THE BOLE OF DIFFUSION AND OSMOTIC PRESSURE IN PLANTS LIVINGSTON % §. p. PtU pfearg |£foritj (ttaralma J^tate College QHfel Cop. I ?*.* N.C. STATE UNIVERSITY D.H. HILL LIBRARY S00273093 O Date Due Nov27*| 280 c isoct 43 XI fi 1977 -^TT- r H2K i 1 — j Owl _M 1977 -0-4 W X — - 1 XUi c nap t*^H^-« L. B. Cat. No. 1 137 THE DECENNIAL PUBLICATIONS OF THE UNIVERSITY OF CHICAGO THE DECENNIAL PUBLICATIONS ISSUED IN COMMEMORATION OP THE COMPLETION OF THE FIRST TEN YEARS OF THE UNIVERSITY'S EXISTENCE AUTHORIZED BY THE BOARD OF TRUSTEES ON THE RECOMMENDATION OF THE PRESIDENT AND SENATE EDITED BY A COMMITTEE APPOINTED BY THE SENATE EDWAED CAPPS STAKE WILLAED CUTTING EOLLIN D. SALISBUBY JAMES BOWLAND ANGELL WILLIAM I. THOMAS SHAILEE MATHEWS CAEL DAELING BUCK FEEDEKIC IVES CAEPENTEE OSKAE BOLZA JULIUS STIEGLITZ JACQUES LOEB THESE VOLUMES ARE DEDICATED TO THE MEN AND WOMEN OF OUR TIME AND COUNTRY WHO BY WISE AND GENEROUS GIVING HAVE ENCOURAGED THE SEARCH AFTER TRUTH IN ALL DEPARTMENTS OF KNOWLEDGE 1243/ DIFFUSION AND OSMOTIC PRESSURE THE ROLE OF DIFFUSION AND OSMOTIC PRESSURE IN PLANTS BY BURTON EDWARD LIVINGSTON OF THE DEPARTMENT OF BOTANY THE DECENNIAL PUBLICATIONS SECOND SERIES VOLUME VIII CHICAGO THE UNIVERSITY OF CHICAGO PRESS 1903 o VJ Copyright 1903 BY THE UNIVERSITY OF CHICAGO ill & a State College PREFACE With the ever-increasing tendency to regard an organ- ism as a complex of physical and chemical processes which may one day be analyzed and understood, there has neces- sarily gone hand in hand a tendency toward more and more accurate and quantitative investigation of the physics and chemistry of the cell itself. Among the various groups of physical and chemical phenomena that have been found to play important roles in the life-process, and which, there- fore, have been interrogated for answers to physiological questions, none has stood out within the past few years as more fundamentally important than those connected with diffusion and osmotic pressure. This field has thus far only been touched upon, and it would seem, judging from researches which have recently appeared, that the best and most far-reaching work therein is probably yet to come. The present volume will deal with the past and present of diffusion and osmotic pressure from the standpoint of plant physiology. It has a double raison d'etre. First, it was felt that there was need of some direct and not too exhaustive account of the essential physical facts and theories of the subject. The interest of the physical chemist here has lain mainly in the light which these phenomena have been able to throw upon the ultimate nature of matter and upon elec- trolytic processes. It has thus been difficult for the student of physiology who is not at the same time well versed in physical chemistry to obtain the information required for the prosecution of work in this field. Secondly, it seemed desirable to bring together in a general review the literature of this subject in its biological aspects, so that the promising and unpromising points for future research might become IX 12437 Peeface more apparent. The volume will thus naturally fall into two Parts, the first dealing with the purely physical aspect of these phenomena, and the second attempting to present in a more or less unified whole the physiological results which have so far appeared in this connection, together with their bearing upon each other and upon the vital problem as a whole. Chapter IV of Part II was presented to the Faculty of the Ogden Graduate School of Science of the University of Chicago in candidacy for the doctor's degree in 1901. The author wishes here to express his thanks to Professor C. R. Barnes, of this laboratory, and to Professor Jacques Loeb, of the Hull Physiological Laboratory, for much valu- able aid. Professor Barnes has kindly read the manuscript and has made many suggestions. The author alone is, how- ever, responsible for whatever new departures are to be found in the book. B. E. L. The Hull Botanical Laboratory, The University of Chicago, October*!, 1902. TABLE OF CONTENTS Preface ------ ix PART I. PHYSICAL CONSIDERATIONS Introduction 1 Chapter I. Matter and Its States - 3 i. Fundamental Theories of the Nature of Matter. a) The Atomic Theory. b) The Kinetic Theory. ii. The Three States of Matter. Chapter II. Diffusion and Diffusion Tension 9 i. Gases. a) Simple Gases. b) Mixed Gases, /n} Liquids. (a) Simple Liquids. *■ b) Mixed Liquids, in. Solids. a) Simple Solids. b) Diffusion of Two Solids. Chapter III. Liquid Solutions - 16 % Liquids in Liquids. ii. Gases in Liquids, in. Solids in Liquids, iv. Terminology for Solutions of Differing Concentration. Chapter IV. Ionization - - - - - - - 23 i. Of Gases. ii. Of Solutes in Liquid Solutions. Chapter V. Osmotic Phenomena - 25 i. Osmotic Pressure of the Solute. a) Non-electrolytes. b) Electrolytes. c) Colloids. d) Osmotic Pressure in General. xi xii Table of Contents ii. Diffusion Tension of the Solvent. in. Experimental Demonstration of Osmotic Pressure. Chapter VI. Measurement and Calculation of Osmotic Pressure - - - 35 1. Measurement of Osmotic Pressure. a) Direct Method. b) Indirect Methods. 1. Freezing-point. 2. Boiling-point. 3. Vapor Tension. 11. Calculation of Osmotic Pressure. a) Non-electrolytes. b) Electrolytes. PART II. PHYSIOLOGICAL CONSIDERATIONS Introduction 47 Chapter I. Turgidity - - 49 1. Protoplasm and its Limiting Membranes. 11. Plasmolysis. in. The Permeability of the Protoplasmic Layers. a) Test by the Plasmolytic Method. b) Direct Test of Penetrability. c) Absorption Test. d) Test by Toxicity. e ) Test by Accumulation. /) Test by Metabolic Processes. g) Outward Permeability. h) Variations in Permeability, iv. Action of the Protoplasmic Membrane, a) The Filter Theory. b) The Solution Theory. c ) The Chemical Theory. v. The Nature of the Osmotically Active Solutes, vi. The Maintenance of Turgidity in Spite of Permeability to Certain Solutes, vii. The Relation of Turgidity to Vital Activity. a) The Retention of Form. b) Mechanical Support. c) Growth. d) Curvature. e) Work. viii. Summary of the Chapter. Table of Contents Xlll Chapter II. Absorption and Transmission of Water - - 91 i. Absorption of Water. II. Transmission of Water. a) Water Loss. (1) Evaporation. (2) Water Pores and Nectaries. (3) Exudation. (4) Summary of Water Loss. b) Upward Movement of Water in Trees and Other Tall Plants. in. Summary of the Chapter. Chapter III. Absorption and Transmission of Solutes - - 115 i. Absorption of Gases. ii. Absorption of Dissolved Solids and Liquids. in. Transmission of Solutes. ^Chapter IV. Influence of the Osmotic Pressure of the Sur- rounding Medium upon Organisms 124 i. Introductory. ii. Presentation of Material. a) Influence upon Growth and Form. b) Influence upon Reproduction. c) Influence upon Irritability. (1) Changes of Irritability. (2) Osmotaxis. d) Analogy between the Effects of High Osmotic Pres- sure of the Medium and Those Produced by Other Water-Extracting Processes. in. Summary of the Chapter. Index 145 PART I PHYSICAL CONSIDERATIONS INTRODUCTION In the following treatment of the physical phenomena of diffusion and osmotic pressure no attempt is made to be exhaustive. Certain aspects only of the present conceptions of these matters among most physicists and chemists 1 are discussed, and every discussion is presented with the sole aim of clearing the way for the physiological discussions which are to follow. Thus, for example, the whole subject of atomic and molecular weights and their experimental determination — so important to the chemist, but not pri- marily interesting to the physiologist — is entirely omitted. Also it may be added that no attention is given to a his- torical treatment of this part of the subject, the excellent chemical treatises which are now available rendering this unnecessary. In the present Part very little is original with the author, excepting the mode of presentation. The various texts and the original papers have been drawn upon wherever it has seemed expedient. Footnotes give the names of the authors i A general confusion among younger students with regard to the way in which these conceptions take the form of theories makes it seem expedient to call atten- tion to the following points : A scientific theory does not pretend to state the truth. It may sometimes state a part of the truth, but this is not primarily its aim. Its aim is to connect the facts together in the most logical and plausible manner pos- sible, and thus to aid the further advancement of our knowledge. Its M employment has its origin in the organization of the human mind, which handles abstract truths much less easily by themselves than by the help of an illustrative image A hypothesis [or theory] can neither be proved nor disproved. It is merely a tool which is rejected when found to be no longer serviceable What the 'real' nature of matter is, is to us a matter of complete ignorance, as it is of complete indifference." (Ostwald-Walkee, Outlines of General Chemistry [London, 1893J, pp. 5, 7.) A principle, on the other hand, does attempt to state the truth; it is a generalization and induction from a great number of known facts. When a fact is discovered which cannot be included under a principle, then that principle falls to the ground and ceases to be a principle. What was at first a theory may at length, by the accumulation of evidence, come to be a principle. 1 Introduction to whom we are indebted for the more important points. The student of this subject will find the following standard texts .very helpful: Nernst, W. Theoretical Chemistry from the Standpoint of Avo- gadro's Rule and Thermodynamics. Translated by C. S. Palmer. London, 1895. Ostwald, W. Lehrbuch der allgemeinen Chemie, comprising : Vertvandschaftslehre, Leipzig, 1887 ; Stoechiometrie, Leip- zig, 1891; Chemische Energie, Leipzig, 1893. Solutions. Translated by M. M. P. Muir. London, 1891. Outlines of General Chemistry. Translated by James Walker. London, 1895. Manual of Physico- Chemical Measurements. Translated by James Walker. London, 1894. The Principles of Inorganic Chemistry. Translated by Alex- ander Findlay. London, 1902. Remsen, I. Principles of Theoretical Chemistry. New York, 1897. Walker, J. Introduction to Physical Chemistry. London, 1899. Reychler, A. Outlines of Physical Chemistry. Translated by J. McCrae. London, 1899. Dastre, M. A. "Osmose," in Traits de physique biologique, publie* sous la direction de MM. D'Arsonval, Gariel, Chauveau et Marey. Tome I. Paris, 1901. Cohen, Ernst. Vortrdge fur Aerzte ilber physikalische Chemie. Leipzig, 1901. Jones, H. C. The Elements of Physical Chemistry. New York, 1902. Whetham, W. C. D. Solution and Electrolysis. Cambridge, 1895. Kohlradsch, F., and Holborn, L. Das Leitvermogen der Elektro- lyte. Leipzig, 1898. Traube, J. Physico- Chemical Methods. Translated by W. L. Hardin. Philadelphia, 1898. Blitz, Henry. Practical Methods for Determining Molecular Weights. Translated by Jones and King. Easton, Pa., 1899. Hamburger, H. J. Osmotischer Druck und Ionenlehre. Wies- baden, 1902. CHAPTER I MATTER AND ITS STATES I. FUNDAMENTAL THEORIES OF THE NATURE OF MATTER a) The atomic theory. — The whole structure of modern physical science is based upon the atomic theory. This theory supposes every mass of matter to be composed of numerous ultimate, indivisible particles, called atoms, which possess a peculiar attraction for one another. Atoms differ in the amount and nature of this attractive force, those of every chemical element being in this way different from those of every other; but all atoms of the same element, when under the same conditions, are exactly similar. Owing to their chemical attraction, atoms seldom exist free as such, but are prone to unite into groups, thus causing the neutralization of their mutual attraction. The groups so formed are called molecules. If the atoms composing the molecules of any substance are alike (i. e., of the same element), the element is said to be in the molecular condition — as opposed to the atomic or nascent condition, in which the atoms are not united to one another, but exist free as such. When the atoms forming a molecule are of different chemical elements, a compound is said to be formed. The physical and chemi- cal properties of molecules are very different from those of their component atoms, and they are also very different from those of any molecules which can be formed in any other way. But all molecules which are formed of the same elements and in the same manner are exactly similar under the same conditions. It thus appears that the smallest par- ticle of a compound which can exist and still retain the properties of that compound is the molecule ; break this up, 3 Diffusion and Osmotic Pressure and free atoms or new groups of atoms, with new properties, will result, the original compound having been destroyed by the process of separation. Atoms may also unite in such a way that their mutual attractive forces are but partially neutralized, thus forming incomplete parts of molecules, called ions. Under certain special conditions molecules may split into two or more ions, and some of these cases of ionization or dissociation, as the process of splitting is called, have proved very important in the development of the sub- ject of osmotic pressure. In some cases an ion may consist of a single atom which has split off from some molecule. Briefly, then, acording to the atomic theory as now made use of, the nature of any mass is dependent upon that of its component particles, these particles being atoms, molecules, or ions. The same mass may contain, at the same time, all three kinds of particles. b) The kinetic theory of matter. — According to the kinetic theory, the particles composing any mass, whatever their nature mav be, are in constant motion. This necessi- tates their being considered, not as packed closely one against another to make up the mass, but as separated from one another by continuously varying spaces. The continuous motion of the particles is probably for the most part a vibratory motion. They are supposed to move in straight lines and in the same direction until a collision occurs, when they rebound according to the principle of the reflection of moving bodies. It thus becomes necessary to consider, for comparison, the average distance apart of these particles, or their average or mean free path. This has been demonstrated to be much greater than the diameter of a single particle. A rough conception of the state of affairs within a mass of matter may be obtained by comparing the mass to a cage of angry bees. The insects in such a cage fly in straight Matter and Its States lines to and fro, striking against each other and against the walls of the cage, ever varying their distances apart yet always remaining equally distributed throughout the cage, t. e, always keeping their average distance apart the same.' Thus far nothing has been said of any restraining force to counteract in a measure the motion of the particles and keep them from flying apart indefinitely. Such a force might be roughly compared to the walls of the cage just referred to, for it is these restraining walls which prevent the indefinite' enlargement of the swarm of angry insects. More accu- rately, the restraining force in the illustration is the sum of the reactions produced by the several impacts of the movin- insects against the rigid walls. There is, indeed, such an active restraining force present in all masses of matter; it is ordinarily made evident, however, only in liquids and solids. This force is the cohesion of the particles themselves. It is probably akin to gravitation, in exhibited larger bodies, and is an inverse function of the square of the average dis- tance apart of the moving particles. That is, the mutual attraction exerted by two particles decreases at the same rate as the square of their distance apart increases. It will thus be seen that this force becomes negligible at a comparatively small distance from any particle. But the particles of liquids and solids are so near to one another that their cohe- sive force is sufficient to overcome, to a certain extent, their energy of motion and to hold most of them within certain fixed limits of space. The science of thermo-dynamics rests upon another sup- position of the kinetic theory of matter, namely, that the tem- perature of any body is directly due to the kinetic energy of its vibrating particles. Since the mass of any particle remains constant, and the kinetic energy of any moving body is, at any instant, one-half the product of its mass and the square of its velocity (KE = \MV l ), it is seen that the average kinetic 6 Diffusion and Osmotic Pkessuke energy of a particle varies with the square of its average velocity. We neglect here, as comparatively unimpor- tant, all other forms of motion which a particle may possess, such as that of rotation, and consider only its transla- tory motion. Therefore, whenever the temperature of a quantity of matter is raised by any means, the average translatory velocity of its particles is increased. Now, the force of impact of a moving body is proportional to its momentum, which is equal to the product of its mass and velocity at the time of impact. But since one particle may strike another particle at any point in its free path, here again the average velocity must be considered. Therefore, since the mass of a particle is a constant quantity, any increase in the average velocity will cause a corresponding increase in momentum, and also in the force of impact. But the force of recoil is practically equal to the force of impact, and this latter force is the repellent force which tends to separate the particles. Thus, with rising temperature the repellent force is increased, the force of cohesion is more and more nearly overcome, and the particles become more and more widely separated. Also, with the rapid decrease in the cohesive force incident upon the increase in its acting distance, a limit is soon reached beyond which the force tending to cause separation is greater than the other, and the particles fly apart indefinitely. In this condition we say the substance is a gas. If it was a liquid or solid at the lower temperature, it has now been vaporized by heat. II. THE THREE STATES OF MATTER Matter exists in three states — the gaseous, the liquid, and the solid. In gases the kinetic energy of the particles is so great that the cohesive force is entirely overcome and the particles tend ever to increase their distance apart. From this it necessarily follows that a mass of gas in a closed Matter and Its States vessel will completely fill it, no matter if the vessel be many times the size of the original volume of gas. This is an observed fact. If such a gas is gradually cooled (/. <°., allowed to do work on some other body and thus to part with some of the kinetic energy of its particles), a condition will be reached wherein the cohesive force is greater than the repellent, and the par- ticles will remain together in a definite volume. As long as the two forces involved are nearly equal, the average free path will still be relatively great, and although the particles cling together, yet they will move very freely upon one another — a condition imperfectly simulated by the component grains in a mass of sand. In this condition the substance is said to be a liquid. Here the particles move so readily upon one another that a mass of liquid still takes the form of the containing vessel, as far as that is possible without increase in volume. In this regard liquids are very differ- ent from gases. Also, oh account of the freedom of motion on the part of the particles making it up, and on account of the downward pull of gravity, the free surface of a liquid is usually approximately level. There are, indeed, certain phenomena of surface tension and adhesion which make it possible for free liquid surfaces to exist in other positions than the horizontal, but the present subject does not lead to a discussion of these. It is necessary to call attention, how- ever, to the fact that, on account of the action of the cohesive force, a peculiar surface layer of particles is formed about a liquid mass, a sort of thin skin or film, which possesses con- siderable tensile strength, and which is much less easily penetrated than the internal mass. By a continuation of the process of cooling (which must ever be thought of as a process of causing the body to give up kinetic energy by doing work, such as warming another cooler body) the liquid particles may be brought still closer 8 Diffusion and Osmotic Pressure together, until cohesion becomes so strong, and hence the friction of particle upon particle so great, that the free movement upon each other just described comes practically to an end. The body is now a solid and will retain its form without surrounding walls. The particles are still in violent vibration, however. It should be stated here that the ideal gas, liquid, or solid does not exist ; the hardest substances show some tendency to flow like liquids, and the most fluid substances exhibit some friction of their component particles upon one another. CHAPTER II DIFFUSION AND DIFFUSION TENSION I. GASES a) Simple gases. — As has been indicated already, it is a fundamental property of all gases that they tend to fill com- pletely any vessel in which they may be inclosed. Thus, if a cubic centimeter of oxygen is measured out at ordinary temperature and at atmospheric pressure, and is then passed into a sealed vacuum chamber, it will completely fill the chamber, no matter how large the latter may be. This process of expansion is called diffusion. Of course, in dif- fusing, the particles of which the gas is composed become distributed throughout a greater space, and hence the gas becomes less dense. This property is often stated as follows : "The particles of gases tend to separate indefinitely.' ' Because of this tendency to expand, an outward pressure, called gas pressure, is exerted by a gas upon the walls of any chamber in which it may be confined. Gas pressure is supposed to be caused by the continuous bombardment of the walls of the inclosing vessel by the vibrating gas par- ticles. If a gas be inclosed in a chamber with elastic walls, the size of the chamber will depend upon the number of particles of gas present (i. e., its concentration) and upon the kinetic energy of the particles themselves (7. " che '>. intra-abdominalen Drucke. anf die Resorp ion^n do" :S«lS.He ""lea ^ "? G PldsiJ Wl^' , ?" ° ^ t,on ° f the copper fe"«>y«nW membrane c :zz, v-oicSii " Vol. XIT«PP. mS. Meth ° de ~ AEal,Z0 d » T "*«k»ft.'V«-r»./. .ta M. 4 lB Sw B ' Eolatioaof Nutrient Salts to Turgor -" *«■ «-.** 4 G. Kraus, Stoffwechsel der Crassulaceen, 1886. XXXVH n^ EIES '«^f er d ' Bed T eutu ^ d - PAanzensfturen," etc., Bot. Zci,,. Vol. S,M 6 °' ? 'T° N ^ A / ENBUEG ' "Losungscoocentration und Turgorregulation bei den Schimmelpilzen," JoArfc/. «,«». i? * M Vol. XXXVI (1901), pp. 3S1-4LU 84 Diffusion and Osmotic Pkessuee niger, when grown in strong nutrient solutions of sugar, etc., escapes plasmolysis by an enormous increase in concentra- tion of the cell sap, this being produced in part by true carbohydrates, but mainly by some unidentified substance, which is, however, probably closely related to the latter group. Maquenne 1 found by the freezing-point method that the expressed juice of seedling peas six days old had an osmotic pressure such that the average molecular weight of the solutes must be in the neighborhood of 239. This shows that the active substance has a much larger molecule than glucose (mol. wt. 180). The juice failed to show the pres- ence of glucose or cane sugar by tests made with Fehling's solution and with phenylhydrazin acetate. Helianthus seed- lings, however, showed the presence of glucose, and the sap had an average molecular weight of only 136. Here, then, the pressure is largely produced by some molecule much smaller than that of glucose. Stange 2 found that the cells of growing root tips of Pisum, Lupinus, etc., which had been grown in strong KN0 3 solutions, showed no accumulation of that salt, although there was a marked increase of it in the parenchyma farther up. The turgor pressure was the same in the root tip as elsewhere in the plant. This argues that the increase in turgor in the growing region, which prevents plasmolysis where the culture is in a strong solu- tion, must be due to some other substance or substances. A collection of analyses of various plant parts, which may be taken as at least some indication of the nature of the active substances in the sap, is given by Mayer. 3 DeVries 4 made 1L. Maquenne, "Sur la pression osmotique dans les graines germees," Compt. rend., Vol. CXXIII (1896), pp. 898 ff. 2B. Stange, "Beziehungen zwischen Substratconcentration, Turgor und Wachsthum bei einigen phanerogamen Pflanzen," Bot. Zeitg., Vol. L (1892), p. 253. 3 A. Mayer, Lehrbuch der Agriculturchemie, Heidelberg, 1895, pp. 307 ff. *H. DeVeies, u Eine Methode zur Analyse der Turgorkraft," Ja/u-6./. wiss. Bot., Vol. XIV (1884), pp. 427-601. TURGIDITY 85 analyses of the sap itself and his results show that different plants vary much as to the nature of their osniotically active substances. Heald 1 has recently determined the electrical conductivity of sap expressed from the roots, stems, and leaves of various plants. The amount of electrolytes thus indicated is in reasonably close agreement with the amount of ash found by incineration. This only goes to show that most of the electrolyte molecules are dissociated in the sap, and are therefore active in conducting the current. Any conclusions with regard to the osmotic pressure of the sap which are based on conductivity methods must be absolutely unreliable, unless it is first ascertained that there are no non-conductors present, and also that the electrolytes present are in the ionic condition. But it is probably impossible to find a natural plant juice whose solutes are all electrolytes. Therefore Heald's method cannot be of use in determining osmotic pressures. The freezing-point, boiling-point, and vapor ten- sion methods are applicable to this problem, however. Maquenne's work on the freezing-points of plant juices 2 has recently been added to by Sutherst. 3 The latter author has merely given the freezing-points, without determining the weight per liter, so that his results will not be available for any determination of the nature of the solutes. In the following table, taken from his paper, I have calculated the osmotic pressures to facilitate comparison: TT , , t pv r»f Os. Pressure in Vegetable marrow: * r - Ft - mm . of Hg. Leaf and stalk -0.75° 6,880 Fruit -0.75° 6,880 Swede turnip: Entire plant - - -1.0° 9,173.2 IF. de F. Heald, "The Electrical Conductivity of Plant Juices," Science, N. S., Vol. XV (1902), p. 457; idem, same title, Bot. Gaz., Vol. XXXIV (1902), pp. 81-i*2. 2 See above, p. 84. 3 W. T. Sutherst, " The Freezing Point of Vegetable Saps and Juices," Chem. News, Vol. LXXXIV (1901), p. 234. 86 Diffusion and Osmotic Pressuke „ , -pi,, -p. Os. Pressure in Celery : * r - Ft * mm. of Hg. Green stalk and leaf - - - -1.4° 12,842.48 White parts -0.75° 6,880 Carrot: Leaf and stalk - -1.2° 11,007.84 Koot - -1.0° 9,173.2 Cabbage: Outer leaf - - - - - -1.1" 10,090.52 Heart - -0.85° 7,797.22 Apple, fruit - - ... -1.4- 12,842.48 Pear, fruit ------- -1.75° 16,053.2 From the evidence at hand it may therefore be concluded that there is great variability among different plants with regard to the particular substances called into requisition to maintain the turgor pressure. There seems to be a gen- eral tendency for these to be of an organic nature, and to possess rather complex molecules. 1 VI. THE MAINTENANCE OF TURGIDITY IN SPITE OF PERMEA- BILITY TO CERTAIN SOLUTES It might be supposed that the fact of greater or less per- meability of the protoplasm to various solutes would lessen the value of the osmotic explanation of the phenomenon of turgidity. This, however, does not necessarily follow. While certain substances are diffusing in and out of a cell, its turgidity may be maintained by the presence within the vacuole of some other osmotic substance or substances to which the protoplast is impermeable, or very slightly permeable. It is probable that this is what occurs in living plant cells. These effective osmotic substances are usually of the nature of carbohydrates, plant acids, and mineral salts. They are probably secreted into the vacuole by the activity of the surrounding protoplasm. How this can occur is not yet known. The process must involve movement of 1 Cf. Pfeffee-Ewart, Physiology of Plants, Cambridge, 1900, p. 141. TURGIDITY 87 solute particles from a lower concentration to a higher, against their own diffusion tension. It may be that these substances are freed from the protoplasm in a certain form, and that, after entering the vacuole, they polymerize or change their nature in some way, according to special con- ditions there existing. The accumulation of many substances within the vacuole (e. g., anilin dyes 1 and caifein) is surely due to a chemical reaction after the substance has passed the protoplasmic layers. VII. EELATION OF TURGIDITY TO VITAL ACTIVITY Only because of the existence of the phenomenon of tur- gidity has the plant organism been able to develop into what it is. In several ways turgidity is absolutely essential, and in many others advantageous, to vital activity as it is now exhibited in plants. a) The retention of form. — By means of turgor pressure the delicate fluid or semi-fluid plasma of the cell is kept pressed out against the cellulose wall, and the plasmic mem- branes are thus kept in a uniform state of tension, upon which condition some of their physical properties doubtless depend ; as has been seen, for instance, great variations in turgor may so affect the protoplast as utterly to change its permeability to certain solutes. b) Mechanical support. — All parenchymatous tissues and nearly all filamentous and uni-cellular forms owe most of whatever rigidity they possess to the stress set up between the internal osmotic pressure and the elastic force of the stretched cell walls. A heavy weight might be supported upon a pile of inflated footballs if they were properly placed, but if the individual balls were leaky they would no longer be available for such a purpose. In a similar manner the plasmolysis of any thin-walled tissue is accompanied by a ilbid., pp. 119 ff. 88 Diffusion and Osmotic Pressure more or less marked collapse. Also, the existence of differ- ences in the turgor, and hence in the tissue tensions, of dif- ferent parts of the body in higher plants, increases the mechanical strength of the whole structure to a very marked decree. The difference in tension between the pith and cor- tex of many herbaceous stems is an illustration of this fact. c) Growth. — Exactly what the relation between growth and turgor may be cannot yet be stated, but there is good evidence to show that in the presence of turgidity growth is accelerated and in its absence retarded. Increase in thick- ness of the cell wall cannot take place unless the protoplast is kept turgid, and thus closely applied to it, as was shown by Keinhardt. 1 Other evidence along this line is that obtained by Klebs, 2 when he succeeded in causing a new cellulose wall to form within the old one by keeping the pro- toplast in a state of plasmolysis. Also the work of Bower 3 needs to be considered here. This author brings out very clearly the fact that there is a close connection between wall and protoplasm, by a study of the strands and fibers which remain joining the protoplast to the wall in a plasmolyzed cell. He thinks that the attachment of the protoplasm, which results in the formation of these strands, is closely con- nected with the process of wall-formation. The strands are not usually opposite on the two sides of a common wall, and thus apparently have no relation to pores through the wall. Experiments on the effect of light and temperature led Copeland 4 to the conclusion that growth regulates turgor rather than turgor growth. i M. O. Reinhaedt, u Plasmolytische Studien zur Kenntniss des Wachsthums der Zellmembran," Festschrift fur Schwendener (1899), p. 425. 2 G. Klebs, " Beitrage zur Physiologie der Pflanzenzelle," Unters. aus d. bot. Inst, zu Tubingen, Vol. II (1888), pp. 489-568. 3 F. O. Bower, " On Plasmolysis and its Bearing upon the Relations between Cell-wall and Protoplasm," Quart. Jour. Microsc. Sci., Vol. XXIII (1885), pp. 157-457. *E. B. Copeland, Ueber d. Einfluss von Licht u. Temperatur auf den Turgor, Halle a. S., 1896. TURGIDITY 89 d) Curvature. — Since most plant curvatures are due to modified growth, it is to be expected from what has just been said that turgor must play an important part in these phenomena. A discussion of the relations of this question will be found in Pollock's paper 1 on root curvature. To enter into this much mooted question would be going too far afield from the present subject. Other more rapid curvatures (such as those of the pulvini of the leaves of the Mimosa, the stamens of Berberis, various tendrils, etc.), which are not due to growth, have already been mentioned. They owe their existence entirely to turgor changes. e) Work. — Turgor is also of great benefit to the plant in that it gives it a means of doing work, of overcoming resistance. The lifting of sidewalks and buildings and the splitting of cliffs by the roots of trees are examples of the extent to which this pressure may be developed. For it must be remembered that the growing region of any plant is always mechanically weak; it owes practically all its power of resistance to the turgidity of the cells. Pf effer 2 has made many tests and measurements in this field, and Rodewald 3 has applied mathematics to the problem, showing how osmotic pressure is a very considerable source of energy to the plant. Of course the energy ultimately comes from the heat of external objects. VIII. SUMMARY OF THE CHAPTER Turgidity is the immediate result of osmotic pressure within the cell. It arises from pressure developed within the cell sap of solutes which are unable to penetrate the surrounding ij. B. Pollock, "The Mechanism of Root Curvature," Bot. Gaz., Vol. XXIX (1900), pp. 1-63. 2 W. Pfeffer, "Druck und Arbeitsleistungdurch wachsendo Pfianzen," Abhandl. d. k. sdchs. Ges. d. Wiss, zu Leipzig, math.-physik. Klasso, Vol. XX (1893), p. 285. 3 H. Rodewald, "Ueber die durch osmotische Vorgauge moglicke Arboitsleistuug der Pfianzen," Ber. d. deutsch. bot. Ges., Vol. X (1892), pp. 83-93. 90 Diffusion and Osmotic Pkessure plasmatic membranes or which penetrate them very slowly, the concentration (and hence the pressure) being greater within the cell than in the external solution. The internal concentration is probably kept up by the chemical activity of the protoplasm itself, substances of the nature of soluble carbohydrates or organic acids being formed and secreted into the vacuole. How such a movement of solutes against the direction of their own diffusion tension can occur is not yet explained. Perhaps they change their nature after leav- ing the protoplasmic layer and entering the vacuole. Turgor pressure may vary in different cells through wide limits (from less than two to more than a hundred atmospheres) and in the same cell the variation during different periods of growth may be almost as great. Turgidity is influenced by variations in the amount of water at hand and also by various conditions which affect the permeability of the protoplasm directly. CHAPTER II ABSORPTION AND TRANSMISSION OF WATER I. ABSOKPTION OF WATER As has been seen in the previous chapter, it is absolutely essential that every living mass of protoplasm be saturated with water. This is so primarily on account of the fact that the energy transformations which are designated as vital phenomena occur solely in aqueous solutions. It is also true because of the fact that water is actually used in these transformations ; it is chemically combined with other sub- stances to form carbohydrates, proteids, etc. Therefore it becomes necessary that every active cell be not only satu- rated with water, but also that it be in connection with an external supply of this material. Especially is this so where water is lost by evaporation. It is possible, of course, for a plant cell to become hermetically sealed within a water-proof wall (e. g., fungus spores, etc.), but as long as it is active and growing it cannot be so shut off from the outward sup- ply of water. If the cell be naked and immersed in water, the supply of this substance is always at hand and simply diffuses into the protoplasm as it is used in metabolism. If the organ- ism be in contact with a moist substratum throughout most of its surface, as is the Myxomycete plasmodium, the absorption of water takes place from the imbibed water of the substratum. When the cells are surrounded by cellu- lose membranes, these are kept saturated by diffusion from without, and the protoplasm absorbs its needed water from them. The cellulose walls of ordinary cells act like the porous and imbibed substratum against which the Myxomy- 91 92 Diffusion and Osmotic Pressure cete plasmodium lies. Thus, if the plant body is not exten- sive and is mostly in contact either with free aqueous solu- tion or with some moist substance, the problem of how it obtains water is a simple one. Also, mere diffusion from one part of the organism to another, and that for only short dis- tances, is sufficient to account for all the water transport to be met with in such plants. But where the plant body elongates upward from its sub- stratum (a phenomenon occurring in all forms, from the sporophores of fungi to the stems of higher plants), it becomes more difficult to point out exactly how every living cell is kept saturated with water. In the case of fungi the rhizoidal part of any filament is in direct contact with the substratum, and here the solution of the substratum is con- tinuous with that of the protoplasm, through the saturated cellulose membrane. The sporophore is in connection with absorbing rhizoids and through these it absorbs not only what water is needed for growth, metabolism, etc., but also what is needed to replace that lost by evaporation. For if the loss by evaporation is not made up, death, or at least suspension of activity, must ultimately ensue from desicca- tion. The cellulose walls which are exposed to the air are more or less thickened, thus causing water loss by evapora- tion to be much less pronounced than it would otherwise be. Indeed, fungi are seldom found growing in localities where evaporation is very marked. In the case of algal filaments, which grow upward from the substratum, and in that of ele- vated capsules in liverworts and mosses, water transport is to be explained in the same way. In the higher green plants, except in the case of aquatic plants, conditions become much more complex. Here the most active parts, the leaves and expanding buds, are often removed many meters from the soil out of which the supply of water must come. In these plants special organs of Absorption and Transmission of Water 93 absorption, the roots, and a special region of transmission of water, the xylem, have been developed. But the great expanse of exposed surface would render it utterly impossible that the living cells of one of the larger plants be kept saturated with water, were it not for the development of various kinds of thickened walls and even layers of dead cells (e. g., cork) covering the exposed sur- faces. By these means evaporation is greatly reduced. But to carry on the photosynthetic process it is essential that carbon dioxid be freely absorbed from the air. Now the only manner by which this gas can reach the interior of the living cells is by passing into solution in the imbibed water of the cellulose walls and then diffusing through the tissues as a solute. The complicated structures of stomata and air chambers bring about a condition of things such that thor- oughly saturated cellulose walls are exposed to the air, while at the same time a minimum amount of evaporation is allowed to go on. The plant would be a more economical machine, in some ways at least, if it could avoid evapora- tion entirely, but this is impossible if imbibed walls are to be exposed to the atmosphere. By far the greater quantity of the water absorbed through the roots finds its way out of the green plant through the leaves, and is of no direct material use. This evaporation plays an important part in keeping the green parts cool when they are subjected to the direct rays of the sun. It is probable also that much of the energy for raising water in the plant comes from this molecu- lar diffusion, which we call evaporation. The same process produces a current of liquid up the stem and thus aids in the transmission of solutes. A comparatively small amount of the absorbed water is used as food material in the processes of photosynthesis, growth, and general metabolism. In the lower forms without chlorophyll the exposure of a wet membrane to the atinos- 94 Diffusion and Osmotic Pressure phere is not so essential, nor is it essential that an extensive surface be exposed at all. A comparatively small area will suffice for the evaporation of the gaseous products of respi- ration, etc., or these may be allowed to diffuse outward into the solutions of the substratum. Thus, in colorless parasites and saprophytes are found reduced surfaces covered with leathery tissues which contain few or no openings. How- ever, water is so plentiful in some habitats that many of the forms found in such places have never acquired any special protection against evaporation. In aquatics the effects of evaporation are not present; absorption and transmission of water take place by direct diffusion, perhaps mainly through the roots, however. Since both cell wall and protoplasm are permeable to water, this substance will diffuse into a cell when, for any reason, its diffusion tension is less within than it is outside. Thus, if the surrounding medium is a very dilute solution, all cells must absorb water until an equilibrium is estab- lished between the expanding solutes within the vacuole and the elastic cellulose wall without. By imbibition the cell wall is kept saturated with water also, so that there is direct water communication between adjacent cells even where there is no protoplasmic connection. This form of water absorption is universal in all organ- isms. It makes no difference how complex the form, the individual cells stand in the same relation to water external to them as does the Myxomycete plasmodium moving about in its moist substratum. Of course in the higher forms the cells are fixed in position. In the latter the important special condition is that here the moist substratum in which any cell lives is often the adjacent living tissue of the same plant. In a complex tissue, if one cell loses water faster than its neighbors, water diffuses from them into it and equilibrium is maintained. Absorption and Transmission of Water 95 In all organisms except the very lowest the power to absorb moisture from outside the body is possessed by only relatively few cells, whose external position fits them for thN Ihus,some submerged aquatics may perhaps absorb equally throughout the whole extent of their comparatively thinem dermis. Partially submerged forms can absorb only through those surfaces which are under water. Ordinary land plants absorb through the surface layers of the younger portions of their roots, the surface layer being often greatly extended by the development of root hairs. But in any event, no matter where the water passes from the substratum into the plant body, absorption always takes place in the same way The cellulose membranes are kept wet by imbibition, and water diffuses into the protoplasm from them. The forces which cause the entrance of water into the plant are, then, partly those of adhesion and surface tension, and partly those of simple diffusion. That the rate of root absorption varies with the tempera- ture of the soil when the changes in temperature are grad- ual, as was demonstrated by Vesque, 1 shows that this absorption is an osmotic phenomenon. With a sudden rise in temperature this author found that absorption is dimin- ished temporarily, and with a sudden fall it is augmented. This is probably due to the increased pressure of the inclosed gas bubbles at higher temperatures. II. TRANSMISSION OF WATER a) Water loss.— Within the single cell transmission of water occurs mainly by simple diffusion, aided, no doubt, by streaming movements of the protoplasm. In more complex forms, like liverworts, and in simple tissue masses of the higher plants, diffusion from cell to cell through the Bat- l J. Vesque, "De l'influence de la temperature du sol sur I'absorption de Feau paries racmes," Ann. set. nat. bot, Ser. VI, Vol. VI (1877), pp. 169-3 96 Diffusion and Osmotic Pressure urated walls brings about all the transfer that is required. The process may often be hastened by pits and protoplasmic connections. In higher plants phenomena occur which are more difficult of explanation. In order to discuss the trans- mission of water in such cases, it will first be necessary to consider briefly the ways in which water is lost by the plant. (1) Evaporation. — As has been stated already, by far the greater part of the water absorbed by the plant is lost by evaporation. Water is continually evaporating into the air- spaces of the plant body and diffusing out into the external atmosphere through stomata and lenticels, and to some extent through the epidermis itself. The vacuoles of the leaf parenchyma furnish this water to the cell walls, and thus their solutions become more concentrated as evaporation con- tinues. Of course this means that water must diffuse into these cells from cells farther back, where the concentration is not as great. Eventually these cells draw water osmotic- ally from the xylem strands. The source of energy for this process of concentration of leaf solutions is the heat of the surrounding atmosphere, which causes the aqueous molecules to break away from the films covering the parenchyma walls next to the air chambers. (2) Water pores and nectaries. — Water pores and nec- taries are curious instances of the passage of liquid water out of the plant body. No completely satisfactory explana- tion of these occurrences has yet been given. Osmotic action surely plays an important part here, but as yet no accurate means of determining the exact process has been devised. The difficulty lies in the fact that in these cells the move- ment of the water is in the opposite direction from that which would be expected from the principles of osmotic action. The following hypothesis may help to bring the facts together : The modified cells bordering a water pore are perhaps irritable in a peculiar way. It may be that the protoplasmic Absorption and Transmission of Water ( M sac, upon being stretched beyond a certain limit, changes its nature in some way so as to become permeable to the con- tained solutes. If this were the case, turgidity would rise to the critical point, and then, when the change in permeability took place, there would result an exudation of cell sap through the plasmic membrane, the contraction of the previously stretched cellulose wall forcing the solution through the now permeable layers. This exudation might be equal in all directions, or might take place mainly in the direction of the water pore, as if the portion of the protoplast lying next the modified air cavity were the only part to become permeable at the assumed critical point. It seems probable that, if such an occurrence takes place at all, the change is brought about uniformly, and that the exudation from the vacuole is equal in all directions. But, since the adjacent tissues of the leaf are engorged with water, a marked flow could take place only in the free direction, namely, out- ward into the cavity of the pore itself. After the cellulose walls had ceased to contract, external pressure would be removed from the protoplast, the flow of liquid would cease, the internal pressure would have fallen below that at the assumed critical point, and it is not at all inconceivable that under these conditions the protoplasm might return to its original condition of semi-permeability toward the contained solution. If this were so, absorption from the surrounding tissues would again take place, and turgidity would gradually return until the critical point was again reached, when the process of external discharge would again occur. Evaporation from the surface of the exuded droplet, which must become rapid as soon as the latter is pressed into the air space, would concentrate the solution, and thus osmotically prevent resorption of the extruded water. More than that, with the increasing concentration it would act as a plasmolyzing solution to extract still more water from 98 Diffusion and Osmotic Pressure within. Experiments are needed to determine whether or not this sort of extraction of water does really take place in water pores. At any rate, the important point lies here, that there is an original exudation of cell sap through the proto- plasm. That in some cases at least the exudation is truly a portion of the sap, and not pure water, has been shown by Bonnier 1 in the case of honey-dew, and by Dandeno 2 in the case of guttation drops. Also Moll 3 showed that when shoots whose leaves bore water pores were injected from below with the juice of Phytolacca decandra, the exudation always con- tained the color. Since water cannot pass from the xylem to the outside without traversing the cells bordering upon the pore, the exuded water bearing the coloring matter must have pased through these cells. The only serious difficulty with this hypothesis as an explanation of the action of water pores lies in the assumed continually decreasing concentration of the cell sap. But there is good evidence that the protoplasm of the plant cell is continually discharging substances into the vacuole, which increase the osmotic pressure therein. There is apparently no reason why we may not postulate this same property, per- haps in an unusually marked degree, in the case of the cells just discussed. In the case of nectaries there is exhibited an apparently similar set of phenomena. Wilson's work 4 shows that after the dissolved substances of the nectar (mainly sugar) have once passed out of the cells and into the cup of the nectary, 1 G. Bonnier, " Recherches experimentelles sur la miellee," Rev. gen. bot., Vol. VIII (1896), pp. 1-22. 2 J. B. Dandeno, "An Investigation into the Effects of Water, etc., on Foliage Leaves," Trans. Canad. Inst., Vol. VII (1901), pp. 238-350. 3 J. W. Moll, " Untersuchungen uber Tropfenausscheidung u. Injection von Blat- tem," Verslagen en Mededeel. d. k. Akad. v. Wetensch. te Amsterdam, Vol. XV (1880), pp. 237-337. * W. P. Wilson, " The Cause of the Excretion of Water on the Surface of Nec- taries," Unters. aus d. bot. Bist. zu Tubingen, Vol. I (1881), pp. 1-22. Absokption and Transmission of Water 99 the high osmotic pressure caused by evaporation will fully account for the outward passage of water which keeps the nectar of flowers and leaves in the liquid condition on the dry est days. Nectaries may even be artificially formed in this way ; if minute granules of sugar are placed upon the hyphse of Mucor, droplets of water will soon form and increase in size until they run down and fall off. The same may be done with leaves ; a small mass of sugar upon the epidermis will soon extract enough water osmotically to make a large drop- let. The method of extraction of the water in these arti- ficial cases is clear enough after a small portion of the sugar has been put into solution. Water to form the first minute amount of solution cannot be withdrawn from the leaf- cells by osmotic action. But all cellulose membranes, even cuticularized ones, are more or less saturated with water of imbibition. Now, the sugar particles resting upon the leaf come in contact with these moist membranes and immedi- ately begin to dissolve in the water therein imbibed. After the start is once made, be it ever so infinitesimal, osmotic action will accomplish the outward flow of water from the comparatively weak solution within the cells to the saturated one on the surface. But in the case of the natural nectary the original exuda- tion of the sugar-containing sap has not yet been accounted for. Perhaps this can be explained in a manner parallel to that just postulated for the case of the water pore. At a certain stage in its development the glandular tissue of the nec- tary may undergo a change such that, through a rapid increase in soluble content, the osmotic pressure of the sap of its component cells rises suddenly. This rise in turgor pres- sure may act as a stimulus upon the protoplasm, and the latter may, in turn, respond by a change in its structure, so that it becomes permeable to solutes as well as water. If this be true, the contraction of the stretched cellulose walls must 100 Diffusion and Osmotic Pressure press the cell sap out of the numerous cells of the gland, and this may accumulate on the external surface. Owing to evaporation this droplet of exuded sap must immediately be°in to increase in concentration, and from that time on Wilson's observations are sufficient to explain the maintenance of liquid in the nectary. It has been shown, 1 however, in certain cases, that if the nectar be removed, new nectar will be secreted. According to the above hypothesis, this must be due to another increase of turgidity and another exudation of cell sap. Thus it must be supposed that, after the first discharge, the conditions again come into equilibrium, and the plasmic membranes again become semi-permeable, only to repeat the former process of excretion if the nectar is again removed. Per- haps the evaporating, and therefore concentrated, solution on the surface acts as a plasmolyzing agent, keeping the turgidity of the gland cells down by extraction of water. If this were so, the removal of the nectar might easily cause an increase in turgidity, which might, in turn, bring about the response of exudation. Wilson found the shining droplets of water which occur on certain molds to be of the same nature as the artificial droplets which he was able to produce by sprinkling the hyphse with pulverized sugar. When these natural drop- lets are removed they again return, but this does not occur if the hyphae are carefully washed with distilled water. Careful examination of the places where droplets had been removed without washing showed minute crystals of sugar upon the surface. Thus, if the air becomes dry, evapo- ration may become so great that the droplets disappear, but as soon as evaporation is checked, the crystals of sugar lying upon the surface go into solution in the imbibed water of i H. Haupt, " Zur Secretionsmechanik der extrafloralen Nektarien, M Flora, Vol. XC (1902), pp. 1-41. On the general subject of secretion, see W. Pfeffee, Studien zur Energetik der Pflanzen, Leipzig, 1892. Absorption and Transmission of Water 101 the cell wall and cause a renewed outward flow of water, and the renewal of the droplet. The recent work of Haupt on extra-floral nectaries adds to our knowledge of gland action. This author finds that the secretion of sugar in these nectaries begins only when the gland has attained a certain stage of development, and then only when transpiration is relatively slight. This makes it appear as though the protoplasmic layers become permeable to sugar at a certain phase in the series of devel- opmental changes, providing there be at that time a great accumulation of water in the cells. This last provision means that the cell walls and plasmic membranes are strongly stretched. After secretion has begun an increase in humid- ity causes an increase in the excretion of water from the gland, but that of sugar remains constant. Hence it may be con- cluded that the humidity, i.e., the amount of water evapo- rated, has no direct effect upon the permeability of the protoplast to sugar. For the beginning of the secretion, Haupt finds that a certain minimum temperature is neces- sary. The temperature very probably affects the physical nature of the protoplasmic layer. That the red-yellow por- tion of the solar spectrum influences the activity of these glands in a profound manner has been already mentioned. 1 It is important to note in this connection that, while pro- toplasm is generally to be regarded as semi-permeable toward many solutes, yet there is evidence from almost all parts of the plant kingdom that it is often permeable to such substances as sugar, acids, inorganic salts, etc. The pene- tration of these substances is usually a slow process, how- ever. Now the variation in the permeability of different protoplasts, even of the same plant, may be taken as evi- dence that slight differences in the protoplasm may cause rather great differences in permeability. This makes the i See p. 78. 102 Diffusion and Osmotic Pressure alteration in permeability which is postulated above in ref- erence to water pores and nectaries less difficult of supposi- tion than it might appear at first thought. Also, the varia- tions in permeability which are known to occur in certain cells, e.g., those which can be brought about in almost any cell by chemical agents like H g Cl 2 , and by change in tem- perature, for example the cold plasmolysis of Spirogyra, render it at least possible that excess of turgor pressure may alter permeability. Indeed, a similar phenomenon to the one postulated was observed by Oltmanns 1 in the case of Fucus, and it is more than probable that the exudations from certain "sensitive" pulvini often, if not always, contain solutes. (3) Exudation. — If the stem of a plant be severed near its base and a mercury manometer be attached to the por- tion connected with the roots, an exudation of water under pressure may often be demonstrated. This so-called exuda- tion pressure is not uniform, but varies in an irregular man- ner, sometimes sinking below the zero point of the scale and at other times amounting to an atmosphere or more. A similar pressure is reported in various parts of the plant, even high up in the tops of trees, as was noted by Molisch 2 in the case of Cocos and Arengo. A series of experiments upon this subject was performed by "Wieler, 3 who came to the conclusion that the power of exuding sap is a general one among plant cells. Other experiments which are not nearly so satisfactory are those made by Kraus* and IF. Oltmanns, " Ueber d. Bedeutung der Koncentrationsanderung des Meer- wassers f iir d. Leben d. Algen," Sitzungsber. d. k. preuss. Akad. d. Wiss. zu Berlin, Jahrg. 1891, p. 183. 2 H. Molisch, "Die Secretion des Palmenweins und ihre Ursachen," Oesterr. bot. Zeitschr., Vol. XLIX (1899), p. 74. Also see W. Figdok, "Untersuchungen uberd. Erscheinung des Blutungsdruckes in den Tropen," Sitzungsber. d. kais. Akad. d. Wiss. zu Wien, math.-nat. hist. Klasse, Vol. CVII (1898), p. 640. This is noted in Oesterr. bot. Zeitschr., Vol. XLVIII (1898), pp. 359, 360. 3A. Wielee, " Das Blnten der Pflanzen," Cohns Beitrdge, Vol. VI (1893), pp. 1-211. *C. Keaus, "Untersuchungen iiber den Saftedruck der Pflanzen," Flora, Vol. XL (1882), pp. 2 ff. Absorption and Transmission of Water 103 Pitra. 1 The former found that various parts of stems and roots exuded water when cut out from the plant and placed partly submerged in wet sand. Details of the experi- ments are not given, and there are many circumstances in the somewhat superficial account which lead the reader to doubt whether the author was dealing with true bleed- in^ • for there are other factors, such as the expansion of gases in the wood, which may cause exudation under cer- tain conditions. Pitra's experiments are much more con- vincing than those of Kraus, but it is still somewhat doubtful whether very much true bleeding was observed by either of these authors. Pitra makes one observation which is of interest here, however. If a cut shoot be inverted with its leaves under water and its stem in air, bleeding from the cut surface of the stem will ensue. That is, if leaves are placed in a position to absorb water, they can do so in a manner entirely similar to that exhibited by roots, and a leaf-pressure corresponding to the normal root-pressure seems to be developed. The observations of Pitra in this regard have been substantiated by Molisch, 2 who finds the same to hold true if the leaves are not placed in water but are surrounded by moist air. The evidence seems to be good that bleeding may occur (1) in the case of cut stumps to which active roots are still attached ; (2) at the cut surface where the crown or inflor- escence has been removed from certain palms (Molisch), and (3) at the cut surface of certain stems whose leaves are sub- merged in water (Pitra). It may occur in other parts, but it seems that true bleeding has not been unquestionably demonstrated elsewhere in tall plants. Exudation pressure has often been ascribed to osmotic i A Pitra, "Versuche fiber d. Druckkraft d. Stammorgane bei d. Erscheinun- gen des Blutens U. Thranens," Jahrb.f. wiss. Bot, Vol. XI (1877), pp. 43,-o.JU. 2 H. Molisch, " Ueber localen Blutungsdruck und seine Ursachen," Bot. Zatg., Vol. LX (1902), pp. 45-63. 104 Diffusion and Osmotic Pressure phenomena occurring within the cells. 1 If this be the true explanation, we have no exact knowledge of the processes by which these phenomena occur. That osmotic pressure within the vacuoles might cause movement of an exuded solution under pressure, through intercellular spaces or through the channels of the xylem, need not be questioned ; this can be simulated in the laboratory with the ordinary thistle tube of molasses closed with animal membrane. But to explain the phenomenon of exudation pressure it must be shown how it comes about that this solution gets into the channels at all. A small amount of solution might be exuded in a manner similar to that supposed above in connection with nectaries and water pores. But the absence of evaporation within the plant body will deprive us of that source of energy to which has been ascribed the maintenance of a relatively high concentration in the exudate of nectaries. However, it is probable that even in nectaries and water pores this discovery of Wilson is not of fundamental impor- tance. The main desideratum is to know how the original exudate comes to get through the otherwise only semi- permeable protoplasm ; if a little can be exuded there seems to be no reason why more could not pass out in the same way. In all the cases of exudation the exudate is known to be a solution; the solutes of the vacuoles pass out and appear in the exudate. This is an unquestionable indica- tion that the protoplasmic membrane is permeable to them. An explanation of this phenomenon was elaborated by Pfeffer 2 and later, apparently independently, by Fuchs. 3 This depends upon the assumption of some sort of vital activity within the cells. These authors point out that in order to have a current of water through a cell, it is only i W. Pfeffer, Osmotische Untersuchungen, Leipzig, 1877, p. 223. 2 Ibid., pp. 222-5. 3 K. Fuchs, "Zur Theorie der Bewegung des Wassers imlebenden Pflanzenkor- per," Beih. Bot. Centralbl., Vol. X (i901), pp. 305-8. Absorption and Transmission of Water 105 necessary that the concentration of the cell sap at the point of entrance be higher than at the point of exit. If such a condition could be maintained, a slight movement of water would undoubtedly take place, though in the writer's judgment it would be very inadequate in amount. But the insurmountable difficulty in this explanation is the continu- ous maintenance of this difference of concentration in differ- ent parts of the same cell. This could only be accomplished by the active absorption and precipitation or fixation of the active osmotic substances in one region of the protoplast, and their secretion and solution in another part. Such a supposition in its simplest form involves the carrying back of these substances by the protoplasm, for example, from one end of the cell to the other, with as great rapidity as they can diffuse in the opposite direction through the cell sap. This could only occur with enormous expenditure of energy on the part of the protoplasm, and it is difficult to imagine any adequate source for this energy. Still another proposed explanation of the passage of water through a cell in any given direction has been offered by Pf effer. 1 He points out that, if the membrane where the water enters be less permeable to solutes than that where it escapes, a continuous flow will take place. This is undoubtedly true, but the flow would be still more marked if the second mem- brane were removed altogether; for it can act only as an obstruction to the upward expansion of the inclosed liquid as water is taken in through the semi-permeable membrane below. The resulting system is such as would be obtained if the thistle tube osmometer used for illustration of osmotic pressure were to have its stem plugged with cotton. The cot- ton would hinder the rise of the liquid column, but would not stop it altogether. Copeland 2 has actually constructed a i W. Pfeffer, Osmotische Untersuchungen, Leipzig, 1877, p. 225. 2E. B. Copeland, "An Artificial Endodermis Cell," Bot. Gaz., Vol. XXIX (1900), pp. 437-9. 106 Diffusion and Osmotic Pressure piece of apparatus by which a current is maintained through a cell, different parts of whose wall are unequally permeable. That exudation pressure depends upon vital activity seems evident from the fact that it ceases with death. Another line of evidence which points toward the necessity of active protoplasts for exudation is that brought forward by Wieler, 1 when he records that, if a bleeding plant is deprived of oxy- gen, exudation stops. He observed the same result when the plant was anaesthetized with chloroform. The exuded liquid varies in its concentration in different plants, usually becoming weaker as bleeding continues, but there seems to be no discovered relation between the exudation pressure and the concentration of the exudate. 2 The whole subject of exudation and sap pressure is viewed in an entirely new light by Molisch 3 in his last paper on these phenomena. He presents convincing evidence that in all cases where true bleeding has been observed it is a phe- nomenon connected with the stimulus of wounding or with the formation of new tissue, such as callus, over the wound surface. Thus, the method of decapitation and of boring, for the study of exudation pressure, becomes at least of very doubtful use in the investigation of the normal pressure within the plant. It is impossible to insert a manometer into a stem without making a wound, and, according to Molisch's conclusions, this wound itself is sufficient to cause a pathological condition of the neighboring tissue such that exudation ensues. In the light of these considerations, then, it seems extremely doubtful whether there is exhibited in the normal, uninjured plant any such phenomenon as that of sap pressure. Where exudation occurs in the pathological wound tissues it must be due to some such change in per- meability of the protoplasts as was postulated above. i A. Wieler, " Das Bluten der Peahen," Cohns Beitrage, Vol. VI (1893), p. 158. 2 Ibid., pp. 65, 69. 3H. Molisch, "Ueber localen Blutungsdruck und seine Ursachen," Bot. Zeitg., Vol. LX (1902), pp. 45-63. Absokption and Transmission of Water 107 (4) Summary of ivater loss. — By evaporation pure water is lost from the plant. Thus the osmotic pressure of the fluids in the leaves and near evaporating surfaces is increased and other water diffuses to these regions, thus tending to re-establish an equilibrium of diffusion tension. Water is eventually taken from the xylem vessels and, since these are not lined with protoplasm, a mass movement of water is set up, flowing upward through the xylem strands. This carries with it whatever dissolved substances have been extruded into these strands from the roots. Evaporation cools the leaves. Leaves, etc., are able to extrude solution upon their surface through specialized openings, the glands and water pores. The exact process by which this extrusion takes place is not known ; it is probably an osmotic one, perhaps coupled with some periodic change in permeability of the protoplasts. Leaf cells will act in the same way in the opposite direction without regard to water pores (Pitra, Molisch). Root cells are able to take in water and solutes and then to pass them on into the xylem. It seems that sap pressure, whether in roots or in wound tissue, must be explained along the same general lines as the action of glands and water pores. Whether a periodic external exudation occurs in roots is not known, but it seems not improbable that at some times roots may let out solutes. Indeed, Czapek 1 and Molisch 2 have observed extrusions from root hairs which are not unlike those from water pores. It may be that some sort of a peri- odic extrusion of solutes is a fundamental property of proto- plasm. If this be true, it is a phenomenon not unlike that exhibited in pulsating vacuoles. b) The upward movement of ivater in trees and other tall plants. — Owing to almost insurmountable difficulties in IF. Czapek, "Zur Lehre von den Wurzelausscheidungen," Jahrb. f.wiss. Bot., Vol. XXIX (1896), pp. 321-90. 2 H. Molisch, "Ueber Wurzelausscheidungen und deren Einwirkung auf or^-a- nische Substanzen," Sitzungsber. d. kais. Akad. d. Wiss. zu Wien, math.-nat. hist. Klasse, Vol. XCVI (1887), pp. 84-109. 108 Diffusion and Osmotic Pressure experimentation, an exact knowledge of the manner in which water is raised in tall stems has not yet been reached. Various hypothetical explanations of the observed phenomena have been offered, but no one of them has been thoroughly established. Imbibition, capillarity, the lifting power of evaporation exerted upon a cohering water column, physical osmosis, and undefined " vital activity," have all been invoked to explain the phenomena of the ascent of sap in trees. It is not intended to take up here the discussion of any of these hypotheses excepting those which deal with osmotic pressure and diffusion. It is well known that the xylem is the conducting region for water. Since the trachese, which mainly compose it, are dead and contain no protoplasmic lining, there cannot be attributed to them any active part in lifting the water which they contain. It has been proposed 1 as a partial explana- tion of this rise of water that the exudation pressure which is made externally apparent where a plant is cut or broken may be normally active within the xylem and may thus fur- nish a part of the needed force. This is a very plausible theory if not pushed too far. It can hardly be supposed that if this pressure is con- cerned in raising water in the xylem it is exclusively applied at the base of the stem or in the roots. It is much easier to suppose that the various groups of cells exerting exudation pressure act as a set of relay pumps, each group taking the iThis theory, as far as I know, was first clearly put by Godlewski in his paper entitled " Zur Theorie d. Wasserbewegung in den Pflanzen,' 1 Jahrb.f. wiss. Bot., Vol. XV (1884) , pp. 569-630. Westerhaieb had somewhat the same idea a year previous to this, but did not develop it as well. His article is " Zur Kenntniss der osmotischen Leistungen des lebenden Parenchyms," Ber. d. deutsch. bot. Ges., Vol. I (1883), pp. 371-81. But the best exponent of the pumping action of parenchyma was Jaxse, whose ideas are expressed in a paper entitled " Die Mitwirkung des Markstrahlen beider Wasser- bewegung im Holze," Jahrb.f. tviss. Bot, Vol. XVIII (1887), pp. 1-69. The last-named author elaborated Godlewskrs theory and supported it with experiment. Many more citations might be made; the literature is very voluminous. For a very complete dis- cussion of this subject, see E. B. Copeland, " The Rise of the Transpiration Stream," Bot. Gaz., Vol. XXXIV (1902), pp. 161-93, 260-83. Absorption and Transmission of Water 109 sap from the adjacent tracheae and passing it on upwards. But if sap has already passed through a set of these active cells in a manner similar to that described in connection with water pores, its concentration cannot be lower than that of the cell sap of these cells when they are stretched to their utmost ; indeed, from the loss of pure water to other cells along its path its concentration is apt to be even higher. If this same sap, now in the trachese, is to enter another set of such active cells and be pressed still higher up, the sap of the latter must necessarily possess a higher osmotic concentration than the fluid to be absorbed; and after it has been pressed out of them into tracheae still farther up the stem, it must have gained in concentration. Thus any such explanation of the rise of sap in stems involves a gradually increasing concentration of the sap as it passes upward. There seems to be no experimental evidence of this as a fact. It is true that the sap in leaves is more concentrated than that in the stem, but there seem to have been put on record no observations of a gradual increase in concentration toward the summit of a tree. Evaporation from the leaves would account for the observed fact. The above presentation will stand for various hypotheses which have been proposed in this regard, all involving some sort of a periodic variation in permeability. Godlewski and Janse have attempted to locate the active cells in the cortex or medullary rays of woody stems, but these attempts have apparently failed. In fact, the whole idea that the ascent of sap in tall stems has any necessary dependence upon the presence of living cells may be doubted very much on the following experimental grounds: In his study of bleeding Wieler l came to the conclusion that this process, while it is probably a general property of i A. Wieler, "Das Bluten der Pflanzen," Cohns Beitrage, Vol. VI (1S93), pp. 1-211. 110 Diffusion and Osmotic Pressure protoplasm, yet plays no leading role in the lifting of water up tall stems. As far back as 1853 T. Hartig 1 showed that a poison (ferric pyrolignate) would pass up the stem of a tree for over 12 meters. He bored five radial auger-holes in the tree trunk near its base, all meeting at the center. These holes were filled with the poison solution and then plugged. When the tree was cut down, the star-shaped stain of the poison was found in a cross-section over 12 meters above the holes. Strasburger 2 performed the same experiment more thoroughly. Trees were cut off and set into tubs of poison, such as aqueous solutions of CuS0 4 and picric acid. The poison ascended to the leaves, a distance of twenty-one meters in the tallest tree. Of course, if these violent protoplasmic poisons ascend the trunk, they must kill all cells lying in their path. Therefore the living cells of the stems cannot be necessary for the rise of sap. But after the leaves had been killed the stem ceased to absorb more solution, or absorption took place very slowly. This may be explained by the fact that most leaves collapse and dry upon being killed. The cause of the rise of sap is perhaps the evaporation from the surface of the leaves, and in order that it should rise the leaves must be in their normal turgid condition. Evaporation may thus result in concen- tration of the solution on the surface of the walls of the parenchyma, thus causing an outward osmotic flow of water from the cells, the solutions of which in turn become more concentrated and extract water from cells lying still farther within the plant. This process may be thought of as con- tinuing until water is finally extracted from the zylem. Here the osmotic withdrawal of water would probably become l T. Hartig, " Ueber die endosmotischen Eigenschaften der Pflanzenhaute," Bot. Zeitg., Vol. XI (1853), pp. 309-17. 2E. Steasbuegee, Ueber den Bau und die Verrichtungen der Leitungsbahnen in den Pflanzen, Jena, 1891; idem, " Ueber das Saftsteigen," Histologische Beitrage, Vol. V, Jena, 1893. Absorption and Transmission of Water 111 a mechanical tension upon the minute films and columns reaching to the foot of the tree. Strasburger's experiments seem to show that atmospheric pressure cannot play a part here ; for his tallest tree was as high as a column of water which would balance a pressure of two atmospheres. However, it is to be remembered that the water in a tree trunk is not in continuous columns, but that the columns are divided by air bubbles. That the leaves play the part just ascribed to them is practically proved. Besides the experiments of Strasburger may be cited that of Dixon, 1 wherein he showed that when the leaves at the top of a tall shoot were killed, the upward passage of water was checked, even though the stem were still uninjured. The hypothesis that evaporation is the source of the energy required in raising water dates back, in its general form, to Dutrochet. 2 The main points of this idea are the entrance of the water below and its evaporation above. The point which has caused most trouble lies in the lack of proof that water columns such as are found in the tree have cohesion enough to be drawn up by evaporation to a height far exceeding that to which this liquid would be supported by one atmosphere. That it will cohere somewhat beyond the height to balance a pressure of one atmosphere has been shown by Bohm, 3 Askenasy, 4 and Copeland. 5 The former of these showed that evaporation from the surface of a twig of Thuya attached to an upright tube of water, the lower end 1H. H. Dixon, "Note on the Role of Osmosis in Transpiration," Proceed. Roy. Irish Acad., Sec. Ill, Vol. Ill (1898), pp. 767-75. 2 M. H. Dutrochet, M&moires pour servir a Vhistoire anatomique ct physiolo- gique des v4gStaux et des animaux, Brussels, 1837. 3 J. Bohm, u Capillaritat und Saftsteigen," Ber. d. deutsch. hot. Ges., Vol. XI (1893), pp. 203-12. *E. Askanasy, "Beitrage zur Erklarung des Saftsteigens," Vcrhandl. d. naturl.- mel. Verein zu Heidelberg, N. F., Vol. V (1896), pp. 429-48. r 'E. B. Copeland, "The Rise of the Transpiration Stream," Bot. 0a*., Vol. XXXIV (1902), pp. 161-93, 260-83. 112 Diffusion and Osmotic Pressure of which dipped in mercury, was sufficient to lift a column of the latter liquid higher than the barometer column at the time of the experiment. Askenasy was able to construct apparatus whereby he could demonstrate a pressure of 90 cm. of mercury arising from evaporation of water from a satu- rated plaster of Paris plate. Copeland constructed a column of plaster of Paris 3 mm. in diameter and 12.4 m. high, which terminated below in a mercury manometer and above in a Cu 2 Fe(CN) 6 membrane. The whole column was as nearly saturated with water as might be (there were many air-bubbles, however), and the membrane above was covered by an exposed solution of CuS0 4 . Evaporation from the surface of this solution caused, in five days, a suction on the manometer below of 301 mm. of mercury. Essentially this apparatus is a bundle of minute water columns held in the pores of the plaster, but broken here and there by air bubbles. There was certainly enough water in the column to give a pressure of more than 459 mm. of mercury (1 atmosphere minus 301 mm.). If this be true, the suction set up by evap- oration above surpassed the pressure of an atmosphere. This theory of sap ascent seems to be gaining ground, and it is quite probable that the idea of Godlewski and Janse will eventually be put entirely aside. The experiments of Strasburger and Dixon show that osmotic pressure must be active in the leaves in order that sap may ascend at its usual rate. Another proof that living protoplasm is necessary in leaves lies in the fact that transpiration can be influenced by anaesthetics. 1 Such reagents act in a similar manner to the 1 For a discussion of the relation of anaesthetics to transpiration see H. H. Dixon, "On the Effects of Stimulation and Anaesthetic Gases on Transpiration," Proceed. Roy. Irish Acad., Ser. Ill, Vol. IV (1898),pp.618-26; H.^umelle, "Influence des anesthetiques sur la transpiration chlorophyllienne," Rev. gen. bot., Vol. II (1890), pp. 417-32; idem, " Nouvelles recherches sur l'assimilation et la transpiration chlorophylliennes," ibid., Vol. Ill (1891), pp. 241-88, 293-305 ; A. Schneider, "The Influence of Anaesthetics on Plant Transpiration," Bot. Gaz., Vol. XVIII (1893), pp. 57-69; A. Woods, "Some Recent Investigations on the Evaporation of Water from Plants," ibid., Vol. XVIII (1893), p. 304-10. Absorption and Transmission of Water 113 poisons used by Strasburger, though, of course, in not so marked a manner. They probably cause partial plasmolysis in the leaf cells and thus disturb diffusion and evaporation. Still another point which may be construed in this same manner is the observation of Kossaroff ' that an increase in the amount of C0 2 in the water in which are placed cut twi^s, with and without leaves, is accompanied by a marked falling off in water absorption. What can be the reason for this we are unable even to conjecture, but it certainly appears as though the chemical nature of the membranes were involved. It cannot be due to the osmotic pressure of the dissolved C0 2 , since this gas penetrates all protoplasts with the utmost ease. Possibly the C0 2 may precipitate in the wood and form bubbles which plug the water channels ; but this, too, seems unlikely. It seems more probable that the dissolved gas affects the protoplasmic membranes in the leaves, causing some change in their osmotic properties. The whole problem of water ascent remains a puzzling one, one which must wait for solution until the development of better and more exact methods of experimentation. In the meantime it will probably be more profitable to devote atten- tion to some of the more definite and restricted problems of the cell itself. In the present condition of our knowledge of plasmic membranes, for example, it is almost foolhardy to attempt to settle such a complex question as the one just briefly reviewed ; but once knowing the nature of these plasmic mem- branes, it is not improbable that the solution of the problem of water transport will follow as the simplest corollary. III. SUMMARY OF THE CHAPTER In general, the process of water absorption and water movement may be stated as follows : The imbibed water of the cell walls, the water of the protoplasm itself, and the 1 P. Kossaroff, "Die Wirkuns? der Kohlens&ure auf den Wassertransportin den Pflanzen," Bot. Centralbl., Vol. LXXXIII (1900), pp. 138-44. 114 Diffusion and Osmotic Pressure water of the substratum in which the organism is growing, are to be regarded as one continuous mass of liquid. Thus, if the diffusion tension of water in any part of the plant becomes less than it is at any other point, diffusion takes place and equilibrium is restored. In the same way, if the diffusion tension within the plant falls below that of the substratum, diffusion of water into the plant must imme- diately occur. This process of simple diffusion is sufficient to account for absorption and for transport in the simple plant bodies and in any small portion of larger bodies. But in the more complex bodies of higher plants this is not sufficient. Just how the sap is raised in trees is not surely known. There are at present two main theories to account for it: (1) It is supposed to be raised by periodic pumping action of living cells in the trunk. (2) It is supposed that evaporation and the resulting osmotic concentration in the leaves will draw it up from the roots, the cohesion of the minute water columns being supposed to be of sufficient magnitude to prevent their being broken by the strain. CHAPTER III ABSORPTION AND TRANSMISSION OF SOLUTES With the exception of the naked amoeboid cells occur- ring in certain stages of the life histories of a few fungi and algse, together with the cell complexes constituting Myxo- mycete plasmodia, plant cells are unable to engulf solid food. The presence of the cellulose membrane makes this fact very evident. In order to be absorbed into a cell, any sub- stance must first be in the form of an aqueous solution. Even where the process of engulfing takes place, the food does not truly pass inside the protoplasmic body until it is dissolved; around each food body in a plasmodium is a vacuole lined by a plasmic membrane which is probably identical in nature and origin with that covering the exte- rior of the protoplasmic mass. In such cases the food is digested in this vacuole and the products of digestion are absorbed through this membrane. I. ABSORPTION OF GASES There are, in general, two forms of material in solution which are absorbed by the plant, namely, gases and solids. As has been seen, gases enter into aqueous solution when they are simply brought into contact with the solvent. All the natural water on the surface of the earth contains in solution oxygen, nitrogen, argon, carbon dioxid, etc. The first and the last of these gases are the only ones which are important in plant metabolism. The moist cellulose mem- brane and the protoplasts are all permeable to these dis- solved gases, and, being soluble in water, they will diffuse wherever it can diffuse. Thus there must be a tendency to equalize the diffusion tension of oxygen and carbon dioxid 115 116 Diffusion and Osmotic Pressure (and of the other two gases also, though they are not used in metabolism) throughout the water in the substratum and that contained within the plant. And, since these two masses of water are continuous, simple diffusion will account for the exchanges which take place between the dissolved gases of the soil and those of the roots of the land plant. The same sort of diffusion takes place between the internal solution of the water plant and the surrounding water. Not only may absorption thus take place, but also the giving off of gaseous waste products. In the case of aquatic sapro- phytes and parasites, oxygen is absorbed and carbon dioxid given off. In that of aquatic green plants this process occurs in darkness, but in light, carbon dioxid is absorbed, while oxygen is given off. The roots of land plants are always absorbing oxygen and eliminating carbon dioxid. But by far the greater portion of the gaseous exchange in land plants and semi-aquatics takes place, not through the soil water, but through the wet membranes which are in contact with the air. This is especially so in green plants, whose leaves are peculiarly constructed so as to expose moist cellulose membranes in air chambers which are in connec- tion with the outer air through stomata. 1 By a number of researches 2 it has been shown that dry walls are but slightly, if at all, permeable to gases and that the more moist they are the more readily are they permeable. i F. F. Blackman, " Experimental Researches on Vegetable Assimilation and Respiration": II, "On the Paths of Gaseous Exchange between Aerial Leaves and the Atmosphere," Phil. Trans. Roy. Soc, London, B., Vol. CLXXXVI(1895), pp. 503-62; H. T. Browne and F. Escombe, " Static Diffusion of Gases and Liquids in Relation to the Assimilation of Carbon and Translocation in Plants," ibid., Vol. CXCIII (1900), pp. 223-92. 2N. J. C. Muller, " Untersuchungen uber d. Diffusion atmospharischer Gase in derPflanzeundder Gasausscheidungunter verschiedenen Beleuchtuugsbedingungen," Jahrb.f. wiss. Bot., Vol. VII (1869), pp. 143-92; E. Lietzmaxn, " Ueberdie Permeabili- tat vegetabilischer Zellmembranen in Bezug auf atmospharische Luft," Flora, Vol. LXX (1887), pp. 339-86; J. Wiesxer and H. Molisch, "Untersuchungen uber die Gasbewegung in der Pflanze," Sitzungsber. d. kais. Akad. d. Wiss. zu Wien math.- nat. hist. Klasse, Vol. XCVIII (1889), Abth. 1; P. Clausen, " Ueber die Durch- lassigkeit der Tracheidenwande fur atmospharische Luft," Flora, Vol. LXXXVIII (1901), pp. 422-69. Absorption and Transmission of Solutes 117 During the hours of sunlight, when the process of pho- tosynthesis is going on, carbon dioxid is being combined with water to produce carbohydrates within the chlorophyll - bearing cells. Thus, at this time the diffusion tension of this gas in the solutions of these cells is much lower than it is in the outer air and in the air chambers. Therefore, there must be a constant diffusion stream of carbon dioxid moving through the stomata into the air chambers, going into solution in the imbibed water of the moist cell walls wherever it touches them, and diffusing as a solute through the tissues of the plant to the places of low diffusion tension. The oxygen which is given off in photosynthesis finds its way to the outer air in the same manner as that by which the carbon dioxid enters. The oxygen tension becomes higher in the green cells than in the outer air, and a diffu- sion stream of this gas is at once set up in the direction of the air chamber, where it goes out of solution and then dif- fuses as a gas through the stomata into the outer air. Dur- ing the night when photosynthesis has ceased this oxygen stream slackens and stops, as does also the incoming stream of carbon dioxid. What oxygen is used in respiration at this time enters from the outer air, and the carbon dioxid produced by this process finds its way out in a manner exactly similar to that in which the other gas escapes during the day. Epidermal tissues of leaves and stems are also imbibed with water, but the amount of water which they can hold is comparatively small on account of the fact that the external walls are heavily impregnated with waxy substances. Hence, as would be expected, a small amount of gaseous exchange between the atmosphere and the plant takes place directly through these membranes. 1 IF. F. Blackman, "Experimental Researches," etc.: II, "On the Paths of Gaseous Exchange between Aerial Leaves and the Atmosphere," Phil. Traits. Roy. Soc., London, B., Vol. CLXXXVI (1895), pp. 503-62. 118 Diffusion and Osmotic Pressure II. ABSORPTION OF DISSOLVED SOLIDS AND LIQUIDS It is probable that the protoplasmic membranes of plant cells are, normally at least, slightly permeable to all sub- stances which the organism needs to absorb. We have direct evidence of the permeability of protoplasm to many of these substances; this evidence was presented in chap, i of this Part. How it comes about that a cell may retain turgor and still be permeable to solutes was also discussed there. The substances which are diffusing into a plant cell at any moment cannot be the ones which are producing the turgor pressure. While certain organic molecules are main- taining the turgor, for instance, many inorganic ions may be diffusing into the cell, because the partial diffusion tension due to them is lower within than it is without. It is also possible that permeability changes from time to time, so that a substance which cannot penetrate the protoplast at one time may do so at another. Turgidity is maintained, and the protoplasmic layer kept stretched and in contact with the imbibed cellulose walls, by the osmotic pressure of certain substances which are probably formed within the cell and to which the protoplast is but slightly permeable. 1 Thus it is clear that the absorption of solid and liquid solutes from the surrounding solution is, like that of gases, merely a phenomenon of diffusion, the particles moving toward that part of the solution where lowest diffusion ten- sion of that substance obtains. Water plants possibly absorb solutes through all parts of their submerged surfaces. Land plants can absorb them only where they are in contact with the moist substratum, mainly through the roots and root hairs. The so-called power of selection of absorbing organs deserves some attention here. It is observed that some plants absorb much more of certain substances than others, JO. H. VON Mayenburg, " Ldsungsconcentration und Turgorregulation bei den Schimmelpilzeu," Jahrb.f. wiss. Bot., Vol. XXXVI (1901), pp. 381-420. Absorption and Transmission of Solutes 119 and that different plants absorb different amounts of the same substance. Failure to absorb a solute which is plenti- ful in the external solution may be due to either one of two causes : either the protoplasmic membrane is impermeable to that substance, or its diffusion tension within and without are equal. If a substance is not used in metabolism, it may simply remain in the cell sap, at the same concentration as is the surrounding medium, or it may be precipitated or condensed in the cell sap or in the protoplasm, and thus con- tinue to accumulate. An example of this last possibility is met with in the case of the storage of carbohydrates. Sugar diffuses into a cell and is there polymerized into insoluble starch. This process continually removes the sugar from solution, so that inward diffusion must continue as long as starch can be formed, and as long as sugar is plentiful out- side. Another example of accumulation is the case cited by MacDougal, in which large quantities of metallic copper were found in the cells of an oak tree (see p. 69). If the substance is used in metabolism, as are N0 3 ions, for instance, there must also occur a continuous diminution of the internal diffusion tension of these particles, which can only be met by inward diffusion from without. If a substance is being rapidly used, this inward diffusion will be correspondingly rapid ; if it is but slowly used, absorption will be correspondingly slow; and all this adjustment of absorbing power may thus take place without any change in the permeability of the plastic membranes. It is probable that most cases of selective power are to be explained in this way. Just as there is a great difference in the use of the various absorbed solutes by different plants, so also there must be a corresponding difference in the amounts absorbed. Thus, Demoussy, 1 using equivalent quantities of several salts, found i E. Demoussy, " Absorption elective de quelques elements mineraux par les plants," Compt. rend., Vol. CXXVII (1900), pp. 970-73; cf. P. Bockget, " Sur Fabsorption de l'iode par les vegetaux," ibid., Vol. CXXIX (1899), pp. 7(38-70; idem, same title, Bull. Soc. chim. de Paris, Ser. Ill, Vol. XXIII (1899), pp. 40, 41. 120 Diffusion and Osmotic Pressure that KN0 3 was taken out of a mixed solution two to six times as fast as NaN0 3 or Ca(N0 3 ) 2 . He also observed that wheat plants absorb K ions two to three times as fast as those of Ca in equivalent solution. On the other hand, maize absorbs somewhat more Ca than K. Pfeffer 1 per- formed a series of experiments with fungi, which gave the general result, that the plant takes out of a mixed solution those solutes which are the best food for it. These would naturally be the ones whose diffusion tension would be first to diminish in the active cells. There may be substances, however, such as certain poisons, 2 which react upon the membranes in such a way as to cause them to become impermeable. The membranes must, of course, be more or less permeable to such sub- stances at first, else they could not react upon them. But such cases are very rarely met with. The so-called select- ive power is thus probably active usually, not in the absorb- ing organs, but in the cells where the substances are used in metabolism. III. TRANSMISSION OF SOLUTES An internal atmosphere exists in the plant, occupying the intercellular spaces, which are in communication throughout its body and which connect with the lenticels of the bark and with the air chambers and stomata wherever these occur. By means of this internal atmosphere, gaseous oxygen may reach the more deeply lying parts of the body, and the gaseous product of respiration in such parts may find its way to the surface. Thus, it is probable that the comparatively slow process of hydro-diffusion of gases is replaced by the much more rapid gas diffusion wherever the internal atmos- 1W. Pfeffer, "Ueber Election organischer Nahrstoffe," Jahrb.f. wiss. Bot, Vol. XXVIII (1895), pp. 205-68. 2 Pulst has recently shown that, while copper ions are readily absorbed by Mucor, Aspergillus, and Botrytis, they are not taken in by Penicillium. See C. Pulst, " Die Widerstandsfahigkeit einiger Schimmelpilze gegen Metallgifte," Jah; b. f. wiss. Bot., Vol. XXXVII (1902), pp. 205-63. Absorption and Transmission of Solutes 121 phere makes this possible. Mechanical movements of the plant, by wind, etc., probably hasten this diffusion by creat- ing currents. Apart from this gaseous diffusion, all transmission of solutes, whether gaseous, solid, or liquid, must take place in the form of aqueous solution. Diffusion of solutes from cell to cell takes place in accordance with the principles of diffusion, the tendency being ever to equalize the diffusion tension of all solutes throughout the extent of the solution. In order to emphasize the fact already referred to that there is no relation between the turgidity produced by one solute and the diffusion of another into or out of the same turgid cell, the following example may be taken: Suppose an artificial cell whose membrane is impermeable to dissolved sugar but permeable to K and N0 3 ions. Such a membrane may be made from copper ferrocyanid. Let this cell be filled with a solution of sugar and potassium nitrate so made up that the partial pressures of the two substances are equal to each other, say five atmospheres. The total pressure of the solution is then ten atmospheres. Now let this cell be placed in distilled water. Since the membrane is permeable to KN0 3 , this salt will immediately begin to diffuse outward, and diffusion will continue until its diffusion tension is prac- tically as great outside as it is within the cell. No osmotic pressure will be manifested by the KN0 3 , excepting the small amount due to friction in the membrane. But the sugar cannot pass out of the cell, and must therefore exert its full pressure upon the walls, making the cell turgid and manifesting a stretching force of nearly five atmospheres. Of course, as the cell becomes turgid a comparatively small amount of water will enter from without, and thus dilute the internal solution of sugar. So far the illustration shows that it is possible for turgidity to be maintained while a substance is diffusing out of the 122 Diffusion and Osmotic Pressure cell. This is just what probably occurs during the trans- mission of substances from one cell to another by diffusion. After a time, however, the artificial cell will come into equilibrium. The diffusion tension of the sugar is then just equaled by the resilience of the walls, and that of the inclosed KN0 3 by the diffusion tension of the same salt in the surrounding medium. No further changes of concen- tration will occur until some alteration is made in the con- ditions. Now let a few crystals of potassium nitrate be added to the external solution. They dissolve immediately and diffuse equally as far as the solution extends. But now the diffusion tension of this salt has been raised in the sur- rounding medium while it remains the same within the cell. However, since the membrane is permeable to KN0 3 , this condition cannot last long; inward diffusion of K and N0 3 ions will soon equalize the tension within and without. Thus it is shown that a solute may diffuse not only out of but also into a cell, the latter remaining turgid meanwhile, through the action of another solute to which the osmotic membrane is impermeable. Mass movements of the sap in stems, caused by changes in temperature, mechanical bending (as by the wind), etc., may aid very much in keeping the various solutes equally dis- tributed throughout the inclosed solution. The mass move- ment occasioned in the solutions by evaporation from above (possibly also by sap pressure) must also aid in this. Within the cells the streaming movements of the proto- plasm must act in the same manner, and the protoplasmic connections between adjacent cells probably sometimes set up mass currents which aid in the transmission of solutes from one cell to another. The latter consideration is prob- ably of relatively great importance in the case of the trans- mission of carbohydrates and other plastic materials through the phloem region of stems. But by far the most impor- Absorption and Transmission of Solutes 123 tant factor in the distribution of solutes throughout the plant body, whether this be the plasmodial mass of a Myxomy- cete or the great body of a pine tree, is probably simple diffusion. If it were not for the phenomenon of turgidity, the plasmic membrane would not be in condition to allow dif- fusion to take place readily. But the membranes do not di- rectly aid in the transmission of solutes ; they only hinder it. CHAPTER IV THE INFLUENCE OF THE OSMOTIC PRESSURE OF THE SURROUNDING MEDIUM UPON ORGANISMS I. INTRODUCTORY Although many researches have been carried out to determine what may be the influence upon the organism of the medium in which it is grown, it is only within the last few years that osmotic pressure has been investigated in this regard. Most experimenters have varied the chemical nature of the medium in which plants and animals were grown, and have argued from their experiments that the presence or absence of certain chemicals brings about cer- tain effects within the organism. But if osmotic pressure can have any effect upon the behavior of the living being — and sufficient evidence has now been accumulated to show that it does have a very marked effect — then the results of all such researches must be considered as very questionable. Nearly all the published accounts of the influence of nutrient salts upon growth, reproduction, etc., in plants are subject to this criticism, that, while the author supposed he was varying a single factor, he was in reality varying at least two. Of course, any conclusions reached from research of this kind are not to be relied upon. There are always two ways in which a nutrient fluid may affect the organisms placed in it, and these two ways corre- spond to the two entirely different sets of properties which are possessed by every solution. The solution may affect the animal or plant chemically, on account of its chemical properties, or it may have a physical effect, on account of its physical properties. Of course, since both sets of prop- 124 Influence of the Medium 125 erties coexist in the same solution, it is possible and prob- able that the organism may often be affected in both ways at the same time. By the chemical properties of a solution are meant the chemical nature of the solute or solutes. It is to be expected that a solution of cuprous sulfate will affect organic beings differently from a solution of cane sugar or one of sulfuric acid; these solutions are chemically very different. By physical properties are meant such qualities as viscosity, transparency, surface tension, osmotic pressure, etc. The latter is the only one of these which it is necessary to con- sider here. This property of osmotic pressure has been shown to be of general importance to the living being grown in ordinary nutrient solutions, but it has long been neglected in experiments with such solutions. Experi- menters with nutrient fluids have varied the chemical nature of their solutions without taking into account the fact that in so doing they were very probably varying the osmotic pres- sure also. When these workers have dealt with very weak solutions only, it is evident that the error thus introduced is practi- cally negligible ; the osmotic pressure must be very slight in all cases. Thus Ono 1 showed that various mineral salts which are usually considered as poisons have an acceler- ating effect upon the growth of certain fungi when the solutions are very dilute. In this case the osmotic pressure is of such a low order that it may be left out of account. But suppose a case of another sort. It is also well known that a stronger solution of such a salt as cuprous sulfate will produce almost instant death. Shall it be argued, then, that life or death depends upon the number of Cu and S0 4 ions which may penetrate the living cells? Or shall it be i N Ono, " Ueber die Wachsthumsbeschleunigungeiniger Algen und Pilz.ylurch chemische Reize," Jour. Coll. Set. Imp. Univ. T^o, Vol XIII (1900), Part I: B* Mag., Vol. XV (1900), p. 75; reviewed in Bot. Gaz., Vol XXX (1900), p. 4— 126 Diffusion and Osmotic Pressure argued that it is merely a question of osmotic pressure of the solution, and that the chemical nature of the cuprous sulfate has nothing to do with the response ? A third possibility is to conclude that both these factors, always possessed in common by any solution, are active in bringing about the observed result. Obviously these two observa- tions, that the organism lives in a weak solution of CuS0 4 and that it dies in a stronger one, are not sufficient to settle the question. There are several different ways in which a plant cell may be affected by a solution into which it is plunged. If the solution be concentrated, it may have two effects : (1) Chemically, the solute may produce a response in the protoplasm by diffusing into it, and reacting with it in some way as yet not understood. Thus, the effect of a solution of HgCl 2 upon plant protoplasm is very different from that of cane sugar. (2) Physically, the solution may affect the cell by plasmolyzing it, or partially plasmolyzing it, or by reducing its active turgor pressure. It has been seen that this effect consists primarily in extracting water from the cell. Secondarily, it results in an increased concentration of the contained solution. This latter may again result in a chemical effect upon the living protoplasm, but of this we know absolutely nothing as yet. If the solution be a weak one, its physical effects will be just the reverse of those just mentioned, while its chemical effects will often be the same, but perhaps less marked. Physically, it will allow more water to diffuse into the cell, and there will result a rise in turgidity. In order to answer the question stated above with regard to the nature of the effect produced by different concentra- tions of cuprous sulfate, experiments upon the same organism must be performed with other salts and with non-electrolytes, such as cane sugar, glucose, etc. In making these solutions Influence of the Medium 127 extreme care must be used to have them of exactly the same osmotic concentration as those of CuS0 4 , which were pre- viously used. Then, if the organism lives in all the dilute solutions, no matter of what substance, it may be concluded that the determining factor is one of osmotic pressure. If, on the other hand, the organism lives in the concentrated solutions of cane sugar, glucose, KN0 3 , NaCl, etc., but dies even in a weak solution of HgCl 2 or CuS0 4 , it must be concluded that the conditions of the medium which deter- mine life or death are of a chemical nature. It is thus pos- sible to analyze the effects of a solution by using a number of different solutes. The primary effect upon an organism of an increase in the osmotic concentration of the surrounding medium is extraction of water, that is, it is a drying effect. The pri- mary effect of a decrease in the osmotic concentration is the reverse, it adds water to the organism. II. PRESENTATION OF MATERIAL Following is a review of the several lines of experimental evidence which have been brought forward in connection with the question of the effect of variations in the concen- tration of the medium upon the living being. Since there is so little to be presented, the work upon both animals and plants will be included. The material at hand will be discussed under four heads: (1) The effect upon growth, (2) the effect upon reproduction, (3) the effect upon move- ment, and (4) the analogy between the effects of high osmotic pressure of the medium and those produced by other water-extracting processes. a) Variations in the osmotic pressure of the surround- ing medium: their influence upon the growth and form of organisms. — A number of observations upon various organ- isms have been made, all tending to the general conclusion 128 Diffusion and Osmotic Pkessure that growth takes place more slowly in a concentrated solu- tion than in a weaker one. Loeb 1 found that the regenera- tion of decapitated tubularian hydroids occurs much more slowly in a concentrated than in a dilute solution. The optimum concentration lies considerably below the normal concentration of sea-water, in which these animals live natu- rally. Similar results were obtained by Yung, 2 working on tadpoles, and also by J. L. Frazeur (with annelids) and P. E. Sargent (with Dero vaga) in the laboratory of C B. Daven- port. 3 The first of a very important series of observations dealing with the effect of external concentration upon cell division was made by Loeb 4 when he discovered the fact that, in fer- tilized Arbacia eggs which were placed in sea-water whose concentration had been raised by the addition of NaCl, the nuclei divided a number of times without the usual accom- paniment of the segmentation of the entire egg. When these eggs with segmented nuclei were returned to normal sea-water, segmentation of the cytoplasm occurred suddenly, the number of segments corresponding, in general, to the number of parts into which the original nucleus had divided. Loeb concludes from these experiments that the extraction of water by high osmotic pressure causes a falling off in the irritability of the protoplasm. Whereas, a part of the nor- mal process of cleavage, namely, that pertaining to the nucleus, is carried out, the remainder of it, segmentation of the egg, fails to occur in the strong solution. The cyto- plasm fails to perform its part, although the nucleus is still 1J. Loeb, Untersuchungen zur physiologischen Morphologie der Thiere. II: Organbildung und Wachsthum, Wiirzburg, 1892. 2 e.Yung, "De l'influence des variations du milieu physico-chimique sur le developpement des animaux," Arch, des sci. phys. et nat., Vol. XIV (1885), pp. 502-22. 3C. B. Davenport, Experimental Morphology, New York, 1899, p. 365. * J. Loeb, "Ueber Kerntheilung ohne Zelltheilung," Arch. f. Entwickl. d. Organismen, Vol. II (1895) , pp. 298-300. Influence of the Medium 129 active. Sperms which were placed in the strong solution lost their irritability, but regained it upon being returned to sea-water. Another instance which seems to show the partial loss of irritability by protoplasm from which water is osmotically extracted, is stated by the same author in the same article. Hearts of ascidians, crustaceans, and of embryo and adult vertebrates all beat less rapidly in strong solutions than in weak ones. Morgan 1 repeated Loeb's experiments on Arbacia eggs with practically identical results. He made the added observation that the free nuclear division described above occurs in concentrated solutions, even though the eggs have not been fertilized. This author also gives valuable cyto- logical notes on the nature of the free nuclear division. This paper by Morgan has been followed by several others by Loeb, 2 the results of which may be brought together as follows : Unfertilized eggs of Arbacia, Strongy- locentrotus, and Asterias, can all be made to develop parthe- nogenetically, if they are first placed for a time in sea-water, the concentration of which has been raised by addition of either an electrolyte (such as NaCl or MgCL) or a non- electrolyte (such as cane sugar or urea). They must then be returned to normal sea-water. In this artificial par- thenogenesis development continues until the animal is in the Pluteus stage. This is as far as the development of normally fertilized embryos can be carried in aquaria. The author concludes that " there can be no doubt that the essen- i T. H. Moegan, " The Effect of Salt Solutions on Unfertilized Eggs of Arbacia," Science, N.' S., Vol. VII (1898), p. 222. \ 2 J Loeb "On the Nature of the Process of Fertilization and the Artificial Production of Normal Larva, (Plutei) from the Unfertilized Eggs of the Sea Urchin," Am. Jour. Physiol.,\oL III (1899), pp. 135-8; idem ■ » On the Artificial Production of Normal Larvae from the Eggs of the Sea Urchin (Arbacia), tfttd.. Vol III (1900), pp. 434-71; idem, "On Artificial Parthenogenesis in Sea Urchins, Science, N. S., Vol. XI (1900), pp. 612-14; idem, " Further Notes on Artificial Par- thenogenesis and the Nature of the Process of Fertilization, Am. Jour. Physiol., Vol. IV (1900), pp. 178-84; idem, "Artificial Parthenogenesis in Annelids (Chaetopte- rusj," Science, N. S., Vol. XII (1900), p. 170. 130 Diffusion and Osmotic Pressure tial feature in this increase in the osmotic pressure of the surrounding solution is a loss of water on the part of the e gg" Furthermore, the same author showed that a similar arti- ficial parthenogenesis may be brought about in the case of Chretopterus, a marine annelid. Here another method of treatment would also bring it about, namely, a slight increase in the amount of potassium in the medium without any increase in its concentration. This is termed by Loeb chemical fertilization in contrast with physical fertilization, the form described above. Chemical fertilization by potas- sium is, so far, impossible in the eggs of Echinoderms. Very recently Loeb and Neilson 1 have shown that chemical ferti- lization by means of hydrogen ions is possible with eggs of Asterias, and the same sort of fertilization, but with cal- ciumions, was brought about by Loeb and Fischer 1 in the case of eggs of the marine annelid, Amphitrite. These phenomena are interesting here mainly as they show that a chemical influence can bring about the same effect as extrac- tion of water. That plants grow less rapidly in concentrated solutions than in more dilute ones was first stated by Jarius, 2 who worked on the germination of seeds. With growing plants, Stange 3 showed that Pisum, Phaseolus, Lupinus, etc., increase in thickness more rapidly in concentrated solutions, while more rapid growth in length occurs in dilute ones. He suggests that the effect of the solution may be different upon the meristematic cells of the growing point and upon 1 J. Loeb, M. Fischer, and H. Neilson, " Weitere Versuche uber kttnstliche Parthenogenese," Vorlaufige Mittheilung, Pflugers Arch. f. d. ges. Physiol., Vol. LXXXVII (1901), pp. 594-6. 2M. Jaeitjs, "Ueberd. Einwirkung von Salzlosungen auf d. Keimungsprocess d. Samen einiger einheimischer Culturgewacb.se," Landwirtsch. Versuchs-Stat., Vol. XXXII (1886), pp. 149-78. 3 B. Stange, " Beziehungen zwischen Substratconcentration, Turgor und Wachs- turn bei einigen phanerogamen Pflanzen," Bot. Zeitg., Vol. L (1892) , p. 253. Influence of the Medium 131 those of the cambium. Vandervelde 1 experimented upon the germination of seeds which had been soaked twenty-four hours in various concentrations of several salts. As the solution became stronger the number of seeds to germinate decreased, but after a certain minimum of germination was reached, the number germinating again increased. This author suggests that the failure to germinate in the inter- mediate concentration is due to the penetration of the salts, while in stronger solutions little or no imbibition of water took place, and the seeds when planted were practically the same as when put into the solution. This subject has been recently taken up again by Buffum and Slosson. 2 These authors show that not only is imbibition of seeds greatly retarded by a concentrated solution (as was known before), but also germination and the growth of the plant are retarded in the same manner. This is true without regard to the chemical nature of the dissolved substance. Both electro- lytes and non-conductors were used. Retardation of growth is not proportional to concentration, however, for an osmotic pressure of one hundred atmospheres retards growth only about twice as much as a pressure of ten atmospheres. Regarding the maximum concentrations in which fungus growth can occur, investigations have been made by Eschen- hagen and by Raciborski. Eschenhagen 3 found that this maximum was different for different fungi studied, but the concentration was about the same for different salts, seeming to show that it was a purely osmotic effect. For Penicillium the maximum concentration is about that of a five-normal i A. J J Vandervelde, " Ueber den Eiafluss des chemischen Reagentien und des Lichtes auf die Keimung der Samen," Bot. Centralbl, Vol. LXIX (1897), pp. 337-42. 2 E E Slosson and B. C. Buffum, "Alkali Studies II," Bulletin 39, Wyoming Aaric Exp Sta. (1898) ; B. C. Buffum, "Alkali Studies III," Ninth Annual Report, Wyoming Agric. Exp. Sta. (1899); E. E. Slosson, "Alkali Studies IV," ibid. (1899); B. C. Buffum and E. E. Slosson, "Alkali Studies V," Tenth Annual Report, Wyoming Agric. Exp. Sta. (1900) . 3 F. Eschenhagen, Ueber den Einfluss von L6sungen verschiedener Koncentra- tionaufdas Wachsthumvon Schimmelpilze, Stolp, 1889. 132 Diffusion and Osmotic Pressure cane-sugar solution, that is, about 111.5 atmospheres. Raci- borski 1 found the maximum concentration for growth of Basidiobolus was that of a 6 per cent, solution of NaCl, or about seventeen atmospheres. Yasuda 2 published an account of some experiments upon infusoria which have a bearing here. He finds that these organisms are able to adjust themselves to solutions of quite high concentration, and that, in general, the limit of their power of adjustment seems to be at about the same osmotic pressure, no matter what salts are used. In other words, the limit to adjust- ment is apparently an osmotic one and depends upon with- drawal of water. The experiments of the present author 3 upon the physi- ology of polymorphism in Stigeoclonium need to be con- sidered here. In the stronger solutions (pressure from 323.7 cm. to 647.4 cm. Hg.) this alga takes the form of groups of spherical cells with somewhat gelatinous walls. Multiplication takes place rather slowly, cell division occur- ring in all directions and the daughter-cells immediately rounding up so far as they are not hindered by adjacent cells. In weak solutions (pressure below 161.8 cm. Hg.) the behavior is entirely different. The daughter-cells elongate into branching filaments composed of cylindrical cells and having the typical appearance of Stigeoclonium. Growth is much more rapid here than in the strong solutions. If fila- ments are transferred to a strong solution, the cells round up and break apart, thus producing the other form. 1 M. Raciborski, " Ueber den Einfluss Susserer Bedingungen auf die Wachs- thumsweise des Basidiobolus ranarum," Flora, Vol. LXXXII (1896), pp. 107-32. 2 Atsushi Yasuda, "Studien uber die Anpassungsfahigkeit einiger Infusorien an concentrirte LOsungen," Jour. Coll. Sci. Imp. Univ. Tokyo, Vol. XIII (1900), pp. 101-40. Reviewed in Bot. Gaz., Vol. XXX (1900), p. 285. 3 B. E. Livingston, (1) " On the Nature of the Stimulus Which Causes the Change of Form in Polymorphic Green Algse," Bot. Gaz., Vol. XXX (1900), pp. 289- 317; idem, (2) " Further Notes on the Physiology of Polymorphism in Green Algae," ibid., Vol. XXXII (1901), pp. 292-302. Some parts of the discussion here given are quoted from these articles. Influence of the Medium 133 In the first series of cultures several modifications of Knop's solution were used. This solution consists of: Ca (NO,),, four parts; MgS0 4 , KN0 3 , and K 2 HP0 4 , each one part, with the addition of a trace of iron. In order to determine whether a change in the concentration of this solution would affect the plant in a chemical or physical way, four modified solutions were made up, each being defi- cient in one of the four constituent salts. The deficient salt was reduced to one-tenth its normal quantity, and, the decrease in osmotic pressure thus brought about having been calculated, a sufficient amount of each of the three other salts was added to increase the pressure by an amount equal to one-third of the calculated decrease. Thus were obtained four solutions, all of which had the same osmotic pressure, but each of which was deficient in one salt. The calculations for the pressure corrections were made both by the now obsolete method of De Vries, and by assum- ing that, in the concentrations used, ionization was complete. Solutions made by both methods gave the same results upon the plant, and after a first trial the second method of cal- culation was exclusively used. During the summer of 1901 the pressure of nearly all these solutions was tested by the freezing-point method. A table of the results so obtained will be found in the second paper cited on this subject. The error introduced by the assumption of complete ioni- zation was found to be too small to interfere with the accuracy of the results in any degree, the discrepancy between the real and the calculated pressures lying well within the limits of the threshold of stimulation for this alga. . , The cultures showed that all four modified solutions, and the normal Knop's solution also, influence the plant in exactly the same manner. The form of the alga is always deter- mined by the osmotic concentration of the medium, and is not affected by the varying proportions of the constituent salts. 134 Diffusion and Osmotic Peessure In the second series of cultures, besides the normal and modified Knop's solutions, two non-electroytes, cane sugar and lactose, were used, and it was found that here also the concentration is the controlling factor in the response of the plant. It must be noted, however, that to prevent the formation of filaments a somewhat higher concentration is required of these sugars than of the inorganic salt. This is perhaps due to a more ready absorption of the sugars and consequent rise in internal concentration. Whatever may be the cause of this phenomenon, it seems to be in accord with that noted by van Rysselberghe, 1 namely, that cells of Tradescantia, etc., develop a greater turgidity in salt solu- tions than in those of sugar. It is thus shown conclusively that the changes in the growth of this plant which result from changes in the con- centration of the medium are entirely dependent upon its osmotic pressure. This means that they are dependent upon the amount of water contained within the cells, for the strong solutions extract water, while the weak ones allow it to be absorbed. In a weak solution vegetative growth is very much more rapid than in a strong one. This may be due to the fact that in a strong solution the water content of the protoplasm is reduced in amount below the limit for optimum lability. When the plant grows fastest and best it is in the filamen- tous form. In the weak solution, where activity seems to be at a maximum, the ions of the electrolytes, which are essen- tial for metabolism, are not plentiful. This may suggest how the cylindrical form of cell with its increased surface 2 may 1 Van Rysselberghe, " Reaction osmotique des cellules vegetales a la concentra- tion du milieu," Mem. cour., pub. par l'Acad. roy. de Belg., Vol.LVIII (1898), pp. 1-101. 2 In a cylinder, the lateral surface is greater than that of a sphere of the same volume, as long as the ratio of the length to the diameter equals or exceeds 2.727. In typical filament cells of this alga, the ratio of the diameters is 3, and it is often 4 and even greater. It is seldom less than 2.8. Thus, it is shown that the filament cell offers more surface to the surrounding medium through its lateral walls alone than does the palmella cell of equal volume. Influence of the Medium 135 be advantageous. At any rate, we may be sure that the greater surface of the cylinder puts the plant into better con- dition for exchange of material with its surrounding medium. On the other hand, the more concentrated solution not only withholds water from the cells, but presents a demand upon them for water. The cell meets this in part by offering as small a surface as possible to the solution. In this case, although the requisite ions may be present, and even in the right number, the scarcity of water in the protoplasm may so decrease the lability that rapid growth is impossible. We shall see that there is also a corresponding falling off in the reproductive activity in strong solutions. Perhaps this response is attributable to the increased general activity in weak solutions. It has no relation to the form of the cell, since zoospores are produced from both spherical and cylin- drical cells, as well as from those of intermediate shape. It is to be emphasized that in the stronger solutions cell division and growth are not only retarded, but the direction of the dividing planes is curiously changed. Whereas in the weak solutions the cylindrical cells divide only by walls in one direction, the spherical cells of cultures in the more con- centrated solutions divide in all directions. Whether this is due to the change in form of the cell, or directly to the water content of the protoplasm, cannot yet be decided. What may be the mechanics of the rounding up of cylin- drical cells when placed in a concentrated solution is one of the most important problems suggested by this research. The fact that the dead cellulose membrane is almost entirely reshaped during this process, without being dissolved, ren- ders it probable that the change in form is directly caused by some change in turgidity within the cell. In a rounding cell the membrane moves and changes its form, and, since it is entirely inert, the source of this motion must be either in the activity of the protoplasmic body itself, or it must be in the 136 Diffusion and Osmotic Pressure effective turgor pressure of the mass of liquid within. But since protoplasm and cellulose wall can be parted so readily during plasmolysis, the first alternative is well-nigh untenable. If the wall be forced into the spherical shape by a change in the pressure from within this must be brought about by a change in the volume of the contained liquids. Now, this slight change in volume which might produce a change in the tur- gidity of the cell is most probably due to an alteration in the amount of cell sap within the vacuole. When the surround- ing medium suffers change in concentration, a change in the volume of the vacuole may come about through the proto- plasmic sac either secreting liquid or acting merely as a semi-permeable membrane. When filaments are placed in a concentrated solution their behavior suggests at once partial plasmolysis. Water may be extracted, the effective turgor pressure on the walls may be decreased, and by the forces of surface tension and cohesion the protoplasm may tend to round itself up into a sphere. If this be true, we have an explanation of the lateral bulging which accompanies the longitudinal shrinking of the cellulose envelope. If the protoplasm tended to assume a spherical form within the cylindrical wall, the pressure upon this would be decreased first at the angles. At the same time, it would be relatively increased upon the lateral walls near their middle. Thus would come about a bulging of the lateral walls outward, and hence a shortening of the cell and a draw- ing of the end walls toward each other. But the internal pressure is to be counted as almost nothing at the angles, while it is still considerable in the middle of each end wall. So the margins of the end walls would approach the middle of the cell more rapidly than do their central portions, and splitting of the common membrane of two adjacent cells would necessarily ensue. Several facts were observed in the cultures which seem to support some such hypothesis as the Influence of the Medium 137 one just stated. I have placed filaments in a solution where they were completely plasmolyzed and killed, without any change in form. In solutions a little less concentrated they are not plasmolyzed, but round up rapidly and soon die, often in the palmella condition. With a still lower pressure the filament cells round up more slowly and live. Another fact suggesting this idea is that floating filaments can resist a stronger solution, and can resist it longer, than sunken ones. The former are to some extent in contact with the air, and thus present less surface than the latter to the liquid. Still another observation bearing upon this hypothesis of partial plasmolysis is that cylindrical cells are the only ones which are able to change their form after they have become mature. A spherical cell must remain so till it divides, even if it be in a solution of very low pressure. Raciborski 1 made what must be regarded as essentially the same observation as the one just discussed upon Basi- diobolus, concerning the rounding up of cells and the change in direction of cross walls. He states that in strong solutions the cells became rounded and separated from one another, and that walls formed in all directions. Although he paid little attention to osmotic phenomena, yet it can hardly be doubted that Basidiobolus ranarum behaves in much the same way as does Stigeoclonium. In a recent paper Beauverie 2 has described some interest- ing effects of the osmotic concentration of the medium upon fungi and higher plants. The concentration of his nutrient fluids was raised by the addition of NaCl — a very question- able method, especially in view of the proof brought forward by True 3 that this salt sometimes has a poisonous action. 1 M. Raciboeski, "Ueber d. Einfluss ausserer Bedingungen auf d. Wachsthums- weise des Basidiobolus ranarum," Flora, Vol. LXXXII (1896), pp. 107-32. 2 J. Beauverie, "Influence de la pression osmotique du milieu sur la forme et la structure des vegetaux," Compt. rend., Vol. CXXXII (1901), pp. 226-29. 3 R. H. True, " The Physiological Action of Certain Plasmolyzing Agents," Bot. Gaz., Vol. XXVI (1898), pp. 407-16. ■v 138 Diffusion and Osmotic Peessure Beauverie found that fungus hyphae which normally grow upon the surface of the nutrient fluid, and even rise into the air, lose this habit in concentrated solutions, and remain, for the most part, submerged. Growth continues in these cases, but seems not to be as marked as in weak solutions. Of the hyphse which do rise into the air from the concentrated medium, the cells are much shorter and broader than those which rise from a weak solution. Details are not given in the published account, but apparently we have here a very similar response to the one which was obtained in the case of Stigeoclonium. The same author has also experimented upon flowering plants, e. g., Pisum and Phaseolus. Grown in a strong solution, the stems of these plants are short and thick, and the roots show a remarkable development of cork tissue on their surfaces, with a slight development of pith. He also states that, while in weak solutions an upward bending of the roots normally occurs, in strong solutions these grow vertically downward. The upward bending in weak solutions has been ascribed heretofore to aerotropism. Perhaps the extraction of water which occurs in the strong solution changes the irritability of the roots so that they no longer respond normally to lack of oxygen. In the stems of Pisum and Phaseolus is perhaps presented another case of cells failing to elongate in a solution which extracts water. Much more experimentation is needed, however, before we can relate these responses in higher plants with those of algaB and fungi. b) The influence of external concentration upon repro- duction. — Raciborski 1 states that concentrated solutions check the formation of zygospores in Basidiobolus. In concentrated solutions Stigeoclonium 2 failed to produce any ! M. Raciborski, "Ueber d. Einfluss ausserer Bedingungen auf d. Wachsthums- weise des Basidiobolus ranarum," Flora, Vol. LXXXII (1896), pp. 107-32. 2B. E. Livingston, (1) "On the Nature of the Stimulus Which Causes the Change of Form in Polymorphic Green Algse," Bot. Gaz., Vol. XXX (1900), pp. 289- 317; idem, (2) "Further Notes on the Physiology of Polymorphism in Green Algae," ibid., Vol. XXXII (1901), pp. 292-302. Influence of the Medium 139 zoospores, but these were formed in great numbers, and very rapidly, in the weak solutions. Since the formation of zoosporis is to be regarded as the result of protoplasmic activity, this fact is added evidence that the cosmotic extraction of water reduced the general activity of the pro- toplast. c) TJie influence of external concentration upon irrita- bility. (1) Changes in irritability. — That Loeb observed a loss of irritability in Echinoderm sperms when these were placed in a concentrated solution, and a return of it when they were brought back to normal sea-water, has already been noted. Eichter 1 states that zoospores of Tetraspora lose their activity in strong solutions, but regain it on being returned to normal sea-water. The writer found that zoospores of Stigeoclonium lose their power of movement in concen- trated solutions. Of interest here is also the observation of Engelmann 2 that the cilia of the epithelial cells which line the frog's oesophagus become much more active in pure water or a very weak solution than in a solution of the same concentration as the fluids of the animal's body. Loeb 3 gives a very striking account of the reversal of a tropism by osmotic extraction of water. At ordinary tem- peratures the larvse of Polygordius and certain Copepods are partly positively and partly negatively heliotropic. Above 25° C. they all react negatively, while below 10° C. the response is reversed, and they all become positively heliotropic. If NaCl is added to the normal sea-water in which these animals are living, they all react positively to light ; if distilled water is added, they all react negatively. 1 A. Richter, " Ueber die Anpassung der Sftsswasseralgen an KochsalzlOsungen " Flora, Vol. L (1892), pp. 4-56. 2T. W. Engelmann, " Ueber die Flimmerbewegung," Jena Zeitschr. , Vol IV (1S68), pp. 321-479. 3 J. Loeb, " Ueber kunstliche Umwandlung positiv heliotropischer Thiere in negativ heliotropische und umgekehrt," Pflilgers Arch.f. d. ges. Physiol., Vol. LIV (1893), pp. 81-107; idem, Physiology of the Brain, New York, 1900, p. 198. 140 Diffusion and Osmotic Pressure A similar reversal of tropism, in this case of geotaxis, was observed in Chromulina woroniniana by Massart. 1 Thus, by osmotically changing the amount of water in the protoplasm the irritability of these organisms can be reversed. (2) Osmotaxis. — The concentration of the medium acts as a directing stimulus upon the motions of certain free- swimming organisms. This form of response has been named osmotaxis, in analogy to other similar responses to light, heat, chemicals, etc. An organism is said to be posi- tively osmotactic when it swims from the weaker to the stronger solution where these are brought into contact. It is negatively osmotactic when it swims in the opposite direction. Since the effect of high concentration of the medium is to extract water from the cell, it will be seen that there must be an identity of nature between this response and that of hydrotropism. An organism is positively hydrotropic when it bends away from a dryer and toward a moister atmos- phere. This phenomenon is exhibited in roots, fungus -sporophores, etc. It corresponds to negative osmotaxis, in which the organism swims from a region where water is extracted from its body to one where absorption can take place more freely. Since the conditions under which the two responses are made manifest are so very different, it is probably well to retain the word "osmotaxis." Rothert 2 has recently devoted an article to the discussion of this subject. The following facts are mainly derived from this source, Stahl 3 showed that Myxomycete plas- modia, which had become accustomed to a certain concen- tration, would be repelled by any other concentration, either higher or lower. They are thus negatively osmotactic. 1 J. Massart, " La sensibility a la concentration chez les fitres unicellulaires marins," Bull. deVacad. roy. de Belgique, Ser. Ill, Vol. XXII (1891), pp. 148-67. 2 W. Rothert, " Beobachtnngen rind Betrachtungen fiber taktische Reizerschei- nungen," Flora, Vol. LXXXVIII (1901), pp. 371-421. 3E. Stahl, u Zur Biologie der Myxomyceten," Bot. Zeitg., Vol. XLII (1884), pp. 145 ff. Influence of the Medium 141 Massart, 1 experimenting upon certain bacteria, found that they were negatively osmotactic to solutions of many different substances. The proof that the phenomenon is osmotaxis and not chemotaxis lies in the fact that the organ- isms were repelled always at the same osmotic concentration, irrespective of the chemical nature of the solute. Rothert found Treptomonas agilis positively osmotactic toward solu- tions which are so concentrated that they kill the organism by plasmolysis. Only such solutes are available for experi- ments upon osmotaxis as are known to be unable to pene- trate the protoplasm of the organism to be tested. Of course, if penetration occurs, the difference in concentration within and without the cell can last but a short time ; it will soon be equalized by the inward diffusion of the solute. d) The analogy between the effects of high osmotic pres- sure of the medium and those produced by other water- extracting processes. — Attention has already been called to the fact that a lowering in temperature is often accompanied by giving out of water. Thus Spirogyra filaments when cooled in olive oil may be seen to give off water before freez- ing. It seems probable that in this case the protoplasm becomes more permeable at these low temperatures and thus the solute escapes with the solvent. If this is true, we can- not look upon cold plasmolysis as producing a concentration of the cell sap. It would only decrease its volume. Of course, a part of the shrinkage in such a case could be accounted for by the diminution of osmotic pressure due to cooling. If the original internal pressure were p at t°C, then it would decrease to p ~ o 7 q , , at t'°Q. The pres- sure of the external solution will decrease according to the 1 J. Massart, " La sensibility a la concentration chez les ©tres unicellulaires marins," Bull, de Vacad. roy. de Belgique, Ser. Ill, Vol. XXII (1891), pp. 148-67 ; idem, "Sensibilite et adaptation des organismes h la concentration dee solutions salines," Arch, de biol., Vol. IX (1899), pp. 515-70. 142 Diffusion and Osmotic Pressure same principle, but since its original pressure, say s, was much smaller than p, its decrease for the same fall of tem- perature will not be so great as that of the internal solution. This may be shown thus: t'p t's > 273 -f* 273 + £ ' when P > s . Thus, the internal and external osmotic pressure will be more nearly the same at a low temperature than at a higher one. The two pressures should become equal at absolute zero. No measurements have been made to determine whether the decrease in volume of the Spirogyra vacuole is propor- tional to the approach of the external and internal concen- trations toward each other. This should not be a difficult thing to settle. But, as has already been stated (page 75), there is cryoscopic evidence that the extruded liquid is not pure water. The identity of the responses obtained by Loeb with Copepods and Polygordius larvae when these were subjected to cold and to high concentrations, has also been noted (page 139). A similar change of tropism occurs among those plant lice which exist in two forms, one winged and the other wingless. The growth of wings in the wingless form can be called forth either by low temperature or by allowing the plants upon which the animals are feeding to dry, thus depriving the latter of water. While in the wingless condition these lice are negatively heliotropic, but upon devel- oping wings they become positively so. Here is a reversal of tropism brought about by withdrawal of water, but this experiment also shows that, although the general protoplas- mic activity may be depressed by this treatment, yet certain special activities (e. g., those involved in wing formation) may be accelerated. Influence of the Medium 143 It is generally known that lowering of the temperature of an animal heart causes the beating to become less rapid. This is perfectly parallel to the falling off in heart activity in strong solutions, as observed by Miss Shively and recorded by Loeb (page 129). In my own experiments on Stigeoclonium, 1 it was found that the organism responds to drying on a porous plate in exactly the same way as it does to change from a weak to a strong solution. Recently, Greeley 2 has shown that by cooling Stentor ccerulcus the same cessation of activity and rounding up was brought about as when the animals were subjected to the action of concentrated solutions. However, the effect of the solution was not reversible, for the animals could not be revived. The same author has shown that cold plasmolysis in Spirogyra is reversible, that a rise in temperature brings the plasmolyzed alga back to its normal condition. During the summer of 1901 Greeley 3 was able to pro- duce artificial parthenogenesis of Echinoderm eggs by merely keeping them for a time at a low temperature. In these cold-fertilized eggs, development went as far as in normally fertilized ones under artificial conditions. In general, then, it may be concluded that there is a striking analogy between the responses obtained in these various organisms by treating them with strong solutions and by extracting water from them in any other way. How much further we may go in this, remains for future experi- ment to show. i B. E. Livingston, " Further Notes on the Physiology of Polymorphism in Green Algae," Bot. Gaz., Vol. XXXII (1901), pp. 292-302. 2 A W Greeley, -On the Analogy between the Effects of Loss of Water and Lowering of Temperature," Am. Jour. Physiol., Vol. VI (1901), pp. 122-8. 3 A W Greeley, "Artificial Parthenogenesis Produced by a Lowering of the Temperature," Am. Jour. Physiol., Vol. VI (1902), pp. 296-304. 144 Diffusion and Osmotic Pressure III. SUMMARY OF THE CHAPTER As far as investigation has gone, it has been found that growth is accelerated in weak solutions and retarded in con- centrated ones. The term "growth" here includes, not only enlargement, but also the process of cell division. Also, in some cases at least, the direction of new walls is profoundly influenced by the concentration of the surrounding medium. In general, all vital processes are retarded in concentrated solutions. Reproduction, being a peculiar form of cell divis- ion, appears in some cases to be entirely dependent upon the osmotic pressure of the surrounding medium. Irritability is also greatly influenced by external pressure. Not only is this function retarded in concentrated solutions, but in some ^ forms the direction of response to a given stimulus may be ^ ?J reversed by a sudden change in the osmotic surroundings. ^ xj The comparative concentration of the external and internal ^ solutions acts, in many cases, as a stimulus upon the organ- £ ism, giving rise to the phenomena of osmotaxis. All the effects of high concentration of the surrounding liquid seem to be due to extraction of water from the living cells. They may be due either to a drying-out process or to decrease in turgidity. That they are sometimes due to the former is proved by curious analogies between the vari- ous processes which extract water from the protoplasm. Whether or not this extraction of water from the protoplasm itself is the direct cause of the responses to concentrated solutions, is not yet known. The effect may be a chemical one, due to the increased concentration of the contained solutions. f INDEX Absorption : of gases, 115 ; of solids and liquids, 118; of solutes, 115. Acids : influence of, on permeability, 61, 74 ; penetrating power of, 64. Acetanilid, in plasmolysis, 63. Acetone, in plasmolysis, 63. Action of protoplasmic membrane, 80. Alcohol, ethyl, in plasmolysis, 63. Alcohols: penetrating power of, 71; aliphatic, in plasmolysis, 63. Alkalies, penetrating power of, 64. Ammonia, penetrating power of, 64. Ammonium carbonate, 77. Ammonium chlorid, ionization of, 23. Amphitrite, eggs of, 130. Amylase, penetrating power of, 71. Anesthetics, effect of, on permeability, 78. Anilin, in plasmolysis, 63. Anilin dyes, penetrating power of, 66. Animal cells, permeability of, 64. Antipyrin, penetrating power of, 64. Apple, pressure of sap, 86. Arbacia, eggs of, 128. Arengo, exudation pressure of, 102. Arrhenius, 18, 24. Artari, 70. askenasy, 111. Aspergillus: permeability of, 67; so- lutes of, 83. Asterias, eggs of, 129, 130. Asci, bursting of, 54. Atmosphere, internal, 120. Atomic theory, 3. Avogadro, principle of, 11. Bacteria, plasmolysis of, 58, 61. Bacterium termo, plasmolysis of, 61. Bases, influence of, on permeability, 61, 74. Basidiobolus, in 'osmotic solutions, 132, 138. Bean, permeability of, 65. Beauverie, 137. Beckmann, 37, 39. Beet: permeability of, 62, 64, 78; solutes of, 83. Begonia, permeability of, 62, 73, 76. Berberis, pulvini of stamens in, 76. Blackman, 116, 117. Bleeding, 102 ; theory of, 104. Blood corpuscles, 54, 57. Blue, methyl, penetrating power of, 66, 77. BOHM, 111. Bonnier, 73, 98. Bouilhac, 70. Bourget,69, 119. Bower, 88. Boyle, principle of, 10. Browne and Escombe, 116. buffum and slosson, 113. burgarszky, 18. Bursting of cells, 54. Cabbage, pressure of sap, 86. Caffein: in plasmolysis, 63; penetrating power, 64, 77. Calcium : absorption of, .67, 120 ; pene- trating power of, 69. Calcium nitrate, absorption of, 120. Campbell, 66. Caoutchouc, membrane of, 82. Carbon dioxid: absorption of, 93, 115; from roots, 72; influence of, on trans- piration, 113; penetrating power of, 70. Carbonates, penetrating power of, 64. Carrot, pressure of sap, 86. Celery, pressure of sap, 86. Cell wall, permeability of, 55. Chaetomorpha, permeability of, 62. Chaetopterus, chemical fertilization of, 130. Chemical theory of semi-permeabil- ity, 82. Chenopodiaceae, absorption of iodin by, 69. Chlorids, penetrating power of, 78. Chromulina, reversal of tropism in, 140. Chloral hydrate, in plasmolysis, 63. Cholesterin, 81. Cilia, in osmotic solutions, 139. Clausen, 116. Cocos, exudation pressure of, 102. Codium, permeability of, 78. Coefficients, isosmotic, 56. Cohnheim, 83. Colloids, 27, 49. Conductivity of- saps, 85. Copeland, 77, 83, 88, 105, 108, 111. COPELAND AND KAHLENBERG, 69. Copepods, reversal of tropism in, 139. 145 146 Diffusion and Osmotic Pressure Copper, accumulation of, 69. Copper ferrocyanid membrane, 82, 112. Copper sulfate, 110. Coupin, 68. Crystalloids, 49. Curcuma, permeability of, 62. Curtis, 54. Curvature, role of turgidity in, 89. Cynara, 78. Czapek, 72, 74. Dandeno, 22, 68, 71, 72, 98. Davenport, 31, 128. Death, theory of, 75. Demoussy, 67, 119. Dero vaga, regeneration of, 128. Devaux, 69. De Vries, 55, 56, 61, 62, 64, 65, 74, 77, 83, 84. Diffusion, of gases, 9. Diffusion tension, of solvent, 30. Digestion, outside the body, 71, 72. Dixon, 111, 112. dutrochet, 111. Dyes, anilin, penetrating power of, 66. Ectoplast, 51, 53, 80, 82. Eggs, parthenogenesis of, by cold, 143. Electrolytes, in plasmolysis, 56. Elodea, permeability of, 66. Embryo, permeability of, 72. Endosperm, permeability of, 72. Engelmann, 139. Environmental factors, 47. Enzymes, penetrating power of, 71, 72. Epidermis, permeability of, 117. ESCHENHAGEN, 131. Ether, ethyl, in plasmolysis, 63. Eerera, 59. Euphorbia, nectaries of, 79. Evaporation, 110. Exudation : nature of, 73; from wounds, 102; from glands, 71,96. Exudation pressure, theory of, 104. Fagopyrum, solutes of, 83. Fehling's solution, 65. Ferric pyrolignate, 110. Fertilization by cold, 143. Fick, 18. Filter theory of semipermeability, 80. Flusin, 82. Form, retention of, 87. Formaldehyde, 63. Frazeur, 128. Fuchs, 104. Fucus, permeability of, 74. Fungi, in osmotic solutions, 138. Furfurol, 63. Gases: diffusion tension of, 9; mixed, 11 ; absorption of, 115 ; transmission of, 120. Gay-Lussac, principle of, 10. Glycerin, in plasmolysis, 56, 62, 63, 64, 67, 77, 79. Glucose: in turgor, 84; penetrating power of, 61, 65, 67, 70, 79. Godlewski, 108. gogorza and gonzalez, 54. Graham, 18. Gram molecule, 20. Greeley, 75, 143. Growth, role of turgidity in, 88. Gryns, 57. Gunnera, solutes of, 83. Guttation, 73, 74, 98. Hamburger, 57, 83. Hansteen, 66. Hartig, 110. Haupt, 79, 100. Hearts : in osmotic solutions, 129 ; effect of cold upon, 143. Heald, 69, 85. Hedin, 57, 83. Helianthus: permeability of, 65, 74; solutes of, 83, 84. Hilburg, 78. HOber, 18, 83. Honey-dew, 98. Imbibition, 93. Infusoria, in osmotic solutions, 132. Internal atmosphere, 120. Iodin, penetrating power of, 68, 69. Ions, in absorption, 53. Ionization: of gases, 23; of solutes in liquids, 24. Irritability, changes of, 139. Jarius, 130. Janse, 62, 65, 77, 108. Jennings, 63. Jumelle, 112. Jung, 128. Kahlenberg, 22. Kinetic theory of matter, 4. Klebs, 62, 88. Knop's nutrient solution, 132. kohlrausch, 42. KOHLRAUSCH AND HOLBORN, 42, 43. Koppe, 57. KOSSAROFF, 113. KOvesi, 83. Krabbe, 75. Kraus, C, 102. Kraus, G., 83. Index 147 Laurent, 67, 71. Leaves : absorption by, 68, 103 ; exuda- tion from, 99; guttation of, 74; permea- bility of, 71. Lecethins, 81. Lehman, 69. Leitzmann, 116. Lemna, permeability of, 76. Lice, reversal of tropism in, 141. Lidforss, 54. Liebermann, 18. LlLIACE^E, 69. Liquids : absorption of, 118 : diffusion of, 12, 13. Livingston, 132, 138, 143. LOb, 57. Loeb, 53, 74, 128, 129, 139. Loeb, Fischer, and Neilson, 130. Lupinus: in osmotic solutions, 130; so- lutes of, 84. MacDougal, 69, 119. Maquenne, 74, 84, 85. Massart, 61, 140, 141. Matruchot and Molliard, 70, 75. Matter: nature of, 3; states of, 6. Mayer, 84. Medium, influence of, 124. Meerburg, 36, 82. Membranes : cellulose, 51 ; copper f erro- cyanid, 82. Membranes: protoplasmic, 49; action of, 80. Mercuric chlorid : effect on permea- bility, 74 ; penetrating power, 68. Methyl blue, penetrating power of, 66, 77. Methyl cyanid, in plasmolysis, 63. Methyl vtolet, penetratingpower of, 66. Mimosa, pulvini of, 77. Mohl, 98. Molds, bursting of, 54. Molisch, 65, 72, 75, 102, 103, 106. Morgan, 129. Morse and Horn, 36. Mccor, exudation from, 99. Muller, 116. Muscle, permeability of, 74. Myriotonie, 59. Naccari, 18. Nathansohn, 78. Nectaries: artificial, 99; conditions for secretion of, 101 ; theory of, 96. Nernst, 18. Nernst-Palmer, 37, 38, 40. Nitrates, test for, 65. Noll, 54. Nostoc, permeability of, 70. Nuclei: frozen, dried, etc., 76; in osmo- tic solutions, 128. Oltmanns, 74, 102. Onion : permeability of, 78 ; solutes of, 83. Ono, 69, 125. Osmotaxis, 140. Osmotic pressure: in general, 28; de- monstration of, 32; of electrolytes, 27; of non-electrolytes, 25; indirect meas- urement, by freezing-point, 37; by boil- ing-point, 38; by vapor tension, 39; calculation of, for electrolytes, 42; for non-electrolytes, 41; compared to dry- ing, 141. Ostwald, 42. Ostwald-Walker, 16, 40, 42. Overton, 63, 71, 81. Oxygen: absorption of, 115; effect of, on permeability, 78; penetrating power of, 70. PARAMOsciA, ? permeability of, 63. Parthenogenesis: 129; by cold, 143. Pear, pressure of sap of, 86. Peas, solutes of, 84. Penicillium, in osmotic solutions, 131. Permeability of protoplasm: 60, 64, 67, 68, 69, 70, 118; outward, 63, 71 ; varia- tions in, 72 ; effect of on turgidity, 86. Pfeffer, 35, 50, 55, 64, 66, 68, 77, 78, 104, 105, 120. Phaseolus: in osmotic solutions, 130, 138 ; permeability of, 65, 78 ; solutes of, 83. Phenol, in plasmolysis, 63. Phloroglucin, in plasmolysis, 63. Phosphoric acid, from roots, 72. Photosynthesis, 117. Phytolocca, penetrating power of sap of, 98. Picric acid, 110. Pilobolcs, bursting of, 54. Pisum: in osmotic solutions, 130, 138; solutes of, 83, 84. Pitra, 103. Plant lice, reversal of tropism in, 142. Plasmolysis: in general, 54, 60; by cold, 75; of bacteria, 58; effect of on growth, 88; on permeability, 74. Platinum chlorid, 65. Poisons, permeating power of, 68. Pollen grains, bursting of, 54. Polygordius, reversal of tropism in, 139. Polimorphism : in Basidiobolus, 137; in fungi, 138; in Stigeoclonium, 132. Potassium: absorption of, 67, 120; pene- trating power of, 69. Potassium chlorid, in turgor, 83. 148 Diffusion and Osmotic Pressure Potassium nitrate : absorption of, 120 ; in plasmolysis, 56, 61; in turgor, 83; penetrating power of, 65, 68, 74, 77. Pressure: exudation, 102; gas, 9; os- motic (see Osmotic pressure). Protoplasm, 49. Protoplasmic membranes, action of, 80. Puriewitch, 78. Quercus, copper in, 69. Quincke, 82. Raciboeski, 132, 137, 138. Reinhardt, 88. Reproduction, in osmotic solutions, 138. Richards, 68. Richtee, 139. Rise of water, 107. Root hairs, exudation from, 75. Roots, permeability of, 69. Rothert, 140. Salts, inorganic, penetrating power of, 67, 71. Sambucus, permeability of, 76. Sap : cell, 52 ; expressed, conductivity of, 85. Sargent, 128. Scheffer, 18. Schneider, 112. schwendener, 76. Seeds, in osmotic solutions, 130, 131. Selective power, 118. Siphoned, bursting of, 54. Skertschley, 69. Slosson, 131. Sodium, penetrating power of, 69. Sodium chlorid : in plasmolysis, 61 ; penetrating power of, 68, 74; in me- dium, 137. Sodium nitrate: absorption of, 120; penetrating power of, 79. Solids, absorption of, 118 ; diffusion of, 14. Solanum, absorption by, 69. Solutes: absorption of, 115: active, na- ture of, 83 ; transmission or, 120. Solution, Knop's, 132. Solutions: denned, 16; properties of, 124; molecular, 21 ; normal, 21; of gases in liquids, 17; of liquids in liquids, 16; of solids in liquids, 18; terminology for, 20. Solution theory of semipermeabil- ity, 80. Sperms, in osmotic solutions, 129, 139. Spirogyra : cold plasmolysis of, 75, 141 ; permeability of, 62, 65. 68. Stahl, 140. Stange, 84, 130. Stentor, cold plasmolysis of, 75, 143. Stevens, 69. Stichococcus, permeability of, 70. Stigeoclonium : drying out of, 143; in osmotic solutions, 132, 138; zoospores in solutions, 139. Stomata, 93. Strasburger, 110. Stratiotes, permeability of, 62. Strongylocentutus, eggs of, 129. Sucrase, penetrating power of, 71. Sugar, cane: in plasmolysis, 61; pene- trating power of, 65. Sunflower, permeabilty of, 65. Support, mechanical, by turgidity, 87. Surrounding medium, influence of, 124. Sutherst, 85. Tamman, 36. Temperature: influence of, on absorp- tion, 95; influence of, on permeabilty, 75 ; influence of, on turgor, 77. Tension, diffusion, of solvent, 30. Theories: of matter, 3; of semipermea- bility, 82. Thunbergia, permeability of, 71. Thuya, transpiration of, 111. Tonie, 59. Tonoplast, 52, 53, 80, 82. Tradescantia, permeability of, 56, 62, 76. Transmission: of solutes, 115, 120; of water, 95. Transpiration, 93, 96. Transpiration stream, 107. Traube, 50. Tropism, reversal of, 139. True, 68, 137. turgescence, 52. Turgidity: nature of, 49, 52, 53; main- tenance of, 86, 121 ; relation of, to activ- ity, 87. Turgor, 52, 55, 74. Turnip, pressure of sap of, 85. Urea: in plasmolysis, 62; penetrating power of, 77, 79. Vacuole, 52. Vandeevelde, 131. Van Rysselberghe, 76, 134. Van't Hoff, 25, 37, 40. Vegetable marrow, pressure of sap of, 85. Vesque, 95. Vicia : nectaries of, 79 ; roots of, 65. Violet, methyl, penetrating power of, 66. voigtlander, 18. Von Mayenburg, 67, 83, 118. Walden, 36. Index 149 Walker, 40. Wall, cell, 51. 52. Water : absorption of, by plant cells, 91 ; absorption of, by muscle, 53 : in cellu- lose wall, 52 ; loss of, 95 ; transmission of, 95. Water pores, 74, 96. Westermeier, 108. Whetham, 43. Wielee, 65, 102, 108. wlesner and molisch, 116. Wilson, 98. Wladimiroff, 58. Woods, 112. wortmann, 66. Yasuda, 132. Zea, solutes of, 83. Zoospores, in osmotic solutions, 139. iluiiUHUlllflJiiliiUlfHlUiliUllliliHltlnHiflliitlil lltliilHllllllliIIH^B^^HllilHliilili Ili iiiiiiiiiiiimi^H illilil IttllllillllHUItlf III llllllllllll III I IIIIIIIH1I illlllnl II II I IIIIIIIIIIIIMlHHlllllllllllllllllllll HI