§ 237. We come now to aggregates of the lowest order. Already something has been said (§ 217) concerning the forms of those morphological units which exist as independent plants. But it is here requisite briefly to note the modifications undergone by them where they become components of larger plants. [Illustration: Fig. 254.] Of the numerous cell-forms which are found in the tissues of the higher plants, it will suffice to give, in Fig. 254, representing a section of a leaf, a single example. In this it will be seen that the cells forming the upper and lower surfaces, _a_ and _b_, have differences of shape related to differences in the incidence of forces: they are more or less flattened in relation to the environment. The underneath cells at _c_, form a class which, similarly exposed to light at their outer ends, and, as we may assume, largely developed in adjustment to their active assimilative functions, are, by mutual pressure, made to grow more in the direction of their lengths than in the direction of their breadths. Then on the other side we see that the cells _d_, next above the outer layer, while approximately similar, become more and more dissimilar as they diverge from the surface, and are quite irregular in the interior _e_, where there is no definiteness in the conditions to which they are exposed. Thus the divergences of these cells from primordial sphericity are such as correspond with unlikenesses in their circumstances. And throughout the more complex modifications which the cells of other tissues exhibit, the like correspondences hold. [Illustration: Figs. 32–35.] Among plants of a lower order of aggregation, we have already seen how cells become metamorphosed as they become integrated into masses having definite organizations. The higher _Algæ_, exemplified in Figs. 32, 34, 35, show this very clearly. Here the departure from the simple cell-form to the form of an elongated prism, is manifestly subordinated to the contrasts in the relations of the parts. And it is interesting to observe how, in one of the branches of Fig. 32, we pass from the small, almost-spherical cells which terminate the branchlets, to the large, much-modified cells which join the main stem, through gradations obviously related in their changed forms to the altered actions their positions expose them to. [Illustration: Figs. 19–23.] More simply, but quite as conclusively, do the inferior _Algæ_, of which Figs. 19–23 are examples, show us how cells pass from their original spherical symmetry into radial symmetry, as they pass from a state in which they are similarly-conditioned on all sides, to a state in which two of their opposite sides or ends are conditioned in ways that are like one another, but unlike the ways in which all other sides are conditioned.