vii.] THE BRACKEN FERN. 67 



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 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. 



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

 with its under surface uppermost. Examine with i 

 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- 

 dral chlorophyll-containing cells, but at a part 

 (the cushion) a little behind the depression 

 marking the growing point it is several cells 

 thick. 



(3. The rootlets: composed of a series of cells 

 which contain no chlorophyll. 



c. The antheridia and archegonia: the former can 

 just be seen with an inch objective as minute 

 eminences on the under surface of those parts of 

 the prothallus which consist of a single layer 

 of cells, especially among the root-hairs ; the 

 latter are partly imbedded in the cushion. 



52 



