presence of a new substance which was reiK'Ucnt to 
 the parasite. The relation between parasite and host is strikingly 
 brought out by Massee's experiment, in which he claims that a 
 purely saprophytic fungus was induced to become a destructive 
 parasite upon the leaves of a species of Begonia b>- injecting the 
 leaves with a sugar solution. The fungus flourished u\kh\ the 
 leaves treated in this way and produced spores. These six>rcs 
 were sown upon leaves similarly treated and this was repeated for 
 
246 
 
 REPRODUCTION OF PERISPORIALES 
 
 twelve generations, at which time he states that the spores would 
 grow upon the untreated leaves. Perhaps this is the explanation 
 of some epidemics or the occasional sudden appearance of a plant 
 disease. A variety of cirucmstances might cause plants to form 
 substances attractive to the parasite or to fail to develop repellent 
 
 Fig. 157. Sexual reproduction of a powdery, mildew: A meeting of the 
 male, aji, and female, o, gametangia. B, fertilization, the male gamete, m, 
 is seen approaching the female. C, section of young ascocarp showing the 
 early germination of the gametospore, which has become surrounded by 
 hyphae derived from the mycelium. D, later stage, the gametospore has 
 developed several cells and the second cell from the end, as, will produce 
 the ascus. — After Harper. 
 
 materials. In either case they would become susceptible to the 
 disease. 
 
 The spores are formed in chains (Fig. 156, c) from the end 
 of the erect hyphae that project from the surface of the leaf in 
 thick masses, causing the powdery appearance and the popular 
 name of these parasites. These spores germinate quickly and 
 rapidly spread the fungus. 
 
 The reproductive organs have in this group become still 
 further reduced and appear as short branches as shown in Fig. 
 157, A. The solution of the walls at the point of contact of 
 these organs permits the male gamete to pass over and fuse 
 with the female (Fig. 157, B). The growth of the gametospore 
 forms a limited number of cells and one of them, usually the 
 
DEVELOPMENT OF PLANTS 247 
 
 second from the end, will develop one or several asci (Fig. 157, 
 C, D). As in Pemcilliiim, this growth becomes envelofK'd by a 
 mass of hyphae that originate from the hyphae bearing the 
 gametospore and from adjacent strands of the nucelium. These 
 ascocarps appear at maturity as black specks and in the majority 
 of forms they are provided with hair-like outgrowths that are 
 very regular and characteristic of the genera (Fig. 158). The 
 ends of these hairs are rather mucilaginous and may assist in 
 the dissemination of the ascocarps. The ascospores are resting 
 
 .\ 
 
 - 7 
 
 ^§^.Af^^ 
 
 Fig. 158. Forms of ascocarps found among the powdery mildews: .1, 
 Phyllactinia with needle-like appendages enlarged at the base. B, Micro- 
 sphaera, appendages dichotomous at apex. C, Uncinida, apix^ndagcs coiled 
 at apex. D, Erysiphe without appendages and crushed to show escaping 
 asci. E, an ascus containing six ascospores. 
 
 spores adapted to enduring drought and cold as in Pcnicillium, 
 which they resemble in their discharge and germination. 
 
 92. Reduced Ascomycetes. — Space will only permit the 
 consideration of two other groups from this enormous alliance of 
 the Ascomycetes. These forms are here considered partly 
 because of their economic importance and partly because they 
 illustrate the reduction that may go on in the plant body. 'I'hcse 
 two groups of fungi have become so changed and simplifuHJ in 
 their structure and life history as to render impo-MMr a cuess 
 as to their relationship to other forms. 
 
248 
 
 REDUCED ASCOMYCETES 
 
 92A. Order g. Exoascales or Peach Curl. — This group in- 
 cludes a small number of parastic fungi that are especially destruc- 
 tive to peach and plum trees, causing distortion of the leaves 
 and fruit, known as leaf curl and bladder plum (Fig. 159). There 
 is apparently no trace of a reproductive process, the mycelium 
 spreading through the leaf develops directly a rudimentary 
 hymenium of numerous asci beneath the cuticle, which is finally 
 
 Fig. 159. Fig. 160. 
 
 Fig. 159. A branch from peach tree, showing the distortion of the leaves 
 caused by the fungus, Exoascus. 
 
 Fig. 160. Section of a leaf showing numerous cells rupturing the cuticle 
 and developing into asci, as. 
 
 ruptured by their growth (Fig. 160). The ascospores are dis- 
 charged into the air by the bursting of the asci and carried by 
 the wind to other plants. The damage in the United States to 
 peach trees alone is estimated at $2,000,000 to $3,000,000 
 annually. 
 
 93. Order h. Yeast or Saccharomycetes. — These fungi are 
 unicellular plants with little suggestion of mycelial growth. 
 Ordinarily they consist of rather oval cells which multiply rapidly 
 by a budding suggestive of conidia formation in preceding orders 
 (Fig. 161). The new cells that push out from the side of the 
 mother cell readily drop off, but in rapid growth they may 
 remain attached in chains (Fig. 161, D). Under certain condi- 
 tions, as the exhaustion of the food supply, the cells become 
 transformed into asci and the contents of each cell rounds ofT 
 into one or more ascospores (Fig. 161, E). The ascospores are 
 
DEVELOPMENT OF PLANTS 
 
 -Mv 
 
 freed by the decay of the ascus and when concHiions are favcjrable, 
 grow into the characteristic yeast cells, as shown in Fig. i6i, F. 
 
 (a) Fermentatio7i.- — These microscopic plants must be num- 
 bered among those plants that are of the greatest economic value. 
 Their importance is due to the fact that they decompose sugars 
 upon which they feed into COo and alcohol, a change called fer- 
 mentation. The extensive brewing and distilling industries all 
 
 Fig. i6i. The yeast plant, Saccharomyccs: A, single plain, ly, yuxui 
 producing three buds. C, section of two plants showing buds and nuclear 
 division. Z), chain of plants due to rapid budding and growth. E, forma- 
 tion of ascospores. F, germination of an ascosporc and the formation of 
 new plants by budding, — After Wager. 
 
 over the world are dependent upon the growth and peculiar action 
 of these microscopic plants. When yeast plants are i>laced in 
 solutions containing sugar in the form of molasses or prepara- 
 tions of rye, corn, barley, potatoes, etc., and slightly warmed, the 
 growth of the yeast produces a vigorous fermentation. CarlKjn 
 dioxide rises to the surface, forming a froth)' scum while the alco- 
 hol accumulates in the fluid. Beers and ales are fermented bever- 
 ages of this nature, while whiskies, brandies, alcohol, etc., are 
 obtained from the fermented mass by removing a part of the 
 water by distillation. Wines and cider are weak alcoholic bev- 
 
250 THE NATURE OF YEAST 
 
 erages formed by the yeast plant from the sugars in the juices 
 expressed from grapes and apples. In this case, the yeast is 
 generally allowed to find its way naturally into the fluids. So 
 minute are these plants that they are distributed in the air 
 throughout the world and no weak sugar solution can be exposed 
 for any length of time without being inoculated by them — a fact 
 that has been apparently taken advantage of by man in all times 
 and places. When these fermented beverages are bottled before 
 the decomposition of the sugar is complete, then a further gen- 
 eration of CO2 sets up a pressure in the bottle or cask that causes 
 the popping of the cork when the bottle is opened and the sparkle 
 of the fluid owing to the escaping gas. These forms show the 
 same range of variability in the work which they perform as 
 noted in the bacteria of decay. In the case of cider our govern- 
 ment experts have separated several forms of yeast that are now 
 offered for distribution and which give distinctive flavors to 
 cider. So in brewing and bread making enough has been done 
 to show that there are numerous forms of yeast that differ 
 materially in their power to induce fermentation as well as in 
 the nature of the fermentations that they bring about. 
 
 (b) Bread Making. — The most important use of the yeast plant 
 is its application to the "raising" of bread. Flour contains, in 
 addition to starch, a little sugar, and this amount is increased in 
 bread making by a ferment, diastase, in the flour which changes 
 starch into sugar. The flour is mixed with water containing 
 yeast plants into a dough and placed in a warm place, when it 
 begins to rise. This means that the yeast plants begin to grow 
 and decompose the sugar into CO2 and alcohol. The dough 
 prevents the escape of the gas which collects in bubbles in the 
 dough, causing it to swell. Baking further expands the gas and 
 also drives off the water and alcohol, leaving the bread light and 
 porous. There are many forms of yeast plants which differ in 
 their power of producing fermentations and in the flavor which 
 they impart. Consequently, different forms are used for differ- 
 ent purposes. Bread yeast is a form that has been selected be- 
 cause of the quickness of its action and the flavor that it imparts 
 to bread. Bread yeast is grown in large vats and put up with 
 starch in cakes, known as compressed yeast. 
 
DEVELOPMENT OF PLANTS 251 
 
 Class C. Basidiomycetes or HAsii)iA-F(jkMi\(; Fingi 
 
 94. General Features. — This class contains two iinporiant 
 groups of fungi; one of which inchidcs very deslriKiive para- 
 sites, and the other comi)rises those conspicuous and fainihar 
 saprophytes known as nuishrooms, bracket fungi, puff balls, 
 etc. The striking feature of tlie class is the formation of spores 
 on club-like hyphae called basidia (sing, basidium) (Fig. 171, D). 
 These organs are often developed in rather complex outgrowths 
 of the mycelium which may be fleshy or w^oody, as in the mush- 
 room and bracket fungi (Figs. 170; 173, B). There is iif) known 
 sexual reproduction, save possibly in the first order mentioned 
 below w^here a vegetative union of cells appears to ha\'e been 
 substituted for a fertilization process which was originalh- like 
 that seen in the red algae. The more important orders are the 
 following : 
 
 95. Order a. Uredinales or Rusts. — These parasites are well 
 known by the streaks and blotches of yellow or black rust which 
 they produce on the leaves and stems of a great variet>' of plants. 
 About 2,000 species are known in the United States. They are 
 among the most destructive parasites, causing great damage to 
 wheat, oats, apples, quinces, roses, carnations, etc. The yearly 
 loss from grain rust alone is estimated at considerably over 
 $18,000,000 in the United States. They exhibit a degree of 
 variation not paralleled among other plants, more than U\c dif- 
 ferent kinds of spores being formed by some species in their 
 life history. This is due, doubtless, to the influence of the 
 climate, the spores varying with the season (spring, sunnner and 
 fall) and the plants upon which the fungus grows, for one and the 
 same fungus may grow^ upon different plants, pnxlucing one or 
 more kinds of spores on each. Forms infesting dilYerent sixties 
 of plants are said to be heteroecious and those living ujion but one 
 species are termed autoecious. 
 
 {a) The Life History of Puccinia.—Scvcnxl si)ecies of this 
 genus infest wheat and illustrate the many forms that may 
 appear in the life history of a rust. ( )ne phase of the life of this 
 parasite appears upon the leaves of the barberry. During May 
 and June the mycelium growing in the leaves forms rounchsh 
 
252 
 
 DEVELOPMENT OF A RUST 
 
 bodies which rupture the epidermis and finally open out into cups 
 filled with chains of yellowish spores. An examination of 
 Fig. 162 shows that these spores are formed in rows at the end of 
 hyphae and surrounded by a layer of rather thick- walled hyphae. 
 This stage of the rust is known as the cluster cup or aecial stage 
 and the spores are called aeciospores. Often smaller spore-bear- 
 
 FiG. 162. Cluster cups as seen in section of leaf of spring beauty, Clay- 
 tonia. At right one of the cups is ruptured, exposing the aeciospores. 
 Below a small cup, pycnium, is discharging pycniospores that are possibly 
 functionless male gametes. 
 
 ing cups, known as pycnia (sing, pycnium), are associated with 
 this phase of the fungus. These small spores, while capable of 
 germinating, do not appear to enter into the life history of the 
 fungus by producing a new parasite. They have been looked 
 upon as male gametes that originally effected fertilization in a 
 female organ from which developed the spore-bearing cluster cup, 
 the process being similar to that noted in the Red Algae. On 
 the other hand they are regarded by some authorities as spores 
 that have lost their power to germinate and so breed the fungus 
 asexually. The process of spore formation in the cluster cup, 
 as will be seen from the outline given below, is so different from 
 that of the algae and is known in so few forms that any interpre- 
 tation of the phenomena should be deferred for the present. 
 
DEVELOPMENT OF PLANTS 
 
 253 
 
 The spore-producing hyphae in the young aecidia form a rather 
 loose stratum of somewhat elongated cells (Fig. 162, A, i). These 
 cells divide, forming an upper series of cells which are smaller 
 and narrower than the lower or basal cells. These upper cells are 
 sterile, ultimately disappearing, and they have been coniixircd 
 
 Fig. 162.4. Spore formation in a cluster cup: i, appearance of the hyphae 
 in a young cup. 2, one of the cells from Fig. i, after it has divided into 
 the sterile cell and the large basal cell. 3, early stage in the fusion of the 
 basal cells of two adjacent hyphae. 4, the two nuclei of the fused cells have 
 divided and two of them are passing to the base of the cells and two into the 
 fused region of the cells. 5, the upper portion of the fused area is cut off, 
 forming the spore mother cell. This cell will divide once, forming a spore 
 and a small sterile cell. 6, a chain of spores and sterile'cells is being formed 
 as noted in 5. — After Christman. 
 
 to remnants of the tubular outgrowth of the female gametan.uium 
 noted in the red algae. 
 
 The basal cells enlarge, become inclined towards one anuthcr 
 in pairs and finally meet at their upper ends, the walls dissolving 
 at the point of contact. The nuclei of the two cells now pass 
 to the fused region of the cells and divide, forming four nuclei. 
 Two of these wander back into the base of their resjx^ctive cells, 
 while the other two pass to the upper portion of the fused area 
 
254 FORMATION OF SPORES IN THE RUSTS 
 
 and become separated from the basal nuclei by a transverse 
 wall (Fig. 162, A, 3-5). The upper cell formed in this manner is 
 the spore mother cell and it divides at once, forming an aeciospore 
 and a small intercalary cell. As soon as this process is completed 
 the two nuclei at the base of the fused cells move up again into 
 the fused portion of the cells, which region has elongated in the 
 meantime, and divided as before. In this way a chain of binu- 
 cleate aeciospores is formed, separated by small intercalary cells 
 (Fig. 162, A, 6) that soon disintegrate. 
 
 As soon as the cluster cups are ruptured the aeciospores are 
 scattered by the wind, and singularly, will only germinate and 
 infest the seedlings and young leaves of the wheat in which they 
 develop a mycelium similar to that of the barberry, but note that 
 the cells of this mycelium are binucleate like the aeciospores. 
 However, this mycelium forms in place of cluster cups, groups of 
 erect hyphae at the ends of which single reddish spores are formed 
 (Fig. 163, C). This growth ruptures the epidermis, exposing 
 the spores in rusty lines of blotches (Fig. 163, B). These yellow- 
 brown blotches account for the popular name of Rusts given to 
 this group of fungi. This is the summer or uredinal stage of the 
 parasite. The spores are known as urediniospores. This phase 
 of the fungus is a very destructive one, for the spores are formed 
 in great numbers and provided with thin walls. They are widely 
 distributed by the wind and germinate at once (Fig. 164, A) 
 on other wheat plants which soon show the rusty brown streaks 
 of urediniospores. In this way, the pest spreads with great rap- 
 idity from a single center of infection. This formation of ure- 
 diniospores goes on during the summer, sapping the vitality of 
 the plant and, in severe cases of infection, materially interferes 
 with the maturing of the grain. 
 
 Later in the season, this same mycelium forms in the leaves 
 of the wheat quite a different type of spore. They are formed in 
 the same manner as the urediniospores but are provided with 
 thick dark walls and from one to several spores are developed at 
 the end of the hyphae (Fig. 163, D). Consequently, when the 
 epidermis is ruptured, these spores form rusty black blotches on 
 the leaves. This third stage is known as the telial, since it ends 
 
DEVELOPMENT OF PLANTS 
 
 255 
 
 the season's growth. These spores, called tc'H(jsi)orc's, arc resting 
 spores and tide the fungus over tlie winter. Furthermore these 
 spores are uninucleate. The two nuclei which have character- 
 ized all cells from the aecial stage on, actually fuse, forminj^^ one 
 nucleus as the teliospores mature. Some regard this as a dchiNed 
 fertilization. The teliospores germinate in the sj)riHg (juite 
 
 }fifi:| 
 
 Fig. 163. Fig. 164. 
 
 Fig. 163. The summer and fall stages of a rust, Puccinia: A, rust blotches 
 on leaf of wheat. B, portion of leaf magnified, showing rupturing of the 
 epidermis due to the formation of spores. C, urediniospores or the summer 
 spores which effect a rapid distribution of the parasite during the summer. 
 D, teliospores or fall spores which are dormant during the winter. 
 
 Fig. 164. Germination of uredinio- and tclio-spores: A, the thin-wallcd 
 urediniospore sending out hyphae from thin places in its wall. This is ef- 
 fected as soon as it is carried by the wind to a moist leaf. 13, teliosporc ger- 
 minating in the spring and forming a short hypha, from the end i>f whirh four 
 cells have been cut off that are forming the basidiospores, b. 
 
 independent of any plant and being deixMideut wyxm llu- lood 
 stored in the spore, they only form short hyphae which usually 
 become divided into four cells (Fig. 164, B). This structure is 
 known as the basidium. Each cell of the basidiuin sends out a 
 delicate tube into the end of which the cell contents passes, thus 
 
255 FORMS OF THE RUSTS 
 
 forming a small spore, known as the basidiospore. This basldial 
 stage completes the life history of the fungus for the basidiospores 
 are carried to the leaves of the barberry and begin again the life 
 cycle of the parasite by forming the cluster cups. It is held by 
 many that this sequence in spore formation represents an alter- 
 nation of generation. According to this view the aecium is the 
 result of a sexual process. Therefore the aeciospore would be 
 the beginning of the sporophyte or asexual generation, which 
 terminates in the teliospore. The germination of the teliospore 
 marks the beginning of the sexual generation and it is interesting 
 to note that we have four cells produced as in the tetraspores of 
 the red algae. 
 
 It is not surprising that this story was not unravelled for a 
 long time and that these different stages of the parasite were 
 known as distinct species. The first clue to the relationship was 
 gained in England where it was observed that the wheat fields 
 to the leeward of the barberry bushes were especially infested with 
 rust. For this reason, a law was passed early in the history of 
 Massachusetts compelling the destruction of the barberry bushes. 
 This suggestion of relationship between the cluster-cup stage of 
 the barberry and the rust of the wheat finally led to the inocu- 
 lation of wheat plants with aeciospores and this resulted after 
 a week or more in the appearance of the characteristic rusty 
 streaks on the leaves of the wheat. 
 
 (b) Features in the Life History of other Rusts. — Considerable 
 variation characterizes these rusts, not all of them having so 
 elaborate a life history as that outlined above. One of the most 
 common species of Puccinia affecting the wheat is perennial in 
 the wheat and possibly in other grasses where it produces ure- 
 diniospores in the spring, thus, the aecial, telial and basidial 
 stages are eliminated. In the apple rust, the uredinial stage is 
 missing. This disease produces on the leaves of various members 
 of the apple family, yellow patches in which are formed tube-like 
 cluster cups (Fig. 165, A). The aeciospores are only capable of 
 infesting the juniper, in the branches of which they produce 
 gall-like swellings (Fig. 165, B) known as cedar apples and also 
 sometimes bushy outgrowths known as witches' brooms. In the 
 
DEVELOPMENT OF PLANTS 
 
 '57 
 
 spring, variously shaped masses of h>[)liae hearing nunu-rous 
 teliospores radiate out from these galls and in tiie early spring 
 rains these strands swell up, forming conspicuous yellow, jelly- 
 like masses. The basidiospores are developed in the jelly and 
 infest the leaves of the apple, thorn, shadbush, etc. In some 
 of the genera of rusts all the stages appear upon the same plant 
 as in the May apple and jack-in-thc-pulpit. In the early spring 
 
 ^FiG. 165. A rust, Gymnosporangtiwi, that infests the juniper and mem- 
 bers of the apple family: A, cluster cups on leaf of thorn apple. B, tclio- 
 spore stage on red cedar, juniper. 
 
 the stems and leaves may often be seen infected with the yellow 
 cluster cups which are followed later by the dark-colored teli- 
 ospores. In other cases, teliospores only are produced, as in the 
 hollyhock, and this was doubtless the original form of the spore. 
 Numerous physiological species have been reported among the 
 rusts as in the case of the powdery mildews, and in both groups 
 we find some of these specialized forms able to grow upon dilTer- 
 ent plants if an Intermediate plant is used to "bridge" them over. 
 Thus a physiological species that grows normally upon the oat 
 but not upon rye or wheat may be made to do so b\- first growing 
 it upon barley. Spores derived from the barle>' will now infest 
 rye or wheat. 
 
 96. Order b. Ustilaginales or the Smuts.— TlRse para>iies 
 ^^ -more primitive than the rusts to which they are doubtless 
 17 
 
258 
 
 FORMS OF SMUTS 
 
 related. They produce very generally but one form of spore that 
 may possibly be compared to the teliospore. They are com- 
 mon and exceedingly destructive parasites, affecting especially 
 the flowers and fruits of corn and other cereals as wheat, oats 
 and barley (Fig. i66). The damage b}- smuts to the corn crop 
 
 Fig. i66. Fig. 167. 
 
 Fig. 166. A common smut, Ustilago, transforming the kernels of corn 
 into sooty black pustules. 
 
 Fig. 167. The formation and germination of the spore of a smut: A, 
 the formation of the spores from the mycelium in the kernel of corn. B, 
 germination of a spore and the appearance of the basidiospore. 
 
 of the United States exceeds $2,000,000 annually. In the case 
 of corn, the parasite keeps pace with the growth of the plant 
 without producing serious damage until the flowers appear, when 
 the mycelium increases greatly in the affected ears and "tassels" 
 producing sometimes enormous malformation, especially in the 
 ears, which appear as glistening white blisters or pustules. Later, 
 these bodies change to a sooty black, owing to the transformation 
 of the cells of the mycelium (Fig. 167, ^) into black greasy spores. 
 These spores are scattered in clouds upon the breaking of the 
 pustules and germinate much after the manner of the telio- 
 spores of the rusts, infesting the young and delicate part«^ cf 
 
DEVELOPMENT OF PLANTS 259 
 
 other corn plants by means of their basitli(jspores (V'l^. 167, B). 
 In other cases, the teliospores germinate in the sprinp:, infesting 
 the seedlings. This appears to be always the case in the de- 
 structive wheat, rye, oat and barley smuts. Infection only 
 takes place when the spores come in contact with the seed . ( 'on- 
 sequently, in these cases, the fungus can more rea(hl>- bi- fought 
 by treating the seed with some fungicide as formahne and copix»r 
 sulphate. 
 
 97. Order c. Agaricales or Mushrooms and Toadstools.— 
 This is the most familiar group of the basidiomycetes and its 
 12,000 odd species are commonly referred to as mushrooms and 
 toadstools. These fungi are largely saprophx'tes, li\ing upon 
 the humus in the soil and upon decaying wood. Se\eral of the 
 genera are exceedingly destructive to trees and cause great hjss 
 to the lumber industry. In the majority of cases, the fungi 
 appear unable to attack the living portion of the trees and only 
 thrive upon dead tissues, as the heart wood, wliicli they quickly 
 disorganize and render worthless. As in all the ])rc'ce(Hng groups 
 of fungi, the real plant body is a delicate mycelium that spreads 
 through the soil or decaying substances, as may readilx* be dem- 
 onstrated by splitting open a tree infested with one of the fungi 
 (Fig. 168). The "spawn" that is sold in seed stores for plant- 
 ing in mushroom beds is a dried mass of decaying leaves and 
 straw mixed with earth, in which the nncelium of tlie nuishrooni 
 has been allowed to grow. Often the h\phae of the m>(\hinn 
 lose their delicate character and become woven togetlu-r, forming 
 rather dense w^oody strands or plates, a dexclopment frecjuently 
 seen in the timbers of mines and under the bark of decaying trees. 
 In certain species the mycelium is phosphorescent and the cause 
 of the pale light that sometimes apjiears upon moist deca>ing 
 wood, the so-called fox wood. Tlie nurrliuin l)ear> at \ariuus 
 places the complex fleshy or woody IkkIv conunonly known as 
 the mushroom or toadstool. If the mycelium grows in a n-gular 
 manner, radiating outward in all directions, then the nuishrooms 
 will have a similar arrangement. The older central iK)rtions of 
 the mycelium will finally die off, while the newer i>ortions con- 
 tinue to radiate outward and produce the nuishrooms, which con- 
 
26o 
 
 DEVELOPMENT OF A MUSHROOM 
 
 sequently appear in a more or less regular circle. In this way, 
 the fairy rings, which are often held in superstitious awe, come 
 about. 
 
 The Mushroom or Toadstool. — The common umbrella type of 
 mushroom consists of a stalk or stipe and a cap or pileus, on 
 
 Fig. i68. 
 
 Fig. 1 68. The mycelium of one of the Agaricales forming white masses 
 as its spreads through wood. 
 
 Fig. 169. Development of a mushroom: 3, early appearance of the mush- 
 room as a ball of hyphae on the strands of the mycelium, i, section of one 
 of these spherical masses of hyphae, showing the circular openings in which 
 the gills are developed. 2, a later stage with the gills formed and the velum, 
 vl, appearing as a delicate membrane. 
 
 the underside of which are located radiating plates or gills (Fig. 
 170, A). This structure originates on a strand of the mycelium 
 as a very small ball composed of a mass of interw^oven hyphae 
 (Fig. 169, 3). Soon, however, the hyphae of this mass begin 
 to grow in a very regular manner. At first the growth results 
 in the formation of a cavity in the upper part of the ball that 
 extends completely around it (Fig. 169, i). Hyphae now grow 
 
DEVELOPMENT OF PLANTS 
 
 261 
 
 down into this cavity in regular lines, forming the radiating 
 plates or gills noted in Fig. 170. This development divides the 
 ball of hyphae into an upper part or pileus and a basal region, 
 the stalk or stipe. As this growth proceeds the mass of hyphae 
 extending from the margin of the pileus to the stijx,* becomes 
 
 P 
 
 Fig. 170. Habit of a poisonous mushroom, AmaniUi: A, the mature 
 mushroom — s, stipe; p, pileus; g, gills; a, annulus; v, volva, a jurt of which 
 appears in patches on the top of the pileus. C, young form of the Amantta, 
 the volva beginning to break. D, later development, the volva completely 
 ruptured, disclosing the pileus, stipe and velum, vl. — H. O. Hanson. 
 
 drawn out and ultimately forms a rather thin membrane known 
 as the velum (Fig. 169, 2, vl). This entire growth is often 
 enveloped by a sheath-like mass of hyphae, the volva (Fig. 170, 
 C, D). At this stage of development, the \-oung mushroom is 
 
262 GROWTH OF A MUSHROOM 
 
 rather spherical in shape and so small that it is quite concealed 
 in the ground or by surrounding vegetation. When conditions 
 are favorable for further growth, as after a rain, each cell of this 
 miniature fungus absorbs moisture and rapidly expands, thus 
 causing the mushroom to spring up as by magic and reach its 
 full growth in a few hours. As the stipe elongates, the pileus 
 spreads out like an umbrella, rupturing the veil, a part of which 
 clings to the stalk (Fig. 170, A, a) in the form of a ring, the 
 annulus, and a part may also hang in the form of ragged fila- 
 ments from the edge of the pileus. If a volva is formed, this 
 is also ruptured by the elongation of the stipe, forming a cup at 
 the base of the stipe, and usually portions also remain attached to 
 the top of the pileus as scales and patches (Fig. 170, A, D). 
 The structure of the mushroom is very simple. The stipe con- 
 sists of a mass of nearly parallel hyphae that show little modi- 
 fication save at the surface of the stem, where they are sometimes 
 more compactly arranged (Fig. 171, A, B). By cutting across 
 the gills, so that we can look into the ends of them, it will be 
 seen that the hyphae extend down the center of the gills and also 
 continually radiate out on either side, forming a compact layer 
 of rather elongated cells on the surface of the gills, known as the 
 hymenium (Fig. 171, C). A magnified view of a portion of this 
 hymenium shows that it is composed of paraphyses and basidia 
 (Fig. 171, D). These so-called paraphyses are potential basidia 
 and continue to develope as such during the life of the mushroom. 
 The basidia are not divided as in the smuts and rusts, but the 
 spores are formed in the same manner at the end of two or four 
 small tubes that grow out from the apex of the basidia. It 
 should be stated that a series of intermediate forms exist that 
 connect the divided basidia with the present form. The spores 
 are mature and begin to drop off as soon as the pileus opens, 
 when they are scattered by the wind and develop under favorable 
 conditions into a new mycelium. If the pileus of a freshly 
 opened mushroom is placed on a sheet of gray paper and tightly 
 covered wuth a dish, so as to exclude all air currents, the spores 
 will fall directly upon the paper and in a few hours form an 
 exact copy or a spore print of the gill arrangement. There 
 
DEVELOPMENT OF PLANTS 
 
 263 
 
 are several very widely distributed and familiar families of the 
 Agaricales, 'distinguished by the arranj^emcnt and distribution of 
 the hymenium. 
 
 A. Thelephoraceae. — These fungi form mfml.r.mcu-. l.•:.t].,•r^• 
 
 Fig. 171. Structure of a mushroom: .1 ami B, cross ami longitudinal 
 sections of a portion of the stipe, showing the character and arrangement 
 of/the hyphae that make up the mushroom. C, tangential view of the gills 
 — p, pileus; h, hymenium appearing as a dark band on the surface of the 
 gills. D, a portion of the hymenium enlarged— 6, basitlia; pa, ixiraphyscs; 
 s, basidiospores. 
 
 or woody incrustations or shell-like strucliirrs (»r branching 
 bodies on soil or wood (Fig. 172, A, B). The hymenium forms 
 a smooth or slightly wrinkled surfac^^ oii ih<- tnultr side or ex- 
 posed surface of the fungus. 
 
264 
 
 FAMILIES OF AGARICALES 
 
 B. Clavariaceae or Coral Fungi. — This family includes the fairy 
 clubs and coral-like fleshy masses of various colors (Fig. 172, C). 
 The hymenium covers the branches. 
 
 C. Hydnaceae or Prickly Fungi. — These fungi form masses of 
 widely various forms, but provided with spine-like outgrowths 
 upon which the hymenium is developed (Fig. 172, D). 
 
 
 \C 
 
 c^ ^s 
 
 Fig. 172. Forms of the Agaricales: A, Stereum, top view. B, underside, 
 showing the characteristic smooth hymenial surface of the Telephoraceae. 
 
 C, a coral fungus, Clavaria, with hymenium confined to the tips of the branches. 
 
 D, a prickly fungus, Hydnum, with the hymenium on spine-like projections. 
 
 D. Polyporaceae or Pore Fungi. — This group includes largely 
 leathery, woody or corky fungi in which the hymenium is de- 
 veloped on the surface of pores. They contain some of the most 
 destructive of the timber-destroying fungi. Many of the genera 
 of this family form shelf-like outgrowths on trees and are there- 
 fore known as bracket fungi (Fig. 173, B-D). Professor Duller 
 estimated that a single pore in one species of this group which he 
 found growing upon a decaying elm, produced no less than 
 1,700,000 spores and that the entire bracket, about 250 sq. cm. 
 in area, formed over eleven billion spores. The combined annual 
 output of the ten brackets growing upon this tree exceeded by 
 fifty times the population of the globe. This enormous spore 
 
DEVELOPMENT OF PLANTS 
 
 265 
 
 production is characteristic of nearly all of the A^aricalcs and it 
 is not surprising that a bit of dead wood or piece of lumber can 
 not be exposed without being infested with the sy)ores. 
 
 E. Boletaceae or Fleshy Pore Fungi. — The members of this 
 family are generally characterized by a stalk and a pileus which 
 
 
 Fig. 173. Pore forms of the Agaricales: A, Boletus, showing flesii\ \)<^vv- 
 bearing layer, p. B, top view of a woody, bracket form, Eljvingia. The 
 concentric lines represent the annual growth. C, section of a similar form, 
 showing three layers of pores that represent three year's growth. D, en- 
 larged view of under surface, showing one of the pores with hymenial layer. 
 
 bears pores, the latter being easily separable as a layer from the 
 pileus (Fig. 173, ^)- 
 
 F. Agaricaceae or GUI Fungi. — These fungi more commonly 
 assume an umbrella form, although some of them are of the 
 shelving bracket type (Fig. 170). The hymcnium is arranged 
 on the surface of gills or plates. Man>- of them arc highly 
 prized for the table, though they contain comparativch- little 
 nourishment, and must be regarded as relishes rather than fotxls. 
 While the majority of the 5,000 species of the famih' are edible, 
 some of them contain the most deadly poisons. No rule can be 
 given that will enable the collector to separate these fungi into 
 the two mythical groups of poisonous toadstools and edible 
 mushrooms. Each species must be known individually beft^re it 
 is safe to use them for food. All forms with a vulvii should l^e 
 most carefully identified, because this is a feature of the deadly 
 amanitas (Fig. 170), which are among the most poisonous plants 
 known. 
 
266 CHARACTERISTICS OF THE PUFF BALLS 
 
 The remaining orders of Basidiomycetes are characterized by 
 the concealment of the basidia in cavities and are for this reason 
 collectively known as the Gasteromycetes, meaning stomach 
 fungi. Among the more common orders may be mentioned: 
 
 98. Order d. Lycoperdales or Puff Balls. — These familiar 
 
 Fig. 174. Cluster of common puff balls, Ly coper don. At left three older 
 ones have opened, permitting discharge of basidiospores. 
 
 fungi are developed, as in the Agaricales, on strands of the 
 mycelium, which often form extensive net-like threads in rotten 
 stumps, logs, sawdust and humus (Fig. 174). The pufT balls 
 vary in size from a pea to over a foot in diameter. When young, 
 they consist of white cheesy masses of hyphae which form in 
 
 "^ Fig. 175. Diagram of a section of one of the puffballs, showing the thick 
 skin of periderm and the irregular cavities which are lined with basidia. At 
 the base the larger, sterile cavities of the stipe are shown. 
 
 the interior of the puff ball a series of irregular cavities lined with 
 basidia and on the exterior, a rather firm skin or periderm (Fig. 
 175). At maturity, the inner hyphae break up, leaving only a 
 
DEVELOPMENT OF PLANTS] 267 
 
 dusty mass of spores and in somes cases firmer hyphae, the capil- 
 litlum. The skin ruptures in various ways, often Ijy one or 
 more pores at the top, and the least touch now causes the spores 
 to sift out in smoke-Hke puffs, hence the popular name of puff 
 balls. In the earth stars (Fig. 176) the outer layer of the jx^ri- 
 
 FiG. 176. One of the puff-balls popularly known as "earth stars," show- 
 ing the outer periderm splitting into star-like sections and the inner jK-ri- 
 derm opening by a pore. 
 
 derm splits into rather regular star-like segments or vahes which 
 are hygroscopic. In damp weather these valves roll back, in 
 some species to such an extent as to lift the puff ball from the 
 ground, when it may be set rolling by the wind and thus bring 
 about a better discharge of the spores. 
 
 Fig. 177. A common bird's-ncst-fungus, Crucihulum: A, habit of fungus 
 on branch of hickory. B, one of the cups in section, showing the Xo\\v^\ hyphae 
 
 surrounding the spore-bearing cavities. 
 
 99. Order e. Nidulariales or Bird's-Nest Fungi. — These 
 
 minute and curious fungi may be found growing: upon twigs or 
 upon bare ground in old fields, or upon dried dung {\''\^- 177. -'H- 
 
268 
 
 DEVELOPMENT OF PHALLALES 
 
 They differ chiefly from the puff balls in that the spore-bearing 
 cavities are surrounded by tougher hyphae. Consequently, 
 when the periderm of these little cup-shaped bodies opens, these 
 tougher parts appear as minute eggs in a nest (Fig. 177, B). 
 
 100. Order f. Phallales or Stink Horns. — These fungi first 
 appear as egg-like structures on rather coarse strands of the 
 mycelium which traverse decaying vegetation. These bodies 
 consist of a white skin-like periderm which encloses a stipe and 
 
 Fig. 178. A common form of the Phallales, Phallus: A, the so-called egg- 
 stage which in B, sectional view, is seen to consist of an outer periderm, p, 
 and within is a central stipe, s, capped with a pileus. The spore-bearing, 
 cavities of the pileus are shaded. C, stipe and pileus have emerged from the 
 periderm, which forms an irregular sac about the base of the stipe. D, later 
 stage after the spore-bearing layer has dissolved, revealing the cavities on 
 surface of pileus. 
 
 pileus (Fig. 178, A, B). The basidia are formed in honeycomb- 
 like cavities on the outer surface of the pileus. As soon as the 
 spores are matured the stipe quickly elongates, rupturing the 
 periderm and lifting the pileus into the air (Fig. 178, C). In 
 structure and coloration, the Phallales are among the most attrac- 
 tive of the Basidiomycetes, but the majority of the forms are 
 regarded with aversion, since the spore-bearing layer melts 
 down into a slimy mass which emits a carrion-like stench. 
 
CHAPTER VII 
 DIVISION II. BRYOPHYTA. THE LIVICRWORTS AND MOSSES 
 
 loi. Adaptive Features of the Bryophyta. — \Vc have now 
 reached the point in the evolution of the pkxnt where it has left 
 the aquatic condition of its algal ancestors and become largely 
 terrestrial. As has been stated, the fungi do not enter into the 
 scheme of evolution of higher types. They are looked upon 
 as a side line, derived doubtless from various groups of the algae, 
 that have undergone a peculiar series of variation owing to their 
 parasitic and saprophytic habits and that are in no way connected 
 with higher plants. In the Bryophyta we take up again the line 
 of ascent. Naturally we would expect that a change from the 
 aquatic to terrestrial conditions would be attended with profound 
 changes, tending to adapt the plants to the terrestrial conditions. 
 The results of the stimulus of the new surroundings are seen in 
 the production of hair-like outgrowths, rhizoids, that anchor the 
 plant to the substratum and bring it into proper relation with the 
 soil. So long as the plant lived in the water any of its cells might 
 serve as absorbing organs, but it is evident that the provision 
 must now be made for procuring the crude materials which 
 are no longer in direct contact with the plant. The exposure of 
 the plant to the atmosphere results also in the formation of a 
 cuticle and of mucilage-producing cells which tend to retain the 
 moisture and so adapt the plant to its drier environment. It 
 will also be noted that modifications are beginning to ai)ix\ir in 
 the arrangement of the chlorophyll-bearing cells, with the result 
 that they are sometimes distributed in the same favorable jxisi- 
 tions for photosynthesis, as you have noted in the higher plants. 
 However, the Bryophyta are only imperfectly adapted to ter- 
 restrial conditions and the entire plant body often assists in the 
 absorption of moisture and mineral substances. For this reason 
 most of them are moisture and shade i)lants, some indeed being 
 still aquatic, and they remain rather small and inconspicuous, 
 
 269 
 
270 CHARACTERISTICS OF THE BRYOPHYTA 
 
 owing to the fact that they have not as yet varied and produced 
 tissues that can supply them with an adequate amount of mois- 
 ture and protect them against the cHmatic changes. Owing to 
 their habit of growing together in colonies, they often become 
 conspicuous and cover the bare earth, logs and tree trunks 
 with swards and mats that constitute one of the most attractive 
 features of the forest vegetation, and in northern regions they 
 often form the most characteristic feature of the vegetation 
 over large districts. 
 
 102. General Characteristics of the Bryophyta. — Certain 
 groups of the Bryophyta are characterized by thalloid plant 
 bodies, i. e., without distinction of stem, root or leaf, while many 
 have developed into creeping or erect plants with leaf-like organs. 
 All are distinguished by a marked localization of the growing 
 zone, the elongation of the plant being due to the repeated divi- 
 sions of a single apical cell. The reproduction of the Bryophyta 
 shows such marked departures that we realize that a wide gulf 
 separates even the simplest of them from the Thallophyta. 
 Notable among these departures is the absence of zoospores. 
 Only in a single genus are nonciliated bodies formed and dis- 
 charged from certain cells as in the case of zoospores. In this 
 connection it is worth your time to look back and note the 
 trend of plant life. The lowest forms of the green algae were 
 essentially motile. Then the stationary condition arose and 
 the motile state represented by the zoospores became less and 
 less conspicuous. In the bryophytes we perhaps have in the 
 spores, referred to above, an interesting example of the last 
 trace of the motile phase of plant life. The suppression by the 
 sexuafplant of the spore method of reproduction is due to several 
 causes. They are able to increase their numbers in a vegetative 
 manner. Special branches or ordinary shoots become detached 
 from the parent plant by decay of the older parts or by other 
 means and develop into new plants. Buds, called gemmae, 
 consisting of one or more cells, are also very commonly formed 
 on various parts of the plant and becoming detached grow into 
 new plants (Fig. i86). Possibly also the higher organization of 
 the Bryophyta, which renders them more capable of meeting 
 
DEVELOPMENT OF PLANTS 271 
 
 conditions that would be fatal to the lower forms, tended to 
 make them independent of spore reproduction.V. But especially 
 has this change been brought about by the fact that the ganicto- 
 spore in germinating develops an asexual plant or generation with 
 enormous possibilities in the way of spore production, so that the 
 increase and distribution of individuals is now fulh- i)rovidcd for 
 by the asexual plant and consequently largeh- remcned from the 
 sexual plant. This shifting of the responsibility for the increase 
 in number of individuals from the sexual to the asexual generation 
 is one of the most significant departures in the evolution of plants. 
 We will be interested to note how this change came about and 
 how it steadily gains in importance in the remaining divisions. 
 
 The sexual reproduction of the bryophytes reveals a scries of 
 variations that are only remotely suggestive of the algae, but that 
 indicate very clearly relationship with higher plants. These 
 features will be considered in the discussion of the various groups. 
 There are two classes of Bryophyta: A, The Hepaticae or Li\er- 
 worts; B, the Musci or Mosses. 
 
 Class A. Hepaticae or Liverworts 
 
 103. Classification of the Liverworts. — While these plants are 
 the simplest and least conspicuous of the Bryophyta, they are the 
 most interesting, because among them will be found reminders of 
 the algae and at the same time they present many features that 
 are suggestive of the mosses and ferns. They occur in very 
 moist places and in deep woods, on rotting logs and moist shady 
 banks. A few are aquatic, floating uix)n still ponds and streams, 
 and some have become adapted to dry conditions. There are 
 three orders: (a) Marchantiales or Thallose Liverworts; (6) 
 Jungermaniales or Leafy Liverworts; (c) Anthocerotalcs or 
 Horned Liverworts. 
 
 104. Order a. Marchantiales or Thallose Hepatics.- Ihc 
 liverworts of this order are characterizrd b\- llat, prostrate and 
 rather fleshy bodies which usual !>• grow upon tlu' earth, to which 
 they are attached by numerous rhizoids. Owing to tiie frecjuent 
 division of the apical cell of the thallus into two equal parts, there 
 result two growing points which thus produce the equal forking 
 
272 
 
 STRUCTURE OF RICCIOCARPUS 
 
 or dichotomous branching of the thallus, so characteristic of 
 these plants (Fig. 179, A, B). The appearance of many of these 
 hepatics is suggestive of the algae. Especially is this true of the 
 aquatic Ricciocarpus and Riccia. 
 
 (a) Structure of Ricciocarpus. — An examination of the struc- 
 ture of one of these will show, however, that extensive changes 
 have been induced in even the simplest forms. The new stimuli 
 to which the terrestrial conditions expose them cause a remark- 
 able series of transformations In the cells that are cut off from the 
 
 Fig. 179. Forms of semiaquatic Marchantiales: A, Riccia, showing the 
 dichotomous branching of the thallus. B, Ricciocarpus. The sexual or- 
 gans are concealed in furrows that appear as radiating lines in the center 
 of the branches. C, diagram of a cross-section of a branch, showing the 
 male gametangia in the bottom of one of the furrows. 
 
 apical cell. The upper cells of the thallus, as soon as they are 
 formed at the growing point, are exposed directly to the air and 
 light, and they develop chlorophyll and grow up into vertical 
 rows or plates just as you have already noticed in the palisade 
 chlorenchyma of the leaf (Fig. 180, ^). At an early period thege 
 rows of green cells become separated so that air spaces arise 
 between them, and thus the chlorophyll-bearing cells are brought 
 into direct contact with the atmosphere and enabled to carry on 
 photosynthesis to the best advantage. The terminal cells of 
 these rows enlarge considerably and form a rudimentary epider- 
 mis. It is also interesting to note that the direct contact of the 
 cells with the atmosphere results for the first time in the forma- 
 
DEVELOPMENT OF PLANTS 
 
 273 
 
 tion of a cuticle. In most species the air spaces arising among the 
 upper cells of the thallus increase greatly in size and the epidermal 
 cells by vertical divisions keep pace with this enlargement and 
 thus arch over the air cavities, forming, however, a small (jpening 
 in the epidermal layer that permits a free circulation <>{ the air 
 (Fig. 180, B). These openings are suggestive of stomata and 
 function in the same w^ay, though they have originated in an 
 entirely different manner. The lower cells of the thallus are 
 compact and doubtless serve as storage cells, being nearly or 
 quite destitute of chlorophyll. The stimulus of the soil causes 
 
 Fig. 180. Structure of the thallus of Ricciocarpus: A, section of the 
 thallus, showing the apical cell, x, forming cells that by further division de- 
 velop into plates of cells separated by air spaces, j. At the left the plates 
 thus formed are seen curving over the apical cell. J5, an older portion of 
 the upper part of the thallus. The air spaces, _;, are greatly enlarged and 
 the upper cells of the vertical plates have divided, arching over the air spaces 
 but leaving small openings which permit the entrance of air for ph"''><v'i- 
 thesis.— I. D. Cardiff. 
 
 the lower epidermal cells of the thallus to form numerous smooth 
 or pitted rhizoids that are suggestive of the root-hairs of tin- hiulur 
 plants, anchoring the plant to the substratum and assisting in the 
 absorption of the earth substances. Plates of cells similar in 
 origin to those occurring on the u\)\wv surface are also found 
 on the under side of the thallus (Fig. 180, A). These plates 
 curve up around the growing point and dcnibtlcss protect it 
 against drought, in which work they are assisted by the mucilage 
 18 
 
274 
 
 REPRODUCTION OF RICCIOCARPUS 
 
 cells or glands. The development of these glandular cells on the 
 plates, as well as on the other parts of the thallus, adapt the 
 plants to the drier terrestrial conditions to which they are 
 exposed, as was the case with the mucilaginous walls of the 
 Schizomycetes and Z^^genmatales. 
 
 (b) Sexual Reproduction of Ricciocarpus. — The gametes are 
 produced in more complex gametangia than we have as yet seen. 
 These organs are developed upon the upper surface of the thallus 
 
 Fig. i8i. The origin and structure of the male gametangia or antheridia: 
 A, section of the thallus, showing the apical cell, x, and the early stages, 
 a, h, in the development of the antheridia. B, older antheridium with cells 
 dividing vertically. C, later stage in which the wall cells are differentiated. 
 D, mature antheridium of Marchantia, showing the numerous cells that 
 develop the male gametes and the wall cells, w. E, greatly enlarged view 
 of a male gamete after discharge from the antheridium. 
 
 and in some species appear as lines radiating from the center of 
 the plant. The male gametangium originates from one of the 
 superficial cells of the thallus, which at first continues to divide 
 transversely after the manner of the vertical plates of chlorophyll- 
 bearing cells (Fig. i8i, A). The cells of these vertical plates 
 soon begin to divide vertically and thus form an elliptical mass 
 of cells (Fig. iSi, B, C). As this growth goes on the outer or 
 wall cells become larger than the others and generally develop 
 chlorophyll, while the inner cells divide repeatedly and become 
 
DEVELOPMENT OF PLANTS 275 
 
 very numerous with dense granular contents (Fig. iHi, D). Each 
 of the latter cells produces a single male gamete that is motile by 
 means of two cilia (Fig. 181, E). This complex gametangium is 
 suggestive of the structure we have seen in Ectocarpus (Fig. 119, 
 B). The male gametangium is commonly called the antheridium, 
 and the male gametes are often termed si)erins (»r antherozoids. 
 The gametes are discharged from the antheridium owing to the 
 absorption of water by the walls of the cells in the ui)per portion of 
 the antheridium which become mucilaginous at maturity and ex- 
 panding, rupture this region of the antheridium, thus permitting 
 the discharge of its contents. The walls of the gamete producing 
 cells also become mucilaginous and by the absorption of moisture 
 they may assist in the extrusion of the gametes. The j)ressure of 
 the surrounding tissues upon the antheridium is also an important 
 factor in this discharge. The mucilaginous mass that is extruded 
 from the antheridium in this manner, rapidly dissolves in the 
 water and sets free the male gametes or sperms. The female 
 gametangium is also quite different from the single-celled game- 
 tangia which w^e have noticed among the algae and fungi. It 
 originates as in the case of the antheridium, but de\elops into a 
 flask-shaped body, consisting of a long neck of several rows of 
 cells surrounding a central row, called the canal cells, and an en- 
 larged basal region in w^hich is developed the female gamete (Fig. 
 182). The female gametangium is commonh- known as the 
 archegonium and the female gamete is sometimes called the 
 oosphere or egg. The female gamete presents essentially the 
 same characteristics as seen in man>- of the algae, as Vaucheria, 
 Oedogonium, etc., but it is surrounded by a jacket of cell which 
 constitutes the basal portion of the archegonium instead of being 
 contained in a single cell. Possibly the archegonium has come 
 about from a multicellular gametangium as seen in ilu- brown 
 algae, owing to the sterilization of all but one of the cells. .Arche- 
 gonia, with two or more gamete-like cells, are sometimes found 
 in the mosses and ferns and there is evidence tending to show 
 that the primitive gametangia contained both male and female 
 gametes. As soon as the female gamete is formed, the apical or 
 lip cells of the neck open in the same manner as noted in the 
 
276 
 
 ARCHEGONIUM OF RICCIOCARPUS 
 
 antheridium and the canal cells become mucilaginous, thus 
 forming a passage way to the female gamete (Fig. 182, F). The 
 male gametes, attracted, It is supposed, by cane sugar, developed 
 in the archegonlum, swim down the canal of the neck, and one 
 unites with the female gamete, as shown In Fig. 182, F, where 
 
 Fig. 182. Development of the female gametangium or archegonium: 
 A-D, stages in the development of the archegonium from a single cell. E, 
 nearly mature archegonium just before the female gamete is formed — en, 
 canal cells. F, lip cells have opened and the canal cells have dissolved, thus 
 forming a passage way to female gamete. A male gamete has entered and 
 is seen fusing with the nucleus of the large female gamete. — After Garber. 
 
 the two gametes are seen fusing. Note that mucilage retained 
 in the neck protects the female gamete against loss of water and 
 also that the attenuated form of the male is of advantage In 
 enabling it to work Its way through this substance. 
 
 (c) Germination of the Gametospore and Spore Formation. — ' 
 The gametospore, resulting from this fusion, becomes surrounded 
 by a cell wall, but does not function as a resting spore as in some 
 of the algae. It germinates at once and by repeated divisions of 
 
DEVELOPMENT OF PLANTS 277 
 
 its nucleus forms a globular mass of cells, the capsule, wiihin the 
 archegonium which keeps pace with its growth (Fig. 183, A, B). 
 The wall cells of this capsule are soon cJistinguishal)le, owing to 
 their watery contents and shape, from the other cells, which arc 
 rather cubical and densely granular. The growth and division 
 of the granular cells continues until about 400 have been formed, 
 when they round off, become separated from one another and 
 increase greatly in size (Fig. 183, C). These large r.Hs, r.ill,<| 
 
 
 ■:-n vA 
 
 Fig. 183. Germination of the ganu'tosport": -1, lias^il portion of an arche- 
 gonium, showing the germinating gametospore in the lour-cell stage, B, 
 later growth, forming a capsule with wall cells, w, which inclose large gran- 
 ular cells. C, a portion of the capsule, showing spore niotht-r cells roun' m .• 
 ofif and floating in the fluid of the capsule. D, the mother cells dJM ...s 
 and forming four spores each. — II. O. Hanson. 
 
 Spore mother cells, form four spores each, as in the tctrasporcs 
 of the red algae (Fig. 183, D). The delicate walls of the cap- 
 sule break down as soon as the spor<>^ in- m nnnd. leaving them 
 
278 
 
 THE SPORES OF RICCIOCARPUS 
 
 free in the archegonium, and later they are set free by the decay 
 of the latter organ. These spores in some forms are provided 
 with thick walls and are, therefore, resting spores adapted to 
 carrying the plant over unfavorable conditions for growth. In 
 other cases the spores have thin walls and germinate at once. 
 The advantage of the transference of the resting stage from a 
 single gametospore to the numerous spores derived from the 
 gametospore is manifest. 
 
 {d) Germination of the Spore. — The germination of the spore 
 is usually indicated by the formation of chlorophyll, and this is 
 
 Fig. 184. Germination of the spores: A, spore. 5, first division of the 
 germ tube. C, early form of the thallus, due to the formation and subse- 
 quent division of the apical cell. 
 
 followed by the rupture of the outer spore coat and the protru- 
 sion of the inner as a delicate papilla or germ tube (Fig. 184). 
 Usually from this tube a small, hair-like outgrowth is soon 
 formed which penetrates the soil as the first rhizoid. The germ 
 tube continues to elongate and often forms a chain of cells by 
 successive transverse divisions, but eventually by oblique divi- 
 sions an apical cell is formed that develops the characteristic 
 thallus (Fig. 184, C). Thus we arrive again at the starting point 
 in the life history of these simple plants. 
 
 (e) Noteworthy Departures in the Life History. — It should be 
 stated that the gametophyte of certain species of the Marchan- 
 
DEVELOPMENT OF PLANTS 279 
 
 tiales may live for long periods without producing tlie gametes. 
 In fact, it frequently multiplies by means of buds and branches 
 which become detached and grow directly into new plants. How- 
 ever, as soon as conditions are favorable the sexual organs and 
 gametes will appear. There are several features in this hisi(jry 
 that must be kept clearly in mind. In tlic first place, tiie 
 gametospore is not discharged from the i)lant as in the case of the 
 green or brown algae, but remains permanenth- in the arche- 
 gonium, where it continues to be nourished by the plant during 
 its germination and the formation of its spores. Therefore, it 
 develops essentially as a parasite upon the plant, as is the case 
 among the red algae. This retention and nourishment of the 
 germinating gametospore in the plant is perliaps the most 
 important of any of the variations that appear in plant life. 
 Owing to this relationship the gametospore attains a larger growth 
 with increased power of spore production. In Ricciocarpus the 
 capsule is about 500 times as large as the gametospore. In this 
 way the development of many new plants is made possible by a 
 single fusion of gametes. This is a very significant feature in 
 terrestrial plants, since the fusion of the gametes is effected with 
 more difficulty, owing to the absence of aquatic conditions. 
 Without doubt this change was induced by the transference of 
 foods from the sexual to the asexual plant deri\ed from the 
 gametospore. This loss of food gradually prevented the sexual 
 plant from producing spores or other bodies designed to multiply 
 its numbers and we will finally see that the sole work of the sexual 
 plant is limited to the production of gametes. Note also that 
 the gametospore not only has a larger growth but a longer life. 
 This is of the utmost importance because it became exi)osed in 
 this way to a new series of stimuli that affected it jirofoundly and 
 that resulted in the evolution of the higher types of plants. 
 Among the green algae the actual germination of the gameto- 
 spore is limited to a few hours at the most and this is effected in 
 the water, where the conditions are exceptionally uniform. Tlie 
 germination of the gametospore in the case of the li\erworts is 
 prolonged over several weeks, and more important still is the 
 fact that this growth occurs on the land where it is exposed to a 
 
280 THE TWO PHASES IN PLANT LIFE 
 
 wide range of stimuli that cause it to var}^ in a most remarkable 
 manner. So we must bear in mind the character of this rather 
 simple growth derived from the gametospore of Ricciocarpus and 
 compare it with the variations that appear in the succeeding 
 forms. 
 
 It is also evident that the fusion of the gametes in the simple 
 liverworts results in the production of a cell, the gametospore, 
 that is radically different in its nature and possibilities of growth 
 from any of the cells of the parent plant or liverwort. This was 
 not apparent in the simpler forms of the algae, w^here the gameto- 
 spore produced directly a plant like the parent, as in Vaucheria 
 and Fucus. In Spirogyra, Ulothrix, Oedogonium and Coleo- 
 chaete, we saw the first indication of the real nature of the game- 
 tospore. It did not develop into a plant like the parent, but 
 uopdecdr cells or zoospores. The reason for this difference in 
 behavior is possibly due to the higher organization of the gameto- 
 spore. In the lower forms its first divisions result in the forma- 
 tion of cells that are of the same nature as the cells of the parent 
 plant, but in higher forms its composition is more complex and as 
 a consequence it forms a number of cells before cells like those of 
 the parent plant are developed. For example, in the Red Algae, 
 certain of the fungi and in Ricciocarpus the gametospore gives rise 
 to a varying number of cells that are different from those of the 
 parent plant, but finally spore mother cells are formed from which 
 are derived spores that are of the same nature as those of the 
 parent plant and that consequently produce new plants like the 
 parent. As has been stated on page 138, one difference between 
 the cells derived from the gametospore and those of the parent 
 plant is to be found in the number of chromosomes which they 
 contain, the former having twice as many chromosomes as the 
 latter. The doubling of the chromosomes occurs when the two 
 gametes unite to form the gametospore and this number is re- 
 tained during the germination of the gametospore until the divi- 
 sion of the spore mother cells, which results in the formation of 
 spores with only one half the number of chromosomes. In the 
 lower algae this reduction must occur in the first divisions of the 
 germinating gametospore, which therefore corresponds to the 
 
DEVELOPMENT OF PLANTS 281 
 
 spore mother cell, but in the Rid Algae and Rkciocarpus a con- 
 siderable growth intervenes before the spore mother cells apfx-ar 
 and the reduction of the chromosomes takes place. In other 
 words, as we ascend the scale of plant life, the formal ion oi the 
 spore mother cells and the reduction of the chromosomes is pre- 
 ceded by an ever-increasing growth of the gametospore. This 
 postponement in the formation of the spore mother cells, owing 
 to the larger and larger growth of the gametospore, will steadily 
 progress in the following studies. 
 
 These simple liverworts, like Ricciocarpus, show very clearly 
 two phases or generations in their life history'. The thallose 
 
 Fig. 185. Diagram of the life history of Ricciocarpus. The upix-r ix>r- 
 tion of the figure represents the sexual generation and the lower portion, 
 the asexual. The former generation begins with the formation of the spores, 
 sp, from the mother-cell and ends with the formation of the gametes, g. The 
 asexual generation begins \vith the gametospore, gm, and ends with the sjwrc 
 mother cells, sm. 
 
 plant is the gametophyte or sexual generation because it lK»ars 
 sexual cells or gametes. The capsule is the sporophyte or asex- 
 ual generation because it can only produce spores. The gameto- 
 phyte begins with the spore and ends with the formation of the 
 gametes. The sporophyte begins with the gametospore and ends 
 with the division of the spore mother rrll (Fig. 185). It may 
 appear to you now as strange to regard the few cells of the cap- 
 sule, the majority of which become spore mother cells, as a plant. 
 But w«^ ^all directly see this microscoi)ic plant assuming larger 
 proT[89. Diagl? a result of its better nourishment and the stimuli 
 t- buds or^posed. It will form a larger mti.1 I.nvcr numU^r 
 
282 STRUCTURE OF MARCHANTIA 
 
 of cells and the spore mother cells will not appear until late In Its 
 growth and they will be confined to definite regions and only con- 
 stitute a small part of It. We now call the gametophyte the plant 
 because it Is many times larger than the sporophyte, but we shall 
 see the sporophyte become quite as conspicuous as the gameto- 
 phyte, as in the mosses, and finally It will become much larger, 
 as in the ferns. Then we shall call the sporophyte the plant. 
 
 (/) Structure and Life History of Marchantia. — Let us now 
 examine a higher type of the Marchantiales, as Marchantia, and 
 note the advances that have been made over simple forms like 
 Ricciocarpus. Marchantia (Fig. i86) Is of common occurrence 
 
 
 ,-v-'--5v^ 
 
 "S' 
 
 '>-5= 
 
 ^r- '"-'--^ 
 
 ■r-^-'^r^ 
 
 
 
 'r@ '-^^ 
 
 ^^^- : 
 
 ;:<;~.^^; 
 
 &' 
 
 
 •:?© "- 
 
 "* 
 
 Fig. i86. Fig. 187. 
 
 Fig. 186. Thallus of Marchantia bearing three cup-like organs that con- 
 tain buds or gemmae and also four erect branches that contain the anther- 
 idia in their upper surfaces in radiating lines. 
 
 Fig. 187. Portion of the surface of the thallus of Marchantia, enlarged 
 showing the rhomboidal air chambers and air pores. 
 
 in moist places, often appearing in greenhouses on the earth of 
 flower pots, and It also forms luxuriant beds on the damp ground, 
 especially where logs and brush have been burnt. The thallus 
 shows the same general features that w^e have noticed in Riccio- 
 carpus, being rather fleshy and creeping over the ground, to which 
 it Is attached by numerous rhizolds. The simple air chambers 
 noted in Ricciocarpus are much enlarged and appear as diamond- 
 shaped or rhomboidal plates on the surface of the thallus (Fig. 
 187). Thin sections across the thallus show that these air 
 chambers have a complex structure (Fig. 188). Thev '-mate 
 in the upper cells of the thallus and as they enlarge -^^o^s 01 ^^g 
 covered by a well-developed epidermis which fo-^ponas to tn 
 
DEVELOPMENT OP^ PLANTS 
 
 283 
 
 like pore over the center of each cavity. From ilie l)on<jm of 
 the chamber numerous delicate chlorophyll-bearing cells arise. 
 This arrangement of the tissues is again suggestive of the chior- 
 enchyma of the leaf and it is manifestly i)rolective and adapted 
 to photosynthesis. The structure of the lower cells of the thallus 
 
 Fig. 188, Section through the center of the thallus of Marchantia, show 
 ing one of the air chambers and chimney-like pores in the epidermis — ch- 
 palisade-like chlorenchyma arising from bottom of air chamber. The lower 
 cells of the thallus are nearly colorless and filled with watery solutions or 
 mucilage, r, rhizoids; /, leaf-like plates of cells. 
 
 and the distribution of the rhizoids and ventral plates are essen- 
 tially as in Ricciocarpiis. 
 
 Marchantia, as in many of the liverworts and some mosses, 
 multiplies extensively b}^ means of buds or gemmae. Tlu'\- are 
 
 Fig. 
 
 Diagram of a section of one of the cups shtnvn in Fig. 186 — g, 
 
 buds or gemmae associated with small glandular •-•"-. 
 
284 
 
 REPRODUCTION OF MARCHANTIA 
 
 borne in cup-shaped receptacles and consist of small lense-shaped 
 masses of cells which are freed from the thallus by the swelling 
 of the mucilage secreted by glandular hairs growing among them 
 (Fig. 189). These minute bodies grow directly into new plants 
 and so serve to rapidly multiply the plant. 
 
 The reproductive organs appear upon different plants which 
 
 a— 
 
 Fig. 190. Thallus of Marchantia bearing several erect branches with 
 lobed or umbrella-like tops. The archegonia are situated on the underside 
 of the umbrella, between the lobes, a, the early appearance of a branch, 
 in which condition fertilization is effected. 
 
 may therefore be distinguished as antheridial or male plants and 
 archegonial or female plants (Figs. 186, 190). Such a distribu- 
 tion of the sexual organs is termed dioecious, meaning in two 
 households, whereas Ricciocarpus and others are said to be mon- 
 oecious, because the sexual organs are developed upon the same 
 plant. The antheridia and archegonia are developed, as in Ric- 
 ciocarpus, upon the upper surface of the thallus, but they are 
 confined to special portions of it which appear at first as mush- 
 room-shaped outgrowths (¥\g. 190, a). These outgrowths, how- 
 
DEVELOPMENT OF PLANTS 
 
 28> 
 
 ever, are but modified branches of the lliallus and l)ear the 
 sexual organs upon their upper surfaces in Hnes radiating from 
 the center of the branches. FertiHzation is effected, as in all 
 forms, by means of dews and rains which flood these organs with 
 water. An examination of a young an hegonial l)ran( li will show 
 how admirably it is constructed to ensure fertilization. The 
 archegonia radiating in lines have their necks strongly rurvcfl 
 
 Fig. 
 
 Fig. 191. P'iG. 192. 
 
 [91. Section of an archegonial branch similar to a, Fig. 190 — ar, 
 
 archegonia; in, involucre or curtain that hangs down on cither side of the 
 rows of archegonia; ac, air chambers; th, thallus. 
 
 Fig. 192. Section of a young antheridial branch: an, anthcridia sunken 
 in cavities of the branch, which is also provided with air chambers similar 
 to those of the normal thallus. Some of the anthcridia have discharged 
 their gametes, as at x. 
 
 upwards and away from the center of the branch. These lines 
 of archegonia are separated by ridges which thus act as a water- 
 shed, deflecting into the upturned necks any water containing 
 gametes that may chance to fall upon them. After fertilization 
 these mushroom-shaped branches elongate, raising the sexual 
 organs up into the air, where the antheridial branch assumes the 
 form of a lobed disc, while the archegonial branch terminates in 
 an umbrella-like structure (Figs. 186, 190). Owing to the ex- 
 tended growth of this latter branch, the archegonia come to lie 
 on the under side of the structure between the fmger-like out- 
 growths, where they are completeh- hidden and jirotecteti by 
 fringed curtains, the involucre, that hang down from the fingers 
 
286 
 
 SPOROPHYTE OF MARCHANTIA 
 
 (Figs. 191, 195, yl). The antheridia retain their original position 
 upon the branch, where they appear in cavities, as shown in 
 Fig. 192. 
 
 The most important and significant departure in Marchantia is 
 seen in the germination of the gametospore which divides into an 
 inner and outer cell as in Ricciocarpus (Fig. 193, B). The inner 
 
 Fig. 193. Germination of the gametospore: A, section of a mature ar- 
 chegonium with canal cells dissolved, thus forming a passageway to the 
 large female gamete, g, B, sectional view of base of archegonium, show- 
 ing the germinating gametospore in two-cell stage. The perianth, p, is 
 seen growing up about the archegonium. C, later stage in growth of the 
 gametospore. The lower cell shown in B is forming stalk cells, while the 
 outer cell has produced densely granular cells that will later by further di- 
 vision form spore mother cells and elaters. 
 
 cell, however, by a series of divisions, forms a rudimentary stalk 
 and the lower part of it, known as the foot, comes into close 
 contact with the tissues of the plant from which it absorbs nour- 
 ishment (Figs. 193, 194). The outer cell divides, forming a 
 spherical mass of cells that resembles the capsule of Ricciocarpus. 
 Some of these cells of the capsule develop as spore mother cells, 
 
DEVELOPMENT OF PLANTS 
 
 287 
 
 while others, known as elaters, elongate greatly and serve to con- 
 duct the foods absorbed by the foot to the spore mother cells, and 
 finally they become spirally thickened (Fig. 194, C). These 
 elaters arise through the sterilization of certain of the spore- 
 
 FiG. 194. Structure of the nearly mature sporophyte: A, continuation 
 of the growth in Fig. 193, C, showing the formation of a foot, /, stalk, and 
 capsules, c, which contain elongated dark cells, the elaters, and the spores. 
 The archegonium has been ruptured by the elongation of the stalk and is 
 not shown in the figure, p, perianth. B, enlargement of the base of A, 
 showing the attachment of the foot (indicated by darker lines) to the tissues 
 of the antheridial stalk. C, an elater and spores. 
 
 Fig. 195. Archegonial branch with mature sporophytes: .1 , branch with 
 the dark capsules of several sporophytes projecting beyond the curtains of 
 the involucre. B, diagram of a branch as seen in section. On the right 
 one of the capsules of a sporophyte ruptured, exposing the elaters and spores. 
 On the left the curtain-like involucre only is shown. 
 
288 
 
 SPOROPHYTE OF MARCHANTIA 
 
 producing cells. This is an important departure from Riccio- 
 carpus and we see here the beginning of the tendency towards 
 the sterilization of sporogeneous cells that becomes more and 
 more pronounced in higher forms and that 
 plays an important part in the evolution of 
 plant life. During the germination and growth 
 of the gametospore, a delicate membrane (the 
 perianth) grows up about the archegonium 
 and doubtless assists the involucre in pro- 
 tecting it against drying winds ( Fig. 193, 
 p). When the spores are mature, the cells 
 of the stalk elongate, rupture the archego- 
 nium and push the capsule beyond the cur- 
 tains of the perianth and involucre, so that 
 it is exposed to the air (Fig. 195). The cap- 
 sule now ruptures, exposing the spores and 
 elaters to the air. The elaters, a word mean- 
 ing lifters, are very hygroscopic; they coil and 
 uncoil with the least change in the humidity 
 of the air and thus doubtless assist in the grad- 
 ual exposure of the spores to the air currents. 
 Fig. 196. One of The spores germinate as in the lower liver- 
 simpler thalloid J un- ^.^^^g ^j^^g ^^ g^^ ^j^^^ ^^^ sporophyte, 
 germaniales, FaUavi- 
 cinia, showing the consisting of a foot, steraand capsule, is more 
 
 rhizoidal growth on complex and larger than in Ricciocarpus. 
 the under surface of -pj^ig jg doubtless due to the better nourish- 
 ment which it receives as a result of the de- 
 velopment of a more efficient absorbing organ, 
 the foot. 
 
 105. Order b. Jungermaniales or Leafy 
 Hepatics. — By far the larger number of 
 hepatics belong to this order. They are 
 especially abundant in moist tropical countries, where they often 
 cover the stems and leaves with a rich vegetation, and with us 
 they are of common occurrence on dripping rocks and in deep 
 woods on the moist bark of trees and decaying logs or damp 
 earth. In the lower form the thallus is very simple and delicate 
 
 the thallus and a 
 mature sporophyte 
 arising from a cup- 
 like perianth which 
 is surrounded at its 
 base with involu- 
 crate leaves. — H. O. 
 Hanson. 
 
DEVELOPMENT OF PLANTS 
 
 289 
 
 (Fig. 196). In other genera there are indications of a Icjljin^^ in 
 the thallus which becomes more pronounced in some forms and 
 leads by gradual gradations to genera in which the lobes appear 
 as distinct leafy organs arranged in two rows upon a stem-Hke 
 
 Fig. 197. One of the leafy Jungermanialcs, Porella: A, branch of the 
 plant bearing several sporophytes. B, under surface of a branch, showing 
 the lobing of the leaves and a row of minute scale-hke leaves. C, portion 
 of a branch bearing an archegonium surrounded by cup-like perianth with 
 minute involucrate leaves at base. D, branch with cone-like anthcridial 
 branchlets. At the left a single leaf is shown with globular antheridium 
 in its axis. E, section of a branch similar to C. The archegonium is seen 
 surrounded by the perianth and below the involucrate leaves. The sjxjro- 
 phyte is nearly mature and ready to elongate. It consists of a round cap- 
 sule containing elaters and spore mother cells that are dividing to form four 
 spores each. Below the capsule is the stalk or seta which ends in a foot 
 buried in the tissues of the branch. At the right is an unfertilized arche- 
 gonium that shows the original position and size of this organ. — H. O. Hanson. 
 19 
 
290 STRUCTURE OF LEAFY HEPATICS 
 
 axis (Fig. 197). Usually a third row of rudimentary leaves, 
 associated with rhizoids, appears upon the under side of the thal- 
 lus (Fig. 197, B). Although the Jungermaniales tend to become 
 highly modified into a leafy body the tissues remain practically 
 unchanged in all forms. Even in the leafy genera, which com- 
 prise the majority- of forms in this order, there is little evidence 
 of such a differentiation of the tissues as characterize the Mar- 
 chantiales. Without doubt, the simple thallose Jungermaniales 
 are the most primitive forms of the hepatics and probably stand 
 nearer the ancestral type than any other form. The Marchan- 
 tiales represent a line in which the tissues of the thallus have 
 attained a considerable degree of differentiation, but in the 
 present order, the evolution led to a marked change of form with 
 but slight modification of the tissues. This is apparent in the 
 highest forms where the leaves, consisting of little more than a 
 single layer of chlorophyll-bearing cells, are arranged in two 
 rather oblique rows upon delicate stems. The under surfaces 
 of these leaves are generally lobed and often form sacs con- 
 taining water (Fig. 197, B). This peculiarity of the leaves, 
 together with the rudimentary third row^ of leaves that are 
 associated with the rhizoids, makes a sharp contrast between 
 the upper 'and lower surfaces and serves to distmgui^ them 
 from the mosses, with which they are often com used . 1 n fact, 
 they are often called scale mosses, owing to the moss-like ap- 
 pearance of their dorsal surface and the close contact formed 
 with bark or other surfaces over which they creep. This habit 
 of pressing the leaves against the substratum tends to retain 
 the moisture and also enables the leaves to absorb it directly 
 through their delicate cell walls. The overlapping of the 
 leaves in many genera, as well as their lobing, would serve the 
 same purpose and enable the plants to live under drier conditions 
 than would otherwise be possible. Doubtless, these departures 
 have been of great advantage to the plants and in part account 
 for the common occurrenx:e of these leafy forms.j ■ It should be 
 stated that the leafy hepatics are regarded as forms that have 
 been evolved from the simple thallose forms in quite recent 
 geological times and owing to their better adaptation to present 
 
i 
 
 DEVELOPMENT OF PLANTS 291 
 
 conditions they have become the most numerous of all the 
 Hepaticae. The cause of the differentiation of ilu- tissues in 
 the two groups is doubtless explained by their relation to mois- 
 ture. The leafy hepatics are easily wetted and the entire i)lant 
 body can readily absorb moisture. Therefore the tissues remain 
 delicate and fewer rhizoids are required to absorb the moisture. 
 The Marchantiales are not wettable, and as a consequence they 
 must develop wick-like strands of rhizoids as well as the elabo- 
 rate storage tissues of the thallus. Both groui)s, and indeed all 
 hepatics, must remain prostrate, because they are dcfx-ndent 
 upon surface water and have not as yet developed an absorbing 
 and conducting apparatus that will permit of any other position. 
 Reproductive Features of the Jungermaniales . — Asexual repro- 
 duction is of the same vegetative type as has been seen in the 
 preceding groups. The archegonia and antheridia are also of 
 essentially the same character as seen in Marchantiale^ They 
 are borne upon the dorsal surface of the thallus or upon more 
 or less modified branches, the archegonia often arising upon 
 the apex of the branch and the antheridia appearing as rather 
 ^^^ spherical bodies in t he a xis of the leaves (Fig. 197, C, D). The 
 archegonia are developed in rather conspicuous cup-like or leafy 
 outgrowths of the thallus. This structure, termed the jx^rianth 
 (Fig. 197, C), assumes very characteristic forms in tlie different 
 genera and is generally associated with modified lea\es, the 
 involucre. By these devices the archegonia gain the same 
 protection against drying out as was secured to them b>- the 
 cavities in which they were developed in the preceding group. 
 The gametospore develops after the manner noted in Marchatitia, 
 forming a capsule with elaters, but its stalk or seta reaches much 
 larger dimensions, owing doubtless to the weli-(le\ cloiu-d to<.)t 
 (Fig. 197, E). The majority of the genera also are characterized 
 by having many of the cells of the capsule sterilized so that 
 the number of spore-producing cells becomes further n-duied. 
 When the spores have been matured the seta rai)i(lly elongates 
 to several times its original length, rupturing tlu* archegonium 
 and lifting the capsule high in the air (Figs. i<)(); n)7. -'0- 
 The capsule usually breaks open into four \al\es which are hygro- 
 
292 DEVELOPMENT OF ANTHOCEROS 
 
 scopic, closing over the spores In damp weather and opening 
 in dry weather to expose them to the wind. These spores germi- 
 nate and begin the life history of a new gametophyte. As in 
 some of the Marchantiales a filamentous alga-like growth is 
 first formed by the germinating spore before the characteristic 
 plant is reproduced. 
 
 1 06. Order c. Anthocerotales or Horned Liverworts. — This 
 small group of four genera is the most interesting of any of the 
 hepatics because it presents features that are suggestive of the 
 algae and also of a relationship with the mosses. ' The thallus 
 is of a primitive type, often with simple lobings and therefore, 
 suggestive of relationship with the simpler Jungermaniales (Fig. 
 198). A peculiar feature of the order is the occurrence of mucil- 
 
 FiG. 198. One of the Anthocerotales, Anthoceros, bearing four pod-shaped 
 sporophytes, j. The one on the right, s, has opened at the top and is dis- 
 charging the spores, but elongation and the formation of spores continue 
 below owing to its basal growth. 
 
 age cavities which communicate with the air through small 
 openings on the under side of the thallus. These cavities are 
 always occupied by one of the blue-green algae, Nostoc, which 
 possibly assists the plant in the retention of water owing to their 
 mucilaginous character. Another interesting feature of the order 
 is the occurrence usually of a single chloroplast in each cell, as in 
 Coleochaete and several other genera of the algae. The archegonia 
 and antheridia, while resembling those of the preceding orders, 
 
DEVELOPMENT OF PLANTS 293 
 
 are sunken In the tissues. This arrangement doubtless protected 
 the reproductive organs just as did the Involucre and perianth 
 in the preceding groups. v,, - ---.,,^^^ 
 
 The most suggestive variation of the Anthocerotales appears 
 in the development of the sporophyte. We have noted that the 
 asexual generation in forms like Ricciocarpiis was a microscopic 
 plant consisting of a delicate spore-bearing capsule. In Mar- 
 chantia the sporophyte is still minute but differentiated into a 
 foot, stalk and capsule, and In the Jungermanlales this becomes 
 much more pronounced. In the Anthocerotales the sporophyte 
 assumes a growth and dIfTerentiatlon that Is much more extensive 
 and complex and it also presents features that Indicate a rela- 
 tionship w^ith the mosses. Furthermore It shows certain features 
 in its development that also arose In the ferns. The germination 
 of the gametospore gives rise to a rather cylindrical or pod-shaped 
 body (Fig. 199), in which all the cells have become sterile save a 
 dome-shaped layer. The basal portion of the sporophyte de- 
 velops into a massive foot, often provided with rhizoldal-like 
 outgrowths, which serve as a very efficient absorbing organ. 
 The upper portion of the sporophyte presents a remarkable series 
 of differentiations. The outer part of it consists of chlorophyll- 
 bearing cells in which, for the first time, genuine stomata appear 
 (Fig. 199, ch). Within this zone of chlorenchyma is a dome- 
 shaped layer of spore mother cells often alternating with sterile 
 cells which in some genera develop as elaters. In the center of 
 the sporophyte is a mass of rather elongated cells, the columella, 
 which assist in conducting food from the foot to the forming 
 spores. Directly above the foot Is a region of rapidly di\iding 
 cells which causes the sporophyte to elongate by basal growth and 
 push out of the archegonium before the spores are mature. This 
 is a radical departure from the other liverworts In the mode of 
 development of the sporophyte and we will see essentially the 
 same manner of growth occurring In the mosses. As the sporo- 
 phyte develops, the spores in the upper part of it mature and the 
 sporophyte splits Into two valves (Fig. 198, s'), thus permitting 
 the scattering of the spores. The lower portion, however, may 
 continue to elongate for several months in some species, forming 
 additional spores. 
 
294 
 
 SPOROPHYTE OF ANTHOCEROS 
 
 It is evident that this sporophyte would become a self-support- 
 ing plant if its foot should reach through the gametophyte and 
 absorb substances from the soil. /In comparing the sporophyte 
 
 Fig. 199. Fig. 200. 
 
 Fig. 199. Section of a young sporophyte of Anthoceros emerging from 
 the involucre-like outgrowth of the thallus — sp, dome-shaped spore form- 
 ing layer of cells; ch, chlorenchyma with stomata; h, foot or absorbing region; 
 c, region of growth. At right, surface view of stoma. 
 
 Fig. 200. A common moss, Funaria: A, two plants with root-like rhi- 
 zoids at base and radially arranged leaves. Rising above the leaves are 
 the stalks or setae and capsules of two sporophytes. B, magnified view of 
 a plant, showing the early appearance of the sporophytes as a delicate stalk 
 still covered by the enlarging archegonium or calyptra. C, a plant bearing 
 antheridia in a rosette of leaves at apex of stem. D, enlarged view of the 
 upper portion of the sporophyte, showing the twisting of the stalk that assists 
 in sifting the spores through the fringe of teeth, peristome, that encircles the 
 mouth of the capsule. 
 
 in forms like Ricciocarpus, Marchantia and the leafy hepatics 
 with Anthoceros, we see that it has undergone a gradual evo- 
 lution and has finally reached a point where it only needs to 
 
DEVELOPMENT OF PLANTS 
 
 295 
 
 come in contact with the soil to become an independent i)laiit 
 since it is provided with all the tissues necessary for photosyn- 
 thesis and absorption. " Note also that the growth of the sporo- 
 phyte is becoming more prolonged, that the spore mother cells 
 appear later in its development and comprise but a small i)orlion 
 of it, and finally, that the sporogenous and sterile tissues alter- 
 nate, forming a dome-like zone of cells. Structures very sugges- 
 tive of these features will appear among the mosses and ferns. 
 
 Class B. Musci or Mosses 
 
 107. General Characteristics. — The mosses are by far the 
 largest group of the Bryophyta and show marked advances over 
 the hepatics in the development of the gametophyte and sporo- 
 phyte. Particularly noticeable is the size and differentiation of 
 the sporophyte which now becomes among the majority of the 
 genera of nearly equal importance with the gametophyte (Fig. 
 200). Variations appear in the mosses that have been successful 
 in adapting them to a great variety of conditions, ranging from 
 submerged aquatics to pronounced xerophytes that live upon 
 exposed rocks. As a result, they are of common occurrence 
 everywhere. The mosses, however, do not show any variations 
 that have led to higher types of plants. In fact, they are re- 
 garded as a highly specialized group, like the Jungcrmaniales, 
 that have branched off from the hepatics in recent geological 
 times and become the dominant representatives of the Bryojihyta 
 owing to their better adaptation to present conditions uj^on the 
 earth. This recent derivation of the mosses is indicated by the 
 remarkable uniformity of structure that characterizes the entire 
 group. 
 
 While the plants themselves are rclati\ely small, they often 
 form large swards or mats, owing to the extensi\e branching 
 and prolonged growth of the stems and their rapid multiplication 
 through the dying off of the older parts and the independent 
 growth of the branches that are thus set free. Owing to this 
 habit of growth and multiplication the mosses often furnish the 
 conspicuous features of the vegetation. Especially is this true 
 in northern regions and in bogs and liarrens \\ lu-re great sections 
 
296 NATURE OF BOG MOSSES 
 
 of the country are carpeted with them. The mosses are leafy 
 stemmed plants like the Jungermaniales, but they usually 
 possess the decided advantage of having erect stems and radi- 
 ally arranged leaves (Fig. 200). Even in the prostrate forms 
 where but two rows of leaves can be developed, the resemblance 
 to the leafy hepatics is only superficial. The moss leaves are 
 not lobed and the central portion is usually traversed by a strand 
 of cells which serve for conduction like the vascular bundles of 
 higher plants (Fig. 206, B). The differentiation of the tissues 
 of the stem also shows a marked advance over preceding forms. 
 A central conducting region of elongated cells that may be 
 compared to a rudimentary vascular system and a cortical zone, 
 often with thickened cells and rudimentary epidermis, are fre- 
 quently to be seen (Fig. 206, A). Asexual reproduction is al- 
 most entirely confined to the detachment of branches as stated 
 above. In only a few genera have the formation of gemmae been 
 noticed. The sexual reproduction and the development of the 
 sporophyte present some interesting features that will be noticed 
 in the following orders. 
 
 108. Order a. Sphagnales. Bog or Peat Mosses. — A single 
 genus, Sphagnum, of numerous poorly defined species is the sole 
 representative of this order and in several respects it occupies 
 an intermediate position between the hepatics and the mosses 
 proper. These pale-green mosses (Fig. 201) grow on bogs and 
 moors and other places where they are subject to drainage con- 
 taining organic matter, as humic acid, derived from the decay of 
 plant and animal life. The majority of plants are unable to 
 endure these conditions, which are popularly referred to as sour, 
 and as a result, you will always find associated w^th the bog 
 mosses a rather limited and peculiar variety of plants such as 
 several genera of heaths, sedges, orchids, pitcher plants {Sar- 
 racenia) and other insectivorous plants like the sundew (Drosera) . 
 For some reason the ordinary plant is not able to procure its food 
 from these sour bogs and this may explain the common occurrence 
 in such places of insectivorous plants, and of certain trees and 
 shrubs that are associated with mycorrhiza. The sphagnums 
 grow luxuriantly in such places, the low^er portions of the stem 
 
DEVELOPMENT OF PLANTS 
 
 297 
 
 dying off and the upper portion branching and conlinuing the 
 growth from year to year. In this way, ponds are gradually 
 covered with a layer of mosses which become rather insecurely 
 bound together by the subsequent introduction of other plants 
 whose roots or stems traverse the covering formed by the mosses. 
 So frail is this covering at first, that a very slight jar will cause 
 the surface to tremble and for this reason these places are known 
 
 Fig. 201. Fig. 202. 
 
 Fig. 201. The bog moss, Sphagtium, bearing three sporophytcs and 
 numerous lateral branches covered with closely overlapping leaves. Note 
 that some of these branches envelop the stem in wick-like strands. 
 
 Fig. 202. Structure of the leaf of Sphagnum: A, section of leaf, showing 
 its single layer of cells that consists of large empty cells alternating with 
 small chlorophyll-bearing ones. B, surface view of the cells, the larger empty 
 cells being marked with spiral bands and often perforated with minute open- 
 ings. 
 
 as quaking bogs. The constant dropping of the deca>ing vege- 
 tation to the bottom of the ponds, assisted by the drainage 
 material, gradually fills them, and in time they may become 
 quite dry. The acid character of these bogs prevents the entrance 
 of those organisms which promote decay. As a result, plants 
 
298 STRUCTURE OF SPHAGNUM 
 
 and animals that fall into these bogs may only partly decay and 
 remain for ages in a wonderful state of preservation as is shown 
 by the recovery of skeletons of animals and clothing of men that 
 belonged to a prehistoric period. It is because of this condition 
 that the bog mosses are of great economic importance. The 
 dead portions of the moss plants, as well as that of the associated 
 plants, do not entirely decay, and consequently there is slowly 
 formed a compact mass of material rich in carbon. This is cut 
 or pressed into blocks, forming peat. This material is an im- 
 portant fuel in Europe and the vast deposits of it in this country 
 will doubtless be utilized in the future. It has been recently 
 shown that the material of some of these bogs is of great value 
 as a source of pulp for paper. 
 
 (a) The Structure and Reproduction of Sphagnum. — The leaves 
 of the bog moss are arranged in compact spirals around the 
 stems and consist of a single layer of cells as In the leafy hepatics 
 (Fig. 202, B). These cells, however, are of two kinds, large 
 and empty cells with spirally thickened walls which are generally 
 perforated with small pores and very narrow cells containing 
 chlorophyll. This distribution of the cells explains the pale^ 
 green color characteristic of the sphagnums. Large and spirally 
 marked cells, similar to those of the leaf, may also occur in the 
 cortex of the stem. The closel}" packed leaves enable the bog 
 mosses to take up water like a sponge and the sphagnums are 
 of considerable commercial value for this reason, being exten- 
 sively employed by horticulturists to keep plants moist during 
 shipment. They are also used extensively in stables in place 
 of straw and they render them almost odorless owing to their 
 absorption of liquids and gases. Perhaps these large cells may 
 serve as floats, enabling the plant to bridge over ponds and they 
 may enable the plant to endure the acid waters in which these 
 plants grow. In this connection, it is noteworthy that rhizoids 
 are entirely lacking and that water and other substances are 
 absorbed by the outer cells of the stems and by certain branches 
 which hang down in wick-like strands close to the main stem 
 (Fig. 201). 
 
 The antheridia and archegonia are developed much as in the 
 
DEVELOPMENT OF PLANTS 
 
 299 
 
 leafy hepatics, the former organs appearin^r as stalked bodies 
 in the axils of the leaves on short cone-like hranclies anrl the 
 archegonia originate on the tips of short l)ranches. The ganieto- 
 spore germinates very much as in Anthoccros (Fig. 203, 6). 
 It does not, however, have as prolonged a grcnvlli, and at nia- 
 
 FiG. 203. The sporoph^'tc of Sphagmim: 6, the young sporophyte sepa- 
 rated from the archegonium with essentially the same dififcrcntiation of 
 parts as noted in Anthoceros, Fig. 199. 5^, diagram of a later development 
 of the sporophyte in the archegonium. The enlarged foot, b, is embedded 
 in the apex of the moss branch, p, and the spores, sp, form a dome-shaped 
 layer in the upper part of the capsule. 5, the naked stem, p, of the moss 
 branch, surrounded at base with spirally arranged leaves, has elongated, 
 lifting the mature sporophyte into the air. The enlargement of the stem, 
 b, is due to the growth of the foot region of the sporophyte; ca, remains of 
 the ruptured archegonium or calyptra; o, Hd or opercuhim of the capsule. 
 — After Schimper. 
 
 turity consists of a well-developed foot emhc-dded in ilie tissues 
 of the gametophyte and a spore-bearing capsule (Fig. 203, 5J). 
 The stomata and chlorenchyma which were so consj)icu()Us in 
 Anthoceros are less perfectly represented, hut the spore mother 
 are cells still developed in a dome-shajH-d zom-. 'i'his rather 
 minute sporophyte at maturity- barely breaks through the arche- 
 gonium, known in this condition as the cahjitra, whicii covers 
 the capsule as a cap. The absence of a conspicuous stalk or seta 
 
300 
 
 SPOROPHYTE OF SPHAGNUM 
 
 in the sporophyte of Sphagnum is provided for by the elongation 
 of the upper part of the moss stem which pushes the sporophyte 
 above the leaves and exposes the capsule to the winds quite as 
 
 Fig. 204. Fig. 205. 
 
 Fig. 204. Germination of the spore of Sphagnum: A, early growth of 
 the spore. B. later development — sp, spore; &, bud developing into moss 
 plant. C, margin of the thallus, showing the origin of the bud. — After Camp- 
 bell. 
 
 Fig. 205. The twisted stalk moss, Funaria: A, two sporophyte-bearing 
 plants, the remains of the archegonium or calyptra still attached to the cap- 
 sule on the right. B, enlarged view of plant with the young sporophyte, 
 still enclosed in the archegonium, just emerging above the leaves. C, the 
 male plant bearing the antheridia in a conspicuous rosette of leaves. D, 
 upper portion of the sporophyte, showing the twisting of the stalk or seta 
 that assist in sifting the spores through the teeth, peristome, that encircle 
 the mouth of the capsule. 
 
 effectually as the seta of the hepatics and mosses (Fig. 203, 5). 
 The spores are freed by the forcing off of a circular lid, the 
 operculum, as is the case among the majority of the mosses 
 (Fig. 203, 0). This is effected by a ring of thin- walled cells, 
 
DEVELOPMENT OF PLANTS 301 
 
 the annulus, that lies in the groove just below llie opcrcuhini, 
 As the walls of the capsule dry out at maturity lhe>' shrink, 
 compressing the air w^ithin the capsule until the tension ruptures 
 the delicate ring of cells and throws out the spores with an 
 audible report. No elaters are associated with tlie spores in any 
 of the mosses. 
 
 The germination of the spores results in the formation of a 
 short chain or filament of cells which soon develop into a flat 
 and rather irregular thallus consisting of a single layer of cells 
 and attached to the ground by numerous rhizoids (Fig. 204). 
 This thallus may produce from its marginal cells other filaments 
 which behave like the original one and thus serve to increase the 
 number of thalli. The archegonia and antheridia are not de- 
 veloped upon these thalli, but there are first formed as in most 
 of the Jungermaniales, bud-like growths which develop into leafy 
 stems of the sphagnum (Fig. 204, b). These stems soon become 
 independent plants through the decay of the thallus and finally 
 bear the reproductive organs. 
 
 109. Order b. Bryales. The Higher or True Mosses.— The 
 great majority of plants, commonly known as mosses, belong 
 to this order. As a rule, these plants are of a higher type than 
 any of the other Bryophyta (Fig. 205). The stems are frequently 
 erect wdth radially arranged leaves and show consideral)k' ditttr- 
 entiation of tissues, as is apparent in the cross-section of the stem 
 of one of the higher forms, shown in Fig. 206, A. Particularly 
 noticeable are the thin-walled cells located in the central strand 
 of the stem which serve to conduct the fluids and are therefore, 
 comparable to the tracheal tissue of the higher plants. A similar 
 conducting tissue is often seen in the central strand that appears 
 in the leaves of the majority of the Bryales (F'ig. 206, B). In 
 connection with this higher development of the plant, attention 
 should be directed to the more efficient organs of absorption. 
 The rhizoids that spring from the base of the erect stems or from 
 the under side of forms with creeping stems are often larger than 
 those noted in the hepatics, and generally they are multicellular 
 and often twisted together into root-like strands (iMg. 206, C). 
 These features doubtless account in part f(^r the larger size that 
 
302 
 
 STRUCTURE OF THE BRYALES 
 
 is attained by many of the Bryales. It is noteworthy, however, 
 that the devices for absorption and conduction of water are not 
 as yet sufficiently developed to enable these plants to attain 
 any considerable height. Erect forms rarely exceed a few centi- 
 meters in height, but prostrate forms creep over the ground in- 
 definitely. Some mosses are short lived and many are per- 
 ennial, continuing their apical growth from year to year. Some 
 can live only in the wettest of places or as submerged aquatics, 
 
 Fig. 206. Structural features of the moss plant; A, cross-section through 
 the basal region of a moss stem, showing rhizoidal outgrowths from epi- 
 dermis, cortical and central conducting tissues. B, leaf of Mniiim with mid- 
 vein of elongated conducting cells. C, rhizoids twisted together into root- 
 like strands. D, cross-section of a leaf of Polytrichiim, showing the partial 
 folding in of the margins of the leaf to protect the delicate plates of chloro- 
 phyll-bearing cells against drought. 
 
 while others are capable of enduring almost complete desiccation 
 and revive quickly when moistened. An interesting device of 
 service in adapting these plants to periods of drought appears 
 in the leaves of some of the higher forms, as the hair cap moss, 
 Polytrichum. Plates of green cells parallel to the midvein project 
 from the upper surface of the leaves, thus greatly increasing the 
 surface of the leaves for photosynthesis and also serving as water 
 
DEVELOPMENT OF PLAN FS 
 
 303 
 
 Fig. 207. Male reproductive organs: i, male plants of J'olylrichum 
 bearing anthericlia -^in cup-like buds. 6, section of apex of stem of Funaria 
 — an, antheridia;'^,fparaphysis. 7, male gametes, the one at left still en- 
 veloped in mucilaginous remains of mother cell wall. 
 
 9 
 
 Fig. 208. Female reproductive organs: 8, l()ni;iiiitliiMl MCtion i>f a|x-x 
 of stem, showing arrangement of archcgonia. 9, arihcgonium enlarged, 
 the canal cells just beginning to dissolve — g, the female gamete. 
 
304 
 
 SPOROPHYTE OF THE BRYALES 
 
 reservoirs (Fig. 206, D) . During drought, this delicate apparatus 
 is protected by the coiling of the leaves into needle-like rolls 
 which result in the exposure only of the thickened epidermal cells. 
 The sexual organs are developed on the apices of the main or 
 lateral branches and they are protected by modified leaves which 
 are often colored and form a more or less conspicuous bud or 
 cup (Fig. 207, i). This is especially true of the antheridial 
 buds, the plants being monoecious or dioecious. The antheridia 
 and archegonia are essentially of the same structure as noted in 
 
 Fig. 209. Germination of the gametospore: 10, base of the archegonium 
 in which the gametospore has germinated, forming a mass of cells with apical 
 growing cell, x. ii, later growth of the gametospore. The sporophyte, 
 spr, still enveloped by the archegonium, ar, appears as a cylindrical mass 
 of cells with foot, h, penetrating the stem of the moss plant. At right an 
 unfertilized archegonium. See Fig. 205, B. — After Sachs. 
 
 previous groups and they are usually associated with modified 
 leaves known as paraphyses (Figs. 207, 6; 208). The germina- 
 tion of the gametospore and the development of the sporophyte, 
 while presenting many features in common with the hepatics 
 and Anthoceros in particular, shows a remarkable series of varia- 
 tions that are of decided advantage to the plant. The gameto- 
 spore in its early growth forms a spindle-shaped mass of cells, 
 
 I 
 
. DEVELOPMENT OF PLANTS 305 
 
 the basal portion of which reaches down into the stem oi tlie 
 moss plant and forms a well-developed absorbing organ or foot, 
 while the upper portion elongates by means of an apical cell 
 (Fig. 209, 10). Later, the growth becomes basal as in Atitho- 
 ceros. For a time, the archegonium keeps pace with ilie elonga- 
 tion of the young sporophyte (Fig. 209, 11), but finally it is 
 ruptured and lifted up as a cap, called the calyptra, on the apex 
 of the young sporophyte. This is one of the most essential 
 differences between the mosses and hepatics. In tlie lat ter group, 
 with the exception of the Anthocerotales, the spores were matured 
 in the archegonium. In the Bryales, the spores are usually 
 not formed until the seta has elongated considerably. As the 
 sporophyte elongates, the upper part enlarges and finally forms 
 a complex capsule which remains covered for a varying length 
 of time by the calyptra. The hairy calyptra of Polytrichiim 
 (this name means many hairs) is due to the development of a 
 felt of hairs upon the archegonium which forms a protective 
 covering to the young sporophyte against the loss of water. 
 
 The capsule and calyptra assume va^-ious forms and positions. 
 In some cases the capsule is quite erect and completely covered 
 by the calyptra (Fig. 210, i), and again it may be more or 
 less inclined or even pendulous and the calyptra laterally placed 
 like a cap (Figs. 205, A\ 211, 5). Removing the calyptra >ou 
 notice that a lid or operculum is situated on top of the capsule 
 while at the bottom is a somewhat enlarged region, the apophysis, 
 provided with stomata that communicate with the interior re- 
 gions of the capsule (Figs. 210, 2; 211, 5). When the opercu- 
 lum is removed a circle of minute teeth-like structures, the peri- 
 stome, appears that more or less completely closes the mouth of 
 the capsule (Fig. 205, D). In some mosses a delicate membrane, 
 the epiphragm, lies just below the peristome, which assists in 
 closing the mouth of the capsule (Fig. 210, 2B, ^A). We are 
 now interested to know the meaning of these structures and to 
 learn how they have come about. By examining a longitudinal 
 section of the nearly mature capsule you will see that the spore- 
 producing tissue is relatively small and appears as a hollow 
 cylinder. The progressive sterilization that we haw seen going 
 
3o6 
 
 CAPSULE OF BRYALES 
 
 on has here resulted in the transformation of the upper part of 
 the dome-shaped zone of sporogenous cells into vegetative cells 
 (Figs. 210, 3; 211, 6). The sporogenous tissue is usually 
 
 sur- 
 
 FiG. 210. The mature sporophyte of Polytrichum: i, moss plant bearing 
 sporophyte. lA, calyptra removed, showing capsule, which has curved to 
 one side. 2, capsule, the calyptra, 2A, removed, showing the operculum or 
 lid and the enlarged apophysis, a, at base. 2B, capsule with operculum re- 
 moved, showing the teeth-like peristome and the epiphragm, which has 
 been lifted up at one side. 3, section of nearly mature capsule — sp, cyl- 
 inder of spore-forming cells that is surrounded on its inner and outer sides 
 by loosely arranged chlorophyll cells; 0, operculum; r, annulus; p, peri- 
 stome; e, epiphragm; a, apophysis. 3^, stoma of apophysis. 3^, cross- 
 section of capsule — sp, spore-forming cells. 4/I, portion of mouth of cap- 
 sule, showing position of peristome and epiphragm in dry weather. 4^, 
 peristome pressed down on epiphragm, closing the capsule in damp weather. 
 — H. O. Hanson. 
 
DEVELOPMENT OF PLANTS 
 
 307 
 
 rounded on the outside, and less commonly on the inside, by 
 loose chlorophyll-bearing cells which extend down to the base 
 of the capsule where chlorenchyma and stomata are developed. 
 This lower region of the capsule, the apophysis, is often enlarged 
 and provided with air spaces and stomata, thus constituting the 
 chief photosynthetic apparatus of the sporophyle (Fi^^. 210. :^A ). 
 
 F:o. .11. Structure of capsule of ^"--^5' "^^J^ V^ l,;; ^i 
 removed. 6, section of nearly mature ^f P^^^^-J^,^,;^^:;,^^ .. o,xt. 
 
 roundea on outside by loosely arranged ^h oroplnll-bcarm, c . ^,^ ^ 
 
 culum; .. annulus; p, peristome; a. ^pophys .- J>. ^ ^ .,.^^.,„,, ,,„, 
 portion of the capsule, showing the ^-^^-^.;^^ he cpi<lcrnus by 
 of the peristome. ^ which --/"^^f ,,^\;':;;t Hnc should ^ 
 a double row of cells; ^^ spore-formmg c^, he It 1 ^^^^^^ ^^^^^^ 
 
 four roundish cells at the left. 7. '^ '^^^^^L of the ,K-ristom..- ^•••- 
 thus forming the inner and outer teclh-hke >ogmcnt^ 1 
 
 Sachs. 
 
3o8 
 
 THE PERISTOME OF THE CAPSULE 
 
 The entire surface of the capsule is protected by a well-developed 
 epidermis but at the top a lid or operculum is cut off by a ring 
 of rather delicate mucilage-bearing cells, the annulus, which 
 swells at maturity and so assists in casting off the operculum 
 (Fig. 210, 3, r). Below the operculum a layer of cells, known 
 as the peristome, extends across the capsule and becomes greatly 
 thickened on its outer and usually on its inner walls. This layer 
 of cells is firmly attached to the walls of the capsule by rather 
 short thick-walled cells (Figs. 210, 3, p] 211, 6-8, p). When 
 the spores are mature, the more delicate cells break down, leaving 
 little more than a loose mass of spores and the peristome within 
 the capsule. The cells of the peristome break apart into teeth- 
 
 FiG. 212. Germination of the spore: 3, early stage in the germination, 
 4, character of the branching, alga-Hke filaments that are finally developed 
 from the spore — r, rhizoids which penetrate the ground; b, bud which will 
 develop into a leafy moss plant. 
 
 like segments (Figs. 211, 7) and being very hygroscopic, co- 
 operate with the contracting capsule in forcing off the operculum. 
 After the removal of the operculum the mouth of the capsule is 
 covered by the peristome which now appears as a fringe of teeth 
 radiating out from the walls of the capsule (Fig. 210, 4). The 
 teeth of the peristome are always in multiples of two and form 
 one or more, usually two layers, accordingly as only the outer or 
 
DEVELOPMENT OF PLANTS 309 
 
 both surfaces of the peristome cells are thickened. These teeth 
 are so variously marked and fashioned that they furnish one of 
 the most characteristic features of the various genera of the 
 mosses. The peristome closes the mouth of the capsule in wet 
 weather, but on dry days it opens in a variety of ingenious ways 
 in the various species of mosses so as to effect a gradual scattering 
 of the spores. The seta also contributes in the distribution, 
 being very elastic and often hygroscopic (Fig. 205, D). 
 
 The spores germinate and produce profusely branching chains 
 or filaments of green cells, called the protonema (Fig. 212). In 
 a few instances, thalloid structures are developed as in Sphagnum. 
 The protonema absorbs the earth substances through delicate 
 rhizoids and spreads over the ground, appearing to the eye as a 
 green mould or fine algal cells. Goebel presents some evidence 
 tending to show that the mosses may have been derived from 
 branching forms of the green algae. Buds finally appear on these 
 branches at numerous places and develop the leafy stems of 
 the moss plant (Fig. 212, h). This in part accounts for the dense 
 colonies so characteristic of the mosses. As soon as the young 
 plants are established in the soil by means of rhizoids that spring 
 from the basal parts, the protonema withers away, although in 
 some genera it is perennial and continues to produce new plants 
 from year to year. 
 
 no. The Relationship of the Bryophyta. — It appears probable 
 that there have been three lines of variation or evolution among 
 the Bryophyta that have had their origin in some simple thalloid 
 form most nearly related to the lower Jungcrnianialcs. One line 
 has resulted in the highly differentiated thallus of the Marchanti- 
 ales with its simple sporophyte. Another, the Jungermaniales, 
 retaining the simple structures of the primitive thallus has modi- 
 fied its form into a leafy plant and developed a more complex 
 sporophyte, while the variations of the third line, or Anthocero- 
 tales, have resulted in slight alterations of the primitive type of 
 the thallus but in profound modifications of tlie sporophyte. The 
 variations of the sporophyte in the latter group appear to have 
 been very advantageous to the present conditions upon the earth 
 and perhaps led to the mosses. The important feature in this 
 
310 EVOLUTION OF THE SPOROPHYE 
 
 evolution, already noted among the algae, is the increasing im- 
 portance of the sporophyte. Among the green algae it was de- 
 pendent upon the amount of food stored in the gametospore. 
 In the Bryophyta, it is better nourished, owing to its essential 
 parasitic habit. This led to a larger growth and more complex 
 development and it is especially important to note that the sporo- 
 phyte became more profoundly modified by the stimuli of external 
 conditions than did the gametophyte. For example, we see in 
 the sporophyte of Anthoceros and of the mosses, a plant that is 
 practically dependent upon the gametophyte only for water and 
 the earth substances. So the way is prepared for a study of 
 the next division where the sporophyte finally comes to absorb 
 its crude materials directly from the soil and thus becomes an 
 independent self-supporting plant. 
 
CHAPTER VIII 
 
 DIVISION III. PTERIDOPHYTA OR FERNS 
 
 III. Nature of the Ferns. — The members of this division differ 
 greatly in character and Hve under a wide range of conditions, 
 although, like the Bryophyta, they usually frequent moist and 
 shady places, as glens, ravines and damp woods, or in tropical 
 countries they often cover the moist rocks and tree trunks with 
 a luxuriant and attractive vegetation. The most noticeable fea- 
 
 Fig. 213. The Christmas fern, Polystichiim, with young leaves uncoiling 
 in the spring at the tip of the stem and further back the fully developed 
 leaves of the past season. The older portion of the stem is covered with 
 the petioles of leaves that have died off and numerous roots are seen arising 
 from the stem and bases of the petioles, — H. O. Hanson. 
 
 311 
 
312 SUPREMACY OF THE SPOROPHYTE 
 
 true in the fern is the large development and complex character 
 of the plant (Fig. 213). This is due to a more efficient absorb- 
 ing and conducting apparatus, the root and vascular bundles. 
 This conducting system was foreshadowed in the elongated cells 
 that appear in the sporophyte of Anthoceros and of the mosses. 
 Here the elongated cells are much more specialized and are united 
 into large bundles, termed the vascular bundles, that appear in 
 the leaves as "veins" and in the stems they are variously related 
 and often constitute a conspicuous portion of these organs, as 
 is the case in the higher plants, see page 76. The fern plant does 
 not correspond to the moss plant. It is the sporophyte which has 
 become independent of the gametophyte. The sexual generation 
 is a small thallus, commonly called the prothallium, that is quite 
 as primitive as the simplest Jungermaniales. Its chief function is 
 the formation of gametes (page 282) and it is capable of nourish- 
 ing the sporophyte only for a short time in the majority of ferns 
 
 Fig. 214. The thalloid gametophyte or prothallium, p, of a fern from 
 which a young fern plant is being developed. This sporophyte has already 
 developed two roots, r, that penetrate the soil, and two leaves, /, are ap- 
 pearing, thus making the fern at an early age independent of the gameto- 
 phyte; rh, rhizoids. 
 
 (Fig. 214). The gametophyte of the Bryophyta never attained 
 any considerable proportions owing to its inability to vary and 
 produce adequate absorbing organs and conducting tissues. 
 This naturally limited the development of the sporophyte which 
 
DEVELOPMENT OF PLANTS 313 
 
 was parasitic upon it. In the Ptcridophyta, however, the 
 sporophyte is not at such a disadvantage since it actually becomes 
 independent of the gametophyte owing to the development of a 
 root w^hich puts it in communication with the earth substances. 
 This radical departure in its mode of life acted as a profound 
 stimulus and cooperated in inducing marked variations that were 
 so beneficial in character as to cause the sporophyte to assume 
 large proportions and become differentiated into a highly or- 
 ganized plant. The stem elongates through the repeated divi- 
 sion of a single apical cell, as in the Bryophyta, but the stem and 
 also the leaves and roots in addition contain vascular bundles 
 and an arrangement of tissues already noticed in the higher 
 plants. 
 
 The gametophyte in many of the Pteridophyta is very sugges- 
 tive of the simpler thalloid hepatics. Owing to the fact that it 
 is no longer permanently burdened with the nutrition of the 
 sporophyte, it becomes greatly reduced in size and length of life. 
 In fact, in several of the more specialized ferns, the gameto- 
 phyte may be reduced to a few cells and the entire development 
 may take place within a day. The reproductive organs and the 
 germination of the gametospore are suggestive of the correspond- 
 ing features noted in the Bryophyta. The more important orders 
 of the Pteridophyta are: i. Ophioglossales. 2. Filicales or Com- 
 mon Ferns. 3. Equisetales or Horsetails. 4. Lycopodiales or 
 Club Mosses. This sequence is followed because it brings out 
 more clearly the trends or tendencies that arose step by step in 
 the evolution of plant life. As a matter of fact the club mosses 
 appear to be the most primitive forms and they are the most 
 suggestive of the Bryophyta; while the common ferns are the 
 most recent in development. Enough has been said to show that 
 these forms are very remotely related to the ancestors of the Bry- 
 ophyta and that these four orders of Pteridophyta are very dis- 
 tantly connected. 
 
 Order~i. Ophioglossales 
 
 112. General Characters of the Sporophyte. — This order con- 
 tains three genera which are probably but a remnant of an 
 earlier and widely distributed group. Only two, Ophioglossum 
 
314 STRUCTURE OF OPHIOGLOSSALES 
 
 and Botrychium, are of common occurrence (Fig. 215). They 
 are of unusual Interest because they present many features sug- 
 gestive of the Uverworts and also of the more specialized ferns 
 and seed plants. This remark is not intended as necessarily 
 implying relationship between the three groups, but that analo- 
 gous structures have arisen in each, due either to inheritance 
 from allied ancestors or to the operation of stimuli upon plants 
 that are only slightly or not at all related. The sporoph^^te 
 consists of a short stem with thick flesh^^ roots that are associated 
 with mycorrhiza. The leaves are simple or divided and usually 
 appear singly, ensheathing the apex of the stem which always 
 remains in the soil. One of the most remarkable features about 
 these ferns is the development in the stems, roots and leaves 
 of the same tissues that we have noted in the higher seed plants. 
 The vascular bundles have the same origin as in the seed plants. 
 The woody portion (xylem) of the conducting system appears as 
 a solid strand in the center of the very young stem and this is 
 surrounded by the cellulose part (phloem) of the system. Early 
 in the development of the stem delicate cells (pith) replace the 
 central cells of the xylem so that the conducting system now 
 appears as a hollow cylinder. As the stem elongates this cylinder 
 is broken into segments by certain strands from it passing out 
 into the leaves. The spaces formed by the departure of these 
 strands are termed foliar gaps. The segments of the con- 
 ducting system are termed the vascular bundles and since the 
 phloem is outside the xylem they are known as collateral bundles. 
 In some cases these bundles develop a cambium which adds new 
 cells to the xylem and phloem so that the arrangement of tissues 
 in the stem is strikingly suggestive of the dicotyledonous stem 
 with its pith; concentric rings of xylem, cambium and phloem; 
 medullary rays; cortex; cork; etc. (Fig. 216, 40). It is note- 
 worthy also that the cells added to the xylem by the cambium 
 resemble those appearing in the pines. The stem structure of 
 some of the simpler representatives of the next order resembles 
 that of the Ophioglossales and it has been suggested that the 
 ancestors of these two lines of Pteridophyta may have been the 
 starting point of the seed plants. 
 
DEVELOPMENT OF PLANTS 
 
 315 
 
 The spores are borne in peculiar modified outgrowths of the 
 leaves that assume a cylindrical shape in Ophioglosstim and be- 
 come more or less branched in the species of the Botrychium 
 (Fig. 215). The structure of these organs presents many fea- 
 tures that recall the sporophyte of Anthoceros (Figs. 215, B; 
 199). The outer portion of the cylinder consists of chlorophyll- 
 bearing cells which communicate with stomata in some of the 
 forms at least. The spore mother cells are also distributed as 
 
 Fig. 215. The two common genera of the Ophioglossales: A, the addcf 
 tongue fern, Ophioglossum, with single leaf ensheathing the short stem and 
 producing a spore-bearing spike. B, section of the spike — sp, the spore- 
 forming cells arranged in groups or sporangia. The spores are exposed by 
 the breaking apart of the cells between the dark lines. C, stoma from epi- 
 dermis of spike. D, the grape fern, Botrychium. The leaf is much divided 
 and also forms a branched spore-bearing organ. E, two sporangia, showing 
 the manner of opening for discharge of spores. 
 
 in Anthoceros^ the essential difference being that they are sepa- 
 rated by a larger number of sterile cells and a larger number of 
 spore mother cells are also grouped together, forming rather con- 
 spicuous sacs or sporangia (Fig. 215, sp). The spores are formed 
 
3i6 
 
 NATURE OF THE SPORANGIA 
 
 from the mother cells as in the Bryophyta and are discharged 
 from the sporangia through a transverse cleft (Fig. 215, E). It 
 is interesting to note in a closely allied order occurring in the 
 
 ■^.zX- 
 
 Xcr T >-\ r 
 
 ^^^^^^ 
 
 Fig. 216. Cross-section of a stem of Botrychium: p, pith; x, xylem: 
 w, medullary ray; c, cambium; ph, phloem; e, endodermis; cr, cortex. — 
 After Jeffrey. 
 
 tropics, that the sporangia are developed directly upon the leaves 
 (Fig. 217) instead of upon special branches and that they are 
 sometimes associated together in groups, known as sori (sing. 
 
 Fig. 217. Arrangement of the sporangia of an allied order, Marattiales; 
 A, leaflet of Archangiopteris with sporangia on surface of leaf and arranged 
 in groups or sori. B, magnified view of a portion of the leaflet. C, section 
 of leaf, showing two sporangia, the left-hand one in section. 
 
 sorus) and provided with thickened cells, the annulus — charac- 
 ters that will become conspicuous in the next order. These 
 sporangia originate, however, as in Ophioglossum and open by 
 transverse clefts, which operation is promoted by the unequal 
 drying of the thick- and thin-walled cells of the annulate forms, 
 (a) The Gametophyte. — The gametophyte or sexual genera- 
 
DEVELOPMENT OF PLANTS 
 
 317 
 
 tion is not often met with, owing to the fact that, Hke the sporo- 
 phyte, its development is associated with mycorrhizal fungi and 
 consequently it is usually completely buried in the soil and usually 
 quite devoid of chlorophyll. The various stages in the germina- 
 tion of the spores have never been observed, but the mature gam- 
 etophyte of Botrychium was found a few years ago by Jeffrey 
 
 Fig. 218. The gamctophyte and young sporophyte of Botrychium: A, 
 tuberous appearance of the gametophyte — e, a young sporophyte or em- 
 bryo developing in one of the archegonia. B, a section of gametophyte — 
 ar, archegonia; an, antheridia. C, two antheridia in section. D, male 
 gamete. E, archegonium before dissolution of canal cells. F, gametospore 
 in two-cell stage of germination. G, young sporophyte with roots and first 
 leaf developed but still attached by foot to the round gametophyte, gm. 
 A. F.— After Jeffrey. 
 
 and carefully studied. It appeared as a rather tuberous body 
 that is provided with numerous rhizoids and a growing apical 
 cell as in the Bryophyta (Fig. 218, A, B). The archegonia and 
 antheridia are usually borne upon the upper surface of the game- 
 
3i8 NATURE OF THE GAMETOPHYTE 
 
 tophyte and in origin and appearance are rather suggestive of 
 Anthoceros. The antheridia consist of a number of mother cells 
 quite buried in the tissues of the thallus, being covered by one 
 or two layers of cells which are destroyed or break open when 
 the male gametes are mature (Fig. 218, C). These gametes are 
 quite different from any yet seen, being spirally coiled and pro- 
 vided with numerous cilia (Fig. 218, D). The archegonia are 
 similar to those noted in Anthoceros, consisting of a rather short 
 neck w^ith two central canal cells and a swollen base sunken in 
 the tissues of the gametophyte (Fig. 218, £). The female gamete 
 is formed at the base of the canal cells which finally become 
 mucilaginous and thus form an open passage-way for the entrance 
 of the male gametes as soon as the lip cells have opened. It has 
 been shown in the common ferns that malic acid is present in 
 this mucilaginous substance which strongly attracts the male 
 gametes so that they crowd into the canal, often completely chok- 
 ing it. So we see that the gametophyte of this fern (though 
 differing in form) is essentially of the same character as in the 
 Bryophyta, its subterranean character and absence of chlorophyll 
 being due to its association with fungi. It should be stated that 
 lobes are produced from the end of the tuberous gametophyte 
 of one of the species which become green on reaching the surface 
 of the soil and in a closely allied order or tropical ferns, the 
 sexual generation is normally a green thallus strikingly like some 
 of the simpler Jungermaniales. 
 
 {h) The Germination of the Gametospore. — ^The early develop- 
 ment of the sporophyte resembles Afithoceros in many respects. 
 The gametospore divides into two cells (Fig. 218, 7^), from the 
 lower of which a large foot is formed, and from the upper cell 
 the short stem and root arise. Later, the first leaf or cotyledon 
 is formed from the stem. This development of the sporophyte 
 goes on very slowly within the archegonium where it lives as a 
 parasite for a long time. Eventually the root ruptures the arche- 
 gonium or calyptra and comes in contact with the soil. The 
 cotyledon now grows upward and makes its appearance above 
 the ground as the first green leaf (Fig. 218, G). The sporophyte 
 thus becomes a self-supporting plant, although it probably re- 
 
DEVELOPMENT OF PLANTS 319 
 
 mains in part dependent upon the ganietophyte for several years. 
 At this stage of development the young sporoplnte is a very 
 simple type of fern, but gradually more efficient roots are de- 
 veloped and each year the stem sends up a larger and larger leaf 
 until the adult size is reached when the spore-bearing branch is 
 formed (Fig. 215), thus making possible again a new series of 
 gametophytes. 
 
 (c) Comparison of the Adder Tongue Ferns with Preceding 
 Groups. — The most noteworthy difference betAveen these simple 
 ferns and the Bryophyta is the larger development of the sporo- 
 phyte and its final independence of the gametophyte. This is 
 doubtless due to the development of true roots which made pos- 
 sible a continuous and abundant supply of the crude materials 
 from the soil. This change acted as a stimulus which promoted 
 variations in the sporophyte while the light and various climatic 
 factors also assisted to a very marked degree. Among the algae, 
 the gametophyte is the dominant generation, the sporophyte 
 being represented often by the gametospore. This is essentially 
 the relationship among the majority of the Hepaticae where the 
 gametophyte performs all the work of food construction and the 
 sporophyte is a minute parasitic plant upon it. In Anthoceros 
 and the mosses, the sporophyte is longer lived and more highly 
 organized, but the gametophyte is still the more important gen- 
 eration as it performs the major portion of the work. Among the 
 ferns, this relationship and the distribution of labor is completely 
 turned about. The sporophyte becomes the larger plant and the 
 principal center of photosynthesis, while the gametophyte re- 
 mains small and gradually becomes very short lived. 
 
 There is also to be noted the longer postponement in the for- 
 mation of the spores. In certain of the algae the gametospores 
 produce the spores directly on germinating while in Ricciocarpiis 
 a small mass of cells is first formed and certain of these cells 
 soon become spore mother cells. In Marchantia, a larger number 
 of cells are formed by the gametospore and the spore mother cells 
 originate after a few weeks in a definite region of the sporophyte. 
 These features are more noticeable in the Jungermaniales and 
 especially in the long-lived sporophyte of Anthoceros. Recall 
 
320 DECLINE OF THE GAMETOPHYTE 
 
 also the considerable growth of the sporophyte of the Bryales 
 and the final development of the spore mother cells In a definite 
 and restricted region In the capsule. In the Pterldophyta, the 
 sporophyte not only becomes larger and more highly differenti- 
 ated, but the formation of the spores is often deferred for years 
 and the spore mother cells are localized in special organs. This 
 postponement of the spore formation ma}^ appear at first sight 
 as a disadvantage since the sporophyte is exposed to many dan- 
 gers during the long period of preparation for Its work. However, 
 the delay Is more than compensated for by the large number of 
 spores that are finally formed and also by the fact that the sporo- 
 phyte does not entirely perish as In previous cases, but lives on, 
 sending up annually new spore-bearing leaves. It would appear 
 that a point had at last been reached In the evolution of the 
 sporophyte where it Is so well organized as to ensure Its exist- 
 ence. It can consequently with safety defer the formation of 
 spores until well-developed roots, stems and leaves have been 
 formed. The independent existence of the sporophyte brings 
 out very clearly the two phases In the life history of the plant 
 which we call the alternation of generations. The formation of 
 the spores marks the beginning of the gametophyte or sexual 
 generation, which ends with the formation of the gametes. The 
 formation of the gametospore through the fusion of the gametes 
 starts the sporophyte or asexual generation which ends with the 
 division of the spore mother cells. 
 
 Why has not the gametophyte varied as well as the sporophyte? 
 Perhaps the simplicity of its structure has become fixed by in- 
 heritance from a long line of algal ancestors. The occurrence 
 in the Bryophyta and Pterldophyta of motile male gametes and 
 the consequent necessity of water for fertilization, points to the 
 inheritance of this peculiarity from aquatic ancestors. It is evi- 
 dent that there can be no considerable modification of the gameto- 
 phyte, as long as this feature Is retained, without greatly decreas- 
 ing the possibilities of fertilization. Consequently the gameto- 
 phyte must remain of necessity a primitive structure. The 
 sporophyte, on the other hand, was accustomed from the first to 
 terrestrial conditions, or to -such conditions as were unfavorable 
 
DEVELOPMENT OF PLANTS 321 
 
 to the growth of the gametophyte. Among the algae we have 
 noticed that it may carry the plant over the more or less com- 
 plete drying up of the water or unfavorable temperatures. Per- 
 haps these exposures to a variety of stimuli which the gameto- 
 phyte never experienced or in a lesser degree enabled the sporo- 
 phyte to respond with more profound variations when it became 
 parasitic upon the gametophyte and exposed directly to the 
 light and air as was the case among the Bryophyta. Certainly 
 we know that abundance of foods, light, temperatures, etc., 
 are among the important stimuli in causing variations. The 
 simple sporophyte of the Bryophyta was exposed to just such 
 forces as these and steadily gained in complexity until in the 
 Bryales it nearly equaled in importance the gametophyte. The 
 sporophyte of the Pteridophyta, owing to the development of the 
 root, experiences a new stimulus, i. e., that of the soil which, co- 
 operating with the stimuli of the light, etc., causes it to assume 
 much larger proportions than the gametophyte and become the 
 dominant generation in the life history of the plant. Perhaps 
 another factor played an important role in securing the supremacy 
 of the sporophyte. We have seen in paragraph 58, The Signifi- 
 cance of Fertilization, that the sporophyte is the result of the 
 fusion of two gametes w^hich may vary in character. The con- 
 stant introduction of variant characters into the asexual genera- 
 tion in this way may have cooperated in its increasing complexity. 
 
 Order 2. Filicales or Common Ferns 
 
 113. General Features. — The great majority of plants popu- 
 arly known as ferns belong to this order. They are a highly 
 specialized group that have branched off from some primitive 
 fern stock in recent geological times and owing to their variations 
 being highly adapted to present conditions upon the earth, they 
 lave become very numerous and widely distributed — probably 
 exceeding 3000 species. In temperate climates the majority 
 live upon the ground in moist and shady regions and some are 
 aquatic or xerophytic. They attain their greatest abundance 
 in the mountainous district of tropical countries where they occur 
 in astonishing profusion and variety upon the moist rocks and 
 
322 
 
 LEAVES OF FI Lie ALES 
 
 trunks of trees, as well as upon the earth. The leaves are the 
 most striking feature of the Filicales. They are usually large and 
 divided and are characterized by being coiled when young (Fig. 
 219). This is due to the stronger growth of the outer cells which 
 causes an inward rolling of the leaf. The growth of the leaf is 
 very slow, often requiring three years for its formation in tem- 
 
 FiG. 219. Christmas fern, Polystichum, with prostrate stem bearing at 
 the tip young coiled leaves covered with chafify scales and further back large 
 leaves of the previous season. The older portions of the stem are covered 
 with the petioles of the dead leaves. — H. O. Hanson. 
 
 perate regions. During the season preceding its expansion, the 
 petiole and blade are completely formed and appear in crosier- 
 like coils more or less covered with chaffy scales (Fig. 220, c). 
 This development enables the leaves to expand with surprising 
 rapidity in the spring when the more rapid enlargement of the 
 cells on the inner side of the leaf cause it to uncoil. There is 
 
DEVELOPMENT OF PLANTS 
 
 323 
 
 another feature about these leaves that compels our admiration. 
 Comparatively few leaves are produced annually from the tip 
 of each stem — in the preceding order usually but one. This 
 
 Fig. 220. Stem in early spring freed from all of its leaves save the young 
 ones, c, near the tip: r, roots; v, vascular bundles in base of petiole; x, region 
 from which the cortex has been removed to show the vascular bundles from 
 the leaves uniting to form a lattice work. 
 
 appears to be the rule in underground stems, since the difficulty 
 and danger of sending the tender young leaves up through the 
 soil is minimized by the development of one or a few leaves which 
 
 Fig. 221. Cross-section of fern stem: v, a concentric vascular strand, the 
 large cells of the xylem being surrounded by the phloem. Each strand is 
 surrounded by a compact layer of cells, the endodcrmis; st, stercome; cr, 
 cortical region. 
 
 form large blades after reaching the air. In the two remaining 
 orders of Pteridophyta, branches of the underground stems rise 
 above the ground and these bear numerous small leaves, also a 
 rule for stems of this kind. The tissues of the leaves are differ- 
 
324 
 
 STRUCTURE OF THE STEM 
 
 entiated into an epidermis, stomata, chlorenchyma and vascular 
 bundles as in the higher plants (Fig. 223). The stems are more 
 usually prostrate, creeping rhizomes that branch sparingly and 
 so gradually give rise to colonies of ferns. One of the most 
 attractive features of certain tropical districts is the tree ferns 
 with erect stems of palm-like appearance which lift their great 
 crowns of leaves 30 to 50 feet in the air. The vascular bundles 
 are more commonly of the concentric type (Fig. 221). Perhaps 
 we should repeat that a bundle is composed of two types of elon- 
 gated cells, thick walled woody cells (the xylem) and thin walled 
 
 Fig. 222. Arrangements of the sporangia: A, lobe of leaf of Dryopteris 
 with sporangia grouped in circular sori, s. B, sorus enlarged, showing the 
 shield-like membrane or indusium, in, covering the sporangia, sp. C, lobe 
 of leaf of Asplenium with elongated sori, s. 
 
 cellulose cells (the phloem). In the fern t^^pe of concentric 
 bundle the phloem surrounds the xylem. These bundles are 
 united in the stem in a variety of ways. In the simplest case 
 we find a central core of xylem surrounded by phloem which 
 gives off strands or vascular bundles to the leaves and roots. In 
 the majority of ferns this core of xylem is modified by the develop- 
 ment of rather delicate pith-like cells in its center, consequently 
 it appears as a ring in cross section or as a cylinder in longitudinal 
 view. This ring is broken into segments owing to the fact that 
 
DEVELOPMENT OF PLANTS 325 
 
 portions of it turn out here and there, thus forming the vascular 
 bundles that extend into the leaves. Such an arrangement is 
 seen in Botrychium (Fig. 216). These spaces formed in the ring, 
 called foliar gaps, are often quite large so that the cylinder of 
 vascular tissue in the stem presents the appearance of a lattice 
 work — note the vascular tissue at the right-hand end of the stem 
 in Fig. 220. In cross section such a cylinder would appear as a 
 series of more or less widely separated strands (Fig. 221). This 
 latter figure shows that strands arising from the lattice-like 
 cylinder may also extend into the pith-like region — note the three 
 large strands in Fig. 221, v. This explains the confusing 
 arrangement of strands often seen in the cross section of fern 
 stems and previously noted in monocotyledons, p. 96. Certain 
 cells of the cortex and pith often become modified into strengthen- 
 ing cells of stereome (Fig. 221, st). Roots arise near the base 
 of the leaves, and in some of the tree ferns form a thick mat-like 
 covering on the stems. They originate from the endodermis 
 of the bundles and possess a root cap and radial arrangement of 
 the vascular bundles as in higher plants. 
 
 (a) Structure and Character of the Sporangia. — The sporangia, 
 instead of being produced in the tissues of special branches as 
 in Ophioglossum, are borne in curiously constructed capsules, 
 usually situated on the under surface of the ordinary green 
 leaves (Fig. 222). The sporangia-bearing leaves are usually 
 called sporophylls, meaning spore-bearing leaves. In some cases 
 the sporophylls are highly modified, being entirely given up to 
 spore production and therefore quite different from the green 
 leaves (Fig. 227). The sporangia are usually associated in 
 groups or sori (sing, sorus) on the vascular bundles and pro- 
 tected by a membranous outgrowth of the epidermis, known 
 as the indusium (Fig. 222, B). Each sporangium originates 
 usually from a single epidermal cell, which by repeated divisions 
 (Fig. 223) produces a capsule or sporangium that contains the 
 spores. A sporangium of the shield fern contains 48 spores and 
 there are fully 100 sporangia to a sorus and 20 sori on each lobe 
 or pinna of the leaf. A well-developed leaf would have on an 
 average 50 pinnae and a healthy plant would l^car about 10 
 
326 
 
 SPORANGIA OF FILICALES 
 
 leaves. Therefore, Bower estimates that the shield fern produces 
 annually upwards of 50 million spores. This makes a striking 
 comparison when we consider the spore production in the Bryo- 
 
 FiG. 223. Section of a leaf of Woodwardia, showing two sori: i, indusium; 
 sp, sporangia arising from the epidermis and in various stages of develop- 
 ment. Note the epidermis, stoma, 5, palisade and spongy chlorenchyma, 
 and vascular bundles, as in higher plants. 
 
 phyta, and it is evident that ample provision is made for the 
 maintenance of the race notwithstanding the postponement and 
 specialization in spore production. 
 
 The sporangia vary considerably in structure in the various 
 
 Fig. 224. Character of sporangia: A, simple type of sporangium of 
 Osmunda with rudimentary annulus, an, of a few thickened cells. B, com- 
 mon type of sporangium — an, annulus; /, lip cells. 
 
 genera. In the simplest forms, they consist of a uniform layer of 
 wall cells inclosing a mass of cells, the majority of which become 
 spore mother cells, producing four spores each. In other cases, 
 a few of the wall cells are thickened, forming a rudimentary 
 
DEVELOPMENT OF PLANTS 327 
 
 annulus (Fig. 224, A), but in the majority of our common ferns, 
 the sporangium consists of a dcHcatc stalk, supporting a rather 
 spherical spore-bearing capsule (Fig. 224, B). The walls of this 
 capsule consist of a single layer of thin-walled cells except for a 
 row of thickened cells, the annulus, that extends from the stalk 
 over the capsule nearly to the opposite side (Fig. 224, j5, an). 
 At the latter point, the annulus ends in a few rather weakened 
 cells, two of which, the lip cells, are conspicuous for their larger 
 size (Fig. 224, B, I). It is to be noticed that the cells of the an- 
 nulus are thickened on their inner and radial walls while the outer 
 walls remain comparatively thin. When the spores are mature 
 and lie loose in the capsule, the cells of the annulus that have 
 been filled with water up to this time begin to dry out. As the 
 volume of water in these cells lessens through evaporation, the 
 cohesion of the water pulls upon the thin outer walls, causing 
 them to bend in, assuming a U-shaped appearance. This tends 
 to pull the two radial walls of each cell together and shorten 
 the outer circumference of the annulus. This contraction pulls 
 open the sporangium at the weak lip cells, the split extending 
 back through the lateral cells of the capsule which is finally 
 drawn back to such an extent that the annulus becomes nearly 
 straight. Finally, when the water is nearly withdrawn from the 
 cells, the tension becomes so great that air is drawn in through 
 the thin walls of the cells. The entrance of air breaks the water 
 adhering to the walls and which had kept them pulled in as 
 described above and consequently each cell returns instantly to 
 its original form. This causes the capsule to snap back to near 
 its original position with a quick jerk, thus throwing out the 
 spores to a considerable distance. This motion of the capsule 
 can be approximated by mounting sporangia that have been 
 soaked in a drop of water and watching them under a micro- 
 scope while a drop of glycerine placed in touch with one side 
 of the mount is drawn in with a filter paper applied to the oppo- 
 site side. The denser glycerine will draw the water out of the 
 cells by osmosis and set up a tension in the cells of the annulus 
 just as did the evaporation of the water. 
 
 The form of the sorus and its relation to the indusium and 
 
328 
 
 FORMS OF FILICALES 
 
 particularly the structure of the annulus are important charac- 
 teristics to be observed in identifying the ferns. In the flowering 
 ferns, Osmunda, the large slight-stalked sporangia are pro- 
 vided with a rudimentary annulus of a few thickened cells lo- 
 cated at one side of the capsule (Fig. 224, A). The entire leaf 
 or certain leaflets are completely covered with the sporangia, 
 little more than the vascular bundles remaining (Fig. 225, A). 
 
 Fig. ?25. Common forms of the Filicales: A, the flowering fern, Os- 
 munda, showing below two green leaflets and above two sporangia-bearing 
 leaflets. At left a cluster of sporangia magnified. The first leaves in the 
 spring only bear sporangia; those appearing later have only green leaves. 
 B, chain fern, Woodwardia. C, Christmas fern, Polystichum. D, bladder 
 fern, Felix. E, hay-scented fern, Dennstaedtia — s, sorus enlarged. F, Woodsia 
 — After Sprague. 
 
 Osmunda belongs to the lowest group of the Filicales and shows 
 certain analogies in the arrangement and structure of its collateral 
 vascular bundles with the Ophioglossales. The majority of 
 ferns in temperate climates are characterized by sporangia of 
 the type shown in Fig. 224, B, and they are distinguished by 
 the form and arrangement of the sori and indusia. The shield 
 fern, Dryopteris, has a rather circular or curved sorus covered 
 
DEVELOPMENT OF PLANTS 
 
 329 
 
 by an indusium that is laterally attached at a single point (Fig. 
 222, A). The Christmas fern, Polystichum, has a circular sorus 
 with centrally attached indusium (Fig. 225, C). The spleenwort 
 fern, Asplenium, is characterized by elongated sori arranged 
 obliquely to the midrib upon the upper side of the veinlets. 
 The indusium is attached on one side of the sorus along its entire 
 length (Fig. 222, B). The chain fern. Woodwardia, differs 
 from Asplenium in having the sori arranged in chain-like rows 
 
 Fig. 226. Ferns without indusia or possessing false ones: A, leaflet of 
 bracken fern, Pteridium. B, maiden-hair fern, Adiantnm. C, polypod fern, 
 Polypodium. — After Sprague. 
 
 parallel to its midrib (Fig. 225, B). In several genera of ferns 
 the indusium is partly or entirely inferior. Thus in the bladder 
 fern, Filix, the partly inferior indusium covers the circular sorus 
 like a hood (Fig. 225, D), while it is wholly inferior in the hay- 
 scented fern, Dennstaedtia, forming a cup (Fig. 225, E) and in 
 Woodsia the indusium is roundish or star-like (Fig. 225, F). 
 
330 
 
 GAMETOPHYTE OF FILICALES 
 
 Several ferns are distinguished by false indusia that are formed 
 by the more or less modified margins of the leaf. In the bracken, 
 Pteridmm, the entire membranous margin of the leaf curves over 
 the closely crowded sori (Fig. 226, A), and in the maiden-hair 
 fern, Adiantum, the sporangia are at the ends of the veins and 
 covered by reflexed portions of the leaf (Fig. 226, B). The 
 indusia are lacking in some forms, as in the beech fern, Phegoteris, 
 and in the polypody, Poly podium (Fig. 226, C). These genera 
 are distinguished by the fact, among others, that the leaves of 
 Polypodium drop off, leaving a scar as in our deciduous trees. 
 
 Fig. 227. The sensitive fern, Onoclea: A, portion of normal green leaf. 
 B, a spore-bearing leaf. C, two views of one of the round lobes of B, showing 
 the views and the sori on inner side of the lobe. — After Bailey. 
 
 This feature is possibly due to its more or less epiphytic habit. 
 This is the only really xerophytic fern of temperate regions. 
 The sporophyll of the sensitive fern, Onoclea, bears a striking 
 outward resemblance to Botrychium, but the sporangia-like bodies 
 are really leaf lobes rolled up and each bears several round 
 sori on its inner side (Fig. 227). In this and several other genera, 
 see Osmunda, the work of photosynthesis is given over to large 
 green leaves that do not produce sporangia. 
 
 (b) The Gametophyte. — In the majority of ferns the spore 
 germinates by rupturing the outer coats and producing a germ 
 tube from which one or more delicate rhizoids are cut off. The 
 germ tube elongates, forming a short chain of cells which soon 
 develop by apical growth into a flat thalloid structure, commonly 
 called the prothallium, that is attached to the ground by numer- 
 
DEVELOPMENT OF PLANTS 
 
 331 
 
 ous rhizoids (Fig. 228). This gametophyte often becomes heart- 
 shaped (Fig. 228, C), owing to the more rapid growth of the 
 cells that are cut off from the apical cell. In some genera 
 branched filamentous or narrowly thalloid growths are developed 
 that resemble the protonema of the mosses or the thallose hepat- 
 
 FiG. 228. Fig. 229. 
 
 Fig. 228. Gametophyte of the Filicales: A, germination of the spore. 
 B, early appearance of the thalloid structure of the gametophyte owing to 
 the formation of an apical cell, x. C, mature gametoph>'te — aw, anthcridia; 
 ar, archegonia: r, rhizoids; v, apical cell or growing point. 
 
 Fig. 229. Structure in reproductive organs: A, antheridium as seen in 
 section, just before the discharge of the gametes. B, male gamete. C, 
 section view of archcgonium — g, female gamete. 
 
 ics. The gametophyte usually lives but a few months, although 
 in some species they may endure for years, multiphing exten- 
 sively by gemmae, and so form conspicuous mats upon the moist 
 trunks and rocks. The archegonia and antheridia are usually 
 borne upon the same gametophyte. Some genera, however, are 
 
332 THE YOUNG SPOROPHYTE 
 
 strictly dioecious, producing small antheridial or male gameto- 
 phytes and larger archegonial or female gametophytes. Small 
 male gametophytes occur not uncommonly among any of the 
 genera, owing doubtless to their poorer nourishment. The 
 sexual organs are developed upon the under side of the gameto- 
 phyte (Fig. 228, C), probably because this position is of advantage 
 in keeping them in contact with any water that may fall upon the 
 earth ^and thus preventing their drying out. The antheridia 
 develop before the archegonia, often appearing while the gameto- 
 phyte is quite small. They usually project from the surface of 
 the thallus as spherical bodies covered with a single layer of 
 chlorophyll-bearing cells which inclose usually from thirty-two 
 to sixty-four gamete-bearing cells (Fig. 229, A), The male 
 gametes are discharged as in the Bryophyta — the swelling of the 
 antheridium causes the rupture or throwing off of the apical cell 
 and the extrusion of the inner cells as a mucilaginous mass. The 
 gametes are large spirally-coiled bodies provided with numerous 
 cilia and the larger posterior coils enclose a delicate sac containing 
 the remains of the nourishment stored in the mother cell (Fig. 
 229, B). The archegonia are developed on the older prothallia, 
 usually just back of the growing point (Fig. 228, C). The neck 
 of the archegonia, consisting of four rows of cells, projects from 
 the under surface of the prothallium and is usually curved back 
 from the growing point, while the basal portion containing the 
 female gamete remains buried in the tissues of the prothallium 
 (Fig. 229, C). When the female gamete is mature the canal 
 cells become mucilaginous and the lip cells open as in the arche- 
 gonium in the mosses. 
 
 (c) Germination of the Gametospore. — The gametospore ger- 
 minates in a week or more after fertilization, usually dividing very 
 regularly first by a vertical and later by a transverse division into 
 four cells. Each of these cells forms a primary organ of the young 
 sporophyte. Of the two outer cells, the one near to the grow- 
 ing point of the prothallium (Fig. 230, A,c)hy repeated divisions 
 produces the first leaf or cotyledon, while the other one forms 
 the first root (Fig. 230, A, r). Of the two lower cells, the one 
 directly below the leaf cell gives rise to the stem (Fig. 230, A, s) 
 
DEVELOPMENT OF PLANTS 
 
 333 
 
 and the other one to the foot (Fig. 230, A, f). The growth 
 of these four cells results at first in the formation of a rather 
 globular sporophyte, but soon the growing point of the young 
 
 Fig. 230. Germination of the gametospore: A, section of archegonium, 
 after fertilization, showing the four-celled stage of the germinating gameto 
 spore. The cell r by repeated division forms the first root, c forms the first 
 leaf, s forms the stem and / the foot; x, apical cell of prothallium. B, later 
 stage. The young sporophyte rupturing the archegonium or calyptra- 
 lettering as in ^. 
 
 root grows through the calyptra, turns downward and pene- 
 trates the soil (Fig. 230, B). This is followed by emergence 
 
 Fig. 231. Older sporophyte that has developed two roots, r, and is un- 
 folding the second leaf, /, but still attached to the withering gametophyte 
 or prothallium, p; rh, rhizoids. 
 
334 EQUISETALES OR HORSETAILS 
 
 of the cotyledon, which, however, curves upward and spreads 
 out its blade to the light (Fig. 231). Note in Fig. 230 that the 
 necessities of the plant have determined the position of these 
 four organs. The foot is formed in contact with the bulk of the 
 food in the prothallium, the root next the ground which it reaches 
 at once, the leaf easily reaches the light since the root has already 
 ruptured the calyptra, and since the leaf is near the margin of 
 the prothallium, and finally the stem is in a position of advantage 
 (Fig. 230, B) for growing straight up or horizontally. The young 
 sporophyte, which has up to this time drawn its nourishment by 
 means of the foot from the gametophyte, is now in a position to 
 care for itself. The prothallium soon withers, leaving the sporo- 
 phyte an independent and self-supporting plant. The first leaf is 
 small and usually bears little resemblance to those of the mature 
 plant, but as the stem elongates, new leaves are formed which 
 gradually become larger and each succeeding one resembles more 
 closely the adult form. The foot disappears with the withering 
 of the gametophyte, and this is soon the fate of the primary root, 
 but numerous secondary roots are formed along the stems as it 
 continues to creep along on or near the surface of the soil. This 
 growth goes on from two to several years before the plant is 
 prepared to develop sporangia upon the leaves, page 282. 
 
 Order 3. Equisetales. The Horsetails 
 
 114. General Characters. — This small group of Pteridophyta, 
 comprising but a single genus of about twenty-five species, is but 
 a remnant of an extensive group of plants that flourished in the 
 coal period and that formed conspicuous features of the vegeta- 
 tion at that time. The structure of these earlier forms has been 
 so perfectly preserved in fossil remains that they give a better 
 idea of the relationship of the group than could be obtained from 
 the living plants. Forms allied to Equisetum doubtless formed 
 during the coal period of the earth large forests and attained a 
 height of sixty to ninety feet and perhaps three feet in diameter. 
 The species that survive to-day are rush-like plants that rarely 
 exceed a foot in height, though a single tropical form supports 
 its delicate stem upon other vegetation and so attains a length 
 
DEVELOPMENT OF PLANTS 
 
 335 
 
 of over thirty-five feet. Species of Eqidsetum are of common 
 occurrence in nearly all countries, living in shallow ponds, 
 swamps, and marshes or drier soils. 
 
 At first sight they show little suggestion of fern relationship 
 (Fig. 232) and impress one as being singularly out of harmony 
 
 Fig 232. A common horsetail, Equisetum arvense: a, the green branch- 
 ing plant that lives through the summer-/, scale leaves; r, rhizome or under- 
 ground stem with tuberous storage organs, b, early spring shoot that bears 
 a spike or strobilus of modified spore-bearing leaves, sp. This stem is ot a 
 light brown color and withers after the spores are shed.— H. O. Hanson. 
 
 with our common plants, as though they were indeed relics of 
 a past age. The large leaves, which were so characteristic of 
 the Filicales, are reduced to minute papery scales in this order 
 
336 STRUCTURE OF EQUISETUM 
 
 and take little or no part in photosynthesis. These scale leaves 
 are arranged with great regularity at the nodes of the stem, and 
 owing to their close association they grow together, forming a 
 papery sheath with teeth-like points about the stem (Fig. 232, I). 
 It is noteworthy that in allied fossil forms large chlorophyll- 
 bearing leaves occurred. 
 
 The stems are of two kinds, subterranean rhizomes that branch 
 
 Fig. 233. Cross-section of a portion of the stem showing its grooved 
 character and stereome confied to the ridges: a, air spaces; e, endodermis, 
 inside of which are shown three bundles. 
 
 extensively through the soil, and aerial stems that arise as 
 branches from the rhizomes. The aerial stem is simple or 
 branched and is characterized by nodes made conspicuous by the 
 sheathing teeth-like leaves and strongly furrowed internodes. In 
 such species as branch, these organs originate with great regular- 
 ity in the axils of the leaves and perforating the sheathing leaves 
 produce a bushy symmetry that caused the name of Equisetum 
 or horsetail to be applied to these plants. The epidermal cells 
 are hard and rough, owing to the abundant deposit of silica in 
 the cell walls. For this reason certain species were used in early 
 times for scouring purposes and so they became popularly known 
 as scouring rushes. Complete silicious casts of the epidermal 
 
DEVELOPMENT OF PLANTS 337 
 
 walls and stomata can be obtained by treating the tissue as noted 
 in the diatoms. The arrangement of the leaves and branches and 
 the distribution of the vascular bundles and other tissues presents 
 a mathematical regularity unexcelled in any plant. As in the 
 Bryophyta, the elongation of the stem is effected by a single 
 apical cell which cuts off with extreme regularity the cells from 
 which the various tissues and organs of the plant are formed. 
 The very rudimentary vascular bundles are arranged around a 
 large pith (Fig. 233). They rarely show secondary growth 
 through the activities of a cambium, although this was a marked 
 feature of many of the extinct species. These bundles lie directly 
 below each ridge of the stem and run downward through the 
 internode to the next lower node, where they divide into two equal 
 parts. One of these branches joins a similar branch from the 
 adjacent bundle and continues straight down to the next node, 
 where the branching is repeated. The rudimentary bundles of 
 the leaves also join on to the bundles of the stem at the node. 
 Owing to this regularity of branching, the leaves and vascular 
 bundles alternate in each succeeding node and the vascular system 
 assumes a cylindrical form composed of oblong six-sided figures. 
 Thick-walled stereome cells are developed in the ridges and con- 
 stitute the principal mechanical tissue of the stem, while the 
 stomata and the chlorophyll apparatus are largely confined to 
 the grooves. The stems are very light, owing to absorption of 
 the larger part of the pith and certain regions of the cortex (Fig. 
 233, cl)- 
 
 (a) The Spore-bearing Leaves or Sporophylh. — The sporangia 
 are formed only upon special organs, probably leaves or sporo- 
 phylls, that are arranged in a compact cone or strobilus (plu. 
 strobili) at the end of the stem (Fig. 232. sp). In some species the 
 strobilus is borne upon a special branch that does not contain 
 chlorophyll, though variously colored, and that withers away as 
 soon as the spores are shed (Fig. 232, b). In other cases the stro- 
 bilus appears at the tip of the ordinary green shoot. This vari- 
 ation is doubtless associated with the season of the year at which 
 the strobili appear. The species characterized by the special 
 branches produce these earl}- in the spring when the temperatures 
 22 
 
338 SPOROPHYLLS OF EQUISETUM 
 
 are not favorable for the work of the green shoots. Note in this 
 connection the significance of the coloration. Species with the 
 strobili upon the ordinary plants form these structures later in 
 the season when the temperatures are favorable for photosyn- 
 thesis. The development of the sporangia upon specialized 
 sporophylls that are grouped together at definite points on the 
 plant is one of the significant departures that appears in the evo- 
 lution of the plant. This distribution of labor that is seea in the 
 setting aside of certain groups of leaves for spore production 
 will appear in all the succeeding groups ; and the arrangement and 
 structure of the sporophylls will steadily become more and more 
 complex until a point is reached where they are popularly called 
 a flower, although this term can be just as correctly applied to 
 the strobilus appearing in Egidsetum and succeeding groups. 
 The sporophylls of Equisetum originate at the nodes, as in the 
 case of the scale leaves, but as they enlarge the apical portion 
 spreads out like a shield. The internodes do not elongate and 
 separate the sporophylls to any considerable extent, consequently 
 the shields become six-sided through contact with the adjacent 
 sporophylls. 
 
 {h) Character of Spora^tgia and Spores. — The sporangia appear 
 as rather elongated sacs on the under side of the shields (Fig. 
 234, 2) and at maturity open by a longitudinal cleft. The 
 structure of the spores is of especial interest. The outer wall 
 of these spores is thickened in spiral bands, and owing to the 
 dissolution of the thin portion of the wall separating these bands, 
 the outer coat uncoils and appears as four bands, with spoon-like 
 ends, attached to the spore at one point (Fig. 234, 3). These 
 bands or elaters are very hygroscopic and their movements assist 
 in rupturing the sporangium, but their special significance is seen 
 in the fact that the elaters become entangled and so several spores 
 are cariied away together by the wind. The meaning of this 
 arrangement will appear directly. 
 
 (c) Germination of the Spore. — The gametophyte produced from 
 these spores (Fig. 235) is an irregularly lobed thallus more 
 suggestive of the irregular thallus of an hepatic or the leaves 
 of a moss plant, or the lobed gametophyte of certain species of 
 
DEVELOPMENT OF PLANTS 
 
 339 
 
 the Ophioglossales than of the prothalHum of the Filicalcs. The 
 most important feature about the gametophytes is the fact that 
 they are as a rule dioecious and of two sizes, the smaller ones 
 bearing only antheridia and the larger only archegonia. This 
 difference in the nature of the gametophytes is largely due to 
 nutrition as noted among the Filicales, the well-nourished ones 
 being the female. In fact antheridia may appear upon the female 
 gametophyte as a result of insufficient nourishment during its 
 
 Fig. 234. Fig. 235. 
 
 Fig. 234. Sporophylls and spores of Egtiisetum: 2, sporophylls viewed 
 from outer and inner side, showing form and attachment of sporangia and 
 the central stalk attaching the sporophyll to the strobilus. 3, spores with 
 elaters expanded in a and partially coiled in b. 
 
 Fig. 235. Female gametophyte of Eqiiisetum bearing several archegonia 
 and leaf-like lobes. At right male gamete. — After Sadebeck. 
 
 later development. Thus we see that the germination of these 
 spores, which are apparently exactly alike, is controlled by a defi- 
 nite stimulus, just as was the case in the formation of zoospores 
 and gametes among the lower green algae (page 185). It is 
 noteworthy that some of the extinct species of this group actually 
 stored more food in certain spores than in others, so that they 
 came to differ somewhat in size. This habit is well established 
 in some of the living Filicales and will also be noted in the next 
 order. As a consequence of this tendency, the nature of the 
 gametophyte developed from the spore is no longer a matter of 
 chance. The larger spores, called megaspores, by reason of the 
 more abundant food produce female gametophytes, while the 
 smaller spores, microspores, form small gametophytes bearing 
 only antheridia. The significance of this tendency to produce 
 
340 REPRODUCTION OF EQUISETUM 
 
 megaspores and microspores will be seen in the disussion of the 
 fourth order, but attention is called to it here because the Equise- 
 tales show very clearly how such a condition came about. It 
 would appear probable that the spores of Eqidsetum, although 
 exactly alike as far as we can see, must have already undergone 
 some physiological change which predisposes them to develop 
 either male or female gametophytes. If this is not the case it 
 would be difficult to explain the common occurrence of dioecious 
 gametophytes in Equisetum and the rarity of such an occurrence 
 among the Filicales. It would appear reasonable to suppose that 
 the living substance in the spore of the Filicales is not so highly 
 organized and therefore not so readily influenced by external con- 
 ditions, while the composition of the spore of Equisetum is of 
 such a nature that the amount of food placed at its disposal pro- 
 foundly affects its germination and development. Perhaps this 
 physiological differentiation of the spores led to their appro- 
 priating different amounts of food during their formation in the 
 sporangium, and so they finally came to be distinguished as 
 large and small spores, as noted above. The antheridia and 
 archegonia present essentially the same features as were noted 
 in the Filicales, and fertilization is effected in the same manner- 
 (d) The Germination of the Gametospore. — The most note- 
 worthy departure in the germination of the gametospore is seen 
 in the limited growth of the stem. The early stages are similar 
 to those of the Filicales, but after the stei^ has formed a few 
 nodes with three leaves each, it is replaced by a stem that devel- 
 ops at its base. This second stem attains a somewhat larger size, 
 but is finally replaced in the same manner by a third shoot. In 
 this way several stems are formed until finally one is developed 
 that penetrates the ground and forms the characteristic rhizome 
 of the mature sporophyte. These plants with their scale leaves 
 reduced to protective organs, sunken stomata and chlorenchyma 
 confined to the grooves of the internodes, present an extreme 
 form of xerophytic structure and form a sharp contrast with the 
 large and usually thin-leaves Filicales. While they occur in dry 
 localities, to which conditions their structures admirably adapt 
 them, they are of more common occurrence in moist and wet 
 
DEVELOPMENT OF PLANTS 
 
 341 
 
 places. This peculiar distribution of the species of Eqiiisehim 
 has not been explained. It is evident that these plants, like the 
 rushes and sedges of our marshes and shallow ponds, are often 
 exposed to intense heat and light, Avhich would cause an excessive 
 transpiration. Possibly these plants are not able to absorb 
 water rapidly owing to the limited amount of conducting tissues 
 and to the exclusion of the atmosphere by the water which sur- 
 rounds the roots (see pages 45, 54), consequently they are at 
 the same disadvantage and require the same protective devices 
 as plants living in arid localities. 
 
 115. 
 
 Order 4. Lycopodiales. The Club Mosses 
 General Characters. — The members of this group are 
 
 popularly known as the club mosses owing to their small moss- 
 like leaves and the arrangement of the spore-bearing leaves into 
 
 Fig. 236. A common club moss, Lycopodium annotinum, with creeping 
 stem and erect branches covered with small moss-like leaves: s, strobilus; 
 r, roots. — H. O. Hanson, 
 
342 THE LYCOPODIALES 
 
 club-like stroblli or cones (Fig. 236) . The gametophyte or sexual 
 generation is also suggestive of the mosses. Particularly is this 
 irue of the male gamete, which is small and not spirally coiled 
 andjt is apparentl}^ biciliate (Fig. 240). 
 
 The Lycopodiales are largely tropical, though a small number 
 of^forms are of common occurrence in temperate regions. The 
 stems are erect or creeping and rather small, but owing to their 
 prolonged growth and extensive branching they often form con- 
 
 FiG. 237. Cross-section of a portion of the stem of Lycopodium, showing 
 the centrally arranged vascular bundles. 
 
 spicuous and attractive colonies. The tissues of the stem do 
 not materially differ from those of the common fern, though the 
 vascular bundles from the leaves generally unite in the stem to 
 form a central core of xylem surrounded by phloem. This 
 solid core of xylem may be broken up in various ways by the 
 development in it of masses of pith-like cells. Sometimes these 
 pith cells form plates alternating with the xylem (Fig. 237). 
 The work performed by the leaves, as in the Equisetales, is 
 usually of two kinds, namely, photosynthesis by the green foliage 
 'leaves and spore production by the leaves that usually form 
 
DEVELOPMENT OF PLANTS 
 
 343 
 
 strobili at the tips of certain branches. But a single sporangium 
 is associated with each of these sporophylls (Fig. 239). 
 
 The club mosses are a very much larger group than the Equise- 
 tales, but like the latter group, they are a remnant of a highly 
 developed and widely distributed race. Fossil remains indicate 
 that the ancient allies of these plants were conspicuous features 
 of an earlier vegetation, with palm-like trunks 100 feet in height 
 and three feet in diameter, and bearing a crown of long narrow 
 leaves that attained a length of three feet. They reached their 
 
 Fig. 238. Fig. 239. 
 
 Fig. 238. Phylloglossum Dnimmondi. — After Pritzcl. 
 
 Fig. 239. Strobilus and sporophylls of Lycopodium: 2, strobilus. 3, a 
 leaf or sporophyll from the strobilus enlarged and showing attached spo- 
 rangium, 4, a spore greatly magnified. 
 
 greatest abundance in the coal age and thence gradually declined, 
 being crowded out by the more specialized seed plants. There 
 are two important families of the Lycopodiales: i, Lycopodiaceae; 
 2, Selaginellaceae. 
 
 116. Family i. Lycopodiaceae. — With but one exception the 
 members of this family belong to the genus Lycopodium, com- 
 monly known as the club moss, ground or running pine, ground 
 
344 SIGNIFICANCE OF PHYLLOGLOSSUM 
 
 fir or hemlock (Fig. 236). These plants are very well repre- 
 sented in our open woods, where they often form conspicuous 
 colonies owing to the extensive branching and prolonged growth 
 of the stems which creep over or through the ground, sending 
 up numerous, erect branches and giving off roots that branch 
 with great regularity. Various species of Lycopodium are ex- 
 tensively gathered for decorations since the aerial portions of 
 the stems are thickly clothed with small moss-like leaves. The 
 second genus of the family (PhyUoglossum) contains but one 
 species, which is found in New. Zealand and Australia. It is of 
 interest because it is the most simple fern known (Fig. 238), 
 consisting of a few narrow leaves, rudimentary strobilus and 
 poorly developed tissues. The simple strobilus of this plant is 
 suggestive of the sporophyte of Anthoceros and it also recalls the 
 spore-bearing spike of Ophioglossum. If the sterile cells separat- 
 ing the spore-forming cells in Anthoceros had increased in number 
 so as to project somewhat and thus form rudimentary leaf -like 
 outgrowths, then it Avould have resembled the strobilus of PhyU 
 loglossum. To be sure the sporangia of the latter plant are borne 
 upon the leaves but in the next family they arise from the stem 
 as in Anthoceros. Ophioglossum shows the increase of the sterile 
 cells between the spore-bearing cells but this did not result in 
 leaf development; so it occupies an intermediate position in this 
 trend of development. It is certainly a striking fact that these 
 three groups, though widely separated, exhibit the same type of 
 development. 
 
 The sporangia of the Lycopodiaceae are large considering the 
 size of the leaf, and usually appear on specialized leaves that are 
 arranged in strobili on the ends of erect branches (Fig. 239). 
 The sporangia open by a longitudinal cleft and the spores are 
 produced in such quantities in some of the forms that they are 
 of commercial importance, being used for flashlight effects and 
 sold as lycopodium powder. 
 
 (a) The Gametophyte of the Lycopodiaceae. — The most inter- 
 esting feature about the club mosses is the sexual generation, 
 because it indicates that these forms may have been derived from 
 the same ancestry as the mosses or at least from a very primitive 
 
DEVELOPMENT OF PLANTS 
 
 345 
 
 stock. In certain species the spores produce a rather cyhndrical 
 erect gametophyte which terminates in a number of radially- 
 arranged green leaf-like lobes, among which the archegonia and 
 antheridia are produced (Fig. 240). This structure has been 
 compared to a miniature leafy moss plant. This suggestion of 
 relationship is strengthened by the resemblance of the male 
 gametes to those of the mosses, the sperms, so far as they have 
 been observed, being small, almost straight bodies, and provided 
 with two cilia (Fig. 240). The archegonia may also be more 
 primitive than those of the ferns in possessing several neck canal 
 
 Fig. 240. Gametophyte of Lyco podium as seen in section: an, antheridia; 
 ar, archegonia. The young sporophytc or embryo developed in one of the 
 archegonia consists of a foot, /; a root, r; and a stem, st, bearing a leaf on 
 either side; s, the suspensor, which has pushed the sporophyte early in its 
 development into the tissues of the gametophyte. Above a single male 
 gamete is figured. — After Bruchmann. 
 
 cells and five instead of four rows of neck cells. The gameto- 
 phyte appears in all species examined to be associated with 
 symbiotic fungi, as in Ophioglossales. The spores will not de- 
 velop unless they come in contact with a suitable fungus and this 
 has led in many forms to a subterranean and chlorophylless 
 development of the, gametophyte. 
 
 ih) The Germination of the Gametospore. — The germination of 
 the gametospore is characterized by new departures that will 
 continue on through the seed plants. The gametospore enlarges 
 and divides into two cells by a transverse wall. The outer of 
 
346 EMBRYO OF LYCOPODIUM 
 
 the two daughter cells takes no part in the formation of the 
 sporophyte, though elongating and often dividing several times. 
 These cells are termed the suspensor (Fig. 240, s) and function 
 in pushing the lower daughter cell down into the nourishing tissue 
 of the gametophyte. The lower daughter cell, by a series of 
 divisions that at first resemble the common ferns, forms the 
 young sporophyte. This consists of a stem with one or two 
 cotyledons, a massive foot and finally, at a late period in the 
 development in the sporophyte, of a root (Fig. 240). The tardy 
 development of the root has been cited as an indication of the 
 origin of the lycopods from very primitive ancestors, in which 
 the formation of the root had not become established. It may 
 also be due to the abundant food stored in the gametophyte and 
 hence the development of the root might well be delayed until 
 this store is in part exhausted. The growth of the sporophyte is 
 very slow and it remains as a parasite upon the gametophyte for a 
 long time, even for years in some of the subterranean forms 
 (see Ophioglossales) . The elongation of the stem and root is 
 effected by the division of several cells rather than by one apical 
 cell, as in previous cases, a feature to be noted in the seed plants. 
 117. Family 2. Selaginellaceae. — This family includes but a 
 single genus, Selaginella, of over 600 species. Only a few forms 
 occur in the temperate regions, the majority being confined to 
 tropical countries, where they often form one of the most attrac- 
 tive features of the forest vegetation owing to the symmetry of 
 their branching and the rare delicacy of their foliage (Fig. 241). 
 For these reasons they are extensively cultivated and familiar 
 objects in conservatories and florists' shops. The so-called res- 
 urrection plant, Selaginella lepidophylla, lives in the very arid 
 sections of the southw^estern United States, and during drought 
 reduces its surface to a nest-like ball by rolling up its branches 
 into tight coils. In this condition it appears as a brownish dead 
 mass. When moistened, the absorption of water causes the 
 branches to quickly uncoil and also renders the tissues trans- 
 lucent, so that the green color of the chloroplasts can be seen. 
 These reactions occur even in the dead plants and so create the 
 impression that they have returned to life. Many of the species 
 
DEVELOPMENT OF PLANTS 347 
 
 of Selaginella have a creeping habit Hke that of Lycopodium, 
 and the aerial stems are covered with leaves arranged in several 
 rows, very frequently in four rows of two large and two smaller 
 leaves (Fig. 241, i^). Usually but a single chloroplast appears 
 in each cell, as already noticed in several algae and in the Antho- 
 
 FiG. 241. A common cultivated Selaginella: 1, habit of the plant — s, 
 strobili; b, a branch bearing roots, r. lA, portion of stem, showing leaf ar- 
 rangement. — H. O. Hanson. 
 
 cerotales. The roots are frequently developed from the end of 
 naked branches that extend from the stems to the ground (Fig. 
 241, r). 
 
 (a) Sporangia and Spores. — The sporangia arc borne on the 
 stems in the axils of the leaves which form a strobilus, as in 
 Lycopodium. However, they differ radically from those of the 
 Lycopodium, in that two kinds of spores are formed, small spores 
 or microspores and larger ones or megaspores. The formation 
 of two kinds of spores, or heterospory, is not confined to the 
 Lycopodiales. It occurs among certain genera of the Filicales 
 
348 
 
 SPOROPHYLLS OF SELAGINELLA 
 
 and, as has been stated, characterizes some of the fossil Equise- 
 tales. The megaspores are generally formed at the base of the 
 strobilus and the microspores occupy the upper sporangia (Fig. 
 242). The sporangia are called microsporangia and megaspor- 
 angia accordingly as they contain small or large spores and for 
 the same reason the leaves may be designated as micro- and 
 
 Fig. 242. Sporophylls and spores of Selaginella: 2, strobilus with lower 
 sporophylls separated for spore dissemination. 3, megasporophyll with 
 sporangium containing four megaspores. 4, microsporophyll. 5, mega- 
 spore enlarged. 6, microspore equally magnified. 6A, more enlarged view 
 of the microspore. The triangular surfaces of the spores show that these 
 spores have been formed in tetrads from a spore mother cell as in previous 
 groups. 
 
 mega-sporophylls. The two kinds of spores originate in the 
 same manner as previously noted and the difference in size is 
 due to the amount of food which they receive. In the case of 
 the microspores, the numerous mother cells of the sporangia form 
 four spores each in the usual manner: but in the megasporangia 
 only one of the mother cells divides in this manner, and in some 
 cases only one spore is formed. The other mother cells do not 
 develop and are ultimately consumed in nourishing the mega- 
 spores. As a result of the large amount of food transferred to 
 the megasporangium, both it and especially the one to four 
 spores become much larger than the others (Fig. 242, 3-6). 
 The physiological differentiation of the spores noted in Equisetum 
 
DEVELOPMENT OF PLANTS 349 
 
 has here gone a step further. They are so constituted that they 
 now appropriate all the food in their respective sporangia. 
 
 (b) The Germination of the Spores. — The most important and 
 suggestive feature in the life history of Selaginella appears in 
 the germination of the spores. The spores germinate in the 
 sporangia and not after being shed, as in the other orders. This 
 growth is somewhat different from previous cases in that at first 
 there is no outgrowth of the spore but simply the formation of a 
 varying number of nuclei within the spore. Later these nuclei 
 develop cell w^alls and subsequent growth ruptures the spore. 
 In this way the microspore forms two cells while in the sporangia, 
 a small one often compared to a remnant of the male gameto- 
 phyte or prothallium and a large cell which ultimately forms a 
 single antheridium. At about this stage of germination the 
 microsporangium opens by a vertical cleft, permitting the scatter- 
 ing of the spores. The larger cell of the microspores now forms 
 a central mass of gamete mother cells which are surrounded by a 
 single layer of wall cells (Fig. 243, i). The male gametes are of 
 the same character as noted in Lycopodium and are set free 
 by the disintegration of the wall cells. The megaspores begin to 
 germinate even before they have reached their full size. The 
 nucleus of a megaspore divides repeatedly, forming numerous 
 nuclei which become arranged about the walls of the young spore. 
 Subsequently walls are formed about the nuclei at the apex of 
 the spore (Fig. 243, 2) and by further division a mass of cells 
 results, in the outer part of which the first divisions of the arche- 
 gonia arise. At this stage of development, or earlier, the mega- 
 sporangium opens by a vertical cleft permitting the discharge of 
 the spores. It must be borne in mind that this origin of the 
 gametophyte is of a radically different nature from any case pre- 
 viously noted. Heretofore the gametophyte always had an inde- 
 pendent existence and the sporophyte for varying periods of time 
 was a parasite upon it. In Selaginella you note the beginning 
 of a reversal of this relation, for the sporangia remain green 
 and continue to nourish the spores during their germination; in 
 other words, during the early stages in the development of the 
 gametoph^'te. We see that the gametophyte is becoming para- 
 
350 
 
 GAMETOPHYTE OF SELAGINELLA 
 
 sitic upon the sporophyte (see page 281). After the spores are 
 shed they complete the growth of the gametophyte by forming 
 the archegonia and filHng the space within the spores with a soUd 
 mass of cells. This growth ruptures the spore walls at the apical 
 regions, thus exposing the archegonia to the male gametes (Fig. 
 243, 3). The archegonia are rudimentary structures consisting 
 
 Fig. 243. Gametophyte and young sporophyte of Selaginella: i, section 
 of a microspore that has nearly completed its germination — p, a single cell 
 that is possibly a remnant of the large gametophyte or prothallium of pre- 
 vious groups of ferns; aw, antheridium consisting of a layer of wall cells 
 which enclose the gamete mother cells. lA, a male gamete. 2, section 
 of a megaspore, showing the stage of germination that is usually attained 
 in the sporangium. 3, mature female gametophyte that has ruptured the 
 wall of the megaspore, thus exposing the archegonia, ar, one of which has 
 developed a young sporophyte or embryo with root, r; stem bearing two 
 leaves, st, foot, /, and the suspensor, s. 4, young sporophyte with root, 
 stem and leaves emerging from the gametophyte. 
 
 of but two neck cells, and a single canal cell leads to the female 
 gamete. 
 
 It is evident that these variations are of great advantage in 
 ensuring the perpetuation of the species. Especially is the 
 nourishment of the young gametophyte by the highly organized 
 sporophyte a distinct gain. In previous types the formation of 
 the gametophyte was dependent upon favorable conditions, such 
 as moisture, light, temperature, etc., and it should be said that 
 
DEVELOPMENT OF PLANTS 351 
 
 only in rare cases do the spores fall in such favorable situations 
 and meet with such suitable conditions as to enable them to 
 mature archegonia and antheridia, and so provide for a new sporo- 
 phyte. In Selaginella the formation of the gametophyte is en- 
 sured by the food supplied to it by the sporophyte. Thus the 
 germination of the spore is practically independent of its sur- 
 roundings and it can complete its growth under a variety of condi- 
 tions, even in the dark. It should also be noted that this relation- 
 ship of the two generations results in a marked reduction in the 
 size of both the male and female gametophyte, since they are no 
 longer burdened with the work of food construction. This re- 
 duction will proceed very rapidly as we advance through the 
 remaining groups. 
 
 (c) Germination of the Gametospore. — The germination of the 
 gametospore is very much like that of Lycopodium. The sus- 
 pensor is much longer and the young sporophyte consists of 
 a stem bearing two cotyledons, a well-developed foot and a root 
 that is developed after the other organs (Fig. 243, 3). The foot 
 continues to absorb food from the gametophyte after the stem 
 and root have emerged, and in this condition the relation of the 
 sporophyte to the gametophyte is strikingly suggestive of a 
 sprouting seed (Fig. 243, 4). In this connection it should be 
 remembered that the young sporophyte of certain species remains 
 in the gametophyte during the winter or during a drought in a 
 resting or dormant condition, thus resembling very closely the 
 seed structure to be seen in the next group. It may also be 
 stated that fertilization and even the formation of the young 
 sporophyte may begin before the spores are discharged from the 
 sporangium, thus helping us to understand how it came about 
 that the megaspores were permanently retained in the sporan- 
 gium as is the case among seed plants. In one of the large 
 groups of Pteridophyta that have become extinct, it is note- 
 worthy that both the gametophyte and young sporophyte were 
 retained in the sporangium so that the seed-like structures were 
 formed . 
 
CHAPTER IX 
 
 DIVISION IV. SPERMATOPHYTA OR SEED PLANTS 
 
 1 1 8. General Characters. — This is the largest group of the 
 vegetable kingdom and includes all our common trees, shrubs 
 and herbs. The relationship of these plants is very imperfectly 
 understood and the various classes, orders, etc., into which they 
 are divided are in part artificial and do not therefore represent 
 completely the alliances of the groups. You have noticed that 
 the sporophyte is the most important feature in the life history 
 of the fern and that the gametophyte, subordinate from the start, 
 becomes very inconspicuous in some of the forms. This in- 
 equality of the two generations is more noticeable in the seed 
 plants where a progressive series of variations result in a more and 
 more complex external and internal differentiation of the various 
 organs of the sporophyte and in a steady reduction of the gameto- 
 phyte. Heterospory that appeared in several groups of the Pteri- 
 dophyta, becomes a constant characteristic of the seed plants. 
 The spores originate much as in the preceding group but the 
 megaspore is nourished and permanently retained in the sporan- 
 gium where it not only forms the female gametophyte (see Selagi- 
 nella) but also develops the young sporophyte. At this point, 
 the most characteristic feature of the Spermatophyta is seen. 
 The young sporophyte or embryo usually ceases to grow while 
 still in the gametophyte and passes into a resting or dormant con- 
 dition and the sporangium which has become variously modified 
 for the purpose of protecting the embryo is discharged from the 
 sporophyte. This sporangium containing the embryo is called 
 the seed. Contrast with these features two important differences 
 that appeared in Selaginella. First: The spores of these ferns 
 are retained and nourished in the sporangia only during the 
 early development of the gametophytes. Second: The young 
 sporophyte of Selaginella — it may also be called the embryo — 
 does not normally pass into a resting state, but stead il grows on 
 
 352 
 
DEVELOPMENT OF PLANTS 353 
 
 nto the mature ^porophyte. The seed plants arc divided into 
 two subdivisions based upon the relation of the megasporangia 
 CO the sporophylls: I. Gymnospermae, with sporangia, hence 
 with the seed, on the surface of the sporophyll. 2. Angiospcrmae, 
 with sporangia, hence with the seed, enclosed by the sporophyll. 
 [t should be stated that this association of these two subdivisions 
 
 s artificial, the Gymnosperms being more nearly related to the 
 Pteridophyta than to the Angiosperms. 
 
 Subdivision i. Gymnospermae. Plants with Naked Seeds 
 
 119. Origin of the Gymnospermae. — This group is principally 
 represented to-day by the cone-bearing trees. In very ancient 
 geological times, several groups of primitive gymnosperms, now 
 extinct, flourished and formed an extensive and varied forest 
 vegetation. Fossil remains of these various groups have been 
 A'onderfully well preserved and show that they were undoubtedly 
 related to the Pteridophyta. The modern group of gymnosperms 
 !ias been derived from these ancient lines and but a remnant 
 las survived the changes occurring upon the earth and the com- 
 petition with the more highly specialized Angiospcrmae. A few 
 ;^enera, however, are well represented to-day and they are of 
 ^^reat commercial importance as lumber. The sporophylls of 
 che gymnosperms are arranged in a strobilus as in the Equise- 
 tales and Lycopodiales, but the microspores and megaspores are 
 not associated, being developed in separate strobili. Attention 
 will be directed to only two of the more important orders: The 
 Cycadales or Cycads and the Pinales or cone-bearing trees. 
 
 120. Order a. Cycadales or Cycads. — These plants are strictly 
 cropical, though two genera are subtropical, Zamia or coontie 
 'n Florida, and Cycas, often called the sago palm, in China and 
 Japan. Many of them are extensively cultivated in green-houses 
 Dwing to their peculiar growth and attractive foliage (Fig. 244). 
 The cycads include less than 100 species of what was in geolog- 
 cal times, a very extensive alliance. They are of special in- 
 terest as howing unmistakable fern characters and because they 
 are the most primitive of our extant seed plants. The stems 
 of these plants are rather tuberous, though certain species are 
 
354 
 
 THE CYCADS 
 
 decidedly palm-like in appearance, with stems 12 to 60 feet high. 
 The vascular bundles are collateral and arranged around a large 
 pith as noted in Botrychium. A slight enlargement of the stem 
 is brought about owing to the weak growth of the cambium of 
 these bundles, but in some genera the principal increase is effected 
 by the formation of new bundles outside of those first formed. 
 Fern types of bundles also occur in the leaves, cortex and strobili 
 of certain species. The foliage leaves are large and leathery and 
 form a rosette, alternating with scale leaves, at the apex of the 
 
 fefe:^:::^ 
 
 -i^^i^ 
 
 Fig. 244. Zamia, a cycad common in southern Florida, with strobilus of 
 megasporophylls. — H. O. Hanson. 
 
 Stem. The bases of these leaves form an armor-like plate over 
 the surface of the stems. In certain genera, the young leaves 
 are coiled as in the ferns. 
 
 (a) The Sporophylls and Sporangia of the Cycads. — The spor- 
 angia are borne on more or less modified leaves arranged in 
 large strobili at the top of the stem, though rarely on the apex. 
 Unlike preceding cases these strobili are of two kinds, the one 
 bearing only megasporangia, the other microsporangia. Further- 
 more these two kinds of strobili are on different plants. The 
 sporangia are either scattered over the sporophyll or arranged in 
 
DEVELOPMENT OF PLANTS 
 
 355 
 
 groups suggestive of the sori of the ferns, in some forms even 
 showing a rudimentary annuUis. There is a considerable vari- 
 ation in the form of the sporophylls and the distribution of the 
 sporangia. For example, in Cycas, the megasporophylls are 
 loosely associated and only slightly modified, the sporangia being 
 developed on their margins (Fig. 245, 3^3). The microsporo- 
 
 FiG. 245. Sporophylls and sporangia of the Cycads: 2, strobilus of Zamia. 
 2A, cross-section of strobilus of Zamia, showing arrangement of microsporo- 
 phylls. 2B, microsporophyll enlarged, showing sporangia arranged in sori. 
 2C, sorus of three sporangia that have opened. 2D, microsporophyll of 
 Cycas. 3, cross-section of a strobilus of Zamia, showing arrangement of 
 megasporophylls. t,A, megasporophyll enlarged with two sporangia. The 
 one on the right shown in section; ms, megasporc; t, integument; sp, spor- 
 angium; m, micropyle. t,B, megasporophyll of Cycas with laterally ar- 
 ranged megasporangia. — H. O. Hanson. 
 
 phylls are small and more compactly arranged, the sporangia 
 being associated in sori on the lower surface of the sporophylls 
 (Fig. 245, 2D). In Zamia, the strobili and sporophylls are quite 
 suggestive of Eqiiisetum, the sporangia being developed on the 
 inner side of shield-like sporophylls (Fig. 245, 2-3.I). 
 
 The microspores originate by the division of certain cells just 
 beneath the epidermis and are discharged from the sporangium 
 very much as in the Ophioglossales. The megasporangia. 
 
356 
 
 THE MEGASPORANGIUM 
 
 however, present several new departures that must be borne 
 in mind. A rather thick coat or integument covers the sporan- 
 gium save for a small opening called the micro pyle (Fig. 245, 3^!). 
 The integument will be a feature in all the succeeding groups and 
 it has been compared to a highly modified indusium. It origi- 
 nates, however, from the base of the sporangium rather than 
 from the epidermis as in the common forms. As in Selaginella, 
 only one of the mother cells takes part in the formation of spores 
 and but one megaspore (instead of four) is formed. This mega- 
 spore does not escape from the sporangium, owing to the large 
 
 Fig. 246. Section of a megasporangium of Zamia, showing the female 
 gametophyte just before fertilization: i, integument differentiating into a 
 fleshy outer and hard inner coat; m, micropyle; p, cavity in which three 
 microspores are shown germinating; g, female gametophyte which has de- 
 veloped from the single megaspore. At the micropylar end of the gameto- 
 phyte are two archegonia opening into the archegonial chamber, ar. Two 
 neck cells lead to large female gamete, 0; sp, remains of the sporangium not 
 yet consumed by the growth of the gametophyte. At the right a few of the 
 jacket cells which supply the female gamete with food are enlarged. — After 
 Webber. 
 
 number of cells surrounding it and consequently it does not form 
 a thick protective coat. It has been suggested that these cells 
 enveloping the megaspore are due to the transformation of the 
 spore mother cells, which make up the bulk of the fern sporan- 
 
DEVELOPMENT OF PLANTS 357 
 
 glum, into nourishing cells. This structure of the megasporan- 
 gium is characteristic of all the spermatoph>'ta and the arrange- 
 ment is of decided advantage since it provides the megaspore 
 during its formation and germination with an abundance of 
 food and moisture. 
 
 (b) The Gametophyte of the Cycads. — The megaspore germi- 
 nates as in Selaginella. Numerous free nuclei are first formed 
 that are arranged around the walls of the spore and by the 
 later formation of walls a tissue is developed that gradually 
 fills the enlarging spore. Archegonia now appear at the micro- 
 pylar end of this gametophyte and become sunken in pits, called 
 the archegonial chamber, owing to a continued growth of the 
 adjacent cells of the gametophyte (Fig. 246). While this female 
 gametophyte is very suggestive of the one noted in Selaginella 
 it should be observed that the archegonia are more simple in 
 structure, usually possessing but two neck cells and that the 
 central canal cells of the neck are entirely lacking. The female 
 gamete is also much larger (Fig. 246, 0) than any heretofore 
 noticed and it is nourished by specialized cells, the so-called 
 jacket cells, that surround it and supply it through minute pores 
 with an abundance of food. 
 
 It is evident that the buried position of the archegonia must 
 necessitate some new departure in order to bring the male ga- 
 metes to the archegonia. The microspores have already begun 
 their germination at the time when they are discharged from the 
 sporangium and being very light, they are carried by the winds 
 to strobili containing megasporangia. When the megasporo- 
 phylls are mature, they spread apart slightly, permitting the 
 microspores to rattle down to the sporangia where they are 
 caught, at least in some forms, by a mucilaginous secretion 
 from the micropyle. This substance in drying contracts and 
 pulls the spores in large numbers through the micropyle into 
 a chamber formed in the upper part of the sporangium (Fig. 
 246, p). As has been stated, the microspore begins to germinate 
 in its own sporangium and by the time that it reaches the cavity 
 of the megasporangium its nucleus has already divided twice, 
 forming a rudimentary male gametophyte of three cells that may 
 
358 
 
 MALE GAMETOPHYTE OF ZAMIA 
 
 be compared with that of Selaginella (Fig. 247, A)\ the ceir(g) 
 representing the small cell, and the cell (a) corresponding to the 
 large antheridial cell of the gametophyte of Selaginella (Fig. 243, 
 i), while the cell {t) called the tube cell, represents a new depar- 
 ture in the evolution of the male gametophyte. The tube cell 
 
 Fig. 247. Male gametophyte of Zamia: A, stage of germination of the 
 microspore attained in the sporangium. See text for explanation of figures. 
 B, formation of tube for absorbing of food from megasporangium. C, spore 
 end of gametophyte showing the antheridial cell dividing into a body cell, 
 h, and a single wall cell, w. D, the body cell has divided, forming two cells 
 which become the male gametes. E, spore end of male gametophyte, show- 
 ing the spirally ciliated gametes. — After Webber. 
 
 grows out into the tissues of the megasporangium, forming a 
 tubular structure, often branching extensively, and absorbs food 
 for the nourishment of the antheridial cell (Fig. 247, B). This 
 latter cell finally divides, forming a rudimentary antheridium con- 
 sisting of but a single wall cell {w) and a body cell {h). The body 
 
DEVELOPMENT OF PLANTS 359 
 
 cell usually divides but once forming two cells, in each of which 
 are organized two huge male gametes with spirally arranged 
 cilia (Fig. 247, C-E). Owing to the growth of the numerous tube 
 cells, the tLssues above the archegonial chamber become dis- 
 organized and absorbed so that the end of the gametophyte 
 containing the gametes bends Into the cavity thus formed and 
 comes into close proximity to the archegonia (Fig. 246). Owing 
 to the swelling of the cells {g) and {w) assisted also by the accu- 
 mulation of fluids in the tube, the male gametophyte is ruptured 
 at its basal or spore end and the male gametes are discharged into 
 the archegonial chamber where they swim about in the fluids 
 probably emitted 'from the tubes at the time of their discharge. 
 This fluid has been observed by Chamberlain to cause a shrinking 
 of the neck cells In one species of Dioon and so we see how the 
 male gametes may enter the archegonia. 
 
 The most noticeable departure in the sexual generation of 
 the cycads is the very considerable r eduction of the male game- 
 tophyte which consists of a delicate tube containing a few cells | 
 from one of which the male gametes are formed as in the ferns. 
 You also notice that the gametophytes have become entirely 
 parasitic upon the sporophyte, the male gametophyte at first upon 
 the sporophyte which bore it and later upon the megasporangium. 
 Attention may be called to a suggestive departure In the develop- 
 ment of the female gametophyte that has been observed In one 
 of the cycads. In case fertilization is not effected, the gameto- 
 phyte ruptures the sporangium and projects as a green tissue, as 
 in certain heterosporous ferns. 
 
 (c) Development of the Sporophyte. — The germination of the 
 gametospore differs In two Important respects from the ferns. 
 Its nucleus gives rise by repeated divisions to numerous nuclei 
 which arrange themselves around the walls of the gametospore, 
 becoming especially numerous at Its lower end, and finally form 
 cell walls (Fig. 248, A). This structure is called the i:)ro-embryo. 
 From the lower cells of the pro-embryo is organized a massive 
 suspensor (Fig. 248, B), which pushes the outermost cells of the 
 pro-embryo deep Into the tissues of the gametophyte where they 
 develop the young sporophyte or eml)r\o, which consists of a 
 
36o 
 
 THE EMBRYO OF CYCADS 
 
 rudimentary root, stem and two cotyledons (Fig. 248, C). In 
 the ferns the gametospore divided into two cells which by further 
 division formed the sporophyte. The second departure now ap- 
 pears. The sporophyte or embryo ceases to grow and it is 
 admirably protected during its resting period by the integument 
 which has become differentiated into a stony middle layer and into 
 a fleshy inner and outer layer. This completed structure, con- 
 
 FiG. 248. Development of the sporophyte or embryo of Cycas: A, sec- 
 tion of archegonium, showing a large number of cells lining the walls of the 
 germinating gametospore. This is the pro-embryo stage. B, later stage 
 in which the pro-embryo has developed a suspensor, s, which pushes the 
 embryo-forming cells, e, into the nourishing tissues of the gametophyte. C, 
 still later development, showing the greatly coiled suspensor and the young 
 sporophyte or embryo, e, with two cotyledons. D, section of a seed of Zamia: 
 i, integument which is fleshy without and stone-like within; w, micropyle; 
 g, gametophyte; e, dormant sporophyte or embryo. — A-C. — After Treub. 
 
 sisting of the modified integument, the embryo, and the gameto- 
 phyte, which has increased in size until it has absorbed all the 
 tissues of the sporangium, we call the seed (Fig. 248, D). At this 
 stage of development, the seed falls from the strobilus and may 
 remain in a dormant condition for years. 
 
 The formation of the seed is the most characteristic feature 
 of the Spermatophyta and the most important advance that is 
 to be noted in the evolution of plant life. This vaiiation has 
 given the seed plants an advantage over all other terrestrial 
 groups and made them the dominant plants upon the earth. Not 
 
DEVELOPMENT OF PLANTS 
 
 361 
 
 only are the male and female gametophytes better provided for 
 than in Selaginella, owing to their complete parasitism on the 
 well developed sporophytes, but the formation of the young 
 sporophyte or embryo is also ensured by the abundance of food 
 placed at its disposal. The most important advantage, however, 
 possessed by the seed plant appears in the modification of the 
 integument which renders it impervious to gases and fluids. 
 This change so effectually seals up the embryo as to protect it 
 for years in some cases against conditions unfavorable for growth. 
 The modification of the integument was doubtless the chief 
 factor that led to the seed habit, since it cuts off the supply of 
 
 Fig. 249, Renewal of growth of the sporophyte or embryo in the seed: 
 7, section of the seed, showing the base of the cotyledons extending from the 
 seed, thus pushing out the stem and root, the latter organ (shown in part) 
 curving down into the ground. The free ends of the cotyledons are enlarging 
 as they absorb the food stored in the gametophyte. 6, seedling six months 
 old with first normal leaf. — After Sachs. 
 
 oxygen and fluids from the sporoph3'te or embryo and thus 
 stops its growth. When conditions are favorable for growth 
 (see page 129), the integument becomes permeable to fluids and 
 gases and the embryo continues its development. The cotyle- 
 dons remain in the seed to absorb the food stored in the gameto- 
 phyte, but their basal portions elongate, pushing out the root 
 which soon becomes established in the soil, while the stem which 
 is carried out with the root extends up into the air, and de- 
 
362 THE CONE-BEARING TREES 
 
 velops the leaves (Fig. 249). This renewal of growth of the 
 embryo, or sprouting of the seed, is often erroneously termed the 
 germination of the seed. 
 
 The maidenhair tree, Ginkgo biloba, a native of China and 
 Japan and now extensively cultivated throughout the world, is 
 the sole survivor of another ancient order of gymnosperms that 
 have many points in common with the cycads and ferns. The 
 tall shaft of the stem, wide-spreading branches and the arrange- 
 ment and character of the tissues are more suggestive of the next 
 order, the cone-bearing trees. The leaves, however, are fern- 
 like in appearance and are deciduous, unlike most of the Gymno- 
 spermae. The arrangement of the sporangia, the development of 
 the gametophytes, and fertilization are of the same nature 
 as in the cycads. It is noteworthy that fertilization may be 
 effected after the megasporangia have fallen, thus approaching 
 the condition seen in Selaginella. 
 
 121. Order b. Finales or Cone-bearing Trees. — This order is 
 by far the largest group of gymnosperms and includes our well- 
 known evergreen or cone-bearing trees. They are largely con- 
 fined to temperate regions and are more numerous in the northern 
 than in the southern hemisphere. The shores of the Pacific are 
 especially favorable for their development and on our western 
 coast they attain dimensions equalled only by the giant Eucalyp- 
 tus of Australia. Though less ancient in origin than the cycads, 
 they have declined in the same way as the latter order and only a 
 remnant of this very extensive group survives today. However, 
 their variations have been more beneficial than in the case of the 
 cycads and as a result nearly 300 species are adapted to present 
 conditions upon the earth and constitute an important part, and 
 commercially the most valuable part of our forest vegetation. 
 (a) Structural and Adaptive Features of the Finales. — One of 
 the variations that is of great benefit to many of these plants 
 is their habit of forming buds (see page 71), which appear in this 
 order in more perfect form than any of the preceding. By 
 means of the scale leaves of the buds all the organs that are to 
 be produced each season are protected against drying winds. 
 The terminal bud of the tree is frequently the most vigorous and 
 
DEVELOPMENT OF PLANTS 
 
 363 
 
 as a result spirc-likc stems with very symmetrically arranged 
 branches are often formed. The growth of the Pinales is usually 
 prolonged and vigorous, the age of some of the giant trees of 
 California being estimated at over 5,000 years, and they may 
 attain the height of over 300 feet and a diameter of 30 feet. The 
 elongation of the stem is effected by a group of actively dividing 
 cells at the apex, while the increase of girth is brought about 
 by the activities of a cambium, as in the dicotyledons (see page 
 89). In fact, all the features seen in the cross-section of the 
 stem as the cork, cortex, phloem, cambium, xylem with its an- 
 nual rings of growth and medullary rays and the pith are sugges- 
 tive of the dicotyledons, save that the tissues are more simple. 
 This is particularly noticeable in the xylem, which is composed 
 
 Fig. 250. A, radial section of the xylem of Finns — /, tracheids; p, border 
 pores or thin places in cell wall to promote transfer of fluids; m, medullary 
 ray of two cells accompanied by tracheids, mt. B, tangential section, show- 
 ing medullary rays, one cell broad and from throe to nine cells high, p, bordered 
 pores, C, cross-section of the outer cells of pine leaf, showing heavy epi- 
 dermal cells, e; well-developed cuticle, c; sunken stoma, s; strengthening 
 stereome cells, st, and chlorenchyma, m. 
 
 of very regularly formed tracheids with peculiar bordered pores 
 on their radial walls to promote the rapid transfer of fluids to 
 and from the medullary rays (Fig. 250, A, B). The character- 
 istic uniform and even grain of pine, spruce, etc., is due to the 
 simplicity of the tracheids which compose the wood. This struc- 
 
364 STRUCTURE OF THE FINALES 
 
 ture of the wood explains the ease with which the timber is 
 worked and the common name of "soft wood" that is appHed 
 to it. The leaves are xerophytic in character and usually rather 
 small and often needle-Kke. The strongly thickened epidermis 
 is strengthened with strands of stereome that give them a leathery 
 character (Fig. 250, C). The stomata are sunken and the 
 centrally-placed vascular bundles communicate with the rather 
 irregular chlorenchyma cells by means of tracheids, and thin- 
 walled parenchyma cells instead of small veins as in the higher 
 plants. The leaves in the majority of the genera remain on the 
 branches for several years, and for this reason the members of 
 the Finales are commonly called "evergreens." 
 
 These plants generally flourish in localities where there is an 
 abundance of moisture and at first sight the modifications of the 
 leaves noted above are not in harmony with such surroundings, 
 being decidedly characteristic of plants growing in arid regions. 
 The minute tracheids which constitute the conducting system of 
 the Finales cannot, however, transport the fluids absorbed from 
 the soil as rapidly as the large cells noted among the dicotyledons. 
 Consequently smaller amounts of Avater are placed at the disposal 
 of the leaves, and were it not for the fact that they are admirably 
 adapted to lessen transpiration these plants would doubtless have 
 disappeared from the earth long ago. The ability of the leaves 
 to diminish the loss of water may also account for the common 
 occurrence of the cone-bearing trees in northern and mountainous 
 districts where the soils are cold and root absorption is therefore 
 lessened and where the vegetation is especially exposed to the 
 drying winds of winter (see page 38). 
 
 Nearly all the Finales are characterized by the presence of 
 resin passages or ducts in the stems and leaves. Whenever the 
 stems are cut or injured these passages pour out a thick resinous 
 liquid that effectually heals the wound. This doubtless explains 
 in part the freedom of these trees from the attack of insects and 
 wood-destroying fungi. It may possibly account for the inabil- 
 ity of many of these trees to sprout from the stump when cut. 
 Hardwood forests will very generally perpetuate themselves by 
 means of sprouts that arise from the stumps, but our evergreen 
 
DEVELOPMENT OF PLANTS 
 
 36 = 
 
 forests are rapidly disappearing, because with few exceptions the 
 trees are able only to develop from seeds. Possibly the rapid 
 covering of living cells of the stump by the resinous substances 
 excludes the atmosphere so effectually as to prevent any further 
 growth (page 75). This resinous liquid is obtained in large 
 quantities from certain species of pine by removing the bark and 
 a little of the wood from the side of the trees when the liquid 
 exudes and is caught in a pan placed at the bottom of the cut 
 surface or in a pocket made in the tree. This substance is re- 
 moved from time to time and distilled, yielding turpentine and 
 resin. Balsam is a resin obtained from the balsam fir (Abies), 
 (b) The Sporangia of the Finales, — The sporangia resemble 
 those of the cycads in origin and structure and are developed 
 upon sporophylls that are usually grouped in strobili (Figs. 251, 
 
 Fig. 251. Microsporophylls of the pine: i, branch of pitch pine with 
 leaves spirally arranged in fascicles of three and also with several strobili, 
 St. At tip of branch the bud, 6, is shown elongating and bearing numerous 
 fascicles of leaves that are still enclosed by paper bracts. 2, surface and side 
 view of microsporophyll, showing arrangement of sporangia, sp. 2a, micro- 
 sporophyll of juniper bearing three sporangia. — II. O. Hanson. 
 
 252). The microsporangia are formed on rather small, delicate 
 and often brightly colored sporophylls that perish as soon as the 
 spores are scattered (Fig. 251, 2). The microsporophylls bear 
 one or several sporangia on their under surface and are hygro- 
 
366 
 
 SPOROPHYLLS OF PINALES 
 
 scopic, curving away from each other when the sporangia are 
 ripe, thus permitting the opening of the sporangia and the gradual 
 distribution of the spores by the wind and closing when mois- 
 tened to protect the spores against wetting. In some of these 
 genera the outer coat of the microspore is lifted away from the 
 inner coat in such a way as to form two sac-like outgrowths 
 which render them more buoyant and adapted to distribution by 
 the wind (Fig. 254). The miscrospores are produced in such 
 enormous numbers that the ground in the vicinity of the ever- 
 
 FiG. 252. Strobili of megasporophylls: i, a branch that has developed 
 in the spring from a bud. The fascicles of leaves are just emerging from 
 their papery bracts and at the right is shown a strobilus, st, consisting of 
 minute spine-tipped scales. 2, growth of strobilus shown in i, twelve months 
 later. 3, appearance of strobilus shown in 2, six months later. — H. O. Hanson. 
 
 green forests is often covered with the yellow spores, the so- 
 called "sulphur snow." This extravagant formation of micro- 
 spores is a necessity owing to the small chance of any one of the 
 microspores being carried to the megasporophylls and so ensur- 
 ing fertilization. Were it not for the fact that the microspores 
 need be carried only comparatively short distances, this order 
 
DEVELOPMENT OF PLANTS 367 
 
 would doubtless become extinct. The gymnosperms, however, 
 live in colonies and the strobili of mega- and micro-sporophylls 
 usually occur upon the same plant, thus necessitating but a short 
 transfer of microspores. The megasporophylls usually endure 
 for a year or more and become quite large, forming the leathery 
 or woody scales of the cone- or strobilus (Fig. 252). The mega- 
 sporangia are usually developed on the upper side of the sporo- 
 phyll (Fig. 253, A), the number formed varying in the different 
 genera. In the majority of the Pinales, the sporangia appear 
 upon curious outgrowths which become large and form the 
 conspicuous scales of the cone. This structure is generally 
 regarded as a greatly modified branch or shoot with which two 
 sporophylls are fused. It generally arises from the axil of a 
 bract just below it and with which it may be more or less united — 
 see the spruces, pines, firs, etc. (Fig. 253, B). The origin and 
 structure of the megasporangium and megaspore are essentially 
 as noted in the cycads. There is only slight indication, however, 
 in a few of the forms of the development of a cavity for the 
 reception of the microspores. Several mother cells may form 
 megaspores, but only one of these megaspores ever develops and 
 germinates. 
 
 (c) The Gametophyte of the Pinales and Fertilization. — The 
 female gametophyte is formed as previously noted in the cycads. 
 By the repeated divisions of the nucleus of the megaspore numer- 
 ous free nuclei are formed that become arranged around the walls 
 of the megaspore. Later cell walls are developed about these 
 nuclei, and by further division the entire space within the enlarg- 
 ing spore is filled with tissue (Fig. 253, C). A few or a large 
 number of archegonia are developed at the micropylar end of 
 the gametophyte. The archegonium consists of two or several 
 neck cells, and a large female gamete which is surrounded by 
 nourishing cells as in the cycads. It is noteworthy in a few of the 
 forms that the archegonia begin to appear very carl>' in this 
 growth outlined above and that the bulk of the female gameto- 
 phyte is developed subsequent to their origin. The possible 
 significance of this will be more apparent in the Angiosperms, 
 where we will see the female gamete arising with the third division 
 
368 
 
 GAMETOPHYTE OF FINALES 
 
 of the nucleus of the megaspore, the gametophytic tissue being 
 developed at a later period. You have noticed that there is no 
 chamber formed above the archegonia, and it would be a natural 
 inference that the male gametophyte must be of a somewhat 
 diflferent character from that of the cycads in order to met this 
 
 Fig. 253. Megasporangia of pine: A, scale-like outgrowth of sporoph^-ll 
 bearing two sporangia. B, section of A, taken through one of the sporangia, 
 showing a small bract, s, and large scale-like outgrowth which bears the 
 sporangia, sp. The sporangium is surrounded by an integument, i, and con- 
 tains a single megaspore, m, as in the Cycads. mc, microspores that have 
 been drawn through the micropyle to the sporangium, into which their tube 
 cells are growing. C, section of megasporangium enlarged and diagrammatic, 
 showing the female gametophyte at the time of fertilization — g, female gamet- 
 ophyte, showing two archegonia with large female gametes, fg. At the left 
 the tube of the microspore, mc, has entered the neck of the archegonium and 
 the two male gametes have been discharged into the large sac of the female 
 gamete. On the right the tube cell with male gametes is seen approaching 
 the neck of the archegonium. i, integument; sp, remains of sporangium 
 not disorganized by the gametophyte; / shows the disorganization caused by 
 the tube cells as they grow towards the archegonia. 
 
DEVELOPMENT OF PLANTS 
 
 369 
 
 new departure. The microspores which have already begun to 
 germinate when discharged from their sporangia are carried by 
 the wind to the megasporophylls which are sHghtly spread apart 
 at this time, permitting the microspores to rattle down to the 
 megasporangia. The microspores fall into the micropyle either 
 by reason of the position and construction of the megasporangia, 
 which may be so placed that the spores naturally roll down into 
 the micropyle, or the microspores may be drawn through the 
 micropyle by mucilaginous excretions as in the cycads. The 
 
 Fig. 254. Male gametophyte of the pine: 6, section of microspore, show- 
 ing the two air sacs, s, formed by the lifting up of the outer wall of the spore. 
 
 8, stage of germination of the spore at time of discharge from its sporangium 
 — t, tube cell; a, antherldial cell, above which are seen two black lines, the 
 remains of cells formed by earlier division of the nucleus of the microspore. 
 
 9, continuation of germination of the microspore after reaching the mega- 
 sporangium — /, tube cell forming a branching tube that disorganizes and ab- 
 sorbs the cells of the megasporangium. The antheridial cell (a) of 8 ha- 
 divided into a wall cell, w, and a body cell, b. 10, end of tube cell as it aps 
 proaches the female gamete — m, m, male gametes. The nuclei of the tube 
 cell and wall cell are also seen (somewhat disorganized) in the end of the tube. 
 — After Coulter and Chamberlain. 
 
 early stages in the germination of the microspore are essentially 
 as in Cycas, though genera ly more reduced. The microspore on 
 reaching the megasporangium has germinated, forming a tube 
 
 24 
 
370 MALE GAMETOPHYTE OF FINALES 
 
 cell and an antheridial cell, but the one or two cells, which corre- 
 spond to the cell (g) of the gametophyte noted in the cycads, are 
 quickly disorganized and appear as faint lines (Fig. 254, 8). 
 In some forms several of these prothallial cells are formed so that 
 the male gametophyte is more primitive in this respect than the 
 cycads. On the other hand other groups do not apparently 
 form a prothallial cell at all. So the tendency is towards the 
 elimination of these cells because they are no longer of service 
 in setting free the gametes. The tube cell develops as in the 
 cycads, but it is directed in its growth so that one of its branches 
 finally reaches one of the archegonia, when it pushes aside the 
 neck cells and fuses with the cell membrane of the large female 
 gamete (Figs. 253, C; 255, A). In the meantime the antheridial 
 cell has divided into a wall cell and body cell (Fig. 254, 9), and 
 two motionless male cells are formed from the body cell. It is 
 to be noted here that the body cell does not divide, forming cells 
 in which male gametes are organized, as in all preceding cases. 
 It forms two free nuclei and these with their associated cytoplasm 
 function as gametes. It is now apparent why the tube cell 
 extends to the female gamete and also why the archegonial 
 chamber is not developed. The male gametes and the wall cell, 
 now quite unattached, are carried down to the end of the tube 
 (Fig. 254, 10) by the cytoplasmic currents and pass into the cyto- 
 plasm of the female gamete through an opening that is formed 
 in the end of the tube. Chemical attraction now draws one of 
 the male gametes to the nucleus of the female and their fusion 
 results in the formation of the gametospore. The remaining 
 cells of the male gametophyte are apparently disorganized 
 (Fig. 255, A). Several instances have been reported where the 
 male cells differ in size, indicating the loss of function in one of 
 these cells. On the other hand in those groups where the 
 archegonia are closely associated and where, therefore, the tube 
 cell may spread over more than one archegonium, no such reduc- 
 tion appears. The functions performed by the tube cells in the 
 Finales will be a constant characteristic of all the members of 
 the next subdivision or Angiospermae, and it should be noted that 
 this important modification of the male gametophyte probably 
 
DEVELOPMENT OF PLANTS 
 
 371 
 
 arose at first as an al)sorbing organ to supply the antlieridial cell 
 with food, as is the case in the cycads. In the more advanced 
 types the archegonia became more effectually enclosed in the 
 sporangial tissues and the tube cell assumed in addition the 
 function of a conduit for the male gametes which lost their 
 
 /-.^, 
 
 D 
 
 Fig. 255. Development of the pro-embryo of pine: A, section of an ar- 
 chegonium at time of fertilization. One of the male gametes, cf , is seen 
 fusing with the female, 9. The second male gamete, cf, the tube nucleus, 
 and the wall cell are also shown near the neck of the archegonium. B, the 
 gametospore has germinated, forming four ceils, which are passing to the 
 upper end of the sac. C, the cells shown in B, arranged at end of sac and 
 in process of division (only two cells in this sectional view). D, later stage, 
 the pro-embryo of four plates of cells. E, the second plate or suspensory 
 cells, s, of the pro-embryo elongating, thus pushing the embryo-forming cells, 
 e, into the tissues of the female gametophyte. — A after Ferguson; B-E after 
 Coulter and Chamberlain. 
 
 motility as a result of this new method of transport to the 
 female gamete. 
 
 (d) Development of the Sporophyte. — The germination of the 
 gametospore of the pine will illustrate the more important 
 features in the process. The nucleus of the gametospore divides 
 a var^dng number of times, commonly forming four nuclei (Fig. 
 255, B), which pass to the upper end of the spore and arrange 
 
 1 
 
372 
 
 EMBRYO OF FINALES 
 
 themselves in a plate. By successive division these cells are 
 increased until usually four plates of four cells each are formed 
 
 Fig. 256. Development of the young sporophyte or embryo: 2, diagram 
 of a section of a megasporangium, showing the formation of elongated sus- 
 pensory cells and numerous embryos, e; i, integument; sp, sporangium nearly 
 consumed by the growth of the gametophyte; g, gametophyte, the central 
 portion of it disorganized by growth of embryos, e. ^A, section of a nearly 
 mature embryo; s, suspensor; r, root cap; c, cotyledons; sf, stem. 35, external 
 view of embryo. 
 
 which become surrounded by cell walls save in the case of the 
 lower plate, which Js in direct contact with the rich food of the 
 spore (Fig. 255, C-D). This meager growth corresponds to the 
 
 Fig. 257. Sectional view of pine seed: i, hard integument; g, gameto- 
 phyte, often called the endosperm, which has completely consumed the spor- 
 angial tissues. The embryo consists of a root, r, ensheathed in a large root cap 
 cotyledons, c, and stem, s. 
 
DEVELOPMENT OF PLANTS 373 
 
 pro-embryo of Cycas (Fig. 248, A). In this connection It should 
 be noticed that only a portion of the gametospore, as In the 
 cycads, is used In forming the embryo. In all the preceding 
 groups, as the mosses, ferns, etc., the gametospore in germinating 
 behaves as an ordinary cell, dividing into two cells which con- 
 tinue the process. In the Finales the larger portion of the 
 gametospore serves as a storehouse for food, cell formation being 
 confined to the upper end of it (Fig. 255, B, C). With one ex- 
 ception this is the only group characterized by this peculiar ger- 
 mination which is more suggestive of the growth of the animal 
 ^gg where a large portion of the ^gg is reserve food for the nourish- 
 ment of the embryo. The nourishment of the female gamete 
 by the jacket cells is also suggestive of certain animals in which 
 the ^gg cells are formed in a similar manner. The embryo is 
 developed from the uppermost plate of cells which are pushed Into 
 the cells of the gametophyte by the elongation of the suspensory 
 cells just below them (Fig. 255, £). Each of these four cells 
 may form an embryo (Fig. 256, 2) or they function collectively 
 in this growth. The result is the same in all cases, rarely more 
 than one embryo being found In a seed. The mature embryo 
 consists of a stem bearing two or several laterally-placed coty- 
 ledons and a root with root cap (Fig. 256, 3). Attending this 
 formation of the embryo, pronounced changes occur in the 
 sporangium. It steadily increases in size and the integuments 
 become modified into a hard coat, or the outer layer may be 
 pulpy, as in some of the cycads. The gametophyte also increases 
 in size, and as the embryo matures it absorbs all the tissues of the 
 sporangium, thus filling the space within the Integument (Fig. 
 257). The cells of the female gametophyte are often called the 
 "endosperm." By these growths the seed is formed and pre- 
 pared for its dormant state, as in the preceding order. In some 
 cases the integument forms a membranous outgrowth, which 
 assists in the distribution of the seeds (Fig. 258, C). It Is inter- 
 esting to note that the seeds are so attached to these wings that 
 they rotate in falling to the ground, thus retarding their fall and 
 making possible a wider flight. The stimulus resulting from the 
 fusion of the gametes also produces extensive changes outside of 
 
374 
 
 THE SEED OF FINALES 
 
 the sporangia. The sporophylls or their outgrowths often be- 
 come greatly enlarged, forming the hard, woody scales of the 
 cones (Fig. 258, B), or they may become fleshy and fuse, forming 
 a berry-like fruit, as in the juniper (Fig. 260, 7). When this 
 growth has been completed, the woody sporophylls or scales of 
 the strobilus dry out, and becoming hygroscopic, they spread 
 apart on dry days, thus permitting the scattering of the seeds 
 by the winds (Fig. 258, A). Many of the fleshy fruits are eaten 
 by birds and the hard nut-like seeds are distributed in this w^ay. 
 
 Fig. 258. A, mature strobilus of pine with open scales to permit the 
 scattering of the seeds. B, scale from strobilus showing the winged seeds 
 developed from the two sporangia. C, a seed with wing-like outgrowth, 
 as it escapes from strobilus. 
 
 The stages in the development of the seed, outlined above, are 
 very much prolonged in many of the pines. The microspores on 
 reaching the megasporangia, in the spring, develop only a short 
 tube cell during the first season and not until about the first of 
 July of the following season are the male and female gametes 
 mature and ready for fertilization. The seeds are matured during 
 the following season, over two years after the appearance of 
 the strobilus. Accordingly, three stages in the development 
 of the strobilus of megasporophylls may be seen on certain 
 species of pine in the early summer — very small ones that received 
 the microspores in the spring, larger ones in which fertilization 
 and the formation of the embryo is being effected and older stro- 
 bili in which the seeds are approaching maturity (Fig. 252). 
 When the conditions are favorable the embryo renews its growth. 
 
DEVELOPMENT OF PLANTS 
 
 375 
 
 The root is pushed out through the micropyle and bends down 
 into the soil, the hard integument often being cracked open by 
 the SwelHng of the cells. The cotyledons remain within the seed 
 until they have absorbed all the food from the gametophyte, 
 when they are withdrawn and become erect (Fig. 259). 
 
 Fig. 259. Renewal of growth of the embryo. At the left a seed has been 
 cut across to show the relation of parts during the early growth. The in- 
 tegument has been ruptured by the swelling of the seed and the protrusion 
 of the root which is curving down into the soil. The cotyledons, c, remain 
 in the seed absorbing the food from the gametophyte or endosperm, g. At 
 the right a later growth with the cotyledons partially withdrawn from the 
 seed after the absorption of its food. 
 
 (e) The Principal Genera of Finales. — The more important 
 genera of Pinales may be distinguished as follows: Pinus or 
 pine, leaves long and needle-like, borne in fascicles on short stems 
 that are quite concealed by papery sheathing scales (Fig. 251, i). 
 Larix or larch, short needle-like leaves clustered in tufts on short 
 lateral branches. This is the only northern member of the order 
 with deciduous leaves. Picea or spruce, leaves angled or four- 
 sided, radiating from all sides of stem, petioles remaining on 
 branchlet after leaves fall, thus causing the rough appearance of 
 the branchlets (Fig. 260, 9). Tsiiga or hemlock, leaves fiat in 
 two rows, the petiole remaining on tlie stem after fall of leaf; 
 
376 
 
 FORMS OF FINALES 
 
 the strobilus is ovoid with rather leathery scales. The spruce 
 and hemlock have pendent strobili. Abies or fir, leaves flat in 
 two rows, the stems being smooth after leaf fall, strobili erect on 
 the branches. Taxodium or bald cypress, leaves flat in two rows 
 
 Fig. 260. Common examples of the Finales: 5, Thuja or arbor vitae. 
 6, Strobilus of Chamaecyparis or southern white cedar. 7, strobilus of Ju7ii- 
 perus or red cedar with fleshy scales fused into a berry-like fruit. 8, branch 
 of Taxus or yew. The seeds are produced singly in the axils of leaves on short 
 lateral branches and nearly enveloped by a thick fleshy cup that becomes 
 bright red. 9, Picea or spruce. 
 
 and deciduous, strobilus globose with thick, spirally-arranged and 
 shield-like scales. Thuja, arbor vitae, or northern white cedar, 
 leaves scale-like, closely appressed to the branches in four rows, 
 strobili ovoid with flat tough scales (Fig. 260, 5). Chamaecyperus 
 
DEVELOPMENT OF PLANTS 377 
 
 or southern white cedar, leaves resembling Thuja, but cones 
 globose with shield-like opposite scales (Fig. 260, 6). Jtiniperus 
 or juniper, pointed scale-like leaves, opposite or in whorls on 
 stem, scales of strobili becoming fleshy and fusing to form a 
 berry-like fruit (Fig. 260, 7). 
 
CHAPTER X 
 
 SUBDIVISION 2. ANGIOSPERMAE. PLANTS WITH ENCLOSED 
 
 SEEDS 
 
 122. Origin of the Angiospermae. — These plants, hke tke 
 Gymnospermae, produce seeds, but they differ so essentially from 
 the latter group in the character and structure of the sporophyte 
 and gametophyte as to justify their separation into a distinct 
 division. They are retained here as a group coordinate with 
 the gymnosperms out of deference to common usage. It has 
 been noticed that the seed habit arose quite independently of 
 the Spermatophyta (see page 351) and it appears probable that 
 the Angiospermae represent an independent line of development. 
 Though the most recently evolved plants, their origin is uncertain 
 and the meager evidence points to their derivation from a stock 
 related to the more primative Filicales rather than to the Gym- 
 nospermae. The variations of this modern group of plants have 
 been many and so successful that they have crowded out many 
 of the lower forms and become the dominant plants upon the 
 earth. They exceed all other groups combined in variety and 
 number of forms, approximately, 125,000 species — certainly a 
 striking contrast to the other vascular plants w^hich comprise 
 about 450 gymnosperms and 4000 pteridophytes. The angio- 
 sperms are adapted by their variations to practically all con- 
 ditions that will sustain life, ranging from aquatics to xerophytes, 
 from terrestrials to epiphytes and from photosynthetic to sapro- 
 phytes and parasitic plants. The important features of the 
 plant body have been considered in the opening chapters of the 
 book. 
 
 123. The Sporophylls of Angiosperms. — The flower is often 
 considered as one of the most characteristic features of the 
 angiosperms, but this structure contains as its essential organs 
 one or more sporophylls and the term flower could be applied 
 quite as well to the association of these organs in the Pterido- 
 phyta and gymnosperms. 
 
 378 
 
DEVELOPMENT OF PLANTS 
 
 379 
 
 The sporophylls, like all organs of the j^lant, show a wide range 
 of variation and they are generally associated with more or less 
 modified leaf-like organs which serve to protect them. These 
 leaf-like organs are known as the floral envelope or perianth and 
 doubtless arose in many forms through the sterilization and modi- 
 fication of the sporophylls (Fig. 261 A, i). The microsporo- 
 phylls, often called stamens, usually consist of a stalk or fila- 
 ment and a four-lobed spore-bearing part, the anther (Fig. 261 A, 
 2). In cross-section, the anther is seen to consist of four spo- 
 
 FiG. 261 A. Flower and sporophylls of Angiospcrnis: i, flower of Sedum 
 with leaf-like perianth, p; microsporophylls, s; megasporophylls, c. 2, 
 microsporophyll of the buttercup, showing four-lobed anther and filament. 
 
 3, diagram of a cross-section of an anther, showing the breaking down of the 
 tissue about the four sporangia and the beginning of the opening of the anther. 
 
 4, one of the sporangia from a young anther, as seen in cross-section — m, 
 spore mother cells. The large cells surrounding the mother cells are nour- 
 ishing cells, known as the tapetum, and disorganize as the spores mature. 
 At the right a mother cell forming four microspores, the upper one being 
 characteristic of dicotyledons and the lower of monocotyledons. 
 
380 
 
 SPOROPHYLLS OF ANGIOSPERMS 
 
 rangia in which the microspores originate in fours as in the 
 preceding groups (Fig. 261 A, 3, 4). At maturity, the two 
 sporangia on each side of the anther usually merge into one 
 cavity, owing to the breaking down of the intervening tissue. 
 The anther opens in a variety of ways, as by slits and pores, 
 permitting the scattering of the spores (Fig. 261 A, 3). The 
 
 Fig. 261B. Megasporophyll and megasporangium : i, megasporophyll of 
 buttercup — s, stigma; st, style; 0, ovary. 2, longitudinal section of mega- 
 sporophyll — mg, megasporangium; m, megaspore, the mother cell having 
 formed four spores in series, but the end one only develops; i, integument; 
 mi, micropyle. 
 
 microsporophylls are often externally quite suggestive of those 
 noted in certain of the Pinales, but the megasporophylls are 
 essentially different from any other group considered, in that 
 the sporangia are inclosed in a cavity formed in the sporophyll. 
 This peculiarity is one of the important characteristics of the 
 angiosperms, as the name indicates. The megasporophyll, often 
 termed the pistil or carpel, is usually a rather elongated, flask- 
 shaped organ with a hollow swollen base and consists of a stigma, 
 style and ovary (Fig. 261 B, i) within which are produced the 
 megasporangia or ovules. Such a structure would result if the 
 edges of the leaf-like megasporophylls of previous groups were 
 inroUed so as to form a closed organ. The megaspores originate 
 
DEVELOPMENT OF PLANTS 
 
 381 
 
 in these sporangia as already noted in the g^-mnospcrms and 
 usually but one megaspore is developed in each sporangium (Fig. 
 261^,2). 
 
 124. Development of the Flower of Angiosperms. — The sporo- 
 phylls are variousl}^ associated in groups tliat are commonly 
 called flowers. In its simplest form, the flower may be defined 
 as a minute branch or receptacle bearing one or more sporo- 
 phylls. Such a type is illustrated in the cat- tail (Fig. 262, B, C) 
 
 Fig. 262. Forms of primitive flowers: A, inflorescence of Typha or cat- 
 tail — mi, region bearing only flowers with microsporophylls; mg, flowers with 
 megasporophylls; h, bract. B, flower consisting of two microsporophylls 
 which are sessile on a short stalk that has numerous hairs. C, flower consist- 
 ing of one megasporophyll — s, stigma; 0, ovary surrounded with haiirs. D 
 early appearance of the inflorescence of Salix or willow. E, inflorescence 
 bearing only megasporophylls. F, flower, of a single megasporophyll with 
 forked stigma — -h, bract; n, nectar gland. G, inflorescence bearing only 
 microsporophylls. H, flower of two microsporophylls. 
 
 where the flower consists of one or a few sporophylls associated 
 with hairs, and also in the willow where the sporophylls are 
 developed in the axil of a minute bract (Fig. 262, 7^, H). It 
 should be noted in the cat-tail that numerous spirally arranged 
 
382 PRIMITIVE FLOWERS 
 
 hairs are associated with the sporophylls. These are supposed 
 to represent sterile sporophylls and this is borne out by the fact 
 that primitive flowers are characterized by just such an arrange- 
 ment of their sporophylls, and also by the fact that in an allied 
 genus spirally arranged sporophylls actually occur. These simple 
 types of flowers are often developed in large numbers upon an 
 elongated stem (Fig. 262, A, D-G) and are rather suggestive 
 of a strobilus although the individual flower really corresponds to 
 the strobilus, as will be seen especially in those types where 
 the sporophylls are numerous and arranged spirally upon the 
 receptacle of the flower (Fig. 264). A group or cluster of flowers 
 is called an Inflorescence in contradistinction to the solitary 
 flower developed at the end of a branch or stem. It Is notice- 
 able in these primitive anglosperms that the micro- and mega- 
 sporophylls are usually borne in separate flowers or inflorescences 
 (compare Finales) which are developed on separate plants as 
 in the willow or on different parts of the same plant as in the 
 cat-tail. This arrangement is probably associated with the 
 fact that the advantages of crossing or the transfer of the micro- 
 spores of one flower to the megasporophylls of another is effected 
 by the wind. In higher types, which Include the great majority 
 of anglosperms, the micro- and mega-sporophylls are developed 
 in the same flower which is therefore said to be perfect since it 
 contains both of the organs essential for seed production (Fig. 
 261 A, i). A type like the willow is termed imperfect because 
 the flower lacks one kind of sporophyll. Crossing Is effected 
 in the perfect type of flower by the earlier ripening of the mlcro- 
 or mega-sporophylls and often also, by the arrangement of the 
 organs of the flower which is of such a nature that the microspores 
 cannot readily reach the mega-sporophylls of the same flower. 
 Insects are usually the agents for the transport of the micro- 
 spores in such cases. Flowers In which the microspores mature 
 and are shed from the anthers before the stigmas of the mega- 
 sporophylls are ready to receive them, are called protandrous, 
 meaning that the microspores w^hich develop the male gametes, 
 are the first to mature. If the mega-sporophylls become recep- 
 tive before the anthers open, the flower is said to be protogynous, 
 
DEVELOPMENT OF PLANTS 
 
 383 
 
 meaning that the pistil matures first. It must also be borne in 
 mind, although the devices for effecting a crossing are almost 
 universal among the various groui:)s of angiosperms, that there 
 are equally elaborate provisions for the transference of the 
 microspores of perfect flowers to the stigmas of their own flower. 
 This is called autogamy and would appear to be a provision for 
 setting seed in case crossing fails. 
 
 From the above discussion we might characterize the primitive 
 
 Fig. 263. Development of the perianth: -1, inllorescencc of Qnercus or 
 oak — mi, inflorescence with flowers bearing only microsporophylls; mg, 
 inflorescence with flowers bearing megasporophylls. B, flower of oak, en- 
 larged, consisting of several microsporophylls and a perianth of minute scale- 
 like organs. C, flower of Erythronium or fawn lily. The perianth of six con- 
 spicuous leaf-like organs. D, flower of Melandrytim or day pink — ca, calyx of 
 green sepals; c, corolla of five delicate petals. 
 
 flowers as consisting usually of a large and indefinite number of 
 sporophylls, spirally arranged upon the receptacle which is suf- 
 ficiently elongated as to permit the separate attachment of each 
 organ. Following the development of this type of flower, there 
 appeared as the next advance, minute outgrowths about the 
 sporophylls, known as the perianth. In its simplest form this 
 
384 
 
 EVOLUTION OF THE FLOWER 
 
 consists of a few scales as in the sweet flag, oak, etc. (Fig. 263, B), 
 but in higher forms, the perianth appears as the conspicuous 
 leafy portion of the flower as in the Hly (Fig. 263, C). Finally 
 flowers appear in which the leaves of the perianth become dif- 
 ferentiated into an outer calyx composed of several green sepals 
 and a corolla of larger, more delicate and often brightly-colored 
 leaves, called the petals (Fig. 263, D). We have now reached a 
 point where the flower is said to be complete, consisting of all 
 the organs that are normally associated in the flower. 
 
 Fig. 264. Fig. 265. 
 
 Fig. 264. Flower of strawberry with elongated receptacle bearing nu- 
 merous spirally arranged sporophylls: A, open flower. B, flower in section, 
 showing arrangement of parts upon receptacle, r, which forms a shallow cup 
 at base bearing the perianth and microsporophylls. C, the fruit or enlarged 
 receptacle bearing the minute spirally-arranged megasporophylls. 
 
 Fig. 265. Flowers with shortened receptacles: A, flower of Pyrola with 
 calyx and corolla arranged in whorls or cycles. B, section of flower, showing 
 all the organs in cycles. C, flower of geranium. D, flower with corolla 
 removed to show the coherence of the five megasporophylls that results from 
 the shortening of the receptacle. 
 
 As stated above the simpler type of flower is characterized 
 by numerous sporophylls, spirally arranged and separately at- 
 tached to an elongated receptacle. One of the most important 
 variations that appeared in the evolution of the flower is asso- 
 ciated with the shortening of this receptacle. This is brought 
 
DEVELOPMENT OF PLANTS 
 
 ^^0 
 
 about by the checking of the apical growth of the receptacle and 
 is often associated with a more or less extended growth of its 
 basal region. These changes affected the flower in a most pro- 
 found way. A crowding resulted, organs were reduced in number 
 and their spiral arrangement upon the receptacle became so 
 flattened that the various sets of organs appear to arise in whorls 
 or cycles (Fig. 265, A, B). These cyclic flowers are characteristic 
 of all the higher orders of angiosperms and no one character is of 
 
 
 \ <■' 
 
 Fig. 266. Forms of adhesion that result from shortening of receptacle: 
 A, flower of rose. B, section of flower, showing the lower portion of recep- 
 tacle forming a cup about the megasporophylls, mg, and bearing the other 
 organs of the flower. C, inflorescence of comfrey, Symphytum. D, flower 
 enlarged in section to show adhesion of microsporophylls, ;;//, to the tubular 
 corolla. 
 
 more value in enabling us to state whether a plant is of high or 
 low rank. As the cyclic habit became established, so the number 
 of organs in each whorl became constant. Thus at a certain 
 point in the evolution of the monocotyledons you will find the 
 organs usually in threes, whereas in cyclic dicotyledons whorls 
 with four or five organs are the rule. 
 
 The crowding of the organs on the receptacle also caused them 
 to develope e7t masse (as a unit), not as separate organs. This 
 
386 EVOLUTION OF THE FLOWER 
 
 tendency is especially noticeable in the megasporophylls, which 
 very frequently form a compound megasporophyll (Fig. 265, 
 C, D), and in the same way the sepals and petals may appear as 
 a more or less tubular calyx and corolla (Fig. 266, C). The 
 crowding also led to the mass growth of the organs of adjacent 
 sets or whorls. This is often seen in the case of the petals and 
 microsporophylls, the latter organs appearing to arise from the 
 corolla (Fig. 266, D). 
 
 The receptacle plays an important role in all cases of mass 
 growth. As stated above, its apical portion ceases to elongate 
 at an early period while the basal part continues active, forming a 
 cup about the ovaries. The sepals, petals, and even the micro- 
 sporophylls are often so associated with this basal growth of 
 the receptacle that they arise en masse about the ovaries. This 
 growth may be so slight that a careful examination of a section 
 of a flower is required to detect it or a conspicuous cup-like struc- 
 ture may be formed, as in the rose (Fig. 266, A, B). This type 
 of flower is termed perigynous, meaning that the receptacle and 
 other organs form a more or less conspicuous cup about the 
 ovaries. In simpler types, as the spiral flowers, it can be seen 
 that the megasporophylls arise at the top of the receptacle and 
 that each of the other organs arises at a point just below the 
 next inner one. Such flowers are called for this reason hypo- 
 gynous, meaning below the ovaries. 
 
 Very frequently the basal growth of the receptacle also involves 
 the ovaries which becomes distinct from the mass growth at 
 various stages in their development. Consequently the other 
 organs of the flower appear to arise from the sides or from the 
 top of the ovaries; the flowers being partially or completely 
 epigynous, meaning that the organs of the flower are developed 
 upon the ovary (Fig. 267, B, D). Often in completely epigynous 
 flowers the receptacle elongates very slightly and the basal 
 portion grows up around the apex forming a cup-like cavity 
 which is roofed over by the megasporophylls. The sporangia 
 or ovules usually arise from the walls of the cavity thus formed 
 and not from the walls of the megasporophylls at the top of the 
 cavity (Fig. 267, D). The calyx, corolla and microsporophylls 
 
DEVELOPMENT OF PLANTS 
 
 387 
 
 arise from the top of the structure thus formed and these organs 
 may be developed separately or there may be varying degrees of 
 mass growth as noted in the perig^nous flower. 
 
 Another feature to be noted in connection with the evolution 
 of the flower is its symmetry. In the lower types, the organs 
 
 Fig. 267. Fig. 268. 
 
 Fig. 267. Adhesion due to basal growth of receptacle: A, flower of saxi- 
 frage. B, section of flower, showing megasporophyll partially inclosed by 
 receptacle. C, inflorescence of red currant. D, section of flower, showing 
 the receptacle forming a sporangial cavity that is covered at the top by the 
 megasporophylls. The other organs arise from the top of this structure,, 
 the microsporophylls adhering to the corolla. 
 
 Fig. 268. Irregular or zygomorphic flower of honeysuckle: ca, calyx; 
 c, corolla of five unequal cohering petals, s, stigma. 
 
 of a set are alike and radially arranged about the center of the 
 flower (Fig. 267, A). This is the regular or actinomorphic type 
 of flower, meaning radially symmetrical. In many of the orders 
 of angiosperms one or more members of a set are different from 
 the others, thus destroying the radial symmetry (Fig. 268). 
 This is the irregular or zygomorphic type of flower, meaning 
 yoke-form. Such flowers can be cut into two similar halves in 
 but one plane. 
 
 It must not be understood that evolution of the flower has 
 
388 GAMETOPHYTE OF ANGIOSPERMS 
 
 progressed steadily through the various changes outHned above 
 and that consequently in the following lessons we can begin with 
 the most primitive type and proceed by regular steps to the 
 highest forms. The various orders of angiosperms have doubt- 
 less been derived from several distinct stocks and they have 
 not only varied in different degrees but especially will it be noted 
 that some orders have a tendency to emphasize certain forms of 
 these variations, while in other alliances, the variations will 
 proceed along quite different lines. These various modifications 
 have been retained because they were of advantage to the plant 
 (p. 144). The cause of the variations is unknown. Similar 
 lines of variations also appear among the insects, and singularly 
 these modifications of the flower and insect have been mutually 
 beneficial the one to the other. In the simpler forms of flowers 
 the microspores are carried by the wind, as in the gymnosperms. 
 In the higher types odor and nectar glands appear and bright 
 colors w^hich serve to attract insects. This type of flower becomes 
 modified so that it is adapted to special types of insects — all 
 others being excluded. In this w^ay the microspores are carried 
 with greater certainty from one flower to another of the same 
 kind. 
 
 125. The Gametophyte of the Angiospermae. — The angio- 
 sperms have developed along quite distinct lines, but they show 
 such a remarkable uniformity in the development and character 
 of the gametophyte generation that this feature of their life 
 history may be considered at this point, as it applies to all forms. 
 The megasporangium, also called the ovule, originates in the 
 cavity of the ovary at various points know^n as the placenta (Fig. 
 269, A). The structure of the sporangium and the formation 
 of the megaspore is very similar to that of the gymnosperms. 
 More often two integuments are formed and the sporangium or 
 the stalk which supports it becomes curved so that it very fre- 
 quently is turned completely over (Fig. 269, B). The germina- 
 tion of the megaspore results in a female gametophyte which is 
 very much more reduced than in the case of the gymnosperms. 
 As the megaspore enlarges, disorganizing the cells of the sporan- 
 gium, also called the nucellus, its nucleus divides and the daugh- 
 
DEVELOPMENT OF PLANTS 
 
 389 
 
 ter nuclei move to each end of the spore (Fig. 270, A). Each 
 of these nuclei di\'ides twice, forming four nuclei at either end 
 of the spore (Fig. 270, B, C). This generally completes the 
 nuclear divisions in the gametophyte. Only a few cases are 
 
 Fig. 269. Development of the megasporangium and megaspore: A, sec- 
 tional view of the pepper-grass, Lepidium. This flower has two cohering 
 sporophylls, only a portion of the right-hand one being shown, s, stigma 
 with protruding cells to receive the microspores; mg, megasporangium con- 
 taining a single spore mother cell, inc. Two integuments, i, are growing up 
 about the sporangium. B, later stage of development, the megasporangium, 
 mg, becoming inverted and completely covered by the integument. The 
 mother cell of A has formed four daughter cells in series and not in tetrads 
 as in the Pteridophyta. The innermost cell of the series, ms, only matures 
 as a megaspore; m, micropyle;/, stalk or funiculus of sporangium. 
 
 known where a larger number of nuclei are formed — compare 
 with the gymnosperms. A nucleus from each of these groups, 
 called the polar nuclei, now approach each other and fuse, 
 forming a single large nucleus that is usually called the endo- 
 sperm nucleus. This rudimentary growth represents the female 
 gametophyte. The three outer or micropylar cells are not pro- 
 
390 
 
 GAMETOPHYTE OF ANGIOSPERMS 
 
 vided with walls and consist of a rather larger cell, the female 
 gamete, and two nourishing cells, the synergids or helpers (Fig. 
 271). The inner group or antipodal cells usually have walls and 
 they are either soon disorganized and absorbed by the enlarging 
 gametophyte or they may remain as permanent features of the 
 gametophyte for a long time and even increase greatly in number, 
 serving to nourish the gametophyte by absorbing food from the 
 sporangium. The endosperm nucleus plays a very important 
 role in the development of the sporophyte, for as soon as fertiliza- 
 
 FiG. 270, Germination of the megaspore: A, first division of the mega- 
 spore. B, second division of the nuclei. C, final division of the nuclei. 
 
 tion has been effected it forms by repeated division a mass of cells 
 that completely fills the entire space within the enlarging spore, 
 thus providing food for the nourishment of the sporophyte. At 
 first sight it would appear impossible to compare the various cells 
 of this peculiar gametophyte with the tissues of the female gam- 
 etophyte of the gymnosperms or ferns. It has been suggested 
 that the female gamete and the synergids are the remains of three 
 archegonia, only one of which is usually capable of being fertilized, 
 and that the antipodal cells are a remnant of the numerous vege- 
 tative cells of the gametophyte. The peculiar formation of the 
 endosperm nucleus through the fusion of the two polar cells is 
 looked upon as a nourishing device to give the endosperm nucleus 
 
DEVELOPMENT OF PLANTS 391 
 
 the power to grow and form a tissue that supplements the anti- 
 podal cells and so takes the place of the nourishing gametophyte 
 of the gymnosperms. According to this view, the endosperm is 
 a delayed prothallial growth which does not take place until 
 fertilization is effected. There is considerable evidence to justify 
 
 Fig. 271. Section of a megasporangium of lily, showing the mature female 
 gametophyte: 9, female gamete, below which are two synergids; />, the 
 two polar nuclei uniting to form the endosperm nucleus; a, antipodal cells; 
 mi, micropyle; i, integuments; /, funiculus in which a vascular bundle, i\ 
 has been formed to transport foods to the sporangium. 
 
 this conclusion. In the Ptcridophyta the archegonia are formed 
 at the close of the prothallial development. Among the species 
 of the Gymnospermae there are several examples indicating that 
 the archegonia are formed at earlier and earlier stages in the 
 development of the gametophyte (p. 367). Possibly we have in 
 the Angiospermae a final condition where the homologucs of the 
 
392 GAMETOPHYTE OF ANGIOSPERMS 
 
 archegonia are developed at the very start of the gametophytic 
 growth. The nicety of such a sequence of development is alto- 
 gether admirable. In the ferns, if fertilization is not effected, 
 the prothallial growth is wasted, but in the angiosperms there is 
 no prothallial growth without fertilization. 
 
 The male gametophyte presents several features suggestive of 
 the gymnosperms. The germination of the microspores usually 
 begins within the sporophylls, and by the time that they are shed 
 and carried to the stigma, their nuclei have already divided once 
 and each spore consists of a large tube cell and an antheridial cell 
 (Fig. 272, B). There is very rarely a trace of the vegetative 
 
 
 />1 C B ^^^ 
 
 Fig. 272. Germination of the microspore: A, mature microspore of lily, 
 By first stage of germination — t, tube cell; a, antheridial cell. C, final divi- 
 sion, in this case effected while in the microsporangium — t, tube cell; a, an- 
 theridial cell forming directly two male gametes. D, diagram showing the 
 formation of the tube which grows down the style and finally reaches the 
 female gametophyte. The two male gametes, g, are shown passing down 
 the tube; t, tube nucleus. All the figures in sectional view. 
 
 cells of the gametophyte, as in the cycads and pines, since the 
 necessity for these cells no longer exists. The stigma is generally 
 provided with minute outgrowths or papillae derived from the 
 epidermal cells which serve to hold the microspores (Fig. 269, s). 
 These cells of the stigma usually secrete a sugary solution which 
 nourishes the microspores and causes a continuation of their ger- 
 mination. It is noteworthy that these microspores may be made 
 to germinate by placing them in sugar solutions, but the approxi- 
 
DEVELOPMENT OF PLANTS 393 
 
 mate strength of the solution on the stigma must be determined 
 in order to prepare a solution suitable for their development. 
 The tube cell, stimulated by the secretion of the stigma, ruptures 
 the outer wall of the spore, which is provided with one or more 
 thin places to favor this growth, and protrudes as a delicate tube. 
 This tube, owing to the fact that it is repelled by the oxygen of 
 the air, grows down into the tissues of the style which are really 
 a continuation of the stigma. These tissues of the style are usu- 
 ally looser and provided with abundant foods which are deposited 
 in the cells just ahead of the elongating tube to nourish and direct 
 it in its growth. In this way the tube is directed down the style 
 to the cavity of the ovary where, owing to the attractive influence 
 of the organic substances in the sporangium, possibly in the syn- 
 ergids, it usually turns out into the cavity of the ovary, enters 
 the micropyle and works its way through the sporangium, and, 
 unlike the Pinales, enters the female gametophyte generally 
 alongside of one of the synergids (Fig. 273). The antheridial cell 
 usually divides, forming directly two motionless male cells during 
 the elongation of the tube, and in other cases these two cells are 
 already formed when the microspores are discharged from their 
 sporangia. In none of these cases is there any indication in the 
 division of the antheridial cell of the formation of a wall cell as in 
 the gymnosperms, so that the male gametophyte of the Spermato- 
 phyta presents a very regular series of reductions from the cycads 
 to the pines and thence to the angiosperms, where it consists of a 
 tubular growth containing three naked cells (Fig. 272, D). This 
 development of the male gametophyte requires from a day to 
 several months and is quite independent, apparently, of the dis- 
 tance that it has to traverse in reaching the female gametophyte. 
 The male cells are often somewhat elongated and even spirally 
 coiled and carried to the end of the tube, as in the Pinales. 
 
 126. Fertilization.— The end of the tube finally ruptures, owing 
 to the tension of the fluids that gradually accumulate in it, and 
 the male cells are forcibly expelled into the sac-like cavity 
 of the female gametophyte (Fig. 273). One of the male cells 
 passes over to and fuses with the female gamete, thus forming 
 the gametospore; and the other male cell unites with the polar 
 
394 
 
 SPOROPHYTE OF ANGIOSPERMS 
 
 nuclei, thus forming the endosperm nucleus through a triple 
 fusion. While this process is probably in the nature of a rein- 
 forcement, enabling the endosperm nucleus to perform its work, 
 it is noteworthy that the qualities of the male parent are trans- 
 
 FiG. 273, Section of the micropylar end of the megasporangium, show- 
 ing the process of fertilization. The tube, t, has passed through the micro- 
 pyle, entered the female gametophyte and ruptured, discharging the male 
 cells. One, cf, is shown fusing with the female gamete, 9, and the other 
 one, (^', is uniting with the two polar nuclei, thus making a triple fusion in 
 the formation of the endosperm nucleus; s, one of the synergids; i, integu- 
 ments. 
 
 mitted to the endosperm cell just as though this fusion were a 
 sexual process. 
 
 127. The Germination of the Gametospore. — After fertiliza- 
 tion the endosperm nucleus divides repeatedly, and usually the 
 resulting nuclei become arranged about the walls of the sac-like 
 gametophyte which may now be called the embryo sac (Fig. 
 274). Later the endosperm cells develop walls and by further 
 division completely fill the embryo sac with cells. This mass of 
 cells is called the endosperm and the method of its development 
 is exactly similar to that of the female gametophyte of the gym- 
 
DEVELOPMENT OF PLANTS 
 
 395 
 
 nosperm, and it serves the same purpose, namely, to nourish the 
 young sporophyte or embryo. Recall, however, that it originates 
 in a different manner and at a different time in the life cycle. 
 The germination of the gametospore and the formation of the 
 embryo vary so greatly that only a very general statement can 
 be made. Following fertilization, the gametospore becomes sur- 
 rounded by a cell wall and attached to the wall of the embryo 
 sac (Fig. 275, A). It now begins to elongate and its nucleus 
 divides several times, forming a row of cells (Fig. 275, A, B). 
 This is the proembryo, the terminal cell being known as the 
 embryo cell and the remaining cells as the suspensor. Less 
 
 Fig. 274. Sectional view of megasporophylls of Lepidium shortly after 
 fertilization (see Fig. 269); en, early development of endosperm cells about 
 wall of embryo sac; p, young sporophyte developing from gametospore; 
 mi, micropyle; s, stigma;/, funiculus. 
 
 commonly the proembryo is a mass of cells without any differ- 
 entiation. This appears as a radical departure from the gymno- 
 sperms where the formation of free nuclei characterized the 
 germination of the gametospore. But in passing from the cycads 
 to the pines there is a steady decline in the number of free cells 
 formed and indeed in one case in the latter group the proembryo 
 
396 
 
 SPOROPHYTE OF ANGIOSPERMS 
 
 consists of a row of cells. The synergids, which are partly con- 
 sumed during the entrance of the tube cells and the process of 
 fertilization, usually become entirely disorganized and absorbed 
 during these early stages of germination. The embryo is formed 
 by the repeated divisions of the embryo cell of the pro-embryo 
 
 B A 
 
 Fig. 275. Stages in the germination of the gametospore of Lepidtum, 
 sectional view: A, micropylar end of embryo sac, showing the enlarging 
 gametospore provided with cell wall and attached to wall of sac. B, later 
 growth — s, suspensor; e, embryo cell; en, endosperm cells. C, pro-embryo 
 after first division of embryo cell. D, further divisions of embryo cell, show- 
 ing formation of an epidermis and a central stem region. E, later growth, 
 two growing regions, the cotyledons, appearing on the sides of the stem. F, 
 microp^dar end of the embryo sac in which the embryo cell has formed a small 
 plant or embryo, consisting of two cotyledons, c; stem, st, which terminates 
 in the root, r; the endosperm cells, en, are being absorbed by the enlarging 
 embryo; s, suspensor. 
 
 assisted to a varying extent by one or more of the adjoining cells 
 of the suspensor (Fig. 275, C-F). The remaining cells of the 
 suspensor are ultimately disorganized or they may increase 
 greatly in size and themselves become the principal means of 
 
DEVELOPMENT OF PLANTS 397 
 
 absorbing food and transferring it to the embryo. The structure 
 of the mature embryo varies greatly. In some genera, as in the 
 orchids, Indian pipe, etc., it remains rudimentary, consisting of 
 only a few cells. Among the monocotyledons, the embryo cell 
 frequently produces the single cotyledon, while the next under- 
 lying cell of the suspensor forms the root and the laterally-placed 
 growing point of the stem (Fig. 279). The pro-embryo of dico- 
 tyledons is frequently a filament of cells of varying length and 
 the embryo cell, by a regular series of divisions, gives rise to the 
 stem, two laterally-placed cotyledons and all of the root, save the 
 tip, which is formed from the cells adjoining the embryo cell 
 (Fig. 276, A). 
 
 128. The Fruit and Seed. — Various changes occur in the 
 sporangium during the growth of the embryo. More frequently, 
 perhaps, the embryo sac enlarges, absorbing all the cells within 
 the integument, and it becomes filled with endosperm cells. In 
 a case like this, the embryo either remains small and embedded 
 in the endosperm (Fig. 279), or the embryo may entirely con- 
 sume the endosperm, the food in this case being stored in the 
 cotyledons (Fig. 276, A). Less commonly, the embryo sac ab- 
 sorbs only a portion of the sporangial tissue (often called the 
 perisperm) and consequently the embryo is associated with a 
 varying amount of endosperm, which in turn is surrounded with 
 sporangial cells or perisperm, as in the water lily (Fig. 276, B). 
 The integument usually undergoes pronounced changes during 
 this growth, becoming hard and tough to protect the parts within, 
 and often developing appendages of various kinds, as hairs and 
 wings, to promote the distribution of the seed (see gymnosperms, 
 page 374). 
 
 The stimulus of fertilization extends beyond the changes 
 wrought in the sporangium. This is particularly noticeable in 
 the megasporophyll and often in adjacent parts which keep pace 
 with the growth of the sporangium and often undergo remarkable 
 transformations. The result of this total growth is called the 
 fruit, while the term seed is restricted to the modified sporangium 
 with its integument and embryo. The megasporophyll may form 
 a firm coat that is closely attached to the seed, as in the corn and 
 
398 
 
 SEED AND FRUIT OF ANGIOSPERMS 
 
 other grasses. Such a fruit is called a grain. Again the tough 
 walls of the megasporophyll are free from the seed, as in the 
 buttercup, forming a fruit known as the akene. The sporophylls 
 may become papery or hard and split open to scatter the seed. 
 Where a single megasporoph^^U behaves in this way and opens by 
 two' valves, the fruit is called a pod or legume, example the bean, 
 
 f 
 
 Fig. 276. Seed structure: A, section of a nearly mature seed of Lepid- 
 ium. The stem of the embryo is differentiated below into a hypocotyl, hy^ 
 and above into an epicotyl, pi, commonly known in the seed as the plumule, 
 r, root with root cap; c, the two cotyledons, which are bent over, lying one 
 upon the other; v, vascular bundles extending through the stem into the 
 cotyledons, where they form a network of veins; en, remains of endosperm. 
 B, section of seed of water lily (after Conard) — e, embryo, surrounded by a 
 layer of endosperm cells; mg, cells of the megasporangium; i, integument. 
 
 or if by one valve the fruit is termed a follicle, example the 
 peony. Frequently wing-like processes develop from the mega- 
 sporophyll, as in the maple and ailanthus, which are of service in 
 distribution and in other ways, as in the manufacture of foods, 
 these organs often being green during the development of the 
 embryo. Such fruits are known as key fruits or samaras. In 
 other cases the sporophyll becomes fleshy, forming a berry as in 
 the currant or the inner layer forms a pit or stone, while the outer 
 
DEVELOPMENT OF PLANTS 399 
 
 layer forms the pulp and skin, as in the cherry and peach, fruits 
 known as drupes. In many fruits the receptacle becomes fleshy 
 and forms the major portion of the fruit, as in the fig, apple, 
 strawberry, etc., and in the pineapple, the axis of the inflores- 
 cence, as well as parts of the flower, become fleshy. These vari- 
 ous devices serve primarily to preserve the life of the embryo 
 and some of them also assist in distributing the seed. 
 
 When conditions are favorable for growth, which may not be 
 till several years after the seeds have been scattered, the embryo 
 renews its growth. The region below the cotyledons, the hypo- 
 cotyl, or base of the cotyledon itself, elongates and pushes out 
 the root which grows down into the soil, and by a later growth 
 the stem tip, with or without the cotyledons which may remain 
 In the seed, elongates and develops the characteristic stem, 
 branches, leaves and sporophylls of the parent plant. The struc- 
 ture and variations of the mature sporophyte have been presented 
 in the first part of the work (pages 7-145), and will be further 
 considered in the following studies. The terms sporangium, 
 microsporophyll and megasporophyll have been retained up to 
 this point in the work in order to keep clearly before you the 
 progress and relationship of the variations that have attended the 
 evolution of plant life. Now that we have arrived at the highest 
 and largest group of plants, where the sporophylls have become 
 highly modified and curiously associated in clusters called flowers, 
 more familiar names will be used in discussing them, i. e., ovule 
 for the megasporangium and its integument, stamen for micro- 
 sporophyll and pistil for megasporophyll. The terms "carpel" 
 and "pistil" are synonymous when these organs a:e distinct — • 
 i. e., free from one another; but when they cohere to form a 
 compound pistil the term "carpel" is employed to indicate the 
 number of pistils that are united, as in the willow the pistil is 
 composed of two carpels, in the lily of three carpels, etc. 
 
 While the sexual generation, mode of reproduction, and the 
 character of the tissues are very similar in all angiosperms, two 
 very distinct lines of variation or classes are to be noted: A, 
 the Monocotyledones, represented by the grasses, lilies and or- 
 chids; B, the Dicotyledones, represented by our common trees, 
 roses, mints and daisies. 
 
400 MONOCOTYLEDONES 
 
 Class A. Monocotyledones 
 
 129. General Characteristics. — This group of angiosperms con- 
 tains about 25,000 species that constitute a very natural alliance, 
 owing to the uniformity and simplicity of their structures. These 
 features may be due to the uniform conditions under which these 
 plants live. The majority of Monocotyledones are moisture- 
 loving plants and are therefore exposed to very constant condi- 
 tions, which would naturally result in less stimulation and conse- 
 quent variation than in the case of plants exposed to the varying 
 conditions of drier soils. Some, to be sure, have become adapted 
 to dry and even arid regions, owing to peculiar modifications of 
 their leaves or stems, as in the grasses, certain bulbous plants, etc. 
 
 The leaves are smooth and simple, lance-shaped or linear in 
 outline, sessile and often attached to the stem by sheathing bases. 
 The veins do not end in free branches on the margins of the 
 leaves, and as a result of this closed venation the leaves are usu- 
 ally entire and destitute of teeth or lobed margins. In many 
 instances the principal veins are quite parallel, but whatever the 
 arrangement, the prominent veins are connected by very minute 
 veinlets that form an inconspicuous network or reticulation 
 throughout all parts of the leaf (Fig. 277). The stems are com- 
 posed largely of parenchyma, through which are scattered numer- 
 ous vascular bundles as in some ferns (Fig. 278) — see page 324. 
 These bundles rarely develop a cambium (Fig. 60) and conse- 
 quently the stem does not increase materially in diameter, and 
 usually it is columnar in appearance. In many instances the 
 stems are reduced in size and are subterranean, the bulb and 
 rhizome being common forms of stems which send up annually 
 short-lived aerial branches. As a rule the stems do not branch, 
 owing to the failure of the buds in the axils of the leaves to 
 develop. 
 
 The flowers are also of rather simple and uniform structure, 
 in the simplest cases being imperfect and consisting of either 
 spirally arranged stamens or pistils without perianth. In the 
 higher types spiral flowers with perianth and both kinds of sporo- 
 phylls appear, and these give place to forms in which the organs 
 are arranged in successive whorls of three parts each, the latter 
 
DEVELOPMENT OF PLANTS 
 
 401 
 
 type of flower being especially common. Perigynous, epig>^nous 
 and irregular flowers are of less common occurrence. The most 
 conspicuous feature of the monocotyledons is the embryo, which 
 has a single terminal cotyledon and a laterally developed stem tip 
 
 I 
 
 Fig. 
 
 Fig. 278. 
 
 Fig. 277. Leaf of Solomon's seal with closed venation, entire margins, 
 and sessile upon the stem, i. e., without petiole. 
 
 Fig. 278. Cross-section of stem of rush: v, vascular bundles; st, stereome. 
 
 (Fig. 279). Generally the root and stem tip are set free from 
 the seed by the elongation of the basal portion of the cotyledon, 
 while the cotyledon remains in contact with the endosperm as a 
 food-absorbing organ, being often highly modified to perform 
 this work. An examination of a few of the more important 
 orders will give a better idea of the characteristics and \'ariations 
 of the monocotyledons. 
 
 130. Pandanales, the Cat-tail Order. — The cat-tail, TypJia, 
 may be taken as an illustration of this order (Fig. 280). These 
 plants live in wet and marshy places and present several varia- 
 tions that adapt them to such conditions. The main stem is a 
 rhizome that branches through the mud and sends up each spring 
 leafy stems. The rhizome grows on from \ear to year, and 
 owing to the decay of the older portions, the branches become 
 independent plants. In this way a single plant may spread and 
 26 
 
402 
 
 THE PANDALES 
 
 thickly populate an extensive marsh. The aerial stems live but 
 for a year and are rather weak, but owing to the sheathing bases 
 of the leaves they become sufficiently rigid to support a heavy 
 foliage and withstand the winds. Notice also the extreme light- 
 ness of these organs. This is due to the larger air spaces which 
 also permit a ready interchange of gases from the leaves to all 
 parts of the plant, even throughout the submerged rhizomes. 
 You would naturally expect to find this structure in all aquatics 
 since the roots at least are submerged and all living cells require 
 an interchange of gases (see page 54). The leaves are long 
 
 mi 
 
 Fig. 279. Sectional view of seed of Veltheimia. The embryo consisting 
 of a large cotyledon, c, and laterally placed stem, s, below which is the root, 
 e, endosperm; mi, micropyle;/, funiculus. 
 
 and narrow and point nearly straight up in the air. This pre- 
 vents shading and permits the association of the plants in dense 
 colonies. The leaves are covered with a waxy coating or bloom 
 to prevent the adhesion of water and the plugging of the stomata. 
 This device is often to be seen on the leaves of plants that are 
 subject to heavy dews or rains. Moisture is frequently not 
 evaporated from the leaves until near midday, and if the stomata 
 become filled with water, there can be no interchange of gases 
 for photosynthesis during this time. 
 
 The flowers are of a very primitive t^-pe, consisting of naked 
 
DEVELOPMENT OF PLANTS 
 
 403 
 
 sporophylls arranged in a compact inflorescence that assumes a 
 spike-like structure at the tip of the stem. The sporophylls are 
 protected until mature by modified sheathing leaves (Fig. 280, A), 
 Each flower in the upper portion of the spike contains only two 
 or three stamens supported upon a short stalk, which is asso- 
 ciated with hair-like outgrowths (Fig. 280, B), while the lower 
 flowers of the spike bear a single pistil each (Fig. 280, C), which 
 
 Fig. 280. 
 
 Fig. 281. 
 
 Fig. 280. Inflorescence and fruit of Typha: A, inflorescence — h, pro- 
 tecting bracts curving away from the sporophylls; mi, region of staminate 
 flowers; mg, region of pistillate flowers. B, staminate flower consisting of 
 two stamens sessile upon a short stem. C, pistillate flower of one carpel — 
 5, stigma; 0, ovary containing a single ovule. D, appearance of A in the 
 fall — mi, region occupied by staminate flowers; mg, the pistils have increased 
 greatly in size during the ripening of the seed. 
 
 Fig. 281. The mature fruit of Typha: 5, remains of the stigma; 0, ovary; 
 P, elongated pedicel bearing numerous hairs. Compare Fig. 280, C. 
 
 consists of a large flat stigma, style and ovary containing a single 
 ovule and supported upon a hairy stalk or pedicel. Note the 
 
404 THE PANDANALES 
 
 association of numerous hairs with both the stamens and pistils. 
 These organs are supposed to be sterile sporophylls so that we 
 have here a very good illustration of a primitive type of flower. 
 The microspores are carried to the stigmas of the pistils by the 
 wind. 
 
 Such wind pollinated flowers are characterized by several fea- 
 tures well illustrated in the cat-tail. The microspores must be 
 produced in large numbers since the chance of one reaching the 
 stigma of another plant rapidly decreases as the distance tra- 
 versed by the spores increases. This probably accounts for the 
 growth of such plants in dense colonies since the close proximity 
 of the plants greatly increases the chance of the microspores 
 reaching the stigmas. The Finales, grasses, willows, oaks, etc., 
 are other illustrations of a very large series of plants, some 
 10,000 in number, that have a similar habit. Wind pollinated 
 flowers are inconspicuous and simple in structure. You will 
 notice that showy perianths, nectar and perfume glands are 
 developed only in such flowers as utilize insects for the distri- 
 bution of the microspores. The stigmas of these flowers are 
 usually large and hairy or brush-like and conspicuously exposed 
 so as to increase the chances of catching the microspores, thus 
 reducing the dangers of this rather risky method of crossing. 
 You will also observe that wind pollinated flowers are usually 
 characterized by having imperfect flowers, the stamens and pistils 
 being developed on different parts of the plant, or on different 
 plants. By this arrangement, the advantages of crossing are 
 secured and it also happens that the microspores are either 
 scattered before the stigmas of neighboring flowers are mature, 
 or, more frequently after they have withered and are therefore 
 no longer capable of catching and nourishing the spores. In 
 this way it comes about that the microspores, even when close 
 to the pistillate flowers as in Typha, are often only of service 
 when carried to some earlier flow^ering plant whose stigmas are 
 mature. In the cat-tail, the microspores are shed a day or so 
 before the stigmas on the same spike are mature and so there 
 must result the benefit that comes from crossing two more or 
 less widely separated plants (page 144). The stamens soon 
 
DEVELOPMENT OF PLANTS 405 
 
 perish after the discharge of their spores, but the pistils Increase 
 greatly In size during the ripening of the seed and form con- 
 spicuous brown cylinders (Fig. 280, D). As winter approaches, 
 the pedicel below the ovary and the hairs attached to It become 
 greatly elongated and the sporophyll is thus transformed Into 
 a very stable parachute that is easily detached from the spike 
 and capable of floating the seed in even the lightest winds (Fig. 
 281). If this fruit chances to fall In a marsh, the torpedo-like 
 seed after a time falls from the ruptured ovary and sinks in the 
 water. When the growth of the seed Is renewed, the base of 
 the cotyledon elongates, pushes off the lid at the end of the seed 
 and curves down so as to bring the root of the embryo in con- 
 tact with the mud. Hairs now develop from the lower part of 
 the cotyledon, anchoring the young plant to the ground while 
 the tip of the cotyledon remains in the seed until the food is 
 absorbed, when it is withdrawn. In the meantime, roots have 
 developed and penetrated the soil and the stem tip begins to 
 elongate, lifting up the cotyledon and making possible the for- 
 mation of other leaves. The curious screw-pines, Pandanits, a 
 large group from the oriental tropics and frequently seen In con- 
 servatories, belong to this order, also the bur reed {Sparganiiim), 
 common about marshes and water ways, which Is the highest 
 member of the order. 
 
 131. Graminales, the Grass and Sedge Order. — This Is one of 
 the largest groups of the Spermatophyta, some 7,000 species 
 being known, and in number of Individuals it exceeds all others 
 (Fig. 282). These plants are almost universally distributed 
 over the earth and have become adapted to almost every condi- 
 tion of climate and soil. The ability of these plants, which are of 
 aquatic origin, to establish themselves upon the drier and more 
 diversified land surface doubtless promoted variations and ac- 
 counts in part for the enormous display of forms. The Grami- 
 nales and the palms (Princlpales) which belong to an order 
 related to Typha are about the only groups of monocotyledons 
 that are associated In sufficient numbers to form striking features 
 of the vegetation of a country. This is particularly true of the 
 Graminales in all lands, where they constitute the principal vege- 
 
4o6 
 
 THE GRAM IN ALES 
 
 tation of hills, plains, meadows and marshes. These plants are 
 also of the greatest economic importance, furnishing a variety of 
 foods, as wheat, r^^^e, barley, oats, rice, corn, hay, etc. The 
 bamboo also belongs to the order. The stem in the majority 
 of forms is a rhizome (as in Typha), which branches extensively 
 through the soil and sends up numerous aerial branches (Fig. 282, 
 B). These interwoven rhizomes, with their numerous roots, 
 form the firm swards of meadows and prairies. The aerial stems 
 are models of mechanical construction. The ability of the long 
 
 Fig. 282. Habit of growth of one of the grasses: A, aerial stem termi- 
 nating in a branched inflorescence. B, underground stem or rhizome send- 
 ing up a new shoot from one of the nodes. C, section through the node of 
 a stem that has been placed horizontal, showing the sheathing leaf base, /, 
 and the beginning of the upward curvature of the stem. 
 
 hollow stems of the grasses to support the heavy head of grain 
 must appeal to every one. An examination of Fig. 282 will 
 show you how the stem is reinforced by the long sheathing base 
 of the leaves while tough, elastic strands of stereome fibers are 
 so distributed as to give the best mechanical support. An un- 
 
DEVELOPMENT OF PLANTS 
 
 407 
 
 usual localization of the growing regions also characterizes the 
 grasses which enables them to lift up again^their stems if they 
 by any means become prostrate. In case the stem becomes pros- 
 trate, the stimulus of gravity causes a renewaPof growth in the 
 cells at the base of the node on the side of thc^stem next to the 
 ground, thus causing the stem to curve up (Fig. 282, C). You 
 can easily demonstrate this peculiar localization of growth by 
 cutting off a growing stem of grass and placing^it horizontally 
 with one end embedded in moist sand and noting the curvature 
 
 Fig. 283. Inflorescence of a grass: i, tip of stem with (lowers arranged 
 in spike-like inflorescences. 2, a single spike enlarged at time of flowering. 
 4, diagram showing structure of spike. At base two sterile bracts, above 
 three flowers, each enclosed by an outer firm bract and an inner more delicate 
 bract. /, lodicules. At apex of spike a sterile flower. 5, another species 
 of grass, showing the scattering of the microspores. 
 
 of the node after a day. The leaves show wide variation in 
 structure and are doubtless one of the factors that have enabled 
 these plants to live under a variety of conditions. For example, 
 where the plants are exposed to drought or drying winds the 
 
4o8 THE GRAMINALES 
 
 epidermal cells are very much thickened, stomata often sunken 
 in narrow furrows and practically all species have the power of 
 rolling up the leaves in dry seasons and thus reducing the leaf 
 surface and lessening transpiration. 
 
 The flowers are exceptionally alike in structure and show about 
 the same state of floral development. While resembling those 
 of Typlia in some particulars, they present several features 
 that indicate a decided advance over the previous group. Fig. 
 283 illustrates the common types of inflorescence found among 
 the grasses. The flowers are arranged on elongated branches, 
 but instead of a large bract ensheathing the inflorescence (see 
 Typlia), each flower is inclosed by one or more small bracts 
 so arranged as to form a spike-like structure of overlapping 
 bracts (Fig. 283, 2). The two kinds of sporophylls may be 
 arranged in separate spikes or on different parts of the same 
 spike, or, as in many grasses, the stamens and pistils are developed 
 in the same flower. One or more of the lower bracts of a spike 
 are usually without flowers and above them are one to several 
 flowers enclosed by secondary bracts (Fig. 283, 4). Removing 
 the secondary bracts, the flower proper is seen (Fig. 284, 3^, 
 3jB). This consists in many of the grasses of three stamens 
 and one pistil and usually two small scale-like organs, the lodi- 
 cules, which assist by their expansion in forcing open the pro- 
 tecting bracts at the time of flowering. The stamen consists 
 of a long filament attached to the middle of the anther so that 
 the latter organ can swing at the end of the filament, an arrange- 
 ment known as the versatile anther. The stigma is often 
 brightly colored and of a delicate feathery character, indicating 
 that the plants are wind pollinated. The ovary contains a 
 single ovule. The manner of flowering of the numerous genera 
 of this order varies. In many cases the stigmas are first ex- 
 truded from the bracts and can therefore be crossed only with 
 the microspores from some earlier flowering plant. In other 
 instances, stigmas and anthers are extended together and in 
 some genera no injury results from the transfer of the micro- 
 spores to the stigmas of the same flower. It is worth any one's 
 time to note the time of flowering, the region on the spike where 
 
DEVELOPMENT OF PLANTS 
 
 409 
 
 it begins and the manner of opening of the bracts and extension 
 of anther and stigma. Many open between 6 and 7 A. M. on 
 pleasant days. Owing to the swelling of tlie huHcules, or of 
 the base of the bracts, these latter organs are forced apart, per- 
 
 i-f 
 
 Fig. 284. Fig. 285. 
 
 Fig. 284. Flower and fruit of grass: 2,A, a single flower with the two 
 enveloping bracts opened, exposing the stamens and pistil with feathery 
 stigmas. 3jB, flower with outer firm bract removed — /, lodiculcs; st, stigma. 
 6, mature fruit or grain — e, region of embryo. 7, section through base of 
 grain, showing the root, stem leaves, and scutellum, sc, or absorbing organ 
 of the embryo; en, endosperm. yA, diagram of a few of the outer cells of 
 the scutellum, sc. 
 
 Fig. 285. Inflorescence of one of the sedges, Carex: p, spike of pistillate 
 flowers, each pistil is surrounded by a papery sac, through which the style 
 and stigma protrude; s, spike bearing staminate flowers. Note the triangu- 
 lar stem, a characteristic of this large genus of over 1,000 species. 
 
 mitting the extension of the stigma and anthers. The lilamcnt 
 quickly elongates and soon curves so as to allow the anther to 
 swing back and forth in the wind. The two lobes of the anther 
 now curve apart and open at their ends by a narrow slit, forming 
 
410 THE ARALES 
 
 a cup in which the microspores collect (Fig. 283, 5). As the 
 rocking of the anther in the wind sifts out the spores, more 
 rattle down into the cup and so a very gradual scattering of the 
 spores is effected. This entire process may be effected in a few 
 minutes, and in the case of the wheat each flower is said to lasc 
 for onl}^ 15 minutes. The Graminales includes two families: 
 The Graminaceae, or grass family, and the Cyperaceae or sedge 
 family. The grass family is by far the more important and is 
 distinguished by the usually hollow aerial stems, two rows of 
 leaves with sheathing bases split and the fruit is generally a 
 grain (Fig. 284, 6). The sedges are largely paludose and of 
 little value, the stems and leaves being often silicified. They 
 usually have solid stems, three rows of leaves with sheathing 
 bases entire, more simple flowers and the fruit is usually an 
 akene (Fig. 285). 
 
 132. Arales, the Aroid Order. — These plants are largely tropi- 
 cal and are characterized by large and generally net-veined leaves 
 and showy inflorescences and fruit (Fig. 286). They present 
 an odd series of striking forms quite different from other mono- 
 cotyledons. Many of them are familiar plants owing to their 
 extensive cultivation in green-houses, as the calla lily (Richardia), 
 Caladium, often w^th variegated leaves, and the gigantic leaves of 
 Dracontium and Colocasia, known as the elephant ear, and the 
 curiously perforated leaves of the Mo?istera. Many species of 
 Anthurium are climbers and reach to the top of the highest 
 trees, sending out with great regularity from the successive nodes, 
 naked branches that may reach the ground and form roots. 
 The order is represented in temperate regions by a few very 
 familiar genera as jack-in-the-pulpit (Arisaema), skunk cabbage 
 (Spathyema) , golden club {Orontium), sweet flag (Acorns), water 
 arum (Calla). The duckweeds that float upon nearly every 
 pond are extremely reduced allies of the order, representing the 
 smallest seed plants known, Wolffia being an oval, rootless plant 
 scarcely one millimeter in diameter. 
 
 The inflorescence is usually covered, as in Typha, with a bract 
 known as the spathe. This organ is variously colored and often 
 the most attractive feature of the plant. The coloration and the 
 
DEVELOPMENT OF PLANTS 
 
 411 
 
 odors and glands that are developed in some of the forms indicate 
 that these plants are pollinated by insects. The flowers are 
 arranged upon a fleshy axis, the spadix, and in the simplest forms 
 are quite comparable to those of Typha, consisting of one or 
 a few stamens or a single pistil (Fig. 286, B-D). These im- 
 
 FiG. 286. Inflorescence of the Arales: A, habit of the jack-in-the-pulpit. 
 B, diagram of the inflorescence with spathe, s, opened on one side to show 
 the spadix, sp, bearing staminate flowers at the base. C, a pistillate flower 
 consisting of a single naked carpel. D, a staminate flower of four two-lobed 
 anthers. £, section of the inflorescence of one of the arums — p, compart- 
 ment containing pistillate flowers; s, staminate flowers. 
 
 perfect flowers ma^' be developed on dilYerent parts of the same 
 spadix or on difYerent spadices. In more advanced types, the 
 flowers are perfect, containing one or several pistils that may be 
 united and stamens which are surrounded by a very rudimentary 
 
412 THE ARALES 
 
 perianth. The relation of these plants to insects is a very inter- 
 esting one. The brightly colored spathe serves as an allurement 
 and it also protects the microspores against wetting. The spores 
 of comparatively few plants can endure w^ater and it will be 
 interesting to observe in the following lessons the devices that 
 appear to guard against this danger. The spathe also affords 
 shelter to the insect and the temperature is higher within the 
 spathe since the food is being rapidly oxidized while the spores 
 are forming. The temperature of the larger spadices may ap- 
 proach blood heat, being lo to 25° C. higher than the outside 
 air. Insects are not slow to avail themselves of these advantages 
 and they also find an abundance of food in the microspores and 
 glands that are often developed upon the spathe. Many aroids 
 are characterized by the foul odors of decaying flesh and it is 
 noteworthy that the coloration of the spathe is in singular har^ 
 mony with these odors, resembling the hues of putrid meat. As 
 a result of these variations, the lower orders of insects visit these 
 flowers in great numbers and unwittingly serve in the transfer 
 of the microspores; 250 carrion beetles and over 1,000 midges 
 have been reported as taken from a single spathe. You can 
 readily demonstrate that small insects are attracted to the spathe 
 by examining the inflorescence of the skunk cabbage and jack- 
 in-the-pulpit. 
 
 Common Examples of the Arales. — In the skunk cabbage 
 {Spathyema foetida), which is of common occurrence along 
 muddy streams, the inflorescence may appear as early as Febru- 
 ary, while the leaves are still rolled up into a compact bodkin to 
 enable them to penetrate the soil. Later the leaves unroll and 
 become very large, being good types of aroid leaves with broad 
 blades, strongly net-veined and evidently adapted to abundant 
 moisture and rapid transpiration. The spathe is shell-like, mot- 
 tled with purple, green and yellow and encloses a fleshy oblong 
 spadix that is entirely covered with flowers. You will find that 
 small insects are attracted to these spathes in great numbers and 
 spiders often take advantage of this fact and spin their webs 
 out of sight in the darker recesses of the spathe. In this plant, 
 the flowers are decidedly in advance of those noted in Typha, 
 
DEVELOPMENT OF PLANTS 413 
 
 being perfect, and in addition containing a perianth of four 
 sepals which arch over the sporophylls. The crossing of the 
 flowers is effected in a variety of ways. In some cases the in- 
 conspicuous stigmas first push out from between the sepals so 
 that microspores must be brought from another earlier flowering 
 plant. In other cases, the stamens at the top of the spadix are 
 first extruded and shed their spores; while at the bottom of the 
 spadix the reverse condition obtains, the stigmas being in a re- 
 ceptive condition. This might result in a crossing of the upper 
 flowers with the lower, and later, when the lower flowers put 
 out their stamens and the upper their stigmas, insects could effect 
 a reverse crossing. As the seeds mature the spadix and sepals 
 become large and spongy, enclosing the ovaries which are finally 
 set free by the decay of the spadix, as fleshy berries. This fruit 
 is admirably adapted to the aquatic habit of these plants and 
 readily floats in the water, owing to the spongy outer tissues, and 
 so bring about the distribution of the seeds. The jack-in-the- 
 pulpit has a surer method for effecting a crossing, since the 
 sporophylls are usually borne on separate spadices. You will 
 often find, however, spadices with a few stamens situated just 
 above the pistillate flowers. The flowers are very simple, con- 
 sisting of a single naked pistil or of four nearly sessile stamens 
 (Fig. 286, C-D). The fruit is a shining bright red berry. 
 
 In some of the tropical genera the insects are held in the 
 spathe by hairs that extend obliquely downwards, thus permit- 
 ting the entrance but blocking the exit of the insects until cross- 
 ing of the flowers has been effected. In one of the species of 
 Arum there are two sets of hairs that divide the spathe into two 
 compartments containing respectively staminate and pistillate 
 flowers (Fig. 286, E). As soon as the spathe opens the insect 
 ladened with microspores from another flower makes his way to 
 the lower chamber, where the stigmas arc in a receptive condi- 
 tion. Here he is held a prisoner and rubs the microspores upon 
 the stigmas as he wanders about in the compartment. When the 
 microspores in the upper chamber begin to discharge, the lower 
 set of hairs wither, giving him entrance to this chamber, where 
 he feeds upon the microspores and becomes covered with them. 
 
414 
 
 THE LILIALES 
 
 N 
 
 Finally the upper set of hairs wither and he is free to leave the 
 spathe and repeat his work in another inflorescence. 
 
 133. Liliales, the Lily Order. — The members of this order 
 comprise nearly 5,000 species that are widely distributed and 
 extensively cultivated for their show^y flowers. We have now 
 reached a point in the evolution of the flower where it has become 
 perfect and the protective spathe of preceding orders is replaced 
 by a well-developed perianth. The floral axis also becomes short- 
 
 FiG. 287. The fawn lily, Erythronium americaniim: A, habit of the plant. 
 5, the bulb, showing the origin of the stem and leaves shown in A; r, run- 
 ners that penetrate the soil forming new bulbs at their tips. C, pistil of 
 three carpels, at the right the fruit, a capsule, opening to scatter the seeds. 
 
 ened and we pass from the spiral series of flowers with a vari- 
 able number of organs to the cyclic flowers with a definite 
 number of organs arranged in whorls. In the lily order there 
 are five whorls of organs of three members each, the stamens 
 being arranged in two whorls. Note also that this crowding 
 usually results in a compound pistil of three carpels (Fig. 287, 
 C). These plants are largely perennia l, with underground 
 stems in the form of bulbs or rhizomes. This feature adapts 
 
DEVELOPMENT OF PLANTS 415 
 
 many of them to dry and steppe regions, from which source many 
 of our cultivated hHes have been obtained. It will be seen that 
 this habit of storing food in underground stems enables these 
 plants to develop their leaves, flowers and fruit during the short 
 rainy season, after which the entire aerial portion withers away 
 and their life lies dormant in the buried stems. This haljit is 
 equally serviceable if these plants come into competition with 
 larger forms, as in forests where phmts with bulbs and rhizomes 
 may complete their annual growth before the grosser vegetation 
 that would crowd them out is fail ly started (see page 44) . 
 
 (a) The Fawn Lily, Erythronium americamim. — This species 
 may be examined as typical of the order (Fig. 287). This plant 
 has received the atrocious name of adder's tongue, which is 
 offensive and far-fetched, and also of dog-tooth violet, although 
 it is not a violet at all. Burroughs has suggested the very appro- 
 priate name of trout lily, since the mottled leaves often form 
 conspicuous beds on shady banks of streams; but to those who 
 have experienced the spring time in the north country, the term 
 fawm lily seems singularly appropriate. The leaves of the fawn 
 lily spring from deep-seated bulbs that are formed in a peculiar 
 way. The seed germinates on the surface of the soil and forms 
 a very small bulb and a single grass-like leaf. During each suc- 
 ceeding season a larger leaf and bulb are formed, and w^hen of 
 sufficient size, the bulb sends out one or more runners that pene- 
 trate the soil and develop new bulbs at their tips (Fig. 287, B). 
 In this way the bulbs become deep-seated and rapidly increase 
 in numbers, and after several years they attain sufficient size to 
 develop two leaves and a flower. The mottled leaves have the 
 same habit of rolling up in emerging from the ground, as noted 
 in the skunk cabbage. It is to be observed that the position 
 assumed by the mature leaf of many plants is often strikingly 
 correlated with the extent of the root system. In the Liliales 
 generally, which do not have extensive lateral roots, the hang of 
 the leaves is such as to direct the water that falls upon them 
 towards the center of the plant, a feature doubtless of consider- 
 able advantage to plants living in semi-arid regions. The flower 
 is of a decidedly higher type than any previously studied and it 
 
4i6 
 
 THE LILIALES 
 
 presents several features of special interest The fl • , 
 
 ally placed orslightly pendulous. TheSan^ s " '*'''■ 
 
 developed, consisting of two whorls of th u '^°"^P"^"°"sly 
 
 these organs are not as rfJlt dff ''"""'''" ""^"^ *''°"^h 
 corolla. The stamens n re ,1 ' ^■'^^'•^"t'^ted into calyx and 
 
 n.e™wseach:rt;:\ e :;r^^^^^ '-. r t-'- °^ ^^- 
 
 ^^ a compound pistil. The :;^LtTtrstif .^^l;; 
 
 Fig. 288. 
 
 Lower forms of the Liiialpc- /t . 
 .ng grass-like appearance of the stem and i„1' '°'""'°" '"'^- ^'"'"''' ^ho"- 
 showing a ,i„ type. C. whL'rnrbA ""^^r ^-/°— ;-^^''' 
 show.ng the partial coherence of the carpus Tr-prf.itt^T.-pTK 
 
DEVELOPMENT OF PLANTS 417 
 
 with each other, so that the flower lias three planes of symmetry. "^ 
 
 At maturity the walls of the ovary become i)aper>' and split Tx 
 down the side, thus freeing the seeds. This form of fruit is 
 known as the capsule, and is of very common occurrence in the 
 order (Fig. 287, C). The development of a conspicuous perianth 
 is a noteworthy departure. This structure protects the micro- 
 spores and replaces the bracts and spathe of preceding orders, 
 but owing to its peculiar coloration and form, it also serves as 
 an attraction to special kinds of insects. In the preceding,^^ ^ 
 forms that attracted insects, the inducements were usually ^"^ yt, 
 in the nature of shelter and foods, which were largely the micro- 
 spores that were offered freely to all. Such flowers are princi- 
 pally visited by a low order of stupid flies and beetles that are 
 rather promiscuous feeders and are quite as likely to go from 
 a flower of one species to that of a difl'erent kind, or to some 
 other object and so defeat the principal object of the flower. 
 With the development of the perianth, however, the flower is 
 equipped with a device that primarily serves to exclude these less 
 desirable visitors and to attract the more intelligent ones. In 
 Erythronium the perianth is in a horizontal position, so that it 
 offers a natural landing place for the insect, and the inducement 
 is a sugary solution secreted by nectar glands concealed at the 
 base of the perianth. It is an easy task for the more intelligent 
 long-tongued insects, like the bees, wasps and butterflies, to reach 
 the nectar in this type of flower. Such insects come to know by 
 experience how to gather the food from a particular form of 
 flower and consequently they will often confine their attention to 
 a single species during their entire flight; consequently the flower 
 by excluding the less intelligent and slothful insects is more cer- 
 tain of being properly crossed. The development of nectar and 
 also of odor glands is among the important variations that 
 appear in the evolution of the flower. It is chiefly by means of 
 the perfumes derived from these organs that the insect is directed 
 to the flowers. The coloration is also of service, when the insect 
 is near to the flower, thus supplementing the perfume glands by 
 directing him to the proper entrance. Some of the larger and 
 more brightly colored members of this and other orders appa- 
 
4i8 THE LILIALES 
 
 rently depend entirely upon the attraction of their colors, while 
 other forms rely upon perfumes, the flowers being small and in- 
 conspicuous and often hidden. So we have reached a point in 
 the evolution of the flower where it presents a number of varia- 
 tions that are adapted and of benefit to special kinds of insects. 
 Insects have likewise varied and some have become of special use 
 to flowers owing to their peculiar form. In this way certain in- 
 sects and flowers have become dependent upon each other, and 
 as a result of their mutually beneficial variations, the flowering 
 plants and insects have greatly increased in numbers and exceeded 
 all other groups in the plant and animal kingdoms (p. 388). 
 
 (b) Other Forms of the Lily Order. — This order comprises 
 an extensive series of plants that present nearly all the variations 
 that occur in the flower. At the bottom of the series is the Rush 
 family with grass-like stems and leaves (Fig. 288, A, B). The 
 flower has the same structure as noted in Erythronium, but the 
 perianth is composed of small scale-like leaves destitute of nectar 
 glands, and as you would expect, is wind pollinated. The Bunch 
 flower family is a somewhat higher type. The perianth is some- 
 what larger and more delicate then in the rushes and the carpels 
 are often but partially fused (Fig. 288, C, D). The flowers gain 
 in conspicuousness by being associated in compact inflores- 
 cences, but do not develop as a rule the bright colors characteris- 
 tic of large flowers. The family is represented by Tofieldia, 
 swamp pink (Helonias), blazing star {Chamaelirium) , Zygade- 
 nus, hellebore (Veratrum), bellwort (Uvularia), autumn crocus, 
 etc. The Lily of the Valley family is characterized by rather 
 larger flowers than those of the previous family, and they are less 
 clustered and often provided with tubular perianths owning to 
 the mass growth of the perianth segments. The fruit is a berry, 
 unlike the majority of the families of this order. Here belong 
 the asparagus, the false and true Solomon's seal, Indian cucum- 
 ber, Trillium, and the closely related Smilax, etc. The Lily 
 family includes nearly one half of the members of the order and 
 occupies a medium position in the evolution of the group. The 
 flowers are large and of the Erythronium type. Here belong 
 some of the most showy of our native and cultivated plants; the 
 
DEVELOPMENT OF PLANTS 
 
 419 
 
 day lily, Erythronium, tulips, hyacinths, lilies, Fritillaria, onion, 
 aloes and Spanish bayonets ( Yucca) of arid regions. Among 
 the higher genera of the lily family the organs of the perianth 
 often form a tubular structure. In the Amaryllis family we 
 find the same type of flower and fruit as in the lily family, but 
 the basal growth of the receptacle has enveloped the ovary so 
 that* the flower has become epigynous (Fig. 289, A). This 
 
 Fig. 289. Advanced forms of the Liliales: A, Narcissus with inferior 
 ovary, 0. The si v sepals cohere at their base, forming a tube and they also 
 develop an outgrowth at the mouth of this tube, which surrounds the anthers 
 and stigma like a cup. B, flower of Iris. C, section of same, showing in- 
 ferior ovary, 0; stigma, g; and anther, a. 
 
 large family furnishes a number of showy flowers, as the jonquil 
 and daffodil (Narcissus), amaryllis, snowflake, Criniim, star 
 grass, century pant, etc. The flowers of some genera become 
 irregular through the unequal development of certain leaves of 
 the perianth. The Iris family marks the culmination of the 
 variations noted in the order. The types of flower and fruit are 
 like those of the previous family, but the crowding on the recep- 
 tacle has resulted in the obliteration of one whorl of stamens. 
 The structure of the flowers often shows a series of variations that 
 adapt them to insect visitors and crossing, as is well illustrated 
 
420 THE SCITAMINALES 
 
 in the iris (Fig. 289, B, C). The styles have a very unusual 
 form, resembHng a leafy organ with the stigma on the upper side 
 of a small projecting shelf. Beneath each of these curving styles 
 is a stamen. The insect naturally alights upon the broad leaf 
 of the outer whorl of the perianth and the peculiar coloration of 
 this leaf probably directs him to the nectar secreted at the base 
 of the perianth. In reaching this food he crowds down into the 
 tube and rubs off the microspores upon his back. In leaving the 
 flower he cannot hit the stigma, but in visiting another flower it 
 can readily be seen that he will deposit some of the spores upon 
 the stigma situated upon the top of the shelf, thus effecting a 
 crossing. This order includes a large number of showy culti- 
 vated forms, as the iris, fleur-de-lis, crocus, gladiolus, Friesia, 
 blue-eyed grass, etc. 
 
 134. The Scitaminales. — This order is confined largely to the 
 tropics and includes such familiar plants as the banana, the trav- 
 eler's tree, gfnger plant, canna, etc. It is mentioned here as fur- 
 nishing an interesting illustration of the variations that often 
 appear in epigynous flowers as a result of the crowding of the 
 organs upon the receptacle. Tendencies toward irregularity ap- 
 peared in the epigynous families of the Liliales, but in this 
 order these variations become very marked and make less abrupt 
 the transition to the next order. The flowers of the banana are 
 suggestive of the amaryllis family, though somewhat irregular. 
 All of the perianth leaves but one are united and only five per- 
 fect stamens are developed. These flowers are united into groups 
 in the axils of bracts that form large buds at the ends of the stem. 
 
 I ' As the bud elongates, the basal or lower bract is first curved 
 back from the bud, thus exposing the flower cluster, and this 
 expansion goes on until all the flowers are exposed. The lowest 
 flowers in the inflorescence are imperfect, containing only pistils, 
 the central are perfect and the upper ones bear only stamens. 
 
 4 I Only the ovaries of the lower flowers of the inflorescence develop, 
 each "hand" of bananas representing the matured ovaries of the 
 flowers in the axil of the bract. Singularly, seeds do not develo 
 in the edible banana, although you can see minute dark-colored 
 ovules forming three radiating lines in a cross-section of the 
 
DEVELOPMENT OF PLANTS 
 
 421 
 
 fruit. The ability of these plants to propagate themselves by 
 means of buds developed on the underground rhizomes may have 
 resulted in the loss of the seed habit. A great many plants are 
 so successful in propagating themselves by buds, bulbs, runners, 
 etc., that they have ceased to produce seed. In the higher mem- 
 bers of the order, as in the ginger and canna families, the flowers 
 become very irregular through the unequal development of the 
 leaves of the perianth or in some forms on account of the abor- 
 
 FiG. 290. Flower of Canna, the showy petal-like organs being modified 
 stamens (staminodia) while the perianth proper, p, is reduced to green bract- 
 like organs: /, labellum; an, anther on petal-like organ; s, stigma; 0, ovary. 
 B, section of flower, showing ovary, 0, the modified stamen, an, and stigma, 
 s, the other organs being removed, 
 
 tion or transformation of all the stamens save one into petal-like'^- — 
 organs termed staminodia. Usually one of the leaves of the 
 perianth, or one or more of the staminodia, are highly modified 
 and known as the labellum. This organ is so placed as to afford 
 a natural landing place for the insects visiting the flower and also 
 to necessitate crossing (Fig. 290). Observe a bee visiting the 
 flowers of the canna and determine the significance of the position 
 and movement of the labellum and its relation to the stamen and 
 stigma. 
 
 135. Orchidales, the Orchid Order. — The orchids are the 
 
422 THE ORCHIDALES 
 
 highest group of the monocotyledons and their flowers are a 
 source of wonder and admiration, owing to the singular beauty 
 and delicacy of their mechanical construction. Variation in this 
 order has occurred on a gigantic scale, resulting in a larger num- 
 ber of species (over 7,000) than is found in any of the preceding 
 orders. Nevertheless these elaborate variations have not been 
 very successful in enabling them to compete with other plants, 
 and as a result the orchids are rather rare and not at all com- 
 parable in number of individuals with the lilies and grasses. 
 Though more widely distributed than any of the other monocoty- 
 ledons, their variations have adapted them as a rule to peculiar 
 conditions. They are especially abundant in the mountainous 
 districts of the tropics, where they more commonly appear as 
 epiphytes upon the trunks of trees and in the crevices of rocks. 
 Such conditions are met by the development of a thick mantle 
 of cells about the aerial roots, the velamen, which absorbs the 
 moisture from the air and doubtless the enlargement of the leaf 
 base in some of these plants into a bulbous storage organ enables 
 them to anticipate in this way the heavy demands that will be 
 made upon them in the flowering season (Fig. 291). The ter- ^ 
 restrial forms are largely parasitic or saprophytic and associated 
 with mycorrhiza, and this has resulted in some of the forms in the 
 suppression of various organs of the plant as the primary root or 
 even of the entire root system, as is illustrated in the coral root 
 orchid, where the leaves have also become reduced to mere scales 
 and the chlorophyll has disappeared. The dependence upon 
 fungal life has reached such a stage in some forms that the seeds 
 do not germinate unless hyphae come in contact with them 
 (p. 69). The organs of the flowers are subject to such remark- 
 able variations that the various parts may appear at first some- 
 what difficult to recognize. It is evident that the same line 
 of variation noted in the higher families of the Liliales has been 
 continued, for the perianth is arranged upon a compound ovary, 
 but its parts are often sharply differentiated into calyx and 
 corolla. The sepals and petals comprising these two whorls/,- 
 differ greatly in form, but especially to be noted is the oddly CGfn^ 
 structed petal known as the labellum (Figs. 292, 293, /). This 
 
DEVELOPMENT OF PLANTS 
 
 423 
 
 organ is the most striking feature of the orchid and it assumes 
 an almost endless variety of forms and colorations, being a goo4^' 
 illustration of Wallace's law that the most highly modified part 
 shows the greatest variation in coloration. In Cypripedium (Fig. 
 292, A) the labellum assumes the form of a moccasin, in other 
 genera it resembles a vase, boat, tongue, body of insect, etc. It 
 
 
 Fig. 291. An epiphytic orchid growing upon the branch of a tree. The 
 coarse roots, r, are surrounded by a mantle of cells which takes up the mois- 
 ture from the atmosphere, h, storage organs formed from the base of the 
 leaves, enabling the plant to produce flowers and fruit. The smaller stalks, 
 a, are the shriveled remains of these organs after flowering and fruiting. 
 
 is entire or variously lobed, slit, fringed and often prolonged into 
 a tube for the concealment of nectar. The stamens and stigmas 
 are reduced in number and greatly modified, the former organs 
 usually being reduced to one and so fused with the style that the 
 anther is sessile upon it (Fig. 292, B). One or two of the stig- 
 mas are generally modified into a mucilage-secreting organ, called 
 the rostellum (Fig. 293, A). The microspores are usually united 
 
424 
 
 THE ORCHIDALES 
 
 into a waxy mass, pollinium (pi. pollinia) which is attached to 
 a sticky part of the rostellum (Fig. 293, B, C). The remarkable 
 feature about these gaudy flowers is the relation that the label- 
 lum, anther, rostellum and stigma sustain to each other. The 
 position of these organs is such that an insect visiting the flower 
 touches with some part of his body the sticky part of the rostel- 
 lum and the pollinia are thus made fast to him and carried to the 
 stigma of another flower. These devices are so elaborate in 
 many orchids that the microspores can only reach the stigma 
 through the agency of an insect. In the lower types of orchids, 
 as the moccasin flower (Fig. 292, B), a somewhat different 
 
 Fig. 292. A simple type of the Orchidales: A, the moccasin flower, Cy- 
 pripedium — /, labellum; p, the two unmodified petals; s, sepals, two being 
 united below the labellum; h, bract, partially concealing the inferior ovary. 
 B, section of the flower — /, labellum; s, stigma; an, anther; st, shield-like 
 sterile stamen covering the two anthers and stigma; 0, ovary; h, bract. 
 
 arrangement is found. The bee enters the opening in the upper 
 part of the labellum and feeds upon the glands distributed along 
 the bottom. In leaving the flower he forces his way through 
 the small opening on either side of the style and so he first comes 
 into contact with the stigma and later with the anthers, which are 
 two in number and located back of the stigma. In this flower 
 the anthers are surrounded by a sticky mass and the microspores 
 are thus fastened to the insect's body to be carried to another 
 flower. In the higher types of orchids, the insect probing for the 
 nectar touches the sticky discs of the rostellum, to which the 
 
DEVELOPMENT OF PLANTS 
 
 425 
 
 pollinia are attached, and in this \va\' they are fastened to his 
 body and drawn out of the open anthers wlien he leaves the 
 flower (Fig. 293, C). In some cases the polhnia quickly curve 
 after being withdrawn from the anther, with the result that this 
 change of position brings them into line with the stigma of the 
 next flower visited. In other genera certain cells are irritable 
 and in a high state of tension. A touch causes an explosion that 
 results in hurling out the pollinia which always land on the end to 
 which the sticky disc is attached and so become fastened to the 
 
 Fig. 293. Higher type of the Orchidalcs: A, flower of Orchis — /, label- 
 lum; p, the two unmodified petals; s, sepals; r, rostellum to which the two 
 pollen masses, pollinia, in the two-lobed anther, an, are attached by sticky 
 discs; st, stigma. B, enlarged lateral view of the anther which has opened, 
 exposing the pollinia. C, one of the pollinia withdrawn from the anther show- 
 ing adhesive disc at base which is attached to the rostellum. At right a 
 pollinium enlarged, showing the small masses of microspores, massula, that 
 may be separately detached from the pollinium. — After Warming. 
 
 insect. (See Darwin's Fertilization of Orchids for a discussion 
 of the multiplicity of these arrangements.) 
 
 The seeds are among the most rudimentary of the Spermato- 
 phyta, consisting of an undifferentiated, few-celled embryo with- 
 out endosperm and surrounded by a delicate bladder-like integu- 
 ment. They are exceedingly small and produced in enormous 
 numbers; in fact, the seeds of the rattlesnake orchid, Peramium, 
 weigh but two millionths of a grain each and float in the air like 
 
426 DICOTYLEDONES 
 
 dust particles. The fringed orchis (BlepharigloUis) , Arethusa, 
 ladies' tresses (Gyrostachys) , rattlesnake plantain (Peramium), 
 grass pink (Limodorum) , rose pogonia, showy orchis (Galeorchis) , 
 moccasin flower {Cypripedium) are among the common and 
 more showy of our native orchids. 
 
 Class B. Dicotyledones 
 
 136. General Characters. — The structure of these plants is 
 more complex than that of the monocotyledons and their varia- 
 tions have been more extensive and successful, over 100,000 
 species being known. For this reason they are adapted to a 
 greater range of conditions and have become the dominant plants, 
 forming the conspicuous and characteristic features of the vege- 
 tation of the earth. 
 
 The dicotyledons may be short-lived annual plants, or per- 
 ennials, and they include a great variety of climbers, epiphytes, 
 parasites, and saprophytes and comparatively few aquatics. The 
 most extreme forms of xerophytes are also found in this group. 
 The leaves are highly differentiated, usually consisting of a 
 blade and petiole which is often associated with small leaf-like 
 organs, the stipules (Fig. 294). The blade varies in form and 
 is foten characterized by teeth and various forms of lobing. 
 This is due to the fact that the veins usually differ, from those of 
 the monocotyledons in that they repeatedly branch, becoming 
 smaller and smaller and thus forming a network or reticulated 
 venation w^ith free ends on the margins and other parts of the leaf. 
 The stems are markedly different from the previous class, owing 
 to the arrangement of the vascular tissues in a circle about the 
 pith (p. 76) and the formation of a cambium cylinder which 
 brings about an increase in the diameter of the stem (Fig. 295). 
 This arrangement gives the plant a great advantage, permitting 
 the extensive system of branching that characterizes the group 
 and the consequent increase in the display of foliage. 
 
 The flower is subject to the same modifications as noted in the 
 monocotyledons. Among the lower orders, the flowers are quite 
 as simple as those of the primitive monocotyledons and the de- 
 velopment of imperfect and wind pollinated flowers is of common 
 
DEVELOPMENT OF PLANTS 
 
 427 
 
 occurrence. The spiral arrangement of the numerous organs 
 of the flower will also be noted. The majority of the orders of 
 the dicotyledons, however, are characterized by perfect, cyclic 
 flowers and the various sets of organs usually consist of four or 
 five members each. The perianth, when present, is generally 
 differentiated into a green calyx and a variously colored corolla. 
 
 Fig. 294. Fig. 295. 
 
 Fig. 294. Leaf of white birch, the blade, b, traversed by a network of 
 veins that end in free branches (the margin irregularly toothed or dentate) 
 and supported upon a petiole, p. 
 
 Fig. 295. Diagram of a cross-section of a stem of black oak four years 
 old: p, pith; i, 2, 3, 4, annual rings of xylem; c, cambium cylinder; ph, phloem; 
 c, cortex; ck, cork; m, medullary rays. 
 
 The stamens are more frequently arranged In one or two whorls, 
 equalling or twice the number of the sepals, and the pistils usually 
 form a single whorl, equalling or less than the number of sepals. 
 The crowding of the various organs, and the lateral growth of 
 the receptacle results in the reduction in the number of organs 
 and in their mass growth so that in the higher types the calyx 
 and corolla become more or less tubular and the carpels form a 
 compound ovary. The basal growtli of tlie receptacle also causes 
 the frequent occurrence of the perig}nous and epigynous t>pes of 
 flowers. Irregularity of the flower is more common than in the 
 
428 
 
 EMBRYO OF DICOTYLEDONES 
 
 monocotyledons and this feature will be seen to characterize the 
 higher members of many of the orders. 
 
 The most characteristic feature of the dicotyledons is the 
 embryo which usually consists of a root, stem, and two laterally 
 attached cotyledons. The region of the stem above the attach- 
 ment of the cotyledons is known as the epicotyl and frequently ap- 
 pears as a minute bud, the plumule. The region of the stem below 
 the cotyledons, the hypocotyl, terminates in the root (Fig. 296). 
 
 t B 
 
 Fig. 296. Structure of dicotyledonous seeds: A, nearly mature seed of 
 Lepidium. The embryo consists of the hypocotyl, hy, ending below in the 
 root, r, and the root cap and above the epicotyl, pi. Two cotyledons, c, 
 arise laterally from the stem; /, funiculus; mi, micropyle; in, integuments; 
 en, remains of endosperm. B, section of seed of water lily — e, embryo with 
 two cotyledons attached laterally to the minute stem of the embryo and sur- 
 rounded by a layer of endosperm cells; mg, sporangial cells or perisperm; i, 
 integument. 
 
 The elongation of the basal portion of the hypocotyl frees the 
 root from the seed and the growth of the upper region of the 
 hypocotyl pushes up into the air the cotyledons and growing point 
 or epicotyl. The formation of the stem is sometimes due to 
 the elongation of the epicotyl alone, the cotyledons frequently 
 containing the storage foods for the nourishment of the young 
 plant and remaining buried in the seed. 
 
DEVELOPMENT OF PLANTS 429 
 
 In endospermous seeds it should be noted that the cotyledons 
 are not developed as digestive and absorbing organs as observed 
 in the monocotyledons (page 401). The primary root often per- 
 sists, forming the main or tap root of the plant, which manner of 
 growth is not so common in the monocotyledons. The Dicoty- 
 ledons include two rather distinct series: a. The Choripetalae, 
 distinguished by their free petals or lack of perianth, b. The 
 Sympetalae, with united petals. 
 
 Series a. Choripetalae 
 
 137. General Characters. — This group comprises about 61,000 
 species and includes the majority of the trees and shrubs found 
 in the temperate regions. The flowers as a rule are of a simple 
 type but exceedingly variable in structure, so that it is not pos- 
 sible to separate them into so sharply characterized orders as in 
 case of the monocotyledons, or in the following group of Sym- 
 petalae. Furthermore, these orders doubtless represent many 
 parallel lines of development that are imperfectly understood. 
 For this reason, the order of the presentation in the following 
 pages does not attempt to represent the real relationship of the 
 groups. 
 
 138. Salicales, the Willow and Poplar Order. — The willows 
 are almost universally distributed along Avatcr ways, as though 
 demanding for their existence onl}- light and water. The poplars 
 are less restricted and some can endure moderately arid condi- 
 tions. Few plants have greater vitality. A bit of a twig and 
 often a portion of a root is capable of developing buds and so 
 starting the shoot. Trees are often pointed out that have origi- 
 nated through the careless sticking of a twig in the soil and 
 several of the willows are naturally propagated by their twigs 
 which are easily broken off by the winds. Their unusual abilit}' 
 to form numerous buds is well shown in the pollarded willows 
 where the branches have been cut back and a large number of 
 shoots develop about the wound. Perhaps the stimulus of the 
 wound also awakens some of the dormant buds (page 75). The 
 flowers, as in Typha, are arranged on an elongated axis forming 
 a compact inflorescence known as an anient or catkin (Fig. 297, 
 
430 
 
 THE SALICALES 
 
 C, E). The distinguishing feature of this kind of flower cluster 
 is seen in the scale or bract that protects each flower (Fig. 297, 
 
 D, F). The aments and also the leaves are concealed in buds 
 that^re peculiar in that they are protected by a boat-shaped 
 scale (Fig. 297, A). These plants flower very early in the spring 
 and as the ament emerges from the bud the overlapping bracts 
 with their hairy coats form the "pussy willow" stage of the 
 
 Fig. 297. Flowers and seed of the willow (Salix): A, winter appearance 
 of a flowering twig, each boat-shaped scale concealing an ament. B, pussy 
 willow stage of flowering, the aments emerging from the scales and exposing 
 the hairy bracts that conceal the flowers. C, ament of pistillate flowers in 
 full bloom. D, pistillate flower, consisting of a compound pistil of two car- 
 pels — b, bract; n, nectar gland. E, ament of staminate flowers. F, stam- 
 inate flower of two stamens. G, ament of mature pistils which are opening 
 to discharge the seeds. H-I, successive stages in the opening of the pistil. 
 /, a seed with circle of hairs at base forming a parachute for dissemination. 
 
 inflorescence (Fig. 297, B). Soon the bracts spread apart, ex- 
 posing the flowers which are nearly as primitive as the simplest 
 of the monocotyledons, being without perianth, imperfect and 
 usually the two kinds of sporophylls are arranged on different 
 plants. The pistil is compound and composed of two carpels, 
 each of which contains numerous ovules, so that the pistillate 
 
^^ 
 
 J 
 
 DEVELOPMENT OF PLANTS 431 
 
 flowers are not as primitive as those of the cat-tails, where the 
 pistil is simple (Fig. 297, Z)). The staminate flowers show the 
 same primitive characters (Fig. 297, F), consisting of one or 
 more naked stamens. The absence of showy perianth, the ex- 
 travagant production of microspores and the formation of the _ 
 flowers before the leaves become large and so interfere with the 
 distribution of the microspores are all characteristics of wind polli- 
 nated flowers. It is noteworthy, however, that nectar glands 
 are developed in the flowers of the willow (F'ig. 297, D, n) and 
 that the microspores are sticky. Perhaps we have here an illus- / 
 
 tration of one of the earliest variations of the flower thai served asl- 
 
 -4Jie_DEctar iriands 
 
 \ 
 
 an allurement to insects. Certaiii l^, tho n^rtar_^gm.nds and the \ 
 conspicuous display of microsporophylls are a very efficient 
 attraction, as is attested by the variety of insects that swarm 
 about the aments. 
 
 The most efflcient factor in the distribution of the willows and 
 poplars is found in the seed. The pistils mature in the early 
 summer, when the two carpels spread apart (Fig. 297, G-I), 
 permitting the discharge of the seeds, which are provided with 
 a circle of hair at the base (Fig. 297, J). This parachute is 
 not so nicely constructed as in Typha, but it is so efficient that^^-^:;;;; 
 myriad numbers of minute seeds are carried a considerable . ^ U 
 distance and cover everything in the neighborhood of the trees \ \^v^ 
 with their lint-like masses. The name cottonwood is popularly ^*v^ 
 applied to several of the poplars because of the cotton-like clusters 
 of seeds that emerge from their aments. 
 
 139. Fagales, the Beech Order. — This order includes many 
 of the most important hard-wood trees of the temperate regions, 
 comprising the family of the birches, with such representatives 
 as the American hornbeam (Carpimis), hop hornbeam (Ostrya), 
 hazel (Corylus), birch (Betula), alder (Abius) and the beech 
 family, which includes the chestnut {Castanca), beech (Fagus) 
 and oak {Ouerciis). The inflorescence is more commonly an 
 anient as in the preceding order (Figs. 298, A ; 299, A), although 
 in the hazel and in the beech family only one or a few pistils are 
 developed in the bud-like clusters of overlapping bracts (Fig. 
 299» P)- The flowers are imperfect and the two kinds of sporo- 
 
432 
 
 THE FAGALES 
 
 phylls are usually developed upon the same plant. Several bracts 
 are usually associated with sporophylls (Figs. 298, B-F; 299, 
 B-E) so that the flowers are of a higher type than the willows. 
 The innermost of these bracts is often of a delicate structure and 
 has been referred to as a primitive form of the calyx (Fig. 298, 
 B, pr) and when present in the pistillate flowers it adheres to 
 
 
 wm? '1 W" 
 
 Fig. 298. Flowers and fruits of the birch family, order Fagales: A, in- 
 florescence of hornbeam (Carpinus) — s, staminate ament; p, pistillate ament. 
 B, staminate clusters from ament of alder (Alnus), each flower consisting 
 of four stamens attached to a four-parted perianth, pr. C, upper view of 
 the same cluster, showing the numerous bracts associated with the flowers. 
 D, fruit of Carpinus attached to the greatly enlarged three-lobed bract. D, 
 fruit cluster of hazel (Corylus) — b, bract encircling the ovary and within a 
 more delicate bract, pr, adnate to the ovary; s, stigma. F, fruit of Corylus, 
 the bract, b, in E has grown out into a tubular beaked structure that com- 
 pletely envelops the nut. 
 
 the ovary (Figs. 298, E, pr; 300, B, pr). The pistils are pro- 
 vided with long delicate stigmas and are compound, containing 
 several ovules but only one usually matures. The wall of the 
 ovary develops into a tough coat about the seed, forming a fruit 
 
DEVELOPMENT OF PLANTS 433 
 
 known as the nut. Certain bracts of the ilowcr increase greatly 
 in size as the ovary matures and form a conspicuous part of the 
 fruit. Thus in the hornbeam (Fig. 298, D), one of the bracts 
 develops into a large, three-lobed green leaf, in the hop horn- 
 beam the bract forms a papery sac about the nut, in the hazel 
 a leafy husk (Fig. 298), in the birch And alders a woody peg- 
 like structure. In the chestnut and beech the pistils are com- 
 pletely enveloped by prickly bracts or outgrowths that form the 
 bur (Fig. 300, A-C), while in the oak these structures only cover 
 the lower portion of the ovary, forming the cup (Fig. 299, C-E). 
 The flowers of this order are typical of those that are wind polli- 
 nated. Note the small and simple flowers, absence of showy 
 perianth, nectar, and odor glands, dry and light microspores 
 lavishly produced to ensure seed formation and the delicate 
 bushy stigmas for catching the spores. The stigmas appear a 
 day or so before the adjoining microspores are being shed so 
 that a crossing from an earlier flowering plant is necessitated. 
 These plants form the larger part of our deciduous forests and 
 their association in colonics is doubtless connected with the dis- 
 tribution of pollen by the wind, as is also the appearance of the 
 flowers before the leaves are fully developed. The positions as- 
 sumed by the staminate aments is of service in protecting the 
 microspores against wetting and also to assist in their distribu- 
 tion. During the winter, these aments are quite erect but as the 
 flowering stage approaches they become pendulous, the larger 
 bracts protecting the sporophylls, like the shingles on a roof 
 (Fig. 298, A). These bracts, being hygroscopic, remain closed 
 during wet weather but on dry days they curve back, each bract 
 forming a shelf which serves to catch the spores when no winds 
 are stirring and thus prc\'cnt their falling to the ground. 
 
 140. Other Tree Orders Suggestive of the Fagales. — Closely 
 allied to the Fagales is the order of the walnuts, Juglandales, 
 including the black walnut and butternut (Juglatis) and the hick- 
 ories {Ilicoria, Fig. 301). These trees are of very common 
 occurrence in the northern United States and are characterized 
 by aromatic oils and large compound leaves. The flowers are 
 very similar in structure and arrangement to those of the beech 
 28 
 
434 
 
 ORDERS RELATED TO FAGALES 
 
 order, but at maturity the outer part of the wall of the ovary 
 becomes transformed into a fleshy rind and the inner portion 
 forms a hard shell. In the walnut this pulpy, aromatic rind 
 about the nut is finally destroyed by decay, while in the hickories 
 
 
 ^r " 
 
 Fig. 300. 
 
 Fig. 299. The beech family, order Fagales: A, inflorescence of oak {Quer- 
 cus) — s, staminate ament; p, pistillate inflorescence. B, staminate flower 
 surrounded by a perianth of slightly united bracts. C, pistillate flowers 
 with numerous bracts surrounding base of ovary. D, section of flower, the 
 pistil being composed of three carpels and the inner bracts adnate to the 
 ovary. E, fruit of oak, the cup consisting of the modified outer bracts shown 
 in C and D and the nut has developed from the ovary and one of its ovules. 
 
 Fig. 300. Flower and fruit of the beech (Fagus), order Fagales: A, pis- 
 tillate inflorescence, the three-lobed stigmas projecting beyond the bracts. 
 B, section of the inflorescence — pr, inner bracts or perianth surrounded by 
 an outer spiny set. C, the fruit, the outer bracts of B have become hard 
 and spiny and are splitting into four valves, exposing the three-angled nuts. 
 
 the rind becomes leathery and splits into valves, freeing the nut 
 (Fig. 301, C). 
 The order of the nettles, Urticales, also has many points in 
 
DEVELOPMENT OF PLANTS 
 
 435 
 
 common with the beech order and contains several of our com- 
 mon trees as the elm {Ulmtis, Fig. 302), hackberry (Celtis), mul- 
 berry (Morns), osage orange {Toxylon), numennis tropical 
 forms, as the India rubber trees, banyan tree, etc., as well as a 
 variety of valuable herbaceous plants, as the hop and hemp. 
 These plants very generally contain a milky juice, latex, which 
 
 
 ':^m 
 
 B 
 
 Fig. 301. Flower and fruit of the Juglandalcs: A, inflorescence of but- 
 ternut — s, staminate ament; p, pistillate inflorescence. The butternut and 
 black walnut (Juglans) may be recognized by the chambered pith, as shown 
 at bottom of twig. B, section of a pistillate flower — pr, perianth adnata to 
 the two carpels composing the pistil; s, stigma. C, fruit of hickory, the 
 outer part of ovary wall splitting into four valves and exposing the hard inner 
 part, the shell of the nut. 
 
 in the ca-e of Ficiis and Castilloa, two of the india rubber trees, 
 furnishes the raw material of india rubber and in the cow tree 
 of South America yields a saccharine, nutritious milky juice. 
 Tough stereome fibers are characteristic features in many of 
 these plants and furnish the hemp derived from the Cafinabis, 
 and in Japan paper is manufactured from the fibers of the paper 
 
436 
 
 ORDERS HIGHER THAN FAGALES 
 
 mulberry. The leaves are often provided with rough hairs which 
 sometimes contain irritating acids, as in the nettles (page 40). 
 The flowers are of a somewhat higher type than in the preceding 
 orders, and associated in a variety of dense inflorescence. They 
 are rarely perfect, though rudimentary (sterile) stamens may 
 be developed with fertile pistils or vice versa, thus giving the 
 appearance of perfect flowers (Fig. 302, B-D). The calyx does 
 
 
 %-^-' 
 
 Fig. 302. 
 
 Fig. 303. 
 
 Fig. 302. Inflorescence and fruit of elm, order Urticales: A, twig of elm 
 bearing principally staminate flowers. B, a staminate flower enlarged, show- 
 ing lobed perianth enclosing numerous stamens. C, pistillate flower, two- 
 lobed stigma projecting from perianth. D, section of pistillate flower, show- 
 ing aborted (sterile) stamens and pistil (fertile) of two carpels, as indicated 
 by the two stigmas. E, the fruit with perianth still attached. A wing has 
 grown out from the sides of the ovary. 
 
 Fig. 303. A simple type of the Chenopodiales: A, shoot of Mexican tea 
 {Cheno podium), showing character of inflorescence, i7i. B, flower enlarged, 
 showing the perianth, stamens and pistil of three cohering carpels. 
 
 not adhere to the ovary as in the Fagales, though the sepals often 
 cohere (Fig. 302, D). The ovary usually matures as a nut, but 
 in many cases a kind of drupe (page 399) is formed, owing to 
 the perianth becoming fleshy and enveloping the nut. The fruit 
 of the mulberry- Is an aggregation of drupes. In the fig the stem 
 envelops the minute curious flowers and becomes fleshy, forming 
 the edible portion of the fruit. 
 
 141. Orders Showing an Advance over Preceding Forms. — 
 
DEVELOPMENT OF PLANTS 
 
 437 
 
 The buckwheat order, Polygonalcs, containing such famiHar 
 weeds as the sorrel, dock (Ricmex), and knotweed or smartweed 
 (Polygonum), has perfect regular flowers with a distinct perianth. 
 These characters also appear in the allied goosefoot order, Cheno- 
 podiales, with its great array of common weeds, as the goosefoot 
 (C heno podium) , orache (Atriplex), tumbleweed and pigweed 
 
 
 Fig. 304. Advanced type of the Chenopodiales: A, shoot of Melandry- 
 num bearing flower with perianth differentiated into calyx, ca, and corolla ^ 
 c. B, section of flower, showing the relation of calyx to corolla and the con- 
 cealment of the nectar glands at the base of the ovary. Access to the corolla 
 tube is guarded by an outgrowth on the petals, as shown in C, which, assisted 
 by the styles or stamens, so eff^ectually closes the mouth of the tube that 
 only insects with long tongues can reach the honey. 
 
 {Amaranthus) . The flowers are arranged in loose clusters and 
 are for the most part small and wind pollinated (Fig. 303). How- 
 ever, it is noteworthy that some of the genera are adapted to 
 insects, the calyx becoming larger and brightly colored and nectar 
 glands are associated with it. In the higher members of the 
 order, as in the purslane and pink families (which include the 
 spring beauty, Claylouia), the perianth becomes differentiated 
 into a calyx and showy corolla, and the flowers have become in a 
 marked degree adapted to insect visitors (Fig. 304). These 
 
438 
 
 [J THE;RANALES 
 
 orders form a natural transition from the primitive flowers of 
 the willows and beeches . toNfhe large flowers of the next order 
 with their showy perianths, though the structure of the flower 
 is not very indicative of a relationship between them, 
 
 142. Ranales, the Buttercup or Crowfoot Order. — This large 
 and interesting order includes a great variety of our common 
 plants, herbs, and J;^ees, as the white and yellow water lilies 
 (Nymphaea ^;and 'pastalia), buttercups {Ranunculus), marsh 
 marigold (Cg///?(2), windflower {Anemone), Hepatica, rue {Thalic- 
 tru}n) X^i^i^^^h'me {Aqicilegia), larkspur {Delphinium), monks- 
 
 (V Fig. 305. A common type of the Ranales, Ranunculus repens: A, habit 
 'Of the plant. B, early stage of flowering, the stamens clustered about the 
 stigmas. C, late stage of flowering, nearly all the stamens bent over towards 
 tne petals, having discharged their spores. D, petal with nectar gland at 
 base.' E, fruit consisting of numerous spirally-arranged akenes. 
 
 hood {Aconitum), may apple {Podophyllum), magnolia, tulip tree 
 {Liriodendron) , Sassafras, spice bush {Benzoin), etc. In this 
 order we have again reached the point, just as in the monocoty- 
 ledons (see Liliales), where the flowers are more usually solitary 
 and conspicuous, owing to the development of large, showy 
 perianths. While the perianth is more differentiated than in the 
 lilies it is noteworthy in the majority of the forms that the flower 
 has not reached the state where the calyx is clearly separable from 
 the corolla. The flowers are very simple, as is indicated by 
 the regular and hypogynous arrangement of the parts (Fig. 
 
DEVELOPMENT OF PLANTS 439 
 
 305). The receptacle is frequently elongated and consequently 
 the organs of the flower, especially the sporophylls, are numerous 
 and indefinite in number and spirally arranged. Were it not 
 for the perianth, such a type of flower would be ([uite as primi- 
 tive as any of the preceding orders. 
 
 (a) The Buttercup, Ranunculus. — The flower of the buttercup 
 illustrates the more characteristic features of the Ranales (Fig. 
 305, A, B). Here the perianth is apparently differentiated 
 into calyx and corolla. The petals, however, are sometimes 
 regarded as stamens that have become highly modified for the 
 production of nectar. The sporophylls are indefinite in number 
 and spirally arranged. It is interesting to note the relationship 
 of the various parts of this simple flower and their behavior during 
 the period of blooming. When the flower first opens, the anthers 
 are still closed and clustered around the receptive stigmas, afford- 
 ing a natural landing place for the insects (Fig. 305, B). Thus 
 crossing may only be effected with the spores carried from older 
 flow^ers. At a later period the outermost filaments elongate, 
 curving over towards the petals, so that as their anthers discharge 
 there is little chance of autogamy (Fig. 305, C). This position 
 also brings the anther into the pathway leading to the nectar 
 gland at the base of the petal (Fig. 305, D), so that the insect 
 in probing for the nectar is sure to receive some of the sticky 
 microspores on his body. Each day, for about a week, succes- 
 sive series of stamens behave in this manner, and it is not until 
 the innermost anthers are discharging on the last day of bloom- 
 ing of the flower that there is an opportunity' for autogam\'. In 
 many members of this and other orders you will find that the 
 stigmas and anthers of the innermost stamens actually meet, 
 owing to the peculiar curvature of the styles and filaments, thus 
 ensuring autogamy in case crossing has failed. It should be 
 borne in mind that the devices for effecting autogamy, as a last 
 resort, are quite as general and often as elaborate as the pro- 
 visions for bringing about crossing. The microspores are pro- 
 tected against dews and rains by the folding of the perianth and 
 also by the downward curvature of the pedicel at night, whereas on 
 each succeeding bright day the perianth opens again and the 
 
^40 
 
 THE RANALES 
 
 flower becomes nearly or quite erect and turned towards the 
 light, so that it is in a position from which the insect will naturally 
 come. The opening of the flower is due to a growth in the morn- 
 ing of the basal, inner portion of the petals thus bending them 
 away from the center of the flower and thus effecting the opening; 
 while a corresponding growth towards evening of the basal outer 
 part of the petal bends them towards the center of the flower. 
 This growth slowly increases the size of the flowers, as may read- 
 ily be observed by comparing the freshly opened flowers of the 
 anemone, buttercup, etc., with those several days old. All these 
 features are of common occurrence in a great variety of orders. 
 The furit of the buttercup is an akene, each carpel containing but 
 a single ovule (Figs. 305, E; 261 B). 
 
 (b) Some Variations of the Order. — Many variations of this 
 simple structure appear in other members of the order. The 
 
 Fig. 306. Modifications appearing in the Ranales: A, flower of Helle- 
 borus — n, nectar glands due to modifications of stamens. B, columbine — 
 n, honey leaves due to modifications of the petals or possibly of the stamens. 
 
 carpels may be reduced to one, as in sassafras and other groups, 
 and in the green hellebore (Fig. 306, A) they are partially com- 
 pounded and even united with the receptacle in some water lilies, 
 etc. (flowers perigynous and epigynous). The stamens are some- 
 times few in number and they are frequently modified into nectar 
 glands and assume a variety of odd shapes (Fig. 306, A). The 
 showy nectar-bearing petals, honey leaves, of many of the genera 
 are regarded by some as modified stamens, as in the monkshood, 
 columbine, larkspur, etc. (Fig. 306, B). 
 
DEVELOPMENT OF PLANTS 
 
 441 
 
 The parts of the perianth, wliicli are more frefjuenth' in threes 
 or indefinite in number than in fives, are also subject to consider- 
 able variation. Very frequently the corolla is not developed or 
 partially suppressed, and the sepals become colored and petaloid, 
 as in the marsh marigold, hepatica, certain windflowers, rue, 
 spice bush, larkspur, etc. In a few of the genera the perianth 
 becomes greatly modified and even irregular, as in the columl)ine 
 (Fig. 306, B, w), where five of the petals, often regarded as 
 modified stamens, are transformed into tubular hone}' leaves or 
 
 Fig. 307. Highest type of the Ranales: A, inflorescence of monkshood. 
 B, section of a flower — s, helmet-like sepal enclosing nectar organ, n. C, 
 flower of larkspur. D, section of flower — s, spurred sepal enclosing two- 
 spurred nectar organs, n. E, fruit of larkspur consisting of 3 pistils maturing 
 as follicles, 
 
 into spurs and hoods, as in the larkspur and monkshood (Fig. 
 307, B, D). These flowers illustrate ver\' well the progressive 
 coloration that is often associated with the variation of the peri- 
 anth. The simpler forms are usually yellow or white, while 
 flowers with more highly modified parts are pink and pale blue 
 (columbine), blue in higher tyi:)es (larkspur) and ulira-marine 
 blue in the most irregular and highly modified type (monks- 
 hood). Any of these flowers may take on by reversion the lower 
 
442 THE PAPAVERALES 
 
 grades of color so that a blue-colored form may vary and its 
 offspring may produce white or yellow flowers, as in the colum- 
 bine. The remarkable brilliant yellow of the flowers, as seen in 
 the buttercup and marsh marigold, etc., has been regarded as the 
 primitive coloration of the perianth, and it is to be noted that 
 the flowers showing reversion to this color assume a duller 
 yellow. This order furnishes a great variety of showy orna- 
 mental plants, but none with fragrant flowers. Many contain 
 acrid juices and poisonous alkaloids, some of which are of medic- 
 inal value, as aconite, hydrastin, helleborine, etc. Others are of 
 value for the volatile oils, as sassafras, cinnamon, camphor, nut- 
 meg, etc. The simple structure of the more characteristic flowers 
 indicates that this order is a very ancient one and the pronounced 
 tendency to vary would suggest that possibly many of the higher 
 orders of the Choripetalae have been derived from this group. 
 The origin of the monocotyledons from this order has also been 
 suggested, owing to the structural features of the embryo and 
 vascular bundles of certain groups. 
 
 143. Papaverales, the Poppy Order. — This group includes two 
 very well known families: (i) The poppies, with such familiar 
 plants as the poppy (Papaver), sea poppy (Glaucium), celandine 
 (Chelidonium), soldier's cap {Bicuculla), fumitory (Adhimia), 
 corydalis (Capnoides) (Fig. 308). (2) The mustards, includ- 
 ing the peppergrass (Lepidmm), hedge mustard {Sisymbrium) , 
 white and black mustard (Sinapis and Brassica), yellow rocket 
 (Barbarea), cress (Roripa), toothwort (Dentaria), shepherd's 
 purse (Bursa), whitlow grass (Draba), rock cress (Arabis) (Fig. 
 309). The families of the caper and mignonette are also in- 
 cluded in the order. These plants are closely related to the 
 Ranales, as is indicated by their simple and showy flowers, the 
 various organs usually being in multiples of two and quite dis- 
 tinct (flowers hypogynous) save in the case of the carpels. These 
 organs are compound as in the water lilies and more commonly 
 reduced to two, thus forming a sharp contrast with the preceding 
 order. Note also that there is a sharp distinction between calyx 
 and corolla. 
 
 (a) The Poppy Family, Papaveraceae. — The Bloodroot, San- 
 
DEVELOPMENT OF PLANTS 
 
 443 
 
 guinaria, is typical of the simpler nieinbers of this family (I^g. 
 308). These plants form colonies in rather open, rich woods 
 owing to their fleshy rhizomes, and like many plants with storage 
 organs, flower very early in the spring. The flowers endure for 
 about two days, but the largc-lobed leaves are consi)icuous fea- 
 tures of the forest carpet for several weeks wliile tlH-\' are manu- 
 
 FiG. 308. Examples of the poppy family, order Papaverales: .4, habit 
 of the bloodroot at time of flowering — r, rhizome; /, leaf folded about flower 
 stalk. B, leaf unfolded, C, section of flower, showing its hypogynous struc- 
 ture with numerous stamens and pistil of two carpels. D, flower of fumitory 
 (Adlumia) — s, sepal; p, inflated petals; w, winged petals which conceal the 
 sporophylls. 
 
 facturing the food for the next season. The large leaves are 
 tightly rolled about the solitary flower and enveloped in a papery 
 sheath as they emerge from the soil. A juice, latex, is found in 
 the majority of the plants of this order, and in this example it is of 
 a blood-red color, thus giving the latin and popular name to the 
 
444 THE PAPAVERALES 
 
 plant, Sangtiinaria, or bloodroot. The flowers are of the simple 
 open type noted in the buttercup, having two sepals (a character- 
 istic of the family) which fall with the opening of the flower, 
 petals 8-12, stamens indefinite and a compound pistil of two 
 carpels, as is indicated by the two stigmas and two rows of ovules 
 (Fig. 308, C). The stigmas are receptive as soon as the flow^er 
 opens, but the anthers remain closed, and so the spores must be 
 carried from an older flower. The petals close during the late 
 afternoon and when opened on the following day the anthers 
 shed their spores and complete the flowering. 
 
 In higher types of the poppies we find the flowers becoming 
 irregular and the various parts reduced in numbers. Thus, in 
 the soldier's cap (Fig. 308, D), there are two scale-like sepals, 
 four petals in two pairs, the two outer heart-shaped and spurred 
 at the base, while the two inner are narrow and winged on the 
 back, enclosing the sporophylls. The six stamens are somewhat 
 united and arranged into two groups opposite the spurred petals. 
 The carpel has the same structure as in the bloodroot. It is 
 evident that the nectar concealed in this closed type of flower 
 can only be secured by some long-tongued insect. Find out how 
 the bee procures the honey, the purpose of the wings on the 
 narrow petals and the position of the nectar glands. Is the 
 stigma mature before the anthers open? The structure of this 
 higher type of the poppies makes the transition to the mustard 
 family a simple one. 
 
 {h) The Ahistard Family, Cruciferae. — Here we find the 
 sepals and petals four in number, and the latter arranged cross- 
 wise (Fig. 309, Ay B), thus explaining the family name, Crucif- 
 erae. The petals usually have rather long claws (Fig. 309, D) 
 that are so associated with the sepals as to form a narrow tubular 
 perianth that effectually conceals the nectar glands situated at 
 the base of the ovary. There are usually six stamens, but the 
 two outer ones are shorter than the others (Fig. 309, C). The 
 ovary is divided into two compartments by a membranous parti- 
 tion. The fruit is generally a capsule that usually opens by two 
 valves, as seen in Fig. 309, F. This large family of over 1,800 
 species is almost universally distributed over the earth and of 
 
DEVELOPMENT OF PLANTS 
 
 445 
 
 very common occurrence. These plants with tlieir small white 
 or yellow flowers aggregated in conspicuous inflorescences are 
 among the most familiar weeds of waste places, fence rows and 
 barren fields. The flowers are of a more specialized type than 
 in the open blossoms of the buttercups and poppies, and present 
 a variety of interesting devices that can only be hinted at in this 
 lesson. More commonly, perhaps, the stigmas are receptive as 
 soon as the flower opens and the anthers are still closed, thus 
 
 Fig. 309. Examples of the mustard family, order Papaverales: .1, branch 
 of Brassica, one of the turnips. B, a flower enlarged. C, flower in section. 
 D, a petal, showing narrow claw and broad blade. E, pistil. F, fruit, the 
 valves of the two carpels opened to show the seeds. 
 
 ensuring crossing. The short stamens are more freciuenth' util- 
 ized only for crossing since they are below the stigma and so 
 placed as to be in line with the nectar glands that are often located 
 in sac-like enlargements of the perianth. The four long stamens 
 
446 THE PAPAVERALES 
 
 are variously related to the stigmas. Sometimes they are below 
 the stigma and when open are of service only for crossing, but 
 later they elongate and bear some of the spores to the stigma. 
 The reverse position of the sporophylls also occurs, but the out- 
 ward opening of the anthers keeps the microspores from dropping 
 by chance on the stigma until the stigma is finally lifted up to 
 the anther by the elongation of the style, where It Is sure to be 
 dusted with spores, as the anthers open wider and wider. Certain 
 species are characterized by an extraordinary twisting of the 
 filaments, so that for a time the anther is turned away and bent 
 over towards the perianth so as to be in line with the nectar 
 glands, and later, at the close of the flowering period, a reversal of 
 this bending brings the anther in contact with the stigma. These 
 illustrations are sufficient to show that the flowers have many 
 arrangements and exact movements that result in crossing, or, 
 if this fails, in autogamy. You would naturally expect that 
 specialized flowers like the mustards would show higher types of 
 coloration. This is indeed the case in some forms where pink and 
 purple colors appear, but in the majority of the species the un- 
 favorable situations in which they grow have resulted in a reduc- 
 tion in the size of the flowers and also In a decline In color to 
 white and pale yellow. It is noteworthy that the flowers of the 
 radish when cultivated on barren soil develop white, but upon 
 rich soil, pink. These minute flowers have a decided advantage 
 in being grouped In compact inflorescences, a device that you 
 will see copied again and again in the higher orders because it 
 ensures crossing and makes them conspicuous. It should be 
 stated that several of these plants get along very well without 
 crossing. 
 
 Many products of great commercial value are derived from 
 various members of this order. Opium, from which morphine 
 and other alkaloids are obtained, is the dried latex obtained from 
 the Incisions of the unripe capsules of the Turkish poppy. The 
 majority of these plants contain acrid or peppery juices that 
 render certain parts of the plant or their seeds of value as spices, 
 oils, foods, etc. Many of them are biennial, forming a close 
 rosette of leaves the first year and flowers and fruit the second. 
 
DEVELOPMENT OF PLANTS 447 
 
 By cultivation an abnormal devulopment of one part or another 
 of the plant has been induced. This feature is well illustrated 
 in the cabbage, where the elongated stem of the wild plant has 
 become shortened by cultivation and covered with fleshy leaves, 
 forming a large bud or head. Brussels sprouts are a modifica- 
 tion of the cabbage in which the stem becomes more elongated 
 and covered with numerous small heads. The great variet}^ of 
 kales are really headless cabbages in which the leaves remain free 
 from one another, assuming a variety of forms. The cauliflower 
 is a variety of the cabbage in which the inflorescence has been 
 transformed into a fleshy mass of tissue, and in the kohlrabi the 
 stem becomes swollen and herbaceous. The turnip is a related 
 species of the cabbage genus in which the underground portion 
 of the plant is modified. Radish, cress, horse-radish, caper, 
 spices and oils of mustard, etc., are other products of the order, as 
 well as many cultivated flowers, as the wallflower, stock, mig- 
 nonette, etc. 
 
 144. Resales, the Rose Order. — This enormous order, com- 
 prising over 14,000 species, is better known than any other, not 
 only because of its great array of common field plants, but espe- 
 cially because of the large variety of our cultivated fruits and 
 flowers that belong to it. The culti\'ated currant, blackbcrr\', 
 strawberry, apple, pear, cherry, peach, plum, pea, bean, h^'dran- 
 gea, syringa, rose, spiraea, wistaria, laburnum, as well as many 
 native trees and shrubs, as the liquid-ambar, sycamore, witch- 
 hazel, locusts, etc., belong to the rose order.* 
 
 These plants may be looked upon as the most typical of the 
 Choripetalae, just as the Liliales were the most representative of 
 the monocotyledons. The parts of the flower are more com- 
 monly in fives and cyclic (Fig. 310, .4) though the spiral ar- 
 rangement still persists in nearly every family of the order. The 
 most distinguishing feature of the group is seen in the basal 
 growth of the receptacle, thus lifting up the corolla and stamens 
 about the ovary (perigynous flowers) and this growth also fre- 
 quently results in the epigynous type of flower. The simple 
 forms of flowers that appear in man\- of tlie families are very 
 suggestive of the Ranales, the sporoph>lls being free and often 
 
448 
 
 THE ROSALES 
 
 spirally arranged. Thus, in the live-forever (Sediim), we have a 
 flower that has always been cited as the typical flower of the angi- 
 osperms. It is very regular with five whorls of alternating parts 
 of five members each, i. e.: 5 sepals, 5 petals, 5 to 10 stamens 
 (one or two whorls) and usually 5 pistils (Fig. 310, ^). This type 
 
 Fig. 310. Simple forms of the Rosales: A, flower of Sedum, showing the 
 radial symmetry of the flower and five organs in each whorl. B, grass of 
 Parnassus (Parnassia), a member of a closely-allied family. C, flower on 
 first day of bloom, stamens converging over the pistil and encircled by row 
 of modified stamens. D, section of flower, showing slight growth of base of 
 receptacle and consequent adhesion to perianth. This flower is protandrous 
 and the stamens in shedding the spores straighten up one by one on succeed- 
 ing days and curve back towards the petals. The section shows three positions 
 assumed by the stamens. You may calculate the significance of these features. 
 
 closely resembles the arrangement of parts in the buttercups, save 
 for the slight adhesion of the calyx and receptacle (Fig. 310, D). 
 These fleshy plants as illustrated in the houseleek (Fig. 69), hen 
 and chickens, etc., are very common forms of xerophytes. 
 
DEVELOPMENT OF PLANTS 
 
 449 
 
 Owing to their ability to retain moisture (page 45) they can exist 
 in the crevices of rocks and upon dr>' soil, while the formation of 
 roots from a bit of stem or even a leaf, as well as their habit of 
 producing buds on short stems that afterw^ard become detaclied 
 cmd grow into new plants, brings about a sure distribution and 
 explains their popular name, live-forever. 
 
 (a) Some Variations of the Rose Order. — The active growth 
 :>{ the base of the receptacle and the consequent crowding result 
 n an interesting series of departures from the simple type noted 
 
 Fig. 311. Higher forms of the Rosalcs: A, flower of the saxifrage. B, 
 lower in section, showing the partial adhesion of receptacle to ovary. C. 
 nflorescence of currant {Rihes). D, section of flower, showing receptacle 
 'orming the cavity for the ovules which is roofed over by the carpels, epi- 
 ^ynous flower. 
 
 above that lead in very regular gradations to the highest forms 
 of the order. In the higher family of the saxifrages, for example, 
 w^hich includes the saxifrage, false mitorwort (Tiarella), heu- 
 ^hera, bishop's cap {Mitella), golden saxifrage {ChrysospJenium) , 
 the receptacle generally adheres to the ovary (Fig. 311, A, B). 
 30 that the flower is, in a measure, epigynous and the carpels are 
 reduced to two and partly fused. Passing to the families of the 
 hydrangeas, syringas (Philadelphus), currants (Ribes), etc., the 
 receptacle and carpels are quite fused (Fig. 311, C, D) and 
 
450 
 
 THE ROSALES 
 
 the flower Is strictly epigynous. In the rose family, which is 
 very closely connected w^ith the preceding group, this story of 
 change in the evolution of the flower is repeated. In the simpler 
 forms the mass growth of the receptacle and calyx, forms a cup- 
 like structure (perig^^nous flower). This cup may be rather 
 broad and shallow as in the strawberry, cinquefoil and black- 
 berry (Fig. 312), but in higher types the receptacle more or less 
 completely surrounds the pistils as in the spiraea, avens, rose, 
 agrimony (Fig. 313). In the apple family, including the apple, 
 
 Fig. 312. Fig. 313. 
 
 Fig. 312. A member of the rose family with the simple structure of the 
 saxifrages: A, flower of the strawberry (Fragaria). B, section of flower 
 showing slight adhesion of receptacle to calyx and the spiral arrangement of 
 the sporophylls. C, the fruit, akenes spirally arranged on the enlarged and 
 fleshy receptacle. 
 
 Fig. 313. Higher forms of the rose family: C, flower of Agrimonia. D, 
 section of flower, showing the bristle-cove ed receptacle completely surround' 
 ing the pistils. 
 
 peach, quince, shadbush, thornapple, hawthorn, the ovules are 
 completely enveloped by the receptacle (Fig. 314, A-C) as in 
 the currants, while in the plum family, with its plum, prune, 
 cherry, peach, almond, apricot members, the pistils are reduced 
 to one and are quite distinct from the cup-like receptacle (Fig. 
 
 The senna family has essentially the same type of flower as 
 the plum save that the pistil usually contains many seeds (nor- 
 
DEVELOPMENT OF PLANTS 
 
 451 
 
 mally but one develops in the plum) and splits at maturity into 
 two valves, a form of fruit called a pod. For example, in the 
 honey locust (Gleditsia) and the Kenluck\' coffee bean (Gymno- 
 cladtis), the receptacle forms a shallow cup which bears the regu- 
 lar sepals, petals and usually ten stamens about a single pistil 
 (Fig. 315, A). The flowers are realh' monoecious, but in other 
 respects are very suggestive of the plum flower. In the coffee 
 bean tree, however, the petals are not quite equal and this irre- 
 gularity becomes more noticeable in the sensitive pea (Cassia) 
 
 A 
 
 
 Fig. 314. Flowers and fruits of the apple and plum families: .1, inllor- 
 escence of the apple {Malus). B, section of llower, showing adhesion of 
 receptacle to the ovary, epigynous flower. C, sections of the fruit — c, car- 
 pels of the pistil; r, fleshy receptacle. D, flower of cherry (Prunus). E, 
 section of flower — r, cup-like receptacle which falls off as fruit matures. F, 
 fruit in section, showing the outer part of the wall of the ovary as a fleshy 
 rind and the inner part forming the stone or pit which enclosed a single seed. 
 
 (Fig. 315, B-D). In the redbud, or Judas tree (Cercis), the 
 petals are very irregular, two of them being insecurely united into 
 a boat-like structure, known as the keel, which encloses the ten 
 distinct stamens and single ])islil, while two lateralK- placed 
 petals, the wings, inclose the fifth petal known as the standard 
 (Fig. 315, E, F). These three examples from the senna family 
 
452 
 
 THE ROSALES 
 
 make a very easy transition from the regular flower of the plum 
 to the highly modified flowers of the pea family. 
 
 (b) The Pea Family, Papilionaceae. — The pea family (Fig. 
 316) is the highest of the rose order and the largest family, with 
 one exception, of all the angiosperms, comprising over 11,000 
 species. Here we find the same type of flower as in the redbud. 
 
 Fig. 315. Development of the irregular type of flower in the rose order. 
 A, regular flower of the honey locust {Gleditsia). At left staminate flower: 
 At right pistillate with single pistil, c, which develops into long flat pod, s. 
 rudimentary stamens. B, flower of Cassia, showing slightly irregular corolla, 
 C, section of flower — s, stamen; c, pistil. D, the fruit or pod. E, irregular 
 flower of redbud (Cercis) — c, calyx; k, keel enclosing stamens and pistil; w, 
 wings which arch over the standard, 5. F, corolla removed, showing the ten 
 stamens surrounding the simple pistil, c. 
 
 The standard in this famil}^, however, incloses the wings and 
 the stamens, usually ten in number, may be distinct as noted 
 above or united by their filaments into a sheath about the soli- 
 tary pistil; more frequently a single stamen remains free, an 
 arrangement called diadelphous (two brotherhoods). This type 
 of flower is called papilionaceous from its fancied resemblance 
 
DEVELOPMENT OF PLANTS 
 
 453 
 
 to a certain genus of butlertly, Papilio. Tlie nujre important 
 characteristics of this form of flower are very well illustrated 
 in the pea (Fig. 316). The standard is the conspicuous and 
 most highly-colored organ of the flower, overlapping the two 
 wings which in turn practically cover the keel. By carefully 
 removing the keel and wings, it will be seen that these organs 
 are attached to the calyx and receptacle by rather narrow claws 
 (Fig. 316, C) and that they are also locked together by a little 
 process on each wing that fits into a groove on the keel. Nine 
 of the filaments, the tenth being free, form a sheath about the 
 
 Fig. 316. Structure of the sweet pea (Lathyrns): A, flower of the pea 
 — c, calyx; s, standard enclosing the two wings, ic; k, keel. B, section of 
 flower, showing the sporophylls concealed in the keel. C, one of the wings. 
 D, perianth removed, showing relation of sporophylls. 
 
 ovary which terminates In a st3lar brush of upwardh' pointing 
 hairs and in a stigma. The anthers and style are confined in 
 the tip of the keel (Fig, 316, J5, D). The significance of these 
 features will be discovered If you watch a bee probing into the 
 sheath of filaments after the nectar secreted at the base of the 
 ovary. The standard serves to attract and also direct his ap- 
 proach to the flower so that he alights at the proper place, i. e.^ 
 on the keel and wings. The weight of his body causes a slight 
 depression of the keel and the stylar brush sweeps some of the 
 microspores upon his body. It should be added that the flower 
 is ready for the insect as soon as opened since the spores are 
 discharged before the flower blooms. Only a portion of the 
 spores are swept out by the Insect and the dusting may be re- 
 peated on several visitors. The stigma docs not mature until a 
 
454 THE ROSALES 
 
 later period and it is evident that there is a good chance of 
 autogamy as soon as the stigma is mature. Some genera are 
 always autogamous, but it has been observed in many cases that 
 the spores grow more vigorously upon the stigma of another 
 flower than upon the stigma of their own flower, so that if the 
 stigma receives spores from its own and other flowers, the latter 
 spores would develop better and effect fertilization. This is 
 called the prepotency of foreign spores. In some species, au- 
 togamy is impossible owing to the failure of the spores to germi- 
 nate except when transported to other flowers. There was a 
 good illustration of this fact in Australia when the English 
 clover was introduced and flourished exceedingly well, but pro- 
 duced no seed until bees from England were introduced for 
 crossing. Plants transported to foreign countries are often 
 sterile owing to the absence of the insect to which their varia- 
 tions are adapted. There are many variations of the mechanism 
 shown in the pea. In some genera the style and stigma act like 
 a piston, pushing out some of the spores from the apex of the 
 keel with each visit of the insect and in some of these forms the 
 wings are unlocked by the insect when the stamens that have been 
 retained in the keel under considerable pressure are liberated by 
 the dropping of the keel and spring up, scattering the spores with 
 an explosive effect. 
 
 In many of these plants the pod splits at maturity into two 
 valves, each of which twists with a sudden snap in opposite direc- 
 tions, hurling the seeds to a considerable distance (Fig. 317, A). 
 This reaction is due to the tension set up by the difference in 
 the drying of the three strata of cells which compose the walls 
 of the pod. The Papilionaceae contain very many nutritious 
 food plants as the pea, bean and lentils, and also forage plants 
 as the clover, alfalfa and vetches. In temperate regions the 
 majority of the forms are herbaceous, only a few trees and 
 shrubs as the locust (Robinia), broom (Cytissus), dye weed 
 (Genista), false indigo (Amorpha) belong to the family, but 
 the tropics are richly represented by a great variety of woody 
 forms that are valuable for lumber, resins and dyes, as the 
 red sandalwood, licorice root, gum tragacanth, balsam of tolu. 
 
DEVELOPMENT OF PLANTS 
 
 455 
 
 indigo, etc. Related to the pea family are the curious vSensitive 
 plant {Mimosa) and the acacias. One of the species of the 
 Acacia develops enormous spines that are tunnelled by ants 
 that were supposed 'to protect the trees against leaf-destn)>ing 
 insects. Gum arable is obtained from African and Australian 
 species of Acacia. 
 
 One of the most interesting features of the Rosales is the. 
 variety of changes to which the receptacle and ovaries are subject 
 in the ripening of the fruit. In the majority of cases, the pistil 
 ripens as an akene or follicle (splitting along one side to free 
 
 Fig. 317, Fruits of the pea family: .1, fruit or pod of the ground nut, 
 showing the manner of seed dissemination by the snapping back of the vah'es 
 with a twisting motion. B, fruit or lomentum of tick-trefoil {Meihomia) , 
 The lomentum breaks into as many nut-like parts as there are seeds in the 
 fruit. C, hooked bristles on the surface of lomentum. 
 
 the seeds) or as a pod, without any considerable modification in 
 the form of the ovary. In many instances, however, this growth 
 is attended with pronounced alterations of the parts as in the 
 currants and gooseberries where the receptacle forms the ovary 
 and the entire structure becomes succulent, forming a berry. 
 The same organs in the witch-hazel develop into a horn-like 
 capsule, the walls of which, after splitting open, contract and 
 pinch out the hard, smooth seed with great force. In the rasp- 
 berries, the ovaries are transformed into drupes which may be 
 lifted off from the convex receptacle like a thimlile, while in 
 
456 SAPINDALES 
 
 the blackberries, the drupes remain attached to the receptacle 
 which becomes the most edible portion of the fruit. The at- 
 tractive portion of the strawberry is the enlarged succulent re- 
 ceptacle while the objectionable hard particles on its surface are 
 the akenes. The receptacle surrounds the pistils in the agrimony 
 and rose, but in the former genus it is hard and covered with 
 hooked bristles for distribution of the nut-like fruit, while in the 
 rose it is fleshy and often brightly colored, serving in some cases 
 as food for birds and thus effecting seed distribution (Fig. 313, 
 B, D). The most pronounced changes are seen in the apple and 
 plum families. In the former group, the carpels become tough, 
 forming the "core" (Fig. 314, C) and the edible part is the 
 greatly enlarged receptacle. The plum, cherry, peach, etc., rep- 
 resent the ovary alone, the cup-like portion of the receptacle is 
 cast off as the fruit matures and the ovary becomes a drupe 
 (Fig. 314, F). In the pea family, the ovary usually becomes 
 a pod with elastic valves but it is also various^ modified. In 
 the peanut, after the withering of the flower, the pistil is thrust 
 into the ground where it develops as a pod that does not open 
 at all. In other genera, the pod is nut-like as in the clovers, 
 spirally coiled in alfalfa, or separable into nut-like joints that 
 are provided with hooks as in the tick trefoil {Meihomia) (Fig. 
 317, B). Doubtless these methods of distribution, the well- 
 developed seeds with their exceptionally firm integuments, the 
 symbiotic relation of these plants with the bacteria (page 161) 
 and the elaborate mechanism of the iregular flowers have been 
 the causes that have led to the abundance and wide distribution 
 of these plants. It will be noticed in the following orders as in 
 the preceding Orchidales, that the irregular and more highly 
 constructed, flowers are generally represented by a great number 
 of genera and, barring some weakness, as for example, the poorly 
 developed embryos of the orchids and their peculiar habitats, by 
 a great number of individuals in each species. 
 
 145. Sapindales, the Soapberry Order. — This group includes 
 principally shrubs and trees, as the box, sumac {Rhus), smoke 
 tree (Cotinus), holly (Ilex), burning bush (Euonymus), climb- 
 ing bittersweet (Celastrus), maple (Acer), horse-chestnut and 
 
DEVELOPMENT OF PLANTS 
 
 457 
 
 buckeye {Aesculus), etc. The llowers are more coninionl>' small, 
 of a yellow-green color and varioUvsly grouped into inflorescences 
 (Fig. 318, A). We noticed in the rose order as the dominant 
 characteristic the tendency towards the development of the basal 
 region of the receptacle which was associated with the checking 
 of its apical growth. In the Sapindales a similar shortening 
 of the axis without its basal growth leads to quite a different 
 series of variations. As a result of the 'crowding of the parts 
 of the flower upon the shortened receptacle, the distinguishing 
 
 Fig. 318. A common form of the Sapindales: A, inflorescence of sugar 
 maple {Acer saccharum) — s, staminate flowers: p, pistillate flowers. B, 
 staminate (right-hand) and pistillate flower enlarged. C, section of a pis- 
 tillate flower, showing the two sterile stamens and nectar disc at base of fila- 
 ments. D, section of ovary, showing early development of the wings of the 
 fruit and the perpendicular ovules with micropyle pointing down. 
 
 features of the order are seen in the reduction of the number 
 of the organs of the flower, a tendency towards the c\clic arrange- 
 ment of parts and an adhesion of the carpels. For example, 
 the sepals and petals are four to five in number, usually distinct 
 and the corolla may be entirely suppressed. There are two 
 whorls of stamens, five to eight in luiniber, rarely ten, while 
 the pistils are reduced in number, ranging from two to more 
 commonly three or rarely five. Thus we see that the spiral 
 
458 THE SAPINDALES 
 
 type of flower so often noticed in the rose and preceding orders 
 has become reduced as a rule to the cyclic type and that five- 
 numerous sets of organs are not of common occurrence. This 
 reduction of the flower and the suppression of parts is well 
 illustrated in the maples (Fig. 318). The sepals form a five- 
 lobed calyx. The petals are often suppressed and a nectar disc 
 is developed outside the stamens, a distinguishing feature of the 
 order (Fig. 318, C). The two whorls of stamens are suppressed 
 to a varying degree so that from four to eight commonly appear 
 and the pistils are normally reduced to two. More frequently the 
 flowers are imperfect, either the pistils or stamens of each 
 flower being aborted (Fig. 318, B). These two kinds of imper- 
 fect flowers are arranged on the same or different trees and they 
 are adapted to small short-tongued lapping insects which visit 
 these open types of flowers. The intelligent long-tongued bees 
 and butterflies generally avoid such flowers, having learned by 
 experience with colors and odors that a surer supply of food is 
 to be found in those flowers that conceal their nectar and so 
 exclude the promiscuous crowd of insects that swarm about the 
 simpler types. It should be stated that the development of 
 imperfect or incomplete flowers appearing in many of the orders 
 is not to be looked upon as a primitive condition, though common 
 in the lower monocotyledons and dicotyledons, since, throughout 
 the Angiospermae, forms will constantly appear in which one or 
 another set of organs fails to develop. The formation of wind 
 pollinated flowers, however, as in the box maple, is a return to a 
 primitive condition. 
 
 The stimulation of fertilization results in a green wing-like 
 outgrowth on each of the ovaries that assists at first in the manu- 
 facture of food for the embryo and later becomes a dry, mem- 
 branous organ for seed distribution. This fruit, known as a 
 schizocarp or samara, is at first partly loosened from its support 
 and remains attached only by a small stalk (Fig. 319, B), which 
 requires a rather strong wind to snap it. Thus the fruit is 
 freed under conditions that will result in the widest dissemina- 
 tion of the seed. The development of the chlorophyll-bearing 
 tissue in immature fruits to assist in the work of food produc- 
 
DEVELOPMENT OP^ PLANTS 
 
 459 
 
 tion is an economical arrangement of tissues often to be seen. 
 It is noteworthy that these green fruits are often protected by 
 bitter, acrid juices and poisonous proi)erties that finalh- give place 
 to attractive flavors, odors and colors, variations that are of 
 considerable assistance to seed protection and distribution. 
 
 Fig. 319. Fruit of the maple: A, mature fruit of red majile (Acer ruhruju). 
 B, schizocarp of Norway maple. The fruit split in half is still attached to 
 the receptacle by delicate stalks. 
 
 Many of the members of this order contain acid or poisonous 
 juices, as in the scarlet fruit of the sumac, which were a source 
 of acetic acid to the early settlers of this country, or poisonous 
 oils, as in the poison ivy or poison oak {Rhus radicans) and the 
 poison sumac {R. Vernix). The former species is a climbing 
 vine or sometimes a shrubby plant with leaves divided into three 
 leaflets and with nut-like fruits (Fig. 320, B), while the ix)ison 
 sumac is an erect coarse shrub ten to fifteen feet high, with large 
 pinnate leaves with reddish petioles and fruit clusters, as in the 
 poison ivy (Fig. 320, A). Both of these plants contain volatile 
 oils that cause the poisoning. The oil ma>- readily be removed 
 by washing in water containing baking soda, which saponifies the 
 oil, or in alcohol which dissolves it. With alcohol the washing 
 must be thorough in order not to spread the infection. Applica- 
 tions of alcohol containing sugar of lead (50 or 70 per cent, 
 alcohol) are also recommended, which treatment is followed by 
 
460 
 
 GERANIALES— RHAMNALES 
 
 thorough washing in alcohol. As In the preceding orders, the 
 lower members of this group are characterized by a regular alter- 
 nation of the members of each whorl, but in the higher types the 
 sets of organs vary in number and the flowers become irregular, 
 as in the horse-chestnut and balsam weed or touch-me-not. 
 
 146. Orders Suggestive of the Sapindales. — The geranium 
 order, Geraniales, shows essentially the same type and range of 
 variation in the flower as the Sapindales, but there is this rather 
 singular difference, namely, that the ovules, usually pendulous, 
 
 Fig. 320. Two poisonous species of the Sapindales: .4, Rhus vernix, poison 
 sumac. B, Rhus radicans, poison ivy. 
 
 are always turned away from the axis of the ovary with micro- 
 pyle directed upwards, while in the Sapindales the opposite ar- 
 rangement is to be found, micropyle pointing down (Figs. 318, D\ 
 321, D). This order ranges from the regular flowers of the 
 oxalis, flax and geranium families to irregular forms like the 
 nasturtium, milkwort {Polygala), etc. Many of these plants are 
 known by their peculiar juices, oils, gums, as in the spurges 
 {Euphorbia), castor bean (Ricinus), citron, lemon, orange, etc. 
 The fruit of the lemon and orange is a berry in which the outer 
 part of the ovary becomes leathery. The juicy pulp, which en- 
 velops the seeds, is formed from hair-like structures that arise 
 on the inner side of the carpellary walls and by degrees entirely 
 fill them. The navel orange is a chance variation in which a 
 
DEVELOPMENT OF PLANTS 
 
 461 
 
 secondary receptacle, bearing coni})artmenls like the normal fruit, 
 is introduced. This never reaches very large dimensions and 
 may readily be seen at one end of the orange. The original 
 stock from which the navel oranges grown in our country have 
 been derived, was obtained from Brazil. 
 
 The Buckthorn order, Rhamnales, is very closely allied to the 
 
 Fig. 321. Flowers of the Geraniales and Rhamnales: A, inflorescence 
 of Geranium. B, nearly mature fruit consisting of five united carpels. C, 
 discharge of the seed. The carpels snap apart, owing to the tension set up by 
 the drying out of the tissues. D, section of pistil, showing the ovules with 
 micropyle directed upwards. E, flower of the grape (Vitis), a common form 
 of the Rhamnales — ca, calyx, reduced to a rim; c, corolla, which opens at 
 base and falls off as a cap; n, nectar glands within stamens. Compare Sapin- 
 dales. F, corolla free from the receptacle. G, flower freed from corolla, 
 ovary in section, showing micropyle pointing down as in Sapindales. 
 
 Sapindales and includes such familiar plants as the buckthorn 
 (Rha^nmis) , Jersey tea (Ceanothiis), used in revolutionary times 
 as a substitute for tea, the grape ( Vitis), Japanese ivies, \'irginia 
 creeper {Parthenocissiis), etc. The minute green or white flower 
 shows a further reduction in parts, the petals not only being fre- 
 quently suppressed, but the stamens are reduced to one whorl 
 
462 
 
 THE MYRTALES 
 
 and placed opposite the sepals, whereas they alternate with the 
 sepals in the Sapindales (Fig. 331, E-G), and the nectary is 
 within the filaments. 
 
 147. Myrtales, the Myrtle Order. — This order marks a decided 
 advance in the evolution of the flower over preceding groups, 
 owing to the mass growth of the calyx and receptacle and usually 
 of the ovary, the flowers being perigynous and usually epigynous. 
 Another important character is that the flowers are always cyclic, 
 the organs being regularly arranged in five whorls of four or less 
 
 Fig. 322. Lower form of the Myrtales, flower perigynous: A, flower of 
 Lythrum. B, flower in section — c, lobe of the tubular calyx, from the margin 
 of which arise the petals, p. This flower has the long form of style and the 
 short and medium form of stamens. C, flower with medium style and with 
 short and long stamens. D, flower with short style and with medium and 
 long stamens. 
 
 commonly of five members each, though the number of stamens 
 may be greatly increased by the splitting of the original number 
 of the set. The petals are frequently suppressed and the calyx 
 is often highly colored, a variation to be seen in many groups when 
 the corolla is wanting. The carpels, as a rule, contain numerous 
 ovules (Figs. 322, 323). The Myrtales is an important tropical 
 order and represented with us by many familiar plants, such as 
 the meadow beauty {Rhexia), willow-herb {Epilobitim), the large 
 
DEVELOPMENT OF PLANTS 463 
 
 evening primrose family, with one of \vhieh, Oowthcra (I-'ig. 323, 
 A, B), deVries worked out liis theory of mutation, and the 
 common ditch and pond aquatics — the water milfoil (Myrio- 
 phyllum) and mermaid weed (Froserpinaca). 
 
 The purple loosestrife {Lythrum) illustrates the characters of 
 the lower meml)ers of the order before ei)ig\ny has heccjme estab- 
 lished (Fig. 322). The wand-like branches of this introduced 
 plant with their terminal spikes of purple flowers are becoming 
 rather common around the borders of marshes and water ways. 
 The cup formed by the calyx and receptacle bears on its rim 
 alternately with calyx lobes the rather twisted petals and at its 
 base eight to twelve stamens. The pistil is composed of two 
 carpels. This flower has become historic because of the atten- 
 tion that has been given to its devices for crossing. Fig. 322 
 shows that the sporophylls are of three lengths, short, medium 
 and long, an arrangement called heterostyly. It can readily be 
 seen that the three different lengths of the styles correspond 
 exactly with the position of the anthers, consequently whatever 
 part of the proboscis or body of the bee or butterfly in \'isiting 
 these flowers comes in contact with one of the sets of open 
 anthers, exactly the same region of their bodies will touch the 
 stigmas as soon as they visit another flower with a corresponding 
 length of style. The spores from the three lengths of stamens 
 differ in size and color, and it has been demonstrated that better 
 results follow when crossing is effected between sporophylls of 
 the same length, i. e., long with long, short with short, etc. 
 Heterostyly arose in many of the orders, as among some of the 
 knotweeds, buttercups and rose families and in the gentians, 
 primroses, forget-me-not, etc. 
 
 A higher and more characteristic t>'pe of the order is seen in 
 the Oenothera and in the great willow-herb {Clmniaoicrion, Fig. 
 323). This latter plant flourishes in rather dry soils, forming- 
 large colonies by its underground stems and gaining in con- 
 spicuousness through its terminal racemes of large deep purple 
 flowers. The parts of the flower are in fours. The receptacle 
 forms the ovary and the mass growth of perianth and stamens 
 results in the development of a long tube which forms at its 
 
464 
 
 THE MYRTALES 
 
 mouth the Hnear segments of the calyx alternating with the 
 round spreading petals and the stamens. At the time of the 
 opening of the flower the eight anthers are shedding their spores 
 and are in line with the nectaries, while the lobed stigma is 
 closed and bent backward (Fig. 323, C, s). A day later the 
 stigmas assume the position shown in the lower flowers, the lobes 
 curving backward so as to lie in the pathway leading to the 
 nectaries (Fig. 323, C, 0). It is evident that these positions 
 
 Fig. 323. Higher forms of the Myrtales, flowers epigynous: A, flower 
 of Oenothera — 0, ovary. B, enlarged sectional view of flower, showing the 
 receptacle enveloping the ovules. The receptacle grows above the ovary, 
 forming a tube, f, that bears at its summit the lobes of the calyx, c, the petals, 
 p, and stamens. C, inflorescence of the great willow herb (Chamaenerion) 
 — s, closed stigma in young flowers; 0, opened stigmas in older flowers; a, 
 stigma touching anthers in withered flowers. D, capsule opening and dis- ^ 
 charging the seeds. E, a seed enlarged. — C after Kerner, 
 
 must necessitate a crossing if the flowers receive the proper vis- 
 itors. If, for any reason, crossing should fail autogamy results, 
 owing to the continued curvature of the stigmas, which are finally 
 brought into contact with the anthers, in which work the positions 
 of the flowers and stamens cooperate, as shown in the lowest 
 
DEVELOPMENT OF PLANTS 465 
 
 flower (Fig. 323, C, a). The microspores are fastened together 
 in threads by a sticky substance, viscin, so that they adhere to 
 the insect's body and are drawn out from the anthers in net-Hke 
 skeins. This may be tested by applying the moistened finger to 
 the opened anthers; this is also true of the related plants, as the 
 fuchsia, evening primrose, firewood {Epilobium), enchanter's 
 nightshade (Circaea). 
 
 The numerous seeds developed in the capsule are provided 
 at the end with tufts of long white hair (Fig. 323, D, £), which 
 readily transport them and explains the quick appearance of these 
 plants in forest lands that have been devastated by fire or cutting. 
 In addition to the native plants mentioned above, many well- 
 known tropical plants are represented in this order, as the man- 
 grove of our southern swamps and the large myrtle family, often 
 characterized by leathery leaves and aromatic oils. This family 
 furnishes the clove, which is a flower bud, the pomegranate, 
 guava, bay rum, the Brazil nut and the eucalyptus trees, some of 
 which (Australian) attain the greatest height of any tree, over 
 "our hundred feet, with a diameter of twent^'-four feet. 
 
 148. Umbellales, the Carrot Order. — This order marks the 
 consummation in the reduction and mass growth of parts that we 
 have seen steadily progressing through the various orders of the 
 Choripetalae. Notice that the strictly epig^mous, cyclic flowers 
 ^Fig. 324, C, D) are now reduced to four whorls, the five lobes 
 of the calyx alternating with the five petals and these in turn 
 with the four to five stamens. The reduction also appears in the 
 ;Distil which usually consists of two carpels that contain but a 
 
 >ingle ovule each. The order contains two rather small families, 
 ■he ginseng family (Araliaceae) represented by the sarsaparilla 
 or spikenard (Aralia), the ginseng {Panax) and the dogwood 
 
 amily (Cornaceae), including the pepperidge or sour gum 
 trees {Nyssa), dogwood or cornel {Coruus); and the large carrot 
 family (Umbelliferae) of 2,100 species. The carrot family 
 contains many familiar native and cultivated plants which may 
 be recognized by the hollow internodes of the stems, leaves vari- 
 
 )usly lobed and attached by conspicuous sheathing petioles, 
 
 peculiar odors derived from oils and resins, small flowers that are 
 30 
 
466 
 
 THE UMBELLALES 
 
 usually white or yellow and grouped into flat-topped inflores- 
 cences (umbels), which are usually surrounded by bracts, called 
 the involucre (Fig. 324, A, B). These features are well seen in 
 the wild carrot which has become a troublesome weed in pasture 
 lands and meadows. The calyx lobes are very small, a feature 
 likely to be seen in any epigynous flower. The incurved petals 
 alternate with the five stamens and in the center of the flower, the 
 
 vi^ -'^r-}^^ *V'''^ 
 
 
 Fig. 324. A common form of the Umbellales: A, stems of wild carrot 
 (Daucus) with flowers arranged in compound umbels. B, inflorescence with 
 all the umbels removed but one — in, bracts of the involucre. C, forms of 
 flowers, the one on the right being a flower from the margin of the umbel. 
 The corolla is enlarged and irregular, thus adding to the conspicuousness 
 of the inflorescence. D, corolla partially removed to show the epigynous 
 character of the flower and the cushion-like nectary at the base of the styles; 
 0, ovary. E, fruit splitting into two nutlets. F, portion of hollow stem 
 with sheathing of leaf base. 
 
 two Styles broaden out into a conspicuous nectary above the 
 ovaries. The fruit is a schizocarp, splitting into two nut-like 
 parts after the manner of the maple (Fig. 324, E). The flowers 
 
DEVELOPMENT OF PLANTS 467 
 
 on the outside of the umbel become somewhat irregular, (Aving to 
 the unequal enlargement of the outermost petals. These small 
 flowers that individually cannot be seen at a distance of a few feet 
 become the most conspicuous feature of our fields owing to their 
 association in large numbers into compact, lace-like um])els. The 
 aggregation of flowers has been noticed in various groups, as in 
 the mustards, various families of the Rosales, in the Sapindales 
 and buckthorns, of which latter simple regular type the carrot 
 flower would appear to be a natural sequence (compare Figs. 
 321, G; 324, D). But in no group has so successful and varied 
 an arrangement of the flowers been achieved. The advantages 
 of this umbellate arrangement are apparent. The crowding of 
 the flowers is attended with the reduction in their size and in 
 saving of material while they have gained in the number of 
 flowers and in conspicuousness. This arrangement also increases 
 the chances of seed production since the insect in a single visit 
 may cross a score of flowers in crawling over the umbel. Autog- 
 amy is prevented at first by the difl"erence in the maturation of 
 the anthers and stigmas, frequently one or more days interven- 
 ing between these two conditions and in many genera the flowers 
 are imperfect, the stamens, and pistils being arranged on difi"er- 
 ent parts of the umbel and so further assist in crossing. But 
 even if the great variety of small lapping insects that swarm over 
 these flowers were absent the construction of the umbel is such 
 that the flowers are usually able to effect pollination unassisted. 
 The variety of devices for the accomplishment of this work is 
 without parallel in any other group. The opening of the flowers 
 of an umbel proceeds either from the circumference towards 
 the center or from the center outwards. In some genera, as 
 Eryngium, the flowers on the margin open first and the stigmas 
 are ready for crossing while the anthers are closed and bent 
 down upon the petals. On the following day, the inner adjoining 
 set of flowers is in the same condition while the anthers of the 
 marginal flowers have been lifted up by their filaments so that 
 they reach out and come in contact with the stigmas of the ad- 
 jacent inner flowers, the curvature of the styles often assisting in 
 bringing the two organs together. In this way, ample oppor- 
 
468 
 
 THE UMBELLALES 
 
 tunity for crossing with the older flowers of another umbel is 
 first given and later a crossing between the adjacent flowers of 
 the same umbel is almost sure to result. In other cases, as the 
 snakeroot {Sanicula) a few of the simple flowers are perfect 
 while the remaining flowers of the umbel bear only stamens. 
 The perfect flowers are the first to open, being in the same con- 
 dition as the marginal flowers of the preceding example. Later 
 the filaments straighten out, curve away from the stigmas and 
 the anthers discharge their spores and finally drop off. Now the 
 adjoining flowers open and extend their filaments so that the 
 anthers are pushed over to the stigmas of the perfect flowers. In 
 the chervil, a similar crossing is effected after the perfect flowers 
 have shed their anthers by the numerous imperfect flowers grow- 
 ing above them and extending their anthers so that the micro- 
 spores will drop straight down upon the stigmas (Fig. 325). 
 
 Fig. 325. Flowers of chervil: A, perfect flowers only in bloom. The 
 anthers are discharging their spores before the stigmas are receptive. B, 
 later stage — the staminate flowers are now in bloom and shedding their spores 
 upon the mature stigmas of the perfect flowers. — After Kerner. 
 
 In the water parsnip {Stum), and beaked parsley (Anthricsus) , 
 etc., there are two kinds of umbels, one containing principally 
 perfect flowers and the other only staminate flowers. The per- 
 fect flowers blossom first, and shed their spores and anthers be- 
 fore the stigmas are receptive. After all the anthers have been 
 shed, the stigmas mature and continue in this condition for two 
 days, during which time the umbels of imperfect flowers have 
 grown above them and assume such a position that when the 
 anthers finally open there results a rain of microspores upon the 
 stigmas below. 
 
 The members of the carrot family are characterized by the 
 
DEVELOPMENT OF PLANTS 469 
 
 presence of essential oils and resins in the roots, stems, leaves or 
 especially in the fruits which usualK' contain oil cavities between 
 the ribs of the ovary. These substances are the sources of the 
 peculiar odors that make easy the recognition of these plants. 
 These oils give the commercial value to several fruits, as in the 
 caraway, parsley, fennel, anise, coriander. Se\-eral species con- 
 tain very poisonous alkaloids, as the water hemlock (Ciciita), 
 among the most poisonous of our native plants, and the hemlock 
 {Conitim) which perhaps furnished the potion drunk by Socrates. 
 Several valuable food plants, as the carrot, parsnip, celer}', par- 
 sley, are cultivated species of the family. 
 
 Series h. Synipetalae. 
 
 149. General Characteristics. — This group contains the most 
 common and abundant of our flowering plants, approximating 
 42,000 species, and ranks as the most specialized of the angio- 
 sperms. The variations of these plants have been very success- 
 ful, especially to be noticed are the development of underground 
 stems, special types of flower structures and the massing of the 
 flowers in dense inflorescences. As a result of these advantages,, 
 they exceed all other groups in the number of individuals, though 
 they comprise fewer orders as a consequence of the striking 
 uniformity in the floral structure. 
 
 While a few of the simpler meml)ers of this group have dis- 
 tinct petals, the crowding of the organs on the receptacle has 
 resulted, in nearly all the forms, in the sympetalous type' of 
 corolla, in the cyclic arrangement of the parts, and in the reduc- 
 tion in the number of stamens, so that each whorl does not exceed 
 the number of petals, and more frequently one whorl of stamens 
 is partly or entirely suppressed. The pistils are usually less 
 numerous than the petals. The flowers range from regular 
 hypogynous forms to epigynous and irregular l\pes through the 
 same series of variations, as noted in the Choripetalae. The 
 Sympetalae are the most recently evohed of the Angiospermae, 
 as is apparent from their uniformity of structure and they 
 appear to be adapted to temperate and northern conditions where 
 they have preempted the open country and flourished exceedingly, 
 
470 
 
 ERICALES 
 
 forming the most characteristic features of the herbaceous flora. 
 Aquatics are of rare occurrence and but few tree forms appear; 
 notably the ash, persimmon, catalpa, paulownia, etc. Heath- 
 hke shrubs, however, are widely distributed in northern regions. 
 The invasion of the tropics w4th its favorable conditions has led 
 to an enormous increase in some of the groups and the develop- 
 ment also of a great variety of woody plants, as trees and climbers. 
 150. Ericales, the Heath Order. — -This order is the simplest of 
 the Sympetalae and includes a great variety of plants that are 
 largely northern in their distribution. Many are cultivated, as 
 the azaleas, rhododendrons and laurels (Kalmia), and others are 
 familiar plants of bogs and woods as the sweet-pepper bush 
 
 Fig. 326. A simple form of the Ericales: A, the shin leaf (Pyrola) with 
 flower just opened. B, flower with part of perianth removed to show the 
 retention of the anthers by the petals. C, later stage of flowering. The 
 anthers have been released by the spreading of the petals and the flower 
 stalk has inclined so that the spores sprinkle down on the stigma. D, flower 
 in section, showing the relation of parts in autogamy. — After Kerner. 
 
 (Clethra), wintergreen (Chimaphila), Labrador tea (Ledum), 
 Rhodora, Leucothoe, rosemary (Andromeda), leather leaf (Cham- 
 aedaphne), arbutus (Epigaea), checkerberry (Gaultheria) , bear- 
 berry (Arctostaphylos) , huckleberry (Gaylussacia) , blueberry 
 (Vaccinium), heather (Calluna), cranberry (Oxycoccus). The 
 flowers of the heaths are very characteristic and sharply dis- 
 
DEVELOPMENT OF PLANTS 471 
 
 tingulshed from other orders. More'^frequenll}' they consist of 
 five regular whorls of five members each, anthers usually opening 
 by pores and often provided with two horn-like appendages, and 
 the pistil is composed of five carpels, the ovary maturing as a 
 capsule or berry (Fig. 326). Tlie separate origin of each \\ liorl fjf 
 organs is an important feature of the order as contrasted with 
 other sympetalae, each set being as a rule separately attached to 
 the receptacle. While the sympetalous corolla is a step in ad- 
 vance of previous types, it is evident that these regular hypogy- 
 nous flowers are in other respects of a rather simple character. 
 This feature is further illustrated in some of the genera like 
 Clethra, Monotropa and Pyrola where the free or slightly fused 
 petals form a very natural transition from the Choripetalae 
 (Figs. 327, A\ 326). 
 
 The simpler forms of flowers are illustrated in Pyrola with 
 its regular hypogynous and polypetalous flowers, though some 
 species show a slight tendency to sympetaly. The stigmas are 
 mature with the opening of the flower while the anthers are 
 bent back out of the way, their filaments being held under con- 
 siderable tension by the petals (Fig. 326, A, B). A bee laden 
 with spores would efi"ect a crossing as soon as he alighted upon 
 the flower, but in probing after the nectar he presses back the 
 petals, thus releasing the anthers which snap down, spilling the 
 spores upon him. Doubtless autogamy results in many of these 
 forms if crossing fails owing to the expansion of the petals 
 which would release the stamens, and the drooping of the flower 
 which would bring the falling spores in line with the stigma 
 (Fig. 326, C, D). 
 
 Other genera show varying degrees of sympetaly but in the 
 majority of cases, tubular corollas appear (Fig. 327, C) as in the 
 Leucothoe, bearberry, Andromeda, blueberries, etc. The majority 
 of such forms have horned anthers which are shaken by the 
 insect and so assist in sifting out the spores. In the blueberry 
 family, we note that a mass growth of the receptacle and o\-ary 
 has begun (Fig. 327, D) while in Rhodora, rhododendron and 
 azaleas another line of variation appears in the irregular corolla 
 (Fig. 327, F). In the two latter genera, the styles and stamens 
 
472 
 
 THE ERICALES 
 
 extend far out, furnishing a natural landing place for hovering 
 animals as moths and humming birds. In this position the 
 spores cannot be sifted out and we find them fastened together 
 In fours, just as they were formed In the mother cell, by sticky 
 threads which cause them to adhere on the insect's body in 
 fringe-like masses (Fig. 327, D). One of the lower basidiomy- 
 cetes (Exobasidium) infests the leaves and flowers of the azalea 
 and other genera, causing large watery outgrowths that are 
 
 Fig. 327. Common forms of Ericales: A, Indian pipe {Monotropa). 
 Leaves reduced to scales and without chlorophyll, owing to saprophytic 
 habit of plant. B, section of flowers, showing polypetalous corolla. C, 
 inflorescence of the blue berry {Vaccinium). D, section of flower, showing 
 sympetalous corolla and epigynous type of flower. E, a stamen enlarged, 
 opening by pores at end of anther. F, azalea, showing irregular type of flower. 
 G, spores attached by viscid threads. 
 
 often eaten under the impression that they are the fruit of the 
 plant. The members of this order are adapted to a wide range 
 of conditions though more characteristic of north temperate and 
 
I 
 
 DEVELOPMENT OF PLANTS 473 
 
 arctic regions where they constitute the conspicuous features of 
 the tundras, bogs, heaths and moors. Tliese plants are often 
 characterized by their thick leathery, evergreen leaves of pro- 
 nounced xerophytic character and their common distribution 
 in wet moors and bogs presents a most puzzling problem in the 
 association of plants. Many of the heaths have been cultiva<-ed 
 from very early times and they have lent themselves so readily 
 to crossing that perhaps no order furnishes so many forms for 
 decorative and landscape effects. Several yield valuable fruits, 
 as the blueberries, huckleberries and cranberries. 
 
 151. Orders of a Higher Type than the Ericales. — The prim- 
 rose order (Primulales) forms a very natural transition from the 
 Ericales. As the next step in advance, we note the slight associ- 
 ation of the stamens and the corolla (Fig. 328, A) and their 
 frequent reduction in number. The occurrence of vStaminodia, 
 page 421, in certain forms marks the first transition to the 
 reduction of the number of stamens. The pistil is generally 
 composed of five carpels forming a capsule without partitions 
 (unilocular) and containing a central placenta, bearing numerous 
 seeds, that is attached only to the base of the ovary (Fig. 328, A). 
 This is the so-called free central placenta and only occurs else- 
 where in the small bladderwort family of aquatics. This order 
 includes the loosestrife {Lysimachia and Steironema), star flower 
 (Trientalis), shooting star (Dodecatheon) and cultivated forms of 
 the cyclamen and primroses. 
 
 Proceeding to the gentian order (Gentianales), it will be 
 noted that the flowers are variable and not as clearly character- 
 ized, the corolla being sometimes wanting or pol>petalous. The 
 reduction of the stamens to a single whorl and the pistils usually 
 to two in number indicates points of advance that are to be 
 associated with the twisting of the petals in the bud and the 
 opposite arrangement of the leaves as distinguishing features 
 of the order (Fig. 328, B-D). Note should be made of the fact 
 that the two pistils are often quite distinct, doubtless a sur\i\al 
 of a more primitive condition. The Gentianales include the ash 
 {Fraxinns), lilac (Syringa), Forsythia, olive {Olea), valued for 
 fruit and oil, fringe tree (Chionanthus), privet {Ligustnim), jas- 
 
474 
 
 THE POLEMONIALES 
 
 mine, Strychnos, yielding the alkaloid strychnine, oleander 
 (Nerium), the gentians, marsh pink (Sabbatia), the aquatic float- 
 ing heart {Limnanthemiim) and the highly specialized plants of 
 the dogbane and milkweed families. The latter family is dis- 
 tinguished by the microspores being united into club-shaped 
 poUinia that are attached, one from each of the two adjoining 
 anthers, to a peculiar hook, so that the leg of the insect becomes 
 caught in the hook and withdraws the poUinia in pairs. 
 
 m. 
 
 
 
 
 ^ ^k m 
 
 Fig. 328. Primulales and Gentlanales: A, flower of loosestrife {Steiro- 
 nema). Above flower in section, showing cohesion of single row of stamens 
 to base of corolla and the numerous ovules on a free central placenta. These 
 are the most important characteristics of the order. B, fringed gentian 
 {Gentiana), showing leaf arrangement and twisting of petals in bud, b, char- 
 acteristics of the order. C, section of flower. D, fruit, the two carpels 
 separated. 
 
 152. Polemoniales, or Phlox Order. — This group is the richest 
 in the number of species, over 14,600, of any order of angio- 
 
DEVELOPMENT OF PLANTS 
 
 475 
 
 sperms. The advance over previous orders is seen in the more 
 prolonged mass growth of the corolla and stamens, the filaments 
 appearing to rise at a higher point on the corolla (Fig. 330, D) 
 and the stamens are reduced to a single whorl, and frequently less 
 than five in numl^er. The pistils are completely compound and 
 usually composed of but two carpels. This reduction is asso- 
 
 FiG. 329. Fig. 330. 
 
 Fig. 329. Flower of the morning-glory (Ipomoea), showing the tubular 
 corolla characteristic of the Polemoniales. 
 
 Fig. 330. Boraginaceae: C, inflorescence of comfrey (Symphytum). Note 
 the coiled inflorescence, a, a feature of this family, D, section of flower, 
 showing the deeply four-lobed ovary and the stamens cohering high on the 
 corolla and alternating with small tongue-like scales. 
 
 ciatcd with a pronounced irregularity of the corolla in the higher 
 families and a high degree of specialization in the construction 
 of the flower which in part accounts for the occurrence of the large 
 number of individuals. The order is noted for its great number 
 of showy flowers and the large tubular corollas which attain their 
 highest perfection in several of the families. 
 
 (a) The More Important Families of the Phlox Order. — The 
 flowers of the lower families are regular, as in the morning- 
 glory, sweet potato, dodder (Cnscuta), a yellow thread-like para- 
 
476 THE POLEMONIALES 
 
 site that twines about various plants, and the phlox (Fig. 329). 
 The flowers of the rough-leaved borages (Boraginaceae) are 
 also regular but they are distinguished by having the ovary of 
 the two carpels deeply lobed so that the fruit appears as four 
 nutlets and also by their coiled Inflorescences as shown in Fig. 
 330. This family includes the heliotrope, hound's-tongue {Cyno- 
 glossum), forget-me-not (Myosotis), comfrey {Symphytum), and 
 the blueweed (Echium) in which the corolla becomes irregular. 
 The mint family, Labiatae, is world-wide in its distribution 
 and the largest of the order with 3,000 species. These plants 
 are sharply characterized by square stems, opposite leaves, aro- 
 matic oils, nut-like fruits as in the borages, but the corolla is 
 very irregular and usually two-lipped. The stamens, usually 
 two short and two long ones, together with the two lobed style 
 are more frequently concealed under the upper lip of the corolla 
 (Fig. 331). The flowers are generally so placed that the lower 
 lip serves as a landing place for insects and in entering the 
 flower, the top of their bodies rubs against the stigmas or anthers. 
 Crossing Is secured by the difference in the time of the matura- 
 tion of the anthers and stigmas, assisted by a variety of move- 
 ments, such as the alternate curving down of the anthers and 
 stigmas which brings first one set of organs and then another 
 into the pathway of the insect. One form of this arrangement 
 is seen In the sage, where there are but two strangely modified 
 stamens, the two anther lobes being connected by a long-curved 
 rod which is fastened near one end to a short filament so that 
 the anther becomes a lever with the filament as a fulcrum (Fig. 
 332). Only the anther lobes concealed under the upper lip are 
 fertile. As the bee enters the flower, he pushes up the short 
 arm of the lever, thus causing the long arm to swing down, 
 pressing the fertile anther upon his back and dusting him with 
 spores. Later, the style which is at first immature and above 
 this apparatus, grows out and curves down and at maturity 
 comes to lie in such a position as to rub upon the back of a 
 visiting insect at the same point as the anthers. The peculiar 
 forms of the corolla, the relation of the anthers and stigmas and 
 the frequent occurrence of hairs and bristles that direct the In- 
 
DEVELOPMENT OF PLANTS 
 
 477 
 
 Fig. 331. Fig. 332. 
 
 Fig. 331. A common species of the mint family: A, inflorescence of the 
 skull cap (Scutellaria). Note the square stem, opposite leaves. Why are 
 all the flowers facing one way? B, flower enlarged, showing the two-lobed 
 un:ler lip and the three-lobed upper lip which conceals the sporophylls. C, 
 section of the flower. Ovary four-lobed, stamens cohering with the corolla 
 and anthers concealed with the stigma beneath upper lip. Purpose of the 
 crest, c, on the calyx? 
 
 Fig. 332. Flower of the sage (Salvia): A, flower after the anthers have 
 shed their spores. The two-lobed stigma is bending down into the position 
 occupied by them. B, sectional view of the flower, showing four-lobed ovary 
 with nectar glands at the base, stigma not receptive and bent back. As the 
 insect enters the flower he pushes against the sterile lobe of the anther, /, and 
 thus causes the fertile lobe, a, to swing down upon his back. /, filament of 
 anther. 
 
478 THE POLEMONIALES 
 
 sect with the greatest precision to particular parts of the flower 
 with a view to crossing, mark this family of the mints as the 
 most specialized of the order. A fine series of devices are seen 
 in such common mints as the blue curls {Tricho sterna), skull- 
 cap {Scutellaria), hyssop (Agastache), ground ivy {Glecoma), 
 catmint (Nepeta), selfheal (Prtmella), motherwort {Leo7iurus), 
 dead nettle (Lamium), hedge nettle (Stachys), sage (Salvia), 
 bergamot (Monarda), pennyroyal (Hedeoma), mountain mint 
 (Kolleia), bugle weed (Lycopus), etc. Many of these plants 
 are of commercial importance. Mentha yields valuable oils as 
 spearmint, peppermint, and menthol and from other genera are 
 derived oils used in perfumery, medicine and as condiments, 
 as rosemary, lavender, origanum, thyme. Several are cultivated 
 for their flowers and foliage, as the coleus, monarda, stachys, etc. 
 The large family of figworts, Scrophulariaceae, with its 2,400 
 species, closely resembles the mints, but is distinguished by the 
 ovary being undivided and containing many seeds on a central 
 axis (Fig. 333, C). While the simpler forms have almost regular 
 corollas and five stamens, the higher forms are bilabiate with 
 two to four stamens as in the mints. This family contains some 
 of the most showy of our cultivated plants, as the foxglove 
 (Dasystoma and Digitalis), Gerardia, snapdragon (Antirrhi- 
 fium), rattlebox {RJiinanthiis) , toadflax (Linaria), Paulownia, 
 beardtongue (Pentstemon) , monkey flower {Mimultts) ; and the 
 allied Catalpa and trumpet creeper; also many other attractive 
 plants that are not cultivated as the mullen (Verbascum), fig- 
 wort (Scrophularia) , turtle head (Chelone), hedge hyssop (Gra- 
 tiola), speedwell {Veronica), painted cup {Castilleja) , lousewort 
 {Pedicularis) , cow- wheat {Melampyncm) . These plants are 
 among the most characteristic features of our flora, the family 
 being largely confined in its distribution to the north temperate 
 regions. The flower of the toadflax {Linaria) illustrates a com- 
 mon type of the figwort family (Fig. 333). Such flowers are 
 said to be personate, that is bilabiate but with the under lip 
 arched so as to meet the upper lip and entirely closing the mouth 
 of the corolla. This arrangement very effectively protects the 
 microspores and conceals the nectar and it requires a rather 
 
DEVELOPMENT OF PLANTS 
 
 479 
 
 muscular insect alighting upon tlie knohhcd lower lip to force 
 his way into the flower. It is worth any one's time to sit by 
 this plant and examine the mechanism of the flower while the 
 bee is at work. The sporophylls present the same variety of 
 arrangements for crossing as noted in the mints. The nectar 
 glands are situated at the base of the carpels but the nectar does 
 not remain upon the glands as in the majority of flowers, but 
 runs down through a narrow duct between the filaments and 
 collects in the spur-like prolongation of the corolla (Fig. 333, s). 
 
 f-Aflif-'t' ■^■■:- ~ 
 
 
 \l^: 
 
 Fig. 333. Examples of the figwort family: A, inflorescence of the toad 
 flax {Linaria). B, flower viewed from beneath, showing the under lip arching 
 up against the upper, two-lobed lip — s, nectar spur of the corolla. C, section 
 of the flower viewed from the side, showing the undivided ovary with central 
 ovules. Note the stigma and anthers concealed at the lips of the corolla. 
 D, sectional view of flower of Rhinayithus with the four stamens arranged 
 in pairs. E, appearance of the stamens as viewed from the mouth of the 
 corolla. The filaments have been pressed apart, thus separating the opened 
 anthers. 
 
 The yellow rattlebox {RhinanthuSy Fig. 333, D, E) shows an- 
 other form of the flower, common in this family. The general 
 arrangement of the floral organs is the same as in Linaria, but 
 the anthers are in pairs so that when the>- open, owing to the 
 rigidity of the filaments or the pressure of the upper lip, they 
 
48o THE POLEMONIALES 
 
 are kept tightly pressed together hke a pair of sugar tongs, 
 thus preventing the shedding of the spores. Owing to the form 
 of the corolla and often because of numerous hairs and bristles, 
 the insect is directed in such a definite wa}^ to the flower that he 
 comes in contact with appendages of the anthers, or causes a 
 slight deflection of the rigid filaments or of the lip of the corolla, 
 any of which movements are suflicient to separate the anthers 
 and bring down upon him a shower of spores. These flowers 
 are largely protog>mous so that at first only a crossing is possible. 
 In many species, autogamy results from the downward bending 
 of the corolla and style, accompanied by a loosening of the anthers 
 so that during the last days of flowering the spores may fall 
 directly upon the stigma. 
 
 Many of these flowers are characterized by mottlings and 
 blotches. It appears to be a rather general law that the most 
 highly modified parts are variegated in this manner — compare the 
 mints and orchids. Less specialized flowers present an associa- 
 tion of various colors or veinings, a feature that is illustrated 
 in many families, as violets, peas, geraniums, etc., whereas the 
 simplest types of flowers are usually of a uniform color. The 
 fig^vorts have been called a suspicious group, and if not actually 
 poisonous, none at least serve as food. Several are medicinal, 
 as Digitalis, Veronica, Gratiola, etc. Many are parasitic, as the 
 foxglove, Gerardia, eyebright (Euphrasia) and the related and 
 reduced brownish rapes (Thalesia and Orobanche), beech drops 
 (Leptamnium) and squawroot (Conopholis) . 
 
 The curious family of the bladderworts is closely related to 
 the fig^vorts. The bladderwort {Utricularia) is a very common 
 aquatic in still waters and ponds. Certain leaves become modi- 
 fied into elaborate sacs with trapdoors for enticing and capturing 
 small insects which die and decay in these prisons and are ulti- 
 mately absorbed and so contribute to the nourishment of the 
 plant. The butterwort {Pingiiiciila) is another member of this 
 family found in the northern countries and also in our southern 
 states. It has more the appearance of a violet with a rosette 
 of leaves which are provided with glands that secrete a viscid 
 substance for the capture of insects and also digestive fluids. 
 
DEVELOPMENT OF PLANTS 
 
 481 
 
 The contact of the insect acts as a stimulus, causing the leaves 
 to roll up and so bringing to bear upon him a larger surface of 
 digestive glands. One of the species of Pinguicula is used in 
 northern countries to curdle milk in place of "rennet" which 
 contains the similar digesti\'e fluids of calves' stomachs. 
 
 The potato family (Solanaceae) shows many of the irregu- 
 larities of the fig^vorts, but the majority of its 1,700 species have 
 regular flowers as illustrated in Fig. 334. While the family 
 
 Ma-., 
 
 V. 
 
 ^ 
 
 
 Fig. 334. Flower of the potato {Solanum tuberosum), showing the stamens 
 inserted on the tube of the corolla and encircling the style. 
 
 is largely tropical, many of the species are cultivated, as the 
 potato, eggplant, tomato, cayenne pepper (Capsicum). Poison- 
 ous, acrid and narcotic properties are characteristic features of 
 the family. Belladonna and atropine from Atropa, stramonium 
 from Datura and nicotine from Nicotiana tahacum are character- 
 istic drugs. Ground cherry (Physalis), nightshade {Solanum), 
 Petunia, etc., are cultivated forms. 
 
 153. A Transitional Order. — It would appear that all the 
 changes possible in the hypog>'nous type of flowers had been 
 wrought in the members of the Polemoniales and that the next 
 step in advance must be to the epigynous flower. This is seen 
 to be the case with the madder order, Rubiales . Here the 
 flowers are epigynous and the parts are generally in fives though 
 the carpels vary greatly in number. The simplicity in the struc- 
 ture of the flower and often also the form of the inflorescence, 
 as in the elderberry {Sambucus), bedstraw {Galium) and arrow- 
 wood ( Viburnum) is strikingly suggestive of the Umbellales 
 (Figs. 335, A, B] 324). It appears that each step in advance 
 is attained in a very simple way and that only gradually are 
 31 
 
482 
 
 THE RUBIALES 
 
 alterations made that lead up to the specialized types. Look 
 back over the preceding groups and orders and note that many 
 of the irregular types of flowers are preceded by a long series 
 of regular forms. So in the madder order, this new departure 
 of the flower is attended with such a simplicity of structure as 
 to render difficult the separation of some of the genera from the 
 open flowers of the Umbellales which have also arrived at the 
 
 Fig. 335. Simpler forms of the Rubiales: A, inflorescence of the arrow- 
 wood (Viburnum). B, simple, epigynous flower of the elderberry (Sam- 
 hucus) — 0, ovary. C, the twin flower (Linnaea). 
 
 same stage of development, page 465. This would not necessarily 
 imply a relationship, since it repeatedly happens both among 
 plants and animals that identically similar structures arise in 
 groups in no way related. From these simple flowers, that may 
 be no more sympetalous than certain genera of the carrot order, 
 we pass to more pronounced tubular forms in the bluets (Hous- 
 tonia), buttonbush (Cephalanthus), tw^in flower {Linnaea, Fig. 
 335 » C), snowberry (Symphoricarpus) and finally to irregula/ 
 and even labiate types as in the honeysuckle (Lonicera, Fig. 
 268), valerian, teasel (Dipsacus) and scabious (Fig. 336). It 
 is noteworthy that in the three latter genera the flowers become 
 massed in a dense inflorescence, known as a head, which are sub- 
 
DEVELOPMENT OF PLANTS 
 
 483 
 
 tended by modified leaves, the involucre (Fig. 336) and the pistils 
 are reduced to a single fertile carpel with two-lobed style. These 
 variations will become very prominent in the next order. The 
 Rubiales are an important tropical group and furnish the coffee 
 (Coffea) and the cinchonas which yield such drugs as quinine 
 and calisaya. 
 
 Fig. 336. Advanced forms of the Rubiales: A, inflorescence of Scabiosa, 
 at the right showing the involucre, in. B, a single flower enlarged, show- 
 ing the somewhat irregular corolla. At the right the sectional view of the 
 flower shows the calyx, ca, terminating in bristle-like teeth; br, bract-like 
 cup surrounding the calyx. C, irregular flower of valerian — n, nectar sac; 
 ca, rudimentary calyx, which matures in the fruit as a mass of delicate feath- 
 ery organs, known as the pappus. 
 
 154. Campanulales, the Bellflower Order. — This order marks 
 the culmination of the tendencies that we have seen steadily 
 progressing through the monocotyledons and the dicotyledons. 
 The variations have been of so peculiar and successful a nature 
 that no group of plants are so widely distributed and in the 
 more specialized families of so common occurrence. Over 
 14,500 species are known. The parts of the epigynous flowers 
 are arranged in four whorls of usually five members each. The 
 
484 
 
 THE CAMPANULALES 
 
 anthers are aggregated {and usually cohere, forming a sheath 
 about the style which is frequently covered with hairs and acts 
 like a piston rod in the cylinder of anthers. The flowers are 
 protandrous with few exceptions, the anthers opening on their 
 inner sides and discharging the spores upon the style which later 
 sweeps them out as it elongates. The pistils are generally re- 
 duced to a single pne-ovuled carpel that ripens as an akene. 
 Leaving out of consideration the gourds which include the 
 melons, pumpkins, cucumber and gourd, as a family of uncertain 
 
 
 Fig. 337. Lower forms of the Campanulales: A, inflorescence of the 
 bellflower (Campanula). B, section of a young flower. The hairy style 
 is pushing up between the encircling anthers and sweeping the spores out 
 of them. Note the closed stigmatic lobes. C, older flower. The anthers 
 are withering and the stigmas are curving back towards the spore-covered 
 style. D, flower of Lobelia. The tubular corolla is opened at one side, per, 
 mitting the style and encircling stamens, g, to protrude. E, section of flower- 
 showing' the anthers, a, united about the style and stigma. F, relation of 
 stigma to anthers. At right the section shows the anthers cohering about the 
 bushy style, which acts later as a brush sweeping out the spores. At the left 
 the style has grown beyond the anthers and the stigmatic lobes are spreading 
 apart. 
 
DEVELOPMENT OF PLANTS 
 
 485 
 
 alliance, we find a very natural sequence leading to the higher 
 families. For example the lower members of the bell-flower 
 family have nearly or quite regular flowers, parts in five, anthers 
 grouped about the style but not united, fruit a capsule with many 
 seeds as in the harebell {Campanula) and Specularia (Fig. 337, 
 A, C). In the Lobelia the corolla usually becomes irregular 
 and slit down one side, thus approaching the form assumed in 
 the highest groups of the order. The anthers arc united about 
 the style which is two lobed and provided with a whorl of hairs 
 to sweep the spores from the cylinder of anthers. The fruit is 
 a capsule of two carpels (Fig. 337, D-E). This brings us to 
 that great group of familiar plants that were formerly known as 
 the Compositae, but that are now separated into the chicory 
 family (Cichoriaceae) of 1,400 species, the ragweed family 
 (Ambrosiaceae) of 55 species, and the thistle family (Cardu- 
 aceae) of over 11,000 species. The most conspicuous feature 
 of these three families is the aggregation of numerous small 
 
 I 
 
 Fig. 338. Habit of the dandelion (Taraxacum), an example of the Cicho- 
 raceae: a, appearance of the inflorescence or head; b, appearance of the head 
 during the ripening of the seed; in, involucre; c, appearance of the fruit. 
 
486 
 
 THE CAMPANULALES 
 
 flowers in heads subtended by one or more rows of bracts that 
 form a calyx-Hke involucre (Fig. 338, in). This type of in- 
 florescence might readily be mistaken for a single floAver as the 
 buttercup, rose, etc. This tendency to group the flowers in 
 heads and compact clusters has been attained in several orders, 
 notably the mustards, peas, Umbelliferae, mints, scrophularias, 
 and especially in the teasel family, page 482. But in no group 
 has the aggregation been so successful and coupled with such 
 efficient types of flowers. Leaving out of consideration the de- 
 generate ragweeds the individual flowers of a head are very 
 
 Fig. 339. Flowers and fruit of Taraxacum: A, sectional view of inflor- 
 escence — in, involucre. The flowers in the center of the head not as yet in 
 bloom. B, an unopened flower. The thread-like calyx (pappus) and corolla 
 arising from the ovary, 0. C, corolla opening. D, later stage, the style has 
 elongated, sweeping the spores from the sheath of anthers, an, and the two 
 stigmatic lobes are beginning to open. E, flower in full bloom, the stigma 
 lobes recurving. 7^, mature fruit. The pappus is lifted up on a long, slender 
 outgrowth of the ovary, 0. 
 
 uniform in structure. They are epigynous, parts usually in fives, 
 calyx wanting or more often appearing as tufts of hairs, plumose 
 or barbed bristles, and known as the pappus (Figs. 339, 341). 
 The corolla is tubular or partially split open, forming a strap- 
 
DEVELOPMENT OF PLANTS 487 
 
 shaped blade known as the Hgulate corolla (Fijr. 339, R), The 
 anthers are united about the style which is often hairy and pushes 
 out the microspores like a piston (Fi^. 341, B-D). The style 
 is two-lobed, the stigma usualh' appearing as a line on the inner 
 surface of the lobes and the single, one-ovuled carpel matures 
 as an akene. Scale-like bracts, the chaff, are often associated 
 with the flowers (Fig. 341, B). The nectar formed from the 
 glands at the base of the style is concealed at the bottom of the 
 corolla tube. 
 
 (a) The Chicory Family. — This family is distinguished by all 
 the flowers of the head being ligulate (Fig. 339) and mostly yel- 
 low in color and by the possession of milky, bitter or acrid juices. 
 Here belong the goat's beard (Adopogofi), hawkbit (Leontodon), 
 sow thistle (Sonchiis), hawkweed (Hieracitim), rattlesnake root 
 (Nabalus), as well as several introduced and native plants that 
 are cultivated, as the dandelion (Taraxacum), species of chicory 
 (Cichorium), salsify (Tragopogon), lettuce {Lactuca). The char- 
 acter of the head, flower and fruit of this family are well shown 
 in the dandelion (Fig. 339). All the flowers are ligulate, the 
 five lobes frequently to be seen on the margin of the corolla 
 indicating the number of petals, and the calyx assumes the form 
 of minute hairs (Fig. 339, B-E). The five filaments adhere to 
 the tube of the corolla while the anthers form a C3'linder about 
 the style and discharge their spores before the flower opens. The 
 style begins to elongate as soon as the flower opens and pushes 
 out the spores which may be seen as little piles of dust at the 
 top of the anthers in freshly opened flowers. The filaments of 
 this genus are sensitive to touch and curve outward, pulling down 
 the sheath of anthers, thus assisting in sweeping out the spores 
 and exposing the style when an insect irritates them in his (|uest 
 for nectar. This device appears in other genera and is sometimes 
 the only means of exposing the microspores and styles. Autog- 
 amy is at first impossible owing to the fact that the two lobes of 
 the style are closely pressed together efTectually excluding the 
 microspores from its inner, stigmatic, surface. Later the style 
 grows considerably above the anthers and the lobes separate, ex- 
 posing the stigmas for crossing (Fig. 339, E). The association 
 
THE CAMPANULALES 
 
 of this type of flower is more efl^ective than any as yet noticed, 
 and it would appear impossible for any of the numerous insects 
 which frequent these flowers to miss crossing a score of -them at 
 a single visit. Crossing between the flowers of a head is effected 
 in some genera b^^ the movements of the flowers. The inner 
 leaves of the involucre fold over the flowers at night and in rainy 
 weather, protecting them like a perianth. This results in crowd- 
 
 FiG. 340. Inftorescence of the bur-marigold (Bidens), a common ray form 
 of flower of the thistle family: in, involucre. 
 
 ing of the flowers together, and the stigmas are often brought in 
 contact with the spores that have been dusted upon the various 
 parts of adjoining flowers. The outward curving of the stylar 
 lobes may also bring the stigmas in contact with the spore-covered 
 parts of adjoining flowers w^ith a like result. Autogamy is 
 brought about in the dandelion, as in nearly all other members 
 of the order, b}^ the curvature of the lobes of the style which 
 continue to bend back until the stigmatic surface is brought in 
 contact with the spore-covered style (Fig. 341, D). The bloom- 
 ing of the flowers progresses from the margin to the center of 
 the head so that during several days new sets of flowers are ex- 
 posed for crossing. When the period of bloom is passed the 
 involucre closes over the head and remains in this condition until 
 the fruit is mature (Fig. 338, b). During this period, the stalk 
 
DEVELOPMENT OF PLANTS 489 
 
 bearing the head elongates considerably, so as to lift the fruit 
 above the surrounding vegetation — the extent of its elongation 
 depends upon the height of this vegetation. The corolla withers 
 away and the hair-like calyx grows out into a white, delicate 
 pappus, which is lifted upon a beak-like outgrowth of the ovary 
 (Fig. 339, F). The fruit is now mature and in a position f(jr dis- 
 tribution. The involucre opens, the hairs of the pappus expand, 
 loosening the akenes so that the least touch or breath of air floats 
 off the fruit as the most perfect type of parachute (Fig. 338, c). 
 
 (b) The Thistle Family, Cardtiaceae. — This is the largest 
 family of the angiospcrms. The flowers have the same arrange- 
 ment and structure as in the Cichoriaceae, save that the corollas 
 are either all tul^ular, as in the thistle and ironweed (Fig. 342), 
 or the marginal flowers of the head may be ligulate, thus in- 
 creasing the conspicuousness of the inflorescence, as in the asters, 
 daisy, etc. (Figs. 340, 341). The marginal ligulate flowers are 
 termed ray flowers to distinguish them from the inner tubular or 
 disc flowers. The ray flowers may be sterile, though more fre- 
 quently imperfect and provided with pistils. The calyx may be 
 wanting, but more often assumes the form of silky or plumose 
 pappus or of membranous scales for wind transportation or of 
 barbed bristles of various kinds for distribution by animals. In 
 the case of the burdock (Arctium) the involucre is covered with 
 hooks. The Carduaceae includes a great array of plants, many 
 of which are among our troublesome weeds: Ironweed (Ver- 
 nonia), thoroughwort {FAipatoriiim), blazing star {Lacinaria), 
 golden-rod (Solidago), aster, fleabane {Erigeron), cat's-paw 
 (Antennaria) , everlasting {Gnaphaliitm) , rosinweed and com- 
 pass plant {Silphiiim), Spanish needles (Bidens), daisy (Chrys- 
 anthe^mim), groundsel (Senecio), thistle (Carduus). Cultivated 
 for their showy flowers: Tickseed (Coreopsis) , cone flower (Rud- 
 beckia), sunflower (Ilelianthus), cornflower (Cefitaurea), ciner- 
 arias, etc. Medicinal: Wormwood (Artemisia), one species of 
 which yields absinth, milfoil (Achillea), colt's foot (Tussilago), 
 tans}^ (Tanacetum), chamomile (Anthemis), arnica, burdock 
 (Arctium). 
 
 The ragweed family, Ambrosiaceae, is a small group, princi- 
 
490 
 
 THE CAMPANULALES 
 
 pally North American in its distribution, that have become sepa- 
 rated from the Carduaceae through degeneration. The heads 
 contain a few greatly reduced Avind pollinated flowers that are 
 always imperfect and generally lacking in calyx or corolla, or 
 
 
 Fig. 341. Flowers and fruit of Bidens: A, sectional view of the inflor- 
 escence — r, ray flowers; d, disc flowers; in, bracts of the involucre. B, disc 
 flower before opening, the shaded region showing the position of the anthers 
 in the corolla — c, calyx in the form of downwardly barbed bristles; b, bract 
 or chaff associated with the disc flowers. C, early stage in the opening of 
 the flower. The stamens have been lifted beyond the mouth of the opened 
 corolla by the growth of their filaments and the style is elongating, pushing 
 out the spores, which appear in little piles at the top of the anthers. D, last 
 stage in the bloom of flowers. The style has grown beyond the anthers and 
 the stigmatic lobes have reflexed, touching the spore-covered style. E, a 
 sterile ray flower, much less enlarged. F, the fruit, showing the barbed pappus 
 for dissemination. 
 
DEVELOPMENT OF PLANTS 
 
 491 
 
 both, and characterized by free anthers. The more famihar are: 
 The marsh elder (/m), ragAveed (Ambrosia), and cloth-bur and 
 cockle bur {XantJiium). 
 
 These three alliances are apparently the most recently evolved 
 of the angiosperms, and owing to their numerous variations that 
 have been so successful In meeting the present conditions upon 
 
 ,':^'- ^ 
 
 Fig. 342. A common tubular flower of the thistle family: A, inflores- 
 cence of the ironweed (Vernonia). B, sectional view of the inflorescence, 
 only the outer flowers in bloom. C, enlarged view of one of the flowers — c, 
 calyx or pappus. D, fruit. 
 
 the earth, the}^ have become the dominant plants the world over. 
 Their success can be traced to a variety of causes, as the occur- 
 rence in many genera of perennial underground stems, which 
 make them to a degree. independent of climate conditions and the 
 depredations of grazing animals. A great many are avoided 
 because of their protective armor of prickles, bitter, acrid juices 
 or oils and resins. Especially to be noted is the seed-like fruit 
 with its numerous devices for distribution. All these features, 
 among others, explain their extensive distribution in open coun- 
 tries. Another advantage appears in the structure of the flower 
 and the inflorescence. The highest type of a tiower, as in the case 
 of a machine, is the one that is most efficient. The tiower is 
 not so elaborate as the orchid or as those of some other groups, 
 but it is superior because it accomplishes its work with greater 
 certainty and more economically. Note the significance of 
 these facts — the ovary of the orchid consists of three carpels 
 
492 THE CAMPANULALES 
 
 which may produce thousands of seeds, whereas the simple 
 ovary of one of the Compositae contains but a single seed. 
 Reduction has been made in every part without loss of con- 
 spicuousness and variations have also appeared that practically 
 eliminate the uncertainties of crossing and autogamy, and con- 
 sequently the formation and distribution of the seed is ensured. 
 In a word every evolutionary tendency that you have noticed 
 in the development of the flower finds its expression in this 
 group of plants. 
 
INDEX 
 
 Abies, 376 
 
 Absorption, 12, 20, 51, 54 
 
 Acer, 457, 459 
 
 Accompanying cells, 79, 80 
 
 Achlya, 219 
 
 Actinomorphic, 387 
 
 Adhesion, 385, 471 
 
 Adhesive discs, 108, 109 
 
 Adiantum, 329 
 
 Adlumia, 443 
 
 Aecial stage, 252 
 
 Aeciospores, 252 
 
 Agaricaceae, 259, 260 
 
 Agaricales, 259 
 
 Agrimony, 450 
 
 Air chambers, 273, 282, 283, 336, 
 
 Akene, 398, 438, 440 
 
 Albugo, 222, 223 
 
 Alder, 432 
 
 Algae, 173 
 
 blue-green, 164, 233 
 
 brown, 201 
 
 colonial, 178 
 
 conjugating green, 185 
 
 filamentous green, 190 
 
 green, 174 
 
 red, 210 
 
 tubular green, 195 
 
 unicellular green, 174 
 Alga-like fungi, 218 
 Allen, 215 
 Allelomorph, 141 
 Alnus, 432 
 
 Alternation of crops, 67 
 Alternation of generation, 136, 
 194, 201, 211-216, 256, 271, 
 281, 312, 319, 352 
 Amanita, 261 
 Amaryllis family, 419 
 Ambrosiaceac, 485, 489 
 Ament, 429, 430 
 Ammonia, 66, 159, 160 
 
 Angiospcrmae, 378 
 Ankistrodesmus, 182 
 Annual rings, of scars, 75, 73 
 
 of xylem, 91, 92 
 Annuals, 69 
 Annulus, 261, 262, 300, 301, 306, 307, 
 
 316, 326, 327, 355 
 Anther, 116, 117, 379 
 Anther, versatile, 408 
 Antheridial cell, 121, 122, 358, 369, 
 
 370, 392 
 Antheridium, antheridia, 196, 208 
 
 274. 303 
 Antherozoids, 196, 275 
 Anthoceros. 292, 294 
 402 Anthocerotales, 292 
 
 Antipodal cells, 119, 120, 390 
 Antitoxin, 164 
 Apophysis, 305, 306 
 Apple, 450, 451 
 
 rust, 256, 257 
 Aquatic plants, 39, 41, 64 
 Arales, 410 
 Aralia, 30, 465 
 Araliaceae, 465 
 Archangiopteris, 316 
 Archegonium, 275, 276, 303, 350 
 Arcyria, 150, 151 
 Aroid order, 410 
 Arrow-wood, 481, 482 
 Arum, 411, 413 
 Ascocarp, 229 
 Ascomycetes, 228 
 Asco-lichens, 232 
 137, Ascopores, 229 
 279, Ascus, asci, 229, 230 
 
 Asexual generation, 136, 137, 194, 
 
 204, 214, 256, 271, 279, 281, 319, 
 
 320 
 Asexual spore, 123, 172 
 Aspcrgillales, 240 
 Aspergillus, 241, 242 
 
 493 
 
494 
 
 INDEX 
 
 Asplenium, 324 
 Autoecious, 251 
 Autogamy, 383, 439, 446, 454, 470, 
 
 480, 488 
 Azalea, 471, 472 
 
 Bacillus, 155 
 Bacteria, 154, 155 
 
 economic importance of, 157 
 
 exclusion of, 156 
 
 of decay, 157, 158 
 
 of disease, 157, 162 
 
 iron, 161 
 
 nitrifying, 65, 67, 160 
 
 nitrogen fixing, 161 
 
 sulphur, 161 
 
 synthetic, 159 
 Banana, 420 
 Banyan tree, 70 
 Barberry rust, 251 
 Bark, 86, 87 
 Basidial stage, 256 
 Basidiomycetes, 251 
 Basidiospore, 255, 256 
 Basidium, basidia, 251, 263, 255, 258, 
 
 262, 263 
 Basswood stem, 100 
 Bean seed, 130, 131 
 Beard, 44 
 Beech, 431, 434 
 
 ordei, 431 
 Begonia, 2 
 Bellflower, 484 
 
 order, 483 
 Berry, 398 
 Bidens, 488, 490 
 Biennials, 69 
 Bilabiate flowers, 478 
 Birch leaf, 7, 427 
 
 family, 147, 431, 432 
 Black knot, 239, 240 
 Bladderwort, 480 
 Bladderwrack, 206 
 Blade, 7, 8, 21, 27, 426 
 Blakeslee, 227 
 Bleeding, of plants, 61 
 Blood root, 442, 443 
 Blueberry, 471, 472 
 
 Blue green algae, 165 
 
 Body cell, 358, 369, 370 
 
 Bogs, 296, 473 
 
 Boletaceae, 265 
 
 Boletus, 265 
 
 Borages, 476 
 
 Boraginaceae, 475, 476 
 
 Botrychium, 314, 315, 316, 317 
 
 Bower, 326 
 
 Brassica, 445 
 
 Bread making, 20, 250 
 
 Bryales, 301 
 
 Bryophyta, 269 
 
 Buckthorn order, 461 
 
 Buckwheat order, 437 
 
 Buds, 71-75, 73, 362 
 
 adventitious, 75 
 
 axillary, 74 
 Bulbs, 113 
 Buller, 264 
 
 Bunch flower family, 418 
 Bur-marigold, 488, 490 
 Burroughs. 415 
 Buttercup, 438 
 
 order, 438 
 Butternut, 433, 435 
 Butterwort, 480 
 
 Cabbage, 447 
 Cactus, 44 
 
 Calcium cyanamide, 66 
 Callithamnion, 213 
 Callus, 75, loi 
 Calyptra, 294, 299, 305 
 Calyx, 115, 116, 384 
 Cambium, 79, 80, 82, 89, 314 
 
 cylinder, 89, 90, 91, 314, 426 
 Campanula, 484 
 Campanulales, 483 
 Canal cells, 276, 303, 317, 318, 345 
 Canna, 421 
 
 Capillitium, 150, 151, 153 
 Capsule, 277, 305, 306, 307, 326, 414 
 
 417 
 Carbohydrates, 11, 12 
 Carbon, 15 
 
 Carbon dioxide, 12, 15, 21, 49, 159 
 Carbonic acid, 13 
 
INDEX 
 
 495 
 
 Carduaccac, 485, 489 
 Carex, 409 
 Carpel. 115, 116, 399 
 Carpinus, 432 
 Carrot, order, 465 
 
 family, 465 
 
 wild, 466 
 Caruncle, 132, 133 
 Cassia, 35, 451.452 
 Castor bean, 76, 77, 90, 91, 133 
 Catkin, 429, 430 
 Cat-tail, 381, 401, 403 
 
 order, 401 
 Cedar apple, 256, 257 
 Celery, blanching of, 14 
 Cell, crystals, 2 
 
 division of, 56, 57, 58 
 
 intercalary, 253, 254 
 
 nature of, i 
 
 sap, I, 3r 33 
 
 wall, I, 57, 58 
 Cellulose, 1 1 
 Ceraniium, 211 
 Cercis, 451, 452 
 Chaetophoralcs, 190 
 Chaff, 487, 490 
 Chamaccyparis, 376 
 Chamacnerion, 463, 464 
 Characters, nature of, 144 
 Chemical reaction, 3 
 Chemo-synthesis, 162 
 Chenopodiales, 436, 437 
 Chenopodium, 436 
 Cherry, 450, 451 
 Chervil, 468 
 
 Chicory family, 485, 487 
 Chlamydomonas, 178, 179 
 Chlorenchyma, 11, 12, 76 
 Chlorophyceac, 174 
 Chlorophyll, 13-15, 167 
 Chloroplasts, 2, 9, 10, 12, 13, 17 
 Chondrus, 214 
 Choripetalac, 429 
 Chromatin, 56, 57 
 Chromoplasts, 2, 32 
 Chromosome, 56, 57 
 
 reduction of, 136, 137, 214-216, 
 280 
 
 Chromosome, homologous, 141 
 
 Cichoraceae, 485, 487 
 
 Cilium, 151, 152 
 
 Cladonia, 233 
 
 Class, 147 
 
 Classification of plants, 146 
 
 Clavaria. 264 
 
 Clavariaceae, 264 
 
 Claviceps, 237, 238 
 
 Climbing plants, 106 
 
 Closterium, 188 
 
 Club moss, 341 
 
 Club root, 153 
 
 Cluster-cup, 252 
 
 Cohesion, 385 
 
 Coenocytes, 195 
 
 Coleochaetes, 199, 200 
 
 Collateral bundles, 314, 354 
 
 Collecting cells, 9, 21 
 
 Collenchyma, 78, 83, 84 
 
 Colors of flowers, 417, 423, 441, 446. 
 
 480 
 Columbine, 440 
 Columella, 225, 293, 294 
 Comfrey, 385. 475, 476 
 Commensalism, 68, 234 
 Compass plant, 33, 34 
 Compositae, 485 
 Concentric bundles, 323, 324 
 Conducting system, 82, 94, 312 
 Cone, 337. 342, 366 
 Conidium, 228 
 Conn, 156 
 Consumption, 163 
 Contact, reaction to, 63, 106, 107 
 Cordyceps, 238, 239 
 Cork, 42, 76, 86, 87, 314-315 
 Cork cambium, 86, 87 
 Corm, 112, 113 
 Corn, root of, 57 
 
 seedling of, 134, 135 
 Corolla, 115, 116, 384 
 Cortex, 59, 60, 61, 76, 314-315 
 Corylus, 432 
 Coscinodiscus, 169 
 Cosmarium, 189 
 Cotyledon, 124, 125, 126, 131, 132 
 
 133. 318, 332, 360, 398, 428 
 
496 
 
 INDEX 
 
 Creeping stems, 109 
 
 Cribrai a, 150 
 
 Crossing, 115, 138, 382, 404, 412, 413, 
 
 445. 446, 463. 467. 476, 487 
 Crowfoot order, 438 
 Crucibulum, 267 
 Cruciferae, 444, 445 
 Currant, 387, 449 
 Cuticle, 10, 25, 36, 269 
 Cutin, 10, 36 
 Cyanophyceae, 165, 166 
 Cycadales, 353 
 Cycads, 353 
 Cycas, 353, 355 
 Cyclic flowers, 384, 385 
 Cymbella, 170 
 Cyperaceae, 410 
 Cypripedium, 424 
 Cystocarp, 212, 213 
 Cytoplasm, 2 
 
 Daldinia, 239, 241 
 
 Dandelion, 485, 486 
 
 Darwin, 64, 132 
 
 Dasya, 211 
 
 Daucus, 466 
 
 Decay, 158 
 
 Deciduous, 42 
 
 Decompositions, in living substance, 
 
 22-23 
 Decussate leaf arrangement. 27, 28 
 Delesseria, 211 
 Dennstaedtia, 328 
 Desert plants, 44 
 Desmidium, 188 
 Desmids, 188, 189 
 De Vries, 144, 463 
 Diadelphous, 452 
 Diastase, 20, 250 
 Diatomaceae, 167 
 Diatoms, 167, 168 
 Dichotomy, 207, 271, 272 
 Dicotyledons, 96, 125, 426 
 Dioecious, 284, 339 
 Diphtheria, 164 
 Disease, 162 ■ 
 Disc flower, 489, 490 
 Divisions, 148 
 
 Dodder, 64 
 Dodge, 229 
 Dominant factor, 143 
 Dx-upe, 398 
 Dryopteris, 324 
 Duckweed, 410 
 Ducts, 79 
 
 Earth star, 267 
 
 Ectocarpus, 202, 203 
 
 Egg cell, 119, 120, 197, 275 
 
 Elaters, 286, 287, 338, 339 
 
 Elderberry, 481, 482 
 
 Elfvingia, 265 
 
 Elm, 436 
 
 Embryo, 124, 125, 359, 360, 396 
 
 cell, 124, 125, 395, 396 
 
 sac, 124, 394, 395 
 Empusa, 227 
 
 Endodermis, 59, 61, 77, 316, 325 
 Endosperm, 124, 125, 133, 373, 391, 
 394. 398 
 
 nucleus, 119, 120, 123, 389, 390, 
 
 394 
 
 Energy of plant, 13, 15, 23, 162 
 
 Enzyme, 17, 20, 128, 157 
 
 Epicotyl, 131, 132, 398. 428 
 
 Epidermis, 8, 9, 26, 36, 272 
 
 Epigynous, 386, 387 
 
 Epiphragm, 305, 306 
 
 Epiphyte, 64, 422 
 
 Equisetales, 334 
 
 Equisetum, 335, 339 
 
 Ergot, 237, 238 
 
 Ericales, 470 
 
 Erysiphe, 244, 254, 247 
 
 Erythronium, 383, 414, 41 S 
 
 Etiolation, 14 
 
 Eucalyptus, 33 
 
 Eumycetes, 216 
 
 Euphyceae, 173 
 
 Evaporation, 25 
 
 Evening primrose, 29, 463, 464 
 
 Evolution of Algae, 174, 190 
 of Angiospermae, 378 
 of Bryophyta, 269, 290, 295, 309 
 of Eumycetes (true fungi), 218, 
 220 
 
INDEX 
 
 497 
 
 Evolution of llowcr, 381, 388, 426 
 of Gymnospermae, 353 
 of Pteridophyta, 312, 319 
 of sex, 178, 180, 194, 198, 204, 
 
 340 
 of sexuality, 171, 177, 181, 185, 
 204 
 
 Excretion, 20, 22, 53 
 
 Exoascales, 248 
 
 Exoascus, 248 
 
 Exobasidium, 472 
 
 Eye spot, 175, 176 
 
 Factor, hereditary, 138 
 
 distribution of, 139 
 
 dominant, 143 
 
 nature of, 144 
 
 recessive, 143 
 Fagales, 431, 432 
 Fagus, 434 
 Fairy rings, 259. 260 
 Family, 147 
 Farmer, 209 
 Fecundation, 122, 123 
 Felix, 328 
 
 Fermentation, 157, 158, 249 
 Ferments, see enzymes. 
 Ferns, 311 
 
 adder tongue, 314, 315 
 
 Christmas, 311, 322 
 
 common, 321 
 
 flowering, 328 
 
 grape, 315 
 
 shield, 324, 325 
 Fertilization, 122, 123, 209, 276, 393 
 
 significance of, 137, 144 
 Fertilizers, of soil. 66 
 Fibrillac, of spindle, 57, 58 
 Ficus, 435 
 Figwort, 478 
 Filament, 116, 165, 379 
 Filicales, 321 
 Flower, 338, 378, 381 
 
 coloration of, 417, 423, 441, 480 
 
 cyclic, 384, 385 
 
 development of, 381 
 
 perfect and imperfect, 382 
 
 spiral, 383 
 32 
 
 Flower, structure of, 115, 116 
 Foliar gaps, 314, 325 
 Follicle, 398 
 Food of plants, il, 48 
 
 distribution of, 16 
 
 storage of, 17, 18 
 Foods, formation of, 12 
 Foot, 286, 287 
 Formaldehyde, 13 
 Formative regions, 58, 60, 98, 99 
 Formic acid, 13, 40 
 Forsythia, 28, 20 
 Fragaria, 450 
 Fruit, 127, 374, 397-399 
 Fucus, 206, 207, 208, 209 
 Fumitory, 443 
 Funaria, 294. 300, 303, 307 
 Fungus, 216 
 
 alga-like, 218 
 
 basidia-forming, 251 
 
 bird's nest, 267 
 
 black, 239, 240 
 
 blue-green, 240 
 
 brown, 240 
 
 coral, 264 
 
 cup, 229 
 
 fly, 227 
 
 gill, 263, 265 
 
 pore, 264, 265 
 
 prickly, 264 
 
 sac, 228 
 Funiculus, 118, 389, 395 
 
 Gamctangium, 192, 196, 202, 203, 
 
 208 
 Gamete, 118, 172, 178, 179, 187, 192, 
 
 198, 200, 211, 275 
 Gamete, female, 118, 120, 178, 389 
 male, 118, 121, 122, 178, 318, 
 332, 342, 358, 392 
 Gametophyte generation, 136, 194, 
 215, 274, 276, 281, 310. 
 312, 316-317, 320, 330, 
 339. 345. 349. 388 
 reduction of, 313, 349-351- 
 359. 393 
 female, 119, 120, 332, 339, 349, 
 357, 367. 368, 388, 389 
 
498 
 
 INDEX 
 
 Gametophy te , male, 122, 332, 339, 
 
 349. 357, 369, 392 
 Gametospore, 118, 122, 172, 201 
 
 germination of, 123, 124, 187, 
 188, 193, 197, 198, 200, 213, 
 214, 215, 276, 277, 280, 286, 
 293, 304. 308, 309, 318, 332, 
 333, 340, 345, 351, 359, 37i- 
 395 
 Gasteromycetes, 266 
 Gemmae, 270, 331 
 
 Generations, alternation of, 136, 137, 
 194, 201, 211-216, 256, 271, 279, 
 281, 319, 352 
 Gentian, 474 
 
 order, 473 
 Gentianales, 473 
 Genus, 146 
 Geoglossum, 232 
 Geraniales, 460 
 Geranium, 384, 461 
 
 order, 460 
 Germ tube, 278 
 Gills, 260, 261, 263 
 Ginkgo, 362 
 Ginseng family, 465 
 Gleditsia, 451, 452 
 Gleocapsa, 166 
 Goebel, 309 
 Goosefoot order, 437 
 Grafting, loi, 102 
 Grain, 134, 135, 397, 398, 409 
 Graminaceae, 410 
 Graminales, 405 
 Grape, 461 
 
 blight, 222 
 
 Grass, 406, 407, 409 
 
 family, 410 
 
 order, 405 
 
 Gravity, effect on plant, 62, 104, 106, 
 
 132, 407 
 Ground nut, 455 
 Growth, apical, 56, 98, 200, 270, 304, 
 
 313 _ 
 conditions of, 72, 74, 129 
 definite, 72 
 nature of, 3 
 of stem, 89, 103, 106 
 
 Growth of root, 56 
 
 relation of chemical changes to, 4 
 
 rhythm of, 93 
 Growing point, 56, 98, 270, 313, 346 
 Guard cells, 9, 10, 26 
 Gulf weed, 206 
 Gymnospermae, 353 
 Gymnosporangium, 256, 257 
 
 Hairs, plant, 10, 38, 39, 40 
 
 root, 47, 46, 52 
 Haustoria, 220, 221 
 Hazel, 432 
 Hazen, 177 
 Head, 482, 485 
 Heath order, 470 
 Heartwood, 95 
 Helleborus, 440 
 Helvellales, 231, 232 
 Hepaticae, 271 
 Hepatics, leafy, 288 
 Hepatics, thallose, 271, 272 
 Hereditary substance, 138 
 Heterocyst, 166, 167 
 Heteroecious, 251 
 Heterogamous, 178 
 Heterospory, 347, 352 
 Heterostyly, 462, 463 
 Hickor>^, fruit of, 435 
 
 branch of, 73 
 
 leaves of, 34 
 Hilum, 129, 130 
 Honey locust, 451, 452 
 Honeysuckle, 387 
 Hop, twining, 106 
 Hormogonium, 166 
 Horizontal branches, 28, 29 
 Hornbeam, 432 
 Horsechestnut, 31, 34 
 Horsetail, 334 
 Hybrid, 138 
 Hydnaceae, 264 
 Hydnum, 264 
 Hydrodictyon, 184 
 Hydrolysis, 20 
 Hydrophytes, 39 
 Hymenium, 230, 231, 262, 263 
 Hypha, 68, 217 
 
INDEX 
 
 499 
 
 Hypocotyl, 125, 126, 398, 428 
 Hypocreales, 238 
 Hypogynous, 386 
 Hypoxylon, 239, 240 
 Hysteriographium, 241 
 
 Immunity, 164, 245 
 Indian-pipe, 69, 472 
 Individuality of the plant, 7 
 Indusium, 324, 325, 328, 329 
 Inflorescence, 381, 382 
 Insectivorous plants, 296, 480 
 Insects, crossing by, 115, 388, 412, 
 
 413. 417. 419. 421, 424. 439. 
 
 445. 453. 471, 476, 479 
 
 sight and sense of smell, 417 
 
 Integument, 117, 118, 356, 361, 398, 
 
 425, 428 
 Intercellular spaces, 9, 11, 21, 25, 32, 
 
 273 
 Involucre, 285, 287, 466, 483 
 Ipomoea, 475 
 Iris, 419 
 Iris, family, 419 
 Ironweed, 491 
 Iron bacteria, 161 
 Irritability, 33, 61, 104 
 Isogamous, 178 
 Isthmus, 189, 190 
 
 Jacket-cells, 356, 357 
 Jack-in-the-pulpit, 411 
 Jeffrey, 317 
 Judas tree, 451 
 Juglandales, 433 
 Juglans, 435 
 Juncus, 416 
 Jungermanialcs, 288 
 Juniperus, 376, 377 
 rust, 256, 257 
 
 Keel, 451-453 
 Kelps, 205 
 Kerner, 70 
 Key fruit, 398, 459 
 Klebs, 185 
 Knot, 100 
 
 black, 239, 240 
 
 Labelluni, 421. 422, 423, 424, 425 
 
 Labiatae, 476 
 
 Lactuca, 33, 34 
 
 Laminaria, 205 
 
 Larix, 375 
 
 Larkspur, 441 
 
 Lateral roots, 59, 62, 63 
 
 Latex, 435, 443 
 
 Lathyrus, 453 
 
 Leaf, arrangement of, 27-36 
 
 color, 32 
 
 compound, 30, 31 
 
 fall of, 42, 43 
 
 forms of, 7, 30, 31 
 
 growth of, 33 
 
 lobing of, 30, 31 
 
 movement, 33-36 
 
 reduction of, 43, 44 
 
 size of, 24, 25 
 
 structure of, 7-1 1, 21, 36-42 
 
 work of, 11-26 
 Leaf-scar, 43 
 Legume, 65, 398 
 Lenticel, 71, 76, 87, 88 
 Leotia, 232 
 Lepidium, 124, 389, 395, 396, 398, 
 
 428 
 Lespedeza, 35 
 Lessonia, 205 
 Leucoplasts, 2, 19 
 Lichens, 232 
 
 Asco-, 232, 233 
 Licmophora, 168 
 Light, 4, 13-14, 17, 31, 36, 41, 63. 
 
 104, 106, 112, 176 
 Ligulate flowers, 487 
 Lilac leaf, 9 
 Liliales, 414 
 Lily, fawn, 383, 414 
 
 of the Valley family, 418 
 
 family, 418 
 Lily order, 414 
 Linaria, 478, 479 
 Linin, 56, 57 
 Linkage of factors, 143 
 Linnaca, 482 
 Live -forever, 448-489 
 Liverworts. 271 
 
500 
 
 INDEX 
 
 Liverworts, horned, 292 
 Living substance, i-6 
 Livingston, 183 
 Lobelia, 484, 485 
 Lockjaw, bacillus, 155, 163 
 Lodicules, 407, 408 
 Lomentum, 455 
 Loosestrife, 473, 474 
 purple, 462, 463 
 Lycogala, 150 
 Lycoperdales, 266, 267 
 Lycoperdon, 266 
 Lycopodiaceae, 343 
 Lycopodiales, 341 
 Lycopodium, 341, 342, 343, 345 
 Lythrum, 462, 463 
 
 Macrocystis, 205 
 
 Madder order, 481 
 
 Malus, 451 
 
 Maple, 457, 458, 459 
 
 Marattalies, 516 
 
 Marchantia, 274, 282, 284, 285, 286, 
 
 287 
 Marchantiales, 271 
 Massee, 245 
 Massula, 425 
 
 Mechanical structure, 78, 83 
 Medullary ray, 92, 93, 94, 314-315 
 Megasporangia, 348, 356, 380, 388, 
 
 389, 391 
 Megaspore, 117, 118, 339, 347, 348, 
 367, 368, 380. 388, 390 
 germination of, 118, 119 
 Megasporophyll, 117, 348, 355, 367, 
 
 379. 380, 389, 397 
 Meibomia, 455 
 Melandryum, 383, 437 
 Melosira, 168 
 Mendel, 142 
 Mendel's Law, 142 
 Mesophyll, 9, 10 
 Mesophytes, 39 
 Metchnikoff, 158 
 Micropyle, 117, 118, 355, 356, 368, 
 
 380 
 Microsphaera, 247 
 Microsporangia, 348, 365, 379 
 
 Microspore, 116, 339, 347, 348 
 
 germination of, 120, 122, 349, 
 350, 357. 358, 369. 392, 454 
 Microsporophyl, 117, 348, 355, 365 
 
 379 
 Micrasterias, 188 
 Mildews, downy, 220, 223 
 
 powdery, 244, 245 
 Mimosa, 35, 36 
 Minerals in soils, 48 
 Mint family, 476 
 Mistletoe, 64 
 Mnium, 302 
 Moccasin flower, 422 
 Monkshood flower, 441 
 Monocotyledons, 96, 125, 400 
 Monoecious, 284 
 Monotropa, 472 
 Morchella, 231, 232 
 Morel, 231, 232 
 Morning-glory, 475 
 Mosaics, 32 
 Moss, 295 
 
 bog or peat, 296 
 
 chloroplasts, 17 
 
 scale, 290 
 
 true, 301 
 Mould, black, 224, 225, 226 
 
 brown, 240 
 
 blue-green, 217, 240 
 Mould, water, 218 
 Movements, gases, 12, 21, 45 
 
 leaf, 33-36 
 
 root, 6 1 
 
 stem, 104 
 
 water, 24, 50, 54 
 Mucorales, 224 
 Mushrooms, 217, 259, 260 
 Musci, 295 
 Mustard family, 444 
 Mutation, 144 
 Mycelium, 21"] 
 
 Mycorrhiza, 68, 243, 296, 317, 422 
 Myrtales, 462 
 Myrtle order, 462 
 Myxomycetes, 149, 150, 151 
 Myxobacteriales, 153 
 
INDEX 
 
 501 
 
 Narcissus, 419 
 
 Nectar glands, 381, 388, 404, 430, 
 
 431.437. 439.440,457, 458 
 Nemalion, 213 
 Nereocystis, 205 
 Netted veined leaf, 426, 427 
 Nettle, 40, 434 
 Nidularialcs, 267 
 Nitrate, 66, 160 
 Nitrite, 160 
 Nitrogen, 16, 65, 160 
 Nostoc, 166, 292 
 Nucleoli, 56, 57 
 Nucleus, I, 2, 56 
 Nut, 432, 433 
 Nutations, 103, 104, 106 
 
 Oak, 31, 383, 427, 434 
 
 Odors, 412, 417, 433 
 
 Oedogonium, 197, 198 
 
 Oenothera, 29, 463, 464 
 
 Olive, 154 
 
 Onoclea, 330 
 
 Oogonium, 197, 208 
 
 Oosphere, 197, 275 
 
 Operculum, 299, 300, 305, 306 
 
 Ophioglossalcs, 313 
 
 Ophioglossum, 313, 315 
 
 Orange, 460 
 
 Orchid order, 64, 421 
 
 Orchidales, 421, 423 
 
 Orchis, 425 
 
 Order, 147 
 
 Oscillatoria, 166, 167 
 
 Osmosis, 53 
 
 Osmunda, 326, 328 
 
 Ovary, 116, 117, 380 
 
 Ovule, 116, 117, 118, 380, 388, 389, 
 
 399 
 Oxygen, 3, 13, 15, 21, 22, 75, 121 
 
 Palisade mesophyll, 9, 10, 21, 40, 41 
 
 Pallavicinia, 288 
 
 Palms, 405 
 
 Palmelloid state, 192 
 
 Pandanales, 401 
 
 Pandorina, 179 
 
 Papaverales, 442 
 
 Papavcraceae, 442 
 
 Papillae of stigma, 389, 392 
 
 Papilionaceae, 452 
 
 Papilionaceous flower, 453 
 
 Pappus, 483, 486 
 
 Paraphyscs, 208, 209, 230, 231, 262 
 263, 303 
 
 Parasites, 64, 68, 216, 244, 245, 350 
 
 Parenchyma, 11, 77, 81 
 
 Parmclia, 233 
 
 Parnassia, 448 
 
 Pasteur, 164 
 
 Pea, family, 452 
 sweet, 453 
 
 Peach curl, 248 
 
 Pear, branch of, 73 
 
 Peat, 298 
 
 moss, 296 
 
 Pedicel, 403 
 
 Pediastrum, 184 
 
 Peniclllium, 217, 228, 242 
 
 Perennials, 69 
 
 Perianth, 115, 116, 286, 287, 288, 379, 
 
 383 
 Periderm, 266, 267, 268 
 Perigynous, 385, 386 
 Perisperm, 397 
 Perisporialcs, 244 
 Peristome, 294, 305, 306, 307 
 Perithecia, 229 
 Peronospora, 220, 223 
 Peronosporales, 220 
 Personata flower, 478 
 Petals, 115, 116, 384 
 Petiole, 7, 8, 29, 42, 43, 107, 426 
 
 427 
 Peziza, 231 
 Pezizales, 229 
 Phaeophyceae, 201 
 Phallales, 268 
 Phallus, 268 
 Phegopteris, 330 
 Philotria, 2 
 
 Phloem, 59, 79, 91, 314-315 
 Phosphorescence, 259 
 Phlox order, 474 
 Phosphorus, 16 
 Photosynthesis, 11-16, 23 
 
502 
 
 INDEX 
 
 Photosynthesis, economic importance 
 
 of, 15-16 
 Phycocyanin, 165 
 Phycomycetes, 218 
 Phyllactinia, 247 
 Phylloglossum, 343, 344 
 Phytophthora, 221, 223 
 Picea, 375, 376 
 Pierce, 190 
 Pigments, 32 
 Pileus, 260 
 Pilobolus, 226 
 Pinales, 362 
 
 classification of, 375 
 Pine, 365, 366, 368, 369, 371, 372, 374, 
 
 375 
 wood, 94, 363 
 Pinguicula, 480 
 Pinnularia, 169, 172 
 Pistil, 115, 116, 399 
 Pit, 398 
 
 Pith, 76, 78, 314-315, 324, 342 
 Placenta, 388, 389 
 Placenta, free central, 474 
 Plankton, 168, 169 
 Planktoniella, 169 
 Plant, nature of, i 
 
 cell, I 
 Plasmodium, 151, 152 
 Plasmopara, 221, 222 
 Plastids, 2, 17 
 Pleurococcus, 182 
 Plowrightia, 239, 240 
 Plumule, 125, 126, 398, 428 
 Pod, 127, 130, 398, 451, 454, 455 
 Poison Ivy, 459, 460 
 Poisonous secretions, 39, 40, 53, 265, 
 
 442, 459, 469, 481 
 Poison sumac, 459, 460 
 Polar nucleus, 118, 119, 389 
 Polemoniales, 474 
 Pollen grains, 116, 120, 122 
 Pollinium, 424, 425, 474 
 Polygonales, 437 
 Polypodium, 329 
 Polyporaceae, 264 
 Polysiphonia, 212 
 Polystichum, 311, 322, 328 
 
 Polytrichum, 302, 303, 306 
 
 Pond scum, 185 
 
 Poplar, leaf arrangement, 28 
 
 order, 429 
 Poppy, family, 442 
 
 order, 442 
 Pore, II, 80, 81, 94, 264, 265, 283, 363 
 Porella, 289 
 Potato, 481 
 
 rot, 221 
 Prepotency, of foreign spores, 454 
 Primitive plants, 46, 154 
 Primrose order, 473 
 Primulales, 473 
 
 Pro-embryo, 359, 360, 371, 395, 396 
 Prosenchyma, 11 
 Protandrous, 283 
 Prostrate stems, 109 
 Protection of microspores, 412 
 Proteids, 16 
 Proteid grains, 18, 19 
 Prothallium, 312, 317, 330, 331, 370 
 Protogynouf, 283 
 Protomyxa, 153 
 Protonema, 308, 309 
 Protoplasm, function of, 3 
 Protoplasm, nature of, 2, 3 
 Protoplasmic connection of cells, 3 
 Pruning, lOO, 102 
 Prunus, 451 
 Pteridophyta, 311 
 Pteridium, 329 
 Ptomaines, 159 
 Puccinia, 251, 252, 253, 255 
 Puff ball, 266, 267 
 Pulvinus, 35 
 
 Purposive reactions, 5, 36, 104 
 Pussy Willow, 430 
 Pustules, 258 
 Pycnium, 252 
 
 Pycnidium, pycnidia, 233, 252 
 Pycnidiospores, 252 
 Pyrenoids, 185, 186 
 Pyrola, 384, 470, 471 
 Pyronema, 229, 230 
 
 Quarter-sawed oak, 95 
 Quercus, 146, 383, 434 
 
INDEX 
 
 503 
 
 Ragweed family, 485, 489 
 Ranales, 438, 440 
 Ranunculus, 438. 439 
 Ray, flower, 489, 490 
 
 medullary, 92, 93, 94 
 Rattlebox, yellow, 479 
 Recessive factor, 143 
 Receptacle, 381-386, 439 
 Red algae, 210, 218 
 Red bud, 451 
 Red snow, 176 
 Regeneration of stems, 364 
 Respiration, 8, 21, 54, 88 
 Reproduction, no, 136 
 Resin, 39, 364 
 Resurrection plant, 346 
 Rhamnales, 461 
 Rhinanthus, 479 
 Rhizoids, 269, 273 
 Rhizomes, no, in, 323, 324 
 Rhizopus, 224, 225, 226 
 Rhododendron, leaf, 41 
 Rhodophyceae, 210, 211 
 Rhus, 460 
 Ribcs, 449 
 Riccia, 272 
 Ricciocarpus, 272, 273, 274, 276, 278, 
 
 281 
 Richards, 75 
 
 Ring, annual, 73, 75, 91, 92 
 Rivularia, 166 
 Rock, decomposition of, 49 
 
 disintegration of, 49 
 Rockweed, 206 
 Root, I, 4, 46, 298, 314 
 
 aerial, 64 
 
 anchoring and supporting, 69 
 
 bacteria on, 65 
 
 bending of, 62, 63 
 
 binding action. 70 
 
 cap, 58, 60, 398 
 
 contraction of, 70 
 
 growth of, 47, 56-58, 63 
 
 hairs, 46, 47, 52 
 
 lateral, 59, 62, 63 
 
 modification of, 64, 109 
 
 sensitiveness of, 61, 109 
 
 storage organs of, 69 
 
 Root, structure of, 56, 59, 60, 61 
 
 surface of, 55 
 
 toxic substances from, 53 
 Rootstock, no, in 
 Rosales, 447 
 Rosette, 29 
 Rose, 385 
 
 family ; 450 
 
 order, 447 
 Rostellum, 423, 425 
 Rubiales, 481 
 Rush, 401, 416, 418 
 Rusts, 251 
 
 white, 220, 223 
 
 Saccharomycetes, 248, 249 
 Sac fungi, 228 
 Sage, 477 
 Salicales, 429 
 Salix, 381, 430 
 Salvia, 477 
 Saltpeter, 66, 160 
 Samara, 398, 458, 459 
 Sambucus, 481, 482 
 Sanguinaria, 442, 443 
 Sap, 3. 33. 69, 71, 83 
 Sapindales, 456 
 Saprolegnia, 219, 222, 223 
 Saprolegniales, 218 
 Saprophytes, 153, 217 
 Sapwood, 95 
 Sargassum, 206 
 Saxifrage, 387, 449 
 Scabiosa, 481, 482 
 Scenedesmus, 182 
 Schizocarp, 458, 459 
 Schizophyta, 154 
 Scitaminales, 420 
 Sclerenchyma, 78 
 Sclerotium, 237, 238 
 Scouring rush, 336 
 Scrophulariaceae, 478 
 Scutellaria, 477 
 Scutellum, 135, 409 
 Secondary growth, 85 
 Sedge family, 410 
 Sedge order, 405 
 Sedum, 116. 379, 448 
 
504 
 
 INDEX 
 
 Seed, 351, 352, 360, 372, 397, 398 
 
 development of, 125, 126, 130 
 
 forms of, 126, 127 
 
 functions of, 127 
 
 growth of, 127-136 
 Seedling, castor bean, 132, 133 
 
 corn, 134, 135 
 Seed plant, 352 
 Segregation of factors, 141 
 Selaginella, 346, 347, 348, 350 
 Selaginellaceae, 346 
 Senna family, 450 
 Sensitive pea, 451, 452 
 
 plant, 35, 36, 455 
 Sensitiveness of leaf, 33 
 
 of protoplasm, 4, 5, 152 
 
 of root, 61 
 
 of stem, 104, 106 
 
 of zoospores, 176 
 Sepals, 384, 441 
 Seta, 289, 291, 309 
 Sex, evolution of, 178, 180, 181, 182, 
 
 194, 198, 204 
 Sexual generation, 136, 137, 279 
 Sexuality, nature of, 171, 172, 177, 
 
 181, 185, 204 
 Shade plants, 41, 44 
 Shin leaf, 470, 471 
 Sieve tubes, 79, 80 
 Silicates, 48, 170, 173, 336 
 Silk, of corn, 134 
 Siphonales, 195 
 Skull-cap, 477 
 vSkunk cabbage, 412 
 Sleep movements, 35 
 Slime mould, 149, 150, 151 
 Smallpox, 163, 164 
 Smuts, 257 
 Soapberry order, 456 
 Soils, care of, 54, 55 
 
 formation of, 48-50 
 
 nature of, 50-52 
 
 retention of substances by, 52 
 
 sterility of, 53-55 
 
 structure of, 51 
 
 texture of, 51 
 Solanum, 481 
 Soldier's cap, 443, 444 
 
 Solomon's seal, leaf, 7, 401 
 Soredium, 234, 235 
 Sorus, 316, 324, 325, 355 
 Spadix, 411 
 Spathe, 410, 411 
 Spathyema, 412 
 Spathyema, leaf structure, 41 
 Species, 146 
 
 biological, 245 
 Sperm. 196, 275 
 Spermatophyta, 352 
 Sphaerella, 174, 175 
 Sphaeriales, 239, 240 
 Sphagnales, 296 
 Sphagnum, 296, 297, 299, 300 
 Spindle, of cell, 57, 58 
 Spiral leaf arrangement, 27, 28 
 Spirillum, 155 
 Spirogyra, 185, 186, 187 
 Spleenwort, 329 
 Spongy mesoph\dl, 9, ii, 21 
 Sporangia, 117, 150, 192, 224, 225, 
 
 315, 325, 326 
 Spore, 115, 118, 151, 270 
 
 differentiation of, 339 
 
 germination of, 278, 300, 301, 
 
 330,331,338,349 
 Sporophylls, 115, 116, 117, 337, 338, 
 
 379, 381 
 Sporophyte generation, 136, 194, 215, 
 
 256, 281 
 Sporophyte, development of, 281, 
 286, 291, 293, 304, 310, 312, 318, 
 319, 332-334, 346, 359, 371, 395, 
 396, 399 
 Squash, leaf, 10 
 stem, 79, 80 
 Stamen, 115, 116, 379, 399 
 Staminodia, 421, 473 
 Standard, 451-453 
 Starch, 2, 12, 17, 18, 19 
 
 test for, 14 
 Staurastrum, 188 
 Steironema, 473, 474 
 Stem, branches of, 99, 100 
 dicotyledon, 84, 90, 93 
 function of, 71 
 growth of, 89, 90, 91, 92, 93, 98, 99 
 
INDEX 
 
 505 
 
 Stem, healing of, loi 
 modification of, 105 
 monocotyledons, 84, 96, 400 
 sensitiveness of, 104 
 strengthening tissues of, 83 
 structure of, 71, 73, 77, 90, 91, 
 92. 93 
 
 Stemonitis, 150 
 
 Stereome, 37, 38, 39, 78, 79, 84, 85, 
 
 323, 325 
 Stereum, 264 
 Sticta, 233 
 
 Stigma, 116, 117, 380 
 Stimuli, nature and effects of, 4 
 
 reaction to, 5, 33-36, 40, 61-63, 
 104,^106, 121 
 Stink horns, 268 
 Stipe, 260 
 Stipules, 426 
 Stolon, 109, no 
 
 Stoma, 8, 9, 10, 21, 25, 26, 37, 88, 293 
 Stone fruit, 398 
 Storage organs, 18, 59, 77, no, in, 
 
 113. 132 
 Strawberry, 384, 450 
 Streaming of protoplasm, 151, 152 
 Strobilus, 335, 337. 344, 347, 353. 
 
 365. 366, 374 
 Stroma, 238 
 Style, 116, 117, 380 
 Suber, 86 
 Subdivision, 147 
 Subkingdom, 148 
 Sugar, 12, 16, 17, 120 
 Sulphur, 16 
 
 bacteria, 161 
 Surface reductions, 44 
 
 tension, 51 
 Suspensor, 124, 125, 345, 346, 360, 
 
 371, 395. 396 
 Symbiosis, 68, 234 
 Sympetalae, 469 
 Symphytum, 385, 475, 476 
 Synergid, 118, 119, 120, 390, 391 
 Synthesis, 159, 160, 162 
 
 Tabellaria, 168 
 Tapetum, 379 
 
 Taraxacum. 485, 486 
 
 Taxodium, 376 
 
 Taxus, 376 
 
 Teeth, of peristome, 307, 308 
 
 Tclial stage, 254. 255 
 
 Teliospores, 254, 255 
 
 Teliospore stage, 255 
 
 Tendrils, 107, 108 
 
 Tetraspores, 213 
 
 Thallophyta, classification of, 149 
 
 Thallus, 149, 270 
 
 Thaxter, 153 
 
 Thelephoraceae, 263 
 
 Thistle family, 485, 489 
 
 Thuja, 376 
 
 Tissue, strengthening, 83 
 
 Toadflax, 478, 479 
 
 Toadstools, 259, 260 
 
 Toxins, 158, 162, 163 
 
 Tracheids, 9, 81, 94, 363 
 
 Tradescantia, stoma, 38 
 
 Transpiration, 8, 24, 35 
 
 Transport of water, 59, 79, 134 
 
 Tree ferns, 325 
 
 Truffles, 243 
 
 Tsuga, 375 
 
 Tube, cell, 121, 122, 358, 369-371 
 
 392, 393 
 Tubercles, 65, 161 
 Tuberculosis, 163 
 Tubers, in 
 Turgor, 83 
 Twigs, 73 
 
 Twining stems, 106 
 Typha, 381, 401, 403 
 
 Ulothrix, 190, 191, 193 
 Umbel, 466 
 Umbel lales, 465 
 Uncinula, 245, 247 
 Unilocular pistil, 473 
 Uredinial stage, 254, 255 
 Uredinales, 251 
 Urediniospoies, 254, 255 
 Urticales, 434, 436 
 Ustilaginales, 257 
 Ustilago, 258 
 Utricularia, 480 
 
5o6 
 
 INDEX 
 
 Vaccinium, 472 
 
 Vacuoles, i, 2, 3, 60, 217 
 
 Valerian, 483 
 
 Valves, of diatoms, 170 
 
 Vapor, from plants, 24 
 
 Variation, 105, 144, 176, 190, 197, 
 
 218, 280, 312-313. 320, 388 
 Variegated plants, 14, 32 
 Vascular bundles, 11, 12, 16, 76, 77, 
 
 78, 79, 80, 97, 312, 314, 323, 324, 
 
 337, 342, 354 
 Vaucheria, 195, 196 
 Veins, 7, 9, 11, 301, 302, 312 
 Velamen, 422 
 Veltheimia, 402 
 Velum, 260, 261 
 Venation, 7, 400, 401, 426, 427 
 Veratrum, 416 
 Vernonia, 491 
 Versatile anthers, 408 
 Vessels, 79, 80 
 Viburnum, 481, 482 
 Vitis, 461 
 Volva, 261 
 Volvocales, 174 
 Volvox, 180, 181 
 
 Wallace's Law, 423 
 
 Wall cells, 277, 318, 358, 392 
 
 Walnut order, 433 
 
 Ward, 24 
 
 Water-bloom, 165 
 
 Water, action on rock, 49 
 
 capillary, 50 
 
 composition of, 12 
 
 gravitational, 50 
 
 hygroscopic, 50 
 
 lily, 398, 438 
 
 mould, 218 
 
 net, 184 
 
 Watering of plants, 55 
 Wax of epidermis, 37, 39 
 Weeds, 445 
 Willow, 381, 430 
 
 order, 429 
 Wilting, cause of, 24 
 Wind, pollination by, 388, 404, 433, 
 
 458 
 Winged fruits, 373, 458 
 Wings, of flowers, 451-453 
 Witches' broom, 256 
 Wolfifia, 410 
 Wood, heart, 95 
 
 oak, 95, 427 
 
 pine, 363 
 
 sap, 95 
 
 sections, 93, 94, 95 
 
 spring, 91, 92, 95 
 
 summer, 91, 92 
 W^oodsia, 328 
 Woodwardia, 326, 328 
 Work of plants, 15 
 Working of ponds, 165 
 
 Xanthidium, 188 
 
 Xerophytes, 39, 45 
 
 Xylaria, 239, 241 
 
 Xylem, 59, 76, 78, 79, 91, 314-315 
 
 Yamanouchi, 215 
 Yeast, 248, 249 
 Yucca, 419 
 
 Zamia, 353, 354, 355, 356 
 Zooglea, 155 
 Zoospore, 151, 152 
 Zygnema, 185, 186 
 Zygnematales, 185 
 Zygomorphic, 387 
 
 ^ 
 
Ilfif If nil iMIIIzviyimzfMmltf mI ^liUm^w 
 fltlitt Till illrlTtflftllrinll^it III lilsuiltfllH 
 
 ^^^H^ 
 
 ^ 
 
 1 
 
 ■ 
 
 
 
 ^^B\ 
 
 
 ^k] 
 
 
 1 
 
 ■ 
 
 n 
 
 % 
 
 h 
 
 k.