VII.] THE BRACKEN FERN. 65 



b. Their structure : composed of brownish cells, one 

 row of which has very thick walls, and forms a 

 marked ring (annulus) round the edge of the 

 sporange. 



c. Their mode of dehiscence (look out for one that 

 has opened) : by a cleft running towards the 

 centre of the sporange from a point where the 

 annulus has torn across. 



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

 glass: examine with J 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.] V. 



B. THE PROTHALLUS; SEXUAL GENERATION. 



Prothalli may be obtained by sowing some spores on a 

 glass slide, 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 (rootlets) arise. 



b. Its structure. 



a. The leafy expansion: it consists throughout 

 most of its extent of a single layer of polyhe- 

 M. 5 



