The air vesicles, of course, serve to buoy up the plant when it is submerged, thus enabling the light to penetrate between its fronds to lower portions; and when the plants have been wrenched from their moorings by the force of the waves, they immediately rise to the surface and are drifted on to the shore or accumulate in the eddies of the surface currents. In this way immense masses of floating weeds are formed, the most remarkable being that of the Sargasso Sea in the North Atlantic.
Like other algæ, the melanospores grow by a continued process of cell-division, and when portions of the thallus are worn away during stormy weather, they are renewed by the same process.
The cell-walls of many species are very mucilaginous, the gelatinous covering being either the result of the degeneration of the cell-walls themselves, or the secretion of special glands.
As with the last division, the reproduction of the melanospores may be asexual or sexual. The asexual spores, which are not motile, are formed in some of the surface cells of the thallus. The male and female sexual organs, called respectively the antheridia and the oogonia, are produced in cavities on special portions of the thallus, both kinds being often formed in the same cavity or depression. The latter contains from one to eight little bodies called oospheres. These escape and float passively away when the wall of the oogonia ruptures. The antheridia are also discharged whole, but the minute fertilising elements (antherozoids), which are eventually set free from them, swarm round the oospheres, being attracted by the latter. Soon one of the antherozoids enters the oosphere, and from that moment all attraction ceases, the remainder of the antherozoids floating passively away; and the oosphere, previously naked and barren, now develops a cell-wall, and becomes the fertile progenitor of a new plant.
Starting with the lowest of the melanospores, we first deal with the order Ectocarpaceæ, which is characterised by olive, thread-like, jointed fronds, with spores on the branchlets or embedded in their substance; two kinds of spores often existing in the same plant.
The typical genus (Ectocarpus) contains many British species, though several of them are rare. They are soft and flexible weeds, generally of a dull olive colour, with slimy, tubular fronds, and grow in tufts on other weeds or on mud-covered rocks. Spores of various shapes are scattered over the fronds, and are also contained in pod-like bodies formed of the branchlets. This latter feature is, perhaps, the best distinguishing characteristic of the genus, but it is not an easy matter to identify the several species it contains.
E. tomentosus is very commonly found on Fucus and other weeds, where it forms matted tufts of slender threads of a yellowish-brown colour. The threads are clothed with transparent cilia, and together form a dense, spongy mass. The spores are contained in narrow pods supported on short stalks. E. littoralis is another common species, of a very unattractive appearance. It grows in matted tufts on other weeds, on rocks, mud, or any submerged object, and its spores are contained in linear swellings of the branches. This species thrives well in brackish water, and may be seen far up certain tidal rivers.
Fig. 260.—Ectocarpus granulosus