FERNS. 



257 



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 itself. The sporangium 



Fig. 2 93 . 

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



opens between the lip cells on the front and the lateral walls of 

 the sporangium are torn directly across. The greater mass of 

 spores are thus held in the upper end of the open sporangium, 

 and when the annulus has nearly doubled on itself it suddenly 

 snaps back again in position. While treating with the glycerine 

 we can see all this movement take place. Each cell of the 

 annulus acts independently, but often they all act in concert. 

 When they do not all act in concert, some of them snap sooner 

 than others, and this causes the annulus to snap in segments. 



538. The movements of the sporangium can take place in 

 old and dried material. If we have no fresh material to study 



