12. Plants terrestrial; individual sheaths apparent; cells rather large, 



5-8^ in diameter; gonidia 6-7// in diameter, 10-15^ long.iV. microscopicum 



12. Plants aquatic; individual sheaths not apparent; vegetative cells 

 averaging smaller than above 13 



13. Colonies when old forming irregularly expanded, gelatinous, free- 

 floating masses; cells cylindrical, 3.5-4/i in diameter; hetero- 



cysts ovate or oblong N. carneum 



13. Cells not cylindrical - 14 



14. Trichomas densely and compactly entangled; 



gonidia 6-7 fi in diameter, 7-8/ii long.... N. Linckia 



14. Trichomes not densely entangled -. lo 



15. Cells of various shapes within the same trichome, globose to elon- 

 gate and barrel-shaped; heterocysts globose or oblong, slightly 



larger than the vegetative cells N. spongiaeforme 



15. Cells all spherical or compressed-globose; heterocysts globose, 



larger, up to twice the size of the vegetative cells..... N. comminutum 



Nostoc caeruleum Lyngbye 1819, p. 201 

 PI. 119, Figs. 10, 11 



Colony spherical, planktonic, 5-10 mm. in diameter, appearing as 

 bright, sky-blue globules dispersed through quiet water; colonial 

 sheath firm and tough; trichomes densely entangled; cells sub- 

 spherical or barrel-shaped, 5-7/i, in diameter; heterocysts frequent, 

 subglobose or spherical, S-lO/x in diameter; gonidia apparently 

 never described. 



Rather uncommon in plankton of lakes with eutrophic character- 

 istics. Mich., Wis. 



Nostoc carneum C. A. Agardh 1824, p. 22 

 PI. 119, Fig. 9 (showing an atypical form) 



Colony olive-green or blue-green, globular, becoming irregularly 

 lobed in age and finally expanded soft to firm; trichomes twisted 

 and entangled but not crowded. Cells subglobose to short-cyhndric, 

 sometimes twice as long as wide, 3-4/j, in diameter, 6-8;u. long; con- 

 stricted at the cross walls. Heterocysts ovate, 6-7 fi in diameter, 8/a 

 long. Gonidia ovate to oblong, 6-7/* in diameter, 8-10/x long. 



On the bottom, and floating at the surface of quiet water; forming 

 soft gelatinous expansions when old but retaining a firm tegument. 

 Wis. 



Nostoc comminutum Kuetzing 1849-1869, Tab. Phyc, 2, p. 3 



PI. 119, Fig. 12 

 Colony an irregularly lobed and membranaceous mass, golden- 

 brown in color, the investing mucilage soft but retaining a definite 

 shape when removed from the water. Trichomes twisted or some- 

 what entangled. Cells globose or compressed-globose; 3.7-4/x in 



[522] 



