The next group, the Laurenciaceæ, are cartilaginous and cylindrical or compressed, the frond in the greater portion of them being inarticulate and solid. They contain several species valued by collectors, although some of them are amongst our commonest plants. Their colour is, when perfect, a dull purple or brownish red, but they change under the influence of light and air, while fresh water is rapidly destructive to their tints. ([Fig. 10.])
Fig. 10. Laurencia pinnatifida.
The Chylocladiæ are curiously jointed plants, removed by Agardh to a new genus, Lomentaria, and a new order Chondriæ. Bonnemaisonia asparagoides is the most rare and beautiful of the tribe.
The last tribe of red weeds, Rhodomelaceæ, varies greatly in the structure of the frond, but the fruit is more uniform. Polysiphonia and Dasya contain the finest of the filiform division; the leafy one, Odonthalia, a northern form, is a very beautiful sea-weed both as respects form and colour. Well-grown specimens are not unlike a hawthorn twig, and of a blood red colour.
The plants of the sub-order Melanospermeæ, are, like the red sea-weeds, exclusively marine. They are usually large and coarse, and confined mostly to comparatively shallow water. In the Laminariaceæ we find the gigantic oarweeds already briefly referred to. Lessonia, which encircles in submarine forests the antarctic coasts, is an erect, tree-like plant, with a trunk from five to ten feet high, forked branches, and drooping leaves, one to three feet in length, and has been compared to a weeping willow. Sir Joseph Hooker says, that from a boat there may on a calm day be witnessed in the antarctic regions, over these submarine groves, "as busy a scene as is presented by the coral reefs of the tropics. The leaves of the Lessoniæ are crowded with Sertulariæ and Mollusca, or encircled with Flustra; on the trunks parasitic Algæ abound, together with chitons, limpets, and other shells; at the base and among the tangled roots swarm thousands of Crustaceæ and Radiata, while fish of several species dart among the leaves and branches." Of these and other gigantic melanosperms, flung ashore by the waves, a belt of decaying vegetable matter is formed, miles in extent, some yards broad, and three feet in depth; and Sir J. Hooker adds that the trunks of Lessonia so much resemble driftwood that no persuasion could prevent an ignorant shipmaster from employing his crew, during two bitterly cold days, in collecting this incombustible material for fuel. Macrocystis and Nereocystis are also giant members of this sub-order. Some of the Laminariæ which form a belt around our own coasts not seldom attain a length of from eight to twelve feet. The common bladder-wrack (Fucus vesiculosus) sometimes grows in Jutland to a height of ten feet, and in clusters several feet in diameter. The colour of most of the plants in this sub-order is some shade of olive, but several of them turn to green in drying.
The first group, Ectocarpeæ, is composed of thread-like jointed plants, the fructification of which consists of external spores, sometimes formed by the swelling of a branchlet. The typical genus, Ectocarpus, abounds in species, a dozen or so of which, very nearly allied plants, being found around our own shores. One or two of them are very handsome. There are also some very beautiful plants in the genus Sphacelaria, belonging to this group, several of them resembling miniature ferns. All the Sphacelariæ are easily recognized by the withered appearance of the tips of the fruiting branches. Myriotrichia is a genus of small parasitical plants, the two British species of which grow chiefly on the sea thongs (Chorda).
The Chordariæ are sometimes gelatinous in structure, in other cases cartilaginous. The fruit is contained in the substance of the frond. The genus Chordaria consists of plants which have the appearance of dark coloured twine. There are two British species, one being rather common. Chorda filum, sea-rope, another string-like sea-weed, grows in tufts from a few inches to many feet in length, and tapering at the roots to about the thickness of a pig's bristle. In quiet land-locked bays with a sandy or muddy bottom, it sometimes extends to forty feet in length, forming extensive meadows, obstructing the passage of boats, and endangering the lives of swimmers entangled in its slimy cords, whence probably its other name of "dead men's lines."