1 62 



BOTANY. 



drawn out of these cells there is produced a pressure from with- 

 out, the atmospheric pressure upon the glycerine. This causes 

 the walls of these cells of the annulus to bend inward, because, 

 as we have already learned, the glycerine does not pass through 

 the walls nearly so fast as the water comes out. 



267. "Working of the annulus. — Now the structure of the cells 

 of this annulus, as we have seen, is such that the inner walls and 

 the perpendicular walls are stout, and consequently they do not 



Fig. 137. 

 Rear, side, and front views of fern sporangium, d, <?, annulus ; a, lip cells. 



bend or collapse when this pressure is brought to bear on the out- 

 side of the cells. The thin membranous walls on the back (dorsal 

 walls) and on the sides of the annulus, however, yield readily to 

 the pressure and bend inward. This, as we can readily see, pulls 

 on the ends of each of the perpendicular walls, drawing them 

 closer together. This shortens the outer surface of the annulus 

 and causes it to first assume a nearly straight position, then curve 

 backward until it quite or nearly becomes doubled on itse 



