The primary leaves, following the seed-leaves, are different in many species, from the later ones, and the difference is extremely pronounced in some cases of reduction. Often, when leaves are lacking in the adult plant, being replaced by flattened stalks as in the case of the acacias, or by thorns, or green stems and twigs as in the prickly broom or _Ulex europaeus_, the first leaves of the young plant may be more highly differentiated, being pinnate in the first case and bearing three leaflets in the second instance. This curious behavior which is very common, brings the plants, when young, nearer to their allies than in the adult state, and manifestly implies that the more perfect state of the leaves is latent throughout the life of the plant, with the exception of the early juvenile period. _Eucalyptus Globulus_, the Australian gum tree, has opposite and broadly sessile leaves during the first years of its life. Later these disappear and are replaced by long sickle-shaped foliage organs, which seem to be scattered irregularly along the branches. The juvenile characters manifestly lie dormant during the adult period, and that this is so, may be shown artificially by cutting off the whole crown of the tree, when the stem responds by producing numerous new branches, which assume the [218] shape proper to the young trees, bearing sessile and opposite leaves. It seems quite unnecessary to give further instances. They are familiar to every student. It is almost safe to say that every character has its periods of activity and of inactivity, and numbers of flowers and fruits can be mentioned as illustrations. One fact may be added to show that nearly every part of the plant must have the power of producing all or nearly all the characters of the individual to which it belongs. This proof is given by the formation of adventitious buds. These, when once formed, may grow out into twigs, with leaves and flowers and roots. They may even be separated from the plants and used as cuttings to reproduce the whole. Hence we may conclude that all tissues, which possess the power of producing adventitious buds, must conceal in a latent state, all the numerous characters required for the full development of the whole individual.