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 

 Sphacelana, portion , . ., . ,, , x , 



of plant stowing longi- Chaetophora pisiformis (Chapter 16) among the 

 tudinal division of cells, rv^r^Wr-^ 



Fig. 173- 



and brood bud (pluri- 



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



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 palma/ormis 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 200300 



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 | r a ma (R^ced*; N rth> 



(6-i2 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. 



