296 FORMS OF FILICALES 



sists of a delicate stalk, supporting a rather spherical spore- 

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

 sist of a single layer of thin-walled cells except for a row of 

 thickened cells, the annulus, that extend 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, /). It is to be noticed that the walls of the annulus 

 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 begin to dry 

 out. It is evident that the thin outer walls will be the first to 

 yield and their 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. As soon as the tension is 

 relaxed through the more complete drying of these cells the 

 elasticity of the thick walls 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 

 particularly the structure of the annulus are important charac- 

 teristics to be observed in identifying the ferns. In the flowering 

 ferns, Osmunda, the large slightly-stalked sporangia are pro- 

 vided with a rudimentary annulus of a few thickened cells 

 located at one side of the capsule (Fig. 224, A). The entire 

 leaf or certain leaflets are completely covered with the sporangia, 

 little more than the vascular bundles remaining (Fig. 225, A). 

 Osmunda belongs to the lowest group of the Filicales and shows 

 certain analogies in the arrangement and structure of its col- 



