§ 218. Re-illustrations of the general truths which the forms of these vegetal aggregates of the first order display, are furnished by vegetal aggregates of the second order. The equalities and inequalities of growth in different directions, prove to be similarly related to the equalities and inequalities of environing actions in different directions. Of spherical symmetry an instance occurs in _Eudorina elegans_. The ciliated cells are here so united as to produce a small, mulberry-shaped, hollow ball which, being similarly conditioned on all sides, shows no unlikenesses of structure. An allied form, however, _Volvox globator_, presents a highly instructive, though very trifling, modification. It is not absolutely homogeneous in its structure and is not absolutely homogeneous in its motions. The waving cilia of its component cells have fallen into such slight heterogeneities of action as to cause rotation in a constant direction; and along with a fixed axis of rotation there has arisen a fixed axis of progression. A concomitant fact is that the cells of the colony exhibit an appreciable differentiation in relation to the fixed axis. There is an incipient divergence from spherical uniformity along with this slight divergence from uniformity of conditions. Vegetal aggregates of the second order are usually fixed: locomotion is exceptional. Fixity implies that the surface of attachment is differently circumstanced from the free surface. Hence we may expect to find, as we do find, that among these rooted aggregates of the second order, as among those of the first order, the primary contrast of shape is between the adherent part and the loose part. Sea-weeds variously exemplify this. In some the fronds are very irregular and in some tolerably regular; in some the form is pseudo-foliar and in some pseud-axial; but differing though they do in these respects, they agree in having the end which is attached to a solid body unlike the other end. The same truth is seen in such secondary aggregates as the common Agarics, or rather in their immensely-developed organs of fructification. A puff-ball, Fig. 192, presents no other obvious unlikeness of parts than that between its under and upper surfaces. So too with the stalked kinds that frequent our woods and pastures. In the types which Figs. 193, 194, 195, delineate, the unlikenesses between the rooted ends and the expanded ends, as well as between the under and upper surfaces of the expanded ends, are obviously related to this fundamental contrast of conditions. Nor is this relation less clearly displayed in the sessile fungi which grow out from the sides of trees, as shown at _a_, _b_, Fig. 196. That which is common to this and the preceding types, is the contrast between the attached end and the free end. [Illustration: Figs. 192–196.]