THE FRESH-WATER ALGAE 



147 



92 (93) Colonies macroscopic or microscopic, expanded or intestiform, cells 

 arranged in fours Tetraspora Link. 



Reproduction by division in two directions; zoospores 

 may originate directly from the vegetative cells, and by divi- 

 sion give rise to a new colony; isogametes with two cilia may 

 be formed, also resting spores with heavy brown walls. 



Fig. 153. Tetraspora explanataK\i\.z\Qg. X 250. (After Nageli.) 



93 (92) Colonies pear-shaped, attached, cells irregularly placed near the 

 surface Apiocystis Nageli. 



Chromatophore single, parietal with a pyrenoid. Division in 

 three directions. A spherical zoospore with two cilia may originate 

 from each cell and escape from the gelatinous vesicle. 



Fig. 154. Apiocystis brauniana Nageli. X 78. (After Nageli.) 



94 (91) . Cells spindle-shaped, clustered on the ends of gelatinous stalks. 



Chlorangium Stein. 



Chromatophore one or two longitudinal bands; the cells may 

 detach themselves and become zoospores with two cilia and a 

 pigment spot. Large numbers of motile individuals may be 

 formed in each cell, though copulation is not known. 



Fig. 155. Chlorangium stentorum Stein, a. X about 200. (After 

 Cieokowski.) b. (After Stein.) 



95 (90, 131, 175) Cells with a thick, often indistinct gelatinous covering, 



uniting several together into greater or smaller free swim- 

 ming, rarely attached colonies. Reproduction by fission or 

 I internal division ; in a few instances by zoospores and 

 isogametes Family Palmellaceae . . 96 



96 (102, 107) Cells embedded in more or less cylindrical and definite gelat- 



inous tubes, strands, or stalks which are broader than the 

 cells 97 



97 (100, 10 1) Cells scattered throughout a gelatinous tulje or strand. . . 98 



