Lithocys´tis Allman´ni (fig. 9) is very minute, and not uncommon upon sea-weeds, stones, &c. It consists of a single fan-shaped crustaceous layer of cells, closely investing the body to which it is attached; its fructification is unknown.
Leaving the family of crustaceous Florideæ, we shall now pass to those of softer consistence, although all the marine Algæ contain a considerable quantity of calcareous matter.
Rhodomela´ceæ.—In this family we have the large genus Polysiphónia, in which the frond ([Pl. IV.] figs. 25 and 26) is filamentous, the filaments being apparently jointed and longitudinally striated. The filaments are composed of rings of cells (fig. 27), arranged end to end, and containing dark endochrome. The ends of the colourless cell-walls separating the endochromes of the cells of adjacent rings produce the jointed appearance; while the striated appearance is caused by the dark cells being elongate and the cell-walls thick, so as to form white interspaces.
The fructification consists of capsules (ceramidia), attached to the sides of the branches, containing pear-shaped spores, with tetraspores imbedded in swollen branches of separate plants.
Polysiphónia fastigiáta (fig. 25, a small piece) is common, attached to the fronds of Fucus. Its filaments are rigid, bristle-like, of the same breadth throughout, forked, and forming globular brown or yellowish tufts, from 2 to 4 inches long. The joints are broader than long, each with 16-18 of the dark cells. In the centre of the branches of this sea-weed is a row of curious objects (fig. 26 a), consisting of a dark-coloured body surrounded with irregular spiny marginal processes, and with a colourless short process above and below. These require further investigation.
P. nigres´cens is also common among masses of seaweeds. Its filaments are brown, pinnate, the branches awl-shaped, and the joints about as long as broad.
Dásya coccin´ea ([Pl. IV.] figs. 1 and 2, representing small portions of a filament) is a very common filamentous red sea-weed of the same family. The filaments are 6-8 inches long, and bipinnate,—the larger ones somewhat resembling those of Polysiphonia, in being composed of parallel longitudinal cells, arranged round the centre, but containing also smaller intermediate cells; while the smallest branches (fig. 2), which arise in tufts, consist of a single row of cells, little longer than broad. The fruit consists of ovate capsules (ceramidia), placed at the base of the branches, and containing a round mass of spores. There is also another kind of fructification, occurring on distinct plants; this is formed of one or two rows of tetraspores, immersed in pod-like capsules, called stichid´ia (στἱχος, row).
Delesseria´ceæ.—In this family, the typical or most highly developed genus of which, Delesséria, has beautiful leaf-like rose-red fronds, we shall examine the common Plocámium coccin´eum ([Pl. IV.] figs. 23 and 22). This is of a fine red colour; the fronds are from 2 to 12 inches long, and consist of numerous branched and bushy filaments. These are compressed, with the branchlets arranged in alternate rows on the two margins of the stem. The end branchlets are acute and pectinate (pecten, a comb), or arranged like the teeth of a comb. The cells of which the filaments consist are small and angular, giving the surface the appearance of being elegantly netted under a high power. The fruit (fig. 22) consists of globular capsules, called coccid´ia (κὁκκος, a berry), placed in the axils or forks at which two branches separate, and containing a mass of angular spores. There are also tetraspore-pods (stichidia), as in Dasya; and tetraspores (fig. 24) in little leaf-like altered branches (fig. 21), called spor´ophylles (σπορἁ, seed, φὑλλον, leaf), and antheridia are present.
Rhodymenia´ceæ.—In this family we have Hyp´nea purpuras´cens ([Pl. IV.] fig. 35). The filamentous pale purple frond of this sea-weed is from 6 inches to a foot or more in length, the branches being alternate and spreading. The fructification consists of capsules or coccidia (fig. 32), immersed in the branches, and containing the spores (fig. 34). Tetraspores also occur in the cells of the surface of the filaments.
Ceramia´ceæ.—This is the last family to be noticed. Cerámium nodósum ([Pl. IV.] figs. 10 and 11), which belongs to it, is a most delicate and elegant filamentous sea-weed, commonly found attached to other sea-weeds. The filaments are hair-like or capillary, irregularly dichot´omous; they consist of colourless cells, 3 or 4 times as long as broad, and with thick walls. The junctions of the cells are swollen (fig. 11), and covered with very minute dark red cells, giving them a knotty and jointed appearance to the naked eye or under a low power. The globular capsules, or favel’læ (favus, a honeycomb), containing the numerous spores, are situated at the ends of the branchlets, and the tetraspores (fig. 11) in twos or threes on the outer margins of them.