STIMULUS AND RESPONSE
GENERAL RELATIONS
=144. The nature of stimuli.= Whatever produces a change in the
functions of a plant is a stimulus. The latter may be a force or a
material; it may be imponderable or ponderable; effect, not character,
determines a stimulus. Consequently, reaction or response decides what
constitutes a stimulus. The presence of the latter can be recognized
only through an appreciable or visible response, since it is impossible
to discriminate between an impact which produces no reaction and one
which produces a merely latent one. From this it is evident that
quantity is decisive in determining whether the impact becomes a
stimulus. Plants grow constantly under the influence of many stimuli,
all varying from time to time in amount. Small changes in these are so
frequent that, in many cases at least, the plant no longer appreciably
reacts to them. Such changes, though usually measurable, are not
stimuli. Furthermore, it must be clearly recognized that plants which
are in constant response to stimuli are stimulated anew by an efficient
increase or decrease in the amount of any one of these. As is well
known, however, such increase or decrease is a stimulus only within
certain limits, and the degree of change necessary to produce a response
depends upon the amount of the factor normally present. The entire
absence of a force usually present, moreover, often constitutes a
stimulus, as is evident in the case of light. The nature of the plant
itself has a profound bearing upon the factors that act as stimuli. Many
species are extremely labile, and react strongly to relatively slight
stimuli; others are correspondingly stable, and respond only to stimuli
of much greater force. Some light is thrown upon the nature of this
difference by the behavior of ecads. A form which has grown under
comparatively uniform conditions for a long time seems to respond less
readily, and is therefore less labile than one which is subject to
constant fluctuation. In many cases this is not true, however, and the
degree of stability, i. e., of response, can only be connected in a
general way with taxonomic position.
=145. The kinds of stimuli.= The factors of a habitat are external to
the plant, and consequently are termed _external_ stimuli. Properly
speaking, all stimuli are external, but since the response is often
delayed or can not be clearly traced, it may be permissible to speak of
_internal_ stimuli, i. e., those which appear to originate within the
plant. These, however, are extremely obscure, and it is hardly possible
to deal with them until much more is known of the action of external
stimuli. Of the latter, certain forces, gravity and polarity, act in a
way not at all understood, and as they are essentially alike for all
plants and all habitats, they can here be ignored. Stimuli are
imponderable when, like light and heat, they are measured with reference
to intensity, and ponderable, when, as in the case of water-content,
humidity, and salt-content, they can be expressed in mass or weight. It
is undesirable to insist upon this distinction, however, since the real
character of a stimulus is determined by its effect, and the latter is
not necessarily dependent upon whether the stimulus is one of force or
one of material. There is, however, a fundamental difference between
factors with respect to their relation to the plant. Direct factors
alone are stimuli, since indirect factors must always act through them.
For example, the wind, its mechanical influence excepted, can affect the
plant only in so far as it is converted into the stimulus of increased
or decreased humidity. Consequently, the normal stimuli of the plants of
a formation are: (1) water-content, (2) solutes, (3) humidity, (4)
light, (5) temperature, (6) wind. Soil, pressure, physiography, and
biotic factors influence plants only through these, and are not stimuli,
though exceptions must be made of biotic factors in the case of
sensitive, insectivorous, and gall-producing plants.
=146. The nature of response.= Since plants have no special organs for
the perception of stimuli, nor sensory tracts for their transmission, an
external stimulus acting upon a plant organ is ordinarily converted into
a response at once. The latter as a rule becomes evident immediately; in
some cases it is latent or imperceptible, or some time elapses before
the chain of responses finds visible expression. A marked decrease in
humidity calls forth an immediate increase of transpiration, but the
ultimate response is seen in the closing of the stomata. A response to
decreased light intensity, on the other hand, is much less rapid and
obvious. This difference is largely due to the fact that the functional
response is more marked, or at least more perceptible in one case than
in the other.
Response is the reaction of the plant to a stimulus; it begins with the
impact of an efficient factor, and ends only with the consequent final
readjustment. The immediate reaction is always functional. The nature
and intensity of the stimulus determine whether this functional response
is followed by a corresponding change in structure. The consideration of
this theme consequently gains in clearness if a functional and a
structural response be distinguished. The chief value of this
distinction lies in the fact that many reactions are functional alone;
it serves also to emphasize the absolute interdependence of structure
and function, and the imperative need of considering both in connection
with the common stimulus. For these reasons, the logical treatment is to
connect each stimulus with its proper functional change, and, through
this, with the corresponding modification of structure. For the sake of
convenience, the term _adjustment_ is used to denote response in
function, and _adaptation_, to indicate the response in structure.
=147. Adjustment and adaptation.= The adjustment of a plant to the
stimuli of its habitat is a constant process. It is the daily task, seen
in nutrition and growth. So long as these take place under stimulation
by factors which fall within the normal variation of the habitat, the
problems belong to what has long been called physiology. When the
stimuli become unusual in degree or in kind, by a change of habitat or a
modification in it, adjustment is of much greater moment and is recorded
in the plant’s structure. These structural records are the foundation of
proper ecological study. Since they are the direct result of adjustment,
however, this affords further evidence that a division of the field into
ecology and physiology is illogical and superficial. Slight or
periodical adjustment may concern function alone; it may be expressed in
the movement of parts or organs, such as the closing of stomata or
changes in the position of leaves, in growth, or in modifications of
structure. This expression is fundamentally affected by the nature of
the factor and is in direct relation to the intensity of the latter.
Adaptation comprises all structural changes resulting from adjustment.
It includes both growth and modification. The latter is merely growth in
response to unusual stimuli, but this fact is the real clue to all
evolution. Growth is periodic, and in a sense quantitative; it results
from the normal continuous adjustment of the plant to the stimuli of its
proper habitat. In contrast, modification is relatively permanent and
qualitative; it is the response to stimuli unusual in kind or intensity.
A definite knowledge of the processes of growth is indispensable to an
understanding of modification. In the fundamental task of connecting
plant and habitat, it is the modification of the plant, and not its
growth, which records the significant responses to stimuli. For this
reason the discussion of adaptation in the pages that follow is
practically confined to modification of structure. This is particularly
desirable, since growth has long been the theme of physiological study,
while modification has too often been considered from the structural
standpoint alone. The comparatively few studies that have taken function
into account have been largely empirical; in them neither stimulus nor
adaptation has received anything approaching adequate treatment.
=148. The measurement of response.= The amount of response to a stimulus
is proportional to the intensity of the factor concerned. This does not
mean that the same stimulus produces the same response in two distinct
species, or necessarily in two plants of one species. In these cases the
rule holds only when the plants or species are equally plastic. For each
individual, however, this quantitative correspondence of stimulus and
response is fundamental. It is uncertain whether an exact or constant
ratio can be established between factor and function; the answer to this
must await the general use of quantitative methods. There can be no
doubt, however, that within certain limits the adjustment is
proportional to the amount of stimulus, whereas reaction is well known
to be abnormal or inhibited beyond certain extremes. It is quite
erroneous to think that reaction is independent of quantity of stimulus,
or to liken the stimulating factor to “the smallest spark (which) by
igniting a mass of powder, produces an enormous mechanical effect.”[8]
Such a statement is only apparently true of the action of mechanical
stimuli upon the few plants that may properly be said to possess
irritability, such as sensitive plants and certain insectivorous ones.
Of the normal relation of response to direct factors, water, light,
etc., it is entirely untrue. Axiomatically, there is ordinarily an
essential correspondence, also, between the amount of adjustment and of
adaptation. This correspondence is profoundly affected, however, by the
structural stability of the plant.
From the preceding it follows that the measurement of response and the
relating it to definite amounts of direct factors as stimuli are two of
the most fundamental tasks of ecology. The exact determination of
physical factors has no value apart from its use for this purpose. It is
perfectly clear that precise methods of measuring stimuli call for
similar methods in determining the amount of adjustment and of
adaptation. The problem is a difficult one, and it is possible at
present only to indicate the direction which its development should
take, and to describe a few methods which will at least serve as a
beginning. To cover the ground adequately it is necessary to measure
response by adjustment and by adaptation separately, and in the latter
to find a measure for the individual and one for the species. The one is
furnished by the methods of morphology and the other by biometry.
A primary requisite for any method for measuring adjustment is that it
be applicable to field conditions. Many instruments for measuring
transpiration, for example, are valueless, not because they are
inaccurate, but because the plant studied is under abnormal conditions.
To avoid the latter is absolutely necessary, a fact which makes it
peculiarly difficult to devise a satisfactory field method. After the
latter has been found and applied, it becomes possible to check other
methods by it, and to give them real value. The final test of a field
method is three-fold: (1) the plant must be studied while functioning
normally in its own habitat; (2) the method must give accurate results;
and (3) it must permit of extensive and fairly convenient application in
the field. Until methods of this character, some of which are described
later, have been employed for some time, it is impossible to connect
definite intensities of factor stimuli with measured amounts of
adjustment. Ultimately, it seems certain that researches will regularly
take this form.
Adaptation is primarily indicated by changes in the arrangement and
character of the cells of the plant. Since these determine the form of
each organ, morphology also furnishes important evidence in regard to
the course of adaptation, but form can be connected certainly with
adjustment only through the study of cellular adaptation. In tracing the
modifications of cell and of tissue, the usual methods of histology,
viz., sectioning and drawing, suffice for the individual. It is merely
necessary to select plants and organs which are as nearly typical as can
be determined. The question of quantity becomes paramount, however,
since it often gives the clue to qualitative changes, and hence it is
imperative that complete and accurate measurements of cells, tissues,
and organs be made. These measurements, when extended to a sufficiently
large number of plants, serve to indicate the direction of adaptation in
the species. They constitute the materials for determining biometrically
the mean of adaptation for the species and the probable evolution of the
latter. In its present development, biometry contains too much
mathematics, and too little biology. This has perhaps been unavoidable,
but it is to be hoped that the future will bring about a wise sifting of
methods, which will make biometry the ready and invaluable servant of
all serious students of experimental evolution. This condition does not
obtain at present, and in consequence it seems unwise to consider the
subject of biometry in this treatise.
=149. Plasticity and fixity.= As the product of accumulated responses,
each species is characterized by a certain ability or inability to react
to stimuli. Many facts seem to indicate that the degree of stability is
connected with the length of time during which the species is acted upon
by the same stimuli. It seems probable that plants which have reacted to
sunlight for hundreds of years will respond less readily to shade than
those which have grown in the sun for a much shorter period. This
hypothesis is not susceptible of proof in nature because it is
ordinarily impossible to distinguish species upon the basis of the time
during which they have occupied one habitat. Evidence and ultimate
proof, perhaps, can be obtained only by field and control experiments,
in which the time of occupation of any habitat is definitely known. Even
in this case, however, it is clear that antecedent habitats will have
left effects which can neither be traced nor ignored. Additional support
is given this view by the fact that extreme types, both ecological and
taxonomic, are the most stable. Intense xerophytes and hydrophytes are
much more fixed than mesophytes, though the intensity of the stimulus
has doubtless as great an influence as its duration. Composites,
labiates, grasses, orchids, etc., are less plastic than ranals, rosals,
etc., but there are many exceptions to the apparent rule that fixity
increases with taxonomic complexity. At present it seems quite
impossible to suggest an explanation of the rule. Recent experiments
indicate that there may be ancestral fixity of function, as well as of
structure. It has been found, for example, that the flowers of certain
species always react normally to the stimuli which produce opening and
closing, while others make extremely erratic response. If further work
confirms this result and extends it to other functions, the necessity of
arriving at a better understanding of fixity will be greatly emphasized.
It is impossible to make progress in the study of adaptation without
recognizing the fundamental importance of ancestral fixity as a factor.
E. S. Clements[9] has shown that a number of species undergo pronounced
changes in habitat without showing appreciable modification.
Consequently, it is incorrect to assume that each habitat puts a
structural impress upon every plant that enters it. For this reason, the
writer feels that the current explanation of xerophytic bog plants,
etc., is probably wrong, and that the discrepancy between the nature of
the habitat and the structure of the plant is to be explained by the
persistence of a fixed ancestral type. The anomaly is scarcely greater
than in cases that have proved capable of being explained.
=150. The law of extremes.= When a stimulus approaches either the
maximum or minimum of the factor for the species concerned, response
becomes abnormal. The resulting modifications approach each other and in
some respects at least become similar. Such effects are found chiefly in
growth, but they occur to some degree in structure also. It is
imperative that they be recognized in nature as well as in field and
control experiment, since they directly affect the ratio between
response and stimulus. The data which bear upon the similarity of
response to extremes of different factors are too meager to permit the
formulation of a rule. It is permissible, however, to suggest the
general principle that extreme stimuli produce similar growth responses,
and to emphasize the need of testing its application to adaptation
proper.
=151. The method of working hypotheses.= In the study of stimulus and
response, where the unimpeachable facts are relatively few, and their
present correlation slight, the working hypothesis is an indispensable
aid. “The true course of inductive procedure ... consists in
_anticipating nature_, in the sense of forming hypotheses as to the laws
which are probably in operation, and then observing whether the
combinations of phenomena are such as would follow from the laws
supposed. The investigator begins with facts and ends with them. He uses
such facts as are in the first place known to him in suggesting probable
hypotheses; deducing other facts which would happen if a particular
hypothesis is true, he proceeds to test the truth of his notion by fresh
observations or experiments. If any result prove different from what he
expects, it leads him either to abandon or to modify his hypothesis; but
every new fact may give some new suggestion as to the laws in action.
Even if the result in any case agrees with his anticipations, he does
not regard it as finally confirmatory of his theory, but proceeds to
test the truth of the theory by new deductions and new trials.”[10] In
the treatment of adjustment and adaptation which follows, the method of
multiple working hypotheses is uniformly employed. No apology is felt to
be necessary for this, since the whole endeavor is to indicate the
proper points of attack, and not to distinguish between that which is
conjectural and that which is known. If an hypothesis occasionally seem
to be stated too strongly, it is merely that it appears, after a survey
of the problem from all sides, to explain the facts most satisfactorily.
The final proof of any hypothesis, however, rests not only upon its
ability to explain all the facts, but also upon the inability of other
hypotheses to meet the same test. The discovery and examination of all
possible hypotheses, and the elimination of those that prove inadequate
are the essential steps in the method of working hypotheses.
HYDROHARMOSE
_ADJUSTMENT_
=152. Water as a stimulus.= Plants are continually subjected to the
action of the water of the soil and of the air; exception must naturally
be made of submerged plants. The stimulus of soil water acts upon the
absorbing organ, the root, while that of humidity affects the part most
exposed to the air, viz., the assimilative organ, which is normally the
leaf. But since both are simultaneous water stimuli, a clearer
conception is gained of this operation if they are viewed as two phases
of the same stimulus. This point of view receives further warrant from
the essential and intimate relation of humidity and water-content as
determined by the plant. They are in fact largely compensatory, as is
shown at some length later. In determining the intensity of the two, a
significant difference between them must be recognized. The total
humidity of the air at any one time constitutes a stimulus to the leaf
which it touches. This is not true of the total soil water. Part of the
latter is not available under any circumstances, and can not affect the
plant, at least directly. The chresard alone can act as a stimulus, but
even this is potential in the great majority of cases, since the actual
stimulus is not the water available but the water absorbed. The latter,
moreover, contains many nutrient salts which are in themselves stimuli,
but as they normally have little bearing upon the action of water as a
stimulus they are to be considered only when present in excessive
amounts.
=153. The influence of other factors upon water.= The amount of humidity
is modified directly by temperature, wind, precipitation, and pressure,
and, through these, it is affected by altitude, slope, exposure, and
cover. Naturally, also, the evaporation of soil water has a marked
influence. In determining water-content, atmospheric factors, with the
exception of precipitation, are usually subordinate to edaphic ones.
Soil texture, slope, and precipitation act directly in determining soil
water, while temperature, wind, and pressure can operate only through
humidity. This is likewise true of altitude, exposure, and cover, though
the latter has in addition a profound effect upon run-off. Biotic
factors can affect humidity or water-content only through the medium of
another factor. Light in itself has no action upon either, but through
its conversion into heat within the chloroplast, it has a profound
effect upon transpiration. The following table indicates the general
relation between water and the other physical factors of the habitat.
The order of the signs, ±, denotes that the water increases and
decreases with an increase and decrease of the factor, or the reverse,
∓.
_Humidity_ ± _Water-content_ ±
Temperature ∓ Temperature ∓
Wind ∓ Wind ∓
Precipitation ± Precipitation ±
Pressure ± Pressure ∓
Soil texture 0 Soil texture
Altitude ∓ Porosity ∓
Capillarity ±
Slope ∓ Slope ∓
Exposure ∓ Exposure ∓
Cover ± Cover ±
=154. Response.= The normal functional responses to water stimuli are
absorption, diffusion, transport, and transpiration. Of these,
absorption and transpiration alone are the immediate response to soil
water and humidity, respectively. Consequently they are the critical
points of attack in studying the fundamental relation of the plant to
the water of its habitat. In determining the pathway of the response, it
is necessary to trace the steps in diffusion and transport, but, as
these are essentially alike for all vascular plants, this task lies
outside the scope of the work in hand. As previously suggested, the
relation between absorption and transpiration is strictly compensatory,
though, for obvious reasons, the amount of water transpired is usually
somewhat less than the amount absorbed. Absorption falls below
transpiration when extreme conditions cause temporary or permanent
wilting; the two activities are essentially equal after a growing plant
reaches maturity. In all cases, however, the rule is that an increase or
decrease in water loss produces a corresponding change in the amount of
water absorbed, and, conversely, variation in absorption produces a
consequent change in transpiration. This is strictly true only when the
stimuli are normal. For example, a decrease in humidity causes increased
water loss, which, through diffusion and transport, is compensated by
increased activity of the root surface. Frequently the water supply is
insufficient to compensate for a greater stimulus, and the proper
balance can be attained only by the closing of the stomata. In the case
of excessive stimuli, neither compensation suffices, and the plant dies.
Many mesophytes and all xerophytes have probably resulted from stimuli
which regularly approached the limit of compensation for each, and often
overstepped, but never permanently exceeded it. For hydrophytes, the
danger arises from excessive water supply, not water loss. There is a
limit to the compensation afforded by transpiration, which is naturally
dependent upon the amount of plant surface exposed to the air. No
compensation occurs in the case of submerged plants; floating
hydrophytes possess a single transpiring leaf surface, while the leaves
of amphibious plants behave as do those of mesophytes. The whole
question of response to water stimuli thus turns upon the compensation
for water loss afforded by water supply where the latter is moderate or
precarious, and upon the compensation for water supply furnished by
water loss where the supply is excessive, submerged plants excepted.
=155. The measurement of absorption.= As responses to measured stimuli
of water-content and humidity, it is imperative that the amount of
absorption and of transpiration be determined quantitatively. It is also
extremely desirable that this be done in the normal habitat of the
plant. A careful examination of the problems to be met quickly discloses
the great difficulty of obtaining a direct and accurate measure of
absorption under normal conditions, especially in the field. For this
purpose, the ordinary potometric experiments by means of cut stems are
valueless. The use of the entire plant in a potometer yields much more
trustworthy results, though the fact that the root is under abnormal
conditions can not be overlooked, especially in the case of mesophytes
and xerophytes. While potometric conditions are less abnormal for
amphibious plants, the error is not wholly eliminated, since the roots
normally grow in the soil. The potometer can be made of value for
quantitative work only by checking the results it gives by means of an
instrument or a method in which the plant functions normally. In
consequence, the potometer can not at present be used to measure
absorption directly, though, as is further indicated in the discussion
of transpiration, it is a valuable supplementary instrument, after the
check mentioned has been applied to its use with a particular species.
An estimate of the amount of absorption may be obtained either in the
field or in the control house by taking samples from the protected soil
at different times. Since it is impossible to determine the weight of
the area in which the roots lie, and since the soil water is often
unequally distributed, this method can not yield exact results. An
accurate method of measuring absorption under essentially normal
conditions has been devised and tested in the control house. The
essential feature of the process is the placing a plant in a soil
containing a known quantity of water, and removing it after it has
absorbed water from the soil for a certain period. In carrying out the
experiment, a soil consisting of two parts of sod and one of sand was
used, since the aeration is more perfect and the particles are more
easily removed from the roots. The soil was completely dried out in a
water bath and then placed in a five-inch battery jar. The latter,
together with the rubber cloth used later to prevent evaporation, was
weighed to the decigram. A weighed quantity of water was added, and the
whole again weighed as a check. Two plants of _Helianthus annuus_ were
taken from the pots in which they had grown, and the soil was carefully
washed from the roots. Each plant was weighed with its roots in a dish
of water to prevent wilting, and then carefully potted, one in each
battery jar. A thistle tube was placed in the soil of each jar to
facilitate aeration, as well as the addition of weighed amounts of
water, when necessary, and the rubber cloth attached in the usual manner
to prevent evaporation. The entire outfit was weighed again, and the
weighing repeated at 8:00 A.M. and 5:00 P.M. for five days, in order to
determine the amount of transpiration and its relation to the water
absorbed. The plants were kept in diffuse light to prevent excessive
water loss while the roots were becoming established. At the close of
the experiment, the jar and its contents were weighed finally. The
plants were removed and weighed, the soil particles being shaken from
the roots into the jar, which was also weighed. The results obtained
were as follows:
══╤═════════╤═══════════════════╤════════╤════════╤════════╤══════════
│ Wt. of │Wt. of pot and wet │ Total │_H_{2}O_│_H_{2}O_│ _H_{2}O_
│ pot and │ soil │_H_{2}O_│ left │absorbed│transpired
│dry soil │ │ │ │ │
──┼─────────┼─────────┬─────────┼────────┼────────┼────────┼──────────
│ │ I │ II │ │ │ │
I│1846.0 g.│2218.0 g.│2174.3 g.│372.0 g.│328.3 g.│ 43.7 g.│ 43.7 g.
II│1886.7 g.│2253.2 g.│2221.6 g.│366.5 g.│334.9 g.│ 31.6 g.│ 31.6 g.
──┴─────────┴─────────┴─────────┴────────┴────────┴────────┴──────────
The amount of water absorbed may be obtained directly by subtracting the
final weight of the jar and moist soil from their first weight, but a
desirable check is obtained by taking the dry weight of jar and soil
from the first, and the final weight of these, and subtracting the one
from the other as indicated in the table. A second check is afforded by
daily weighings, from which the amount of water transpired is
determined. Since the two sunflower plants made practically no growth
during the period of experiment, the exact correspondence between water
absorbed and water lost is not startling, though it can not be expected
that the results will always coincide.
This method has certain slight sources of error, all of which, it is
thought, have been corrected in a new and more complete series of
experiments now being carried on. The aeration of the soil is not
entirely normal, as is also true of the capillary movements of the
water, on account of the nonporous glass jar and the rubber cloth. Since
the latter are necessary conditions of all accurate methods for
measuring absorption and transpiration, the resulting error must be
ignored. It can be reduced, however, by forcing air through the thistle
tube from time to time. Sturdy plants, such as the sunflower, are the
most satisfactory, since they recover more quickly from the shock of
transplanting. Almost any plant can be used, however, if repotted in a
loose sandy soil often enough. This permits the root system to develop
normally, and also makes it possible to wash the soil away without
injury to the root. The method is so recent that there has been no
opportunity to test it in the field. It would seem that it can be
applied without essential change to plants in their normal habitats.
Very large herbs or plants with extensive root systems could not be used
to advantage, and to be practicable the experiments would need to be
carried on near the base station. The great value of the method,
however, lies in its use as a check in determining the accuracy of other
methods, and in practice it will often be found convenient and
time-saving to use the latter, after they have once been carefully
checked for different groups of species. This matter is further
considered under measures of transpiration.
[Illustration: Fig. 31. Absorption and transpiration of _Helianthus
annuus_. I and II, plants repotted in soil of known weight and
water-content; III, plant undisturbed in the original soil; IV,
potometer containing plant with cut stem; V, potometer with entire
plant.]
=156. The quantitative relation of absorption and transpiration.=
Burgerstein[11] has summarized the results of various investigators in
the statement “that between the quantitative absorption of water on the
one hand and emission on the other there exists no constant parallelism
or proportion,” and he has cited the work of Kröber, and of Eberdt in
proof. This statement holds, however, only for short periods of a few
hours, or more rarely, a day, and even here its truth still remains to
be conclusively demonstrated. The discrepancy between absorption and
transpiration for a short period is often greater than for a longer
time, but it is evident that a transient change in behavior or a small
error in the method would inevitably produce this result. Eberdt found
the discrepancy for a few hours to be 1–2 ccm. in an entire plant of
_Helianthus annuus_, while for a whole day the water absorbed was 33.57
ccm. and the water lost 33.98 ccm. Kröber’s experiments with cut
branches of _Asclepias incarnata_ showed a maximum difference for 12
hours of 2.5 ccm., but the discrepancy for the first 24 hours was 1 ccm.
and for the second 1.9 ccm. In both cases, the potometer was employed.
Consequently, as will be shown later, Eberdt’s results are not entirely
trustworthy, while those of Kröber, made with cut stems, are altogether
unreliable. Hence, it is clear that the discrepancy is slight for a
period of several days or weeks, and that it may be ignored without
serious error, except in a few plants that retain considerable water as
cell-sap, in consequence of extremely rapid growth. Accordingly, the
amount of transpiration, which may be readily and accurately determined,
can be employed as a measure of absorption that is sufficiently accurate
for nearly all purposes. The truth of this statement may be easily
confirmed. It is evident that the amount of water absorbed equals the
amount transpired plus that retained by the plant as cell-sap, or used
in the manufacture of organic compounds. In plants not actively growing,
the amount lost equals that absorbed, as already shown in the experiment
with _Helianthus_. According to Gain[12], Dehérain has found that a
plant rooted in ordinary soil transpired 680 kg. of water for each
kilogram of dry substance elaborated. In _Helianthus annuus_, the dry
matter is 10 per cent of the weight of the green plant. A well-grown
plant weighing 1,000 grams, therefore, consists of 100 grams of dry
matter and 900 of water. The length of the growing period for such a
plant is approximately 100 days, during which it transpires 68 kilograms
of water. Assuming the rate of transpiration and of growth to be
constant, the plant transpires 680 grams daily, adds 9 grams to its
cell-sap, and 1 gram to its dry weight. The amount of water in a gram of
cellulose and its isomers is about ⅗. Consequently, the total water
absorbed daily by the plant is 689.6 grams. The 680 grams transpired are
98.6 per cent of the amount absorbed; in other words, only 1.4 per cent
of the water absorbed is retained by the plant. From this it is evident
that the simplest and most convenient measure of absorption under normal
conditions can be obtained through transpiration, since the discrepancy
between absorption and transpiration is scarcely larger than the error
of any method applicable to the field. Conversely, the measure of
absorption obtained by the process described in the preceding section
serves also as a measure of transpiration. The determination of the
latter in the field is so much simpler, however, that it is rarely
desirable to apply the absorption method.
=157. Measurement of transpiration.= The water loss of a plant may be
determined absolutely or relatively. Absolute or quantitative
determinations are by (1) weighing, (2) collecting, or (3) measuring the
water absorbed; relative values are indicated by hygroscopic substances.
A number of methods have been employed more or less generally for
measuring transpiration. The great majority of these can be used to
advantage only in the laboratory, and practically all fail to meet the
fundamental requirement for successful field work, namely, that the
plant be studied under normal conditions in its own habitat. The
following is a summary of the various methods, the details of which may
be found in Burgerstein.
1. _Weighing._ This is the most satisfactory of all methods for
determining water loss. It is more accurate than any other, and is
unique in that it does not place the plant under abnormal conditions. On
the score of convenience, moreover, it excels every other method capable
of yielding quantitative results. Various modifications of weighing are
employed, but none of these have all the advantages of a direct, simple
weighing of the plant in its own soil.
2. _Collecting the water transpired._ This may be done by collecting and
weighing the water vapor exhaled by a plant placed within a bell jar, or
by weighing a deliquescent salt, such as calcium chloride, which is used
to absorb the water of transpiration. The decisive disadvantage of these
methods is that transpiration is carried on in an atmosphere far more
humid than normal. If an excessive amount of salt is used, the air is
abnormally dry. In both cases, the water loss decreases until it reaches
a point much below the usual amount. Finally, all methods of this kind
are open to considerable error, and are inconvenient, especially in
field work. They are of relatively slight value in comparison with
weighing.
3. _Potometers._ It has already been shown that the amount of water
absorbed is a close measure of the amount transpired. In consequence,
the potometer can be used to determine the amount of transpiration
provided the absorption is not abnormal. It is rarely and only with much
difficulty that this condition can be met. The use of cut stems and
branches does not meet it, and even in the case of plants with roots,
the results must be compared with those obtained from absorption
experiments made with plants rooted in soil before they can be relied
upon. This necessity practically puts the potometer out of commission
for accurate work, unless future study may show a somewhat constant
ratio between the absorption of a plant in its own soil and that of a
plant placed in a potometer.
4. _Measuring absolute humidity._ The cog psychrometer makes it possible
to determine the increased relative humidity produced within a glass
cylinder or special tin chamber by a transpiring plant. From this result
the absolute humidity is readily obtained, and by means of the latter
the actual amount of water given off. The evident drawback to this
method is that the increasing humidity within the chamber gives results
entirely abnormal for the plant concerned.
5. _Self-registering instruments._ There are various methods for
registering the amount of transpiration, based upon weighing, or upon
the potometer. The Richard recording evaporimeter has all the advantages
of weighing, inasmuch as the water loss is measured in this way, and in
addition the amount is recorded upon a revolving drum, obviating the
necessity of repeated attention in case it is desirable to know the
exact course of transpiration. On the other hand, methods which depend
upon the potometer, while graphic, are not sufficiently accurate to be
of value.
6. _The use of hygroscopic materials._ Hygroscopic substances change
their form or color in response to moisture. As they indicate
comparative water loss alone, they are of value chiefly in the study of
the stomatic surfaces of leaves. F. Darwin[13] has used strips of horn,
awns of _Stipa_, and epidermis of _Yucca_ to construct small hygroscopes
for this purpose. In these instruments the error is large, but as no
endeavor is made to obtain exact results, it is negligible. Filter paper
impregnated with a 3–5 per cent aqueous solution of cobalt chloride is
deep blue when dry. If a strip of cobalt paper is placed upon a leaf and
covered with a glass slip it turns bright rose color, the rapidity of
the change affording a clue to the amount of transpiration.
=158. Field methods.= The conditions which a satisfactory field method
of measuring transpiration must fulfill have already been discussed;
they are accuracy, simplicity, and normality. These conditions are met
only by weighing the plant in its own soil and habitat. This has been
accomplished by means of the sheet-iron soil box, already described
under the determination of the chresard. The method is merely the
familiar one of pot and balance, slightly modified for field use. The
soil block, which contains the plant to be studied, is cut out, and the
metal plates put in position as indicated in section 53. Indeed, it is a
great saving of time and effort to determine transpiration and chresard
in the same experiment; this is particularly desirable in view of the
close connection between them. In this event, the soil block must be
small enough not to exceed the load of a field balance. After the block
is cut and encased, all the plants are removed, except the one to be
studied. If several individuals of the same species are present, it is
an advantage to leave all of them, since the error arising from
individual variations of water loss may, in this way, be almost
completely eliminated. A sheet of rubber or rubber cloth is carefully
tied over the box to prevent evaporation from the soil. A broad band is
passed under the box to aid in lifting it upon the scales. The latter
must be of the platform type, and should have a capacity as great as
consistent with the need for moving it about in the field. Weighings are
made in the usual way, care being taken to free the surface of the box
from soil. The aeration of the soil block is kept normal by removing the
rubber for a few minutes from time to time, or by forcing air through a
thistle tube. Water is also added through the latter, when it is desired
to continue the experiment for a considerable period. After the study of
transpiration is concluded, the rubber cloth is removed, soil samples
taken, and the soil allowed to dry out until the plant becomes
thoroughly wilted. If the box is weighed again, the difference
represents the amount of available water. The per cent of chresard is
also obtained in the usual way by taking samples for ascertaining the
echard, and subtracting this from the holard. Field determinations of
water loss yield the most valuable results when different habitat forms,
or ecads, of the same species are used. There is little profit in
comparing the transpiration of a typical sun plant, such as _Touterea
multiflora_, with that of a shade plant, such as _Washingtonia obtusa_.
But the simultaneous study of plants like _Chamaenerium angustifolium_,
_Gentiana acuta_, _Scutellaria brittonii_ etc., which grow in several
different habitats, furnishes direct and fundamental evidence of the
course of adjustment and adaptation.
Hesselmann[14], in his study of open woodlands in Sweden, has employed a
method essentially similar to the preceding. Young plants of various
species were transferred to pots in the field, where they were allowed
to grow for several months before a series of weighings was made to
determine the amount of transpiration. Since weighing is the measure
used in each, both methods are equally accurate. The one has a certain
advantage in that the pots are, perhaps, more easily handled, while the
other has the advantage of maintaining the normal relation of soil and
roots, a condition more or less impossible in a pot. In both instances
the weighing should be done in the habitat, which was not the case in
Hesselmann’s researches.
The slight value of the potometer, which has had a vogue far beyond its
merits, is indicated by the following table. These results were obtained
from three plants of _Helianthus annuus_; III was left undisturbed in
the pot where it had been growing, IV was placed in a potometer, after
the root had been cut off, and V was an entire plant placed in a
potometer. The amount of transpiration is indicated in grams per square
decimeter of leaf surface. The plants were kept in diffuse light, except
for a period of two hours (8:00 to 10:00 A.M.) on the last day, when
they were in full sunshine at a temperature of 75° F. Plant IV wilted so
promptly in the sunshine that it was found necessary to conclude the
experiment in diffuse light.
═════╤═════╤═════╤═════╤═════╤═════╤═════╤═════╤═════╤═════╤═════╤═════
│ 8 │ 5 │ 8 │ 5 │ 8 │ 5 │ 8 │ 10 │ 5 │ 8 │Total
│A.M. │P.M. │A.M. │P.M. │A.M. │P.M. │A.M. │A.M. │P.M. │A.M. │
─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────
III│ 2.9 │ 7.3 │ 2.4 │ 6.0 │ 1.7 │ 1.6 │ 2.0 │ 3.4 │ 2.0 │ 1.8 │31.1
IV│ 4.7 │ 7.2 │ 2.9 │ 2.3 │ 1.0 │ 0.6 │ 0.9 │ 0.5 │ 0.5 │ 0.4 │21.0
V│ 3.7 │ 5.3 │ 3.2 │ 4.8 │ 2.5 │ 1.6 │ 3.0 │ 2.6 │ 1.6 │ 2.6 │30.9
─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────
The cut plant, IV, lost more water the first day than either of the
others, but the water loss soon decreased, and at the end of the period
was almost nil. The total transpiration for III and V is much the same,
but the range of variation for periods of 12 hours is from +2 to –1
gram. This experiment is taken as a fair warrant that the use of cut
stems in potometers can not give accurate results. It is inconclusive,
however, as to the merits of potometric values obtained by means of the
entire plant, and further studies are now being made with reference to
this point.
=159. Expression of results.= From the previous discussion of the
relation between them, it follows that an expression of the amount of
transpiration likewise constitutes an expression of absorption. It is
very desirable also that the latter be based upon root surface and
chresard, but the difficulty of determining the former accurately and
readily is at present too great to make such a basis practicable. In
expressing transpiration in exact terms, the fact that plants of the
same species or form are somewhat individual in their behavior must be
constantly reckoned with. In consequence, experiments should be made
upon two or three individuals whenever possible, in order to avoid the
error arising from this source.
Water loss may be expressed either in terms of transpiring surface or of
dry weight. Since there is no constant relation between surface and
weight, the terms are not interchangeable or comparable, and in practice
it is necessary to use one to the exclusion of the other. Obviously,
surface furnishes by far the best basis, on account of its intimate
connection with stomata and air-spaces, a conclusion which Burgerstein
(_l. c._, p. 6) has shown by experiment to be true. For the best
results, the whole transpiring surface should be determined. This is
especially necessary in making comparisons of different species. In
those studies which are of the greatest value, viz., ecads of the same
species, it is scarcely desirable to measure stem and petiole surfaces,
unless these organs show unusual modification. The actual transpiring
surface is constituted by the walls of the cells bordering the
intercellular spaces, but, since it is impossible to determine the
aggregate area of these, or the humidity of the air-spaces themselves,
the leaf surface must be taken as a basis. Since the transpiration
through the stomata is much greater than that through the epidermal
walls, the number of stomata must be taken into account. Since they are
usually less abundant on the upper surface, their number should be
determined for both sides of the leaf. The errors arising from more or
less irregular distribution are eliminated by making counts near the
tip, base, and middle of two or three mature leaves. The most convenient
unit of leaf surface is the square decimeter. The simplest way to
determine the total leaf area of a plant is to outline the leaves upon a
homogeneous paper, or to print them upon a photographic paper. The
outlines are then cut out and weighed, and the leaf area obtained in
square decimeters by dividing the total weight by the weight of a square
decimeter of the paper used. The area may also be readily determined by
means of a planimeter.
=160. Coefficient of transpiration.= At present it does not seem
feasible to express the transpiration of a plant in the form of a
definite coefficient, but it is probable that the application of exact
methods to each part of the problem will finally bring about this
result. Meanwhile the following formula is suggested as a step toward
this goal: _t_ = _g_(_u/l_)_LHT_, in which _t_, the transpiration
relation of a plant, is expressed by the number of grams of water lost
per hour, on a day of sunshine, by one square decimeter of leaf,
considered with reference to the stomata of the two surfaces, and the
amount of the controlling physical factors, light, humidity, and
temperature, at the time of determination. For _Helianthus annuus_, this
formula would appear as follows: _t_ = 2(²⁰⁰⁄₂₅₀) : 1 : 50 : 75°. To
avoid the large figures arising from the extent of surface considered,
the number of stomata per square decimeter is divided by 10,000. This
amounts to the number per square millimeter, and time may consequently
be saved by using this figure directly. While this formula obviously
leaves much to be desired, it has the great advantage of making it
possible to compare ecads of one species, or species of the same habitat
or of different habitats, upon an exact basis of factor, function, and
structure.
_ADAPTATION_
=161. Modifications due to water stimuli.= In adaptation, the great
desideratum is to connect each modification quantitatively with the
corresponding adjustment. This is even more difficult than to ascertain
the quantitative relation between stimulus and functional response, a
task still beset with serious obstacles. At the present time, little
more can be done than to indicate the relation of marked adaptations of
organs and tissues to the direct factors operating upon them, and to
attempt to point out among the functions possibly concerned the one
which seems to be the most probable connection between the probable
stimulus and the structure under investigation. In the pages that
follow, no more than this is attempted. The general changes of organs
and tissues produced by water are first discussed, and after this is
given a summary of the structural features of the plant types based upon
water-content.
=162. Modifications due to a small water supply.= A water supply which
may become deficient at any time is compensated either by changes which
decrease transpiration, or by those that increase the amount of water
absorbed or stored. These operate upon the form and size of the organs
concerned, as well as upon their structure. Modifications of the form of
leaf and stem are alike in that they lessen transpiration by a reduction
of the amount of surface exposed to the air. Structural adaptations, on
the other hand, bring about the protection of epidermal cells and
stomata, and often internal cells also, from the factors which cause
transpiration, or they anticipate periods of excessive transpiration by
the storage of water in specialized cells or tissues. In certain extreme
types the epidermis is itself modified for the absorption of water vapor
from the air.
=163. The decrease of water loss.= The following is a summary of the
contrivances for reducing transpiration.
1. _Position of the leaf._ Since the energy of a ray of sunlight is
greatest at the sun’s highest altitudes, those leaves transpire least
which are in such a position during midday that the rays strike them as
obliquely as possible. A leaf at right angles to the noonday sun
receives ten times as much light and heat upon a square decimeter of
surface as does one placed at an angle of 10 degrees. This device for
reducing the intensity of insolation is best developed in the erect or
hanging leaves of many tropical trees. In temperate zones, it is found
in such plants as _Silphium laciniatum_ and _Lactuca scariola_, and in
species with equitant leaves. In such plants as _Helianthus annuus_, the
effect is just the opposite, since the turning of the crown keeps the
leaves for a long time at a high angle to the incident rays. In the case
of mats, it is the aggregation of plants which brings about the mutual
protection of the leaves from insolation and wind.
2. _Rolling of the leaf._ Many grasses and ericaceous plants possess
leaves capable of rolling or folding themselves together when drouth
threatens. In other cases, the leaves are permanently rolled or folded.
The advantage of this device arises not only from the reduction of
surface, but also from the fact that the stomata come to lie in a
chamber more or less completely closed. In the case of those mosses
whose leaves roll or twist, a reduction of surface alone is effected.
3. _Reduction of leaf._ The transpiring surface of a plant is reduced by
decreasing the number of leaves, by reducing the size of each leaf, or
by a change in its form. In so far as the stem is a leaf, a decrease in
size or a change in shape brings about the same result. The final
outcome of reduction in size or number is the complete loss of leaves,
and more rarely, of the stem. Such marked decrease of leaf area is found
only in intense xerophytes, though it occurs in all deciduous trees as a
temporary adaptation. Changes in leaf form are nearly always accompanied
by a decrease in size. Of the forms which result, the scale, the linear
or cylindrical leaf, and the succulent leaf are the most common. Leaves
which show a tendency to divide often increase the number of lobes or
make them smaller.
4. _Epidermal modifications._ Excretions of wax and lime by the
epidermis have a pronounced effect by increasing the impermeability of
the cuticle, and, hence, decreasing epidermal transpiration. It seems
improbable that a coating of wax on the lower surface of a diphotic leaf
can have this purpose. The thickening of the outer wall of epidermal
cells to form a cuticle is the most perfect of all contrivances for
decreasing permeability and reducing transpiration. In many desert
plants, the greatly thickened cuticle effectually prevents epidermal
transpiration. In these also the cuticle is regularly developed in such
a way as to protect the guard cells, and even to close the opening
partially. An epidermis consisting of two or more layers of cells is an
effective, though less frequent device against water loss. When combined
with a cuticle, as is usually the case, the impermeability is almost
complete. Hairs decrease transpiration by screening the epidermis so
that the amount of light and heat is diminished, and the access and
movement of dry air impeded. While hairs assume the most various forms,
all hairy coverings serve the same purpose, even when, as in the case of
_Mesembryanthemum_, they are primarily for water-storage. Hairs protect
stomata as well as epidermal cells: the greater number of the former on
the lower surface readily explains the occurrence of a hairy covering on
this surface, even though absent on the more exposed upper side. In some
cases, hairs are developed only where they serve to screen the stomata.
The modifications of the stomata with respect to transpiration are
numerous, yet all may be classed with reference to changes of number or
level. With the exception of aquatic and some shade plants, the number
of stomata is normally greater on the less exposed, i. e., lower
surface. The number on both surfaces decreases regularly as the danger
of excessive water loss increases, but the decrease is usually more
rapid on the upper surface, which finally loses its stomata entirely. It
has been shown by many observers that species growing in dry places have
fewer stomata to the same area than do those found in moist habitats.
This result has been verified experimentally by the writer in the case
of _Ranunculus sceleratus_, in which, however, the upper surface
possesses the larger number of stomata. Plants of this species, which
normally grow on wet banks, were grown in water so that the leaves
floated, and in soils containing approximately 10, 15, 30, and 40 per
cent of water. The averages for the respective forms were: upper 20,
lower 0; upper 18, lower 10; upper 18, lower 11; upper 11, lower 8;
upper 10, lower 6. Reduction of number is effective, however, only under
moderate conditions of dryness. As the latter becomes intense, the guard
cells are sunken below the epidermis, either singly or in groups. In
both cases, the protection is the same, the guard cells and the opening
between them being withdrawn from the intense insolation and the dry
air. The sun rays penetrate the chimney-shaped chambers of sunken
stomata only for a few minutes each day, and they are practically
excluded from the stomatal hollows which are filled with hairs. The
influence of dry winds is very greatly diminished, as is also true,
though to a less degree, for leaves in which the stomata are arranged in
furrows. Sunken stomata often have valve-like projections of cuticle
which reduce the opening also. Finally, in a few plants, water loss in
times of drouth is almost completely prevented by closing the opening
with a wax excretion.
5. _Modifications in the chlorenchym._ A decrease in the size and number
of the air passages in the leaf renders the movement of water-laden air
to the stomata more difficult, and effects a corresponding decrease in
transpiration. The increase of palisade tissue, though primarily
dependent upon light, reduces the air-spaces, and consequently the
amount of water lost. The development of sclereids below the epidermis
likewise hinders the escape of water. Finally the character of the cell
sap often plays an important part, since cells with high salt-content or
those containing mucilaginous substances give up their water with
reluctance.
=164. The increase of water supply.= Plants of dry habitats can increase
their absorption only by modifying the root system so that the absorbing
surfaces are carried into the deep-seated layers of soil, and the
surfaces in contact with the dry soil are protected by means of a
cortex. Exception must be made for epiphytes and a few other plants that
absorb rain water and dew through their leaves, and for those desert
plants that seem to condense the moisture of the air by means of
hygroscopic salts, and absorb it through the epidermis of the leaf. The
storage of water in the leaf is a very important device; it increases
the water supply by storing the surplus of absorbed water against the
time of need. Modifications for water-storage are occasionally found in
roots and stems, but their chief development takes place in the leaf.
The epidermis frequently serves as a reservoir for water, either by the
use of the epidermal cells themselves, by the formation of hypodermal
water layers, or by means of superficial bulliform cells. The water
cells of the chlorenchym regularly appear in the form of large clear
cells, scattered singly or arranged in groups. In this event, they occur
either as transverse bands, or as horizontal layers, lying between the
palisade and sponge areas, and connecting the bundles. A few plants
possess tracheid-like cells which also serve to store water. In the case
of succulent leaves, practically the whole chlorenchym is used for
storing water, though they owe their ability to withstand transpiration
to a combination of factors.
=165. Modifications due to an excessive water supply.= Water plants with
aerial leaf surfaces are modified in such manner as to increase water
loss and to decrease water supply, but the resulting modifications are
rarely striking. There is a marked tendency to increase the exposed
surface. This is indicated by the fact that, while the leaves of mud and
floating forms become larger, they change little or not at all in
thickness. The lobing of leaves is also greatly reduced, or the lobes
come to overlap. Leaves of water plants are practically destitute of all
modifications of epidermis and stomata, which could serve to hinder
transpiration. The stomata are usually more numerous on the upper
surface, and in the same species their number is greater in the forms
grown in wet places. These facts explain in part the extreme development
of air-passages in water plants, though this is, in large measure, a
response to the increasing difficulty of aeration. The increase of
air-spaces is correlated with reduction of the palisade, and a decided
increase in the sponge. An increase in water supply is indicated by the
absence of storage tissues, and the reduction of the vascular system,
which, however, is more closely connected with a diminished need for
mechanical support.
[Illustration: Fig. 32. Mesophyll of _Pedicularis procera_ (chresard,
15%, light, 1). × 130.]
=166. Plant types.= The necessity for decreasing or increasing water
loss in compensation of the water supply has made it possible to
distinguish two fundamental groups of plants upon the twofold basis of
habitat and structure. These familiar groups, xerophytes and
hydrophytes, represent two extremes of habitat and structure, between
which lies a more or less vague, intermediate condition represented by
mesophytes. These show no characteristic modifications, and it is
consequently impossible to arrange them in subgroups. Xerophytes and
hydrophytes, on the other hand, exhibit marked diversity among
themselves, a fact that makes it desirable to recognize subgroups, which
correspond to fundamental differences of habitat or adaptation. It is
hardly necessary to point out that these types are not sharply defined,
or that a single plastic species may be so modified as to exhibit
several of them. The extremes are always clearly defined, however, and
they indicate the specific tendency of the adaptation shown by other
members of the same group.
=167. Xerophytic types.= With the exception of dissophytes, all
xerophytes agree in the possession of a deep-seated root system, adapted
to withdraw water from the lower moist layers, and to conserve from loss
from the upper dry layers. Reservoirs are developed in the root,
however, in relatively few cases. The stem follows the leaf more or less
closely in its modification, except when the leaf is greatly reduced or
disappears, in which event the stem exhibits peculiar adaptations. While
the leaf is by far the most strikingly modified, it is a difficult task
to employ it satisfactorily as the basis for distinguishing types.
Several adaptations are often combined in the same leaf, and it is only
where one of these is preeminently developed, as in the case of
succulence, that the plant can be referred to a definite type. The
latter does not happen in many species of the less intensely xerophytic
habitats, and, consequently, it is difficult, if not undesirable, to
place such xerophytes under a particular group. The best that can be
done is to recognize the types arising from extreme or characteristic
modification, and to connect the less marked forms as closely as
possible with these. Halophytes differ from xerophytes only in the fact
that the chresard is determined by the salt-content of the habitat, and
not by the texture of the soil. In consequence, they should not be
treated as a distinct group.
[Illustration: Fig. 33. Staurophyll of _Bahia dissecta_, showing extreme
development of palisade (chresard, 3–9%; light, 1). × 130.]
=168. Types of leaf xerophytes.= In these, adaptation has acted
primarily upon the leaf, while the stem has remained normal for the most
part. Even when the leaves have become scale-like, they persist
throughout the growing season, and continue to play the primary part in
photosynthesis. The following types may be distinguished:
1. _The normal form._ The leaf is of the usual dorsiventral character.
In place of a reduction in size, structural modifications are used to
decrease transpiration. With respect to the protective feature that is
predominant, three subtypes may be recognized. The _cutinized_ leaf
compensates for a low water-content by means of a thick cuticle, often
reinforced by a high development of palisade tissue. Such leaves are
more or less leathery, and they are often evergreen also.
_Arctostaphylus_ and many species of _Pentstemon_ are good examples.
Lanate leaves, i. e., those with dense hairy coverings on one or both
surfaces, as _Artemisia_, _Antennaria_, etc., regularly lack both
cuticle and palisade tissue. The protection against water loss, however,
is so perfect that the chlorenchym often assumes the loose structure of
a shade leaf. _Storage_ leaves usually have a well-developed cuticle and
several rows of palisade cells, but their characteristic feature is the
water-storage tissue, which maintains a reserve supply of water for the
time of extreme drouth. Xerophytic species of _Helianthus_ furnish
examples of transverse bundles of storage cells, while those of
_Mertensia_ illustrate the more frequent arrangement in which the water
tissue forms horizontal layers.
2. _The succulent form._ Many succulent leaves are normal in shape and
size, though always thicker than ordinary leaves. Usually, however, they
are reduced in size and are more or less cylindrical in form. The
necessary decrease in transpiration is effected by the reduction in
surface, the general storage of water, a waxy coating, and, often also,
by a very thick cuticle. _Agave_, _Mesembryanthemum_, _Sedum_, and
_Senecio_ furnish excellent examples of this type.
[Illustration: Fig. 34. Diplophyll of _Mertensia linearis_, showing
water cells (chresard, 3–9%, light, 1). × 130.]
3. _The dissected form._ The reduction in surface is brought about by
the division of the leaf blade into narrow linear or thread-like lobes
which are widely separated. The latter are themselves protected by a
hairy covering or a thick cuticle, which is often supplemented by many
rows of palisade, or by storage tissue. _Artemisia_, _Senecio_, and
_Gilia_ contain species which serve as good examples of this type.
4. _The grass form._ Xerophytic grasses and sedges have narrow
filamentous leaves with longitudinal furrows which serve to protect the
stomata. The furrows are sometimes filled with hairs which are an
additional protection, and the leaves often protect themselves further
by rolling up into a thread-like shape. The elongated subulate leaves of
_Juncus_ and certain _Cyperaceae_ are essentially of this type, although
they are usually not furrowed.
5. _The needle form._ This is the typical leaf of conifers, in which a
sweeping reduction of the leaf surface is an absolute necessity. The
relatively small water loss of the needle leaf is still further
decreased by a thick cuticle, and usually also by hypodermal layers of
sclerenchyma.
6. _The roll form._ Roll leaves are frequently small and linear. Their
characteristic feature is produced by the rolling in of the margin on
the under side, by which an almost completely closed chamber is formed
for the protection of the stomata which are regularly confined to the
lower surface of the leaf. The upper epidermis is heavily cutinized and
the lower one often protected by hairs. This type is found especially
among the genera of the _Ericales_, but it also occurs in a large number
of related families.
7. _The scale form._ Reduction of leaf surface for preventing excessive
water loss reaches its logical culmination in the scale leaf
characteristic of many trees and shrubs, e. g., _Cupressus_, _Tamarix_,
etc. Scale leaves are leathery in texture, short and broad, and closely
appressed to the stem, as well as often overlapping.
=169. Types of stem xerophytes.= In these types the leaves are deciduous
early in the growing period, reduced to functionless scales, or entirely
absent. The functions of the leaf have been assumed by the stem, which
exhibits many of the structural adaptations of the former. Warming[15]
has distinguished the following groups:
1. _The phyllode form._ The petiole is broadened and takes the place of
the leaf blade which is lacking. In other cases, the stem is flattened
or winged, and it replaces the entire leaf. This type occurs in
_Acacia_, _Baccharis_, _Genista_, etc.
2. _The virgate form._ The leaves either fall off early or they are
reduced to functionless scales. The stems are thin, erect, and rod-like,
and are often greatly branched. They are heavily cutinized and
palisaded, and the stomata are frequently in longitudinal furrows. This
type is characteristic of the _Genisteae_; it is also found in
_Ephedra_, many species of _Polygonum_, _Lygodesmia_, etc.
3. _The rush form._ In _Heleocharis_, many species of _Juncus_,
_Scirpus_, and other _Cyperaceae_, the stem, which is nearly or
completely leafless, is cylindrical and unbranched. It usually possesses
also a thick cuticle, and several rows of dense palisade tissue.
4. _The cladophyll form._ In _Asparagus_ the leaves are reduced to mere
functionless scales, and their function is assumed by the small
needle-shaped branches.
5. _The flattened form._ As in the preceding type, the place of the
scale-like leaves is taken by cladophylls, which are more or less
flattened and leaf-like. _Ruscus_ is a familiar illustration of this
form.
6. _The thorn form._ This is typical of many spiny desert shrubs, in
which the leaves are lost very early, or, when present, are mere
functionless scales. The stems have an extremely thick cuticle, and the
stomata are deeply sunken, as a rule. _Colletia_ and _Holacantha_ are
good examples of the type.
7. _The succulent form._ Plants with succulent stems such as the
_Cactaceae_, _Stapelia_, and _Euphorbia_ have not only decreased water
loss by extreme reduction or loss of the leaves, and the reduction of
stem surface, but they also offset transpiration by means of storage
tissues containing a mucilaginous sap. The cuticle is usually highly
developed and the stomata sunken. Thorns and spines are also more or
less characteristic features.
[Illustration: Fig. 35. _Polygonum bistortoides_, a stable type: 1,
mesophyll (chresard, 25%); 2, xerophyll (chresard, 3–5%). × 130.]
=170. Bog plants.= Many of the xerophytic types just described are found
in ponds, bogs, and swamps, where the water supply is excessive, and
hydrophytes would be expected. The explanation that “swamp xerophytes”
are due to the presence of humic acids which inhibit absorption and
aeration in the roots has been generally accepted. As Schimper has
expressed it, bogs and swamps are “physiologically dry”, i. e., the
available water is small in amount, in spite of the great total
water-content. Burgerstein (_l. c._, 142) has shown, however, that maize
plants transpire, i. e., absorb, three times as much water in a solution
of 0.5 per cent of oxalic acid as they do in distilled water, and that
branches of _Taxus_ in a solution containing 1 per cent of tartaric acid
absorb more than twice as much as in distilled water. Consequently, it
seems improbable that small quantities of humic acids should decrease
absorption to the extent necessary for the production of xerophytes in
ponds and bogs. Indeed, in many ponds and streams, where _Heleocharis_,
_Scirpus_, _Juncus_, etc., grow, not a trace of acid is discoverable.
Furthermore, plants with a characteristic hydrophytic structure
throughout, such as _Ranunculus_, _Caltha_, _Ludwigia_, _Sagittaria_,
etc., are regularly found growing alongside of apparent xerophytes. Many
of the latter, furthermore, show a striking contrast in size and vigor
of growth in places where they grow both upon dry gravel banks and in
the water, indicating that the available water-content is much greater
in the latter. Finally, many so-called “swamp xerophytes” possess
typically hydrophytic structures, such as air-passages, diaphragms, etc.
In spite of a growing feeling that the xerophytic features of certain
amphibious plants can not be ascribed to a low chresard in ponds and
swamps, a satisfactory explanation of them has been found but recently.
This explanation has come from the work of E. S. Clements already cited,
in which it was found that certain sun plants underwent no material
structural change when grown in the shade, and that the same was true
also of a few species which grew in two or more habitats of very
different water-content. In accordance with this, it is felt that the
xerophytic features found in amphibious plants are due to the
persistence of stable structures, which were developed when these
species were growing in xerophytic situations. When it is called to mind
that monocotyledons, and especially the grasses, sedges, and rushes, are
peculiarly stable, it may be readily understood how certain ancestral
characters have persisted in spite of a striking change of habitat. Such
a hypothesis can only be confirmed by the methods of experimental
evolution, and a critical study of this sort is now under way.
[Illustration: Fig. 36. _Hippuris vulgaris_: 1, submerged leaf; 2,
aerial leaf. × 130.]
=171. Hydrophytic types.= Hydrophytes permit a fairly sharp division
into three groups, based primarily upon the relation of the leaf surface
to the two media, air and water. In submerged plants, the leaves are
constantly below the water; in amphibious ones, they grow normally in
the air. Floating plants have leaves in which the upper surface is in
contact with the air, and the lower in contact with the water.
Transpiration is at a maximum in the amphibious plant; it is reduced by
half in the floating type, and is altogether absent in submerged plants.
Aeration reaches a high development in amphibious and floating forms,
but air-passages are normally absent from submerged forms except as
vestiges. Photosynthesis is marked in the former, but considerably
weakened in the latter. The vascular system, which attains a moderate
development in the amphibious type, is considerably reduced in floating
forms, and it is little more than vestigiate in submerged ones.
[Illustration: Fig. 37. Floating leaf of _Sparganium angustifolium_. ×
130.]
1. _The amphibious type._ Plants of this type grow in wet soil or in
shallow water. The leaves are usually large and entire, the stem well
developed, and the roots numerous and spreading. In the majority of
cases the leaves are constantly above the water, but in some species the
lower leaves are often covered, normally, or by a rise in level, and
they take the form or structure of submerged leaves. This is illustrated
by _Callitriche autumnalis_, _Hippuris vulgaris_, _Ranunculus
delphinifolius_, _Proserpinaca palustris_, _Roripa americana_, _etc._
The epidermis has a thin cuticle, or none at all, and is destitute of
hairs. The stomata are numerous and usually more abundant on the upper
than on the lower surface. The palisade tissue is represented by one or
more well-developed rows, but this portion of the leaf is regularly
thinner than that of the sponge part. The latter contains large
air-passages, or, in the majority of cases, numerous air-chambers,
usually provided with diaphragms. The stems are often palisaded, and are
characterized by longitudinal air-chambers crossed by frequent
diaphragms, which extend downward through the roots.
2. _The floating type._ With respect to form and the structure of the
upper part of the leaf, floating leaves are essentially similar to those
of amphibious plants. They are usually lacquered or coated with wax to
prevent the stoppage of the stomata by water. Stomata, except as
vestiges, are found only on the upper surface, and the palisade tissue
is much less developed than the sponge, which is uniformly characterized
by large air-chambers. The stems are elongated, the aerating system is
enormously developed, and the supportive tissues are reduced. In the
_Lemnaceae_, the leaf and the stem are represented by a mere frond or
thallus, and the roots are in the process of disappearance, e. g.,
_Spirodela_ has several, _Lemna_ one, and _Wolffia_ none.
3. _The submerged type._ Both stem and root have been greatly reduced in
submerged plants, owing to the generalization of absorption and the
density of the water. The leaves are greatly reduced in size and
thickness, chiefly, it would seem, for the purpose of insuring readier
aeration and great illumination. The leaf may be ribbon-like, linear,
cylindrical, or finely dissected. Stomata are sometimes present, but
they are functionless and vestigial. A distinction into palisade and
sponge tissues, when present, must also be regarded as a vestige; the
chlorenchym is essentially that of a shade leaf. The air-chambers are
much reduced, and sometimes lacking; they function doubtless as
reservoirs for air obtained from the water.
PHOTOHARMOSE
_ADJUSTMENT_