8 



MORPHOLOGY 



this means only "other cells," and suggests nothing as to their beha\ior. 

 By means of the heterocysts, therefore, the working cells of a filament 



8 

 Figs. 7, S.—Nostoc: 7, the jelly-like mass in which 

 the filaments are embedded ; 8, filamentous colonies coiled 

 within the gelatinous matrix; four heterocysts shown, 

 dividing the filaments into hormogonia. 



are separated into distinct sections, and these 

 sections are called hormogonia. It has been 

 observed that when colonization occurs, the 

 heterocysts anchor the filament, and that the 

 hormogonia break loose from them and wriggle 

 out through the jelly-like matrix and establish 

 new colonies. So far as observed, therefore, 

 this differentiation of heterocysts seems to 

 be associated with the fragmentation of the 

 filament. 



Nostoc illustrates well an ordinary plant 

 method of enduring an unfavorable season, as 

 winter. At the inception of the period of 

 danger, certain cells of the filament enlarge, heterocysts and the whip- 

 accumulate reserve food, and become thick- "ke extension of the apex, 

 walled. These cells are able to endure cold or drought ; and upon 

 the return of favorable conditions, the heavy wall is broken through 

 and a beginning filament emerges. These resting vegetative cells are 



Fig. 9. — Rivularia: the 

 filaments show the basal 



