284 FRESH-WATER ALGLE OF THE UNITED STATES. 



NOSTOC MACROSPORUM, Menegh. 



Fronds very small, merely a form of the last, rupestre, and 

 often intermingled with it. 



Syn. Hormosiphon macrosporuSj Kg. ; Nostoc pyreniacum, Bipart ; 

 Nostoc ichthyon, Rab. 



NOSTOC SPHAEROIDES, Kg. 



A form not recognized in the United States. 



NOSTOC C03RULEUM, Lyngb. 



Fronds small, globose or subglobose, 1-8 JJL in diameter, 

 fixed or free swimming, blue or greenish blue. 



Diameter of filaments, 4-7 yw. 



On mosses and submerged plants. 



NOSTOC MINUTISSIMUM, Kg. 

 Var. N. PRUNIFORME. 



NOSTOC GREGARIUM, Thur. 

 Yar. N. PRUNIFORME. 



NOSTOC PRUNIFORME, Ag. 



Fronds from the size of a mustard seed to that of a pea, 

 to that of a damson or larger ; olive or dark aeruginous 

 green, when old becoming blackish brown ; periderm cori- 

 aceous. Trichomes loosely interwoven, joints subglobose, 

 compressed, closely connected. Heterocysts globose. 



Diameter of filaments, 4-5 yw. 



The two preceding forms appear to be merely younger 

 conditions of N. pruniforme. Compare note under the 

 generic description, page 279. 



Syn. Ulva pruniformis, Linn.; N. coccymelon, Kg.; Tremella 

 pruniformis, Both. 



Plate CXCVII, figs. 3, 4, young forms (N. minutissimum, 

 Kg. ; N. gregarium, Thur. ) ; fig. 5, well matured forms ; figs. 

 6, 7, moniliform filaments from a frond. 



NOSTOC VERRUCOSUM, Vauch. 



Fronds more or less warty, subglobose or nodulose, brown- 

 ish green, limited at the circumference by a firm periderm. 

 Trichomes slender, somewhat compact, spaced out, joints 

 subglobose, closely connected ; heterocysts spherical. 



Diameter of filaments, 3-4 ^. 



