170 



MORPHOLOGY. 



sporangia, the water should be drawn from under the cover glass 

 with the aid of some bibulous paper, like filter paper, placed at the 



edge of the cover glass on 

 the opposite side from the 

 glycerine. As the glycer- 

 ine takes the place of the 

 water around the sporangia 

 it draws the water out of 

 the cells of the annulus, 

 just as it took the water 

 out of the cells of the 

 spirogyra as we learned 

 some time ago. As the 

 water is drawn out of these 

 cells there is produced a 

 pressure from without, the 

 atmospheric pressure upon 

 the glycerine. This causes 

 the walls of these cells of 

 the annulus to bend in- 

 ward, because, as we have 

 Fig. 20s. already learned, the glycer- 



Section through sorus of Polypodium vulgare \ ne . HnPS lint na« thrniitrli 

 showing different stages of sporangium, and one ,Ue UOeb "° l " aSS tnrou g n 

 multicellular capitate hair. the wal j s nearly so fest 



as the water comes out. 



353. Now the structure of the cells of this annulus, as we 

 have seen, is such that the inner walls and the perpendicular 



Fig. 209. 

 Section through sorus and shield-shaped indusium of aspidium. 



walls are stout, and consequently they do not bend or collapse 

 when this pressure is brought to bear on the outside of the cells. 



