138 



THE BIOLOGY OF A PLANT. 



organs (Figs. 70, 75), described for the first time by Suminski 

 in 186-i, likewise arise from single superficial cells of the pro- 

 thallium. They are situated almost exclusively upon the cushion 

 near its anterior or apical extremity, and hence at the bottom of 

 the anterior depression (Fig. 70). Since they appear later than 

 the antheridia, they are not likely to be fertilized by spermato- 

 zoids 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 dic/togamy. There is reason to believe 

 that important advantages are gained by thus securing cross-fer- 

 tilization and preventing self-fertilization or ' ' breeding in and 

 in." 



In the development of the archegonium the original cell enlarges, be- 

 comes somewhat dome-shaped, and divides by transverse partitions into 

 three cells : a proximal, im- 

 bedded in the tissue of the 

 prothallium, a middle, and a 

 distal dome-shaped cell (Fig. 

 76). The fate of the proximal 

 cell is unimportant. The dis- 

 tal cell gives rise by division 

 to a chimney-like structure, 

 the neck (Figs. 75, 77), which 



B. 



A. 



FIG. 76.-Diagram to illustrate 

 the origin ot an archegonium. 

 A, an early stage; B, a later 

 stage; A, a, the original epi- 

 dermal cell enlarged ; B, o, the FIG. 77. (After Strasburger.) Developing arche- 



basal cell; b, the central or 

 canal cell; c, the neck-cell. 



gonia of Pteris xerrulata. A, young stage ; B, 

 older ; n, neck ; c, canal ; o, oOsphere. 



encloses a row of cells (canal-cells) derived from the original middle cell 

 (Figs. 75, 77). These afterwards become transformed into a mucilaginous- 

 substance filling a canal leading through the neck from the outside to the 

 oosphere (Fig. 77), which also arises from the original " middle" cell at its 



