THALLOPHYTES 263 



tion. They reproduce by cell divisions at rigkt angles to the 

 long axis; the filaments break into^ shorter filaments in which 

 the cells continue to divide. 



The genus Nostoo (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 sifbstance. 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 knovm as 

 hormogonia and appear to be anchored by the hetei'ocysts ; thcv 

 occasionally brealc loose, wriggle out 

 through the jelly and establish new 

 colonies. 



The genus Bivularia is made up of ^ 

 filamentous, colonial species. The basal <} 



cell of each filamemt is .a heiterocyst, (§>i^ 



and the succeeding cells decrease in size, ,,^,,0 „ u 



o . ' Fig. 118. — Cyanophyoese; 



thus producing a whip-like colony. (<^\ GiceooapsBe, m Osciiia- 



The genus Stigon&ma consists of 

 species in which the filaments branch. The branches originate 

 as lateral outgi-owths from individual cells. 



The Chloropliycece or true green algse 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 akmdant 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. Eeproduction 

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

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

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

 plants and {d) by the formation of well-defined sex organs 



