SINGLE-CELLED PLANTS AND COLONIES. 



11. Gelatinous filament-colonies. — In other cases, instead 

 of being held together only by the weak jelly-like portion of 

 the cell-wall, the plants, still practically independent the one 

 of the other, remain connected by the firmer portions of the 

 wall into rows, forming irregularly coiled or serpentine fila- 

 merits, which are embedded in a profuse jelly (fig. 6). The 



Fig. 6. — Nostoc, ^, a' gelatinous colony, irregularly lobed. Natural size. .5, a por- 

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

 separated from the rest of the cells composing the filament, and three intermediate 

 ones) and the jelly belonging to it. Magnified about 400 diam. — After Thuret and 

 Janczewski. 



real independence of the cells, even though they remain con- 

 nected, is shown by the fact that such a chain may be 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, whose purpose seems to be to break the chain 

 into pieces, which work their way out of the jelly and grow 

 into independent colonies. 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 2-5 centimeters in 

 diameter. They may be found adhering to water-weeds as 

 clear- or dirty-green masses, or sometimes floating free 

 {A, fig. 6). 



EXERCISE I. 



Nostoc or Bivularia. — i. Observe the size and form of the colonies 

 and the consistence of the jelly enclosing them. (^11.) 



