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.
Fig. 175.
Portion of plant of Fucus
showing conceptacles in
enlarged ends; and below
the vesicles
(Fucus vesiculosus).
Fig. 176.
Section of conceptacle
of Fucus, showing oogonia,
and tufts of antheridia.
365. Structure of the conceptacles.—On making sections of the fruiting portions one finds the walls of the cavities covered with outgrowths. Some of these are short branches which bear a large rounded terminal sac, the oogonium, at maturity containing eight egg-cells. More slender and much-branched threads bear narrowly oval antheridia. In these are developed several two-ciliated spermatozoids.
Fig. 177.
Oogonium of Fucus
with ripe eggs.