FERNS 



361 



tance around. This row of cells is known as the ring (annulus). 

 The inner walls of the cells are thick and firm. The perpen- 

 dicular walls are thick next the inner wall and taper outward, 

 but are quite rigid. The outer and lateral walls are thin and 

 membranous, this ring standing out quite prominently above the 

 lateral faces of the spore case. On the opposite edge of the 

 spore case from the ring are two cells near the middle of the edge 

 which fit together somewhat like lips. They are called " lip " 

 cells, and the point between 

 them is the " mouth " (or 

 stomium), for when the spore 



Fig. 321. Fig. 322. 



Rear, side, and front views of fern spor- Dispersion of spores from sporangium of 



angium. d, c, annulus; a, lip cells. Aspidium acrostichoides, showing different 



stages in the opening and snapping of the 

 annulus. 



case opens, it opens between these two cells. In the interior of 

 the spore case are usually sixteen spore mother cells in this 

 family. Each of these forms four spores, so that there are 

 sixty-four spores in a spore case. 



516. Opening of the spore case and scattering of the 

 spores. When the spores are ripe, the indusium, or other cover- 

 ing of the sorus, dries and withers, exposing the spore case. 

 These then begin to dry. In doing so the water evaporates from 

 the cells of the ring. The air cannot enter these cells, conse- 

 quently there is great air pressure from the outside as the water 

 slowly escapes. The inner walls of the ring cells, as well as the 

 perpendicular walls between its cells, are firm and do not bend 

 inward. But the outer and lateral walls being thin and mem- 



