ORIGIN OF THE AEGHEGONIA. 



97 



sively upon the cushion near its distal or apical extremity, and 

 hence at the bottom of the anterior depression (Fig. 58). Since 

 they appear later than the antberidia, they are not likely to be 

 fertilized by spermatozoids descended from the same spore. 

 This phenomenon of maturation of one set of sexual organs of a 

 bisexual individual before the ripening of the other set, is a 

 common feature among plants, and is known as dichogamy. 

 There is reason to believe that important advantages are gained 

 by thus securing cross-fertilization and preventing self-fertiliza- 

 tion or ''breeding in and in." 



In the development of the a/rchegonium the original cell en- 

 larges, becomes somewhat dome-shaped, and divides by transverse 

 partitions into three cells : 

 a proximal, imbedded in 

 the tissue of the prothalli- 

 um, a middle, and a distal 

 dome-shaped cell (Fig. 64). 

 The fate of the proximal B 

 cell is unimportant. The 



FIG. 64. Diagram to illus- 

 trate the origin of an arche- 

 gonium. A, an early stage; 

 B, a later stage; A, a, the 

 original epidermal cell en- 

 larged ; B, a, the mother- 

 cell: b, the central or canal 

 cell; e, the neck-cell. 



A 



FIG. 65. (After Strasburger.) Developing archegonia 

 of Pteris serrulata. A, young stage; B, older; n, 

 neck ; c, canal ; o, ocisphere. 



distal cell gives rise by division to a chimney-like structure, the 

 neck (Figs. 63, 65), which encloses a row of cells (canal-cells} de- 

 rived from the original middle cell (Figs. 63, 65). These after- 

 wards become transformed into a mucilaginous substance filling a 

 canal leading through the neck from the outside to the oiJspJiere 

 (Fig. 65), which also arises from the original " middle" cell at its 

 proximal end. The ou'sphere is the all-important female yerm- 

 7 



