The Rhodosperms are comparatively small plants. Some which form velvety cushions on stones, or minute tufts on small Algæ, are only the fraction of an inch high, but the larger kinds range from one to four, six, ten, or twenty inches; probably none exceed two feet. In thickness, some fronds are fine, like jointed and branched hairs, while others are thick, like hog’s bristles or crow quills. Numerous as the forms are, the simple jointed filamentous frond is connected by a series of forms with the highest order of the class. A great portion of the Rhodosperms on the British coasts is composed of the exquisitely beautiful order of the Ceramiaceæ. They abound in every rocky pool, on every piece of wood that has been long exposed to the waves, on rocks and stones, and, above all, they fringe the Zostera marina, or sea wrack, as well as the firmer Algæ, with every shade of red from bright crimson to purple. They are articulated filiform plants, approaching in simplicity of form to the Confervas. The genus Callithamnion, which has thirty species in the British seas, consists of cylindrical jointed threads more or less profusely branched, and distinguished by having the divisions between the joints opaque and of various shades of red and purple, while the joints themselves are transparent and colourless, so that the stem and branches appear to be striped across by alternately white and coloured bands which are often visible to the naked eye, notwithstanding the smallness of the plants and the delicacy of their filaments, as the C. sparsum,—which is a soft purple tuft of jointed threads scarcely one-tenth of an inch high. The Callithamnion corymbosum has a soft jointed filamentous stem, hair-like below, fine as a cobweb above, and excessively branched, with dichotomous branches. In fig. 25 _a_ represents a thread of this plant with tetraspores, much magnified; _b_, a portion of the same, more highly magnified; _c_, a thread with naked nuclei, gongylospermous, that is, filled with a mass of spores, magnified; and _d_, a spore, magnified more highly. The nuclei are naked in all the Ceramiaceæ. [Illustration: Fig. 25. Callithamnion corymbosum.]