Myxophyceae 91 



Plate IV. fig. so, 51. 



Trichomes 2 mic. in diameter, when twisted 75-200 mic. long, having 

 form of slender, flat, strap-like, continuous bands (when untwisted form- 

 ing a complete ring), normally flattened and twisted, with one to four or 

 more turns. 



Minnesota. Frequent in pool near Minneapolis. (Wolle). 



Genus PHORMIDIUM Kuetzing. Phyc. Gen. 190. 1843. 



Filaments simple, forming a woolly or felt-like layer or rarely float- 

 ing, attached at the base with free ends torn and ragged; sheaths thin, 

 transparent, mucous, agglutinated, partly or entirely diffluent; trichomes 

 cylindrical, in some species constricted at joints, never distinctly spiral; 

 apex of trichome often tapering, straight or curved, capitate or not capi- 

 tate; outer membrane of apical cell thickened into a calyptra in many 

 species. 



I Trichomes especially constricted at joints, even moniliform; apex of 



trichome neither curved nor capitate. 



1 Trichomes scarcely 4 mic. in diameter 



(i) Plants living in hot or in brackish water; trichomes 1.2-2.3 niic. 

 in diameter; cells somewhat quadrate P. fragile 



(2) Plants living in salt water; plant mass rose-colored; trichomes 



1.7-2 mic. in diameter; cells longer than the diameter 



P. persicinum 



(3) Plants terrestrial, nestling in pits in rocks; trichomes 1.5 in diame- 



ter; cells somewhat quadrate P. foveolarum 



2 Trichomes 6-8.5 mic. in diameter P. tinctorium 



II Trichomes rarely or scarcely constricted at joints; apex of trichome 



straight or curved, capitate in many species 

 I Trichomes scarcely 3 mic. in diameter 



(i) Plant mass purplish violet, reddish brown or scarlet 

 A Filaments somewhat straight; trichomes slightly constricted at 



joints; transverse walls not granulated P. luridum 

 B Filaments somewhat straight; trichomes fragile, frequently in- 

 terrupted, not constricted at joints; transverse walls rarely 

 visible P- rubnmi 



C Filaments very much twisted; trichomes not constricted at joints; 

 transverse walls marked by four protoplasmic granules 



P. purpurascens 

 D Filaments curved, entangled or arranged parallel with each other, 

 trichomes not constricted at joints; apex of trichome straight, 

 neither tapering nor capitate P. crosbyanum 



(2) Plant mass blue-green or olive. 

 A Plants living in hot water; plant mass expanded, lamellose, com- 

 posed of many superposed papery layers; trichomes .6-.8 mic. 



