Myxophyceae i6l 



(i) Heterocysts and gonidia intercalary 

 A Trichomes free; cells disc-shaped; shorter than their diameter; 

 gonidia seriate, remote from the heterocysts 



Nodularia 

 B Trichomes naked, or with a thin mucous sheath, free or ag- 

 gregated without order to form a flocculent mass; cells equal 

 to or longer than their diameter; gonidia solitary, in pairs 

 or in short series Anabaena 



C Trichomes short, aggregated in parallel bundles to form thin, 

 feathery, plate-like masses Aphanizomenon 



(2) Heterocysts terminal and the* gonidia always contiguous to them 



Cylindrospermum 



II Sheaths thin, membranaceous, persistent; filaments free or agglutinated 

 in a parallel manner 



1 Sheaths not present; trichomes single, endophytic; heterocysts termi- 



nal Richelia 



2 Trichomes single within the sheath; heterocysts intercalary 



Aulosira 



3 Trichomes single within the sheath; heterocysts intercalary and 



terminal Microchaete 



4. Trichomes usually many within the sheath, forming a membranaceous 

 or filamentous mass Hormothamnion 



Genus NOSTOC Vaucher. Hist. Conferves. 203. 1803. 



Plant mass or colony at first globose or oblong, afterwards assuming 

 various forms (globose, foliose, filiform, buUose) in the different species, 

 solid or hollow, mucous, gelatinous or leathery, made up of tangled trich- 

 omes and their more or less dissolved sheaths; filaments flexuous, curved, 

 entangled, coalesced; sheaths sometimes distinct, sometimes invisible; 

 trichomes often torulose; cells depressed spherical, barrel-shaped or cylin- 

 drical; heterocysts intercalary and (in younger plants) terminal; gonidia 

 spherical or oblong, developed centrifugally in series between the hetero- 

 cysts. 



I Plants living in fresh water; forming minute, disc-shaped specks or 



patches on aquatic plants; plant mass growing at the periphery; 

 filaments closely contorted. N. cuticulare 



II Plants living in fresh water, microscopic, granular, aggregated, hav- 



ing the appearance of Aphanocapsa; filaments very closely en- 

 tangled; trichomes scarcely distinct. N. punctiforme 



III Plants living in fresh water, very minute; trichomes 2-3.5 rnic. in 



diameter, distinct. 

 I Plant mass very minute, punctiform; filaments loosely flexuous; 

 trichomes 3-3.5 mic. in diameter; gonidia about 4 mic. in diameter, 

 6-8 mic. in length, oblong N. paludosum 



