154 MORPHOLOGY AND LIFE HISTORY 



larger-celled tissue serves to nourish the young 

 embryo-sporophyte, that develops from the 

 fertilized egg. 



The archegonia are exposed for fertilization 

 by the splitting of the wall of the megaspore 

 along the ridges, but the prothallus itself does 

 not project beyond the walls of the spore. 



When the sporophyte begins to develop from 



the fertilized egg, it continues to grow, without 



any resting period until it is mature. 



3. Diagram the life cycle of Isoetes, as directed for the 



fern (I, n, p. 78). Let MG = male gametophyte; 



FG = female gametophyte; s sperm; e = egg; S 



= sporophyte; mi = microspore; mg = megaspore. 



