310 PLANT STKUCTUKES 



seem to have been derived from asexual spores. At first 

 the pairing gametes are alike, but later they become differ- 

 entiated into sperms or male cells, and eggs or female cells. 



With the establishment of alternation of generations, 

 the asexual spores are restricted to the sporophyte, and the 

 gametes to the gametophyte. With the further introduction 

 of heterospory, the male and the female gametes are sepa- 

 rated upon different gametophytes, which become much 

 reduced. 



With the reduction of the functioning megaspores to 

 one in a sporangium (ovule), and its retention, the seed is 

 organized, and the elaborate scheme of insect-pollination 

 is developed. 



