no 



FRESH-WATER BIOLOGY 



43 (28) Filaments simple, unbranched; with heterocysts; living singly or in 



gelatinous masses, often of definite form. Sheaths very 

 delicate, mostly confluent. Cells generally torulose, in a 

 single row Family, NOSTOCEAE . . 44 



44 (47) Filaments enclosed within a gelatinous mass of definite form. 45 



45 (46) Forming delicate, hollow, cylindrical colonies. 



Wollea Bornet and Flahault. 



Delicate colonies; filaments straight or 

 slightly bent, arranged in tolerably parallel 

 rows, with a common gelatinous envelope; 

 heterocysts intercalary; spores in chains, 

 bordering on one or both sides of the 

 heterocysts. W. saccata Bor. and Flah. 

 occurs in stagnant water. 



FIG. 52. Wollea saccata Bornet and Flahault. 

 a, X 250; b, natural size. (After Wolle.) 



46 (45) Colonies spherical, or of varied form; with the enclosed filament 

 irregularly interwoven and contorted. . . Nostoc Vaucher. 



Forming leathery or slimy gelatinous masses, at first spher- 

 ical or oblong, later of varied form, solid or hollow, and 

 attached or unattached; filaments contorted and curved in 

 all directions; the gelatinous sheath sometimes sharply 

 delimited, more often fused with the enveloping jelly. 

 Cells globular, barrel-shaped, or cylindrical; heterocysts 

 intercalary, or (when young) sometimes terminal; spores 

 globular or oblong, formed in rows in varying number be- 

 tween the heterocysts. Forming free-floating or attached 

 masses, on damp ground, wet rocks, etc. 



FIG. 53. Nostoc commune Vaucher. a, natural size; b, X 465. 

 (Original.) 



47 (44) Filaments more or less straight, free-floating or forming a thin 



mucous stratum 48 



48 (52) Heterocysts and spores intercalary. 49 



49 (50, 51) Filaments naked, or with a thin sheath; single, or aggregated into 



formless, flocculent masses; cells equal to or longer than 

 their diameter Anabaena Bory. 



Filaments straight or drcinate, naked or enclosed 

 in a thin sheath, free floating as single filaments or 

 united to form a thin, slimy stratum; vegetative 

 cells as long or somewhat longer than thick; heter- 

 ocysts numerous and intercalary; spores variously 

 disposed, borne singly or rarely in short chains. 

 A. flos-aquae Breb. and A. circinalis Rabenh. are 

 frequently abundant in fresh-water plankton. 



FIG. 54. Anabaena flos-aquae Brebisson. 

 . (Original.) 



X 465. 



