6o Minnesota Algae 



7 Trichomes 3.5-4 mic. in diameter, not constricted at joints; trans- 



verse walls pellucid, not granulated O. chlorina 



8 Trichomes .6 mic. in diameter, flexible, elongate, tangled, not con- 



stricted at joints O. angustissima 



V Plants living in fresh water, hot water, rarely in salt water; trichomes 

 tapering, more or less pointed, hooked or flexuous, not entirely spiralled 

 (except O. c h a 1 y b e a) ; cells longer or shorter than the diameter, never 

 very short. 



1 Apical cell capitate 



(1) Trichomes 2-3 mic. in diameter; cells longer than their diameter 



O. splendida 



(2) Trichomes 2.5-5 mic. in diameter; cells somewhat quadrate 



O. amoena 



2 Apical cell not capitate 



(i) Plants living in salt water 

 A Trichomes 4.7-6.5 mic. in diameter, flexible, undulating; apex of 

 trichome very gradually tapering, \ery flexuous 



O. subuliformis 

 B Trichomes 4 mic. in diameter, somewhat flexuous, sometimes 

 coiled in a regular circle, very much constricted at joints; apex 

 of trichome tapering, slightly curved, obtuse 



O. salinarum 

 C Trichomes 3-5 mic. in diameter, fragile, straight; apex of trichome 

 briefly tapering, hooked or undulating O. laete-virens 

 (2) Plants living in fresh water, often in hot water, rarely in brackish 

 water 

 A Trichomes 3-5 mic. in diameter; apex of trichome briefly tapering, 

 very sharply pointed, hooked; cells usually longer than their 

 diameter O. acuminata 



B Trichomes 3-4 mic. in diameter; apex of t.richome briefly tapering, 

 very sharply pointed, hooked; cells usually shorter than their 

 diameter O. animalis 



C Trichomes 4-4.5 mic. in diameter, straight, entangled; transverse 

 walls granulated; cell contents violet or sky-blue 



O. violacea 

 D Trichomes 4-6.5 mic. in diameter, here and there interrupted by 

 inflated refringent cells; apex of trichome briefly tapering, 

 hooked or flexuous; cells three times shorter than their diam- 

 eter O. brevis 

 E Trichomes 4-7 mic. in diameter; apex of trichome obtuse- straight, 



rarely slightly curved O. cruenta 



F Trichomes 4-6 mic. in diameter, slightly constricted at joints; apex 

 of trichome briefly and somewhat obtusely tapering, hooked; 

 cells quadrate or one-half as long as wide 



O. formosa 



