The syncladeous Sphagnei are aquatic bog mosses of a pale yellowish green colour. They form but one genus, Sphagnum, which consists of some eight or ten species and several varieties. The species of Sphagnum, or common bog moss, hold an important place in the economy of nature. They are floating mosses, entirely destitute of roots. The stalk of the full grown plant, like that of land mosses, is constructed of three kinds of cells; one forms the exterior or cortical layer, another forms the central pith or axial system, while the third, which is coloured and somewhat ligneous, comes between the other two. The leaves, which have their origin in the cortical layer while it is yet soft, consist of two kinds of cells, one kind being large, elongated, and colourless, and containing spiral fibres loosely coiled in their interior. The membranous walls of these cells have large round apertures by which their cavities have free communication; for certain animalcules, which sometimes live in the cells, have been seen to pass from one to the other. Between these colourless cells are some thick-walled, narrow, elongated green cells, which give the leaf firmness and colour. According to Mr. Wilson, the fascicles, or bundles of branches, are disposed round the stem in imbricated spirals, so that, for every complete spiral formed by five of these fascicles, there are eight spirals formed of twenty leaves, four leaves being inserted between each pair of fascicles. The fructification in all these floating mosses is immersed in the leaves of the stem, the antheridia being globose, and the spermatozoids having spiral motions, both within the cells of the antheridia and when they come out. The urns, which are the product of fructification, and are borne at the top of what appears to be a long footstalk, but is in reality a pedunculate vaginula, are globular, and their lids have been observed to be driven off when the spores are ripe, with such force as to give a distinctly audible report. The common bog moss grows so rapidly that, rootless as it is, it soon covers a pool with its matted bundles of branches, and as in a few years it has no room to spread, the lower stems and branches decay, sink to the bottom, and begin to form a peat moss, while the upper parts grow on, so that new stems and branches are perpetually produced. Multitudes of spores no doubt germinate, and, in this way, the pool is filled up, and a peat moss is at length formed.