HOW TO KNOW THE FRESH-WATER ALGAE 



383a Trichomes aggregated, tapering from a base which is incorporated 

 in a prostrate cushion of cells. Fig. 274 AMPHITHR1X 



Fig. 274. Amphithiix jan- 

 thina (Mont.) Bor. & Flah. 

 (Redrawn from Bornet & 

 Flahault). 



Figure 274 



These tapering filaments 

 are arranged in somewhat 

 parallel fashion to form clusters (but without conspicuous mucilage) at- 

 tached to substrates. There is a weakly developed, prostrate expan- 

 sion of the thallus from which the filaments arise. The lack of hetero- 

 cysts (see Anabaena, Fig. 271) makes this a somewhat anomalous mem- 

 ber of the Rivulariaceae which characteristically has this type of cell 

 at the base of the filament. 



383b Without a prostrate cushion of cells at the base of the trichome. 

 384 



384a Trichomes gregarious, parallel in a colonial mass. See Fig. 274. 

 AMPHITHRIX 



384b Trichomes solitary or loosely clustered, not parallel. Fig. 275 . . . 

 CALOTHR1X 



Fig. 275. a, Calothrix epiphytica West 

 & West; b, C. atricha Fremy; c, 

 C. Braunii Bor. & Flah. 



These tapering filaments are soli- 

 tary or loosely clustered, although 

 sometimes gregarious, forming ex- 

 tensive masses. There is a basal 

 heterocyst and in some species, 

 akinetes. Species are differentiated 

 on the basis of size, presence or 

 absence of akinetes, and by the 

 degree of tapering of the trichome, 

 some being very abrupt. 



160 



