DEVELOPMENT OF PLANTS 327 



annulus (Fig. 224, A), but in the majority of our common ferns, 

 the sporangium consists of a delicate stalk, supporting a rather 

 spherical spore-bearing capsule (Fig. 224, B). The walls of this 

 capsule consist of a single layer of thin-walled cells except for a 

 row of thickened cells, the annulus, that extends from the stalk 

 over the capsule nearly to the opposite side (Fig. 224, B, an). 

 At the latter point, the annulus ends in a few rather weakened 

 cells, two of which, the lip cells, are conspicuous for their larger 

 size (Fig. 224, B, I). It is to be noticed that the cells of the an- 

 nulus are thickened on their inner and radial walls while the outer 

 walls remain comparatively thin. When the spores are mature 

 and lie loose in the capsule, the cells of the annulus that have 

 been filled with water up to this time begin to dry out. As the 

 volume of water in these cells lessens through evaporation, the 

 cohesion of the water pulls upon the thin outer walls, causing 

 them to bend in, assuming a U-shaped appearance. This tends 

 to pull the two radial walls of each cell together and shorten 

 the outer circumference of the annulus. This contraction pulls 

 open the sporangium at the weak lip cells, the split extending 

 back through the lateral cells of the capsule which is finally 

 drawn back to such an extent that the annulus becomes nearly 

 straight. Finally, when the water is nearly withdrawn from the 

 cells, the tension becomes so great that air is drawn in through 

 the thin walls of the cells. The entrance of air breaks the water 

 adhering to the walls and which had kept them pulled in as 

 described above and consequently each cell returns instantly to 

 its original form. This causes the capsule to snap back to near 

 its original position with a quick jerk, thus throwing out the 

 spores to a considerable distance. This motion of the capsule 

 can be approximated by mounting sporangia that have been 

 soaked in a drop of water and watching them under a micro- 

 scope while a drop of glycerine placed in touch with one side 

 of the mount is drawn in with a filter paper applied to the oppo- 

 site side. The denser glycerine will draw the water out of the 

 cells by osmosis and set up a tension in the cells of the annulus 

 just as did the evaporation of the water. 



The form of the sorus and its relation to the indusium and 



