400 



PART III. THE CLASSIFICATION OF PLANTS. 



then proceeds in two planes, giving rise to a flattened plate of cells, 

 further growth being effected by means of a two-sided apical cell. 

 After a time the activity of the apical cell ceases, a periclinal wall 

 beingformed in it; whatever further 

 growth takes place is effected by P ^/^ 



FIG. 265. Diagram of the prothal- 

 lium of a Leptosporangiate Fern : under 

 side ( x 10). ar Archegonia ; an anthe- 

 ridia; h root-hairs. 



FIG. 260. Antheridium of Adiantum 

 Capillus-Veneris (x 650). p Prothallium; 

 a antheridium ; s spermatozoid ; b the 

 vesicle containing starch-grains. 



A 



the marginal cells. At this stage the prothallium becomes some- 

 what heart-shaped, the anterior depression indicating the position 

 of the organic apex. The cells lying anteriorly in the middle line 

 now begin to divide in a plane parallel to the surface, with the 

 result that the prothallium becomes thickened in this region, and 



eventually a 

 "cushion" 

 of tissue, 

 several lay- 

 ers of cells 

 in thickness, 

 is produced, 

 which pro- 

 jects on the 

 lower (ven- 

 tral) sur- 

 face, and 



FIG. 267. Polypodium vulgare. A Young archegonium, not yet bears the 

 open : K' neck-canal-cell ; K" ventral canal-cell : o young oosphere. 

 B Mature archegonium open, (x 210 : after Strasburger.) archegoma. 



