vii.] THE BRACKEN FERN. 69 



b. Their structure. Each is composed of a layer 

 of transparent 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. i. b. a). 

 In this cavity lies, in young specimens, a large 

 nucleated granular basal cell, with two or three 

 smaller granular cells (tieck-cells) above it in the 

 narrow upper part of the cavity; in older speci- 

 mens this upper part is empty, forming a canal 

 leading down to the basal cell. 



4. Examine young Fern in connection with its pro- 

 thallus. 



