8 
MORPHOLOGY 
this means only "other cells," and suggests nothing as to their behavior. 
By means of the heterocysts, therefore, the working cells of a filament 
FIGS. 7, 8. 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 
. . FlG. 9. Rivularia: the 
winter. At the inception of the period of filaments show the basal 
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 
