=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. [Illustration: 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: