The type of the Desmidiaceæ is continued by various modes of bisection, depending upon the genus and species of the plant. In the Closterium Lunula, which has an elongated crescent shape, as in [fig. 10] A, the endochrome or internal matter divides into two equal parts, which retreat from one another at the middle line; and a constriction of the cellulose coat takes place between them, which increases till it closes entirely round the extremities, as in [fig. 10] D; then one of the halves remains at rest while the other moves from side to side, and finally detaches itself from the other with a jerk. In each of these halves a constriction of the endochrome may be seen, dividing it into an obtuse and an elongated part, and for some time the circulating fluid flows round the obtuse end, but the latter gradually assumes the form of the elongated end, the regular circulation of the fluid is established, and in five or six hours after the separation, two young desmids are formed precisely similar to their parent, the Closterium Lunula.
The Cosmarium, another Desmid, consists of a cell of two lobes united by a narrow isthmus. When about to multiply, the isthmus swells into two globular expansions, separated from each other and from the two lobes of the cell, by a narrow neck. These enlargements increase and assume the appearance of half segments of the original cell. In this state the plant consists of four segments lying end to end, the two old ones forming the extremes, with the two new ones in the middle. At last, each of the middle segments gets a new half, which soon acquires the full size and characteristics of the old one. This process, which is accomplished in twenty-four hours, is repeated ere long, and being continued indefinitely, the extreme lobes of the row are thrust farther and farther asunder, and the whole constricted thread or chain of Cosmaria is enclosed in a gelatinous sheath. The last two central lobes contain no portion of the original frond or plant, and may thus be considered to be entirely new individuals.
Many of the Desmidiaceæ multiply by the subdivision of their endochrome into a multitude of granular particles called gonidia, which are set free by the rupture of the cell wall, and of which every one may develop itself into a new cell. The gonidia may be zoospores with cilia and active locomotion, or they may be enclosed in a firm envelope, and become resting spores. The movement of the zoospores at first within the cavity of the cell which gave them their origin, and afterwards externally to it, has frequently been observed in the varied species of the genus Cosmarium, and has been described under the name of the ‘swarming of the granules,’ from the resemblance of the moving mass to a swarm of bees. Their subsequent history is unknown.
In the Pediastrum, a plant consisting of a cluster of cells, the zoospores are not emitted separately, but those formed by the subdivision of the endochrome of one cell into 4, 8, 16, 32, or 64 parts, escape from the parent plant still enclosed in the inner tunic of the cell, and it is within this that they develop themselves into a cluster resembling that in which they originated.
Mr. Thwaites discovered that the Desmidiaceæ are also propagated by conjugation, which would be impossible if the hard coat of the adjacent cells about to unite did not split open; then the whole endochrome in one cell passes into and blends with that in the other cell, so as to form one mass, which soon acquires a delicate membranaceous envelope. At first the mass consists of granular green matter, but when the membrane becomes thicker, it changes to brown or red. This body, which is called a sporangium, is sometimes smooth, sometimes granular, covered with tubercles or rough with spines, according to the nature of the original plants. The filamental species are propagated by conjugation, but the subsequent history of the produce is still obscure, though there is reason to believe that they give rise to plants of different forms, while all the other modes of increase only reproduce a facsimile of the parent.
Desmidiaceæ exist in America, but their distribution is little known. In Europe, their maximum seems to be in the south of England. They abound in small shallow pools that do not dry up in summer, and also on boggy moors. The larger kinds are spread out as a thin gelatinous stratum at the bottom of water, or collected in little tufts; others form a dirty cloud upon the stems and leaves of aquatic plants. They have been found in a fossil state in flint, their spores have been discovered in the grey chalk at Folkestone, and the cells of various species of Closterium and Euastrum are imbedded in the marls of the United States of North America.
The Diatomaceæ, or Brittleworts, are unicellular microscopic plants so numerous that there is hardly a spot on the face of the earth, from Spitzbergen to Victoria Land, where they may not be found. They abound in the ocean, in still and running fresh water, and even on the surface of the bare ground. They extend in latitude beyond the limits of all other plants, and can endure extremes of temperature, being able to exist in thermal springs, and in the pancake ice in the south polar latitudes. Though much too small to be visible to the naked eye, they occur in such countless myriads as to stain the berg and pancake ice wherever they are washed by the swell of the sea; and when enclosed in the congealing surface of the water they impart to the brash and pancake ice a pale ochreous colour.
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.
