2o6 Minnesota Algae 



uous branches each developing a sheath of its own; sheaths homogeneous 

 and colorless, or lamellose and yellowish or brownish, firm, tubular; trich- 

 omes consisting of a single row of cells, one or more included in a sheath; 

 heterocysts and gonidia variously disposed; reproduction by means of 

 vegetative division, homogones and gonidia. 



I Trichomes single within the sheath 



1 Heterocysts not present; filaments free or forming felt-like masses, 



branched; false branches often in pairs Plectonema 



2 Heterocysts present 



(i) False branches usually arising between two heterocysts, single or 



1, in pairs; sheaths delicate or very thick, parallel, or more or 



less diverging towards the apex Scytonema 



(2) False branches usually arising in the immediate region of the 



heterocysts, single; sheaths somewhat thin, flexible, more or 



less fragile Tolypothrix 



II Trichomes or filaments several within the sheath. 



1 Filaments straight, associated in tufts; sheaths thin; trichomes two 



or more within the sheath; heterocysts basal Desmonema 



2 Filaments several contorted within a common tegument, associated 



in a gelatinous stratum; trichomes single within the sheath 



Diplocolon 



Genus PLECTONEMA Thuret. 

 Essai Class. Nostochinees. 375, 375). 1875. 



Filaments free or forming felt-like masses, branched; false branches 

 solitary or in pairs; sheaths firm, colorless or rarely yellowish orange; 

 trichomes frequently constricted at the joints; apex of trichome straight, 

 very rarely tapering; calyptra none. 



I Plants large, caespitose; trichomes 3 mic. and more in diameter. 



1 Plant mass caespitose, rotund, light green; trichomes S-io ™ic. in 



diameter, here and there constricted at joints 



P. tenue 



2 Plant mass caespitose, indefinite, brownish green; trichomes 11-22 



mic. in diameter P. tomasinianum 



3 Plant mass widely expanded, indefinite, blackish, rarely yellowish 



green; trichomes 28-47 ni'c. in diameter, not constricted at joints 



P. woUei 



II Plant mass very thin, not caespitose; trichomes 1-4 mic. in diameter. 



1 Filaments somewhat flexuous, immersed in dead shells; trichomes 



.9-1.5 mic. in diameter, not constricted at joints 



P. terebrans 



2 Filaments somewhat straight, growing among various gelatinous al- 



gae; trichomes 1-1.5 mic- 'n diameter P. nostocorum 



3 Filaments usually strongly flexuous, densely entangled in a rose- 



colored membrane; trichomes i. 2-1.8 mic. in diameter 



P. roseolum 



