gradually to a hair-like point from a basal heterocyst; cells short, 

 rectangular to slightly swollen, with constiictions at the cross walls, 

 5-9|ii in diameter, shorter than wide below, becoming longer than 

 wide in the apical region; heterocyst spherical or subspherical, 

 basal, solitary or in pairs, 6-11/* in diameter; sheaths thin, firm, 

 gradually narrowed with the trichomes; gonidia 1-3 in series, 

 adjacent to the heterocyst, lO.S/i in diameter, 14-16/1, long; filament 

 8-(9)-10-(ll)/t wide at the base. 



Common; attached to large filamentous algae such as Cladophora, 

 Rhizoclonium, and Oedogonium. Mich., Wis. 



Calothrix stelhris Bornet & Flahault 1886, p. 365 

 Filaments solitary or clustered and radiately arranged, bent from 

 the basal swollen portion and tapering to a fine hair from a hemi- 

 spherical, basal heterocyst; sheaths thin, firm and close, colorless; 

 cells 6-7/1 in diameter, constricted at the cross walls, i/^-% times as 

 long as broad; heterocysts either solitary or in a series of 2-3; 

 filaments 15-21/i in diameter at the base. 

 Attached to submerged plants. Wis. 



DICHOTHRIX Zanardini 1858, p. 297 

 In this genus the plant is composed of 2-6 tapering trichomes 

 inclosed within a single sheath for at least a part of their length. 

 They are usually solitary but sometimes form macroscopic, cushion- 

 like masses or feathery tufts on submerged wood and stones, or on 

 moist substrates; trichomes with basal heterocysts ( sometimes inter- 

 calary also), and with dichotomous false branching, a branch ex- 

 tending for some distance within the same sheath as the principal 

 trichome, then emerging in its own sheath and usually rebranching 

 successively; cells quadrate, slightly swollen, or shorter or longer 

 than their diameter, either constricted or not at the cross walls; 

 tapering at least in the distal portion of the trichome; basal hetero- 

 cysts connate or hemispherical; sheaths either thin and close or 

 lamellated, sometimes with a bulbous base, tapering with the 

 trichome or widely diverging toward the apex to form a funnel, 

 according to the species. 



Key to the Species 



1. Filaments 10— 14/i, in diameter, with flexuous, 



spreading branches D. Orsiniana 



1. Filaments larger, with straight or curved branches 2 



2. Filaments coarse, 20— 28/* in diameter; branches bulbous 



at the base D. Hosfordii 



2. Filaments narrower, (12)— 15— 18y, in diameter; 



branches not bulbous at the base D. gypsophila 



[ 554 ] 



