296. Trichomes and lateral branches multiseriate with wide muci- 

 laginous sheaths, or uniseriate with close, firm sheaths; hetero- 



cysts not on the ends of short lateral branches 297 



297. Filaments uniseriate; sheaths firm, close; heterocysts oblong 



to cylindrical H apalosiphon 



( cf. Stigonema ocellatum ) 



297. Filaments multiseriate' (rarely imiseriate in part), with wide 

 mucilaginous sheaths; heterocysts compressed, spheroidal, usually 



lateral on a vegetative cell 298 



298. Branches as wide or nearly as wide as the main filament; hormo- 



gonia not formed in the apices of the branches Stigonema 



298. Branches narrow, erect, with hormogonia formed in their apical 



region _ _ Fischerella 



299. Trichomes without branches 300 



299. Trichomes with false branches 310 



300. Trichomes with a basal heterocyst ( sometimes intercalary hetero- 

 cysts also); sheath firm and definite Microchaete 



300. Trichomes vdthout a firm sheath; heterocysts either always ter- 

 minal or always intercalary 301 



301. Heterocysts always terminal ._ _ .302 



301 . Heterocysts always intercalary—. _-303 



302. Heterocysts at one end, rarely at both ends, of the trichome; 

 gonidia formed adjacent to the heterocysts Cylindrospermum 



302. Heterocysts at both ends of the trichome; gonidia remote from 



the heterocysts Anabaenopsis 



303. Plant formed of many trichomes, often lying parallel to one 

 another 304 



303. Trichomes solitary or, if gregarious, entangled, not parallel - —.306 



304. Trichomes surrounded by copious mucilage, forming hollow, 

 attached, tubular or bullate masses Wollea 



304. Trichomes not forming attached, tubular or bullate masses, but 

 occurring in free-floating bundles or fascicles .305 



305. Trichomes very slightly tapering toward both apices; many 

 within free-floating macroscopic flakes of definite shape; gonidia 

 always solitary Aphanizomenon 



305. Trichomes not tapering toward the apices; forming flakes of 

 irregular outline; gonidia usually in a series (solitary in some 

 species ) Anabaena ( in part ) 



306. Trichomes solitary, planktonic among other algae - 307 



306. Trichomes aggregated in a gelatinous mass of definite or in- 

 definite shape 308 



307. Cells disc-shaped, much shorter than wide; heterocysts com- 

 pressed - Nodidaria 



307. Cells not disc-shaped but globose to cylindrical; heterocysts 



not compressed Anabaena (in part) 



308. Plant mass of definite shape, the periphery of the colonial 

 mucilage forming a tegument Nostoc 



308. Plant mass not of definite shape; colonial mucilage soft, not 

 bounded by a firm tegument 309 



309. Trichomes forming small packets, entangled, sometimes sub- 

 parallel, inclosed in a gelatinous sheath Aulosira 



309. Trichomes not forming packets, individual sheaths lacking, or 



not definite -__. ___ Anabaena ( in part ) 



[594] 



