400 



A TEXTBOOK OF BOTANY 



[Ch. X 



and on wet earth. Its filaments show differentiation be- 

 tween the cells, some of which are empty and of unknown 

 function, called heterocysts, while others acquire thicker 

 walls and become resistant resting spores, in which state 



they are dissemi- 

 nated. These Nostoc 

 filaments are them- 

 selves often aggre- 

 gated together into 

 rounded masses as 

 large as walnuts ; and 

 in Europe these are 

 called " fallen stars," 

 from the glistening 

 appearance of their 

 gelatinous envelopes 

 when freshly swollen 

 after a rain. Rivu- 

 laria is a water form 

 I /TP''*^£j^\ showing a structural 



^•| I ^f">^^iJ\ differentiation of the 



filament as a whole, 

 for a heterocyst oc- 

 curs always at one 

 end, while towards 

 the other the cells 

 taper regularly down- 

 ward in size. These 

 plants also form gela- 

 tinous masses, some- 

 what more specialized in form than in Nostoc, as shown in 

 the figure. Other forms of colonies also occur, some branch- 

 ing, some crudely net-like, and so forth. 



In all Cyanophyceae the cells are of the simplest char- 

 acter, showing no clear differentiation of nucleus or chro- 

 matophores from the cytoplasm. Their characteristic blue- 



Fig. 276. — Cyanophyceae, typical forms. 



Above, left, Gkeocapsa sp., and below, Nostoc 

 paludosum ; center, Oscillatoria principis ; right, 

 Rivularia bullata ; all X 500. Below in center 

 is a colony of Rivularia, natural size. (The first 

 from nature, others after Engler and Prantl.) 



