Research methods in ecology

by Frederic E. Clements

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.

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.