THE GREEN ALG^ (CHLOROPHYCE^E) 207 



From high-tide mark to a little below low-water mark Fucus 

 and Ascophyllum (known as rockweeds) often form dense coat- 

 ings upon rocks. At low tide these rockweeds hang loosely 



over the exposed rocks. Such masses 

 exhibit the dark olive-green color 

 that is characteristic of the group. 

 190. Sargassum and the Sargasso 

 seas. Some of the brown algse may 

 become detached and be carried hun- 

 dreds or even thousands of miles 

 from their original growing places. 

 This is true in the case of Sargassum, 

 some species of which thrive along 

 the shores of tropical oceans. In the 

 North Atlantic Ocean, north of the 

 Canary Islands, is a body of water 

 known as the Sargasso Sea. Its entire 

 area is more or less 

 filled with floating Sar- 

 gassum and other forms 

 of plant and animal life. 

 In other similarly quiet 

 parts of the seas occur 

 large regions filled with 

 floating algse. Sargas- 

 sum, as is also true of 

 some other brown algse, 

 is peculiarly fitted for 

 floating by the presence 

 of " air bladders," which 

 are swollen regions of 

 the leaf -like expansions 

 (Fig. 171). In mid- 

 ocean one may see small floating masses of these plants, which 

 have been carried sometimes hundreds or even thousands of 

 miles from their original homes. 



FIG. 170. The stonewort alga (Chara) 



A, a slightly magnified piece of a plant showing 

 the general appearance ; B, a more highly mag- 

 nified illustration showing the oogonium (o) and 

 the antheridium (a) , hy means of which repro- 

 duction takes place 



