globose, depressed-globose, or ovate, often with intercellular con- 

 nections; heterocysts either intercalary or lateral; branches often 

 developing hormogonia in the distal portion. 



Stigonenia is commonly terrestrial, but there are a few^ aquatic 

 and amphibious species. This genus should be compared with 

 Hapalosiphon, although the two are ordinarily easily separable on 

 the basis of the number of series of cells in the filament. Stigonema 

 spp. are sometimes uniseriate and Hapalosiphon spp. tend to become 

 biseriate, but not throughout the entire plant. Occasionally Hapalo- 

 siphon branches will rebranch as they do in Stigonema. The contents 

 of the Stigonema cell are usually more nearly homogeneous than in 

 Hapalosiphon. The laterally developed heterocysts are typical of 

 Stigonema. One comes to differentiate the two genera by their re- 

 spective combinations of characteristics. 



From Fischerella, Stigonema is differentiated by the slender erect 

 branches of the former which terminate in homogonia. Some authors 

 include species of Fischerella in a subgenus of Stigonema. 



Key to the Species 



1. Filaments uniseriate; individual cell sheaths conspicuous; inter- 

 cellular connections evident- — - - S. ocellatum 



1. Filaments multiseriate - ' 



2. Filamentous habit scarcely discernible; lateral clusters of 

 cells inclosed by lamellate sheaths and forming short, 



buUate branches S. mesentericum 



2. Filamentous arrangement of cells apparent; more elongate branches 3 



3. Filaments 40-70ai wide; plants mostly aquatic; branches 



short and broad _.__ S. mamillosum 



3. Filaments 27-37^ wide; plants mostly terrestrial; 



branches long and curved - S. turfaceum 



Stigonema mamillosum (Lyngb.) C. A. Agardh 1824, p. 42 



PI. 130, Figs. 1-3 

 Filaments much-branched, forming attached, dark green wooly 

 tufts, or scattered in small entanglements among other floating 

 algae, composed of several series of globose or ellipsoid cells; 

 branches short, irregularly developing, narrowed at both the base 

 and in the distal region; heterocysts numerous, compressed-ovate, 

 cut off laterally from the vegetative cells; sheath wide, lamellate, 

 becoming yellowish or olive-brown in age; cells 14-17/^ in diameter; 

 filaments 40-70ia wide; hormogonia developing in special short 



branches. 



Our collections have all been made from soft water habitats. 



Common in several lakes, usually attached to submerged wood; 

 sometimes forming small clots among floating algae. Mich., Wis. 



[547] 



