274 Minnesota Algae 



Filaments 15-20 mic. in diameter at the base, swollen, often curved; 

 trichomes elongated into a colorless, transparent, pointed hair, with long 

 cells; cells in basal portion of trichome somewhat quadrate, becoming 

 when old three or four times shorter than the diameter; cell contents at 

 first blue-green, later violet or yellowish; heterocysts compressed spherical 

 or concave-convex. 



Pennsylvania. Scattered, or in small clusters, in gelatinous coatings on 

 submerged timbers. (Wolle). 



Genus DICHOTHRIX Zanardini. 



Plant. Maris Rubri Enum. 89. 1858. 



Plant mass caespitose, penicillate or pulvinate; filaments more or less 

 dichotomously branched; trichomes often several (two to six) enclosed 

 T.ithin the original sheath or common tegument; heterocysts sometimes 

 basal, sometimes intercalary, in one species not present. 



I Plants living in fresh water. 



1 Sheaths close, gradually tapering at the apex 



(i) Plants living in hot water; filaments 15-25 mic. in diameter, trich- 

 omes 5-6 mic. in diameter D. montana 



(2) Filaments 10-12 mic. in diameter, flexuous, erect, radiating 



D. orsiniana 



(3) Plant mass encrusted with calcium carbonate; filaments 9-12.5 



mic. in diameter, prostrate, not rigid D. calcarea 



(4) Plants living in fresh or rarely salt water; filaments about 15 mic. 



in diameter; trichomes 5-9 mic. in diameter, constricted at joints 



D. baueriana 

 (si Filaments 12-15 mic. in diameter; trichomes 10-15 niic- in diame- 

 ter, not constricted at joints D. olivacea 



2 Sheaths lamellose, funnel-shaped at apex 



(i) Filaments 9-12 mic. in diameter; trichomes 6 mic. in diameter 



D. compacta 



(2) Filaments 13 mic. in diameter; trichomes 6.5-7.5 niic. in diameter 



D. meneghiniana 



(3) Plant mass usually encrusted with calcium carbonate; filaments 



15-18 mic. in diameter; trichomes 6-8 mic. in diameter 



D. gypsophila 



(4) Filaments 25-28 mic. in diameter; trichomes 10-12 mic. in diame- 



ter, bulbously inflated at the bases of the branches; heterocysts 

 light blue in color D. hosfordii 



II Plants living in salt water. 



1 Filaments 15-22 mic. in diameter; trichomes 7-g mic. in diameter; 



heterocysts basal D. rupicola 



2 Filaments 20-30 mic. in diameter; trichomes 17-22 mic. in diameter; 



heterocysts basal and intercalary D. fucicola 



