Mostly, simple plants are too small to be individually visible without the microscope. But, in some cases, these vegetal aggregates of the first order grow to appreciable sizes. In the mycelium of some fungi, we have single cells developed into long branched filaments, or ramified tubules, that are of considerable lengths. An analogous structure characterizes certain tribes of Algæ, of which Codium adhærens, Fig. [4], may serve as an example. In Botrydium, another alga, Fig. [5], we have a structure which is described as simulating a higher plant, with root, stem, bud, and fruit, all produced by the branching of a single cell. And among fungi the genus Mucor, Fig. [6], furnishes an example of allied kind.[3] Here, though the size attained is much greater than that of many organisms which are morphologically compound, we are compelled to consider the morphological composition as simple; since the whole can no more be separated into minor wholes, than can the branched vascular system of an animal. In these cases we have considerable bulk attained, not by a number of aggregates of the first order being united into an aggregate of the second order, but by the continuous growth of an aggregate of the first order.
§ 182. The transition to higher forms begins in a very unobtrusive manner. Among these aggregates of the first order, an approach towards that union by which aggregates of the second order are produced, is indicated by mere juxtaposition. Protophytes multiply rapidly; and their rapid multiplication sometimes causes crowding. When, instead of floating free in the water, they form a thin film on a moist surface, or are imbedded in a common matrix of mucilage; the mechanical obstacles to dispersion result in a kind of feeble integration, vaguely shadowing forth a combined group. Somewhat more definite combination is shown us by such plants as Palmella botryoides. Here the members of a family of cells, arising by the spontaneous fission of a parent-cell, remain united by slender threads of that jelly-like substance which envelops their surfaces. In some Diatomaceæ several individuals, instead of completely separating, hold together by their angles; and in other Diatomaceæ, as the Bacillaria, a variable number of units cohere so slightly, that they are continually moving in relation to one another.
This formation of aggregates of the second order, faintly indicated in feeble and variable unions like the above, may be traced through phases of increasing permanence and definiteness, as well as increasing extent. In the yeast-plant, Fig. [7], we have cells which may exist singly, or joined into groups of several; and which have their shapes scarcely at all modified by their connexion. Among the Desmidiaceæ, it happens in many cases that the two individuals produced by division of a parent-individual, part as soon as they are fully formed; but in other cases, instead of parting they compose a group of two. Allied kinds show us how, by subsequent fissions of the adherent individuals and their progeny, there result longer groups; and in some species, a continuous thread of them is thus produced. Figs. [8, 9, 11], exhibit these several stages. Fig. 10 represents a Scenedesmus in which the individuation of the group is manifest. Instead of linear aggregation, many protophytes illustrate central aggregation; as shown in Figs. [12, 13, 14, 15]. Other instances are furnished by such forms as the Gonium pectorale, Fig. [16] (a being the front view, and b the edge view), and the Sarcina ventriculi, Fig. [17]. Further, we have that spherical mode of aggregation of which the Volvox globator furnishes a familiar instance.
Figs. 7–17.
Figs. 18–23.
Thus far, however, the individuality of the secondary aggregate is feebly pronounced: not simply in the sense that it is small; but also in the sense that the individualities of the primary aggregates are very little subordinated. But on seeking further, we find transitions towards forms in which the compound individuality is more dominant, while the simple individualities are more obscured. Obscuration of one kind accompanies mere increase of size in the secondary aggregate. In proportion to the greater number of the morphological units held together in one mass, becomes their relative insignificance as individuals. We see this in the irregularly-spreading lichens that form patches on rocks; and in such creeping fungi as grow in films or laminæ on decaying wood and the bark of trees. In these cases, however, the integration of the component cells is of an almost mechanical kind. The aggregate of them is scarcely more individuated than a lump of inorganic matter: as witness the way in which the lichen extends its curved edges in this or that direction, as the surface favours; or the way in which the fungus grows round and imbeds the shoots and leaves that lie in its way, just as so much plastic clay might do. Though here, in the augmentation of mass, we see a progress towards the evolution of a higher type, we have as yet none of that definiteness required to constitute a compound unit, or true aggregate of the second order. Another kind of obscuration of the morphological units, is brought about by their more complete coalescence into the form of some structure made by their union. This is well exemplified among the Confervoideæ and Conjugatæ. In Fig. [18], there are represented the stages of a growing Mougeotia genuflexa, in which this merging of the simple individualities into the compound individuality, is shown in the history of a single plant; and in Figs. [19, 20, 21, 22, 23], are represented a series of species from this group, and that of Cladophora,[4] in which we see a progressing integration. While, in the lower types, the primitive spheroidal forms of the cells are scarcely altered, in the higher types the cells are so fused together as to constitute cylinders divided by septa. Here, however, the indefiniteness is still great. There are no specific limits to the length of any thread thus produced, and there is none of that differentiation of parts required to give a decided individuality to the whole.
To constitute something like a true aggregate of the second order, capable of serving as a compound unit that may be combined with others like itself into still higher aggregates, there must exist both mass and definiteness.
§ 183. An approach towards plants which unite these characters, may be traced in such forms as Bangia ciliaris, Fig. [24]. The multiplication of cells here takes place, not in a longitudinal direction only, but also in a transverse direction; and the transverse multiplication being greater towards the middle of the frond, there results a difference between the middle and the two extremities—a character which, in a feeble way, unites all the parts into a whole. Even this slight individuation is, however, very indefinitely marked; since, as shown by the figures, the lateral multiplication of cells does not go on in a precise manner.