SINGLE-CELLED PLANTS AND COLONIES. 



firmer portions of the wall into rows, forming irregularly 

 coiled or serpentine filaments, which are embedded in a 

 profuse gelatinous material (fig. 13). The essential inde- 

 pendence of the individual cells, even though they remain 

 connected, is shown by the fact that such a chain may be 



il^lfe^^lf A 



FIG. 13. Nostoc. A, a gelatinous colony, irregularly lobed. Natural size. /?, a 

 portion of a serpentine filament with five heterocysts (one at each end by which it 

 \vasseparatedfromtherestof the cells composing the filament, and three inter- 

 mediate ones") and the jelly belonging to it. Magnified about 400 diam. After 

 Thuret and Janczevvski. 



broken up into any number of pieces and each piece will 

 retain all its powers. Here and there in the chain there 

 occur cells unlike the rest (/*, fig. 14), called heterocysts, 

 whose function seems to be to break the chain into pieces, 

 from the growth of which independent colonies may arise. 

 The association of considerable numbers of these plants in 

 colonies gives rise to masses of jelly which vary from the size 

 of a pin-head to 25 centimeters in diameter. They may be 

 found adhering to water-weeds as clear- or dirty-green 

 masses, or sometimes floating free (A, fig. 13). 



14. Filaments of loose organization. Of very near kin 

 to these plants are the oscillarias, which have received this 

 name from the pendulum-like swinging of their tips (fig. 15). 

 In them the cells remain connected more extensively and 

 more firmly, so that each is disk-shaped, and the filament is 

 much less easily separated into its component cells. More- 



