PTERIDOPHYTES 



319 



which finally form free male gametes within the microspore 

 (Fig. 184, <?). These male gametes are then liberated by the rup- 

 ture of the spore coat and fertilize the female gametes. In some 

 species of Selaginella fertilization and the early development of 

 the embryo take place within the female sporangium, which has 

 previously opened and admitted the germinated microspores. In 



d e 



FIG. 184. Gametophytes, sex organs, and embryo of Selaginella 



a, megaspore opening to expose the inclosed gametophyte and archegonia preparatory 

 to fertilization ; 6, sectional view of a megaspore and gametophyte ; c, section of a 

 microspore, gametophyte, and sperm mother cells, with sperms above; d, e, three 

 stages in the development of the sporophyte of Selaginella; e, young and differ- 

 entiated sporophyte, with elongating root and stem 



other instances fertilization may take place on the soil, where 

 the female spores complete the germination process. The process 

 of fertilization, as in all other instances among the higher plants, 

 produces an embryo sporophyte which ultimately grows into an 

 adult spore plant of Selaginella. 



Embryo. The young embryo (Fig. 184, d) is early differentiated 

 into the embryo proper and the suspensor, which serves to force 

 the embryo down into the nutritive tissue of the gametophyte. 

 The embryo then develops two cotyledons and a rudimentary 



