MORPHOLOGY. 



locular), and usually occur in the place of lateral branches. The zoospores 

 escape from the apex of the sporangium and are biciliatc, 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 

 tvpc because the jjjant 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 

 Sphacelaria. portion ^, , . ., . ,^, ^ ,, , 



•of plant showing InnKi- Ch:ctoy)hora pisitormis (Chapter lO) among the 

 tudinal division .jf cells, rhlnroohvce-r 

 and brood bud (pluri- '^ ""^™l-'"> '^'^'"^• 



locular sporangium). 363. The Giant Kelps. — Among the brown algEe 



are found the largest specimens, some of the laminarias or giant 



kelps, rivaling in size the largest land plants, 



and some of them have highlv developed tissues. 



Posleisia paluurjormis 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 ex]>ands at the end into a 



broad blade, which becomes split into se\-era1 



smaller blades (fig. 174). Macrocystis pyrijera 



inhabits the ocean in the southern hemisy>hcre, and 



sometimes is found along the north .American 



coast. It is said to reach a length of 200-300 



meters. 



364. Fucus, or Eockweed. — This plant is a more 

 or less branched anrl flattened thallus or " frond." 

 One of tlicm, illustrated in lig, itq, measures 

 i5-3of«i (ft-i2 indies) in length. It is attached to 

 rocks and stones whicli are more or less exposed al I1 

 of the [il.int aredevelopecl sevcnd shurl and more or K 

 called "holdfasis," wliiili, as their nai 



Fig. 1 74. 

 Laniinaria digitata, 



fi irnia cloustoni, Xortli 

 Sea. (Reduced. I 



From the base 

 bed expansions 



d' 



rg.ins ot ,it(,ii hnient. 



Some species (!''. vesiculosus^ have vesicular swelhiigs 111 the thallus. 



