Fucus is the generic name of various species of brown seaweed which form the main vegetation of rocky shores between tide marks. Most common of all is F. vesiculosus (Rockweed, Bladderweed, sea wave, etc.), easily distinguished by its entire edges and paired air vesicles. On account of the large proportion of ash it forms a valuable manure. Besides manure, the only direct chemical utilization of of the Fuci is for the preparation of Iodine. The vegetative body of Fucus is usually a thallus; the branching 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 inward, thus surrounding the inner cells within a network of filaments. The ladders are formed by the simple separation of portions of the tissue, the cavities becoming distended by air. Fertilization takes place when the ripe, fertile fronds are left bare by the tide. The fertilized ovum soon develops a wall, becomes attached, and proceeds to divide and lengthen, forming a root-like attachment at one end, a growing point at the other. This illustration shows a Fucus, 1) a cross-section of a fertile frond, 2) a female conceptacle, 3) a male conceptacle, and 4) Zoospores and Antheridia.