Although the diatoms have a vast variety of forms, they all consist of a simple primordial cell whose external coat of cellulose is so deeply interpenetrated with silex that it is indestructible, a structure which constitutes the peculiar characteristic of the tribe. This primordial cell, as in other plants, contains organizable liquid or protoplasm, through which golden-brown granules are pretty regularly distributed, except in the centre, where they are collected into a nucleus. Round this nucleus they commonly form a ring from which radiating lines of granules diverge to the interior wall of the cell. In each of these there is a double current of granules, similar to the circulation in the Desmidiaceæ; it was discovered by Professor Smith in some of the comparatively large diatoms. At times oil globules are seen in the protoplasm. The golden-brown matter is supposed to be chlorophyll, whose green tint has been changed by the presence of iron, which is assimilated in this group. Such is the internal structure of a race of plants altogether invisible to the naked eye. Their external forms, reproduction and movements, are no less wonderful. The silicious envelope of the simple cell of a Diatom or frustule, as a single plant is usually called, consists of two valves or plates, commonly of the most perfect symmetry, closely applied to each other along a line of junction like the two valves of a bivalve shell, and each valve being more or less concavo-convex, a cavity is left between the two which is occupied by the golden-brown cell described above. The form of the cavity differs greatly, for sometimes each valve is hemispherical, so that the cavity is globular; sometimes it is a small segment of a sphere, resembling a watch-glass, so that the cavity is lenticular; in short, the form of the cavity depends upon that of the valves, which may be heart-shaped, or much elongated, square, triangular, boat-shaped, or furnished with outgrowths, which, however, is rare. [Illustration: Fig. 11.—A, Diatoma vulgare:—_a_, side view of frustule; _b_, frustule undergoing self-division. B. Grammatophora serpentina:—_a_, front and side view of single frustule; _b_, front and end view of divided frustule; _c_, frustule about to undergo division; _d_, frustule completely divided. ]