LIFE HISTORY OF A FERN 



21 



down. The dorsal surface would then be the under 

 surface, and the ventral surface the upper one. Organisms 

 or organs having two such surfaces clearly distinguishable 

 are said to have dorso-ventral differentiation. Among many 

 other structures thus differentiated are foliage-leaves, 

 sporophylls, man, fishes, and other animals. In buds the 

 dorsal surface of leaves is the upper or outer surface; 

 when foliage leaves are fully expanded the dorsal surface 

 is commonly underneath, and the ventral surface above. 



FIG. 21. Archegonia of a fern (Adiantum). A, young archegonium; 

 B, mature; C, top view, showing terminal cells of the four rows of wall 

 cells; v, wall of venter; e, egg; v.c.c, ventral canal-cell; n.c, neck-canal; 

 sp, sperms entering the neck-canal. A and B in longitudinal section. 



14. Reproductive Organs: Archegonia. Examination 

 of the ventral surface of a mature prothallus with a lens 

 will reveal near the notch and on the cushion, several 

 tiny flask-shaped bodies, the archegonia. Each arche- 

 gonium consists of a wall, one cell thick, and contents 

 (Fig. 21). The neck projects away from the surface 



