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 Gametophyte. The megaspores begin 
to germinate "while still in the sporangium. This will be 
