58 Minnesota Algae 



(5) Filaments branched; sheaths solid, closed at the apex, of various 



colors; trichomes densely aggregated within the sheath 



Schizothrix 



(6) Sheaths wide, transparent or yellowish brown; trichomes very 



few within the sheath, very loosely aggregated 



Dasygloea 



(7) Sheaths mucous, not lamellose, always transparent; trichomes 



many within the sheath Microcoleus 



3 Colonies somewhat spherical, elliptical or spindle-shaped; filaments 

 solitary or aggregated in colonies; sheaths thick, gelatinous 

 (i) Sheaths very thick; trichomes usually single or in scattered frag- 

 ments Catagnymene 

 (2) Colonies somewhat spherical; sheaths thick; trichomes curved, 

 radiating Haliarachne 



Genus OSCILLATORIA Vaucher. Hist. Conferves. 165. 1803. 



Trichomes cylindrical, free, motile, without a sheath or rarely enclosed 

 in a very thin, fragile, mucous sheath, often constricted at the joints; apex 

 of trichome straight, curved, or more or less regularly spiralled, often 

 tapering; outer wall of apical cell often thickened, forming a calyptra. 



I Plants living in fresh water, floating; apex of trichome constantly 

 straight, gradually tapering, obtuse, finally capitate; cells somewhat quad- 

 rate or shorter than the diameter, never very short. 



1 Plant mass purple; trichomes 2.2-5 inic. in diameter; cells some- 



what quadrate or longer than the diameter O. prolifica 



2 Plant mass light blue-green; trichomes 4-6 mic. in diameter; cells 



somewhat quadrate or twice as short as the diameter 



O. agardhii 



II Plants living in fresh water, sometimes in hot water; trichomes large 

 or very large; apex of trichome straight, curved or spiral, not at all or 

 briefly tapering, obtuse; cells very short. 



1 Transverse walls never granulated 



(i) Trichomes 16-60 mic. in diameter; apex of trichome slightly taper- 

 ing, somewhat capitate, hooked O. princeps 



(2) Trichomes 12-15 mic. in diameter; apex of trichome tapering, 

 capitate, hooked or loosely terebriform O. proboscidea 



2 Transverse walls frequently granulated 

 (i) Apex of trichome straight 



A Trichomes 10-20 mic. in diameter, constricted at joints; apex of 

 trichome very briefly tapering, somewhat capitate 



O. sancta 

 B Trichomes 11-20 mic. in diameter, not constricted at joints; apex 

 of trichome neither tapering nor capitate 



O. limosa 



