310 PLANT STKDOTUKES 



seem to haye 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 sporojjhyte, and the 

 gametes to the gametopJigte. With the further introduction 

 of heterospo7'ii, the male and the female gametes are sepa- 

 rated upon diiferent gametophytes, which become much 

 reduced. 



^^'ith the reduction of the megaspores to one in a spo- 

 rangium (ovule), and its retention, the seed is organized, 

 and the elaborate scheme of insect-pollination is developed. 



