Other orders of Protophyta have much more special forms, along with much more special attitudes: their homologous parts maintaining, from generation to generation, unlike relations to incident forces. The Desmidiaceæ and Diatomaceæ, of which Figs.[ 2 and 3] show examples, severally include genera characterized by triple bilateral symmetry. A Navicula is divisible into corresponding halves by a transverse plane and by two longitudinal planes—one cutting its valves at right angles and the other passing between its valves. The like is true of those numerous transversely-constricted forms of Desmidiaceæ, exemplified by the second of the individuals represented in Fig. [2]. If now we ask how a Navicula is related to its environment, we see that its mode of life exposes it to three different sets of forces: each set being resolvable into two equal and opposite sets. A Navicula moves in the direction of its length, with either end foremost. Hence, on the average, its ends are subject to like actions from the agencies to which its motions subject it. Further, either end while moving exposes its right and left sides to amounts of influence which in the long run must be equal. If, then, the two ends are not only like one another, but have corresponding right and left sides, the symmetrical distribution of parts answers to the symmetrical distribution of forces. Passing to the two edges and the two flat surfaces, we similarly find a clue to their likenesses and differences in their respective relations to the things around them. These locomotive protophytes move through the entangled masses of fragments and fibres produced by decaying organisms and confervoid growths. The interstices in such matted accumulations are nearly all of them much longer in one dimension than in the rest—form crevices rather than regular meshes. Hence, a small organism will have much greater facility of insinuating itself through this débris, in which it finds nutriment, if its transverse section is flattened instead of square or circular. And while we see how, by survival of the fittest, a flattened form is likely to be acquired by diatoms having this habit; we also see that likeness will be maintained between the two flat surfaces and between the two edges. For, on the average, the relations of the two flat surfaces to the sides of the openings through which the diatom passes, will be alike; and so, too, on the average, will be the relations of the two edges. In desmids of the type exemplified by the second individual in Fig. [2], a kindred equalization of dimensions is otherwise insured. There is nothing to keep one of the two surfaces uppermost rather than the other; and hence, in the long succession of individuals, the two surfaces are sure to be similarly exposed to light and agencies in general. When to this is added the fact that spontaneous fission occurs transversely in a constant way, it becomes manifest that the two ends, while they are maintained in conditions like one another, are maintained in conditions unlike those of the two edges. Here then, as before, triple bilateral symmetry in form, co-exists with a triple bilateral symmetry in the average distribution of actions.
Figs. 4, 5, 6.
Still confining our attention to aggregates of the first order, let us next note what results when the two ends are permanently subject to different conditions. The fixed unicellular plants, of which examples are given in Figs. [4, 5, and 6], severally illustrate the contrast in shape arising between the part that is applied to the supporting surface and the part that extends into the surrounding medium. These two parts which are the most unlike in their relations to incident forces, are the most unlike in the forms. Observe, next, that the part which lifts itself into the water or air, is more or less decidedly radial. Each outward-growing tubule of Codium adhærens, Fig. [4], has its parts disposed with some regularity around its axis; the upper stem and spore-vessel of Botrydium, Fig. [5], display a lateral growth that is approximately equal in every direction; and the stems of the Mucor, Fig. [6], shoot up with an approach to evenness on all sides. Plants of this low type are naturally very variable in their modes of growth: each individual being greatly modified in form by its special circumstances. But they nevertheless show us a general likeness between parts exposed to like forces, as well as a general unlikeness between parts exposed to unlike forces.
Respecting the forms of these aggregates of the first order, it has only to be added that they are asymmetrical where there is total irregularity in the incidence of forces. We have an example in the indefinitely contorted and branched shape of a fungus-cell, growing as a mycelium among the particles of soil or through the interstices of organic tissue.
§ 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.
Figs. 192–196.