281. Trichomes gregarious, apices extended; plants parallel in a 



colonial mass -- Amphithrix ( in part) 



281. Trichomes diverging; plants not parallel Calothrix (in part) 



282. Trichomes with false branching (branches sometimes infre- 

 quent ) Plectonema 



282. Trichomes not branched 283 



283. Sheath containing a single trichome 284 



283. Sheath containing more than 1 trichome 287 



284. Sheath firm and definite, not adherent to sheaths of other plants 



and not confluent Lyngbya (in part) 



284. Sheaths mucous and sticky, confluent with sheaths of other plants 

 (sometimes individual sheaths indistinct unless stained) 283 



285. Trichomes lying parallel, forming planktonic fascicles- .^Trichodesmium 



285. Trichomes not forming planktonic fascicles; sheaths mucous 



and difiluent -286 



286. Trichomes much entangled (sometimes parallel), forming mucous 

 plant masses and often thick layers on substrates; sheaths 

 watery (best demonstrated with chlor-zinc-iodide reagent — see 

 Drouet, 1937, p. 604 )..... -- Phormidium 



286. Plant mass forming erect tufts from an expanse on a substrate; 

 sheaths usually firm; plants mostly terrestrial Symploca 



287. Sheaths firm, usually branching at the ends - 288 



287. Sheaths vdde, mucous, soft and sticky, dividing and frequently 

 anastomosing at the ends 289 



288. Sheath wide; trichomes few and separate, not intertvdned "Dasygloea 



288. Sheath close, trichomes many, intertwined Schizothrix 



289. Trichomes loosely arranged, 2 to 4 in a sheath Hydrocoleum 



289. Trichomes densely entangled, many within a sheath Microcoleus 



290. Trichomes decidedly tapering from base to apex 291 



290. Trichomes not tapering from base to apex — _. 295 



291. Filaments inclosed vdthin a copious mucilage, many united to 

 form a globose or hemispherical attached or free-floating thal- 



lus of macroscopic size - 292 



291. Filaments not inclosed in copious mucilage to form thaUi 294 



292. Sheaths containing 2 or more trichomes— .Sacconema 



292. Sheaths containing but 1 trichome -—293 



293. Trichomes bearing cyUndrical gonidia adjacent to the basal 

 heterocyst; enveloping mucilage not hard and granular (al- 

 though firm in 1 species); thaUi free-floating or attached Gloeotrichia 



293. Trichomes without gonidia; colonial mucilage hard, often gran- 

 ular; thalli attached; trichomes radiate or often parallel in ex- 

 panded gelatinous masses Rivularia 



294. Filaments freely branched, the basal portion of the branches 

 inclosed for some distance within the sheath of the primary 

 filament; not epiphytic Dichothrix 



294. Filaments seldom or not at all branched; branches not inclosed 

 in the sheath of the primary filament; plants usually epiphytic; 

 sohtary or gregarious..— -- Calothrix (in part) 



295. Trichomes exhibiting true branching (branches arising as a 



result of cell division in 2 planes ) 296 



295. Trichomes exhibiting false branching, or wdthout branches 299 



296. Trichomes and lateral branches uniseriate; heterocysts lateral, 



mostly on the ends of short branches '*Nostochopsis 



[593] 



