cylindrical, with either granular or homogeneous contents. In some 

 there are pseudo vacuoles. Of great taxonomic value is the shape 

 and location in the tiichome of the gonidia (akinetes) which are 

 enlarged reproductive cells with thick walls. The outer membrane 

 of the gonidia may be smooth or granular, punctate, etc. 



Key to the Genera 



1. Heterocysts at one or both ends of the trichome — - 2 



1. Heterocysts scattered and intercalary (incidentally at 



the ends of trichomes after fragmentation) 3 



2. Heterocysts at one end of the trichome; 



gonidia adjacent to the heterocysts Cylindrospermum 



2. Heterocysts at both ends of the trichomej gonidia 



intercalary, remote from the heterocysts Anabaenopsis 



3. Trichomes lying parallel 4 



3. Trichomes solitary, or if gregarious, entangled, not parallel —. 6 



4. Trichomes surrounded by copious mucilage, lying in 



gelatinous, hollow tubes or in bullate masses.. Wollea 



4. Trichomes not in hollow tubes; planktonic 5 



5. Trichomes many witliin a free-floating flake of definite shape; tri- 

 chomes slightly tapering at the apices; gonidia solitary Aphanizomenon 



5. Trichomes forming flakes of irregular outline, embedded in mucil- 

 age; trichomes not tapering toward the apices; gonidia in a series 



( rarely sohtary ) Anabaena ( in part ) 



6. Trichomes solitary, planktonic 7 



6. Trichomes aggregated in gelatinous masses 



of definite or indefinite shape — - 8 



7. Cells disc-shaped, much shorter than wide; 



heterocysts compressed Nodularia 



7. Cells not disc-shaped; globose to cylindrical; 



heterocysts not compressed Anabaena (in part) 



8. Plant mass of definite shape, the periphery of the colonial 



mucilage forming a tegument -. Nostoc 



8. Plant mass not definitely shaped, not boimded by a firm tegument 9 



9. Trichomes forming small packets, entangled, sometimes subparallel, 

 inclosed in definite rather firm sheaths Aulosira 



9. Trichomes not forming packets, individual 



sheaths lacking, or not definite .— Anabaena (in part) 



ANABAENA Bory 1822, p. 307 



Filamentous, mostiy gregarious, much entangled and inclosed in 

 amorphous mucilage, solitary and planktonic in a few species, some- 

 times attached and forming films and gelatinous expansions on moist 

 substrates; tiichomes straight, flexuous, or spirally coiled, either with 

 or without a sheath; cells torulose, barrel-shaped, or (in a few 

 species) cylindrical; heterocysts usually numerous and scattered in 

 the trichome, spherical, ovate, or cylindrical; gonidia round, ovate, 



[510] 



