heterocysts and, when mature, gonidia which are either sohtary or 

 in series. 



This genus should be compared with Anabaena in which some 

 species have trichomes similarly inclosed in a thick mucilage but 

 for which there is no definite shape. The colonial mass in Anabaena 

 is soft, spreading, and does not retain its shape when removed from 

 the water. 



Key to the Species 



I. Plants inhabiting the tissues of Ricciocarpus 



and other liverworts N. sphaericum 



(See also N. punctiforme.) 



1. Plants free-living, aquatic, or subaerial 2 



2. Colonies globular masses of microscopic size, 



attached or entangled among other algae N. paludosum 



2. Colonies of macroscopic size 3 



3. Colony a membranous expanse, usually subaerial.— N. muscorum 



(See also N. Linckia and N. commune.) 



3. Colony globular, regular in shape or lumpy and warty; mostly aquatic 4 



4. Trichomes inclosed in a thick, tough, v^Tinkled mucilage, at first 

 globular and soUd, later becoming hollow and irregularly tuber- 

 cular; colonies olive-green or brownish; trichomes densely en- 

 tangled at the periphery of the colonial mass N. verrucosum 



4. Trichomes not in leathery, hollow, tubercular colonies 5 



5. Colonies spherical, planktonic, blue; 



2-5 mm. in diameter —- N. caeruleum 



5. Colonies not definitely planktonic; not blue - 6 



6. Colonies with a firm outer membrane; definite 



in shape; usually globular 7 



6. Colonies without a firm outer membrane or integument; usually 

 soft and amorphous, although retaining their shape when re- 

 moved from the water 11 



7. Plants growing on moist soil 9 



7. Plants growing in water 8 



8. Cells 4-5/11 in diameter; gonidia elliptic or spherical, 5-7^ in 

 diameter; colonies tuberculate or expanded irregularly N. sphaericum 



8. Cells 4-6m in diameter; gonidia spherical, 10^ in diameter; colonies 

 firm, globular pellets,, sometimes several adhering together; 



usually gregarious N. pruniforme 



9. Colonial mass membranaceous, expanded (although at first form- 

 ing globular masses) N. commune 



9. Colonial mass not membranaceous 10 



10. Cells cylindrical; constricted at the end walls, which are truncate; 



gonidia eUipsoid to cylindric, 6-7 /u in diameter, 6-14/i long N. ellipsosporum 



10. Cells barrel-shaped, short; gonidia oval, 



5fi in diameter, 7^ long -N. sphaericum 



II. Trichomes densely entangled, filamentous 



arrangement not clearly discernible N. punctiforme 



11. Trichomes not so densely entangled that the filamentous arrange- 

 ment is not easily discernible 12 



[521] 



