intensity and not by quality. A single exception is afforded by those
aquatic habitats where the depth of water is great, and in consequence
of which certain rays disappear by absorption more quickly than others.
In forests and thickets, where the leaves transmit only the green and
yellow rays, it would appear that the light which reaches the herbaceous
layers is deficient in red and violet rays. The amount of light
transmitted by an ordinary sun leaf is so small, however, that it has no
appreciable effect upon the quality of the light beneath the facies,
which is diffuse white light that has passed between the leaves. Indeed,
it is only in the densest forests that distinct sunflecks do not appear.
Coniferous forests, with a light value less than .005, which suffices
only for mosses, lichens, and a few flowering plants, show frequent
sunflecks. This is convincing evidence that the light of such habitats
is normal in quality. It warrants the conclusion that in all habitats
with an intensity capable of supporting vascular plants the light, no
matter how diffuse, is white light. The direction of the light ray is of
slight importance in the field, apart from the difference in intensity
which may result from it. In habitats with diffuse light, the latter
comes normally and constantly from above. Likewise, in sunny situations,
direction can have little influence, since both the direction and the
angle of the incident rays change continually throughout the day, and
the position of the leaf itself is more or less constantly changed by
the wind. The influence of duration upon the character of light stimuli
is difficult to determine. There can be no question that the time during
which a stimulus acts has a profound bearing upon the response that is
made to it. In nature the problem is complicated by the fact that light
stimuli are both continuous and periodic. The duration of sunlight is
determined by the periodic return of night as well as by the irregular
occurrence of clouds. Since one is a regular, and the other at least a
normal happening, it is necessary to consider duration only with respect
to the time of actual sunlight on sunny days, except in the case of
formations belonging to regions widely different in the amount of normal
sunshine, i. e., the number of cloudy days. In consequence, duration is
really a question of the intensities which succeed each other during the
day. The differences between these have already been shown to fall
within the efficient difference for light, and for this reason the ratio
between the light intensity of a meadow and of a forest is essentially
the ratio between the sums of light intensity for the two habitats, i.
e., the duration. The latter is of importance only where there is a
daily alternation between sunshine and shadow, as at the edge of forest
and thicket, in open woodland, etc. In such places duration determines
the actual stimulus by virtue of the sum of preponderant intensities.
The periodicity of daylight is a stimulus to the guard cells of stomata,
but its relation to intensity in this connection is not clear.
The amount of change in light intensity necessary to constitute an
efficient stimulus seems to depend upon the existing intensity as well
as upon the plant concerned. Apparently, a certain relative decrease is
more efficient for sun plants than for shade plants. At least, many
species sooner or later reach a point where a difference larger than
that which has been efficient no longer produces a structural response.
This has been observed by E. S. Clements (_l. c._) in a number of shade
ecads. For example, a form of _Galium boreale_, which grew with
difficulty in a light value of .002, showed essentially the leaf
structure of the form growing in light of .03, while the form in full
sunlight showed a striking difference in the leaf structure. In
considering the light stimuli of habitats, it is unnecessary to discuss
the stimulus of total darkness upon chlorophyllous plants, although this
is of great importance in experimental evolution and in control
experiment. The normal extremes of light intensity, i. e., those within
which chlorenchym can function, are full sunshine represented by 1, and
a diffuseness of .002, though small flowering plants have once or twice
been found in an intensity of .001. The maximum light value, even on
high mountains, never exceeds 1 by more than an inconsiderable amount,
except for the temporary concentration due to drops of dew, rain, etc.
It seems improbable that the concentrating effect of epidermal papillae
can do much more than compensate for the reflection and absorption of
the epidermis. Experimental study has shown that the maximum intensity
in nature may be increased several, if not many times, without injurious
results and without an appreciable increase in the photosynthetic
response, thus indicating that the efficient difference increases toward
the maximum as well as toward the minimum.
=173. The reception of light stimuli.= Rays of light are received by the
epidermis, by which they are more or less modified. Part of the light is
reflected by the outer wall or by the cuticle, particularly when these
present a shining surface. Hairs diffract the light rays, and hairy
coverings consequently have a profound influence in determining stimuli.
The walls and contents of epidermal cells furthermore absorb some of the
light, especially when the cell sap is colored. In consequence of these
effects, the amount of light that reaches the chlorenchym is always less
than that incident upon the leaf, and in many plants, the difference is
very great. According to Haberlandt[16], the epidermal cells of some
shade plants show modifications designed to concentrate the light rays.
Of such devices, he distinguishes two types: one in which the outer
epidermal wall is arched, another in which the inner wall is deeply
concave. Although there can be no question of the effect of lens-shaped
epidermal cells, their occurrence does not altogether support
Haberlandt’s view. Arched and papillate epidermal cells are found in sun
plants where they are unnecessary for increasing illumination, to say
the least. A large number of shade plants show cells of this character,
but in many the outer wall is practically a plane. Shade forms of a
species usually have the outer wall more arched or papillate, but this
is not always true, and, in a few cases, it is the lower epidermis alone
that shows this feature. Finally, a localization of this function in
certain two-celled papillae, such as Haberlandt indicates for _Fittonia
verschaffelti_, does not appear to be plausible.
The epidermis merely receives the light; the perception of the stimulus
normally occurs in those cells that contain chloroplasts. The cytoplasm
of the epidermal cells, as well as that of the chlorenchym cells, is
sensitive to light, but the response produced by the latter is hardly
discernible in the absence of plastids, except in those plants which
possess streaming protoplasm. The daily opening and closing of the
stomata, which is due to light, is evidently connected with the presence
of chloroplasts in the guard cells. Naturally, the perception of light
and the corresponding response occur in the epidermis of many shade and
submerged plants which have chloroplasts in the epidermal cells. Such
cases merely serve to confirm the view that the perception of light
stimuli is localized in the chloroplast. In conformity with this view,
the initial response to such stimuli must be sought in the chloroplast,
and the explanation of all adaptations due to light must be found in the
adjustment shown by the chloroplasts.
=174. Response of the chloroplast.= The fundamental response of a
plastid to light is the manufacture of chlorophyll. In the presence of
carbon dioxide and water, leucoplasts invariably make chlorophyll, and
chloroplasts replace that lost by decomposition, in response to the
stimulus exerted by light. The latter is normally the efficient factor,
since water is always present in the living plant, and carbon dioxide
absent only locally at most. Sun plants which possess a distinct
cuticle, however, produce leucoplasts, not chloroplasts, in the
epidermal cells, although these are as strongly illuminated as the guard
cells, which contain numerous chloroplasts. This is evidently explained
by the lack of carbon dioxide in the epidermis. This gas is practically
unable to penetrate the compact cuticle, at least in the small quantity
present in the air. The supply obtained through the stomata is first
levied upon by the guard cells and then by the cells of the chlorenchym,
with the result that the carbon dioxide is all used before it can reach
the epidermal cells. This view is also supported by the presence of
chloroplasts along the sides and lower wall of palisade cells, where
there is normally a narrow air-passage, and their absence along the
upper wall when this is closely pressed against the epidermis, as is
usually the case. Furthermore, the leaves of some mesophytes when grown
in the sun develop a cuticle and contain leucoplasts. Under glass and in
the humid air of the greenhouse, the same plants develop epidermal
chloroplasts but no cuticle. This is in entire harmony with the
well-known fact that shade plants and submerged plants often possess
chloroplasts in the epidermis. Although growing in different media,
their leaves agree in the absence of a cuticle, and consequent
absorption of gases through the epidermis. The size, shape, number, and
position of the chloroplasts are largely determined by light, though a
number of factors enter in. No accurate studies of changes in size and
shape have yet been made, though casual measurements have indicated that
the chloroplasts in the shade form of certain species are nearly
hemispherical, while those of the sun form are plane. In the same
plants, the number of chloroplasts is strikingly smaller in the shade
form, but exact comparisons are yet to be made. The position and
movement of chloroplasts have been the subject of repeated study, but
the factors which control them are still to be conclusively indicated.
Light is clearly the principal cause, although there are many cases
where a marked change in the light intensity fails to call forth any
readjustment of the plastids. The position of air-spaces as reservoirs
of carbon dioxide and the movement of crude and elaborated materials
from cell to cell frequently have much to do with this problem. Finally,
it must be constantly kept in mind that the chloroplasts lie in the
cytoplasm, which is in constant contact with a cell wall. Hence, any
force that affects the shape of the cell will have a corresponding
influence upon the position of the chloroplasts. When it is considered
that in many leaves these four factors play some part in determining the
arrangement of the plastids, it is not difficult to understand that
anomalies frequently appear.
It may be laid down as a general principle that chloroplasts tend to
place themselves at right angles to rays of diffuse light and parallel
to rays of sunlight. This statement is borne out by an examination of
the leaves of typical sun and shade species, or of sun and shade forms
of the same species. Cells which receive diffuse light, i. e., sponge
cells, normally have their rows of plastids parallel with the leaf
surface, while those in full sunlight place the rows at right angles to
the surface. This disposition at once suggests the generally accepted
view that chloroplasts in diffuse light are placed in such a way as to
receive all the light possible, while those in sunlight are so arranged
as to be protected from the intense illumination. Many facts support
this statement with respect to shade leaves, but the need of protection
in the sun leaf is not clearly indicated. The regular occurrence of
normal chloroplasts in the guard cells seems conclusive proof that full
sunlight is not injurious to them. Although the upper wall of the outer
row of palisade cells is usually free from chloroplasts, yet it is not
at all uncommon to find it covered by them. These two conditions are
often found in cells side by side, indicating that the difference is due
to the presence of carbon dioxide and not to light. In certain species
of monocotyledons, the arrangement of the chloroplasts is the same in
both halves of the leaf, and there is no difference between the sun and
shade leaves of the same species. The experimental results obtained with
concentrated sunlight, though otherwise conflicting, seem to show
conclusively that full sunlight does not injure the chloroplasts of sun
plants, and that the position of plastids in palisade cells is not for
the purpose of protection. This arrangement, which is known as
_apostrophe_, is furthermore often found in shade forms of heliophytes.
In typical shade species, and in submerged plants, the disposition of
plastids on the wall parallel with the leaf surface, viz., _epistrophe_,
is more regular, but even here there are numerous exceptions to the
rule.
The absorption of the light stimulus by the green plastid results, under
normal conditions, in the immediate production of carbohydrates, which
in the vast majority of cases soon become visible as grains of starch.
The appearance of starch in the chloroplasts of flowering plants is such
a regular response to the action of light that it is regarded as the
normal indication of photosynthetic activity. The mere presence of
chlorophyll is not an indication of the latter, since chlorophyll
sometimes persists in light too diffuse for photosynthesis. The amount
of starch formed is directly connected with the light intensity, and in
consequence it affords a basis for the quantitative estimation of the
response to light. Two responses to light stimuli have a direct effect
upon the amount of transpiration. Of the light energy absorbed by the
chloroplast, only 2.5 per cent is used in photosynthesis, while 95–98
per cent is converted into heat, and brings about marked increase in
transpiration. Furthermore, in normal turgid plants, the direct action
of light, as is well known, opens the stomata in the morning and closes
them at night.
[Illustration: Fig. 38. Ecads of _Allionia linearis_, showing position
of chloroplasts. The palisade shows apostrophe, the sponge epistrophe:
1, sun leaf (chresard, 2–5%, light, 1); 2, shade leaf (chresard, 11%;
light, .012); 3, shade leaf (chresard, 11%; light, .003). × 250.]
=175. Aeration and translocation.= The movements of gases and of
solutions through the tissues of the leaf are intimately connected with
photosynthesis, and hence with responses to light stimuli. Aeration
depends primarily upon the periodic opening of the stomata, for, while
the carbon dioxide and oxygen of the air are able to pass through
epidermal walls not highly cutinized, the amount obtainable in this
manner is altogether inadequate, if not negligible. The development of
sponge tissue or aerenchym is intimately connected with the stomata. The
position and amount of aerenchym and the relative extent of sponge cells
and air-spaces are in part determined by the number and position of the
breathing pores. The disposition of air spaces has much to do with the
arrangement of chloroplasts in both palisade and sponge tissues. Starch
formation is also dependent upon the presence of air spaces, but,
contrary to what would be expected, it seems to be independent of their
size, since sun leaves, which assimilate much more actively than shade
leaves, have the smallest air spaces. From this fact, it appears that
the rapidity of aeration depends very largely upon the rapidity with
which the gases are used. Translocation likewise affects the arrangement
of the chloroplasts and the formation of starch. According to
Haberlandt, it also plays the principal part in determining the form and
arrangement of the palisade cells. Chloroplasts are regularly absent at
those points of contact where the transfer of materials is made from
cell to cell, though this is not invariably true. Since air passages are
necessarily absent where cell walls touch, it is possible that this
disposition of the plastids is likewise due to the lack of aeration.
Translocation is directly connected with the appearance of starch. As
long as all the sugar made by the chloroplasts is transferred, no starch
appears, but when assimilation begins to exceed translocation, the
increasing concentration of the sugar solution results in the production
of starch grains. The latter is normally the case in all flowering
plants, with the exception of those that form sugar or oil, but no
starch. The constant action of translocation is practically
indispensable to starch formation, since an over-accumulation of
carbohydrates decreases assimilation, and finally inhibits it
altogether. In consequence, translocation occurs throughout the day and
night, and by this means the accumulated carbohydrates of one day are
largely or entirely removed before the next.
[Illustration: Fig. 39. Position of chloroplasts in aerial leaf (1) and
submerged leaf (2) of _Callitriche bifida_. × 250.]
=176. The measurement of responses to light.= Responses, such as the
periodic opening and closing of stomata, which are practically the same
for all leaves, are naturally not susceptible of measurement. This is
also true of the transpiration produced by light, but the difficulty in
this case is due to the impossibility of distinguishing between the
water loss due to light and that caused by humidity and other factors.
If it were possible to determine the amount of chlorophyll or glucose
produced, these could be used as satisfactory measures of response. As
it is, they can only be determined approximately by counting the
chloroplasts or starch grains. The arrangement of the chloroplasts can
not furnish the measure sought, since it does not lend itself to
quantitative methods, and since the relation to light intensity is too
inconstant. Hesselmann (_l. c._, 400) has determined the amount of
carbon dioxide respired, by means of a eudiometer, and has based
comparisons of sun and shade plants upon the results. As he points out,
however, light has no direct connection with respiration. Although the
latter increases necessarily with increased nutrition, the relation
between them is so obscure, and so far from exact, that the amount of
respiration can in no wise serve as a measure of the response to light.
As a result of the foregoing, it is clear that no functional response is
able to furnish a satisfactory measure of adjustment to light, though
one or two have perhaps sufficient value to warrant their use. Indeed,
structural adaptations offer a much better basis for the quantitative
determination of the effects of light stimuli, as will be shown later.
In attempting to use the number of chloroplasts or starch grains as a
measure of response, the study should be confined to the sun and shade
forms of the same species, or, in some cases, to the forms of closely
related species. The margin of error is so great and the connection with
light sufficiently remote that comparisons between unrelated forms or
species are almost wholly without value. It has already been stated that
starch is merely the surplus carbohydrate not removed by translocation;
the amount of starch, even if accurately determined, can furnish no real
clue to the amount of glucose manufactured. In like manner, the number
of chloroplasts can furnish little more than an approximation of the
amount of chlorophyll, unless size and color are taken into account. In
sun and shade ecads of the same species, the general functional
relations are essentially the same, and whatever differences appear may
properly be ascribed to different light intensities for the two
habitats. The actual counting of chloroplasts and starch grains is a
simple task. Pieces of the leaves of the two or more forms to be
compared are killed and imbedded in paraffin in the usual way. To save
time, the staining is done _in toto_. Methyl green is used for the
chloroplasts and a strong solution of iodine for the starch grains. When
counts are to be made of both, the leaves are first treated with iodin
and then stained with the methyl green. The thickness of the microtome
sections should be less than that of the palisade cells in order that
the chloroplasts may appear in profile, thus facilitating the counting.
The count is made for a segment 100 μ in width across the entire leaf.
Two segments in different parts of the section are counted, and the
result multiplied by five to give the number for a segment 1 millimeter
in width. Although sun and shade leaves regularly differ in size and
thickness, no correction is necessary for these. Size and thickness
stand in reciprocal relation to each other in ecads, and thickness is
largely an expression of the absorption of light, and hence of its
intensity. In the gravel, forest, and thicket ecads of _Galium boreale_,
counts of the chloroplasts gave the following results. The gravel form
(light 1) showed 3,500 plastids in the 1–mm. segment, the forest form
(light .03) possessed 1,350, and the thicket form (light .002), 1,000.
In these no attention was paid to the size and form of the plastids in
the different leaves, since the differences were inappreciable. When
this is not the case, both factors should be taken into account. Starch
grains are counted in exactly the same way. Indeed, if care is taken to
collect leaves of forms to be compared, at approximately the same time
on sunshiny days, a count of the chloroplasts is equivalent to a count
of the starch grains in the vast majority of cases. Measurements of the
size of starch grains can be made with accuracy only when the leaves are
killed in the field at the same time, preferably in the afternoon.
Counts of chloroplasts alone can be used as measures of response in
plants that produce sugar or oil, while either chloroplasts or starch
grains or both may be made the basis in starch-forming leaves.
Hesselmann (_l. c._, 379) has employed Sachs’s iodine test as a measure
of photosynthesis. This has the advantage of permitting macroscopic
examination, but the comparison of the stained leaves can give only a
very general idea of the relative photosynthetic activity of two or more
ecads. The iodine test is made as follows:[17] fresh leaves are placed
for a few minutes in boiling water, and then in 95 per cent alcohol for
2–5 minutes, in order to remove the chlorophyll and other soluble
substances. The leaves are placed in the iodine solution for ½–3 hours,
or until no further change in color takes place. The strength of the
solution is not clearly indicated by Sachs, who says: “I used an
alcoholic solution of iodin which is best made by dissolving a large
quantity of iodin in strong alcohol and adding to this sufficient
distilled water to give the liquid the color of dark beer.” This
solution may be approximated by dissolving ⅓ gram of iodin in 100 grams
of 30 per cent alcohol. The stained leaves are put in a white porcelain
dish filled with distilled water, and the dish placed in the strong
diffuse light of a window. The colored leaf stands out sharply against
the porcelain, and the degree of coloration, and hence of starch
content, is determined by the following table:
1. bright yellow or leather yellow (no starch in the chlorenchym)
2. blackish (very little starch in the chlorenchym)
3. dull black (starch abundant)
4. coal black (starch very abundant)
5. black, with metallic luster (maximum starch-content)
_ADAPTATION_
=177. Influence of chloroplasts upon form and structure.= The beginning
of all modifications produced by light stimuli must be sought in the
chloroplast as the sensitized unit of the protoplasm. Hence, it seems a
truism to say that the number and arrangement of the chloroplasts
determine the form of the cell, the tissue, and the leaf, although it
has not yet been possible to demonstrate this connection conclusively by
means of experiment. In spite of the lack of experimental proof, this
principle is by far the best guide through the subject of adaptations to
light, and in the discussion that follows, it is the fundamental
hypothesis upon which all others rest. The three propositions upon which
this main hypothesis is grounded are: (1) that the number of
chloroplasts increases with the intensity of the light; (2) that in
shaded habitats chloroplasts arrange themselves so as to increase the
surface for receiving light; (3) that chloroplasts in sunny habitats
place themselves in such fashion as to decrease the surface, and
consequently the transpiration due to light. In these, there can be
little doubt concerning the facts of number and arrangement, since they
have been repeatedly verified. The purpose of epistrophe and apostrophe,
however, can not yet be stated with complete certainty.
The stimulus of sunlight and of diffuse light is the same in one
respect, namely, the chloroplasts respond by arranging themselves in
rows or lines on the cell wall. The direct consequence of this is to
polarize the cell, and its form changes from globoid to oblong. This
effect is felt more or less equally by both palisade and sponge cells,
but the disturbing influence of aeration has caused the polarity of the
cells to be much less conspicuous in the sponge than in the palisade
tissue. While the cells of both are typically polarized, however, they
assume very different positions with reference to incident light. This
position is directly dependent upon the arrangement of the plastids as
determined by the light intensity. In consequence, palisade cells stand
at right angles to the surface and parallel with the impinging rays; the
sponge cells, conversely, are parallel with the epidermis and at right
angles to the light ray. Some plants, especially monocotyledons, exhibit
little or no polarity in the chlorenchym. As a result the leaf does not
show a differentiation into sponge and palisade, and the leaves of sun
and shade ecads are essentially alike in form and structure. The form of
the leaf is largely determined by the chloroplasts acting through the
cells that contain them. A preponderance of sponge tissue produces an
extension of leaf in the direction determined by the arrangement of the
plastids and the shape of the sponge cells, viz., at right angles to the
light. Shade leaves are in consequence broader and thinner, and
sometimes larger, than sun leaves of the same species. A preponderance
of palisade likewise results in the extension of the leaf in the line of
the plastids and the palisade cells, i. e., in a direction parallel with
the incident ray. In accordance, sun leaves are thicker, narrower, and
often smaller than shade leaves.
=178. Form of leaves and stems.= In outline, shade leaves are more
nearly entire than sun leaves. This statement is readily verified by the
comparison of sun and shade ecads, though the rule is by no means
without exceptions. In the leaf prints shown in figures 14 and 15, the
modification of form is well shown in _Bursa_ and _Thalictrum_; in
_Capnoides_ the change is less evident, while in _Achilleia_ and
_Machaeranthera_ lobing is more pronounced in the shade form, a fact
which is, however, readily explained when other factors are taken into
account. The leaf prints cited serve as more satisfactory examples of
the increase of size in consequence of an increase in the surface of the
shade leaf, although the leaves printed were selected solely with
reference to thickness and size or outline. In all comparisons of this
kind, however, the relative size and vigor of the two plants must be
taken into account. This precaution is likewise necessary in the case of
thickness, which should always be considered in connection with amount
of surface. The relation between surface and thickness is shown by the
following species, in all of which the size of the leaf is greater in
the shade than in the sun. In _Capnoides aureum_, the thickness of the
shade leaf is ½ (6 : 12) that of the sun leaf; in _Galium boreale_ the
ratio is 5 : 12, and in _Allionia linearis_ it is 3 : 12. The ratio in
_Thalictrum sparsiflorum_ is 9 : 12, and in _Machaeranthera aspera_ 11 :
12. The thickness of sun and shade leaves of _Bursa bursa-pastoris_ is
as 14 : 12, but this anomaly is readily explained by the size of the
plants; the shade form is ten times larger than the sun form. Certain
species, e. g., _Erigeron speciosus_, _Potentilla bipinnatifida_, etc.,
show no change in thickness and but little modification in size or
outline. They furnish additional evidence of a fundamental principle in
adaptation, namely that the amount of structural response is profoundly
affected by the stability of the ancestral type.
The effect of diffuse light in causing stems to elongate, though known
for a long time, is still unexplained. The old explanation that the
plant stretches up to obtain more light seems to be based upon nothing
more than the coincidence that the light comes from the direction toward
which the stem grows. Later researches have shown that the stretching of
the stem is due to the excessive elongation of the parenchyma cells, but
the cause of the latter is far from apparent. It is generally assumed to
be due to a lack of the tonic action of sunlight, which brings about a
retardation of growth in sun plants. The evidence in favor of this view
is not conclusive, and it seems probable at least that the elongation of
the parenchyma cells takes place under conditions which favor the
mechanical stretching of the cell wall, but inhibit the proper growth of
the wall by intussusception. It is hardly necessary to state that the
reduced photosynthetic activity of shade plants favors such an
explanation. Whatever the cause, the advantage that results from the
elongation of the internodes is apparent. Leaves interfere less with the
illumination of those below them, and the leaves of the branches are
carried away from the stem in such a way as to give the plant the best
possible exposure for its aggregate leaf surface.
[Illustration: Fig. 40. Isophotophyll of _Allionia linearis_, showing
diphotic ecads: 1, light 1; 2, light .012; 3, light .003. × 130.]
[Illustration: Fig. 41. Isophotophyll of _Helianthus pumilus_, showing
isophotic ecad: 1, sun leaf; 2, shade leaf (light .012). × 130.]
=179. Modification of the epidermis.= The development of epidermal
chloroplasts in diffuse light is the only change which is due to the
direct effect of light. This does not often occur in the shade ecads of
sun species, but chloroplasts are regularly present in the epidermis of
woodland ferns and of submerged plants. The slight development of hairs
in sciophilous plants is an advantage, but it must be referred to the
factors that determine water loss. The significance of epidermal
papillae in increasing the absorption of light by shade plants has
already been discussed. The questions as to what factor has called forth
these papillae and what purpose they serve must still be regarded as
unsettled. The increased size of the epidermal cells, which is a fairly
constant feature of shade ecads, seems to be for the purpose of
increasing translocation and transpiration, and to bear no relation to
light. The extreme development of the cells of the epidermis in
_Streptopus_ and _Limnorchis_, which grow at the edge of mountain
brooks, has been plausibly explained by E. S. Clements as a contrivance
to increase water loss. The presence of a waxy coating, such as that
found upon the leaves of _Impatiens aurea_ and _I. pallida_, is clearly
to prevent the wetting of the leaf and the consequent stoppage of the
stomata. In regard to the latter, different observers have noted that
the number of the stomata is greater in sun than in shade leaves. This
holds generally for sun and shade species, but it is most clearly
indicated by different ecads of the same species. In _Scutellaria
brittonii_, the sun form possesses 100 stomata per square millimeter,
but in the shade these are reduced to 40 per square millimeter; the sun
leaf of _Allionia linearis_ has 180 stomata to the square millimeter,
the shade leaf 90. In the stable leaf of _Erigeron speciosus_, however,
the number of stomata is the same, 180 per square millimeter, for
sunlight and for diffuse light. The presence of the larger number of
stomata in the plant exposed to greater loss, which at first thought
seems startling, is readily explained by the more intense photosynthetic
activity in the sun. Since the absorption of gases is the primary
function of the stomata, and transpiration merely secondary, it is
evident that sun plants must have more stomata than shade plants. This
is further explained by the fact that the small air passages of sun
leaves necessitate frequent inlets, which are less necessary in shade
leaves with their larger air spaces. In shade plants, moreover, the
decrease in the number is compensated in some measure by the ability of
the epidermal cells to absorb gases directly from the air.
[Illustration: Fig. 42. Diphotophylls of _Quercus novimexicana_: 1, sun
leaf; 2, shade leaf of the same tree (light .06). × 130.]
=180. The differentiation of the chlorenchym.= The division of the
chlorenchym into two tissues, sponge and palisade, is the normal
consequence of the unequal illumination of the leaf surfaces. Exceptions
to this rule occur only in certain monocotyledons, in which the leaf
tissue consists of sponge-like cells throughout, and in those stable
species that retain more or less palisade in spite of their change to
diffuse light. The difference in the illumination of the two surfaces is
determined by the position of the leaf. Leaves that are erect or nearly
so usually have both sides about equally illuminated, and they may be
termed isophotic. Leaves that stand more or less at right angles to the
stem receive much more light upon the upper surface than upon the lower,
and may accordingly be termed diphotic. Certain dorsiventral leaves,
however, absorb practically as much light on the lower side as upon the
upper. This is true of sun leaves with a dense hairy covering, which
screens out the greater part of the light incident upon the upper
surface. It occurs also in xerophytes which grow in light-colored sands
and gravels that serve to reflect the sun’s rays upon the lower surface.
In deep shade, moreover, there is no essential difference in the
intensity of the light received by the two surfaces, and shade leaves
are often isophotic in consequence. From these examples it is evident
that isophotic and diphotic leaves occur in both sun and shade, and that
the intensity of the light is secondary to direction, in so far as the
modification of the leaf is concerned.
The essential connection of sponge tissue with diffuse light is
conclusively shown by the behavior of shade ecads, but further evidence
of great value is furnished by diphotic leaves, and those with hairy
coverings. The sponge tissue, which in the shade leaf is due to the
diffuse light of the habitat, is produced in the hairy leaf as a
consequence of the absorption and diffraction of the light by the
covering. In ordinary diphotic leaves, the absorption of light in the
palisade reduces the intensity to such a degree that the cells of the
lower half of the leaf are in diffuse light, and are in consequence
modified to form sponge tissue. The sponge tissue of the diphotic leaf
is just as clearly an adaptation to diffuse light as it is in those
plants where the whole chlorenchym is in the shade of other plants or of
a covering of hairs. As is indicated later, all these relations permit
of ready confirmation by experiment, either by changing the position of
the leaf or by modifying the intensity or direction of the light.
[Illustration: Fig. 43. A plastic species, _Mertensia polyphylla_,
showing the effect of water upon the sponge: 1, chresard 25%; 2,
chresard 12%. × 130.]
The preceding discussion makes it fairly clear that sponge tissue is
developed primarily to increase the light-absorbing surface. Because of
its direct connection with photosynthesis, the sponge tissue is the
especial organ of aeration, also, and since it shows a high development
of air spaces for this purpose, it is inevitably concerned in
transpiration. It seems to be partly a coincidence, however, that the
sponge is found next to the lower surface upon which the stomata are
most numerous. This is indicated by artificial ecads of _Ranunculus
sceleratus_, in which sponge tissue is unusually developed, although the
stomata are much more numerous upon the upper surface. Palisade tissue
is apparently developed primarily as a protection against water loss,
particularly that due to the absorption of light by the chloroplast. The
small size of the intercellular passages between palisade cells likewise
aids in decreasing transpiration. The fact that leaves with much
palisade tissue transpire twice as much as shade leaves is hardly an
objection to this view, as Hesselmann (_l. c._, 442) would think. It is
readily explained by the intense photosynthesis of sun plants, which
makes necessary an increase, usually a doubling, in the number of
stomata, in consequence of which the transpiration is increased.
[Illustration: Fig. 44. A stable species, _Erigeron speciosus_: 1, sun
leaf; 2, shade leaf (light .03). × 130.]
[Illustration: Fig. 45. Spongophyll of _Gyrostachys stricta_ (light 1).
× 130.]
=181. Types of leaves.= Isophotic leaves are equally illuminated and
possess more or less uniform chlorenchym. Diphotic leaves are unequally
illuminated, and exhibit a differentiation into palisade and sponge
tissues. They may be distinguished as isophotophylls and diphotophylls
respectively.[18] Isophotic leaves fall into three types based upon the
intensity of the light. The staurophyll, or palisade leaf, is a sun type
in which the equal illumination is due to the upright position or to the
reflection from a light soil, and in which the chlorenchym consists
wholly of rows of palisade cells. The diplophyll is a special form of
this type in which the intense light does not penetrate to the middle of
the leaf, thus resulting in a central sponge tissue, or water-storage
tissue. The spongophyll, or sponge leaf, is regularly a shade type; the
chlorenchym consists of sponge cells alone. For the present at least it
is also necessary to refer to this group those monocotyledons which grow
in the sun but contain no palisade tissue. Diphotic leaves always
contain both palisade and sponge, though the ratio between them varies
considerably. Diphotophylls are characteristic of sunny mesophytic
habitats. They are frequent in xerophytic habitats as well as in
woodlands where the light is not too diffuse. In the case of stable
species, this type of structure sometimes persists in the diffuse light
of coniferous forests. Floating leaves, in which the light is almost
completely cut off from the lower surface, are also members of this
group. Submerged leaves, on the other hand, are spongophylls.
=182. Heliophytes and sciophytes.= The great majority of sun plants
possess diphotophylls. This type is represented by _Pedicularis procera_
(fig. 32). Plants with isophotophylls are found chiefly in xerophytic
places, though erect leaves of this type occur in most sunny habitats.
The staurophyll, in which the protection is due to the extreme
development of palisade tissue, is illustrated by _Allionia linearis_
(fig. 40) and _Bahia dissecta_ (fig. 33). The diplophyll, which is
characterized by a central band of sponge tissue or storage cells, is
found in _Mertensia linearis_ (fig. 34). The form of the spongophyll
that is found in certain monocotyledons is shown by _Gyrostachys
stricta_ (fig. 45). The spongophyll (fig. 38:3, 39:2) is frequent among
plants of deep shade, but as the leaf sections of _Allionia_ (figs. 38,
40) and _Quercus_ (fig. 42) show, the diphotophyll is the rule in shade
ecads.
EXPERIMENTAL EVOLUTION
=183. Scope.= The primary task of experimental evolution is the detailed
study, under measured conditions, of the origin of new forms in nature.
As a department of botanical research that is as yet unformed, it has
little concern with the host of hypotheses and theories which rest
merely upon general observation and conjecture. A few of these
constitute good working hypotheses or serve to indicate possible points
of attack, but the vast majority are worthless impedimenta which should
be thrown away at the start. It is the general practice to speak of
evolution as founded upon a solid basis of incontestible facts, but a
cursory examination of the evidence shows that it is drawn, almost
without exception, from observation alone, and has in consequence
suffered severely from interpretation. With the exception of De Vries’s
work on mutation, sustained and accurate investigation of the evolution
of plants has been lacking. As a result, botanical research has been
built high upon an insecure foundation, nearly every stone of which must
be carefully tested before it can be left permanently in place. In a
field so vast and important as evolution, experiment should far outrun
induction, and deduction should enter only when it can show the way to a
working hypothesis of real merit. The great value of De Vries’s study of
mutation as an example of the proper experimental study of evolution has
been seriously reduced by the fact that the “mutation theory” has
carried induction far beyond the warrant afforded by experiment. The
investigator who plans to make a serious study by experiment of the
origin of new plant forms should rest secure in the conviction that the
most rapid and certain progress can be made only by the accumulation of
a large number of unimpeachable facts, obtained by the most exact
methods of experimental study.
The general application of field experiment to evolution will render the
current methods of recognizing species quite useless. It will become
imperative to establish an experimental test for forms and species, and
to apply this test critically to every “new species.” Descriptive
botany, as practiced at present, will fall into disuse, as scientific
standards come to prevail, and in its place will appear a real science
of taxonomy. In the latter the criteria upon which species are based
will be obtained solely by experiment.
=184. Fundamental lines of inquiry.= There are two primary and sharply
defined fields of research in experimental evolution, namely, adaptation
in consequence of variation (and mutation), and hybridization. The
latter constitutes a particular field of inquiry, which is not
intimately connected with the problems of evolution in nature. In the
study of specific adaptation, two questions of profound importance
appear. One deals with the effects of ancestral fixity or plasticity in
determining the amount of modification produced by the habitat. These
are fundamental problems, and a solution of them can not be hoped for
until exact and trustworthy data have been provided by numerous
experimental researches. It thus becomes clear that the principal, if
not the sole task of experimental evolution for years to come is the
diligent prosecution of accurate and prolonged experiment in the
modification of plant forms. It seems inevitable that this will be
carried on along the lines that have already been indicated. Plants will
be grown in habitats of measured value, or in different intensities of
the same factor. The relation between stimulus and adjustment will form
the basis of careful quantitative study, and the final expression of
this relation in structural modifications will find an exact record in
drawings, photographs, exsiccati, and biometrical measures. The making
of an accurate and complete record of the whole course of each
experiment of this sort is an obligation that rests upon every
investigator. Studies in experimental evolution will prove
time-consuming beyond all other lines of botanical research, and the
work of one generation should appear in a record so perfect that it can
be used without doubt or hesitation as a basis for the studies of the
succeeding generation.
=185. Ancestral form and structure.= The significance of the fact that
some species have been found to remain unaltered structurally under
changes of habitats that produced striking modifications in others has
already been commented upon. It is hardly necessary to indicate the
important bearing which this has upon evolution. The very ability of a
plant to undergo modification, and hence to give rise to new forms,
depends upon the degree of fixity of the characters which it has
inherited. Stable plants are less susceptible of evolution than plastic
ones. The latter adapt themselves to new habitats with ease, and in each
produce a new form, which may serve as the starting point of a phylum.
There is at present no clue whatever as to what calls forth this
essential difference in behavior. This is not surprising in view of the
fact that there have been no comparative experimental studies of stable
and plastic species. Until these have been made, it is impossible to do
more than to formulate a working hypothesis as to the effect of
stability, and an explanation of the forces which cause or control it is
altogether out of the question.
=186. Variation and mutation.= New forms of plants are known to arise by
three methods, viz., variation, mutation, adaptation. The evidence in
support of these is almost wholly observational, and consequently more
or less inexact, but for each there exist a few accurate experiments
which are conclusive. Origin by variation and subsequent selection is
the essence of the Darwinian theory of the origin of species. According
to this the appearance of a new form is due to the accumulation, and
selection, through a long period, of minute differences which prove
advantageous to the plant in its competition with others in nature, or
are desirable under cultivation. Slight variations appear
indiscriminately in every species. Their cause is not known, but since
they are found even in the most uniform habitats, it is impossible to
find any direct connection between them and the physical factors. In the
case of origin by mutation, the new form appears suddenly, with definite
characteristics fully developed. Selection, in the usual sense of the
term, does not enter into mutation at all, though the persistence of the
new form is still to be determined by competition. Mutations are known
at present for only a few species, and their actual appearance has been
studied in a very few cases. Like variations, they are indiscriminate in
character. The chief difference between them is apparently one of
degree. Indeed, mutation lends itself readily to the hypothesis that it
is simply the sudden appearance of latent variations which have
accumulated within the plant. De Vries regards constancy as an essential
feature of mutation, but the evidence from the mutants of _Onagra_ is
not convincing. Indeed, while there can be no question of the occurrence
of mutation in plants, a fact known for many years, the facts so far
brought forward in support of the “mutation theory” fall far short of
proving “the lack of significance of individual variability, and the
high value of mutability for the origin of species.”[19] Mutations do
not show any direct connection with the habitat, but their sudden
appearance suggests that they may be latent or delayed responses to the
ordinary stimuli. Origin by adaptation is the immediate consequence of
the stimuli exerted by the physical factors of a habitat. This fact
distinguishes it from origin by variation, or by mutation. The new form
may appear suddenly, often in a single generation, or gradually, but in
either case it is the result of adaptation that is necessarily
advantageous, because it is the result of adjustment to controlling
physical factors. Origin by adaptation is perhaps only a special kind of
origin by variation, but this might be said with equal truth of
mutation. New forms resulting from adaptation are like those produced
from mutation, in that they appear suddenly as a rule and without the
agency of selection. They are essentially different, inasmuch as their
cause may be found at once in the habitat, and since a reversal of
stimuli produces, in many cases at least, a reversion in form and
structure to the ancestral type.
A valid distinction between forms or species upon the basis of constancy
is impracticable at the present time. It is doubtful that such a
distinction can ever be made in anything like an absolute sense, since
all degrees of fluctuation may be observed between constancy and
inconstancy. In all events, it is gratuitous to make constancy the
essential criterion in the present state of our knowledge. So little is
certainly known of it that it is equally unscientific to affirm or to
deny its value, and even a tentative statement can not be ventured until
a vast amount of evidence has been obtained from experiment.
Accordingly, there is absolutely no warrant, other than tradition, for
limiting the term species to a constant group. In the evolutionary
sense, a species is the aggregate ancestral group and the new forms
which have sprung from it by variation, mutation, or adaptation. It
should not be regarded as an isolated unit for purposes of descriptive
botany; indeed, its use in this connection is purely secondary. It is
properly the unit to be used in indicating the primary relationships
which are the result of evolution.
On the basis of their actual behavior in the production of new forms,
species may be distinguished as variable, mutable, or adaptable. The new
form which results from variation is a _variant_; the product of
mutation is a _mutant_, and that of adaptation, an _ecad_. The following
examples serve to illustrate these distinctions. _Machaeranthera
canescens_, judging from the numerous minute intergrades between its
many forms, is a variable species, i. e., one in which forms are arising
by the gradual selection of small variations. It apparently comprises a
large number of variants, _M. canescens aspera_, _superba_, _ramosa_,
_viscosa_, etc. _Onagra lamarckiana_ is a mutable species: it comprises
many mutants, e. g., _Onagra lamarckiana gigas_, _O. l. nanella_, _O. l.
lata_, etc. _Galium boreale_ is an adaptable species: it possesses one
distinct ecad, _Galium boreale hylocolum_, which is the shade form of
the species.
=187. Methods.= The best of all experiments in evolution are those that
are constantly being made in nature. Such experiments are readily
discovered and studied in the case of origin by adaptation; variants
present much greater difficulties, while mutants are very rare under
natural conditions. The method which makes use of these experiments may
be termed the _method of natural experiment_. The number of ecads which
appear naturally in vegetation is limited, however, and it is
consequently very desirable to produce them artificially, by the _method
of habitat culture_. This method, while involving more labor than the
preceding, yields results that are equally conclusive, and permits the
study of practically every species. The _method of control culture_,
which is carried on in the planthouse, naturally does not possess the
fundamental value of the field methods. It is an invaluable aid to the
latter, however, since it permits the physical factors to be readily
modified and controlled. All these methods are based on the
indispensable use of instruments for the measurement of physical
factors.
METHOD OF NATURAL EXPERIMENT
=188. Selection of species.= Species that are producing variants or
ecads are found everywhere in nature; those which give rise to mutants
seem, however, to be extremely rare. Consequently, mutants can not be
counted upon for experimental work, and their study scarcely needs to
be considered. When a mutant is discovered by some fortunate chance,
the mutable species from which it has sprung, and related species as
well, should be subjected to the most critical surveillance, in the
hope that new mutants will occur or the original one reappear. On
account of the suddenness with which they appear, mutants do not lend
themselves readily to natural experiment, and after they have once
been discovered, inquiry into the causes and course of mutation is
practicable only by means of habitat and control cultures. Among
variable species, those are most promising that show a wide range of
variation and are found in abundance over extensive areas. A species
which occurs in widely separated, or more or less isolated areas,
furnishes especially favorable material for investigation, since
distance or physical barriers partly eliminate the leveling due to
constant cross-fertilization. The individuals or groups which show
appreciable departure from the type are marked and observed critically
from year to year. The direction of the variation and the rapidity
with which small changes are accumulated can best be determined by
biometrical methods. Representative individuals of the species and
each of its variants should likewise be selected from year to year.
After being photographed, these are preserved as exsiccati, and with
the photographs constitute a complete graphic record of the course of
variation. When the latter is made evident in structural feature also,
histological slides are an invaluable part of the record.
Polydemic species are by far the best and most frequent of all natural
experiments. In addition to plants that are strictly polydemic, i. e.,
grow in two or more distinct habitats, there are a large number which
occur in physically different parts of the same habitat. The recognition
of polydemics is the simplest of tasks. As a rule, it requires merely a
careful examination of contiguous formations in order to ascertain the
species common to two or more of them. The latter are naturally most
abundant along the ecotones between the habitats, and, as a result,
transition areas and mixed formations are almost inexhaustible sources
of ecads. Many adaptable species are found throughout several
formations, however, and such are experiments of the greatest possible
value. Not infrequently species of the manuals are seen to be ecads, in
spite of their systematic treatment, and to constitute natural
experiments that can be readily followed. Finally, it must be kept in
mind that some polydemics are stable, and do not give rise to ecads by
structural adaptation. They not only constitute extremely interesting
experiments in themselves, but they should also be very carefully
followed year by year, since it seems probable that the responses are
merely latent, and that they will appear suddenly in the form of
mutants. In natural experiments it is sometimes difficult to distinguish
which form is the ecad and which the original form of the species. As a
rule, however, this point can be determined by the relative abundance
and the distribution, but in cases of serious doubt, it is necessary to
appeal to experimental cultures.
Although habitats differ more or less with respect to all their factors,
the study of polydemics needs to take into account only the direct
factors, water-content, humidity, and light. Humidity as a highly
variable factor plays a secondary part, and in consequence the search
for ecads may be entirely confined to those habitats that show efficient
differences in the amount of water-content or of light. Temperature,
wind, etc., do not produce ecads, and may be ignored, except in so far
as they affect the direct factors. Complexes of factors, such as
altitude, slope, and exposure, are likewise effective only through the
action of the component simple factors upon water and light. The
influence of biotic factors is so remote as to be negligible, especially
in view of the fact that ecads are necessarily favorable adaptations,
and are in consequence little subject to selective agencies. The
essential test of a habitat is the production of a distinguishable ecad,
but a knowledge of the water-content and light values of the habitats
under examination is a material aid, since a minute search of each
formation is necessary to reveal all the ecads. It is evident that
habitats or areas that do not show efficient differences of water or
light will contain no ecads of their common species, and also that
extreme differences in the amount of either of these two factors will
preclude origin by adaptation to a large degree, on account of the need
for profound readjustment. The general rule followed by most polydemics
is that sun species will give rise to shade forms, and vice versa, and
that xerophytes will produce forms of hydrophytic tendency, or the
converse, when the areas concerned are not too remote, and the water or
light differences are efficient, but not inhibitive. Some species are
capable of developing naturally two series of ecads, one in response to
light, the other to water-content, but they, unfortunately, have been
found to be rare. Greatly diversified regions, such as the Rocky
mountains, in which alternation is a peculiarly striking feature of the
vegetation, are especially favorable to the production of ecads, and
hence for the study of natural experiments in origin by adaptation.
=189. Determination of factors.= For the critical investigation of the
origin of new forms, an exact knowledge of the factors of the habitat,
both physical and biotic, is imperative. In the case of variable
species, these factors determine what variations are of advantage, and
thereby the direction in which the species can develop. They are the
agents of selection. With mutants, the factors of the habitat are
apparently neither causative nor selective, though it seems probable
that further study of mutants will show an essential connection between
mutant and factor. In any event, the persistence of a mutant in nature,
and its corresponding ability to initiate new lines of development, is
as much dependent upon the selection exerted by physical and biotic
factors as is the origin of variants. Physical factors are causative
agents in the production of ecads, as has been shown at length
elsewhere. The form and structure of the ecad are the ultimate responses
to the stimuli of light or water-content, and the quantitative
determination of the latter is accordingly of the most fundamental
importance. The measurement of factors has been treated so fully in the
preceding chapters that it is only necessary to point out that the
thorough investigation of habitats by instruments is as indispensable
for the study of experimental evolution as for that of the development
and structure of the formation. Furthermore, it is evident that a
knowledge of physical factors is as imperative for habitat and control
cultures as for the method of natural experiment. In the latter,
however, the biotic factors demand unusual attention, since pollination,
isolation, etc., are often decisive factors in origin by variation and
in the persistence of mutants.
Measurements of adjustment, i. e., functional response to the direct
factor concerned, are extremely valuable, but not altogether
indispensable to research in experimental evolution. This is due to the
fact that a knowledge of adjustment is important in tracing the origin
of new forms only when adjustment is followed by adaptation, and in all
such cases the ratio between the two processes seems to be more or less
constant. In the present rudimentary development of the subject,
however, it is very desirable to make use of all methods of measuring
functional responses to water and light that are practicable in the
field. Certain methods that are difficult of application in nature may
be used to advantage in control cultures, and the results thus secured
can be used to interpret those obtained from natural experiments and
field cultures.
=190. Method of record.= As suggested elsewhere, there are four
important kinds of records, which should be made for natural
experiments, and likewise for habitat and control cultures. These are
exsiccati, photographs, biometrical formulae and curves, and
histological sections. These serve not merely as records of what has
taken place, but they also make it possible to trace the course of
evolution through a long period with an accuracy otherwise impossible,
and even to foreshadow the changes which will occur in the future. The
possibility of doing this depends primarily upon the completeness of the
record, and for this reason the four methods indicated should be used
conjointly. In the case of ecads and mutants, exsiccati, photographs,
and sections are the most valuable, and in the majority of cases are
sufficient, since both ecads and mutants bear a more distinctive impress
than variants do. On the other hand, since variations are more minute,
the determination of the mean and extreme of variation by biometrical
methods is almost a prerequisite to the use of the other three methods,
which must necessarily be applied to representative individuals.
Exsiccati and photographs are made in the usual way for plants, but it
is an advantage to photograph each ancestral form alongside of its
proper ecads, mutants, or variants, in addition to making detail
pictures of each form and of the organs which show modification. In the
collection of material for histological sections, which deal primarily
with the leaf or with stems in the case of plants with reduced leaves, a
few simple precautions have been found necessary. Whenever possible,
material should be killed where it is collected, since in this way the
chloroplasts are fixed in their normal position. In case leaves that can
not be replaced easily have become wilted, an immersion of 5–6 hours in
water will make it possible to kill them without shrinkage. In selecting
leaves, great pains must be taken to collect only mature leaves. When
the plants have a basal rosette, or distinct radical leaves, mature
leaves are taken from both stem and base. In all cases where the two
surfaces of the leaf can not be readily distinguished, the upper one is
clearly marked.
METHOD OF HABITAT CULTURES
=191. Scope and advantages.= By means of experiments actually made in
the field, practically every species that is capable of modification can
be made to produce new forms, the origin of which can be traced in the
manner already indicated. Field experiments of this sort are especially
favorable to the production of ecads from adaptable species. No attempt
has yet been made to apply it to mutable or variable species, but its
ultimate application to these does not seem at all impossible. The chief
advantage of the method of habitat cultures is seen in the great range
of choice in selecting the plant for experiment, and the habitat or area
in which the experiment is carried out. A polydemic species which
already has one or more ecads can be extended to a number of different
habitats of known value, and a complete series of ecads obtained, based
either upon water-content, or light, or upon both. On the other hand, an
endemic species, or one brought from a remote flora, can be placed in as
many habitats as desired, and the appearance of ecads followed in each.
Frequently, results of much value are obtained in a diversified habitat
by growing its most plastic species in those areas which show the
greatest differences in water-content or light intensity. Habitat
cultures give results which are practically as perfect as those obtained
from natural experiments, since the course of adaptation in no wise
depends upon whether the agent by which the seed or propagule is carried
into the new habitat is natural or artificial. Cultures of this kind
further possess the distinct advantage of permitting more or less
modification of the physical factors themselves. However, when it is
desirable to have the factors under as complete control as possible, it
is necessary to use the method of control cultures in the planthouse.
=192. Methods.= All field experiments in evolution are based upon a
change of habitat. The latter is accomplished by the modification of the
habitat itself, or by the transfer of the species to one or more
different habitats, or to different areas of the same habitat. In both
cases the choice of habitats is made upon the basis of efficient
differences of water-content or light. Saline situations do not
constitute an exception, since the chresard is really the effective
stimulus. Cultures at different altitudes, which afford striking
results, appear to concern several factors, but in the final analysis,
water-content and humidity are alone found to be really formative.
Cultures may furthermore be distinguished as simple or reciprocal.
Simple cultures are those in which a species is transferred to one or
more habitats, or in which a habitat is modified in one or more ways.
Reciprocal cultures are possible only with polydemic species, or with
endemics after ecads have been produced by experiment. Modification or
transfer is made in the usual way, but reciprocally, i. e., the original
form is transferred to the habitat of the ecad, and the latter to the
habitat of the former; or the shade in which some individuals of the
ecad are growing may be destroyed, and at the same time individuals of
the type may be shaded. Both transfer and modification may be applied to
the same species, but since the same measured change of factor can be
obtained in either way, the use of both is undesirable, with the
exception of the rare cases where they serve as checks upon each other.
The transfer of a seed or plant is so much simpler and more convenient
that this method is the one regularly used. It sometimes happens,
however, that a change of water-content or light intensity is readily
and conveniently made, and is desirable for other reasons.
It is evident that both transfer and modification require that the
factor records of the various habitats or areas be as full as possible,
at least so far as water-content, humidity, and light are concerned. In
the case of the areas that are to be modified, these factors are
determined before the change is made. Afterward they are read from time
to time during the growing season, and are also checked by readings made
near at hand in the unmodified formation. The readings made in the
beginning should correspond closely to the check readings, but in case
of disagreement the latter are to be taken as conclusive.
=193. Transfer.= After the species to be used for experiment has been
chosen, the various habitats or areas selected, and the direct factors
measured by instruments, the actual transfer of the individuals is made
by means of seeds, preferably in autumn, though the results are
practically the same if seeds are kept over the winter and planted at
the opening of spring. The natural method is to scatter the seeds in the
place selected, as though they had been carried by the usual agents of
migration. The mortality is usually great in such case, however, and the
chances of success are increased by actually planting the seeds. This is
the method which has been used in making cultures of species of the
European Alps on the summit of Mount Garfield in the Rocky mountains.
The number of seeds used is recorded in order to obtain some estimate of
germination and competition. While the use of the seed or disseminule
possesses the great advantage of making the experiment essentially a
natural one, the transfer of rosettes, seedlings, or young plants makes
the results more certain, and consequently saves time, even though the
actual transfer is somewhat more difficult. It is hardly necessary to
point out that the removal of the plant should be made with the greatest
care. The best success is obtained by making the transfer on cloudy or
rainy days, and when shade plants are to be placed in sunny situations,
they should be transplanted late in the afternoon. When the task of
carrying them is not too great, it is a distinct advantage to move a
number of individuals in the same block of earth. The transfer of mature
plants is inadvisable, except for those perennials which can not readily
be secured in an early stage. This naturally does not apply to woody
plants, evergreen herbs, mosses and lichens; the last two may be
transferred at any time with satisfactory results. Each culture is
carefully marked with stakes, and definitely located by means of
landmarks.
[Illustration: Fig. 46. Series for producing hydrophytic forms under
control: 1, amphibious; 2, floating; 3, competition; 4, submerged.]
Reciprocal transfers may be made by means of seed or plant. Since the
experiment is a complex one, all the care possible should be taken to
make sure that the plants become established in the reciprocal
situations, and consequently, it is often advisable to transfer both
seeds and plants. Reciprocal transfer is of paramount value in solving
the problem which bog plants present. A slight modification of the
method makes it possible to obtain experimental evidence of the
polyphyletic origin of species in consequence of adaptation. In an
experiment mentioned elsewhere, the transfer of _Kuhnistera purpurea_ to
the area occupied by _K. candida_, and vice versa, is designed to show
whether one has been derived from the other. If the two species are
moved into an area which contains more water than that usually occupied
by _K. purpurea_, and less water than is found where _K. candida_
habitually grows, the resulting modifications will throw much light upon
the origin of polyphyletic species. In this connection, it hardly needs
to be pointed out that this simple transfer of a species to several
separated areas of a new habitat may often furnish complete proof that a
new form may arise at different times, and at different places.
[Illustration: Fig. 47. Control ecad of _Ranunculus sceleratus_, holard
10% (50 cc.).]
=194. Modification of the habitat.= Efficient changes in the habitat are
brought about by increasing or decreasing the water-content, or by
varying the light intensity between sunshine and the diffuse light of
deep forests. Humidity can not well be regulated except in so far as it
is connected with water-content. Since its effects merge with those of
the latter, its modification is unnecessary. An increase in
water-content is readily brought about by irrigation. A stream may be
dammed and its water allowed to spread over the area to be studied, or
the water may be carried to the proper place by deflecting the stream or
by digging a canal. The construction of earth reservoirs makes it
possible to obtain almost any per cent of soil water by varying the size
of the reservoir or the height of the wall or bank. Near a base station,
such as Minnehaha, where there is a simple system of water-works, the
experimental area may be watered whenever desirable by means of a hose.
Water-content may be readily decreased by drainage, or by the deflection
of a stream. When such means are not available, as in the case of
extensive marshes, hummocks may be used or constructed, and the soil
blocks containing plants placed upon them. By the use of sand or gravel,
the water-content of mesophytic areas can be reduced in a similar
manner, or by surrounding the plant _in situ_ with either of these soils
which hold little water. In meadows, especially, the addition of a large
quantity of alkaline salts decreases the amount of available water,
while the holard may be reduced by denuding the soil about the plants
concerned.
[Illustration: Fig. 48. Control ecad of _Ranunculus sceleratus_, holard
40% (200 cc.).]
In sunny habitats, the light intensity is most easily reduced by means
of cloth awnings, which can be put in place conveniently. It is not a
difficult matter to produce effective shade by using shrubs or small
trees for this purpose. This plan is especially advantageous in habitats
too remote to make frequent visits feasible. When a shrub or tree is
used, the experiment necessarily requires a longer time, though this
disadvantage is partly compensated by the fact that the shelter requires
practically no attention after the shrub is once established. Forest
plantations furnish excellent examples of this kind of experiment. On
the other hand, clearings afford the only examples of habitats modified
in such manner as to increase the light. In nature, the diffuse light in
which shade plants grow is due to the presence of tall plants, chiefly
shrubs and trees, and an increase in the light intensity is possible
only through the thinning-out or removal of the plant screen. This is a
task of considerable magnitude in forests, but it can be readily
accomplished in thickets and at the edges of woodlands. It is quite
practicable to establish a series of awnings or clearings of various
light values, but the labor required is hardly worth while when it is
recalled that the method of transfer makes it possible to take advantage
of the various intensities already found in nature.
METHOD OF CONTROL CULTURES
=195. Scope and procedure.= Control experiments are necessarily carried
on in the planthouse, since factors can be controlled in the field only
with great difficulty. Their greatest value is in connection with
experiments that are being carried on in the habitat, but they also
constitute an invaluable means of independent research, since it is not
at all difficult to approximate the conditions of a habitat, especially
with reference to water-content and light. The essential feature of the
method is that the less important factors are equalized as far as
possible, while the direct factors, water-content and light, are under
the complete control of the investigator. By the equalization of
humidity and temperature is meant experimentation in which all the
plants of each experiment are subjected to the same amounts of these
factors. It is a matter of no importance whatever whether the humidity
and temperature are constant or variable. In the case of soil, which is
not a variable, it naturally happens that the plants are placed once for
all in the same soil mixture. Batteries consisting of thermograph and
psychrograph have been kept in the different control houses, but
although used at first to give some idea of the hourly and daily
fluctuations of temperature and humidity, they have slight bearing upon
the evolution of new forms under control. For use in connection with
supplementary experiments in adjustment and adaptation, the batteries
have proved to be indispensable. Control experiments are regularly made
in series which are planned with reference to as many modifications as
the efficient difference of the factor and the plasticity of the species
concerned permit.
=196. Water-content series.= An account of the experiments which have
been carried on for four generations with _Ranunculus sceleratus_ will
serve to show the application of culture methods to the origin of new
forms in response to varying water-content. This species was chosen
because it grows readily in the planthouse, is plastic, and, since it is
naturally amphibious, permits of much modification in both directions.
The smallest amount of water per day under which the seedlings would
grow was found to be 25 cc. This was taken as one extreme for the
series, and deep water in which the plant could be submerged as the
other. An arbitrary series was tentatively made as follows: 25 cc., 50
cc., 100 c., 150 cc., 200 cc., mud, shallow water, and deep water.
Further study justified these divisions, since the first six gave
efficient differences in water-content, and the resulting forms all
showed differences of structure as well as of growth and form. Seedlings
of the same age, and as nearly alike as possible, were transplanted to
large pots of which there were four for each of the first six; they were
placed in half-barrels for mud and floating forms, and in a barrel for
submerged forms. After a few days, when they had become well
established, the plants in the pots were watered in the amounts
indicated, as often as was necessary to keep the most xerophytic form
alive; the soil for the mud form was kept covered with a thin film of
water; the leaves of the form in shallow water were kept floating on the
surface, and those of the last form submerged just below the surface.
The water in which the submerged form grew was aerated by means of a
spigot near the bottom of the barrel. From time to time water-content
determinations were made of the soil in the pots until it was definitely
ascertained that the holard was practically constant. The nine new forms
obtained by adaptation showed striking differences in vigor and growth,
as may be seen from the figures. In all cases, these were accompanied by
distinct and often striking differences in the number and position of
the stomata, the amount of sponge and palisade tissues, and the
development of air passages. Photographs were made of a typical plant of
each form, and the different leaf structures were preserved in permanent
mounts. The xerophytic and the submerged form were unable to produce
flowers, and it was necessary to develop them anew in each generation.
The other forms fruited abundantly, and the succeeding generations of
each form were produced from plants which had grown the year before in
the same conditions. In addition to the development of a series of new
water-content forms, this experiment was begun in the hope of
determining whether the modifications of a plastic species tend to
become fixed if each new form is grown constantly under the same
conditions. A period of four years is too short, however, to throw much
light upon this problem.
[Illustration: Fig. 49. Floating form of _Ranunculus sceleratus_ grown
under control.]
_Helianthus annuus_ has been used for other series of experiments, in
which alkaline salts or different soils are employed to vary the
water-content. These are more complex and hence are not as satisfactory
as the series described above, but they are valuable for the light they
throw upon the behavior of plants in similar conditions in nature. In
the case of soil, however, the adaptation may be referred to
water-content alone, if thoroughly leached sands and gravels are used,
so that the difference is solely one of water-retaining power.
=197. Light series.= Cloth tents have been found the most satisfactory
means of obtaining different light intensities in the planthouse. The
cloth permits the air to circulate to a considerable degree, and in
consequence the equalization of humidity and temperature is much more
complete than in the glass houses first employed. The cloth tents, or
shade tents as they are called, are cubical, each dimension being 1
meter. The series which has been most used consists of three tents: the
first is made of cheesecloth and has a light value of .1; the second is
of thin muslin, and has a value of .04, while the third is made of dark
cambric and the light is reduced to .01. A more desirable series is one
with five tents, which have approximately the following light
intensities: .1, .05, .01, .007, .003. Plants grown in shade tents
should be repotted as often as they will permit in order to increase the
aeration of the soil. The amount of water given them must also be
decreased as the shade increases. Mesophytic species give the best
results in shade tents, xerophytes thrive less well, and amphibious
plants do not grow at all except in the brightest light. Excellent
results have been obtained with _Helianthus_, _Taraxacum_, _Gaura_, and
_Onagra_, while _Ranunculus sceleratus_ is unable to produce flowers and
seeds in a light intensity of .01.
A number of important supplementary experiments have been made in
connection with light tents. These do not result in the production of
new forms, but they throw much light upon it. Plants have been placed in
the shade tents so that certain leaves would be in the sun and others in
the shade. Young leaves have been fixed at various angles with the stem,
and they have been revolved 90° or 180° in order to change the relation
of their surfaces. Soils of different colors, e. g., loam and sand, have
been used to determine the effect of light reflected from their
surfaces. Shade tents make it possible to illuminate plants from the
top, bottom, or side, and to carry on a large number of fundamental
experiments in adjustment and adaptation.