The Polysiphonia are Algæ, seen in tufts from ten to twelve inches long, of usually much branched jointed filaments, on rocks, corallines, and the smaller Algæ at low water mark. The joints of the filaments contain upright tubes, full of purple or reddish brown matter, which is seen through their transparent walls. The number of these colour tubes vary from four to ten, eighteen, or even twenty, and form the characteristic of the genus. Thus there is a similarity of structure between the Polysiphonia, a genus of the highest order amongst Rhodosperms, and the Griffithsia, which is one of the lowest. The Polysiphonia elongata, which is from six to twelve inches high, has four primary and several secondary colour tubes in the transparent joints of its filaments. Like many of its congeners, this plant does not come to perfection or bear fruit till the second spring. In its youth, it resembles the full grown plant but is smaller, and the colour tubes are not formed in the capillary threads of the tufts, which with many of its branchlets are deciduous, leaving the plant in its naked winter state. With returning warmth, it assumes its perfect form, and in March and April bears fruit, which consists of nuclei in conceptacles, sessile on the branches, either clustered or scattered. The spores are at the top of jointed threads rising from a substance at the base of the nuclei. In some species of this genus, tetraspores only have been found. The Cryptonemiaceæ are the most numerous and diversified of all the orders of the Rhodosperms. Thirty-five genera are widely dispersed throughout the world, chiefly in the northern hemisphere; twenty-four genera at least occur on the east coast of North America; and fifteen genera have representatives in the British seas. This multitude of generic forms is divided into two groups of gelatinous structure, the one having inarticulate fronds composed of articulate threads closely incorporated, the other membranaceous, formed of cells closely incorporated into a foliaceous expansion. Most of these plants have a stratum of cellular tissue, interposed between a spongy matter in the interior of the frond, and the epiderm or external skin, which for the most part consists of a simple layer of minute cells firmly united by their sides, generally forming a mere film; but it may be thin and flexible, thick, tough, or leathery, according to circumstances. The Furcellaria fastigiata (fig. 23 B) has an intermediate layer of cellular tissue between its skin, and a pulpy interior. The frond is cylindrical, smooth, strong, and opaque, repeatedly forked with long narrow forkings. The root is fibrous, and the stem short and tapering. Masses of spores nestle under the skin and swell out the upper forkings, and oblong tetraspores are deeply imbedded in the same. In the Dumontia filiformis the simple undivided stem and branches are filled with a watery jelly.