[454] Dimorphism is of universal occurrence in the whole vegetable kingdom. In some cases it is typical, and may easily be discerned from extreme fluctuating variability. In others the contrast is not at all obvious, and a closer investigation is needed to decide between the two possibilities. Sometimes the adaptive quality is evident, in other cases it is not. A large number of plants bear two kinds of leaves linked with one another by intermediate forms. Often the first leaves of a shoot, or those of accidentally strong shoots, exhibit deviating shapes, and the usefulness of such occurrences seems to be quite doubtful. The elongation of stems and linear leaves, and the reduction of lateral organs in darkness, is manifestly an adaptation. Many plants have stolons with double adaptations which enable them to retain their character of underground stems with bracts or to exchange it for the characteristics of erect stems with green leaves according to the outer circumstances. In some shrubs and trees the capacity of a number of buds to produce either flowers or shoots with leaves seems to be in the same condition. The capacity of producing spines is also a double adaptation, active on dry and arid soil and latent in a moist climate or under cultivation, as with the wild and cultivated apple, and in the experiments of Lothelier [455] with _Berberis_, _Lycium_ and other species, which lose their spines in damp air. In some conifers the evolution of horizontal branches may be modified by simply turning the buds upside down. Or the lateral branches can be induced to become erect stems by cutting off the normal summit of a tree. Numerous organs and functions lie dormant until aroused by external agencies, and many other cases could be cited, showing the wide occurrence of double adaptation. There are, however, two points, which should not be passed over without some mention. One of them is the influence of sun and shade on leaves, and the other the atavistic forms, often exhibited during the juvenile period.