386 



STRUCTURE AND LIFE HISTORIES 



alternate along the axis of the cone (Fig. 281). The 

 larger number of microspores results from the fact that 

 every spore-mother-cell, by tetrad-division, develops 

 spores, while in the megasporangia only one spore-mother- 

 cell develops spores, the other cells serving to nourish that 

 one. The microspores develop only male gametophytes, 

 the megaspores, female. In dissemination, the spores 

 are ejected to some distance from the parent plant 

 (Fig. 282). 



Fig. 282. — Selaginella Martensii. Dissemination of spores. The 

 branch was covered with a glass bell-jar to avoid currents of air. The 

 "dust" is composed of both microspores and megaspores, and indicates 

 the distance to which the spores are projected from the dehiscing spor- 

 angia. X 23. 



346. The Male Gametophyte. — The male gametophyte 

 is developed entirely within the wall of the microspore. 

 The first division gives rise to a vegetative (or sterile) 

 and a fertile cell. The vegetative tissue never develops be- 

 yond the one-celled stage. By several divisions the fertile 

 cell develops a simple antheridium containing four sperms. 

 Each sperm bears two long, slender cilia (Fig. 284). 



347. The Female Gametophs^e. — The megaspores begin 

 to germinate while still in the sporangium. This will be 



