=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. [Illustration: Fig. 173. Sphacelaria, portion of plant showing longitudinal division of cells, and brood bud (plurilocular sporangium).] [Illustration: Fig. 174. Laminaria digitata, forma cloustoni, North Sea. (Reduced.)] =363. The Giant Kelps.=—Among the brown algæ are found the largest specimens, some of the laminarias or giant kelps, rivaling in size the largest land plants, and some of them have highly developed tissues. _Postelsia palmæformis_ has a long, stout stem, from the free end of which extend numerous large and long blades, while the stem is attached to the rocks by numerous “root” like outgrowths, the holdfasts. It occurs along the northern Pacific coast, and appears to flourish where it receives the shock of the surf beating on the shore. Several species of Laminaria occur on our north Atlantic coast. In L. digitata, the stem expands at the end into a broad blade, which becomes split into several smaller blades (fig. 174). _Macrocystis pyrifera_ inhabits the ocean in the southern hemisphere, and sometimes is found along the north American coast. It is said to reach a length of 200-300 meters. =364. Fucus, or Rockweed.=—This plant is a more or less branched and flattened thallus or “frond.” One of them, illustrated in fig. 119, measures 15-30 _cm_ (6-12 inches) in length. It is attached to rocks and stones which are more or less exposed at low tide. From the base of the plant are developed several short and more or less branched expansions called “holdfasts,” which, as their name implies, are organs of attachment. Some species (F. vesiculosus) have vesicular swellings in the thallus. The fruiting portions are somewhat thickened as shown in the figure. Within these portions are numerous oval cavities opening by a circular pore, which gives a punctate appearance to these fruiting cushions. Tufts of hairs frequently project through them. [Illustration: Fig. 175. Portion of plant of Fucus showing conceptacles in enlarged ends; and below the vesicles (Fucus vesiculosus).] [Illustration: Fig. 176. Section of conceptacle of Fucus, showing oogonia, and tufts of antheridia.]