non-lamellated sheaths, scarcely tapering toward the apices; hetero- 

 cysts the same size as the vegetative cells. 



Forming prostrate patches on submerged logs and stones. Wis. 



PLECTONEMA Thuret 1875, p. 375 

 A falsely branched, sheathed trichome, occurring either free and 

 solitary or in wooly mats, or embedded and attached in the mucilage 

 of other algae; cells short, disc-like or barrel-shaped, constricted at 

 the cross walls or not, forming long trichomes (usually) which 

 taper but very little, if at all, toward the apices; branches arising 

 singly or in pairs, often becoming free or extending loosely away 

 from the main filament; sheaths thin and close, or thick in some 

 species, homogeneous or lamellate, usually colorless, sometimes 

 yellowing or becoming brown with age; heterocysts wanting. 



Key to the Species 



1. Filaments coarse, seldom branched, 28-50/n in diameter P. Wollei 



1. Filaments frequently branched, less than 28/u, in diameter - 2 



2. Plants forming tufts or fascicles - P- tenue 



2. Plants solitary or forming mucous layers or expansions 3 



3. Plants inhabiting the mucilage of other 



algae, especially Nostoc --- P- nostocorum 



3. Plants free-living; solitary, gregarious, or forming thin wefts .— - 4 



4. Filaments 3.7-4;u in diameter; purphsh P. purpureum 



4. Filaments less than 3.7/it in diameter 5 



5. Trichomes reddish P- carneum 



5. Trichomes not red P- notatum 



Plectonema carneum (Kuetz.) Lemmermann ex Geitler 1925a, p. 249 

 [P. roseolum (P. Richt.) Gomont] 



Filaments matted and entangled, forming thin, reddish, gelatinous 

 layers; false branches frequent, solitary or in pairs; sheaths thick, 

 uneven and colorless, not colored by chlor-zinc-iodide reagent; cells 

 1.2-1.8iu in diameter, 1.7-5/x long, not constricted at the cross walls, 

 which are marked by 2 granules; apical cell broadly rounded. 



Tychoplankter; on moist aerial substrates. Wis. 



Plectonema nostocorum Bornet in Bornet & Thuret 1880, p. 137 



PI. 126, Figs. 4, 5 



A slender, frequently branched filament in the mucilage of Nostoc 

 and other blue-green algae, or forming small gelatinous masses; 

 cells quadrate or slightly longer than wide, frequently separated 

 from each other, 0.7-1.5^ in diameter, 2-3/t long; sheaths thin, color- 

 less; branches usually sohtary; filaments 2-4/i, in diameter. 



Common in many lakes and swamps, almost invariably found in 



[539] 



