400 



A TEXTBOOK OF BOTANY 



[Cn. 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 

 showing a structural 

 differentiation of the 

 filament as a whole, 

 for a heterocyst oc- 

 curs always at one 

 end, while towards 



FIG. 276. Cyanophycese, typical forms. the Other the cells 



Above, left, Glceocapsa sp., and below, Nostoc taper regularly down- 



paludosum ; center, Oscillatoria principis ; right, , . . ,-,, 



Rivularia bullata ; all X 500. Below in center Ward in S1Z6. Inese 



is a colony of Rivularia, natural size. (The first plants also form gela- 

 from nature, others after Engler and Prantl.) 



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 Cyanophycese the cells are of the simplest char- 

 acter, showing no clear differentiation of nucleus or chro- 

 matophores from the cytoplasm. Their characteristic blue- 



