THE GKEAT GROUPS OF ALG.E 



251 



feet long, whose stalk develops root-like holdfasts (Fig. 218). 

 The largest body is developed by an Antarctic Laminaria 

 form, which rises to the surface from a sloping bottom with 

 a floating thallus six hundred to nine hundred feet long. 

 Other forms rise from the sea bottom like trees, with 

 thick trunks, numerous branches, and leaf-like appendages. 



The common Fucus, 

 or " rock weed," is rib- 

 bon-form and constantly 

 branches by forking at 

 the tip (Fig. 219). This 

 method of branching is 

 called dichotomous, as dis- 

 tinct from that in which 

 branches are put out 

 from the sides of the axis 

 (monopodicd). The swol- 

 len air-bladders distrib- 

 uted throughout the body 

 are very conspicuous. 



The most differenti- 

 ated thallus is that of 

 Sargassum (Fig. 220), or 

 " gulf weed," in which 

 there are slender branch- 

 ing stem-like axes bearing 

 lateral members of various 

 kinds, some of them like 

 ordinary foliage leaves ; 

 others are floats or air- 

 bladders, which sometimes 



resemble clusters of berries; and other branches bear the 

 sex organs. All of these structures are but different regions 

 of a branching thallus. Sargassum forms are often torn 

 from their anchorage by the waves and carried away from 

 the coast by currents, collecting in the great sea eddies 



Fig. 219. Fragment of a common brown 

 alga {Fucus), showing the body with 

 dichotomous branching and bladder-like 

 air-bladders. — After Luerssen. 



