XV.] THE BRACKEN FERN. 457 



marked ring (annulus) round the edge of the 

 sporangium. 



c Their mode of dehiscence (look out for one that 

 has opened) : by a cleft running transversely 

 across the sporangium. 



3. Burst open some sporangia by pressing on the cover- 

 glass : examine, with § obj., the spores which are set 

 free. 



a. Their size : measure. 



b. Their form : somewhat triangular. 



[c. Their structure : a thick outer coat, a thin inner 

 coat, protoplasm, and a nucleus : crush some by 

 pressure on the cover-glass.] 



B. The Prothallus ; sexual generation. 



Prothalli may be obtained by sowing some spores on 

 peat and keeping them warm and very moist for about three 

 months. They are small deep green leaf-like bodies. 



a. The Prothallus. 



1. Transfer a prothallus to a slide, and mount it in water 

 with its under surface uppermost. Examine with 1 

 inch obj. 



a. Its form : a thin kidney-shaped expansion from 

 which, especially towards its convex border, a 

 number of slender filaments {root-hairs) arise. 



b. Its structure. 



u. The leafy expansion : it consists throughout 

 most of its extent of a single layer of polyhe- 

 dral chlorophyll-containing cells, but at a part 

 (the cushion) a little behind the depression 

 marking the growing point it is several cells 

 thick, 



