164. The increase of water supply. Plants of dry habitats can increase their absorption only by modifying the root system so that the absorbing surfaces are carried into the deep-seated layers of soil, and the surfaces in contact with the dry soil are protected by means of a cortex. Exception must be made for epiphytes and a few other plants that absorb rain water and dew through their leaves, and for those desert plants that seem to condense the moisture of the air by means of hygroscopic salts, and absorb it through the epidermis of the leaf. The storage of water in the leaf is a very important device; it increases the water supply by storing the surplus of absorbed water against the time of need. Modifications for water-storage are occasionally found in roots and stems, but their chief development takes place in the leaf. The epidermis frequently serves as a reservoir for water, either by the use of the epidermal cells themselves, by the formation of hypodermal water layers, or by means of superficial bulliform cells. The water cells of the chlorenchym regularly appear in the form of large clear cells, scattered singly or arranged in groups. In this event, they occur either as transverse bands, or as horizontal layers, lying between the palisade and sponge areas, and connecting the bundles. A few plants possess tracheid-like cells which also serve to store water. In the case of succulent leaves, practically the whole chlorenchym is used for storing water, though they owe their ability to withstand transpiration to a combination of factors.
165. Modifications due to an excessive water supply. Water plants with aerial leaf surfaces are modified in such manner as to increase water loss and to decrease water supply, but the resulting modifications are rarely striking. There is a marked tendency to increase the exposed surface. This is indicated by the fact that, while the leaves of mud and floating forms become larger, they change little or not at all in thickness. The lobing of leaves is also greatly reduced, or the lobes come to overlap. Leaves of water plants are practically destitute of all modifications of epidermis and stomata, which could serve to hinder transpiration. The stomata are usually more numerous on the upper surface, and in the same species their number is greater in the forms grown in wet places. These facts explain in part the extreme development of air-passages in water plants, though this is, in large measure, a response to the increasing difficulty of aeration. The increase of air-spaces is correlated with reduction of the palisade, and a decided increase in the sponge. An increase in water supply is indicated by the absence of storage tissues, and the reduction of the vascular system, which, however, is more closely connected with a diminished need for mechanical support.
Fig. 32. Mesophyll of Pedicularis procera (chresard, 15%, light, 1). × 130.
166. Plant types. The necessity for decreasing or increasing water loss in compensation of the water supply has made it possible to distinguish two fundamental groups of plants upon the twofold basis of habitat and structure. These familiar groups, xerophytes and hydrophytes, represent two extremes of habitat and structure, between which lies a more or less vague, intermediate condition represented by mesophytes. These show no characteristic modifications, and it is consequently impossible to arrange them in subgroups. Xerophytes and hydrophytes, on the other hand, exhibit marked diversity among themselves, a fact that makes it desirable to recognize subgroups, which correspond to fundamental differences of habitat or adaptation. It is hardly necessary to point out that these types are not sharply defined, or that a single plastic species may be so modified as to exhibit several of them. The extremes are always clearly defined, however, and they indicate the specific tendency of the adaptation shown by other members of the same group.
167. Xerophytic types. With the exception of dissophytes, all xerophytes agree in the possession of a deep-seated root system, adapted to withdraw water from the lower moist layers, and to conserve from loss from the upper dry layers. Reservoirs are developed in the root, however, in relatively few cases. The stem follows the leaf more or less closely in its modification, except when the leaf is greatly reduced or disappears, in which event the stem exhibits peculiar adaptations. While the leaf is by far the most strikingly modified, it is a difficult task to employ it satisfactorily as the basis for distinguishing types. Several adaptations are often combined in the same leaf, and it is only where one of these is preeminently developed, as in the case of succulence, that the plant can be referred to a definite type. The latter does not happen in many species of the less intensely xerophytic habitats, and, consequently, it is difficult, if not undesirable, to place such xerophytes under a particular group. The best that can be done is to recognize the types arising from extreme or characteristic modification, and to connect the less marked forms as closely as possible with these. Halophytes differ from xerophytes only in the fact that the chresard is determined by the salt-content of the habitat, and not by the texture of the soil. In consequence, they should not be treated as a distinct group.
Fig. 33. Staurophyll of Bahia dissecta, showing extreme development of palisade (chresard, 3–9%; light, 1). × 130.
168. Types of leaf xerophytes. In these, adaptation has acted primarily upon the leaf, while the stem has remained normal for the most part. Even when the leaves have become scale-like, they persist throughout the growing season, and continue to play the primary part in photosynthesis. The following types may be distinguished:
1. The normal form. The leaf is of the usual dorsiventral character. In place of a reduction in size, structural modifications are used to decrease transpiration. With respect to the protective feature that is predominant, three subtypes may be recognized. The cutinized leaf compensates for a low water-content by means of a thick cuticle, often reinforced by a high development of palisade tissue. Such leaves are more or less leathery, and they are often evergreen also. Arctostaphylus and many species of Pentstemon are good examples. Lanate leaves, i. e., those with dense hairy coverings on one or both surfaces, as Artemisia, Antennaria, etc., regularly lack both cuticle and palisade tissue. The protection against water loss, however, is so perfect that the chlorenchym often assumes the loose structure of a shade leaf. Storage leaves usually have a well-developed cuticle and several rows of palisade cells, but their characteristic feature is the water-storage tissue, which maintains a reserve supply of water for the time of extreme drouth. Xerophytic species of Helianthus furnish examples of transverse bundles of storage cells, while those of Mertensia illustrate the more frequent arrangement in which the water tissue forms horizontal layers.