§ 181. These aggregates of the lowest order, each formed of physiological units united into a group that is structurally single and cannot be divided without destruction of its individuality, may, as above implied, exist as independent organisms. The assumption to which we are committed by the hypothesis of evolution, that such so-called unicellular plants were at first the only kinds of plants, is in harmony with the fact that habitats not occupied by plants of higher orders, commonly contain these protophytes in great abundance and great variety. The various species of _Pleurococcaceæ_, of _Desmidiaceæ_, and _Diatomaceæ_, supply examples of morphological units living and propagating separately, under numerous modifications of form and structure. Figures 1, 2, and 3, represent a few of the commonest types. [Illustration: Figs. 1, 2, 3.] [Illustration: Figs. 4, 5, 6.] 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.