112 FRESH-WATER BIOLOGY 



57 (58) Branches generally arising in pairs Scytonema Agardh. 



Filaments consisting of vegetative cells and 

 heterocysts; borne singly in a sheath; sheath 

 tough, lamellose, frequently yellowish or brownish 

 in color; false branches borne generally in pairs 

 between the heterocysts. Forming felt-like masses 

 on wet rocks, etc. 



Fig. 59. Scytonema mirabiU Thuret. X 465- (Original.) 



58 (57) Branches arising as a rule singly Tolypothrix Kiitzing. 



Filaments resembling closely 

 those of Scytonema, but false 

 branches arising singly as a rule 

 instead of in pairs, as outgrowths 

 in the region of the heterocysts; 

 the latter 1-5 in a row; spores (in 

 a few species) elliptical, borne 

 singly or in rows. Occurring 

 among various aquatic plants in 

 ponds and lakes. 



Fig. 60. Tolypothrix lanata Wart- 

 mann. X 465- (Original.) 



59 (55' 56) With basal heterocysts. Two to several filaments enclosed in 

 each sheath Desmonema Berkeley and Thwaites. 



Filaments sometimes slightly branched; 

 heterocysts always basal. On stones, in brooks, 

 and waterfalls. 



Fig. 61. 



Desmonema wrangelii Borzi. X 200. 

 (After Borzi.) 



60 (54) 



Filaments usually stout, bearing true branches; cells rounded, dis- 

 posed generally in more than one row; heterocysts present. 

 Family Stigonemaceae . . 61 



61(62,63) Sheaths thick; firm S tigonema \g^rdh. 



Filaments free-floating or aggregated 

 on the substratum to form felt-like 

 masses; filaments composed partly of two 

 to several cell-rows, sometimes of a single 

 row, enclosed in a thick, lamellose, yellow- 

 ish or brownish sheath, which is often 

 of irregular thickness. Hormogonia 

 formed at the ends of the vegetative 

 branches or in special short branches. 

 Heterocysts commonly lateral, or less often 

 intercalary. Vegetative cells rounded, 

 frequently showing protoplasmic continuity. Growing generally on damp or 



Fig. 62 



nema 



a, Stigonema ocellatum Thuret; b, Stigo- 

 minutum Hassall. X 440. (After West.) 



wet rocks or moss; sometimes free-floating. 



