VII.] THE BRACKEN FERN. 67 



their movements so that their form can be more 

 distinctly seen. 



3. The archegonia. Most numerous towards the concave 

 border of the prothallus. 



a. Their form: chimney-shaped eminences with a 

 small aperture at the apex. 



I. Their structure: composed of a layer of trans- 

 parent cells, containing no chlorophyll, arranged 

 in four rows, and surrounding a central cavity 

 which extends into the cushion formed by the 

 thickened part of the prothallus (a. 1. b. a). In 

 this cavity lies, in young specimens, a large 

 nucleated granular basal cell, with two or three 

 smaller granular cells above it in the narrow 

 upper part of the cavity ; in older specimens this 

 upper part is empty, forming a canal leading 

 down to the basal cell. 



4. Examine young Fern in connexion with its prothallus. 



52 



