It is not to be presumed that every variation in the structure of plant organs is a direct result of adaptations taken on by the plant to protect it from unfavorable factors in its environment. It is the natural, inherent tendency of plants to vary, and when the variation chances to be in a direction that fits it better to its environment, the variation is apt to persist in future generations. There is no apparent reason, however, why in many instances structures may not be present in the plant that are in no sense of direct aid. We should not expect to refer every variation in plant structures to variation in environment. We should, however, expect those species to do best that in their natural
tendencies to vary become so modified as to fit them most perfectly to their surroundings.
Each plant organ must not only be adapted for the kind of work that it has to do, but is must be adapted for doing its best under the external influences which enable it to persist in any given form. The foliage leaf bears a definite relation to light and moisture; the leaves of one plant, however, may have quite different requirements as to light and moisture than the leaves of another. Every traveler in our arid southwest has noticed that the leaves of the trees and shrubs are small and thick, or, in some instances, entirely absent as foliage. The reason for this is very clear. It arises from the necessity of the desert plant to expose a comparatively small surface to the intense sunlight and the desiccating action of the dry atmosphere.