Research methods in ecology

by Frederic E. Clements

ADAPTATION

161. Modifications due to water stimuli. In adaptation, the great desideratum is to connect each modification quantitatively with the corresponding adjustment. This is even more difficult than to ascertain the quantitative relation between stimulus and functional response, a task still beset with serious obstacles. At the present time, little more can be done than to indicate the relation of marked adaptations of organs and tissues to the direct factors operating upon them, and to attempt to point out among the functions possibly concerned the one which seems to be the most probable connection between the probable stimulus and the structure under investigation. In the pages that follow, no more than this is attempted. The general changes of organs and tissues produced by water are first discussed, and after this is given a summary of the structural features of the plant types based upon water-content.

162. Modifications due to a small water supply. A water supply which may become deficient at any time is compensated either by changes which decrease transpiration, or by those that increase the amount of water absorbed or stored. These operate upon the form and size of the organs concerned, as well as upon their structure. Modifications of the form of leaf and stem are alike in that they lessen transpiration by a reduction of the amount of surface exposed to the air. Structural adaptations, on the other hand, bring about the protection of epidermal cells and stomata, and often internal cells also, from the factors which cause transpiration, or they anticipate periods of excessive transpiration by the storage of water in specialized cells or tissues. In certain extreme types the epidermis is itself modified for the absorption of water vapor from the air.

163. The decrease of water loss. The following is a summary of the contrivances for reducing transpiration.

1. Position of the leaf. Since the energy of a ray of sunlight is greatest at the sun’s highest altitudes, those leaves transpire least which are in such a position during midday that the rays strike them as obliquely as possible. A leaf at right angles to the noonday sun receives ten times as much light and heat upon a square decimeter of surface as does one placed at an angle of 10 degrees. This device for reducing the intensity of insolation is best developed in the erect or hanging leaves of many tropical trees. In temperate zones, it is found in such plants as Silphium laciniatum and Lactuca scariola, and in species with equitant leaves. In such plants as Helianthus annuus, the effect is just the opposite, since the turning of the crown keeps the leaves for a long time at a high angle to the incident rays. In the case of mats, it is the aggregation of plants which brings about the mutual protection of the leaves from insolation and wind.

2. Rolling of the leaf. Many grasses and ericaceous plants possess leaves capable of rolling or folding themselves together when drouth threatens. In other cases, the leaves are permanently rolled or folded. The advantage of this device arises not only from the reduction of surface, but also from the fact that the stomata come to lie in a chamber more or less completely closed. In the case of those mosses whose leaves roll or twist, a reduction of surface alone is effected.

3. Reduction of leaf. The transpiring surface of a plant is reduced by decreasing the number of leaves, by reducing the size of each leaf, or by a change in its form. In so far as the stem is a leaf, a decrease in size or a change in shape brings about the same result. The final outcome of reduction in size or number is the complete loss of leaves, and more rarely, of the stem. Such marked decrease of leaf area is found only in intense xerophytes, though it occurs in all deciduous trees as a temporary adaptation. Changes in leaf form are nearly always accompanied by a decrease in size. Of the forms which result, the scale, the linear or cylindrical leaf, and the succulent leaf are the most common. Leaves which show a tendency to divide often increase the number of lobes or make them smaller.

4. Epidermal modifications. Excretions of wax and lime by the epidermis have a pronounced effect by increasing the impermeability of the cuticle, and, hence, decreasing epidermal transpiration. It seems improbable that a coating of wax on the lower surface of a diphotic leaf can have this purpose. The thickening of the outer wall of epidermal cells to form a cuticle is the most perfect of all contrivances for decreasing permeability and reducing transpiration. In many desert plants, the greatly thickened cuticle effectually prevents epidermal transpiration. In these also the cuticle is regularly developed in such a way as to protect the guard cells, and even to close the opening partially. An epidermis consisting of two or more layers of cells is an effective, though less frequent device against water loss. When combined with a cuticle, as is usually the case, the impermeability is almost complete. Hairs decrease transpiration by screening the epidermis so that the amount of light and heat is diminished, and the access and movement of dry air impeded. While hairs assume the most various forms, all hairy coverings serve the same purpose, even when, as in the case of Mesembryanthemum, they are primarily for water-storage. Hairs protect stomata as well as epidermal cells: the greater number of the former on the lower surface readily explains the occurrence of a hairy covering on this surface, even though absent on the more exposed upper side. In some cases, hairs are developed only where they serve to screen the stomata.

The modifications of the stomata with respect to transpiration are numerous, yet all may be classed with reference to changes of number or level. With the exception of aquatic and some shade plants, the number of stomata is normally greater on the less exposed, i. e., lower surface. The number on both surfaces decreases regularly as the danger of excessive water loss increases, but the decrease is usually more rapid on the upper surface, which finally loses its stomata entirely. It has been shown by many observers that species growing in dry places have fewer stomata to the same area than do those found in moist habitats. This result has been verified experimentally by the writer in the case of Ranunculus sceleratus, in which, however, the upper surface possesses the larger number of stomata. Plants of this species, which normally grow on wet banks, were grown in water so that the leaves floated, and in soils containing approximately 10, 15, 30, and 40 per cent of water. The averages for the respective forms were: upper 20, lower 0; upper 18, lower 10; upper 18, lower 11; upper 11, lower 8; upper 10, lower 6. Reduction of number is effective, however, only under moderate conditions of dryness. As the latter becomes intense, the guard cells are sunken below the epidermis, either singly or in groups. In both cases, the protection is the same, the guard cells and the opening between them being withdrawn from the intense insolation and the dry air. The sun rays penetrate the chimney-shaped chambers of sunken stomata only for a few minutes each day, and they are practically excluded from the stomatal hollows which are filled with hairs. The influence of dry winds is very greatly diminished, as is also true, though to a less degree, for leaves in which the stomata are arranged in furrows. Sunken stomata often have valve-like projections of cuticle which reduce the opening also. Finally, in a few plants, water loss in times of drouth is almost completely prevented by closing the opening with a wax excretion.

5. Modifications in the chlorenchym. A decrease in the size and number of the air passages in the leaf renders the movement of water-laden air to the stomata more difficult, and effects a corresponding decrease in transpiration. The increase of palisade tissue, though primarily dependent upon light, reduces the air-spaces, and consequently the amount of water lost. The development of sclereids below the epidermis likewise hinders the escape of water. Finally the character of the cell sap often plays an important part, since cells with high salt-content or those containing mucilaginous substances give up their water with reluctance.