ALTERNATION OF GENERATIONS 243 



421. Mossworts may then be described as green plants 

 in which the gametophyte is a prostrate or erect some- 

 what long-lived plant, producing antherids, and oogones 

 (the latter enclosed in archegones). After fertilization a 

 distinct structure, the sporophyte, is produced, but al- 

 though it rests on and in the gametophyte and obtains its 

 supply of water and much of its food from it there is 

 no organic connection between them. In this sporo- 

 phyte certain internal cells (the "spore mother-cells") 

 divide twice and thus produce internally four spores 

 each. These eventually germinate and produce other 

 gametophytes. 



422. Here it should be noted that the nuclei of the 

 gametophyte cells contain a definite number of chromo- 

 somes, and that on the fertilization of the egg this number 

 is doubled. This double number is maintained in the 

 sporophyte until spores are formed by division into fours, 

 at which time a reduction takes place to the original num- 

 ber. So in this phylum the two generations are separable 

 also by their chromosome numbers in addition to the 

 other more obvious differences. 



423. The antherids are complex structures. They are 

 usually short-stalked, and consist of a layer of large 

 boundary cells within which are very numerous, small, 

 more or less cubical cells, each of which produces in- 

 ternally an elongated, more or less spiral, biciliate sperm. 

 The walls of these spermatogenous cells dissolve, leaving 

 the sperms free within the cavity of the antherid. By the 

 rupture of the apical cells the sperms escape. This 

 occurs only when the antherid is covered with water (rain, 

 dew, etc.)- 



424. The archegone is a flask-shaped, elongated organ, 

 consisting of an enlarged lower part (venter) containing 

 the egg, above which is the slender neck, at first closed at 



