464 APPENDIX. 



characteristic manner; they undergo repeated forking, the 

 ends of the final forkings being free from each other and 

 not united to form a network. 



The fully developed sporangium is oval in outline and 

 biconvex, or lens-like, and has a slender 

 ANNULUS ^ stalk (Fig. 5). Its wall consists of a 



single layer of cells, and the cells round 

 the edge, starting from the stalk and 

 going over the top of the sporangium 

 halfway down the other side, are large 

 and thick-walled. This row of cells is 

 called the annulus, or ring, though it is 

 not a complete ring but is replaced on 

 the lower half of one edge of the spor- 

 angium by thin- walled cells. Inside the 

 SPORE Tl I ** spore-case there is a brown powdery mass 

 Fig. 5.-s P orangium and consisting of the spores, generally sixty- 

 apore of Fern. four in number. 



When the spore-case is ripe, the cells 



of the ring lose water, contract, and thus put a strain on the 

 thin part of the wall of the spore-case. Eventually the ring 

 springs back, straightening itself and becoming curved in 

 the opposite direction ; this takes place suddenly, and the 

 spores are flung out ; the place where the ring is incomplete 

 is called the stomium, or mouth, but as a matter of fact it 

 is merely the place where the tearing open of the spore-case 

 occurs, for the spores are jerked out in all directions when 

 the ring bends back, and the spore-case is soon emptied of the 

 spores. 



If mature but still intact spore-cases are mounted in water 

 on a slide, the explosion may be watched if glycerine is 

 placed at one edge of the cover-glass and drawn through, 

 by means of a piece of filter-paper placed at the opposite 

 edge. The explosion can also be seen if a number of ripe 

 spore-cases are mounted on a dry slide, which is then 

 gently warmed and quickly placed again under the micro- 

 scope. If now you select for observation a spore- case which 

 has burst open and is lying upon the dry slide, and breathe 

 on the slide, you will see the ring returning to its original 

 curved position; then as it dries again it again becomes 

 straightened out. 



