MM. Thuret and Decaisne discovered, by microscopic investigation, that the fuci have a truly sexual fructification, consisting of male and female cells inclosed in these receptacles. In the common Fucus vesiculosus it was found that the male and female cells are either in different individuals, or in different conceptacles on the same individual; whilst in the Fucus platycarpus, both the male and female cells were found to be contained in a globular cavity enclosed in the flattened receptacles which grow at the extremities of the branches. The cavity is lined with jointed hair-like filaments formed of cells, some of which are so long as to project through a pore on the surface of the receptacle in a spreading brush (see fig. 31, where the whole is highly magnified). Towards maturity, the cells of some of these filaments assume an ovoid form; the white viscous, granular matter in their interior acquires an orange hue, and is divided into a multitude of hyaline particles, each having an orange spot and two cilia of unequal lengths, which enable these spermatozoids to swim with great vivacity in the water as soon as they are set free by the rupture of the cell in which they are inclosed. Besides these, dark olive-green female cells, of a large pyriform shape, are fixed to the walls of the same cavity by very short stems; their contents spontaneously divide into eight spore cells, never more; each contains a colourless viscous liquid, which is mixed with protein and yellow-green matter, and is inclosed in a double coat. ‘The coats are united at the base, and when the spores are ready for dispersion, the inner coat bursts through the apex of the outer one, dragging with it a portion of the latter in the form of a little peduncle. The immediate covering of the spores at length bursts, and they are set free.’[49] In Fucus serratus, vesiculatus, and nodosus, swarms of spermatozoids are produced, but M. Thuret has proved by experiment that they never come to anything of themselves, and the unfertilized spores perish. [Illustration: Fig. 31. Vertical section of receptacle of Fucus platycarpus.] When a fertilized spore begins to grow, it assumes a pear shape, and sends out from its narrow end filaments or footstalks containing solid yellow grains at their extremities, where a hook or claw is formed by which it fixes itself to rocks or stones. The spore then divides itself into four equal cells of a brown colour, and by the continued subdivision of these into four, the plant increases in size, and assumes a form corresponding to the genus and species of the spore. Dr. Carpenter mentions that in the Fucaceæ there is also a multiplication by zoospores. These bodies are produced within certain of the cells that form the superficial layer of the frond, and swim about freely for a time in the water after their emission, until they fix themselves and begin to grow; but these are merely gemmæ.