PHOTOHARMOSE

Fig. 32. Mesophyll ofPedicularis 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 lessintensely 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.

Fig. 33. Staurophyll ofBahia 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. Thecutinizedleaf 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.Arctostaphylusand many species ofPentstemonare good examples. Lanate leaves, i. e., those with dense hairy coverings on one or both surfaces, asArtemisia,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.Storageleaves 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 ofwater for the time of extreme drouth. Xerophytic species ofHelianthusfurnish examples of transverse bundles of storage cells, while those ofMertensiaillustrate 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, andSeneciofurnish excellent examples of this type.

Fig. 34. Diplophyll ofMertensia 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, andGiliacontain 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 ofJuncusand certainCyperaceaeare 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 lowerone often protected by hairs. This type is found especially among the genera of theEricales, 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 inAcacia,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 theGenisteae; it is also found inEphedra, many species ofPolygonum,Lygodesmia, etc.

3.The rush form.InHeleocharis, many species ofJuncus,Scirpus, and otherCyperaceae, 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.InAsparagusthe 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.Ruscusis 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.ColletiaandHolacanthaare good examples of the type.

7.The succulent form.Plants with succulent stems such as theCactaceae,Stapelia, andEuphorbiahave 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.

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 ofTaxusin 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, whereHeleocharis,Scirpus,Juncus, etc., grow, not a trace of acid is discoverable. Furthermore, plants with a characteristic hydrophytic structure throughout, such asRanunculus,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 explanationof 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.

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 constantlybelow 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.

Fig. 37. Floating leaf ofSparganium 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 byCallitriche 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 thestoppage 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 theLemnaceae, the leaf and the stem are represented by a mere frond or thallus, and the roots are in the process of disappearance, e. g.,Spirodelahas several,Lemnaone, andWolffianone.

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.

172. Light as a stimulus.In nature, light stimuli are determined by 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 canhave 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 ofGalium 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 thetemporary 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 forFittonia 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 havechloroplasts 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 theplastids. 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 asapostrophe, 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 aregular 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.

Fig. 38. Ecads ofAllionia 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 thebreathing 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.

Fig. 39. Position of chloroplasts in aerial leaf (1) and submerged leaf (2) ofCallitriche 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 dueto 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 donein 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 ofGalium 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:

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 ofthe 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 inBursaandThalictrum; inCapnoidesthe change is less evident, while inAchilleiaandMachaerantheralobing 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. InCapnoides aureum, the thickness of the shade leaf is ½ (6 : 12) that of the sun leaf; inGalium borealethe ratio is 5 : 12, and inAllionia linearisit is 3 : 12. The ratio inThalictrum sparsiflorumis 9 : 12, and inMachaeranthera aspera11 : 12. The thickness of sun and shade leaves ofBursa bursa-pastorisis 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 dueto 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.

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