126 PL ANT- ANIMALS [CH. 



wall of mucilage. So rapidly may the wall form 

 about the encysting green cell, that individuals are 

 sometimes observed in which the flagella may be 

 seen in undulating movement within the enclosing wall. 



The resting cells (Fig. 22) vary remarkably both 

 in form and behaviour. Thus, a single, flagellated cell 

 may come to rest, surround itself with a thin wall 

 and divide longitudinally into two or four daughter 

 cells. Each daughter cell, at first naked, organises a 

 delicate cell- wall, develops flagella and escapes from 

 the deliquescent mother wall as an active, flagellated 

 cell. Or an active cell comes to rest, surrounds itself 

 with a thick wall, takes on a spherical shape and 

 becomes uniformly green (Fig. 22, A). From such cells 

 the pyrenoid and eye-spot disappear. Within each 

 such resting cell, four daughter cells arise, develop 

 flagella and escape (Fig. 22, C). 



A third form of resting cell occurs (Fig. 22, A). 

 It is identical with that just described, except that 

 it is colourless. Like its green counterpart it may 

 divide to form four colourless daughter cells, which 

 may be extruded from the mother cell-wall or divide 

 yet further (Fig. 22, B, C, and Z>). 



Again, paired resting cells occur. Two active 

 green cells settle down together, become pressed 

 against one another, and surround themselves with 

 a common envelope. Such paired resting cells are 

 either green or colourless. 



