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The role of diffusion and osmotic press

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THE DECENNIAL PUBLICATIONS OF
THE UNIVERSITY OF CHICAGO

THE DECENNIAL PUBLICATIONS

ISSUED IN COMMEMORATION OF 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

EDWARD CAPPS

STAER WILLAED CUTTING ROLLIN D, SALISBURY
JAMES ROWLAND ANGELL WILLIAM I. THOMAS SHAILER MATHEWS
CARL DARLING BUCK FREDERIC IVES CARPENTER OSKAR 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

DIFFUSION AND OSMOTIC PRESSURE

THE ROLE OF DIFFUSION AND
OSMOTIC PRESSURE IN PLANTS

BY

BURTON EDWARD LIVINGSTON

a
OF THE DEPARTMENT OF BOTANY

THE DECENNIAL PUBLICATIONS
SECOND SERIES VOLUME VIII

CHICAGO
THE UNIVERSITY OF CHICAGO PRESS
1903

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BY THE UNIVERSITY OF CHICAGO

PREFACE

Wits 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 réles 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 hasadouble raison détre. 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

x PREFACE

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
awhole. 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 J acques
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.
Tax Hutt Boranicat Lasoratory,
The University of Chicago,
October 1, 1902.

TABLE OF CONTENTS

PREFACE - ix

PART I. PHYSICAL CONSIDERATIONS

INTRODUCTION -

Cuarrer I. Matter and Its States - - 3
1. Fundamental Theories of the Nature of Matter.
a) The Atomic Theory.
b) The Kinetic Theory.
u. The Three States of Matter.

Cuapter II. Diffusion and Diffusion Tension - : - 9

1. Gases.

a) Simple Gases.
b) Mixed Gases.

mu. Liquids.
a) Simple Liquids.
b) Mixed Liquids.

ur. Solids.
a) Simple Solids.
b) Diffusion of Two Solids.

Cuarrer III. Liquid Solutions - - + = 16
1. Liquids in Liquids.
ur. Gases in Liquids.
ur. Solids in Liquids.
tv. Terminology for Solutions of Differing Concentration.

Cuaprer IV. Jonization - - - 23

1. Of Gases.
u. Of Solutes in Liquid Solutions.

Cuarrer V. Osmotic Phenomena - 2 - 2
1, Osmotic Pressure of the Solute.
a) Non-electrolytes.
b) Electrolytes.
c) Colloids.
d) Osmotic Pressure in General.
xi

xii TABLE OF CONTENTS

uz. Diffusion Tension of the Solvent.
ir, Experimental Demonstration of Osmotic Pressure.

Cuarrer’ 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.
1. Calculation of Osmotic Pressure.
a) Non-electrolytes.
b) Electrolytes.

PART II. PHYSIOLOGICAL CONSIDERATIONS

InrropuctTiIon - : 2 47

Cuapter I. Turgidity - 49

1. Protoplasm and its Limiting Membranes.
11. Plasmolysis.
mu. 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.
f) Test by Metabolic Processes.
g) Outward Permeability.
h) Variations in Permeability.
tv. Action of the Protoplasmic Membrane.

a) The Filter Theory.
b) The Solution Theory.
c) The Chemical Theory.

/ vy, The Nature of the Osmotically Active Solutes.

’ yt. The Maintenance of Turgidity in Spite of Permeability to

Certain Solutes.

vu. The Relation of Turgidity to Vital Activity.
a) The Retention of Form.
b) Mechanical Support.
c) Growth.
d) Curvature.
e) Work.

vii. Summary of the Chapter.

TABLE OF CONTENTS xiii
Cuarter II. Absorption and Transmission of Water 91
1. Absorption of Water.
11, Transmission of Water.
a) Water Loss.
(1) Evaporation.
(2) Water Pores and Nectaries.
(8) Exudation.
(4) Summary of Water Loss.
b) Upward Movement of Water in Trees and Other Tall
Plants.
zu. Summary of the Chapter.
Cuarter III. Absorption and Transmission of Solutes - 115
1. Absorption of Gases.
i. Absorption of Dissolved Solids and Liquids.
uu. Transmission of Solutes.
Carter IV. Influence of the Osmotic Pressure of the Sur-
rounding Medium upon Organisms - - 124
1. Introductory.
u. Presentation of Material.
a) Influence upon Growth and Form.
b) Influence upon Reproduction.
ce) 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.
um. 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’ 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

1A 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 itsaim. 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 ‘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-WALKER, Outlines of General Chemistry [London, 1895},
pp. 5,7.) A principle, on the other hand, does attempt to state the truth; it isa
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

2 INTRODUOTION

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.

OsrwaLp, W. Lehrbuch der allgemeinen Chemie, comprising :
Verwandschaftslehre, 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.

Watser, J. Introduction to Physical Chemistry. London, 1899.

Reycuter, A. Outlines of Physical Chemistry. Translated by J.
McCrae. London, 1899.

Dastre, M. A. “Osmose,” in Traité de physique biologique,
publié sous la direction de MM. D’Arsonval, Gariel, Chauveau
et Marey. TomeI. Paris, 1901.

Couen, Ernst. Vortrige fiir Aerzte wiber physikalische Chemie.
Leipzig, 1901. ;

Jones, H.C. The Elements of Physical Chemistry. New York,
1902.

Wuersamu, W.C.D. Solution and Electrolysis. Cambridge, 1895.

Koutravscon, F., anp Houzorn, L. Das Leitvermdgen der Elektro-
lyte. Leipzig, 1898.

Travuse, J. Physico-Chemical Methods. Translated by W. L.
Hardin. Philadelphia, 1898.

Buirz, Henry. Practical Methods for Determining Molecular
Weights. Translated by Jones and King. Easton, Pa., 1899.

Hamevurcer, 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 (7. 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

4 DIFFUSION AND OsSmoTIO 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 may 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

MatTTER AND Its STATES 5

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,
2. 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 moving
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 willthus
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=4M V?), it is seen that the average kinetic

6 DIFFUSION AND OsmotTio Pressure

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 q

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 (7. e., 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, on 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 OgmoTio 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
toanend. 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. e., its tem-
perature). Thus, for any temperature and amount of gas,
the distension of such a chamber will cease when the inward
pressure, due to the resilience of the walls and to the pres-
sure of the surrounding atmosphere (unless the chamber be
in a vacuum), becomes equal to the outward pressure, due

to the gas.
8 9

10 DIFFUSION AND OSMOTIC PRESSURE

For a given amount of gas the pressure is constant at a
constant temperature. But change in temperature means
simply change in the kinetic energy of the particles. There-
fore a rise in temperature must cause a corresponding increase
in gas pressure, and a fall a corresponding decrease. Keep-
ing the pressure constant, a rise in temperature produces an
increase in volume, and vice versa. It has been found
experimentally that the volume of a given mass of gas
under constant external pressure varies with its absolute
temperature (273°-+ the given temperature Centigrade).
This is the principle of Gay-Lussac, sometimes called
that of Charles.

But if, in the elastic chamber mentioned above, the tem-
perature be kept constant and the resiliency of the walls
be increased, thus increasing the external pressure on the
gas, the volume will be decreased. As this occurs, how-
ever, the gas will increase in density, and continually more
particles will strike unit area of the wall in unit time. Thus
the internal pressure upon the bag will also be increased,
until at length another state of equilibrium will be reached,
wherein the external and internal pressures are again equal.
But during the readjustment the volume of the gas has
decreased. As long as the temperature (7. e., the kinetic
energy of the particles) is constant, an increase in external
pressure produces a decrease in volume, and a decrease in
external pressure an increase in volume. Experimentally
it is demonstrated that the volume of a given mass of gas
at a constant temperature varies inversely as the external
pressure to which it is subjected. This is the principle of
Boyle.

Still another principle has been discovered for gases.
If the volume and temperature both remain constant, and
if the number of particles is increased (i. ¢., the concen-
tration), the pressure will be correspondingly increased. It

DIFFUSION AND DIFFUSION TENSION 11

is obvious from the theoretical consideration already pre-
sented that this must be true. In this case the kinetic
energy of the particles is not altered, but their number
has been increased, hence the increase in pressure. Also,
for a given concentration and temperature, all gases exhibit
the same pressure. This is called the principle of Avo-
gadro. It is usually stated ina somewhat different way,
namely: Equal volumes of gases, at equal temperature
and pressure, contain the same number of particles. This
principle holds rigorously true only for gases whose con-
centration is rather low. As a gas approaches the liquid
state, the principle of Avogadro, and also those of Boyle
and Gay-Lussac, have to be modified. They apply only to
a theoretically perfect gas.

b) Mixed gases.—In a mixture of several gases each
gas practically exerts its own pressure independently of the
others. Thus the total pressure of a mixture of gases in a
chamber is the sum of the pressures which would be exhib-
ited were the gases separated and each put into a chamber
of the same size as the first one, the temperature of course
remaining constant. The pressures which would be thus
shown are called partial pressures, and the above fact may
be stated more directly, by use of this term, as follows: The
total pressure of a gas mixture is practically the sum of the
partial pressures of its component gases. As a gas nears
the liquid state, this principle also breaks down in part, it too
applying rigorously only to perfect gases.

Also, if two gases be brought together so as to form two
horizontal strata in a chamber, diffusion of each gas will
take place just as completely as if the other gas were not
present. Particles of the lower gas will pass up from the
lower stratum until that gas is equally distributed through-
out the chamber. Downward diffusion of the upper gas will
occur simultaneously, and the result of the two processes

12 DIFFUSION AND OSMOTIO PRESSURE

will be a uniform mixture of the two gases. If this pro-
cess of diffusion is obstructed by a wall placed between
them, the pressures of both gases will of course be exhibited
independently upon the opposite sides of this wall.

II. LIQUIDS

a) Simple liquids.—When a liquid is heated, the kinetic
energy of its particles is increased, until at length the cohe-
sive force which held them together is overcome; then they
fly off from the main mass and tend ever to increase their
distance apart. This is the process of vaporization by heat.
As long as the temperature remains high enough, such mat-
ter will remain in the gaseous state. Also many substances
which are usually liquids can be vaporized at ordinary tem-
peratures. Water, alcohol, and ether are examples of this.
This process, however, is a slow one. It is explained
theoretically in this way: Although the majority of the
liquid particles cannot break away from the main mass at
ordinary temperatures, yet some of them, which reach the
surface with greater kinetic energy than the others, do suc-
ceed in breaking through the firmer surface layer (see p.7),
and so escape as gas particles. If thechamberabove the liquid
be a closed one, so that the evaporated liquid cannot escape,
evaporation soon apparently ceases. If some of the liquid
particles come against the surface layer with sufficient force to
pass through it, it is reasonable to suppose that, after escap-
ing into the chamber above, some of them may again pass
through this film in the opposite direction, and so re-enter
the liquid. Here they come under the influence of the force
of cohesion, which holds the liquid particles together, and,
since they are unable to break forth at once, they remain in
the liquid state. The number which thus re-enter the liquid
will gradually increase as the pressure of the vapor (i. e.,
the number of vapor particles, for the temperature is sup-

DIFFUSION AND DIFFUSION TENSION 13

posedly constant) increases. Thus, an equilibrium will be
established sooner or later, wherein the number of particles
escaping from the liquid in unit time will be just equaled by
the number re-entering it. That is, evaporation is just
equaled by the opposite process, condensation. This is the
condition when evaporation apparently ceases. The gas
pressure with which the liquid particles escape is termed
vapor tension. And when evaporation has apparently ceased,
the gas pressure of the vapor in the space above the liquid
is equal to the vapor tension which the particles exhibit in
leaving the liquid surface. We have thus a means for
measuring the vapor tension of any liquid.

If the temperature rises, the vapor tension rises corre-
spondingly, following the principles of gases. If the
external pressure upon the supernatant mass of vapor be
increased, its gas pressure becomes greater than the vapor
tension of the liquid, and condensation surpasses evaporation,
thus decreasing the number of vapor particles—and hence
the pressure due to them— until equality of tension and
pressure is restored. If two such chambers in which the
supernatant vapor is at different pressures be connected
above the level of the liquid, the substance will distil over
and condense in the chamber which has the lower pressure.
This will continue until the two pressures have been equal-
ized by the resulting change in the relative volumes occupied
respectively by the two masses of liquid, and by the diffusion
of the vapor particles themselves.

b) Mixed liquids—If two different, equally miscible
liquids are brought into contact with each other so as to
form two horizontal strata, diffusion will take place in both
directions, just as in the corresponding case with gases, but
much more slowly on account of the friction and interfer-
ence of the particles: and there will result a uniform mix-
ture in which both kinds of particles are equally distributed

14 DIFFUSION AND OSMOTIO PRESSURE

throughout. This tendency of one liquid to diffuse into
another may be termed diffusion tension; it corresponds to
the vapor tension exhibited by an evaporating gas. Diffu-
sion in liquids is sometimes distinguished from that in gases
by the use of the term ‘“‘hydro-diffusion” to denote the former.
They are, however, essentially the same thing. In the case
just cited, each liquid develops a diffusion tension independ-
ently of the other. Of course, above such a mixture of
liquids there will lie (if the chamber allow it) a stratum
of gas mixture in which each of the two gases has its
own vapor tension, just as though the other gas were not
present.
III, SOLIDS

a) Simple solids.— Continuously raising the temperature
of a solid may result in liquefying it and then in vaporizing
the liquid thus formed; or vaporization may take place
immediately, without the intervention of the liquid phase
at all. In either case the particles break away from the
solid mass and become more widely separated. With the
process of liquefaction, however, we have no concern.

When vaporization of a solid takes place directly, it is
called sublimation. Gum camphor, naphthalene, and ice
below the temperature of melting exhibit this phenomenon.
If the vapor particles are prevented from escaping, an equi-
librium between vapor and solid is ultimately reached, at
which sublimation apparently ceases. At such a point sub-
limation is just equaled by condensation. The whole
process is analogous to that of evaporation from free liquid
surfaces. The pressure of the vapor surrounding a solid
mass of the same substance, when equilibrium is reached,
may be termed, as in liquids, vapor tension.

b) Diffusion of two solids.—If two solid masses of differ-
ent substances are brought together with their adjacent
faces in close contact, there can be demonstrated, in some

DIFFUSION AND DiFrFrusion TENSION 15

cases at least,’ a diffusion of the substances into each other.
The process goes on with extreme slowness, however, and
the details need not be stated here.

1W. Sprine, ‘‘Ueber die chemische Einwirkung der Korper in festem Zustande,”’
Zeitschr. f. physik. Chem., Vol. II (1888), pp. 536-8; Ipem, ‘‘Ueber das Vorkommen
gewisser far den Flissigkeits- oder Gaszustand charakteristischen Eigenschaften
bei festen Metallen,” ibid., Vol. XV (1894), pp. 65-78; W. RoBERTS-AUSTEN, ‘“‘On
the Diffusion of Gold in Solid Lead at the Ordinary Temperature,” Proceed. Roy.
Soc. London, Vol. LXVII (1901), pp. 101-5.

CHAPTER III
LIQUID SOLUTIONS

‘“‘ SoLUTIONS are homogeneous mixtures— mixtures which
allow no separation of their components by mechanical
means. The ability of gases to form such mixtures is un-
‘limited, that of liquids is limited.”* Solid solutions also
exist, but have not yet been shown to play any part in
physiology; therefore they need not be considered here.
Gas mixtures have already been discussed. There remains,
then, only the subject of liquid solutions —a very important
subject in the study of physiology.

I. SOLUTIONS OF LIQUIDS IN LIQUIDS

Not nearly all liquids are readily miscible to form solu-
tions. Many are nearly — perhaps quite — insoluble in one
another. Again, many liquids are mutually soluble in all
proportions (e. g., water and alcohol); others are so only
within certain limits.

When a mixture of two liquids is considered as a solu-
tion, the liquid which preponderates is called the solvent
and the other the solute. If such a solution were brought
into contact with a mass of the pure solvent, diffusion of the
solute would take place into the pure solvent until the solute
were uniformly distributed throughout both layers. At the
same time the pure solvent would diffuse into the solution.
Of course, the interchange of particles between two such
layers would not cease when uniformity of constitution had
been attained throughout; it would still go on, but would
cease to be apparent, having become simply the continuous
motion of the particles composing the uniform mixture. At

10sTWALD-WALKER, Outlines of General Chemistry (London, 1895), p. 117.
16

Liquip SoLutions 17

the beginning of such a process of mixing, however, a defi-
nite diffusion tension exists and can be demonstrated — a dif-
fusion tension produced on the one hand by the solute, and
on the other by the solvent. These diffusion tensions are
identical in their nature with those spoken of in the last
chapter. They increase in amount with rise in temperature,
and, in case there are several solutes, each one has its own
diffusion tension. These facts are found to be fundamental
in the consideration of osmotic pressure.

Above any liquid mixture contained in a closed jar which
it does not fill, there will be a gas mixture of the vapors
of the solvent and of the several solutes. Each body will
have its own vapor pressure, and the total pressure of the
gas mixture will be the sum of its partial pressures.

II. SOLUTIONS OF GASES IN LIQUIDS

Gas solutes behave in the same manner as that just de-
scribed for liquid solutes. The amount of gas going into
solution, when a mass of it is brought into contact with a
mass of liquid solvent, increases with the temperature and
pressure. Diffusion pressures of solvent and solute are
developed here also, and are constant for a given tempera-
ture; they also vary with the absolute temperature. There
may be several gaseous solutes in the same solution, and in
this case each develops its own diffusion tension in the sol-
vent. Above such a solution there will be a gas mixture of
the vapor of the solvent and of the several solutes. Inter-
change of particles will go on continually between the gas
solution above and the liquid solution below, but will not
be apparent for reasons similar to those expressed above for
liquid solutes. Also, if a solution containing a gas solute
be brought into contact with a mass of the pure solvent, dif-
fusion will take place of both solvent and solute, each devel-
oping its own diffusion tension in its own direction, just as

18 DIFFUSION AND OsmoTIO PRESSURE

in the corresponding case with a liquid solute. Equilibrium
and an apparent stoppage of diffusion will be brought about
when diffusion is equal in both directions.

III. SOLUTIONS OF SOLIDS IN LIQUIDS

If a crystal of sugar or salt be put into the water, it dis-
solves. This process of dissolving consists in the flying off
of particles into the water, just as the process of vaporiza-
tion of a mass of naphthalene consists in the flying off of
particles into the air. After the particles of the dissolved
substance (solute) are once free from the solid mass, they
behave in an entirely different manner from that which
characterized them before. While they were in the crystal
they clung together by cohesion. Now they tend to sepa-
rate as much as possible within the limits of the solvent.’
They may or may not pass the surface of the solvent and
enter the air as a gas, but within the solvent they continue
to diffuse until they are uniformly distributed. Diffusion
of the solute in its solvent takes place much more slowly
than does gas diffusion, but in the end it is just as com-
plete. Thus it is evident that within the volume occu-

17, Granam, ‘Ueber die Diffusion von Flissigkeiten,” Liebigs Ann., Vol.
LXXVII (1851), pp. 56-89 and 129-60; see also zbid., Vol. LXXX (1851), pp. 197-201,
The following references may serve to put the reader into contact with the
literature of diffusion: T. GRAHAM, ‘Supplementary Observations on the Diffusion
of Liquids,” Phil. Trans. Roy. Soc. London, 1850, pp. 805-36; A. Fick, ‘* Ueber Dif-
fusion,’’ Pogg. Ann., Vol. XCIV (1855), pp. 59-86; T. Granam, *‘ Anwendung der Dif-
fusion der Fltssigkeiten zur Analyse,” Liebigs Ann., Vol. CXXI (1862), pp. 1-77;
F, VoratLAnpDEr, ‘ Ueber die Diffusion in Agargallerte,” Zeitschr. f. physik. Chem.,
Vol. III (1884), pp. 316-35; J. D. R. Scoerrmr, ‘‘ Untersuchungen fiber die Diffusion
einiger organischen und anorganischen Verbindungen,” I, Ber, d. deutsch. chem.
Gesellsch., Vol. XV (1882), pp. 788-801 ; II, ibid., Vol. XVI (1883), pp. 1903-17; W. NEENST,
“Zur Kinetik der in Losung befindlichen Korper,” Zeitschr. f. physik. Chem., Vol. II
(1888), pp. 613-37; J. D. R. Scuerrer, ‘‘ Untersuchungen fiber die Diffusion w&sse-
riger Lésungen,” tbid., pp. 390-404; S. ARRHENIUS, ‘‘ Untersuchungen aber Diffusion
von in Wasser gelosten Stoffen,” ibid., Vol. X (1892), pp. 51-95; L, LIEBERMANN UND
S. Buraarszxy, “ Beitrage zur Theorie der wisserigen Losungen von Salzgemi-
schen,” ibid., Vol. XII (1893), pp. 188-95; A. NAccanrt, ‘Sulla pressione osmotica,”
Atti della Reale Accad. dei Lincei, Ser. 5, Rendiconti, Classe di Scienze fisiche,
matemat. e naturali, Vol. II (1893), 1 Semestre, pp. 238-9, and 2 Semestre, pp. 136-8;
R. Hosen, “ Ueber ConcentrationsAnderungen bei der Diffusion zweier geldster
Stoffe gegen einander,” Pflugers Archiv, Vol, LXXIV (1899), pp. 225-45.

Liquip SOLUTIONS 19

pied by the solution, dissolved particles exhibit at least one
of the fundamental properties of gas particles, namely, that
of indefinite diffusion. It will be gathered from what was
said under the preceding headings that the same is true for
liquid and gas solutes.

As in gases and in solutions with liquid and gas solutes,
this tendency of the solute to diffuse (diffusion tension) may
be measured. It is found that, for the same temperature
and volume, the same number of particles of different solutes
gives always the same diffusion tension. Thus the solute in
such a solution exhibits another principle of gases, namely,
that of Avogadro. This, too, is true for liquid and gas
solutes. The principle does not hold rigorously for very
concentrated solutions. There is developed here also a dif-
fusion tension on the part of the solvent, which varies with
temperature just as does that of the solute.

If two solutions containing different concentrations of the
same solute in the same solvent are brought into direct con-
tact, it is found that diffusion of solvent and solute will at
length equalize the concentrations of the two solutions, so
that the solute particles will at last be equally distributed
throughout the combined volume. Therefore diffusion of
solute particles must be more rapid from the stronger to the
weaker of the two solutions than in the opposite direction.
That is, the diffusion tension of the solute is greater from a
higher concentration to a lower than from a lower to a
higher. But the diffusion tension of the solvent is greater
in the direction from the lower concentration to the higher.
This is also true in the case of gas and liquid solutes. When
reference is made to the “concentration” of a solution, the
concentration of the solute is always meant.

20 DIFFUSION AND OSMOTIO PRESSURE

IV. TERMINOLOGY FOR SOLUTIONS OF DIFFERING
CONCENTRATION

To designate different concentrations of solutions, the
most common method among physiological writers has been,
until quite recently, that of percentage. An example of this
method will explain its use. A solution is said to be a 5 per
cent. solution of a certain solute in a certain solvent when it
is composed of five parts by weight of solute to ninety-five
parts by weight of solvent. But solutions of different solutes
in the same solvent depend for their physical properties upon
the relative number of solute particles which they contain
per unit volume. A glance at a table of atomic weights will
make it clear that any method by weights which has as its
basis the percentage system cannot readily be adapted to a
discussion of the relative number of molecules contained in
equal volumes of solutions of different solutes. Atomic
weights, and therefore molecular weights, cannot readily be
compared in terms of percentage. As long as physiologists
persist in using this antiquated method in the preparation of
their solutions, so long will they fail to arrive at any far-
reaching principles concerning the chemical and physical
nature of the substances used.

A more scientific method is that based on the relative
number of particles of solute in unit volume of solution.
We cannot, of course, actually count the molecules of any
substance, but from a knowledge of the relative weights of
the molecules of different bodies it is easily possible to get
several masses of different substances, each of which will
contain approximately the same number of molecules. The
weights of such masses must be to each other as the molecu-
lar weights of the respective substances. For instance, 342
grams of cane sugar (mol. wt. 342) must contain the same
number of molecules as 180 grams of glucose (mol. wt. 180),
for the molecular weights give the relative weights of the

Liquip SoLuTiIons 21

two different molecules. Now, if these two masses are placed
in equal volumes of solution, both solutes ought to show the
same diffusion tension. This, indeed, is found to be true,
and the same principle has been shown to be true, as far
as experiment has gone, for solutions of all substances
which do not conduct electricity (non-electrolytes). Solu-
tions of non-electrolytes which contain the same number
of molecules per unit volume have the same diffusion tension
(at the same temperature) and are physically similar. Solu-
tions which conduct electricity exhibit this principle only in
a general way. Their departures from it and the reasons
therefor will be discussed in the next chapter.

The number of grams of a substance represented by its
molecular weight is called a gram-molecule. Gram-molecules
of all substances contain, then, the same number of molecules.
Ifa gram-molecule of some substance be put into solution, and
then this be diluted to one liter, there results a solution which
can reasonably be used asa standard. Such asolution is often
termed a molecular solution. Thus, a molecular solution of
potassium nitrate (mol. wt. 101) is 101 grams of the salt in
a liter of solution. It is as though the substance had been
vaporized and the resulting gas occupied a volume of one
liter.

But the analytical chemist has found it convenient to use
another solution as a standard. He dissolves, to form a liter
of solution, as many grams of the substance in question as
will react chemically with a gram-molecule of a monovalent
compound, This amount of substance is termed a gram-
equivalent. A gram-equivalent of sulphuric acid (H,SO,)
will just neutralize a gram-equivalent of potassium hydroxide
(KOH) or will just decompose a gram-equivalent of sodium
chlorid (NaCl); but it takes two gram-equivalents of either
of the last-mentioned compounds to react completely with a
gram-molecule of sulphuric acid. It follows that “gram-

22, DIFFUSION AND OgsmoTio PRESSURE

equivalent” and “gram-molecule” are synonymous terms
in the case of monovalent compounds, and that a gram-
equivalent of a bivalent compound is one half of its gram-
molecule, of a trivalent compound one-third, ete.

A solution made up so as to contain in one liter a single
gram-equivalent of solute is termed an equivalent normal,
or simply a normal, solution. Unfortunately there is a usage
which terms a molecular solution normal, thus giving rise
to ambiguity for all but monovalent solutes. This ambi-
guity can be avoided only by the careful definition of the
term “normal” by each author using it.’ For all neutral
organic compounds, such as the sugars, and also for monova-
lent electrolytes, a gram-equivalent is the same as a gram-
molecule, and a normal solution must be a gram-molecule in
a liter volume. Thus the sugar solutions described in a pre-
vious paragraph are both normal solutions. No ambiguity
can arise from the use of the term in reference to such com-
pounds.

Regarding acid salts (such as KHSO,, for example), there
is a difference of opinion as to what should be denoted by
gram-equivalent. Some hold (e. g., Kahlenberg) that in
the salt just mentioned gram-equivalent and gram-molecule
are identical. Thus, such a salt might be regarded as a
monobasic acid. On the other hand, Sutton, Fresenius,
Dandeno, and others regard a gram-equivalent of KHSO,
as one-half a gram-molecule. Thus, an equivalent solution
of this salt would contain only one-half gram of hydrogen;
the salt is to be regarded as a monobasic acid, one-half of

whose hydrogen has been replaced. It seems that the latter
is the more truly scientific position.

1For an account of confusion (partly imagined) which has arisen from a lack of
attention to such definition of these terms, see J. B. DANDENO, “ The Application of
Normal Solutions to Biological Problems,” Bot. Gaz., Vol. XXXII (1901), pp. 229-37,

Also see “Open Letters,” one from Louris KAHLENBERG, and an answer from DAN-
DENO, ibid., p. 437.

CHAPTER IV
IONIZATION
I. IONIZATION OF GASES

From the hypothesis of Avogadro it would be expected
that, if a gram-molecule of ammonia and a gram-molecule
of ammonium-chlorid vapor were put into chambers of the
same size, the pressures exhibited at the same temperature by
the two gases would be equal. The latter substance, how-
ever, showsa far greater pressure. Now, since the kinetic
theory supposes that gas pressure is due to the kinetic
energy of its particles, and that the kinetic energy of any
particle is dependent only upon the temperature to which it is
subjected, we must either reject the theory when we come
upon such a case as that just cited, or we must conclude that
there are, in the mass of ammonium chlorid, a greater number
of particles than in that of ammonia. Several lines of
experiment and of reasoning seem to point to this as the
true condition of affairs. The number of molecules is the
same in both masses of gas, but in the ammonium chlorid
it is supposed that many of the molecules split apart into
ammonia and hydrochloric-acid ions (NH; and HCl), and
that, for producing pressure, the ions are as active as would
be the same number of molecules. In this way, if all the
molecules were dissociated, the pressure should be twice
that required by the theory. The ammonium molecule
seems not to dissociate at ordinary temperatures. Many
gases exhibit the phenomenon just described; usually ioniza-
tion is not nearly complete and the pressure is simply raised
above its theoretical value. As the gas becomes more con-
centrated, dissociation becomes less and less complete.

23

24 DIFFUSION AND OsmoTio PRESSURE

The theory just given is called the theory of dissociation.
There are other theories to account for these phenomena,
but this has the widest acceptance at the present time and
serves the purpose of the physiologist better than any other
yet advanced.

II, IONIZATION OF SOLUTES IN LIQUID SOLUTIONS

It has been found that a phenomenon similar to the one
just described occurs in dilute aqueous solutions of electro-
lytes. These solutions uniformly give a higher diffusion
tension than the one required by the theory. The explana-
tion is the same as that given above;’ if ammonium chlorid,
forinstance, be put into aqueous solution, it has been shown
that some of the molecules ionize, thus increasing the diffusion
tension of thesolute. The more dilute the solution, the greater
is the proportion of molecules dissociated, and at infinite dilu-
tion a limit would be reached at which complete dissociation
would occur. This theory is named for its originator, Arrhe-
nius. In very weak solutions it is found that practically all the
molecules are ionized. If several electrolytes are contained
in the same solution, ionization occurs in all of them, but
not always to the same degree as if there were but one
solute; the presence of other molecules and ions seems to
influence the amount of dissociation. This subject has not
yet been sufficiently investigated to permit the formulation
of a general principle. Where a solution contains several
different kinds of ions, it is found that the velocity of diffu-
sion of some ions is much greater than that of others.

Only those substances which conduct electricity when in
solution are dissociated. The whole theory of primary bat-
teries and of electric conduction by liquids depends upon
this principle. Hnough has been said, however, to prepare
the way for what is to follow.

18, AnRHENIDS, ‘‘ Ueber die Dissociation der in Wasser geldsten Stoffe,’”’ Zeit-
sehr. f. physik, Chem., Vol. I (1887), pp. 681-48.

CHAPTER V
OSMOTIC PHENOMENA
I, OSMOTIO PRESSURE OF THE SOLUTE

a) Non-electrolytes.—If a parchment-paper bag be filled
with aqueous sugar solution and, after the opening has
been sealed, the bag be submerged in water, the walls will
soon be distended by an internal pressure. If the original
solutionis strong enough, the walls will be stretched to their
limit of extensibility, and at last ruptured. If, during the
distension of the bag, the water around it be tested, it will
be found to be nearly or quite free from sugar; after the
bag is ruptured, however, we find the sugar diffusing rapidly
to the limits of the water. Therefore parchment paper
hinders greatly the diffusion of the sugar, 7. e., itis only
slightly permeable to dissolved sugar molecules. This fact
forms the basis for an explanation of the phenomenon of
distension and rupture just mentioned. In tending to dif-
fuse indefinitely, the dissolved molecules (there is no disso-
ciation in the case of sugar and other non-electrolytes) bom-
bard the walls of any chamber in which they may be
inclosed.’ The fact that they possess this property of in-
definite diffusion only when within the limits of the solvent
makes it necessary that such a chamber be surrounded by the
pure solvent, and that the solvent permeate its walls. The
pressure thus produced upon the walls of the bag is really
the diffusion tension of the solute. If diffusion could take
place without obstruction, this pressure would not be made
apparent, but would exist none the less. The water itself
exerts but little pressure upon the walls of the bag, since

1J, H. van’t Horr, “ Die Rolle des osmotischen Druckes in der Analogie zwischen
Losungen und Gasen,” Zeitschr. f. physik. Chem., Vol. I (1887), pp. 481-508,

25

26 DIFFUSION AND OSMOTIO PRESSURE

these are almost freely permeable to it, but a diffusion ten-
sion of water exists and can be demonstrated in other ways.
The internal pressure of dissolved sugar molecules forces the
walls of the bag outward through the water, just as a cloth
bag may be distended under water by the expansion of wire
springs inside of it. Of course, in such a process of dis-
tension water enters the bag from without, the bag being
permeable to that substance. If in either case the bag is
not strong enough to bear the pressure, it will burst when its
limit of extensibility is reached. If the parchment bag is
strong enough to withstand the pressure developed within,
an equilibrium will be established, just as in the case of the
expanding gas described in chap. ii. In this condition
of equilibrium the inward pressure (due to the resilience of
the walls of the bag) is just equaled by the outward pres-
sure (due to the bombardment of the walls by the solute
particles).

Such a membrane as parchment paper, which allows the
solvent to pass, but greatly retards or prevents the passage
of solute particles, is said to be semi-permeable; and the
pressure which such a membrane makes evident under the
conditions just described is the osmotic pressure of the
solute. This is merely the diffusion tension of the solute,
made evident by the opposition of the membrane. All pos-
sible gradations exist between membranes which are freely
permeable to solutes and those which retard them or are
impermeable to them. It needs to be noted here, however,
that a theoretically perfect semi-permeable membrane has
not been found. The best ones which have been tested
allow some passage of solute particles. Many animal mem-
branes are nearly semi-permeable in certain solutions, pig’s
bladder being often used. A membrane of copper ferro-
cyanid is almost perfectly semi-permeable in aqueous sugar
solution, but it is permeable to certain salts, e. g., potassium

Osmorio PHENOMENA 27

nitrate. The term “semi-permeable,” therefore, must be
used with reference to a particular solute and its solvent.

Such membranes as that of copper ferrocyanid are termed
“precipitation membranes ;” they are formed by precipitation
from two solutions which react chemically. If a solution
of potassium ferrocyanid and one of copper sulphate be
brought together within the walls of a porous clay cup, such
a membrane (composed of copper ferrocyanid, Cu,Fe
(CN),) will be precipitated within the clay walls. The mem-
brane is then supported by the clay, and the whole cup may
be used for osmotic determinations, Another precipitation
membrane is that formed by gelatin and tannic acid.

b) Electrolytes.— Osmotic pressure is found to be abnor-
mally high in solutions of electrolytes. This is one of the
facts from which the conclusion was drawn that in these
solutions the diffusion tension of the solute is abnormally
great, and hence that dissociation occurs. When the amount
of ionization which takes place in any solution is taken into
account, it is found that these solutions are only apparent
exceptions to the general rule of osmotic pressure. In this
case we can no longer say that the pressure is due to the
bombardment of the membrane by the solute molecules, but
by the solute particles, meaning thereby both molecules and
ions. Solutions having the same number of solute particles
per unit volume have, at the same temperature, the same
osmotic pressure. As far as it has been carefully tested, this
principle has been found to hold for all somewhat dilute
solutions.

c) Colloids.—According to their behavior when in solu-
tion, substances have been classified as crystalloids and col-
loids. Crystalloids produce an osmotic pressure which is
practically equal quantitatively to the gas pressure which
would be produced by the same number of gas particles as
there are of solute particles, occupying the same volume as

28 DIFFUSION AND OSMOTIO PRESSURE

the solution and possessing the same temperature. There
is, however, a group of substances soluble in water, which
do not produce osmotic pressure at all, or produce it in a
very slight degree. These are the so-called colloids, such as
gelatin, gum, silicic acid, aluminium hydroxid, etc. These
substances have very large molecules which diffuse with
exceeding slowness and seem to encounter great resistance
in passing through water. Colloids in their relation to crys-
talloids bid fair to become very important in the advance
of physiological knowledge.

d) Osmotic pressure in general.— Most solutes which
produce osmotic pressure when in solution are either solids
or liquids at ordinary temperatures, when not in solution.
But a gas in solution may also produce osmotic pressure if
a suitable membrane is employed.

Osmotic pressure being, in its origin, perfectly comparable
to gas pressure, the various principles established for gas
pressure have been found to hold for osmotic pressure. The
principles of Boyle, of Gay-Lussac, and of Avogadro, devel-
oped for gases, have all been extended so as to include sub-
stances in solution. Here is convincing evidence that a
solute, as long as it is in solution, is essentially a gas occu-
pying the volume of the solution. The solvent merely pro-
vides conditions under which the pseudo-vaporization which
we call solution can take place. Osmotic pressure is inde-
pendent of the solvent and is dependent only upon the num-
ber of particles of solute (7. e., its concentration) and upon
their kinetic energy (7. ¢., their temperature). The nature
of the solute is immaterial, the number of particles (mole-
cules or ions) per unit volume being, as far as is known, the
only essential factor.

Where several substances that do not react chemically are
in very dilute solution, the osmotic pressure of the mixture
is the sum of the pressures which would be exhibited were

Osmotic PHENOMENA 29

each of the different solutes dissolved separately to form
a volume of solution equal to the original volume. As in
gases, these latter pressures are termed partial pressures.
This principle may be formulated again as follows: The
total osmotic pressure of a dilute solution of mixed solutes is
the sum of the partial osmotic pressures of the component
solutes. As was seen in the last chapter, however, for more
concentrated solutions this principle does not seem to hold.
But more work needs to be done here before we may be
positive.

The principles of Boyle and Gay-Lussac would hold per-
fectly true only for an ideal gas, 7. e., one without any fric-
tion between its particles. Such a gas does not exist, although
hydrogen approaches this condition very nearly. These
principles, however, hold very nearly true for all ordinary
gases as long as they are not nearing the point of condensa-
tion into a liquid. But, as has been stated, in the vicinity
of this critical point, whether it be approached because of
increase in pressure or fall in temperatnre, they do not hold
true. In the study of osmotic pressure it is found that a
similar breaking down of the same principles occurs when
the solute becomes too concentrated. At high concentration
the principles of gas pressure no more apply to osmotic pres-
sure than they do to gas pressure itself.

Just what is the action of the membrane in osmotic phe-
nomena is not known. In many respects it acts like a sieve
or filter, to prevent the passage of large particles, but allow
smaller ones to go through unhindered. In some cases,
however, the chemical nature of the membrane seems to
come into play; it seems to react chemically with the solute
particles, taking them up on one side and giving them off
on the other. But for a discussion of the principles and
general phenomena of osmotic pressure, a knowledge of the
exact method by which the membrane acts seems not to be

30 DIFFUSION AND OSMOTIO PRESSURE

essential. A brief discussion of the different theories which
have been proposed to account for the action of the membrane
will be presented in connection with the treatment of the
protoplasmic membranes of vegetable cells (see p. 80).

II, DIFFUSION TENSION OF THE SOLVENT

The diffusion tension of the solvent has been mentioned
several times during the discussion of solutions, but it is
thought well to bring together in this place the ideas con-
cerning it.

If the vapor tension of pure water be determined and
then that of an aqueous salt or sugar solution, it will be
found that the latter is invariably less than the former, and
this in proportion to the concentration of the solution.
Therefore it must be that particles of the solute hinder the
escape of the solvent molecules. The moving particles of
solute perhaps bring this about merely by moving into the
path of solvent molecules which would otherwise leave the
liquid. Itis probable that there exists also an attraction
between the particles of solute and solvent.

If two solutions whose concentrations are different be
brought into direct contact, as by placing a weak sugar
solution over a stronger one, phenomena similar to those
just discussed may be detected. Water molecules pass
through the common surface in both directions. They are
not vaporized, for they remain in the liquid state, but they
diffuse as liquid molecules. Under such conditions the dif-
fusion of the solvent is always found to be greater from the
weaker to the stronger solution than in the opposite direc-
tion; it will be remembered that the most rapid diffusion
of the solute takes place from stronger to weaker solution.
There is, therefore, a difference in the energy of diffusion
(diffusion tension) of the solvent in the two solutions. This
corresponds to their difference in vapor tension just de-

OsmoTio PHENOMENA 31

scribed. The diffusion tension of the solvent is greatest in
the pure solvent and decreases as the concentration of the
solution increases.

If there were available a membrane permeable to the
solute, but impermeable to the solvent, this diffusion tension
of the solvent might be directly measured. It would be an
osmotic pressure similar to that occasioned by the solute
molecules, but of much greater magnitude and in the
opposite direction. Though there is no membrane which
will make this pressure evident, most of the phenomena of
osmotic pressure show that it exists. No membrane can
allow the solvent particles to pass absolutely without fric-
tion. Thus the question arises: Why is not this pressure of
the solvent made evident to a degree equal to the amount of
force needed to overcome this friction? The answer is,
obviously, that the pressure produced by the solvent on one
side of the membrane is practically equaled by that on the
other side. In solutions where the principles of osmosis
hold true, the dilution of the solvent, due to the presence of
the solute, is negligible.

The following explanation of osmotic pressure has been
given by various authors. The quotation is from Davenport:'
“‘Upon the side containing the greater number of molecules
of salt [solute] fewer water [solvent] molecules will in a
given time strike the membrane than upon the other side;
and since the number passing through is proportional to the
number striking, relatively fewer molecules of water will
consequently pass out, and so there will be a resultant flow
of water to that side; and if the mass of water is confined,
it will exert great pressure.” This explanation is untenable
for several reasons. Not nearly all solutions occupy more
space than the original mass of pure solvent from which they
were prepared. If to exactly a liter of water be added a

1C. B. DavenroRt, Experimental Morphology, Vol. I (New York, 1897), p. 71.

32 DIFFUSION AND OSMOTIO PRESSURE

given quantity of some solute, it cannot be told a priori
whether the resulting solution will occupy the same volume
as the original solvent, or a greater or less volume. Into
this matter it is unnecessary to go farther than to add that
osmotic pressure may be demonstrated as readily in solu-
tions occupying less volume than the original solvent as in
those occupying more. It is obvious that in the former
case there must be a greater number of solvent particles
per unit volume than in the pure solvent. Hence, if the
above explanation can be retained, there should be no
osmotic pressure developed in such a solution; indeed, it
should appear on the side of the pure solvent.

But even if it were possible that the entrance of solvent
particles into the solution was due to such a difference in
concentration of the solvent on opposite sides of the mem-
brane, the explanation just quoted would fail. It is incon-
ceivable that the osmotic membrane should be more
permeable to solvent particles moving in one direction than
to those moving in the other, and it thus becomes impos-
sible to suppose that solvent particles which can pass the
membrane in one direction “will exert great pressure” upon
it in the other. Great hydrostatic pressure cannot be main-
tained in a sieve, nor can osmotic pressure be maintained
upon a membrane by a solvent to which it is permeable.
Any difference between the solvent pressures on the two sides
of an osmotic membrane must be very rapidly destroyed by
diffusion of the solvent through the membrane.

Ill. EXPERIMENTAL DEMONSTRATION OF OSMOTIO PRESSURE

If a parchment-paper bag, like the one used in the illus-
tration of osmotic pressure, were fixed in a firm cage, so that
it could not expand except on one side, and were then filled
with solution and submerged in pure solvent, bulging would
occur on the freeside. It would make no difference whether

OsmMoOTIO PHENOMENA 33

this free side were serving as an osmotic membrane or not,
for pressure produced anywhere in the bag must be trans-
mitted equally and undiminished to all parts of the surface,
in accordance with Pascal’s principle of transmission of
pressure in fluids. This transmission would be accomplished
by the fluid as a whole, solvent and solute acting together.
If, however, the portion where the bulging is supposed to
take place be permeable to the solvent, the immediate pres-
sure which affects the membrane must be due to the solute.
Thus it comes to the same end if we consider the pressure
as transmitted by the solute, acting like a gas; for increased
energy of solvent particles will be transmitted to the solute
particles with which they come in contact.

A very common mode of demonstrating the existence of
osmotic pressure is the following: A piece of animal bladder
or parchment paper is tied tightly over the expanded end of
a thistle tube, and the bulb is filled with molasses, or a strong
aqueous sugar or salt solution. Then the tube is fastened
upright, the bulb being immersed in water so that the liquids
within and without have a common level. Aftera time it is
observed that the solution has risen in the tube, often to the
height of a meter or more if the tube is sufficiently long.
The diffusion tension of the solute particles within the bulb
is of course operative in every direction, but osmotic pressure
is developed and made apparent only within the membrane.
This pressure is transmitted through the liquid to all parts
of the surface of the solution. But the only part of this
surface which is free to move, after the limit of extensibility
of the membrane is reached, is the free surface in the stem
of the tube. We have seen that the free surface of a liquid
is bounded by a peculiar layer or film. Upon this film the
transmitted osmotic pressure is effective, just as though the
film were a piston closely fitting within the bore of the tube.
In this case, since the surface layer is nearly impermeable to

34 DIFFUSION AND OSMOTIO PRESSURE

liquid solvent, the pressure of solvent particles may be imme-
diately effective. The surface film is lifted by the pressure
exerted from below, as a piston in such a position might be
lifted by wire springs situated within the bulb; and, in
rising, be the change in level ever so slight, it increases the
volume of the solution in the tube, thus decreasing the dif-
fusion tension of the solvent within this solution, and also
overcoming to some degree the atmospheric pressure on the
free surface of the solution; and water enters through the
permeable membrane below. The entrance of the water
is due mainly to the diffusion tension of the solvent and in
part to hydrostatic pressure. In the latter sense the push-
ing up of the surface film acts like the raising of a piston in
a pump. With a closed water manometer, or an open one
of mercury, a pressure far surpassing that of an atmosphere
may be obtained where the membrane used is sufficiently
strong. Of course, in such cases hydrostatic pressure as a
cause for the ascent of the column is to be ruled entirely
out of consideration.

Other methods of demonstrating osmotic pressure are in
use, but the explanation just given may be applied, mutatis
mutandis, to any of them.

CHAPTER VI

MEASUREMENT AND CALCULATION OF OSMOTIC
PRESSURE

I. MEASUREMENT OF OSMOTIO PRESSURE

a) Direct method.—The direct method of measurement
of osmotic pressure is very difficult of operation, and deter-
minations thus made are exceedingly tedious processes.
Nor is this method susceptible of sufficient accuracy to
recommend it to physiologists. But since it is the classical
method used by Pfeffer’ in his original investigation of
the subject, and since it has been used since that time by
physical chemists in establishing the principles by which
indirect methods become available, it will be described here
at some length.

A membrane of copper ferrocyanid is precipitated within
the walls of a cup or bulb of porous clay (a filter bulb
serves admirably) by filling the bulb with a solution of po-
tassium ferrocyanid and surrounding it with one of copper
sulphate. The bulb should first be thoroughly cleaned and
freed from air by boiling for some time in water. When
the membrane is well formed (which occurs after fifteen to
forty hours), the cup is filled with the solution to be tested,
closed with a rubber stopper bearing a mercury manometer,
and immersed in water. The osmotic pressure rises for a
number of hours, being indicated by the rise in the mercury
column, and at last, when the membrane has been ruptured
somewhere, begins to descend again. The maximum read-
ing of the mercury column is taken as the osmotic pressure
of the solution. The difficulty of the method lies in getting

1W. PFEFFER, Osmotische Untersuchungen, Leipzig, 1877.

35

36 DIFFUSION AND OsMOTIO PRESSURE

a perfect semi-permeable membrane which will withstand the
high pressures developed. A number of determinations for
the same solution are necessary in order to eliminate erratic
cases.

The direct method of measurement has just been brought
again into prominence by the work of Morse and Horn.'
These authors have succeeded in forming much more perfect
membranes in porous clay cups than have ever been pro-
duced before. Air is first swept out of the pores of the cup
by an ‘“‘endosmotic” current. The cup is filled with a weak
solution of K,SO, and immersed in a vessel of the same
solution until the outer level is near the margin of the cup.
Then a current from a dynamo is passed between a cylindri-
cal copper electrode surrounding the cup and a platinum
electrode within it. As the liquid rises in the cup, it is
removed, and in a short time the air is all removed from the
porous clay. Then the cup is filled with K,Fe(CN), and
immersed as before, but now in a solution of CuSO,. The
current is passed again, and thus the Fe(CN), ions are
driven into the clay from one side, while the Cu ions are
forced in from the other. The resistance of the cup gradu-
ally rises as the membrane is formed, being from fifteen
hundred to three thousand ohms at the time when the mem-
brane is considered as complete.’

b) Indirect methods.—Owing to the difficulties encoun-
tered in the use of the direct method just described, an
indirect method is usually resorted to. These indirect
methods depend upon the general principles, that depression

1H. N. MorsE Anp D. W. Horn, ‘The Preparation of Osmotic Membranes by
Electrolysis,” Am. Chem. Jour., Vol. XXVI (1901), pp. 80-86.

2The most recent investigations into the nature of precipitation membranes are
as follows: G. Tammann, ‘ Ueber die Permeabilitat der Niederschlags-Membranen,”
Zeitschr. f. physik. Chem., Vol. X (1892), pp. 255-64; P. WALDEN, “Ueber Diffu-
sionserscheinungen an Niederschlags-Membranen,” ibid. (1892), pp. 699-782; J. H.
Meersura, “Zur Abhandlung Tammanns: Ueber die Permeabilitat der Nieder-
schlags-Membranen,”’ ibid., Vol. XI (1893), pp. 446-8.

MEASUREMENT AND CALOULATION 387

of the freezing-point, clevation of the boiling-point, decrease
of the vapor tension of solutions, and osmotic pressure are
all related phenomena, and may be obtained one from the
other for any given solution." Three of the most satis-
factory methods for determining osmotic pressure in this
way will be briefly described here:

1. The freezing-point method: The freezing-point of a
solution is always lower than that of the pure solvent. This
depression of the freezing-point is proportional to the num-
ber of solute particles present, and therefore to the osmotic
pressure.

The depression of the freezing-point can best be deter-
mined by means of Beckmann’s apparatus,” which may be
found described in any of the texts on physical chemistry.
A determination of the freezing-point is first made for
distilled water ; this is followed by a determination for the
solution to be tested, care being taken not to disturb the
adjustment of the thermometer between the determinations.
The difference between the two observations will be the
required depression, which may be denoted by A;. The rela-
tion between this quantity and the osmotic pressure is ex-
pressed, for aqueous solutions, by the following equation:

P,= 9178.24, 3

wherein P; is the osmotic pressure at the freezing-point
of the solution, measured in millimeters of mercury.

The osmotic pressure at any desired temperature other
than the froezing-point, say T in the absolute scale, may be
obtained by applying the principle of Gay-Lussac, which

1J. H. van’t Horr, ‘Die Rolle des osmotischen Druckos in der Analogie

zwischen Losungen und Gason,” Zeitschr. f. physik. Chem., Vol. I (1887), pp. 481-508.

2H. Broxmann, “ Ueber dic Methode der Molekulargewichtsbestimmung durch
Gofrierpunktsorniodrigung,” ébid., Vol. IT (1888), pp. 638-45.

3Nprnst-PALMER, Theoretical Chemistry (London, 1895), p. 132, The pressure
here is reduced from atmospheros to millimeters of mercury.

88 DIFFUSION AND OsMOTIO PRESSURE

holds for osmotic pressures of dilute solutions. This opera-
tion is expressed in the following:

P,T,=P,T ,

in which P; is the osmotic pressure, in millimeters of mer-
cury, at required temperature T (absolute), and T; is the
absolute freezing-point of the solution. From the equation

we get:
T
oat mel 2 .

In the case of weak aqueous solutions, the freezing-point of
the solution may be considered, for this calculation, as prac-
tically the same as that of the solvent. Thus 7, = 273°
(the freezing-point of pure water), and 7’ becomes 273 + 7,
where ¢ is the desired temperature in the Centigrade scale.
Now the equation given above becomes:

P,=P,(1 +5751) — P, (1 + 0.003674) .

This is sufficiently accurate for dilute aqueous solutions.

The freezing-point method is the simplest and most satis-
factory method for general use.

2. The boiling-point method: The boiling-point of a solu-
tion is always higher than that of the pure solvent, and its
elevation is proportional to the osmotic pressure at that tem-
perature. The relation between the two quantities for
aqueous solutions is expressed as follows:

P,, = 483204, ,'

wherein P, is the osmotic pressure in millimeters of mercury
at the boiling-point of the solution, and A, is the elevation
of the boiling-point. The determination of the boiling-
point of the solution and of distilled water is best made

1 NERNST-PALMER, Theoretical Chemistry (London, 1895), p. 129, The pressure is
again reduced to millimeters of mercury.

MEASUREMENT AND CALOULATION 89

by Beckmann’s improved apparatus for this purpose,’ a de-
scription of which will be found along with that for the
freezing-point determinations.

The correction for temperature may be made, as in the
last case, by the application of the principle of Gay-Lussac
directly, or by interpolation between P, and P,, the latter
having been determined by the previous method.

The expression for the Gay-Lussac principle is of course
the same, mutatis mutandis, as that given above:

T
sa ai? ’

in which P> is again the pressure in millimeters at the de-
sired temperature 7, in the absolute scale, and 7, is the
absolute boiling-point of the solution. For this calculation
the boiling-point of a weak aqueous solution may be con-
sidered the same as that of pure water. Thus 7; = 873°,
the boiling-point of water, and 7 = 273 + t, where ¢ is the
desired temperature in the Centigrade scale. Making these
changes in the above equation,
273 + t

P,=FP, 3737 °

The method of interpolation is expressed by the follow-

ing equation:
t

PaPrt 7a

T, (P, a P,) ?

where P; is the pressure at the desired temperature, t (Centi-
grade), and the other symbols are the same as above.

8. Method by observed vapor tension: As has already
been stated, the vapor tension of the solvent is decreased by
the presence of a solute. It is found that, for dilute solu-
tions, this decrease in vapor tension is proportional to the

1E. Beckmann, ‘Zur Praxis der Bestimmung von Molekulargewichten nach
der Siedemethode,”’ Zeitschr. f. physik, Chem., Vol. VIII (1891), pp. 223-8,

40 DIFFUSION AND OSmMOTIO PRESSURE

osmotic pressure. This relation is expressed by the follow-
ing equation:
a—’ 0.08197 x 1000s x 760 ,

ais T M

In this P is the osmotic pressure in millimeters at the abso-
lute temperature 7, 7 and 7’ are the vapor tensions observed
at that temperature of the solvent and solutions respectively,
s is the specific gravity of the solution, and M is the molecu-
lar weight of the pure solvent. In the case of dilute
aqueous solutions, s may be put equal to unity (the specific
gravity of the pure solvent instead of that of the solution),
and M is 18 (the molecular weight of water). Making these
substitutions in the above equation, we have:

x—’ 0.08197 x 1000 x 760

iomekar™ 18

or
r—

P=" 34587

TT

The determination of the vapor tensions is best made by
means of the method devised by Ostwald and Walker.’
Two Liebig potash bulbs, one filled with the solution to be
tested and the other with the pure solvent (the latter
weighed), are joined in series and then attached to a weighed
U-tube of pumice moistened with sulphuric acid. A slow
current of air is passed, for six to twelve hours, through the
series. The air first becomes saturated at the tension of the
solution, and then, passing through the second bulb, be-
comes again saturated at the vapor tension of the pure
solvent. A final weighing of the second bulb and of the

1NERNST-PALMER, Theoretical Chemistry (London, 1895), p. 126; also J. H.
van’r Horr, “Die Rolle des osmotischen Druckes in der Analogie zwischen Lésun-
gen und Gasen,” Zeitschr. f. physik. Chem., Vol. I (1887), pp. 481-508.

2J, WALEER, ‘‘ Ueber eine Methode der Bestimmung der Dampfspannung bei
niederen Temperaturen,” Zeitschr. f. physik. Chem., Vol. IL (1888), pp. 602-5; also
OsTWALD-WALKER, Manual of Physico-Chemical Measurements (London, 1894)
p. 188,

y

MEASUREMENT AND CALCULATION 41

sulphuric-acid tube gives the required data. The amount of
vapor removed from the two bulbs respectively is proportional
to the vapor tensions of their contents. Thus if w denote the
loss in weight in the second bulb and w’ the gain in weight

of the sulphuric acid,
wioor—_7

w' wv

Therefore the equation given above may be written:
P= 34587 .
w

This method is difficult of operation and not very satis-
factory. The whole apparatus must be surrounded by a
jacket to keep all the parts at the same temperature; it is
not necessary that the temperature be absolutely constant,
however. The only advantage in this method over those
previously described is that by this means the osmotic
pressure can be determined for the temperature at which
the solution is used, thus avoiding the correction for tem-
perature.

II. CALCULATION OF OSMOTIO PRESSURE

a) When the pressure is produced by a non-electrolyte.
—All solutions of non-electrolytes which contain the same
number of molecules per unit volume of solution give the
same osmotic pressure. From measurements made by Pfeffer
we know that the osmotic pressure of a solution of sugar
containing a gram-molecule per liter is the same as the gas
pressure of a gram-molecule of gas occupying a liter volume.
This pressure is 22.3 atmospheres, or 16,948 mm.of mercury,
at 0° C., or 273° absolute. Thus, if we know the molecular
weight of the solute and the number of grams per liter of
solution, the calculation, on the principle that pressure varies
as concentration, is simple enough. The correction for tem-
perature is carried out by the principle of Gay-Lussac.

42 DiFFUSION AND OSMOTIO PRESSURE

For mixed solutions of non-electrolytes the total osmotic
pressure is the sum of the partial pressures due to the several
solutes respectively.

b) When the pressure is produced by an electrolyte.—
On account of the phenomena of ionization or dissociation,
the calculation of the osmotic pressure of a solution of
an electrolyte becomes somewhat complicated. The amount
of ionization must be known in order to get the relative
number of particles per unit volume. For instance, a gram-
molecule of NaCl, in aqueous solution, occupying a liter
volume, contains more particles than the same volume of a
normal solution of sugar; some of the molecules have sepa-
rated into Na and Cl ions.

The amount of ionization in any simple solution of an
electrolyte can be determined by means of the method of
electrolytic conductivity devised by Kohlrausch.' The con-
ductivity is proportional to the number of free ions, and
hence, knowing the conductivity both at the given concen-
tration and at a concentration where ionization is complete,
we can calculate the amount of ionization. The conductivity
of many solutions has been determined by different authors
in different units. Of course, all are reducible to C. G. S.
units or to the conductivity of mercury, but it is immaterial
for the present purpose what units are used, so long as the
same ones are used throughout the same calculation. As the
solution becomes more and more dilute, the conductivity
approaches a limit. This limit is the conductivity at infinite
dilution, where ionization is complete; it is usually denoted
by A, Allow > to denote the conductivity at the given

concentration. Then > =a, the fraction of the whole

oo
1F, Konneauscn, Leitfaden der praktischen Physik, Ith ed. (Leipzig, 1892),
p. 801; also F. KonLRavusce unp L. HouBorn, Das Leitvermégen der Elektrolyte,
Leipzig, 1898; W. OstwaLp, ‘‘ Ueber Apparate zur Bestimmung der Electrischen
Leitfahigkeit von Electrolyten, Zeitschr. f. physik. Chem., Vol. II (1888), pp. 561-67;
OsTWALD-WALKER, Manual of Physico-Chemical Measurements, London, 1894.

MEASUREMENT AND CALCULATION 43

number of molecules which are dissociated. Thus, if one
out of every ten molecules were dissociated, a would equal 7.

Now, if each molecule forms k ions, and if 7 denote the
ratio of the actual osmotic pressure to that which would be
obtained in the same concentration of a non-electrolyte
solution, 7 may be found from the following:

t=14+(k-l)a.

And if P denote the osmotic pressure developed in a
solution of a non-electrolyte, of the same concentration as
that whose pressure is to be found, P’ being the required
pressure, then

P'’=Pi.

The conductivities of a great many solutions are to be
obtained from published tables.’ It is not necessary to give
the methods for determining these conductivities here.
They are thoroughly and completely discussed by Kohl-
rausch and Holborn. If the osmotic pressure is all that is
required, and data for the conductivity of the given solute
cannot be found in the published tables, then it is more expe-
dient to determine the pressure by means of one of the indirect
methods previously described than to determine the conduc-
tivity. If the proper concentration is not given in the
tables, the conductivity for it is found by interpolation
between the conductivities for the two concentrations nearest
toit. If the table is rather extensive for the solution in
question, so that conductivities for very low concentrations
are given, it is usually safe to take the highest conductivity
as ,. If the table is not so complete, a limit for the con-
ductivity has to be approximated from the trend of the given
data. In using the published tables, it is very important
that one bear in mind the difference between molecular and

1W.C. D. WHETHAM, Solution and Electrolysis (Cambridge, 1895), pp. 218 ff. ;
also F, KonLgausce unD L, Hougorn, Das Leitvermégen der Elekrolyte, Leipzig,
1898,

44 DIFFUSION AND OSMOTIO PRESSURE

equivalent solutions. Most of the tables consider as a
standard solutions containing a gram-equivalent per liter.
These are easily transformed into gram-molecular solutions
by dividing the given concentration by the number repre-
senting the valency of the compound. Thus one-tenth
gram-equivalent per liter of Na,SO, is identical with one-
twentieth gram-molecular solution of the same salt.

For very weak solutions of mixed electrolytes the above
method may be resorted to. But for solutions of mixed
electrolytes and non-electrolytes, and for strong solutions of
electrolytes, no method of calculation has yet been discovered.
The only practical way open in such a case is to resort to the
methods of freezing- and boiling-points. It is often best to
make use of both these methods, and to interpolate between
them for the normal temperature, inasmuch as ionization often
increases rapidly at higher temperatures. Of course, where
chemical reaction occurs between the different solutes, the
osmotic pressure of the solution will not be constant until
chemical equilibrium has been attained.

PART II

PHYSIOLOGICAL CONSIDERATIONS

INTRODUCTION

So important a part do diffusion and osmotic pressure
seem to play in the vital processes of plants, that it is well-
nigh impossible to consider any phase of vegetable physiology
without some reference to these subjects. It is obviously
not to the point, however, to attempt here a discussion of
every phenomenon in plant life into which they enter.
Rather will attention be directed to certain groups of
phenomena wherein diffusion and osmotic pressure seem to
be fundamental factors. Thus, it is hoped, may be formed
a general conception of the trend which modern study is
taking along these lines.

Of the four following chapters, the first three have to do
with osmotic pressure as an internal factor in the life of the
plant; in them are considered the most important effects of
the development of diffusion tensions within the plant body.
In the last chapter are brought together the responses of the
organism to variations in the osmotic pressure of the sur-
rounding medium. Such division of the subject is merely
expedient; it is purely artificial, for the organism and its
surrounding medium are physically almost as truly continu-
ous as are a mass of ice and the water in which it floats.
Also—a fact which is often apparently lost sight of— every
portion of the plant body is a portion of the environment of
every other portion. This is of fundamental importance,
especially in the physiology of multicellular forms. How-
ever, the plant body is a fairly definite thing, and in the
present state of our knowledge the above classification of
environmental factors is perhaps as good as any other.

In the following pages authors are cited for the most

47

48 DIFFUSION AND OsgmoTio PRESSURE

important pieces of research, mainly for the more recent
ones. References are not given for material which may be
regarded as a matter of common knowledge. To those who
wish full citations for the period up to the time of its publi-
cation, Ewart’s admirable translation of Pfeffer’s Physiology
of Plants will be found of great service.

CHAPTER I
TURGIDITY

I. PROTOPLASM AND ITS LIMITING MEMBRANES

ANYTHING resembling an exact knowledge of the nature
of protoplasm is very remote from us as yet, but we may be
fairly certain of this, at least, that, whatever else it may be,
the vital substance is a mixture of many soluble colloids
dissolved in, or impregnated with, an aqueous solution of
many different crystalloids. Colloids are very inactive as
far as diffusion and osmotic pressure are concerned. Thus,
if an internal diffusion tension is developed within a mass
of protoplasm, it must be mainly due to the crystalloids dis-
solved in the contained water. On this account it must
come about that a mass of colloid substance inclosing within
its body an osmotic solution, and surrounded by another
osmotic solution, will act somewhat as though the former
solution were surrounded by a semi-permeable membrane.
Because of their slow rate of diffusion, colloid particles must
in a measure block the way for the diffusion of crystalloid
particles. Hence, if the more concentrated osmotic solution
be within the colloid mass, there will be developed a slight
osmotic pressure within the mass, which will hasten its
normal process of swelling by imbibition. With no truly
semi-permeable membrane about it, no state of equilibrium
can be attained between a colloid body and the surrounding
medium, until, by the slow outward diffusion of the crystal-
loid particles and by the entrance of water, there comes
about a uniform concentration both within and without.

In the author’s experiments with gelatin plate cultures of
Stigeoclonium the following observations were made, which

49

50 DIFFUSION AND OsmoTIO PRESSURE

appear to have a bearing in this connection: A somewhat
concentrated solution of mineral salts was thickened by the
addition of enough gelatin to make a firm mass at ordinary
temperatures. On the surface of this mass were placed single
drops of a dilute solution having the same chemical nature
as the one contained within the gelatin plate, and the whole
was kept in a moist chamber. After four or five hours it
was always noted that the drops of liquid had disappeared;
they had been absorbed into the colloid mass. If, however,
the more dilute solution were contained within the gelatin
plate, and drops of a concentrated solution were placed upon its
surface, it took very much longer for total absorption to occur.
For the first few hours there was usually even an observable
increase in the size of the liquid drops. Eventually absorp-
tion occurred, but it was often at the end of a period of more
than twenty-four hours. Of course, if there had been a
semi-permeable membrane between the drops and the gel-
atin, absorption would not have taken place. The gelatin
mass is not semi-permeable, but seems merely to retard the
process of diffusion of crystalloid solutes.

If a mass of such gelatin, containing a strong osmotic
solution and surrounded by a semi-permeable membrane, be
placed in water or a weaker solution, this membrane will be
stretched by the internal pressure practically as though no
colloid were present, and a state of equilibrium will be
reached only when the resilience of the membrane equals
the osmotic pressure within. This has been demonstrated
experimentally by Traube and Pfeffer.’ In such a case the
osmotic pressure of the colloid is of such an order as to be
negligible.

Now, any mass of protoplasm is very much the same sort
of a colloid mass as the gelatin just described. Its outer

1M. TravBE, “ Experimente zur Theorie der Zellenbildung und Endosmose,”

Arch, f. Anat. u. Physiol., Physiol. Abth., Jahrg. 1867, pp. 87-165. Also PFEFFER-
Ewakrt, Physiology of Plants, Cambridge, 1900, p. 106.

TURGIDITY 51

layer is so transformed (perhaps in many instances by mere
contact with the external solution and with surrounding
objects) that it is almost perfectly impermeable to many
solutes, but remains permeable to water. The protoplast of
every normal vegetable cell is thus surrounded by a more or
less perfectly semi-permeable layer, the ectoplast. If the
ectoplast is ruptured in any way, it is soon re-formed, unless
disorganization of the protoplast ensues.’ In such cases
(e. g., in Myxomycete plasmodia, etc.), where unmodified
protoplasm is brought into contact with surrounding medium,
it is perhaps partially on account of its colloidal nature that
the contained crystalloids are not immediately lost by dif-
fusion, instead of being retained, as they are, until a new
surface layer can be formed.

The internal osmotic pressure, which results when the
inclosed solution is more concentrated than the external one,
tends to stretch the surface’ layer and enlarge the protoplast.
Against this pressure is brought to bear whatever cohesive
and resilient force the ectoplast may possess; but this, from
the semi-fluid nature of protoplasm itself, must be of a
low order. In naked cells this fact prevents the internal
pressure from ever becoming very great; in such cases
rupture and destruction of the protoplasm would inevitably
result. But if the protoplasm is surrounded by a cellulose
membrane, as in the case of the majority of plant cells, this
condition is entirely altered; the swelling of the protoplas-
mic mass is checked at the limit of extensibility of the
inclosing cellulose layer. Pressure upon the ectoplast is
transmitted immediately to the cell wall, and the latter is
stretched according to its extensibility and to the pressure
applied. In the condition of strain resulting from the
interaction of the force of osmotic pressure (the diffusion

1W. Prerres, ‘Zur Kenntniss der Plasmahaut u. d. Vacuolen,” etc., Abhandl.
d.k. sdchs. Ges. d. Wiss. zu Leipzig, math.-physik. Klasse, Vol. XVI (1890), pp. 187-344.

52 DIFFUSION AND OSMOTIO PRESSURE

tension of the solute particles) on the one hand, and that of
resilience of the cellulose membrane on the other, a rigidity
and firmness of the cell as a whole is brought about, just as
a football or bicycle tire becomes rigid and firm upon being
inflated with gas. This rigidity is termed turgescence, or
turgidity. The term “turgor” has also been applied to
this condition, but it is better to reserve this word to express
the osmotic pressure of the internal fluid.’ In order that
the protoplast may retain its osmotic properties, the cellu-
lose wall must be permeated with water. This is absolutely
essential for the development of turgidity, since osmotic
pressure is not active beyond the limits of the solvent.
It is, indeed, true that the cellulose envelope of every active
cell is saturated with water.

Thus far only those cells have been considered which are
completely filled with protoplasm. This is the condition in
young cells, but mature cells are not usually so filled; as
growth progresses, vacuoles of a watery fluid appear in the
protoplasm. These increase in size and fuse together, until at
length there is a single large vacuole within the protoplasmic
mass. The typical cell of plant tissues consists of a cellulose
wall lined internally by a layer of protoplasm, which incloses
a mass of aqueous solution, the cell sap, containing sugars
and various other solutes. The litting layer of protoplasm
is bounded externally, where it comes in contact with the
cell wall, by the ectoplast. Internally, toward the vacuole,
it is bounded by a similar membrane, the tonoplast. The
cellulose wall is readily permeable to water and solutes, but
the protoplasmic lining, with its two somewhat differentiated
limiting layers, normally acts like a semi-permeable mem-
brane, allowing water to pass quite freely, but hindering, and
often seeming absolutely to prevent, the passage of solutes.

1Cf. E. B, CopELanp, “The Mechanism of Stromata,” Ann. Bot., Vol. XVI
(1902), p. 8830; IDEM, ‘‘ The Rise of the Transpiration Stream,” Bot. Gaz., Vol. XXXIV
(1902), p. 173.

TURGIDITY 53

It is in these vacuolated cells that turgidity is developed
to its greatest extent. It may be that continued concentra-
tion of the solution within the protoplasm itself may soon
reach a limit beyond which it cannot go without affecting
those energy transformations which we term vital activity or
life. An alteration in the activities of the protoplasm thus
produced may result in a change in its permeability in one
way or another. And changes of this sort accompanied by
changes in the chemical activity within the protoplast may
account for the formation of the vacuole and the secretion of
osmotically active materials into it. It was shown by Loeb!
that changes in the concentration of different ions in the
protoplasm of animal muscle bring about marked changes in
its power of absorbing water.

At any rate, however the vacuole may arise, the turgidity of
the normal mature plant cell is mainly due to the osmotic
pressure of the cell sap and to the semi-permeability of the
surrounding protoplasmic layer. The part played in the
development of turgidity by the tonoplast and ectoplast and by
the unmodified protoplasm itself, has not been worked out.
Indeed, the semi-permeability of this layer can perhaps be
attained only through the co-operation of the three some-
what distinct layers which make up the lining of the cellu-
lose wall. Although De Vries’ was able to separate the
tonoplast from the remainder of the protoplasmic mass, yet
it soon lost its peculiar properties when the surrounding
protoplasm was killed. Pfeffer® has shown that the tonoplast
and ectoplast are equivalent and are probably formed in the

1J. Lozs, “On Jon Proteid Compounds and Their Role in the Mechanicsof Life
Phenomena”; I, ‘The Poisonous Character of a Pure NaCl Solution,” Am. Jour.

Physiol., Vol. III (1900), pp. 327-38.

2HucGo De Vriss, ‘ Plasmolytische Studien fiber die Wand der Vacuolen,’’
Jahrb.f. wiss. Bot., Vol. XVI (1885), pp. 465-598. The tonoplast is not a special cell
organ, as De Vries was led to suppose.

3.W. Prerrer, ‘Zur Kenntniss d. Plasmahaut u. d. Vacuolen,” etc., Abhandl. d.
k. sachs. Ges. d, Wiss. zu Leipzig, math.-pbysik. Klasse, Vol. XVI (1890), pp. 187-344.

54. DIFFUSION AND OSMOTIO PRESSURE

same manner. In consideration of such facts as these much
stability cannot be predicated of these membranes, and thus,
in a discussion of the osmotic properties of the cell, it will
probably be safer to regard the intra-vacuolar pressure as
arising from the semi-permeability of the lining layer of
protoplasm as a whole.

In a vacuolated cell the osmotic pressure sometimes
becomes so great as to burst the cellulose membrane. This
is notably so in the bursting of the asci in certain ascomy-
cetous fungi and in the explosion of the hypo-sporangial
region in Pilobolus. Many plant cells may be made to burst
in this way by immersing them in a very weak solution or
in distilled water. For example, Lidforss’ found that the
pollen grains of many plants (notably the Liliaceae, as
Funkia, Asphodelus, Anthericum, etc.) exploded in this way
when put into water. Noll’? has shown that when certain
marine Siphoneae are placed in pure water their filaments
are apt to burst. Also Curtis’ noted that when the common
molds were placed in water after having been accustomed
to a concentrated solution, the hyphal tips often burst in the
same manner. Similar observations were made among ani-
mals by Gogorza,‘ who records the bursting of blood cor-
puscles in certain marine forms when they were killed by
being placed in fresh water.

Il. PLASMOLYSIS

When a plant cell is surrounded by a solution of greater
concentration than that contained within its vacuole, the
phenomenon of plasmolysis occurs. The greater osmotic

1B. Liprozss, ‘“ Zar Biologie des Pollens,”’ Jahrb. f. wiss. Bot., Vol. XXIX (1896),
pp. 1-138.

2F.Nouu, “Beitrag zur Kenntniss der physikalischen Vorgange welche den
Reizkramungen zu Grunde liegen,” Arb. d. bot. Inst. in Wurzburg, Vol. III (1888),
p. 496.

3C, Curtis, “Turgidity inMycelia,” Bull. Torr. Bot. Club, Vol. XXVII (1900),
pp. 1-13.

4Gocorza ¥ GonzALEz, “‘Influencia del aqua dulce en los animales marinos,”’
Annales de la soc. espagn. hist. nat., Vol. XX (1891), pp. 220-71.

TURGIDITY 55

pressure of the solutes outside, together with the slight
resilience of the protoplasmic layer, cause a contraction of
the protoplasm resulting in its separation from the inclosing
cellulose wall. If the process of plasmolysis is complete the
vacuole may disappear, practically all the water passing out.
In such cases the protoplasm often takes on the form of a
solid sphere, which lies near the middle of the cell or at one
side. Plasmolysis comes about within a very few minutes
after the cell has been placed in the plasmolyzing solution.
It is partly from the latter fact that the cellulose wall is
known to be permeable to solutes as well as to water. If it were
not so, either plasmolysis would not occur, or the cellulose
membrane would follow the protoplasm in its withdrawal
toward the center of the cell. The cellulose wall does,
indeed, contract to a certain measurable extent, but this is
due entirely to its elasticity; it simply returns to its normal
state of equilibrium when the internal pressure of the turgid
protoplasmic sac is removed.

This fact of plasmolysis has long been known,’ but the
true interpretation of it was due to De Vries’ and Pfeffer.’
After the relation which exists between plasmolysis, turgidity,
and osmotic pressure was once established, it was De Vries*
who pointed out that in the former of these phenomena we
possess a means of measuring the amount of osmotic pressure
in any given cell. His method has been used very largely
in such measurements. It may be described as follows:
If a piéce of plant tissue be placed in a concentrated solution

1¥For a historical treatment of this subject see W. Prerrer, ‘Zur Kenntniss
der Plasmahaut u. d. Vacuolen,” etc., Abhandl. d. k. sdichs. Ges. d. Wiss. zu Leipzig,
math.-physik. Klasse, Vol. XVI (1890), p. 316.

2H. DEVetss, Untersuchungen tlber die mechanischen Ursachen der Zellstreck-
ung, Leipzig, 1877.

3 W. PFEFFER, Osmotische Untersuchungen, Leipzig, 1877, pp. 121 ff.

4H. Dz Vatses, ‘ Eine Methode zur Analyse der Turgorkraft,” Jahrb. f. wiss. Bot.,
Vol. XIV (1884), pp. 427-601. 1pEM, “‘Osmotische Versuche mit lebenden Membranen,”
Zeitschr. f. physik. Chem., Vol. II (1888), pp. 415-32; 1DzM, “‘Isotonische Koeffizienten
einiger Salze,” ibid., Vol. III (1889), pp. 103-12,

56 DIFFUSION AND OSMOTIO PRESSURE

of potassium nitrate, plasmolysis will occur. If tissues with
colored cell sap, such as portions of the lower epidermis of
the leaves of Tradescantia, are used, contraction of the
vacuole may be seen very readily under the microscope. The
coloring matter of the sap fails to pass the protoplasmic
layer, and thus plasmolysis is accompanied by a deepening
of the color of the sap. If the experiment be repeated on
fresh bits of tissue, continually weaker and weaker solutions
of potassium nitrate being used, a concentration of the latter
will at length be reached, such that no plasmolysis will
occur. But plasmolysis indicates that the external solution
is more concentrated than that within the vacuole, and its
failure to appear indicates that the cell sap is more concen-
trated than the external solution. Therefore, it may be
considered that the maximum concentration of potassium
nitrate which does not cause plasmolysis is isosmotic (z.e.,
has the same osmotic pressure) with the cell sap. If we can
choose a plasmolyzing substance to which the protoplasmic
membrane is very nearly or quite impermeable’ (see the fol-
lowing section), this will give a very exact method for
measuring turgor pressure. In this way De Vries was able
to show that, in general, the concentration causing plas-
molysis was always the same, no matter what substance was
used to produce it. There were some exceptions, however,
glycerin being the most notable of those used by him. He
found, too, that certain electrolytes gave extraordinarily high
osmotic pressures. The last is now known to be due to
ionization. The “ isotonic coefficients” given by this author
express approximately the amount of ionization for the con-
centrations which he used. The results are exceedingly
valuable, for they have led to great advance, not only in
physiology, but also in physical chemistry; but since these

1 Dx Vatzs has given some very pointed directions for the critical use of his
method in “Zur plasmolytischen Methodik,” Bot. Zeitg., Vol. XLII (1884), Pp.
289-98,

TURGIDITY 57

coefficients hold true only within certain limits, and since
other more accurate methods are now available for deter-
mining the amount of ionization, a discussion of them is
here omitted.’

On the animal side the method of plasmolysis has been
used by Hamburger and others’ for determining the osmotic
pressures of the fluid contained in blood corpuscles.

Another method for comparing the osmotic pressure of
the fluid contained in red blood corpuscles with that of
the surrounding fluid was devised by Képpe,’ and further
used by Léb‘ and Hedin.’ The total volume of all the cor-
puscles of a given amount of blood was first determined by
separating them from the plasma by means of the centrifuge.
Then a known amount of blood was added to a given volume
of salt solution of known concentration, and the mixture was
shaken thoroughly. The corpuscles were then separated
from the solution on the centrifuge, and their total volume
carefully measured. If the resulting volume was less than
the normal for the given amount of blood, the conclusion

1It will be well for physiology when the practical use of these coefficients dies
out entirely.

2H. J. HAMBURGER, “ Ueber den Einfluss chemischer Verbindungen auf Blut-
korperchen in Zusammenhang mit ihren Molekulargewichten,’’ Arch. f. Anat. u.
Physiol., Physiol. Abth., Jahrg. 1886, pp. 476-87; IDEM, ‘‘ Ueber die durch Salz- und
Rohrzuckerlosungen bewirkten Veranderungen der Blutkorperchen,” ibid., Jahrg.
1887, pp. 31-47; IDEM, ‘‘Die Permeabilitat der rothen Blutkérperchen in Zusammen-
hang mit den isotonischen Coefficienten,” Zeitschr. f. Biol., Vol. XXVI (1889), pp.
414-33; G. Gryns, “Ueber d. Einfluss geloster Stoffe auf d. rothen Blutzellen, in Ver-
bindung mit d. Erscheinungen der Osmose u. Diffusion,” Pfliigers Arch. f. d. ges.
Physiol., Vol. LXIII (1896), pp. 86-119; H. Kopp, “‘Physiologische Kochsalzlésung—
Isotonie—osmotischer Druck,” ibid., Vol. LXV (1897), pp. 492-502,

3H. Korrn, ‘‘Eine neue Methode zur Bestimmung isotonischer Konzentrati-
onen,” Zeitschr. f. physik. Chem., Vol. XVI (1895), pp. 261-88.

4W. Los, “Ueber Molekulargewichtsbestimmung von in Wasser ldslichen Sub-
stanzen mittels der rothen Blutkorperchen,”’ ibid., Vol. XIV (1894), pp. 424-32.

5S, G. Heprn, “Ueber d. Brauchbarkeit der Centrifugalkraft far quantitative
Blutuntersuchungen,” Pfliigers Arch. f. d. ges. Physiol., Vol. LX (1895), pp. 360-
404; IDEM, “Ueber d. Bestimmung isotonischer Konzentrationen durch Zentrifugieren
von Blutmischungen,” Zeitschr. f. physik. Chem., Vol. XVII (1895), pp. 164-70; 1pEM,
“Einige Bemerkungen Koppes Abhandlung: Ueber eine neue Methode, etc.,”” ¢bid.,
Vol. XXI (1896), pp. 272-6.

58 DIFFUSION AND OSMOTIC PRESSURE

was drawn that the corpuscles had lost water, and hence that
the surrounding solution was of higher osmotic pressure
than the internal one. Several slight modifications of the
method were used, and many different solutions were com-
pared, the results being quite uniform with those obtained
by direct observation of the cells by De Vries and Hamburger.

Still another manner of comparing osmotic pressures of
various solutions by means of plasmolytic phenomena in liv-
ing cells is that used by Wladimiroff,’ who brought motile
bacteria into requisition for the purpose. He found that
these organisms cease to be motile when the osmotic pressure
of the surrounding fluid attained a certain magnitude.
Using asa criterion the degree of concentration at which
motion ceased, he compared the osmotic pressures of a num-
ber of solutions. His results are, in general, uniform with
those of the other authors just mentioned. The loss of
motion was due to extraction of water in a manner exactly
analogous to plasmolysis.

In making turgor determinations by the plasmolytic
method the results may be given in various ways. The
usual method has been to give them in terms of a per cent.
solution of potassium nitrate, sometimes of sodium chlorid,
sometimes of sugar, etc. But with this method, whenever it
is desired to compare pressures which have been measured
by means of different plasmolyzing solutions, it becomes
necessary to make calculations which involve the molecular
weights of the substances used. A much better way to
express turgor pressures is in terms of fractions (e. g., tenths)
of a molecular solution. But this, although it suffices for
non-electrolytes, fails utterly for electrolytes, because of the
unequal dissociation of different compounds. A 1, gram-
molecular solution of KNO, will give a much greater osmotic

1A. WLADIMIROFF, ‘ Osmotische Versuche an lebenden Bakterien,” Zeitschr. f.
physik. Chem., Vol. VII (1891), pp. 529438; also Zeitschr. f. Hygien, Vol. X (1891), pp
89-110.

TURGIDITY 59

pressure than a ~) gram-molecular solution of glucose. A
method must therefore be devised which will render it possi-
ble to compare readily the osmotic pressures of electrolytes
and non-electrolytes. This can be done by means of any
unit of pressure. The mercury column may be used, or
large pressures may be expressed in atmospheres. Recently
Errera’* has suggested a special unit for measuring osmotic
pressure, which he proposes to call the tonie. It is to be
equal to the pressure of one dyne upon a surface of one
square centimeter. For larger measurements he suggests
the term myriotome, equal to ten thousand ftonies. It is
difficult to see how this new unit possesses any advantage
over the mercury unit for practical work. For plasmolytic
purposes it is much more convenient to reduce all measure-
ments to terms of a molecular solution of a non-electrolyte.
Thus comparison becomes easy and the absolute pressure per
unit surface can be readily found from the relation

M = 22.3 atmospheres, or 1695 cm. Hg,

where M is the osmotic pressure of a molecular solution of
a non-electrolyte. Of course, in making up solutions of an
electrolyte for use by this method it must be borne in mind
that the desideratum is not a molecular solution of the elec-
trolyte, but a solution whose osmotic pressure will just equal
that of a given solution of a non-electrolyte. Thus a solu-
tion of NaCl whose osmotic pressure is, say, ~> M, must
be considerably more dilute than a 2, molecular solution
of that salt.

III. THE PERMEABILITY OF THE PROTOPLASMIC LAYERS

The often repeated statement that the protoplasmic layer
is not permeable to solutes needs to be modified as follows:
To some substances it is probably absolutely impermeable

1L. ERRERA, “Sur la myriotonie comme unité dans les mesures osmotiques,”
Extr. des Bull. de Vacad. roy. de Belgique, Vol. IIT (1901), pp. 185-53,

60 DIFFUSION AND OsSMOTIO PRESSURE

under certain conditions; to the majority of substances it is
usually very slightly permeable, but under certain conditions
its permeability may increase; and to some substances it is
usually very readily permeable. Further than this, the pro-
toplasm of different plants, and even of different cells in the
same plant, has different osmotic properties. The condition
of things is thus seen to be very complex. It will be of
value to pass in review the most important fragments of evi-
dence which have been accumulated upon this question of
protoplasmic permeability.

a) Test by the plasmolytic method.— There are several
ways of testing the permeability of the protoplasmic sac.
The one most frequently resorted to is that of plasmolysis.
A bit of tissue or a unicellular organism is subjected to the
osmotic action of solutions of the substance which is to be
tested, these being of several different concentrations. If
plasmolysis occurs in a solution of rather high concentration,
this fact is taken as evidence that the protoplasm of the
given cells is either impermeable to the solute or very
slightly permeable. Of course, it is also theoretically pos-
sible that in this case the substance used penetrates the
protoplast to some extent and causes a polymerization or
precipitation of the osmotically active solutes within the sap.
There is no evidence for this phenomenon, however, and its
general improbability throws it out of the category of seri-
ous objections to the plasmolytic method. If, after being
left a short time in the plasmolyzing solution, the cells
regain their normal condition, it shows either that the pro-
toplasm is somewhat slowly penetrated, or else that some
osmotic material has been secreted within the cell. If plas-
molysis occurs at a very low concentration, it is sufficient
proof that the substance enters the protoplasm; for such
plasmolysis is due to alteration in the membrane through
poisonous action, or to a precipitation or some similar change

TURGIDITY 61

within the vacuole, either of which phenomena could not take
place without penetration. If plasmolysis does not occur
even at high concentrations, we have evidence that the pro-
toplasmic sac is not only penetrable to the substance used,
but that this substance has no marked immediate toxic
action.

Cane sugar, glucose, KNO,, and NaCl are usually found
to produce permanent plasmolysis. Plant cells placed in
concentrated solutions of these substances do not, as a rule,
regain their original turgid condition as long as they remain
therein ; no perceptible penetration occurs. However, there
are many cells whose protoplasts are more or less permeable
to these compounds, and there are all gradations between
absolute impermeability and rather slow permeability. One
extreme of this series is Massart’s Bacterium termo,’ which
was not plasmolyzed at all in strong solutions of cane sugar
and KNO,.

But in most cases plasmolysis is the first result of irri-
gating the cells with the test solution, and it is only after
the lapse of some time that the first effect disappears. The
gradual inward diffusion of the external osmotic substance,
or, in some cases, the gradual secretion of an osmotic substance
within the cells, finally brings about an equalization of the
internal and external pressures. Then the original internal
pressure, produced by the solutes within the vacuole,
becomes again effective in producing turgidity. De Vries’
found that the tonoplasts of various plant cells were gener-
ally freely penetrated by acids and alkalies, but that salts
passed these membranes much more slowly. However,
many cells were found which, after being plasmolyzed in a
solution of KNO, or NaCl, gradually returned to their origi-

1 Massart, “Sensibilité et adaption des organismes & la concentration des
solutions salines,” Arch. de biol., Vol. IX (1899),pp. 515-70.

2H. De VRIES, “ Plasmolytische Studien fiber die Wand der Vacuolen,” Jahrb.
f. wiss. Bot., Vol XVI (1885), pp. 465-598.

62 DIFFUSION AND OsmMoTIO PRESSURE

nal condition if left in the plasmolyzing solution. This
author also observed that the presence of an acid or base, or
of any other poisonous substance, made the protoplasm
rapidly permeable to such salts as KNO, and NaCl. The
cells of the epidermis of leaves of Tradeacantia, Curcuma,
and Begonia rex appeared to be impermeable to KNO,,.
The same author’ found the protoplasm of beets to be per-
meable to NaCl. Janse’ found a similar return of tur-
gidity in the case of marine alge (e. g., Chetomorpha)
which were allowed to remain in a solution of KNO, or of
NaCl which plasmolyzed them at first. He also found that
the protoplasts of these alges are permeable to cane sugar.
When plasmolysis was brought about in a solution of this
substance, turgor gradually returned, but this process took
about four times as long here as ina KNO, solution. In
Spirogyra the same general facts were observed, but the
permeability is not as marked here as in the marine forms.
In summing up the results of his second paper, this author
states that he has found the protoplasm of the following five
plants permeable as follows:
Cheetomorpha is permeable to KNO3;, NaCl, cane sugar.
Spirogyra is permeable to KNO;, NaCl, grape sugar.

Tradescantia and Curcuma are permeable to KNO;, NaCl.
Stratiotes is permeable to KNQO3,.

Glycerin and urea have been shown by De Vries* and
Klebs‘ to penetrate nearly all plant cells with great readi-

1H. DE Vries, “Sur la perméabilité du protoplasma des betteraves rouges,”
Arch. néerl,, Vol. VI (1871), pp. 117-26.

2J, M. Jans, ‘ Plasmolytische Versuche an Algen,” Bot. Centralbl., Vol. XXXII
(1887), pp. 21-6; IDEM, ‘‘ Die Permeabilitat des Protoplasma,” Verslag. en Mededeel.
d.k. Akad. v. Wetensch. te Amsterdam, 3 Reihe, Vol. IV (1888), p. 332.

2H. Dr Vrres, ‘Ueber den isotonischen Coefficient des Glycerins,” Bot. Zeitg.,
Vol, XLVI (1888), pp. 229 ff.; 1pEM, ‘“‘ Ueber die Permeabilitat der Protoplaste far
Harnstoff,” ibid., Vol. XLVII (1889), pp. 309 ff.

4G. Kuxss, “ Beitrage zur Physiologie der Pflanzenzelle,” Unters. aus d. bot.
Inst. zu Tibingen, Vol. II (1888), pp. 489 ff.; rpm, ‘ Beitrage zur Physiologie der
Pflanzenzelle,” Ber. d. deutsch. bot. Ges., Vol. V (1887), pp. 181-9.

°

TURGIDITY 63

ness. Plasmolysis occurs, but is of short duration ; Overton!
found that it took from two to five hours for equilibrium to
be re-established in solutions of these substances.

There are exceptions here also, however, for De Vries
found that the cells of the bud scales of Begonia manicata
were almost impermeable to glycerin and urea. Jennings’
states that paramoecia are permanently plasmolyzed in
glycerin.

By an extensive investigation of the plasmolytic behavior
of various organic compounds Overton* found that a great
number of these do not produce plasmolysis at all, so rapidly
do they penetrate the protoplasm. Among these substances
may be named: ethyl alcohol, ethyl ether, formaldehyde,
chloral hydrate, acetone, methyl cyanid, furfurol, caffein,
etc. The list includes practicaily all of the aliphatic alco-
hols and related compounds which are soluble in water, and
also a number of soluble aromatic compounds, such as
anilin, acetanilid, phenol, phloroglucin, etc. Although the
author does not express himself on this point, it seems
probable that the apparent plasmolysis which occurs when
plant cells are placed in strong alcohol is due, not to osmotic
pressure, but to an increased permeability in the osmotic
membranes (due to the poison) and also to an active con-
traction on the part of the protoplasm. The same author
shows that there are all gradations in rapidity of penetra-
tion, from those substances which fail to plasmolyze at all
to those which produce permanent plasmolysis. In all these
cases he found that the protoplasmic sac is as readily per-
meated outward as inward. Practically all liquids which

1E, Overton, “Ueber die osmotischen Eigenschaften der lebenden Pflanzen-
und Tierzelle,” Vierteljahrschr. d. Naturf.-Ges. in Zurich, Vol. XL (1895), pp. 159-84.

2H. S. Jennrnes, “Studies on the Reactions to Stimuli in Unicellular Organ-
isms”: I, ‘Reactions to Chemical, Osmotic, and Mechanical Stimuli in the Ciliate
Infusoria,” Jour. Physiol., Vol. XXI (1897), pp. 258-322.

3E. OvERTon, loc. cit. The table of compounds occurs on p. 181.

64 DIFFUSION AND OSMOTIO PRESSURE

are soluble in water penetrate readily, glycerin being one
of the slowest. It appears as if the power to penetrate
decreased with increasing specific gravity. Overton finds
that the same thing is generally true of animal cells also,
and—what is still more striking—that the amount of
alcohols, etc., which plant and animal cells are able to bear
is nearly the same.

When the solute fails to penetrate the protoplast, the
osmotic concentration necessary for plasmolysis is constant,
no matter what the solute may be. But the more readily it
penetrates, the higher the concentration necessary to bring
about plasmolysis, until at last, as in the alcohols and ethers,
this phenomenon does not truly occur at all.

b) Direct test of penetrability—Another method of
determining the extent of permeability manifested by pro-
toplasm is to identify the diffusing substance after it has
passed the plasmic layer. De Vries’ showed that the pene-
tration of dilute ammonia into the cells of the red beet can
be demonstrated by the reaction of the colored cell sap to
this substance. The red sap changes to blue upon contact
with an alkali. By choosing other cells whose sap contains
red and blue dissolved pigments, Pfeffer’ showed that not
only ammonia, but also the caustic alkalies and alkaline car-
bonates as well as acids (such as tartaric, phosphoric, and
carbonic) pass very rapidly through plant protoplasm. We
may consider that this at least proves that the H and OH
ions penetrate. In some cases a precipitate may be produced
within the vacuole by the reaction of the penetrating sub-
stance with the materials of the cell sap. This is the case
with caffein, antipyrin, and some others.’

1H. De Vries, ‘Sur la perméabilité du protoplasme des betteraves rouges,”
Arch. néerl., Vol. VI (1871), p. 124; 1peM, ‘Sur la mort des cellules végétales,”
ibid. (1871), pp. 245-95.

2W. PFEFFER, Osmotische Untersuchungen, Leipzig, 1877, p. 140.
3 PrErFER-Ewart, Physiology of Plants, 1900, p. 98.

TURGIDITY 65

Another manner of carrying out the direct test is to place
cells in the solution to be tested and, after sufficient time has
elapsed, to treat them with a reagent which will penetrate
and also give a visible reaction with the substance to be
tested. Thus, if penetration took place in the first solution
a microchemical test for the solute should be obtained within
the vacuole. In this way diphenylamin was first used by
Molisch’ to detect KNO; in plant cells. With this reagent
a nitrate is indicated by the appearance of a blue color. It
appears from the work of Molisch, Wieler, De Vries, and
Janse, in which this method was used, that plant protoplasm is
very generally penetrated by KNO; in dilute solution. Ina
similar manner tests have been made with Fehling’s solution,
which prove the penetration of glucose.

Janse* showed in this manner that Spirogyra protoplasts
are penetrated by KNO3. Wieler® worked with entire
plants of the angiosperm group and obtained similar results.
By means of diphenylamin and sulphuric acid he was able to
demonstrate that NO; ions penetrate the protoplasts of seed-
ling beans, sunflowers, etc. By platinum chlorid he also
demonstrated the penetration of K ions. Stems placed in a
sugar solution formed starch, while a control without sugar
failed to do so. It made no difference whether cane sugar
or glucose were used. This proves the power of these sugars
to penetrate the protoplasts in stems. The last-named
author also presents evidence that the roots of seedlings of
Vicia faba are able to absorb glucose from a solution. Of

1H. MouiscH, ‘‘Ueber den mikrochemischen Nachweiss von Nitraten und
Nitriten in der Pflanze mittelst Diphenylamin oder Brucin,” Ber. d. deutsch. bot.
Ges., Vol. I (1883), pp. 150-55; also Sitzungsber. d. kais. Akad. d. Wiss. zu Wien, math.-
nat. hist. Klasse, Vol. I (1887), p. 221.

2J. M. Jansp, “‘ Plasmolytische Versuche an Algen,” Bot. Centralbl., Vol. XXXII
(1887), pp. 21-6; rpmM, “‘ Die Permeabilitat des Protoplasma,” Verslag. en Mededeel,
d. k. Akad. v. Wetensch. te Amsterdam, 3 Reihe, Vol. IV (1888), p. 332.

3A, WIELER, ‘‘ Plasmolytische Versuche mit unverletzten Phanerogamen,” Ber.
d. deutsch. bot. Ges., Vol. V (1887), pp. 875-80.

66 DIFFUSION AND OSMOTIO PRESSURE

course it is possible that, in these cases, the substance in
question does not pass the protoplasm as such, but is modified
at the surface of the ectoplast and penetrates in another
form. For this question there seems to be as yet no method
of attack. Wortmann’ believed he had evidence that the
starch in the endosperm of seeds was not acted upon by an
enzyme but by the protoplasm itself; this, however, has been
disproved.’ Since protoplasm contains so many enzymes of
one sort and another it seems impossible to gather evidence
as to whether a given action takes place within the proto-
plasmic mass or outside of it. It is probable that it occurs
wherever the enzymes are present, whether this be within
or without.

The penetration of anilin dyes has been studied exten-
sively by Pfeffer,’ who showed, for example, that the sap of
living cells may be strongly stained by the inward diffusion
of methyl blue, methyl violet, ete. Certain anilin dyes may
be absorbed into the living protoplasm itself and held there
so as to give a marked stain. Even the nucleus may be so
stained while living by means of dahlia, mauvein, etc.‘

In most of these experiments there is an accumulation of
the stain in the vacuole or within the protoplasm. Thus, if
plants of Elodea canadensis be placed for several days in a
weak solution of methyl blue, they become visibly stained,
while the external solution loses its color. Examination
shows that the protoplasm itself is not colored, but that the

1J, WorTMAnNN, “ Ueber den Nachweiss, das Vorkommen und die Bedeutung des
diastatischen Enzyms in den Pflanzen,” Bot. Zeitg., Vol. XLVIII (1890), pp. 581 ff.

2B. HansTeen, ‘Ueber die Ursachen der Entleerung der Reservestoffe aus
Samen,” Flora, Vol. LXXIX (1894), pp. 419-29,

3 PFEFFER-EWART, Physiology of Plants, Cambridge, 1900, p. 96. References to an
extensive literature are there given, the most important of which is: W, PFEFFER,
“Ueber Aufnahme von Anilinfarben in lebenden Zellen,” Unters. aus d. bot. Inst. zu
Tibingen, Vol. II (1886), pp. 179-331; see also E. OverrTon, “Studien tber d. Aufnahme
der Anilinfarben durch die lebende Zelle,” Jahrb. f. wiss. Bot., Vol. XXXIV (1900),
pp. 669-701.

4D. H. Campsent, “The Staining of Living Nuclei,” Unters. aus d. bot. Inst. zu
Ttibingen, Vol. II (1888), pp. 569-81.

TURGIDITY 67

dye has accumulated in the vacuole, there becoming much
more concentrated than was the original external solution.
This phenomenon will be discussed under e).

Another method by which direct determinations of per-
meability may be made is to analyze the plant or its juice
after the culture has been grown some time in a medium of
known content. In this way von Mayenburg’ found that,
out of a series of substances used in his culture fluids for
Aspergillus niger, only glycerin was absorbed in sufficient
quantity to be worthy of consideration as an osmotically
active solute within the cells.

c) Absorption test.—If the concentration of the sur-
rounding medium is carefully determined and the organisms
whose permeability is to be tested be allowed to grow in it
for a time, decrease in concentration of the medium may be
interpreted to mean that absorption has taken place. This
has been shown to occur in the case of a number of inorganic
salts, but is especially well marked with solutions of glucose
and glycerin. Demoussy’ determined in this way the relative
rate of absorption of potassium and calcium ions by wheat,
maize, etc., while Laurent*® was able to prove the somewhat
unexpected fact, that roots of maize can absorb measurable
quantities of glucose from a solution in which they are
grown. This is probably not a general phenomenon, for if
the protoplasm is permeable to sugar in one direction it is
difficult to see how it could fail to be permeable in the oppo-
site one also, and such a condition must allow outward
diffusion of glucoses and other sugars which are found so
commonly in plant cells.’

10. H. Von Mayvenzure, “ Losungsconcentration und Turgorregulation bei den
Schimmelpilzen,” Jahrb. f. wiss. Bot., Vol. XXXVI (1901), pp. 381-420.

2, Demoussy, ‘Absorption élective de quelques éléments minéraux par les
plantes,” Compt. rend., Vol. CXKXVII (1900), pp. 970-73.

3J. LAvRENT, ‘Sur l’absorption des matidres organiques par les racines,” ibid.,
Vol. CKXV (1897), pp. 887-9.

4Cf. PFEFFER-EWART, Physiology of Plants, Cambridge, 1900, p. 99.

68 DIFFUSION AND OSMOTIO PRESSURE

That ordinary leaves can absorb inorganic salts was shown
in this way by Dandeno.'’ He found that drops of solution
placed upon foliage leaves were completely absorbed if too
rapid evaporation was prevented. When the drop disap-
peared no trace of solute crystals remained upon the leaf
surface.

d) Test by toxicity.— To all protoplasmic poisons must be
accredited power of penetration in a greater or less degree;
if there were no penetration the substance could not bring
about its toxic effect. True’ proved a slight toxicity for
KNO; and NaCl upon Spirogyra; these substances must
therefore penetrate the protoplasts of this plant, though
probably this occurs with difficulty. More recently Coupin®
has prepared a catalogue of the poisonous effects upon wheat
of certain salts in various concentrations. His tables are
useful for comparison. Of course, the fact that a rather
high concentration of a given solute is needed to affect the
plant may mean either that the protoplasm is only slightly
permeable, or that the substance is only slightly toxic.
From Pfeffer‘ we have the fact that mercuric chlorid and
iodin penetrate many vegetable cells and exert a marked
toxic effect.

There are many other proofs that various mineral and
organic substances are able to penetrate the plant protoplast.
Where a noticeable and specific effect is produced upon the
organism by the presence of a given substance in the
medium, there can be no doubt that the substance pene-

1J. B. DANDENO, ‘An Investigation into the Effects of Water and Aqueous
Solutions of Some of the Common Inorganic Substances on Foliage Leaves,” Trans.
Canad. Inst., Vol. VIT (1901), pp. 238-350.

2R. H. True, ‘The Physiological Action of Certain Plasmolyzing Agents,”
Bot. Gaz., Vol. XXXVI (1898), pp. 407-16,

3H. Coupin, “Sur la toxicité des composés du sodium, du potassium, et de
Yammonium a l’égard des végétaux supérieurs,” Rev. gen. bot., Vol. XII (1901),
pp. 177-94,

4W. Prerrer, Osmotische Untersuchungen, Leipzig, 1877, p. 140,

TURGIDITY 69

trates, to some extent at least. Whether the effect be the
death of the plant or only an alteration in its metabolic
processes makes no difference for the present consideration.’

e) Test by accumulation.—The power of penetration of all
inorganic salts, and of many organic compounds also, may
be tested by analysis of plant material which has been
grown in the solution to be tested. The various metallic
ions such as K, Na, Ca, etc., are known to accumulate in the
bodies of higher plants. In this manner Bourget’ demon-
strated a marked absorption of iodin by various plant roots.
There is a great difference in different plants in this regard,
however; the Liliaceae and Chenopodiaceae absorb com-
paratively large quantities of iodin, while Solanum tuberosum,
grown in the same soil, fails to absorb enough for a test.

Great accumulation of copper in plant cells has been
recorded several times. Thus, MacDougal’ describes a case
where a tree of Quercus macrocarpa absorbed copper in
large amounts and caused its precipitation within the wood
in the metallic state.

All these accumulations come about by a chemical change
taking place in the substance after it has entered the cell,

1The following references will be of use to supplement those already given: F.
DE F. HEALD, ‘On the Toxie Effect of Dilute Solutions of Acids and Salts upon
Plants,” Bot. Gaz., Vol. XXII (1896), pp. 125-53; H. M. Ricuarps, ‘‘ Die Beeinflussung
des Wachsthums einiger Pilze durch chemische Reize,” Jahrb. f. wiss. Bot., Vol. XXX
(1897), pp. 665-79; F. L. Stevens, ‘The Effect of Aqueous Solutions upon the Ger-
mination of Fungus Spores,” Bot. Gaz.,Vol. XXVI (1898), pp. 377-406; E. B. COPELAND
AND L. KAHLENBERG, “‘ The Influence of the Presence of Pure Metals upon Plants,”
Trans. Wisconsin Acad. Sci. Arts and Let., Vol. XII (1899), pp. 454-74; N. Ono,
“Ueber die Wachsthumsbeschleunigung einiger Algen und Pilze durch chemische
Reize,” Jour. Coll. Sci. Imp. Univ. Tokyd, Vol. XIII (1900), reviewed in Bot. Gaz.,
Vol. XXX (1900), p. 422; H. Dz Vaux, ‘De l’absorption des poisons métalliques trés
dilués par les cellules végétales,”’ Compt. rend., Vol. CKXXII (1901), pp. 717-20.

2P, Bourcet, “ Sur l’absorption de l’iode par les végéteaux,” Compt. rend., Vol.
CXXIX (1899), pp. 768-70; IDEM, same title, Bull. soc. chim. Paris, Ser. 3, Vol. XXIIT
(1899), pp. 40-41.

3D. T. MacDouaat, “ Copper in Plants,” Bot. Gaz., Vol. XXVII (1899), p. 68. He
cites the following on the same general subject: LEHMAN, ‘Der Kupfergehalt von
Pflanzen und Thieren in kupferreichen Gegenden,” Arch. f. Hygien, Vol. XXVII
(1896), p. 1; J. B. Sxerrscuuy, “Tin Mines of Watsonville,” Report Geologist
Queensland, 1897.

70 DIFFUSION AND OSMOTIC PRESSURE

either within the protoplasm or in the vacuole. If this were
not so, the diffusion tension of the solute would soon become
as great within the cell as without, and thus there could be
no accumulation. But if a substance is precipitated, poly-
merized, or condensed within the cell through the chemical
action of some other body already there, which perhaps arises
as a secretion from the protoplasm, then the internal diffusion
tension of the entering substance will be kept low, and inward
diffusion will continue indefinitely. In this way copper salts
entering the cell are probably reduced to metallic copper.
This fact of accumulation is a very important one in under-
standing the process of absorption of dissolved substances by
the plant.

J) Test by metabolic processes.—The absorption of any
food substance is of course a proof of permeability to that
substance. The immediate effect upon the living green cell
of absence of carbon dioxid, or upon any living cell of
oxygen, shows that these gases, when in solution, enter the
protoplast with extreme ease. Penetration by many usually
solid substances may be proved in this manner; the long
series of experiments upon growth, and especially the forma-
tion of starch by green plants in darkness, may be regarded
as evidence in this matter. Thus, Bouilhac' grew Nostoc in
the dark in a solution of glucose, where it appeared perfectly
healthy, and Artari’ and Matruchot and Molliard* grew
Stichococeus in organic solutions in a similar way. The
absorption of organic food by many alge, and by all saporo-
phytes and parasites, including all of the fungi, may be
mentioned in this connection. In many of these cases the

1R. BovrnHAc, “Sur la culture de Nostoc punctiforme en présence de glucose,”
Compt. rend., Vol. CXXV (1897), p. 880.

2A, Anrart, “Zur Ernahrungsphysiologie der grinen Algen,” Ber. d. deutsch.
bot. Ges., Vol. XIX (1901), pp. 7-10.

3L, Matrucuot ET M. Mouuiagp, “ Variations de structure d’une algue verte
sous influence du milieu nutritif,” Rev. gen. bot., Vol. XL (1902), pp. 114-80, 254-68,
316-32.

TURGIDITY WW

presence of the substance in question is due to digestion
outside the body, brought about by outward diffusion of
enzymes.

Dandeno' has recently shown that inorganic salts are
absorbed in some instances by ordinary leaves when these
are kept covered by a solution by means of a constant spray,
or by submersion. This occurred to such a degree in this
writer’s experiments with Thunbergia that plants of this
form whose roots were supplied with nothing but water, but
whose leaves were sprayed with a solution, were able to
make a good growth. Control plants, which had distilled
water applied to the leaves as well as to the roots, perished
in a much shorter time. Also, drops of solution placed upon
various leaves were completely absorbed if too rapid evapora-
tion was prevented. This observation has been mentioned
under c).

g) Outward permeability.—Many substances which pene-
trate the cell from without have been shown to pass in the
opposite direction with equal ease. This has been especially
emphasized by Overton” in the case of the soluble alcohols,
etc. But in general the outward passage from the plant
body of sugars and the various organic food substances has
not been demonstrated. It must be of rather rare occurrence,
or the phenomena of nutrition, etc., would be impossible.
There are, however, certain cases where exudation occurs,
notably in the case of glandular structures, both in plants
and animals. Laurent,’ however, has demonstrated an out-
ward passage of enzymes (e. g., amylase and sucrase) from

1J. B. DanpENo, ‘‘An Investigation into the Effects of Water and Aqueous
Solutions of Some of the Common Inorganic Substances on Foliage Leaves,” Trans.
Canad. Inst., Vol. VII (1901), pp. 238-350.

2E. Overton, ‘‘ Ueber die osmotischen Eigenschaften der lebenden Pflanzen
und Thierzelle,” Vierteljahrschr. der Naturf.-Ges. in Ztirich, Vol. XL (1895), pp.
159-84.

‘8g. LAvRENT, “Sur l’exosmose de diastases par les plantules,” Compt. rend.,
Vol. CXXXI (1900), pp. 848-51.

72 DiFFUSION AND OSmMOTIO PRESSURE

the roots of maize seedlings, and a similar phenomenon is
very commonly met with in the case of bacteria, yeast fungi,
etc. Molisch’ believed this to be generally true, but Czapek?
has shown that he was probably mistaken. The latter found
that normal roots give off not only CO, but also phosphoric
acid in the form of an acid salt. These substances must of
course pass out through the protoplasm. There seems to be
no doubt from the work of Dandeno® that both organic and
inorganic substances will diffuse out from the cells of foliage
leaves if these are kept covered with water. In these cases
inward and outward diffusion seem to take place in exactly
the same manner.

It is a well-known fact that enzymes, especially diastase,
pass out from the cells of embryos and digest food stored in
the endosperm of seeds.“ This argues the permeability of
the protoplasm of both embryo and endosperm to these
substances. In this connection evidence has also been
presented that the cells of embryo and endosperm are both
permeable to carbohydrates, probably of the glucose group.
Whether these arise from the action of an enzyme derived
from the embryo, or from the action of enzymes formed
within the endosperm itself,’ is of no consequence so far as
permeability is concerned; after the carbohydrates are
formed they diffuse into the embryo.

h) Variations in permeability.—If a turgid cell gives

1H. Moriscu, “‘ Ueber Wurzelausscheidungen und deren Einwirkung auf orga-
nische Substanzen,’’ Sitzungsber. d. kais. Akad. d. Wiss. zu Wien, math.-nat, hist.
Klasse, Vol. XCVI (1887), pp. 84-109.

2F. Czapex, ‘‘Zur Lehre von den Wurzelausscheidungen,” Jahrb. f. wiss. Bot.,
Vol. XXIX, pp. 321-90.

8 Loe. cit., p. 71.

4 Gruss, ‘ Ueber d. Eintritt von Diastase in d. Endosperm.,” Ber. d. deutsch. bot.
Ges, zu Berlin, Vol. IV (1893), p. 286; also B. HANsTEEN, ‘‘ Ueber die Ursachen
der Entleerung der Reservestoffe aus Samen,” Flora, Vol. LXXIX (1894), pp. 419-29.

5K, PurrewITcu, “Ueber die selbstthatige Entleerung der Reservestoffbehalter,”
Ber. d. deutsch. bot. Ges., Vol. XIV (1896), pp. 207-15; IDEM, ‘‘ Physiologische Unter-
suchungen fiber die Entleerung der Reservestoffbehalter,” Jahrb. f. wiss. Bot., Vol.
XXXI (1897), pp. 1-76.

TURGIDITY 73

out pure water, this occurrence must be due to one of two
conditions: (1) the protoplasm may contract with great force,
thus overcoming the osmotic pressure of the contained
solutes and causing the solvent to pass outward through
the membrane, or (2) a relative decrease in the internal
pressure may occur resulting either from an active precipita-
tion or condensation of some of the solutes of the sap, or from
an absolute rise in the external osmotic pressure. Accord-
ing to supposition (1), the osmotic pressure within the cell
remains unchanged, but is in part overcome by the mechani-
cal pressure of the contracting protoplasmic membrane.
According to supposition (2), the internal osmotic pressure is
relatively reduced, and the protoplasm does not exert any
appreciable pressure itself, but is forced inward through the
solvent by the osmotic pressure of the solutes outside the cell
and by the elastic force of the restraining cellulose wall.
It is probable that this last supposition expresses the truth in
many cases where an alteration in turgidity is observed.
The former supposition is not tenable at all; the protoplast
would burst long before concentration of the sap solution
could be brought about by pressure.

If a cell gives out a solution, the cause of this must be a
change in the permeability of the protoplasm, such that it
now allows the outward passage of solutes to which it was
formerly impermeable. The liquid exuded in guttation is
known! to be, not pure water, but a portion of the cell sap.
In the last particular this sort of shrinkage of the vacuole
differs from true plasmolysis, for in that we have the extrac-
tion of pure water. However, the apparent effect upon the
cell is the same; if the volume of the vacuole is in any way
decreased, the protoplasmic sac will contract from its own
elasticity and surface tension, if for no other reason.

1G. Bonnrer, “Recherches expérimentales sur la miellée,” Fev. gen. bot., Vol.
VIII (1896), pp. 1-22; DanpENO has shown that guttation droplets contain both
organic and inorganic solutes. See Trans. Canad. Inst., Vol. VIL (1901), pp. 238-350.

74. DIFFUSION AND OSMOTIO PRESSURE

At different times and under different conditions the
permeability of certain protoplasts apparently changes
greatly. The presence of poisons may cause the protoplasm
to become more permeable to other substances. Thus,
Maquenne' found that HgCl, caused a marked increase in
the permeability of the protoplasm of the cells of Helianthus
seedlings to plasmolyzing agents. Similarly DeVries’ found
that plasmatic membranes which were normally impermeable
to KNO, and NaCl could often be made permeable to them
by treatment with an acid or a base. With animal muscle
Loeb? has shown that acids, bases, and other chemicals exert
a great influence upon the water-absorbing power of the
cells. This may be due to changes in permeability.

Partial or complete plasmolysis may act in the same way.
Oltmanns*‘ was able to cause Fucus cells to give out coloring
matter by placing the tissues in concentrated solutions.
Both of these reactions must consist in an alteration of the
physical (perhaps chemical) structure of the protoplasm.
On the other hand, an increase in turgor above the normal
may cause the same change. This seems to be the case in
the guttation from the water pores of the leaves of the
tomato, balsam, etc. When the turgor pressure in the
cells bordering these water pores passes a certain limit, the
protoplasm apparently becomes altered so that the cell sap
oozes out and appears in droplets on the leaf-tips where
water pores are present. Czapek® describes a similar phe-

1L. MAQuEeNnNge, “Sur la pression osmotique dans les graines germées,”’ Compt.
rend., Vol. CXXIII (1896), pp. 898, 899.

2H. Ds Vaztes, “ Plasmolytische Studien &. d. Wand d. Vacuolen,” Jahrb. f. wiss.
Bot., Vol. XVI (1885), pp. 465-598.

3J. Lozs, “ Physiologische Untersuchungen fiber Ionenwirkung”: I. Mitthei-
lung, “‘ Versuche am Muskel,” Pfliigers Arch. f. d. ges. Physiol., Vol. LXIX (1897), pp.
1-27.

4F, OLttTmMawns, “Ueber die Bedeutung der Concentrationsinderung des
Meerwassers ffir das Leben der Algen,” Sitzungsber. d. k. preuss. Akad. d. Wiss. zu
Berlin, Vol. X (1891), p. 183.

5¥F,. Czapex, ‘Zur Lehre von den Wurzelausscheidungen,” Jahrb. f. wiss. Bot.,
Vol. XXIX (1896), pp. 321-90.

TURGIDITY 75

nomenon in the case of turgid root hairs, from which droplets
of solution are exuded. It is probable that in these cases
we have to do with a phenomenon related to glandular
secretion.

Another potent cause for great increase in protoplasmic
permeability in some instances is lowering of temperature.
If a filament of any common alga be carefully dried exter-
nally and placed in olive oil whose temperature is then
rapidly lowered to the vicinity of 0° C., a film of water may
be seen to form about the filament, and partial plasmolysis
may be observed. When the temperature is again brought
back to normal, the extruded water is again absorbed.
Greeley’ has recently shown, not only that complete plas-
molysis can be produced in Spirogyra by low temperature,
but that the same thing occurs in Stentor coeruleus. Exactly
the same phenomenon is exhibited by Stentor individuals
when water is removed from them by the action of a con-

centrated sugar solution. The animals plasmolyzed by low

temperature return to their normal activity with rise in tem-
perature, but Greeley was unable to cause the same reversal
in the case of the osmotically plasmolyzed individuals. I
have often observed that the liquid exuded from cells of
Spirogyra plasmolyzed by cold is a solution. Its freezing
point is considerably lower than that of pure water.

The theory of death by freezing which was advanced by
Molisch’ accounts for the decline of activity and for final
death at low temperatures by the extraction of water from
the protoplasm until the processes which make up life are
no longer possible. Matruchot and Molliard* have pointed

1A. W. GREELeEy, ‘‘On the Analogy between the Effects of Loss of Water and
Lowering of Temperature,” Am. Jour. Physiol., Vol. VI (1901), pp. 112-28.

2H. Mouiscu, Untersuchungen tiber das Erfrieren der Pflanzen, Jena, 1897;
reviewed in Bot. Centralbl., Vol. LX XIII (1898), p. 149.

3L. Matrocuot AND M. MoLuraRD, “Sur lidentité6 des modifications de
structure produites dans les cellules végétales par le gel, la plasmolyse, et la
fanaison,” Compt. rend., Vol. CK XXII (1901), pp. 495-8.

ae

76 DIFFUSION AND OSMOTIO PRESSURE

out a striking parallelism in the behavior of plant nuclei
which have been either frozen, dried, or subjected to the
osmotic action of a concentrated solution. In all these cases
water was found to be extruded from the nucleus. The
nuclear material took on a peculiar appearance not unlike
that of karyokinetic figures.

Krabbe’ experimented upon the effect of rise in tempera-
ture upon the absorption of water by various plant cells,
finding that the rate of absorption rises with the tempera-
ture. This author supposes the response to be due to a
physical change in the protoplasm, caused by the higher
temperature.

But the best series of experiments on the change in pro-
toplasmic permeability due to temperature variations is that
of van Rysselberghe.? He worked with a variety of plant
cells (Sambucus, Tradescantia, Begonia, Lemna, green alge,
etc.) and found that Krabbe’s general result is true. The ratio
of increase in permeability to water becomes less, however, as
the temperature rises. From 0° C. to 5° C., this ratio is 0.05;
from 5° C.to 18°C., 0.043; and above the last-named tempera-
ture, 0.1. The ratios between the permeability to water at

0° and that at 6°, 12°, 16°, 20°, 25°, 30°

are dy. ede ace 2,45, 6, 7, 7.5, 8.

The total amount of water absorbed by a cell is not changed
by variations in temperature; the rate of absorption alone is
affected. The nature of the protoplasm does not appear to
have any effect on the total amount of water absorbed or
given out by a cell; this is determined by the osmotic pres-
sure of the sap and by the temperature. Permeability to

1G. Krapse, “Ueber d. Einfluss d. Temperatur auf d. osmotische Processe
lebender Zellen,” Jahrb. f. wiss. Bot., Vol. XXIX (1896), pp. 441-98.

2F. VAN RysSELBERGHE, ‘Influence de la température sur la perméabilité du
protoplasme vivant pour 1’eau et les substances dissoutes,” Recueil de l’inst. bot. de
Bruszelles, Vol. V (1901), pp. 209-49; 1pEM, ‘‘ Réaction osmotique des cellules végétales
a la concentration du milieu,” Mem. cour. pub. par Uacad. roy. de Belg., Vol. LVIII
(1898), pp. 1-101.

TURGIDITY : 77

this liquid does not cease altogether at 0° C., as was thought
by Schwendener.’ At 0° C. protoplasm is not only perme-
able to water, but also to KNO,, glycerin, urea, methylene
blue, caffein, and ammonium carbonate. Thus Krabbe’s
idea that below 5° C. nothing but water penetrates is entirely
unfounded. Temperature variations in permeability to
solutes were also observed in many cells by the same author.
These, too, seem to follow the rule for water as stated on the
preceding page.

However, Copeland’ showed that decrease in temperature
caused a rise in tde turgor pressure of moss leaves. This
may not be a direct effect of the temperature upon the pro-
toplast, for the same author found that various agencies
which checked growth also caused a rise in turgor; there is
surely a close relation between growth and turgor. what-
ever this relation may ultimately turn out to be. In this
connection it may be noted that De Vries’ found that, as
growth proceeds, turgor rises, to fall again after the curve
of growth begins to decline.

The extrusion of liquid from the cells of the pulvini of
“sensitive” organs, such as the leaves of Mimosa and the
stamens of Berberis, may be due to a change in permeability
also. There seems to be a question, however, as to whether
the exuded liquid is water or a solution. Pfeffer* considers
this subject at some length, and concludes that solutes prob-
ably do not pass out. He believes that the salts found by
Janse’ in the extruded liquid from pulvini in Mimosa are

18, SCHWENDENER, “ Zur Kritik der neuesten Untersuchungen 4. d. Saftsteigen,”
Sitzungsber. d. k. preuss. Akad. d. Wiss. zu Berlin, Vol. VI (1892), p. 911.

2E. B. COPELAND, Ueber den Evnfluss von Licht u. Températur auf den Turgor,
Halle a. S., 1896.

8H. Dr Verses, “‘ Ueber die Ausdehnung wachsender Pflanzenzellen durch ihren
Turgor,” Bot. Zeitg., Vol. XXXV (1877), p. 1.

4W. Prerrer, ‘Zur Kenntniss der Plasmahaut u. d. Vacuolen, etc.,” Abhandl,
d. kh. sdchs. Ges. d. Wiss., math.-physik. Klasse, Vol. CLXI (1890), pp. 187-344.

5J, M. Janse, ‘' Die Permeabilitat des Protoplasma,” Verslag. en Mededeel. d.
k, Akad. v. Wetensch. te Amsterdam, 1888.

78 DIFFUSION AND OsmoTio PRESSURE

merely extra-cellular material. But Pfeffer shows that there
is surely an extrusion of solutes from the stamens of Cynara.
It may be that all “sensitive” organs do not act alike in
this regard. Hilburg’ states that the cells of the pulvini of
leaves of Phaseolus increase in turgor pressure when sub-
jected to the action of light.

Puriewitch? found that absence of oxygen and the pres-
ence of anzsthetics prevented the giving off of reserve food
from cells of tubers, roots, bulbs, the endosperm of seeds, etc.
He explains this fact as an effect upon the diastatic enzymes
in the various cases, but the onion bulb and the beet root
exhibit the same phenomenon, and it is difficult to see how
organs whose stored food is already in solution could be
affected by an alteration in enzymes—unless, indeed, even
sugars need to be modified by enzyme action before they
can pass the protoplasm. A live onion scale totally submerged
in water will give off no sugar, but if partially exposed to
the air exudation takes place. It is probable that here we
have another case of the influence of a chemical (i. e,,
oxygen) upon the permeability of the membrane. That
enzyme action is necessary for the translocation of the cane
sugar of the beet root is possible and even probable.

The permeability of the protoplast to a certain substance
may change according to the relative concentration of that
substance within and without the cell, as has been shown
recently by Nathansohn.* Codium tomentosum was used in
his experiments. The cells of this plant are permeable to
chlorids, so that the content of HCl is found to be the same
in expressed sap and in the surrounding medium. If the

1C, Hitpure, “‘ Ueber Turgescenczinderungen in den Zellen der Bewegungs-
gelenke,” Unters. aus d. bot. Inst. zu Ttibingen, Vol. I (1881), pp. 23-52.

2K. Purgrewitcg, “ Physiologische Untersuchungen &. d. Entleerung der Reser-
vestoffbehalter,” Jahrb. f. wiss. Bot., Vol. XXXI (1897), pp. 1-76.

3A, NaTHANSOHN, “Zur Lebre von Stoffaustausch,” Ber. d. deutsch. bot. Ges.,
Vol. XIX (1901), pp. 509-12,

TURGIDITY 79

plant be put into a solution without the chlorid, this sub-
stance diffuses out of the cells in twelve to twenty-four
hours, and the reverse occurs if the external solution con-
tains more Cl ions than the internal. Other substances in
the medium will cause the retention of chlorid, apparently
in inverse proportion to their ability to penetrate. The
following table to show this is taken from Nathansohn’s

paper:

Medium Pressure cares ea
Normal Sea H,O Sw! 2.25
Na NO; iSw 67
Na NO; Sw 1.27
Urea, Sw 0.64
Glycerin Sw 1.0
Grape sugar 4Sw 1.45

Of the substances named in the table, those of least pene-
trating power are grape sugar and NaNO;. This is a new
departure, and no definite conclusions can be drawn until
more work has been done.

Very clear evidence of change in permeability of proto-
plasm to solutes is that obtained by Haupt’ in his work on
extrafloral nectaries. In certain plants (e. g., Euphorbia
and Vicia) he found that the secretion of sugar by the nec-
taries was profoundly influenced by light, specifically by the
red-yellow rays of the sun’s spectrum. Secretion of sugar
occurred only in light, and from nectaries already containing
sugar this substance was resorbed in darkness or in blue
light. This must indicate that the nature of the protoplas-
mic membranes here is entirely altered by the etheric vibra-
tions; in light sugar passes outward, while in darkness it
moves in the opposite direction. That plants which had

1 Sw in the table denotes the osmotic pressure of normal sea-water.

2H, Haupt, “ Zur Secretionsmechanik der extrafloralen Nektarien,” Flora, Vol.
X (1902), pp. 1-41.

80 DIFFUSION AND OSMOTIC PRESSURE

been deprived of CO, for many days still exhibited this
response seems to show that the phenomenon stands in no
direct relation to the process of photosynthesis.

Iv. ACTION OF THE PROTOPLASMIC MEMBRANE

If ignorance still prevails regarding the manner in which
a purely physical osmotic membrane acts, it is even more
prevalent regarding the action of the protoplasmic mem-
brane of the living cell. That the tonoplast and ectoplast
are the main factors in the production of semi-permeability
there seems little reason to doubt. There are three possible
ways in which they may act, and of course the same mem-
brane may act in different ways at the same time. These
three possible explanations of semi-permeability may be briefly
stated as follows:

a) The filter theory—The simplest explanation of
osmotic pressure, whether within a living cell or not, is this:
That the semi-permeable membrane acts merely as a sieve
or filter, and that the larger solute particles are prevented
from passing by the smallness of the opening, while the
smaller solvent particles pass with only slightly increased
friction. Since we cannot be sure of the relative sizes
of the different molecules and ions, there is no way of
directly testing this hypothesis. But there are many facts,
both in chemistry and physiology, which make the filter
theory at least very improbable in many instances. Among
the physiological facts which tend in this direction may be
cited the effects of chemicals in varying the permeability of
the protoplasm, especially the observations of Maquenne,
De Vries, and Loeb, already discussed’ and the work of
Overton, to be taken up under b).

b) The solution theory.— According to this theory, the
membrane is to be considered as a solvent in which the pene-

1See above, p. 74.

TURGIDITY 81

trating substances are readily soluble, while those which fail
to penetrate are not. Thus, in the case of an osmotic cell
containing a sugar solution and surrounded by pure water,
it may be supposed that the membrane dissolves water
readily, but cannot dissolve sugar. Thus, water would go
into solution in the membrane on the side of higher diffusion
tension of water (the side of the solvent), and, after diffusing
through the membrane, would be given off on the side of
lower diffusion tension of that substance (the side of the
solution). The solute, not being able to go into solution in
the membrane, merely exerts its expansive force upon it, and
does not diffuse.

From the researches of Overton’ and others it appears
that this is probably the true explanation of many cases of
the development of osmotic pressure in plant and animal
cells. By comparing the numerous substances which he
found able to penetrate the protoplasm, Overton observed
that penetrating power seemed to vary in proportion to solu-
bility in aliphatic oils and ethers. He went still further: A
given substance, like glycerin, may be only slightly soluble
in aliphatic oils, and may penetrate the protoplast slowly;
but if its substitution products (as mono- and dichlor-hydrin,
or mono- or dimethyl-glycerin) are more soluble in this
class of substances, their power of penetrating the plant
protoplast is also found to be greater, and this in proportion
as solubility has been increased by the substitution. From
these and many other observations Overton is led to con-
clude that the peculiar property of the protoplast of being
permeable to certain substances and not to others probably
depends upon the presence within it of some aliphatic oil
or ester, or a mixture of these substances. He goes so far
as to point out that cholesterin and the lecithins, substances

1E. Overton, ‘Ueber die allgemeinen osmotischen Eigenschaften der Zelle,
ihre vermuthlichen Ursachen und ihre Bedeutung fir die Physiologie,” Vierteljahr-
schr. d. Naturf.-Ges, in Zitrich, Vol. XLIV (1899), pp. 88-135,

82 DIFFUSION AND OsmoTio PRESSURE

of this nature which are of general occurrence in animal and

‘plant cells, when in certain mixtures, absorb water. He
suggests that the tonoplast and ectoplast may be merely
layers of such substances, and gives reasons why it is not
plausible to suppose that a simple aliphatic oil plays this
part of surface layer, as wassupposed by Quincke.’ The
difference, however, between oils and lecithins is so slight
that Quincke’s theory is not fundamentally modified by
these facts.

On the purely physical side Meerburg’s work’ on the
passage of fuchsin through membranes of copper ferro-
cyanid seems to present evidence in favor of the solution
theory. The dye could not pass the membrane, even in a
slight degree, until the latter had become fully impregnated
with it. Flusin also showed’ that the rapidity of diffusion
of various substances through a membrane of caoutchouc
varies in the same degree as does the absorptive power of
the membrane for these substances. The same was found
to be true for pig’s bladder. :

c) The chemical theory.—The chemical explanation of
the phenomenon of semi-permeability supposes that the
membrane takes an active chemical part in the transmission
of substances from one side to the other. In all its free-
dom of indefiniteness this theory offers the only escape from
the dark problem of glandular secretion, wherein the solute
moves from the weaker to the stronger solution against its
own diffusion tension. This is one of the most difficult of
physiological problems, and upon it absolutely no light has

1QuINncKE, ‘Ueber Emulsionsbildung und d. Einfluss der Galle bei der Ver-
dauung,” Pyligers Arch. f. d. ges. Physiol., Vol, XIX (1879), pp. 129-44,

2J. H. Mezgpure, “Zur Abhandlung Tammanns: Ueber die Permeabilitat der
Niederschlagsmembranen,” Zeitschr. f. physik. Chem., Vol. XI (1893), p. 446-8.

3G. Fuustn, “Sur l’osmose des liquides & travers une membrane de caoutchouc
vulcanis6,” Compt. rend., Vol. CXXVI (1898), pp. 1497-1500; IDEM, “Sur l’osmose des
liquides & travers une membrane de vessie de pore,” zbid., Vol. CXXXI (1900), pp.
1308, 1309.

TURGIDITY 838

yet been shed. In the realm of pure physical chemistry
there is some evidence that osmotic membranes may some-
times play a chemical part.’

Vv. THE NATURE OF THE OSMOTICALLY ACTIVE SOLUTES -

Within the cell sap there are very many substances in solu-
tion, and thus the question as to what substances the turgor
pressure is due becomes of some importance. DeVries’
found that in the onion bulb-scale and in the beet root this
pressure is chiefly due to stored sugar. He also showed
that the salts KCl and KNO, play important réles in the
maintenance of turgor in leaf-stalks of Gunnera scabra and
the shoots of Helianthus tuberosus, respectively. Copeland®
concluded from experiments with Phaseolus, Pisum, Fago-
pyrum, and Zea, that in these plants the osmotically active sub-
stance is mainly potassium nitrate. On the other hand, Kraus‘
and DeVries* found that in many plants organic acids are
the main source of the osmotic pressure of the cell sap.
The research of von Mayenburg® shows that Aspergillus

10n the action of animal membranes in diffusion phenomena see the following:
H. J. HamBurcer, ‘' Die isotonischen Koeffizienten und die rothen Blutkorperchen,”
Zeitschr. f. physik. Chem., Vol. VI (1890), pp. 319-33; 1DEM, ‘‘ Ueber den Einfluss des
jntra-abdominalen Druckes auf die Resorption in der Bauchhohle,” Arch. f. Anat.
u, Physiol., Physiol. Abth., Jahrg. 1896, p. 302, ibid., p. 428; G. Kovest, ‘‘ Beitrage
zur Lehre der Resorption im Dinndarm,”’ Physiol. Centralbl., Vol. XI (1898), pp.
555-7 and 593-7; O. CoHNHEIM, ‘‘ Ueber Dtinndarmresorption,” Zeitschr. f. Biol., Vol.
XXXVI (1898), pp. 129-53; S. G. Heprn, “‘ Ueber den Einfluss einer thierischen Mem-
bran auf die Diffusion verschiedener Korper,” Pfligers Arch. f. d. ges. Physiol.,
Vol. LX XVIII (1899), pp. 205-61; R. HOsex, ‘‘ Ueber Resorption im Dinndarm,”’ ibid.,
Vol. LXXIV (1899), pp. 246-71. On the action of the copper ferrocyanid membrane see
G. Fuusrn, “Sur l’osmose & travers la membrane de ferrocyanure de cuivre,”’
Compt. rend., Vol. CXXXII (1901), pp. 1110-12.

2H. De Vetss, ‘‘ Eine Methode zur Analyze der Turgorkraft,” Jahrb. f. wiss. Bot.,
Vol. XIV (1884), pp. 427-60.

3E. B. CopeLanp, ‘The Relation of Nutrient Salts to Turgor,” Bot. Gaz., Vol.
XXIV (1897), pp. 399-416.

4G. Kraus, Stoffwechsel der Crassulaceen, 1886.

5H. De Veras, “Ueber d. Bedeutung d, Pflanzensduren,” etc., Bot. Zeitg., Vol.
XXXVII (1879), pp. 847-53; IDEM, ‘‘ Ueber d. Antheil Pflanzensauren,” etc., Bot. Zeitg.,
Vol. XLI (1883), pp. 849-54.

60, H. von Mayenpura, “ Losungsconcentration und Turgorregulation bei den
Schimmelpilzen,” Jahrb. f. wiss. Bot., Vol. XXXVI (1901), pp. 381-420,

84 DIFFUSION AND OSMOTIO PRESSURE

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’ 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’ found that the cells
of growing root tips of Pisum, Lupinus, etc., which had been
grown in strong KNO, 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.’ DeVries‘ made

1L. MaQuenng, “Sur la pression osmotique dans les graines germées,” Compt.
rend., Vol, CXXIII (1896), pp. 898 ff.

2B. Sraner, ‘Beziehungen zwischen Substratconcentration, Turgor und
Wachsthum bei einigen phanerogamen Pflanzen,” Bot. Zeitg., Vol. L (1892), p. 253.

3A, Mayer, Lehrbuch der Agriculturchemie, Heidelberg, 1895, pp. 807 ff.

4H. De Vains, ‘‘ Eine Methode zur Analyse der Turgorkraft,” Jahrb. f. 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 osmotically
active substances.

Heald* 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’
has recently been added to by Sutherst.* 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:
Os, Pressure in

Vegetable marrow: Fr. Pt. mm. of Hg.
Leaf and stalk - —0.75° 6,880
Fruit - = —0.75° 6,880

Swede turnip:

Entire plant —1.0° 9,173.2

1F, pE F. HEAzp, “The Electrical Conductivity of Plant Juices,” Science, N.5.,
Vol. XV (1902), p. 457; IDEM, same title, Bot. Gaz., Vol. XXXTV (1902), pp. 81-92.

2See above, p. 84.

3 W. T. Suraerst, “The Freezing Point of Vegetable Saps and Juices,” Chem.
News, Vol. LXXXIV (1901), p. 234.

86 DIFFUSION AND OSMOTIO PRESSURE

Fr. Pt. Os. Pressure in

Celery: mm. of Hg.
Green stalk and leaf —14° 12,842.48
White parts —0.75° 6,880

Carrot:

Leaf and stalk - —1.2° 11,007.84
Root - —1.0° 9,173.2

Cabbage:

Outer leaf - - -11° 10,090.52
Heart —0.85° 7,797.22
Apple, fruit - —14° 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.’

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

1Cf, PrerFER-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’ and caffein) is surely
due to a chemical reaction after the substance has passed the
protoplasmic layers.

VII. RELATION 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

1Ibid., pp. 119 ff.

88 DIFFUSION AND OSMOTIO 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
degree. 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 Reinhardt.‘ Other evidence along this line is that
obtained by Klebs,” 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’
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‘ to the conclusion that growth regulates turgor
rather than turgor growth.

1M. O. Rernwagpt, “ Plasmolytische Studien zur Kenntniss des Wachsthums
der Zellmembran,” Festschrift fiir Schwendener (1899), p. 425.

2G, Kuzss, “ Beitrdge zur Physiologie der Pflanzenzelle,” Unters. aus d. bot.
Inst. zu Tubingen, Vol. II (1888), pp. 489-568.

3F. O. BowEr, ‘On Plasmolysis and its Bearing upon the Relations between
Cell-wall and Protoplasm,” Quart. Jour. Microsc. Sci., Vol. XXIII (1885), pp. 157-67.

4E. B. CopELann, 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’ 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. Pfeffer’ has made
many tests and measurements in this field, and Rodewald?
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

1J. B. Pouuock, “The Mechanism of Root Curvature,’’ Bot. Gaz., Vol. XXIX

(1900), pp. 1-63.

2W. Prerrer, ‘ Druck und Arbeitsleistung durch wachsende Pflanzen,” Abhandl.
d. k. sdchs. Ges. d. Wiss. zu Leipzig, math.-physik. Klasse, Vol. XX (1893), p. 235.

3H. RoDEWALD, ‘‘Ueber die durch osmotische Vorginge mégliche Arbeitsleistung
der Pflanzen,” Ber. d. deutsch. bot. Ges., Vol. X (1892), pp. 83-93.

90 DIFFUSION AND OSMOTIC PRESSURE

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 leay-
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. ABSORPTION 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 OsSMOTIO 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 asimple 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.

ei A comparatively small amount of the absorbed water is
used as food material in the processes of photosynthesis,
growth, and general metabolism. f] In the lower forms without
chlorophyll the exposure of a wet membrane to the atmos-

94 DIFFUSION AND OsmMoOTIO 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 this.
Thus, some submerged aquatics may perhaps absorb equally
throughout the whole extent of their comparatively thin epi-
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,' 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.

Il. 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 sat-

1J, VEsqus, “‘ De l'influence de la température du sol sur l’absorption de l’eau
par les racines,” Ann. sci. nat. bot., Ser. VI, Vol. VI (1877), pp. 169-209.

96 DIFFUSION AND OSMOTIO 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 97

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 OSMOTIO 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 7s 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’ in the case of honey-dew, and by Dandeno’ in the
case of guttation drops. Also Moll* 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‘ 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,

1G. BonNIEE, “ Recherches expérimentelles sur la miellée,” Rev. gen. bot., Vol.
VIII (1896), pp. 1-22,
2J.B. DanDeno, “An Investigation into the Effects of Water, etc., on Foliage
Leaves,” Trans. Canad. Inst., Vol. VII (1901), pp. 238-350.
8J. W. Mout, “ Untersuchungen fiber Tropfenausscheidung u. Injection von Blat-
tern,” Verslagen en Mededeel. d. k. Akad. v. Wetensch. te Amsterdam, Vol. XV (1880),
Pp. 237-337,

4W. P. Wruson, “ The Cause of the Excretion of Water on the Surface of Nec-
taries,” Unters. aus d. bot. Inst. zu Tubingen, Vol. I (1881), pp. 1-22.

ABSORPTION 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
dryest days. Nectaries may even be artificially formed in this
way ; if minute granules of sugar are placed upon the hypha
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 OsMoTIO 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
begin 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,' 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 cellsap. 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 a 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
hyphe 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

1H. Haupt, ‘ Zur Secretionsmechanik der extrafloralen Nektarien,” Flora, Vol.
XC (1902), pp. 1-41. On the general subject of secretion, see W. PFEFFER, 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, z.¢., 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.’

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

1See p. 78.

102 DIFFUSION AND OsmoTIO 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, ¢.g., those which can be brought about in almost any
cell by chemical agents like H, Cl,, 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’ 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’
in the case of Cocos and Arengo. A series of experiments
upon this subject was performed by Wieler,’ 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

1F, Ottmanns, “Ueber d. Bedeutung der Koncentrationsanderung des Meer-
wassers fiird. Leben d. Algen,” Sitzungsber. d. k. preuss. Akad. d. Wiss. zu Berlin,
Jahrg. 1891, p. 183.

2H. Moutscu, ‘Die Secretion des Palmenweins und ihre Ursachen,” Oesterr.
bot. Zeitschr., Vol. XLIX (1899), p. 74. Also see W. Fiapor, ‘Untersuchungen tber d.
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.
a Wre.er, ‘Das Bluten der Pflanzen,” Cohns Beitrdge, Vol. VI (1893), pp.

4C. Kraus, ‘‘ Untersuchungen fiber den Saftedruck der Pflanzen,” Flora, Vol.
XL (1882), pp. 2 ff,

ABSORPTION AND TRANSMISSION OF WATER 103

Pitra.". 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-
ing; 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,’ 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

1A, Prrra, ‘‘ Versuche fiber d. Druckkraft d. Stammorgane bei d. Erscheinun-
gen des Blutens u. Thranens,” Jahrb. f. wiss. Bot., Vol. XI (1877), pp. 437-530.

2H. Mo.isca, ‘‘ Ueber localen Blutungsdruck und seine Ursachen,” Bot. Zeitg.,
Vol. LX (1902), pp. 45-63,

104 DIFFUSION AND OsmoTiIo PRESSURE

phenomena occurring within the cells. 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’ and later, apparently independently, by Fuchs.°
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

1W. Prerrer, Osmotische Untersuchungen, Leipzig, 1877, p. 223.

2 Ibid., pp. 222-5.

3K, Fucus, ‘‘Zur Theorie der Bewegung des Wassers im lebenden Pflanzenkor-
per,” Beth, Bot. Centralbl., Vol. X (1901), pp. 805-8.

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
Pfeffer.' 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’ has actually, constructed a

1W. PFEFFER, Osmotische Untersuchungen, Leipzig, 1877, p. 225.

Fe E. B. CopE.ann, “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,’
when he records that, if a bleeding plant is deprived of oxy-
gen, exudation stops. He observed the same result when
the plant was anesthetized 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.’

The whole subject of exudation and sap pressure is viewed
in an entirely new light by Molisch* 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.

1A, WIELER, ‘‘ Das Bluten der Pflanzen,” Cohns Beitrdge, Vol. VI (1893), p. 158.
2Ibid., pp. 65, 69.

3H. Moxtscu, ‘‘ Ueber localen Blutungsdruck und seine Ursachen,” Bot. Zeitg.,
Vol. LX (1902), pp. 45-63.

ABSORPTION AND TRANSMISSION OF WaTER 107

(4) Summary of water 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' and Molisch* 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 water in trees and other
tall plants. — Owing to almost insurmountable difficulties in

1F, Czapex, “Zur Lehre von den Wurzelausscheidungen,” Jahrb. f. wiss. Bot.,
Vol. XXIX (1896), pp. 321-90.

2H. Mouiscu, “ Ueber Wurzelausscheidungen und deren Einwirkung auf orga-
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 trachex, 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’ 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

1This theory, as far as I know, was first clearly put by GopLEWsEI in his paper
entitled “ Zur Theorie d. Wasserbewegung in den Pflanzen,” Jahrb. f. wiss. Bot., Vol.
XV (1884), pp. 569-630. WESTERMAIER 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 JANSE, whose ideas
are expressed in a paper entitled “‘ Die Mitwirkung des Markstrahlen bei der Wasser-
bewegung im Holze,” Jahrb. f. wiss, Bot., Vol. XVIII (1887), pp. 1-69. The last-named
author elaborated Godlewski’s 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 tracheze 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 trachex, 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 trachee 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’ came to the conclusion
that this process, while it is probably a general property of

1A, Wrever, “Das Bluten der Pflanzen,” Cohns Beitrdge, Vol. VI (1893), pp.
1-211,

110 DIFFUSION AND OSMOTIO PRESSURE

protoplasm, yet plays no leading réle in the lifting of water
up tall stems. As far back as 1853 T. Hartig’ 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’ performed the same experiment more
thoroughly. Trees were cut off and set into tubs of poison,
such as aqueous solutions of CuSO, 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

1T, Hartia, “Ueber die endosmotischen Eigenschaften der Pflanzenhaute,”
Bot. Zeitg., Vol. XI (1853), pp. 309-17.

2K. STRASBURGER, Ueber den Bau und die Verrichtungen der Leitungsbahnen
in den Pflanzen, Jena, 1891; DEM, “Ueber das Saftsteigen,” Histologische Beitrdge,
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,’ 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.? 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,’ Askenasy,‘ and Copeland.’ 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. Drxon, “Note on the Role of Osmosis in Transpiration,’ Proceed. Roy.
Irish Acad., Sec. ITI, Vol. 1II (1898), pp. 767-75.

2M. H. Durrocuet, Mémoires pour servir & UVhistoire anatomique et physiolo-
gique des végétaux et des animaux, Brussels, 1837.

8J, Boum, ‘“Capillaritat und Saftsteigen,” Ber. d. deutsch. bot. Ges., Vol. XI
(1893), pp. 203-12.

4H, ASKANASY, ‘“‘Beitrage zur Erklarung des Saftsteigens,” Verhandl. d. naturl.-
mei, Verein zu Heidelberg, N. F., Vol. V (1896), pp. 429-48.

5E. B. CopeLAND, “The Rise of the Transpiration Stream,’ Bot. Gaz., 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 3mm. in diameter and 12.4m. high,
which terminated below in a mercury manometer and above
in a Cu, Fe(CN), 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 CuSO,. Evaporation from the
surface of this solution caused, in five days, a suction on the
manometer below of 301mm. 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 459mm. of mercury (1 atmosphere
minus 301mm.). 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
anesthetics.’ Such reagents act in a similar manner to the

1For a discussion of the relation of anmsthetics to transpiration see H. H.
Dixon, ‘On the Effects of Stimulation and Anesthetic Gases on Transpiration,”
Proceed, Roy. Irish Acad., Ser. ITI, Vol. TV (1898), pp.618-26 ; H. JUMELLE, “Influence des
anesthétiques sur la transpiration chlorophyllienne,” Rev. gen. bot., Vol. II (1890),
pp. 417-32; IpmM, “Nouvelles recherches sur l’assimilation et la transpiration
chlorophylliennes,” ibid., Vol. III (1891), pp. 241-88, 293-305; A. SCHNEIDER, “The
Influence of Anesthetics on Plant Transpiration,” Bot. Gaz., Vol. XVIII (1893), pp.
57-69; A. Woops, ‘Some Recent Investigations on the Evaporation of Water from
Plants,” ibid., Vol. XVIII (1893), p. 304-10.

ABSORPTION AND TRANSMISSION OF WateER 118

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 CO, in the water in which are placed cut twigs,
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 CO,,
since this gas penetrates all protoplasts with the utmost ease.
Possibly the CO, 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

1P, Kossarorr, ‘‘ Die Wirkung der Kohlensaure auf den Wassertransport in den
Pflanzen,” Bot. Centralbl., Vol. LX XXIII (1900), pp. 13844,

114 DIFFUSION AND OSMOTIO PRESSURE

water of the substratum in which the organism is growin
are to be regarded as one continuous mass of liquid. Thu
if the diffusion tension of water in any part of the pla
becomes less than it is at any other point, diffusion tak:
place and equilibrium is restored. In the same way, if tl
diffusion tension within the plant falls below that of tl
substratum, diffusion of water into the plant must imm
diately occur.

This process of simple diffusion is sufficient to accour
for absorption and for transport in the simple plant bodic
and in any small portion of larger bodies. But in the moi
complex bodies of higher plants this is not sufficient. Ju:
how the sap is raised in trees is not surely known. Ther
are at present two main theories to account for it: (1) It i
supposed to be raised by periodic pumping action of livin
cells in the trunk. (2) It is supposed that evaporation an
the resulting osmotic concentration in the leaves will dra:
it up from the roots, the cohesion of the minute wate
columns being supposed to be of sufficient magnitude t
prevent their being broken by the strain.

CHAPTER III
ABSORPTION AND TRANSMISSION OF SOLUTES

Witu the exception of the naked ameeboid cells occur-
ring in certain stages of the life histories of a few fungi and
alge, 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-
tior 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 OsmoTio 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." By a number of
researches’ 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.

1F, 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. Brownz and F. Escomsn, ‘‘ Static Diffusion of Gases and Liquids in Relation
to the Assimilation of Carbon and Translocation in Plants,” ibid., Vol. CXCIIL
(1900), pp. 223-92,

2N. J.C. MULurr, ‘ Untersuchungen fiber d. Diffusion atmosph4rischer Gase in
der Pflanzeund der Gasausscheidung unter verschiedenen Beleuchtungsbedingungen,”
Jahrb. f. wiss. Bot., Vol. VII (1869), pp. 143-92; E. LrerzmMann, “ Ueber die Permeabili-
tat vegetabilischer Zellmembranen in Bezug auf atmosphirische Luft,” Flora, Vol.
LXX (1887), pp. 339-86; J. Wiesner and H. Mouiscu, * Untersuchungen tber die
Gasbewegung in der Pflanze,” Sitzwngsber. d. kais. Akad. d. Wiss. zu Wien math.-
nat. hist. Klasse, Vol. KCVIII (1889), Abth. 1; P. CLAusgEn, ‘‘ Ueber die Durch-
lassigkeit der Tracheidenw&nde ftir atmospharische Luft,” Flora, Vol. LXX XVIII
(1901), pp, 422-69.

ABSORPTION AND TRANSMISSION OF SouutEs 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.’

1F,F, Buackman, “Experimental Researches,” etc.: TI, ‘On the Paths of
Gaseous Exchange between Aerial Leaves and the Atmosphere,” Phil. Trans. Roy.
Soc., London, B., Vol. CLXXXVI (1895), pp. 503-62.

118 DIFFUSION AND OsmMoTIO PRESSURE

Il. 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.’

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,

10, H. von MAYENBURG, “ Losungsconcentration und Turgorregulation bei den
Schimmelpilzen,” Jahrb. f. wiss. Bot., Vol. XXXVI (1901), pp. 381-420,

ABSORPTION AND TRANSMISSION OF SoutuTes 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. Ifa 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 NO; 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,’ using equivalent quantities of several salts, found

1E. DEmorssy. “ Absorption élective de quelques éléments minéraux par les
plants,” Compt. rend., Vol. CXXVII (1900), pp. 970-73; cf. P. Bovcrcet, ‘Sur
labsorption de l'iode par les végétaux,’’ ibid., Vol. CNXIX (1899), pp. 768-10; IDEM,
same title, Bull. Soc. chim. de Paris, Ser. III, Vol. XXIII (1899), pp. 40, 41.

120 DIFFUSION AND OSMOTIO PRESSURE

that KNO, was taken out of a mixed solution two to six
times as fast as NaNO; or Ca(NO3).. 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’ 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,’ 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. Prerrer, “Ueber Election organischer Nahrstoffe,” Jahrb. f. wiss. Bot.,
Vol. XXVIII (1895), pp. 205-68.

2PuusT has recently shown that, while copper ions are readily absorbed by
Mucor, Aspergillus, and Botrytis, they are not taken in by Penicillium. See C.
Putst, ‘‘ Die Widerstandsfahigkeit einiger Schimmelpilze gegen Metallgifte,” Jahzb.
f. wiss. Bot., Vol. XXXVII (1902), pp, 205-63,

ABSORPTION AND TRANSMISSION OF SouuTsEs 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 NO, 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
KNO,, 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 KNO,, 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 OSMOTIO 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 KNO, 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 KNOg, this
condition cannot last long; inward diffusion of K and NO,
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' 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 SO,
ions which may penetrate the living cells? Or shall it be

1N. Ono, “ Ueber die Wachsthumsbeschleunigung einiger Algen und Pilze durch
chemische Reize,” Jour. Coll. Sci. Imp. Univ. Tokyé, Vol. XIII (1900), Part I: Bot.
Mag., Vol. XV (1900), p. 15; reviewed in Bot. Gaz., Vol XXX (1900), p. 422.

126 DIFFUSION AND OsmoTIO 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 CuSO,
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 itis 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, 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 CuSQ,, 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, KNO3, NaCl, etc., but dies
even in a weak solution of HgCl, or CuSQ,, 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 OsmMoTIO PRESSURE

that growth takes place more slowly in a concentrated solu-
tion than in a weaker one. Loeb’ 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,’ 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.’

The first of a very important series of observations dealing
with the effect of external concentration upon cell division
was made by Loeb* 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. Los, Untersuchungen zur physiologischen Morphologie der Thiere. II:
Organbildung und Wachsthum, Warzburg, 1892.

2E, Yuna, ‘‘De Vinfluence des variations du milieu physico-chimique sur le
développement des animaux,” Arch. des sci. phys. et nat., Vol. XIV (1885), pp. 502-22,

3C. B. DAvENPORT, Experimental Morphology, New York, 1899, p. 365.

4J. Lous, “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’ 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,’ 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-

17, H. Morean, “The Effect of Salt Solutions on Unfertilized Eggs of Arbacia,”
Science, N.S., Vol. VII (1898), p. 222,

2J. Lorex, ‘On the Nature of the Process of Fertilization and the Artificial
Production of Normal Larve (Plutei) from the Unfertilized Eggs of the Sea
Urchin,” Am. Jour. Physiol., Vol. III (1899), pp. 135-8; rpeM, ‘‘On the Artificial
Production of Normal Larve from the Eggs of the Sea Urchin (Arbacia),” ibid.,
Vol. III (1900), pp. 434-71; rpem, ‘‘On Artificial Parthenogenesis in Sea Urchins,”
Science, N.S., Vol. XI (1900), pp. 612-14; 1pmm, ‘‘ Further Notes on Artificial Par-
thenogenesis and the Nature of the Process of Fertilization,” 4m. Jour. Physiol.,
Vol. TV (1900), pp. 178-84; 1pEM, “ Artificial Parthenogenesis in Annelids (Chwtopte-
Tus),”” Science, N.S., Vol. XII (1900), p. 170.

130 DIFFUSION AND OSMOTIO PRESSURE

tial feature in this increase in the osmotic pressure of the
surrounding solution is a loss of water on the part of the
egg.”

Furthermore, the same author showed that a similar arti-
ficial parthenogenesis may be brought about in the case of
Chetopterus, 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’ 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’ 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,’? who
worked on the germination of seeds. With growing plants,
Stange’ 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

1J. Lorn, M. Fiscuer, AND H. Neruson, “ Weitere Versuche fiber kfinstliche

Parthenogenese,” Vorlaufige Mittheilung, Pyliigers Arch. f. d. ges. Physiol., Vol.
LXXXVII (1901), pp. 594-6.

2M. Janius, “Ueber d. Einwirkung von Salzlosungen auf d. Keimungsprocess d.
Samen einiger einheimischer Culturgewdchse,” Landwirtsch. Versuchs-Stat., Vol.
XXXIT (1886), pp. 149-78.

3B. STanes, “ Beziehungen zwischen Substratconcentration, Turgor und Wachs-
tum bei einigen phanerogamen Pflanzen,” Bot. Zeitg., Vol. L (1892), p. 253.

INFLUENCE OF THE MEDIUM 131

those of the cambium. Vandervelde’ 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.’ 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* 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

14.J. J, VANDERVELDE, “Ueber den Einfluss des chemischen Reagentien und
des Lichtes auf die Keimung der Samen,” Bot. Centralbl., Vol. LXIX (1897), pp. 337-42.

2B. E. Suosson AnD B. C, Burrum, ‘Alkali Studies II,” Bulletin 39, Wyoming
Agric. Exp. Sta. (1898); B.C. Burrum, ‘Alkali Studies III,” Ninth Annual Report,
Wyoming Agric. Exp. Sta. (1899); E. E, Suosson, “Alkali Studies IV,” ibid. (1899) ;
B. C. Burrum anp E. E. Suiosson, ‘Alkali Studies V,” Tenth Annual Report,
Wyoming Agric. Exp. Sta. (1900).

3F. EscHENHAGEN, Ueber den Einfluss von Lésungen verschiedener Koncentra-
tion auf das Wachsthum von Schimmelpilze, Stolp, 1889,

132 DIFFUSION AND OSMOTIO PRESSURE

cane-sugar solution, that is, about 111.5 atmospheres. Raci-
borski’ found the maximum concentration for growth of
Basidiobolus was that of a 6 per cent. solution of NaCl, or
about seventeen atmospheres. Yasuda’ 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*® 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.

1M. Racrporsxi, “Ueber den Einfluss Ausserer Bedingungen auf die Wachs-
thumsweise des Basidiobolus ranarum,” Flora, Vol. LXXXII (1896), pp. 107-32.

2ATsusHr YAsuDA, ‘Studien fiber die Anpassungsfahigkeit einiger Infusorien
an concentrirte Lésungen,” Jour. Coll. Sci. Imp. Univ. Tokyo, Vol. XIII (1900), pp.
101-40. Reviewed in Bot. Gaz., Vol. XXX (1900), p. 285.

3B. E. Livinesron, (1) ‘On the Nature of the Stimulus Which Causes the
Change of Form in Polymorphic Green Algw,’? Bot. Gaz., Vol, XXX (1900), pp. 289-
317; IpEM, (2) “‘ Further Notes on the Physiology of Polymorphism in Green Algs,”
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; MgSO,, KNO,, and K,HPO,, 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 OsMOTIO PRESSURE

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,’ 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’ may

1 VAN RYSSELBERGHE, “ Réaction osmotique des cellules végétales A la concentra-
tion du milieu,” Mem. cour., pub, par 1’Acad. roy. de Belg., Vol. LVIII (1898), pp. 1-101.

2In a cylinder, the lateral surfaceis 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 8, and it is often 4
andeven 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 zodspores 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 inthe
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 OSMOTIO 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. Ifthis 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’ 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’ 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’ that this salt sometimes has a poisonous action.

1M. Racrsorsxi, ‘‘ Ueber d. Einfluss ausserer Bedingungen auf d. Wachsthums-
weise des Basidiobolus ranarum,” Flora, Vol. LXXXII (1896), pp. 107-32.

2 J. BEAUVERTIE, ‘Influence de la pression osmotique du milieu sur la forme et la
structure des végétaux,” Compt. rend., Vol. CKXXII (1901), pp. 226-29.

3R.H.Trvue, ‘The Physiological Action of Certain Plasmolyzing Agents,”’ Bot.
Gaz., Vol. XXVI (1898), pp. 407-16.

138 DIFFUSION AND OsmoTiIc PRESSURE

Beauverie found that fungus hyphe 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
hyphe 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 aérotropism. 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 alge and fungi.

b) The influence of external concentration upon repro-
duction.—Raciborski’ states that concentrated solutions
check the formation of zygospores in Basidiobolus. In
concentrated solutions Stigeoclonium’ failed to produce any

1M. Racrporsxt, “‘ Ueber d. Einfluss Ausserer Bedingungen auf d. Wachsthums-
weise des Basidiobolus ranarum,” Flora, Vol. LXXXII (1896), pp. 107-32.

2B. E. Livinaston, (1) “On the Nature of the Stimulus Which Causes the
Change of Form in Polymorphic Green Algw,” Bot. Gaz., Vol. XXX (1900), pp. 289-
317; IDEM, (2) ‘‘Further Notes on the Physiology of Polymorphism in Green Algew,”
ibid., Vol. XXXII (1901), pp. 292-302.

INFLUENOE OF THE MEDIUM 139

zoéspores, but these were formed in great numbers, and
very rapidly, in the weak solutions. Since the formation
of zodsporis 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) The 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. Richter’ states that zodspores of Tetraspora
lose their activity in strong solutions, but regain it on being
returned to normal sea-water. The writer found that zodspores
of Stigeoclonium lose their power of movement in concen-
trated solutions. Of interest here is also the observation of
Engelmann’ that the cilia of the epithelial cells which line
the frog’s cesophagus 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’ gives a very striking account of the reversal of a
tropism by osmotic extraction of water. At ordinary tem-
peratures the larve 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.

1A. RicuTeR, ‘ Ueber die Anpassung der Sfsswasseralgen an Kochsalzldsungen,”
Flora, Vol. L (1892), pp. 4-56.

2T, W. ENGELMANN, “Ueber die Flimmerbewegung,” Jena Zeitschr., Vol. IV
(1868), pp. 321-479.

3J, Lorn, ‘‘ Ueber ktinstliche Umwandlung positiv heliotropischer Thiere in
negativ heliotropische und umgekehrt,” Pfligers Arch. f. d. ges. Physiol., Vol. LIV
(1893), pp. 81-107; 1pEM, Physiology of the Brain, New York, 1900, p. 198.

140 DIFFUSION AND OsmoTio PRESSURE

A similar reversal of tropism, in this case of geotaxis, was
observed in Chromulina woroniniana by Massart.' 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 osmotamis, 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’ has recently devoted an article to the discussion
of this subject. The following facts are mainly derived
from this source, Stahl’ 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.

JJ. Massart, ‘‘La sensibilité& la concentration chez les é6tres unicellulaires
marins,” Bull, del’acad. roy. de Belgique, Ser. III, Vol. XXII (1891), pp. 148-67.
2W. Roruert, ‘‘ Beobachtungen und Betrachtungen fiber taktische Reizerschei-
nungen,” Flora, Vol. LXXXVIII (1901), pp. 371-421.
r a E. Stagx, “Zur Biologie der Myxomyceten,” Bot. Zeitg., Vol. XLII (1884), pp.
45 ff.

INFLUENOE OF THE MEDIUM 141

Massart,’ 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.,

t’
then it would decrease to ear er at t’°C. The pres-

sure of the external solution will decrease according to the

1 J, Massart, ‘La sensibilité & la concentration chez les 6tres unicellulaires
marins,” Bull. de Vacad, roy. de Belgique, Ser. III, Vol. XXIT (1891), pp. 148-67; IDEM,
“Sensibilité ot adaptation des organismes & la concentration des solutions salines,”
Arch, de biol., Vol. IX (1899), pp. 515-70.

142 DIFFUSION AND OSMOTIO 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
RB—-i~ DB +e’
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 larve 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 latterof 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,’ 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’ has shown that by cooling Stentor
cerulcus 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’® 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.

1B, E. Livrneston, “ Further Notes on the Physiology of Polymorphism in Green
Alge,” Bot. Gaz., Vol. XXXII (1901), pp. 292-302,

2A. 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.

3A. W. GREELEY, ‘Artificial Parthenogenesis Produced by a Lowering of the
Temperature,” Am. Jour. Physiol., Vol. VI (1902), pp. 296-804,

144 DIFFUSION AND OSMOTIO 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 wallsis 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
reversed by a sudden change in the osmotic surroundings.
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.

INDEX

ABSORPTION: of gases, 115; of solids and
liquids, 118; of solutes, 115.

Acrps: influence of, on permeability, 61,
74; penetrating power of, 64.

ACETANILID, in plasmolysis, 63.
ACETONE, in plasmolysis, 63.

AOTION OF PROTOPLASMIO 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.

AN ZSTHETICS, effect of, on pormeability,

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, eges of, 129, 130,

Asct, bursting of, 54.

ATMOSPHERE, internal, 120.

ATOMIC THEORY, 3.

AvoGapno, principle of, 11.

Bacteria, plasmolysis of, 58, 61,
BACTERIUM TEEMO, plasmolysis of, 61.
Bases, influence of, on permeability, 61,

BASIDIOBOLUS, in ‘osmotic solutions, 182,

BEAN, permeability of, 65.
BEAUVERIE, 137,
BECKMANN, 37, 89.

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.
BU, methyl, penetrating power of, 66,

Boum, 111.

Bonntep, 73, 98.

BovurLHac, 70.

BourGer, 69, 119.

Bower, 88.

Boy te, principle of, 10,
Browne AND Escomss, 116,
BuUFFUM AND SLosson, 113.
BurGARSzzKY, 18.

BURSTING OF CELLS, 54.
CABBAGE, pressure of sap, 86.

CaFFEIN: in plasmolysis, 63; penetrating
power, 64, 77.

Caucitum: 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.
CHAETOMORPBA, permeability of, 62.
CHAETOPTERUS, chemical fertilization of,

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.

Ci1uIA, in osmotic solutions, 139.

CLAUSEN, 116.

Cocos, exudation pressure of, 102.

Copium, permeability of, 78.

CorFrrIcIents, isosmotic, 56.

CoHNHEIM, 83.

Co.Lorps, 27, 49.

CONDUCTIVITY OF:SAPS, 85.

CopELAND, 77, 83, 88, 105, 108, 111.

COPELAND AND KAHLENBERG, 69,

Coprrops, reversal of tropism in, 189,

145

146

DIFFUSION AND OsmMoTIC PRESSURE

CopPER, accumulation of, 69.

COPPER FERROCYANID MEMBRANE, 82, 112,
CoPpPER SULFATE, 110.

Courin, 68.

CrYSTALLOIDS, 49.

CurcUMA, permeability of, 62,
Curtis, 54,

CURVATURE, role of turgidity in, 89.
CYNARA, 78.

CZAPER, 72, 74,

DANDENO, 22, 68, 71, 72, 98.
DAVENPORT, 31, 128.

DEATH, theory of, 75.

Demovssy, 67, 119.

DERO VAGA, regeneration of, 128.
Devaux, 69,

DE Vries, 55, 56, 61, 62, 64, 65, 74, 77, 83, 84.
Dr1Frvusron, of gases, 9.

DirrvusiIon TENSION, of solvent, 30.
DIGESTION, outside the body, 71, 72.
Drxon, 111, 112.

DutTrocHer, 111.

Dyzs, anilin, penetrating power of, 66.
EcrToruast, 51, 53, 80, 82.

Eaas, parthenogenesis of, by cold, 143.
ELECTROLYTES, in plasmolysis, 56.
ELODEA, permeability of, 66.

EmMBEYO, permeability of, 72.
ENDOSPERM, permeability of, 72.
ENGELMANN, 139.

ENVIRONMENTAL FACTORS, 47.
ENzyYMES, penetrating power of, 71, 72.
EPrIpEeRMIs, permeability of, 117.
ESCHENHAGEN, 1381.

ETHER, ethyl, in plasmolysis, 63.
ERRERA, 59.

Evruorsia, nectaries of, 79.
EvAPorRATION, 110.

EXxvuDATION: nature of, 73; from wounds,
102; from glands, 71, 96.

EXUDATION PRESSURE, theory of, 104.
FacopyroM, solutes of, 83.

FEHLING’S SOLUTION, 65.

FERRIc PYROLIGNATE, 110.
FERTILIZATION BY COLD, 143.

Fick, 18,

FILTER THEORY OF SEMIPERMEABILITY,

Fuostn, 82.

Form, retention of, 87.
FORMALDEHYDE, 63,

FRAZEUR, 128,

Fucus, 104.

Fucus, permeability of, 74.
Funai, in osmotic solutions, 138.

FuRFUROL, 63.

Gasus: diffusion tension of, 9; mixed,
11; absorption of, 115; transmission of,
120.

GaAy-LussAc, principle of, 10.

GLycERIN, in plasmolysis, 56, 62, 63, 64,
67, 71, 79.

GuvucosEe: in turgor, 84; penetrating
power of, 61, 65, 67, 70, 79.

GODLEWSEI, 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.

Harrie, 110.

Haoupt, 79, 100.

HeEaxts: in osmotic solutions, 129; effect
of cold upon, 143.

HEALD, 69, 85.
Hep, 57, 83.

HELIANTHUS: permeability of, 65, 74;
solutes of, 83, 84.

HiIpoure, 78.

Hoseg, 18, 83.

HONEY-DEw, 98.

IMBIBITION, 93.

InrusoktiA, in osmotic solutions, 182.
INTERNAL ATMOSPHERE, 120,

Iop1n, penetrating power of, 68, 69.
Ions, in absorption, 53.

IonIzATION: of gases, 23; of solutes in
liquids, 24.

IRRITABILITY, changes of, 139.
JaRrIvs, 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.
KouLRauscg, 42.

KoHLRAUSCH AND HoOLzorn, 42, 43.
KO6ppPsE, 57.

Kossanrorr, 113.

KOvEst, 83,

KRABBE, 75.

Krats, C., 102.

Kraus, G., 83.

INDEX

147

LAvreEnt, 67, 71.

LEAVES: absorption by, 68 103; exuda-
tion from, 99; guttation of, 74; permea-
bility of, 71.

LECETHINS, 81.

LEaMaAn, 69.

LEITZMANN, 116.

LEMNA, permeability of, 76.

Lick, reversal of tropism in, 141.

Lrprorss, 54.

LIEBEEMANN, 18.

LILIACES, 69.

Liquips: absorption of, 118; diffusion
of, 12, 13.

LIVINGSTON, 132, 138, 143.

Los, 57.

Logs, 53, 74, 128, 129, 189.

LOEB, FISCHER, AND NEILSON, 130.

Lupinus: in osmotic solutions, 130; so-
lutes of, 84.

MacDovuaat, 69, 119.

MAQUENNE, 74, 84, 85.

Massanrvt, 61, 140, 141.

MAtTRUCHOT AND MOLLIARD, 70, 75.

Matter: nature of, 3; states of, 6.

Mayer, 84.

MeEpivM, influence of, 124,

MEERBURG, 36, 82.

MEMBRANES: cellulose, 51; copper ferro-
cyanid, 82,

MEMBRANES: protoplasmic, 49; action
of, 80.

MERCURIC CHLORID: effect on permea-
bility, 74: penetrating power, 68.

METHYL BLUE, penetrating power of, 66,

METHYL OYANID, in plasmolysis, 63.

METHYL VIOLET, penetrating power of, 66,

Mimosa, pulvini of, 77.

Moat, 98.

Mo ps, bursting of, 54.

Mo isc#, 65, 72, 75, 102, 103, 106.

Moreay, 129.

Morse AND Horny, 36.

Mucor, exudation from, 99.

MULLER, 116.

Muscxg, permeability of, 74.

MyRIOTONIE, 59.

Naccazt, 18,

NATHANSOHN, 78.

Necrarres: artificial, 99; conditions for
secretion of, 101; theory of, 96.

Nernst, 18.

NEENST-PALMER, 37, 38, 40.

NITRATES, test for, 65.

NOLL, 54.

Nostoc, permeability of, 70.

NucueEt: frozen, dried, ete., 76; in osmo-
tic solutions, 128.

OxutTMAnNns, 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-WALEEB, 16, 40, 42.

OVERTON, 63, 71, 81.

OxyGEN: absorption of, 115; effect of,

on permeability, 78; penetrating power
of, 70.

PARAMaECIA,*permeability of, 63.
PARTHENOGENESIS: 129; by cold, 143.
PAR, 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.

EESrens 35, 50, 55, 64, 66, 68, 77, 78, 104, 105,

PHASEOLUS: in osmotic solutions, 130,
ae permeability of, 65, 78; solutes of,

PHENOL, in plasmolysis, 63.

PHLOROGLUCIN, in plasmolysis, 63.

PHOSPHORIC ACID, from roots, 72.

PHOTOSYNTHESIS, 117.

PET OLOOU A: penetrating power of sap
of, 98.

Picric ACID, 110.

PrLOBOLus, 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.

Porsons, permeating power of, 68.

POLLEN GRAINS, bursting of, 54.

ECL TaSED ee, reversal of tropism in,

PoLimorPHism: in Basidiobolus, 137; in
fungi, 138; in Stigeoclonium, 132,

Porassium: absorption of, 67, 120; pene-
trating power of, 69.

POTASSIUM CHLORID, in turgor, 83.

148

DIFFUSION AND OSMOTIC PRESSURE

PoTASSsIUM 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.

QUINCEE, 82.

RACIBORSKI, 132, 137, 138.

REINHARDT, 88,

REPRODUCTION, in osmotic solutions, 138.

RIcHARDS, 68.

RicHTEs, 189.

R1isE OF WATER, 107,

Root HarRs, exudation from, 75.

Roots, permeability of, 69,

RotTuHERT, 140,

Sauts, inorganic, penetrating power of,

SAMBUCUS, permeability of, 76.

Sap: cell, 52; expressed, conductivity of,

SARGENT, 128,

ScHEFFER, 18,

ScHNEIDER, 112.

ScHWENDENER, 76.

SrEps, in osmotic solutions, 130, 131.

SELECTIVE POWER, 118.

SIPHONEA, bursting of, 54.

SEKERTSCHLEY, 69.

SLosson, 131.

SopiuM, penetrating power of, 69.

SopIUM CHLORID: in plasmolysis, 61;

peneetane power of, 68, 74; in me-
ium, 137.

SopruM NITRATE: absorption of, 120;
penetrating power of, 79.

a absorption of, 118; diffusion of,

SoLANuM, absorption by, 69.

SoLurTEs: absorption of, 115; active, na-
ture of, 83; transmission of, 120,

SoututTion, Knop’s, 132.

SoLuTions: defined, 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.

SprroayRa: cold plasmolysis of, 75, 141;
permeability of, 62, 65. 68,

STARL, 140,

STANGE, 84, 130.

STENTOR, cold plasmolysis of, 75, 143,

Strvens, 69,

SricHococcus, permeability of, 70.

StranocLonium: drying out of, 143; in
osmotic solutions, 132, 138; zoospores
in solutions, 139,

STomMATA, 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.

TAaMMAN, 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.

TuuyA, transpiration of, 111.

TonrIE, 59.

TONOPLAST, 52, 53, 80, 82.

TRADESCANTIA, permeability of, 56, 62,

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,

TuURGIDITY: nature of, 49, 52, 53; main-
tenance of, 86, 121; relation of, to activ-
ity, 87.

Tureor, 52, 55, 74.

TuENIP, pressure of sap of, 85,

UREA: in plasmolysis, 62; penetrating
power of, 77, 79.

VACUOLE, 52,

VANDERVELDE, 131.

Van RyssELBERGHE, 76, 134,

Van't Horr, 25, 37, 40.

VEGETABLE MARROW, pressure of sap of,

VESQUE, 95.
VIcIA: nectaries of, 79; roots of, 65.
wigeen, methyl, penetrating power of,

VoOIGTLANDER, 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 muse e, 53: in cellu-
lore galls 52; ; loss of, 95; transmission
of, 95.

WATER PORES, 74, 96.
WESTERMEIER, 108,
WHETHAM, 43,
WIELER, 65, 102, 108,

WiesnER AND Mo.isog, 116.
WILSON, 98.

WLADIMIROFF, 58,

Woops, 112.

WorrMann, 66,

YASUDA, 182.

ZEA, solutes of, 83.

ZOOSPOBRES, in osmotic solutions, 139.