In plants of the third order it is thus with the proximate components: they are subject to mutual influences that are unlike one another and are continually changing. The earlier-formed units become mechanical supporters of the later-formed units, and so experience modifying forces from which the later-formed units are exempt. Further, these elder units simultaneously begin to serve as channels through which materials are carried to and from the younger units--another cause of differentiation that goes on increasing in intensity. Once more, there arise ever-strengthening contrasts between the amounts of light which fall upon the youngest or outermost units and the eldest or innermost units; whence result structural contrasts of yet another kind. Evidently, then, along with the progressive integration of cells into fronds, of fronds into axes, and of axes into plants still more composite, there come into play sundry causes of differentiation which act on the whole and on each of its parts, whatever their grade. The forces to be overcome, the forces to be utilized, and the matters to be appropriated, do not remain the same in their proportions and modes of action for any two members of the aggregate: be they members of the first, second, third, or any other order. § 215. Nor are these the only kinds and causes of heterogeneity which we have to consider. Beyond the more general changes produced in the relative sizes and shapes of plants and their parts by progressive aggregation, there are the more particular changes determined by the more particular conditions. Plants as wholes assume unlike attitudes towards their environments; they have many ways of articulating their parts with one another; they have many ways of adjusting their parts towards surrounding agencies. These are causes of special differentiations additional to those general differentiations that result from increase of mass and increase of composition. In each part considered individually, there arises a characteristic shape consequent on that relative position towards external and internal forces, which the mode of growth entails. Every member of the aggregate presents itself in a more or less peculiar way towards the light, towards the air, and towards its point of support; and according to the relative homogeneity or heterogeneity in the incidence of the agencies thus brought to bear on it, will be the relative homogeneity or heterogeneity of its shape. § 216. Before passing from this _à priori_ view of the morphological differentiations which necessarily accompany morphological integrations, to an _à posteriori_ view of them, it seems needful to specify the meanings of certain descriptive terms we shall have to employ.