HOW TO KNOW THE FRESH-WATER ALGAE 



417a (406) Branches arising in pairs about midway between 2 hetero- 

 cysts (branching rarely solitary). Fig. 302 SCYTONEMA 



Fig. 302. Scytonema Aichangelii 

 Born. & Flah. 



There are several species of 

 this genus which are very com- 

 mon. Although some branch 

 rather seldom, most of them show 

 numerous false branches which 

 arise singly or in pairs between 

 the heterocysts. They are plants 

 of both aquatic and subaerial 

 habitats. Species are differenti- 

 ated by size, shape of cell, and 

 sheath characters. 



Figure 302 



417b Branches arising singly just below a heterocyst or a series of 

 them; (branching sometimes rare and not definite, requiring a 

 search through a number of plants to determine this character). 

 418 



418a Sheath close and firm, 1 trichome in a sheath 419 



418b Sheath usually wide and soft; at least more than 1 trichome with- 

 in a sheath 420 



419a Branches frequent, arising just below the heterocysts which are 

 always intercalary. Fig. 303 TOLYPOTHRIX 



Fig. 303. Tolypothrix distorta Kuetz. 



This genus is differentiated from Scy- 

 tonema (Fig. 302) principally by the 

 fact that the false branch always arises 

 below a heterocyst or sometimes below 

 a series of them. There are several 

 species that are common, differentiated 

 by size and by sheath characters, some 

 of which are thick and lamellate, others 

 thin and soft. 



Figure 303 



419b Branches rare; heterocysts terminal (rarely intercalary also). See 

 Fig. 294 MICROCHAETE 



174 



