On the northern coasts of Britain we may meet with Odonthalia dentata, the blood-red fronds of which are tufted, and arise from a hard, disc-like root. Each frond projects from the axil of a tooth-like projection of the main stem, and is deeply pinnatifid, with a distinct midrib in the lower part, and thin and membranaceous towards the tip. The pinna are dentate, and the spores are in stalked, oval conceptacles in the axils of the pinnæ. The tetraspores are similarly situated in stalked, lanceolate leaflets.
The weeds of the genus Rytiphlæa are very similar to some of the Polysiphonia, the axis of the frond being jointed and transversely striped, but the nodes are less distinct and are not constricted. They are shrub-like weeds, with tufted spores in oval, sessile conceptacles; and tetraspores in spindle-shaped branchlets or in little pod-like leaflets. The principal British species are:—
R. pinastroides, a much-branched and shrub-like weed, of a dull-red colour, which turns black when the plant is dried. The branches have rigid, hooked branchlets arranged in such a manner as to give a combed appearance. This species occurs on the south coast, and is in its prime in very early spring. It is often rendered peculiarly interesting by the colonies of zoophytes and the patches of Melobesia with which it is more or less covered.
R. fruticulosa is another shrubby species, with irregularly branched, interlacing stems. It is to be found in the rock pools of the south and west coasts, and is of a deep-purple colour in the deeper shady pools, but varying to a yellowish tint where exposed to the full light of the sun. The whole of the frond is covered with hooked branchlets, and the weed is peculiar for the fact that, when removed from the rock pool, little glistening beads of water remain attached to the tips of the terminal branches. The tetraspores are situated in distorted branchlets.
R. thuyoides has creeping, fibrous roots, from which arise the erect stems of purple-brown, branched fronds with short spine-like branchlets. It occurs in the shallower rock pools, where it grows attached to rocks or to other weeds. It is in its best condition during the summer, when we may see its oval spore-conceptacles and the tetraspores in distorted branchlets.
The last genus of the Rhodomelaceæ is Dasya, which contains some very graceful and brightly-coloured weeds that are found principally on our south and west coasts. In these the fronds are thread-like or flattened, branched, and without visible joints. The main stem contains many tubes, but the tubular structure is hidden by the outer layer of cells; and the branchlets, which are slender, one-tubed, and jointed, bear little lanceolate pods that contain the tetraspores.
D. ocellata has small tufted fronds, about two or three inches long, attached to a small discoid root. The main stems are densely covered with slender, forked branchlets, those at the tips being clustered in such a manner as to recall the eye-like marks of the peacock’s tail. It grows principally on the mud-covered rocks beyond low-water mark, and is not by any means a common weed. Another species—D. arbuscula—is somewhat plentiful on parts of the Scottish and Irish coasts, but comparatively rare in South Britain. It has a small disc-like root, and stems thickly clothed with short branchlets. The spore-conceptacles are tapering, on short stalks, and the tetraspores are contained in pointed pods on the branchlets. The scarlet Dasya (D. coccinea) may be commonly seen at and beyond low-water mark during late summer, at which time splendid specimens may also be found on the beach after storms. Its stem is thick, proceeding from a discoid root, and is clothed with hair-like filaments; and the branches bear short, slender branchlets that give them a feathery appearance. The tetraspores are contained in elongated, pointed, and stalked pods. There are three other species on the British list, but they are not common weeds.
The last of the three great groups into which the sea weeds are divided is the Melanospermeæ, or olive-spored algæ, the different species of which are generally very readily distinguished by their olive-green or olive-brown colour, for the whole plant, as well as the spores, contains a dark olive colouring matter, in addition to the chlorophyll which is always present.
These weeds are often very large, frequently attaining a length of twenty feet or more in our seas, and from eighty to a hundred feet in warmer parts; and, being also extremely abundant almost everywhere, they form a most conspicuous feature of the shore. They usually grow on rocks and stones, from high-water mark to moderately deep water, but some of the smaller species are pseudo-parasitic on other algæ.
Their form is most varied. Some are minute filamentous plants, consisting only of slender jointed threads, and others are mere shapeless masses; but many of the larger species exhibit a great differentiation of form, having root-like and stem-like structures, and expansions that resemble leaves. The latter, too, often have large vesicles that contain air, sometimes arranged singly along the median line of the frond, or in lateral pairs, or a single vesicle at the base of each segment of the thallus.