MALE FERN. 



349 



cell of the ring is thickened chiefly on its inner and lateral 

 walls, the outer wall being much thinner. The annulus is 

 not a complete ring, being re- 

 placed at its end by broad flat 

 cells (this region is sometimes 

 called the stomium, since dehis- 

 cence of the sporangium- wall be- 

 gins here) ; the rest of the cells 

 forming the wall .are flat and 

 thin- walled. 



Inside the ripe sporangium 

 there are numerous (usually 

 forty - eight ) brown unicellular 



spores ; each spore is kidney- \ V \^ \ I 

 shaped, and when ripe has a \ =- \ "< 



two-layered wall, the outer layer 

 being thick and cutinised. 



490. Dehisceiice of Sporan- 

 gium. Mount some ripe but still 

 intact sporangia in water and, 

 while watching them under the 

 microscope, place a drop of glyce- 

 rine at one edge of the cover- 

 glass and draw it through by 

 means of a piece of filter-paper 

 or blotting-paper placed at the 

 opposite edge; watch the burst- 

 ing of the sporangia. The ex- 

 plosion can be caused by any 

 other method for withdrawing 

 water from the ripe sporangia ; 

 for instance, mount some spor- 

 angia on a dry slide, then warm 

 the slide gently, and quickly ob- 

 serve it again under the micro- 

 scope. 



Dehiscence is due to contraction of the cells of the ring : 

 as these cells lose water, the thinner outer wall bulges 

 inwards ; this pulls the radial walls together, until the 



