CYANOPHYCEA 253 
classifying the genera and species, the thicker kind 
being frequently lamellated. The sheaths are at first 
colourless and transparent, and may remain so, but 
more often they become coloured as described. The 
Fic. 86.—a, Rivularia hospita; b and ¢, hormogonia of Calothrix pulvinata 
highly magnified. (After Bornet and Thuret.) 
heterocysts are special cells situated at the base of 
the trichome or intercalated in its course. They are 
bright green or light yellow in colour, have very little 
solid contents, and are commonly much larger than 
