XV.] THE BRACKEN FERN. 459 



b. Treat with iodine ; this stains them and stops 

 their movements, so that their form can be more 

 distinctly seen. 



3. The archegonia. Make vertical sections of the pro- 

 thallus passing through the cushion ; this is best 

 done while holding it between two pieces of pith. 

 Note in the archegonia — 



a. Their form: chimney-shaped eminences with a 

 small aperture at the apex. 



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 central cell, with two or three 

 smaller granular cells {neck canal-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 central cell. The 

 latter has now divided into two ; the upper 

 smaller cell resulting from its division forming 

 the ventral canal-cell, \^\\\c\\ becomes disorganized, 

 while the larger lower cell is the ovum or oosphere. 



4. Examine a young fern-plant in connection with its 

 prothallus, to the lower side of which it is attached 

 by means of \S\Qfoot. 



