TAXONOMY 



235 



streams, etc. Each filament is unbranched and consists of a row of 

 short cells, one of the terminal cells, called the rhizoid cell, being 

 flongated and serving as an attachment structure. Each cell con- 

 sists of a cell wall of cellulose enclosing cytoplasm, a nucleus and a 

 wide band-shaped green chromatophore, more or less cylindrical in 

 shape. The chromatophore lies next to the cell wall and contains 

 pyrenoids or starch-forming centers. The filament grows in length 

 by the fission of its various component cells. After attaining a 

 certain size it reproduces either asexually or sexually. Asexual 

 reproduction takes place by certain cells becoming altered in their 

 protoplasmic contents, through division, to form rounded or pear- 

 shaped zoospores. Each zoospore contains a red pigment spot and 



FIG. 113. Vaucheria terrestris. anth, antheridium (empty) ; o, oogonia. (Gager.) 



bears four cilia (protoplasmic outgrowths). The zoospores escape 

 into the water by lateral openings in the walls of cells containing 

 them. They swim rapidly about, propelled by their cilia, and ere 

 long attach themselves to various objects and grow into Ulothrix 

 filaments. The sexual method of reproduction is effected through 

 the production of many gametes, in cells of the filament, which re- 

 semble the zoospores in shape but differ from them in being smaller 

 and possessing but two cilia. These escape into the water, and, 

 after swimming about for a short time come together in pairs and 

 fuse with one another. The product of the fusion of each pair of 

 these like gametes is termed a zygospore. The zygospore swims 

 about but finally comes to rest, remaining quiescent for a consider- 

 able length of time. It then enlarges and its protoplasmic content 



