Fucus, the generic name of the various species of brown sea-wrack which form the main vegetation of rocky shores between tide-marks. Commonest of all upon European coasts (save in the Mediterranean), and abundant also in the North Pacific, is F. vesiculosus (Bladderweed, Black Tang, Seaware, Kelp-ware, &c.), easily distinguished by its entire edges and paired air-vesicles. In scarcity of better fodder, oxen, sheep, and deer will eat it from the rocks, and in North Europe it is sometimes boiled for hogs with a little coarse flour. On account of the very large proportion of ash (up to 23 per cent. of the dry weight), it forms a valuable manure, and, although very imperfectly utilised in most places, is regularly harvested as 'varec' or 'vraic' by the farmers of the Channel Isles and their kinsmen of the adjacent mainland. The chemical composition also made it the staple of the industry of kelp-burning (see KELP), once so important as a source of raw material to the soap-boiler and glass-maker. Even more esteemed for these purposes, although unfortunately abounding nearer low-water mark, was the kindred F. nodosus (Knobbed Wrack) with its solitary air-vesicles in the line of the absent midrib. F. serratus (Black Wrack), also very common and easily recognised by its serrated fronds without air-vesicles, was least valued. With these are gathered other less common species, as well as the Laminaria (see SEAWEEDES), exposed by the lowest tides. Besides manure, the only direct chemical utilisation of the Fuci is for the preparation of iodine; and the important proportion of iodine present justifies their ancient medicinal repute in the treatment of scrofulous diseases, the
Quercus marina of ancient pharmacy being F. serratus, and the Æthiops vegetabilis the charred residue of this and its allies. An alcoholic extract is also frequently advertised for the treatment of corpulence.
The genus Fucus and a few closely allied genera (e.g. Fucodium, Himanthalia, Cystoseira, and notably Sargassum, specially described under GULF-WEED), form the family Fucaceæ, which are the highest, and with the allied Laminariaceæ, also the most familiar representatives of the large alliance of brown seaweeds (see the article SEaweeds). The vegetative body is usually a thallus, yet in Sargassum, &c., a distinction of this into stem and leaves is very complete. The branching of Fucus is dichotomous in one plane. Of the inner or medullary cells of the thallus, the outer wall becomes mucilaginous, while the less superficial of the rind cells develop filaments which grow inwards, so surrounding the inner cells within a network of filaments. The bladders are formed by the simple separation of portions of the tissue, the cavities becoming distended by air. A sexual multiplication may be said to be absent, but sexual reproduction is easily observed. A large area at the end of the frond becomes covered with small depressions, which are overgrown until they are spherical flasks with only a minute opening on the surface. The cells lining this flask or conceptacle proceed to divide, and many form barren cellular filaments which, however, instead of turning inwards, as in vegetative growth, grow into the cavity of the flask or even project beyond it as a tuft of hairs. But many are arrested in division while still only two-celled, and the upper of these cells enlarges greatly. In some forms (Cystoseira, Himanthalia) this becomes the ovum, but in others its contents divide into two, four, or in Fucus eight ova; hence it is termed the oogonium. Other filaments again not only lengthen, but branch freely. Their terminal cells become antheridia—i.e. their protoplasm divides into a multitude of spermatozoids. Fertilisation takes place when the ripe fertile fronds are left bare by the tide, the change of specific gravity through evaporation doubtless being of importance in aiding the escape of the sexual products. The outer membrane of the oogonium, like that of a medullary cell, becomes mucilaginous and gives way, and the groups of eight ova, still, however, enclosed within the inner wall, escape from the conceptacle; the antheridia, too, break off and escape to the opening of the conceptacle (perhaps helped by the slight contraction of the volume of this which evaporation must tend to produce). When the tide returns, both ova and spermatozoids break completely free and fertilisation takes place. Cross-fertilisation, always possible even where, as in F. platycarpus, the same conceptacle develops ova and spermatozoa, becomes perfect in the more familiar species, of which the greater prevalence thus becomes more intelligible. The fertilised ovum soon develops a wall, becomes attached, and proceeds to divide and lengthen, soon forming a root-like attachment at one end, a growing point at the other. See SEaweeds; also special articles above mentioned.