HOW TO KNOW THE FRESH-WATER ALGAE 

 406a (402) Trichomes unbranched 407 



406b Trichomes with false branches 417 



407a Individual trichome sheath firm and definite; heterocyst basal 

 (rarely intercalary heterocysts also). Fig. 294. . . MICROCHAETE 



Fig. 294. a, Microchaete diplosi- 

 phon Gom.; b, M. iobusta Setch. 

 & Gard. 



Plants of this genus are mostly 

 epiphytic, with part of the fila- 

 ment lying parallel with the sub- 

 strate and then curving away. The 

 filaments do not taper (or scarcely 

 so) and although there may be in- 

 tercalary heterocysts they usually 

 are basal. There are 4 species 

 described from the United States. 



Figure 294 



407b Individual sheath soft, often indistinct and intermingled with co- 

 lonial mucilage; heterocysts either all terminal or all intercal- 

 ary 408 



408a Heterocysts terminal 409 



408b Heterocysts intercalary 410 



409a Spores adjacent to the heterocysts which usually are at one end of 



the trichome. Fig. 295 CYL1NDROSPERMUM 



Fig. 295. a, Cylindrospermum majus 

 arxzC I OCCoqd Kuetz.; b, C. marchicum Lemm. 



^rxc" CS3^^ ^^b The chief characteristic of this genus 

 cxDoooa30oooooou^ r ^v ) is the location of the heterocysts and 



-BBgr,. ? spores, always terminal and usually 



^■•k>*" only at one end of the trichome which 

 Figure 295 does not taper at the extremeties. The 



plants form gregarious patches or films 

 over submerged vegetation. The spores are formed singly or in series 

 just back of the heterocyst. Frequently a dense 'nest' of spores will 

 be found left where there has been a colony of filaments. Some spe- 

 cies are terrestrial. 



170 



