THALLOPHYTES 263 



lion. They reproduce by cell divisions at right angles to the 

 long axis; the filaments break into shorter filaments in which 

 the cells continue to divide. 



The genus Nostoc (Fig. 118, c) is found on wet soil and 

 rocks and in pools and streams. It consists of strings of spheri- 

 cal cells resembling strings of beads and the whole is embedded 

 in a thick mass of jelly-like substance. They reproduce in the 

 same manner as the Oscillatoria. Scattered along the filaments 

 will be found cells which are somewhat larger than the others. 

 They are thick-walled, have lost their contents and are known 

 as heterocysts. The intervening chains of cells are known as 

 hormogonia and appear to be anchored by the heterocysts ; they 

 occasionally break loose, wriggle out 

 through the jelly and establish new 

 colonies. 



The genus Rivularia is made up of 

 filamentous, colonial species. The basal 

 cell of each filament is ,a heterocyst, 



and the succeeding; cells decrease in size, c 



' . FIG. 118. Cyanophyceas; 



thus producing; a whip-like colony. (). Giopocaps, (6) Oscuia- 



m f / toria, (c) Nostoc. 



The genus btigonema consists of 



species in which the filaments branch. The branches originate 

 as lateral outgrowths from individual cells. 



The Chlorophycece or true green algae are characterized by 

 their green color, which is due to chlorophyll. They are widely 

 distributed throughout the world in streams, ponds, lakes and 

 seas, but are most abundant in the fresh waters. They range 

 from microscopic, unicellular, to visible, multicellular forms, 

 some of which frequently form very conspicuous masses. The 

 protoplasmic contents of the cells are highly organized and 

 chloroplasts are always present in the active cells. Reproduction 

 is (a) by simple cell division, (&) by free swimming zoospores, 

 each capable of producing a new plant, (c) by gametes or sex 

 cells which unite (usually in pairs) before the formation of new 

 plants and (d~] by the formation of well-defined sex organs 



