i68 



MORPHOLOGY. 



locular), 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 repre- 

 sent 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 



of plant sh ? wing long?- Chaetophora pisiformis (Chapter 16) among the 

 tudinal division of cells, rVilr>rnnhvrpa 

 and brood bud (pluri- <-nloropnyceae.^ 

 locular sporangium). 363. The Giant Kelps. Among the brown algas 



immanas or giant 



are found the largest specimens, some of the 

 kelps, rivaling in size the largest land plants, 

 and some of them have highly developed tissues. 

 Postelsia palmcejormis 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 pyrijera 

 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 pig 



or less branched and flattened thallus or "frond." Laminaria digitata, 

 One of them, illustrated in fig. 119, measures lea" 4 (Reduced ^ N rth ' 

 I5-30CW (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. 



