THE FISSION PLANTS. 



61 



First, by division of the cells in a direction transverse to 

 the length of the filament. Second, by the formation of 

 resting spores that develop into new filaments. Third, in 

 a special way by the formation of what 

 are called hormogonia. The parts of 

 the filaments between the heterocysts 

 escape from the gelatinous covering 

 after it has become more or less de- 

 cayed and dissolved. After a time 

 each of these fragments becomes in- 

 vested with a new gelatinous covering FIG. 31. A Nostoc. An- 

 and develops into a new filament by o^^WAft^rCook 1 ) 

 the division of its cells. 1 



The Rivularieae are found as minute masses of dark 

 blue-green color in sluggish streams and in standing 

 water. The filaments radiate from the center of the 

 mass. The masses are seldom more than one or two 

 millimeters in diameter. 



The Oscillatorieae occur as a dark blue-green scum on the 

 surface of stagnant water, about watering troughs, on wet 

 rocks, on Lily pads, and in similar locations. Examined 

 under the microscope, the filaments are seen to oscillate 

 slowly from side to side, a phenomenon that has not as 

 yet been satisfactorily explained. The plants multiply by 



FIG. 32. Oscillaria, magnified. A single plant in the vegetative condition. 

 The filament has resulted from repeated transverse divisions of the cells. 



the transverse division of the cells. Resting spores are 

 also formed, which continue the existence of the plant in 

 the dried state. 



1 The teacher might arrange a series of Practical Studies, involving the 

 observation of these facts, on any species of Nostoc that is available. 



