Fig. 172.
A, Ectocarpus siliculosus; B, branch with a young and a ripe plurilocular sporangium; E, gametes fusing to form zygospore, (B, after Thuret; E, after Berthold.)
360. Ectocarpus.—The genus Ectocarpus represents well some of the simpler forms of the brown algæ ([fig. 172]). They are slender, filamentous branched algæ growing in tufts, either epiphytic on other marine algæ (often on Fucaceæ), or on stones. The slender threads are only divided crosswise, and thus consist of long series of short cells. The sporangia are usually plurilocular (sometimes unilocular), and usually occur in the place of lateral branches. The zoospores escape from the apex of the sporangium and are biciliate, and they fuse to form zygospores.
361. Sphacelaria.—The species of this genus represent an advance in the development of the thallus. While they are filamentous and branched, division takes place longitudinally as well as crosswise ([fig. 173]).
362. Leathesia difformis represents an interesting type because the plant body is small, globose, later irregular and hollow, and consists of short radiately arranged branches, the surface ones in the form of short, crowded, but free, trichome-like green branches. This trichothallic body recalls the similar form of Chætophora pisiformis ([Chapter 16]) among the Chlorophyceæ.
Fig. 173.
Sphacelaria, portion of plant
showing longitudinal division
of cells, and brood bud
(plurilocular sporangium).
Fig. 174.
Laminaria digitata, forma cloustoni,
North Sea. (Reduced.)