THE PIXE 131 



which the pine is one), for, as we have seen, some of the 



relatives of the ferns have two kinds of spores. 



In the keeping of the macrospore in the spore sax:, which 



compels the female plant also to pass its whole life 



withia the macrospore sac. 



In the great reduction in size of the sexual generation — 



a reduction which has gone further in the male plant 



than in the female plant. This reduction had begun in 





I sexual 



generation 



\ 



asexual • 



generation ! 



Fig. 77. — Diagram illustrating the life cycle 

 of the pine. 



the fern, whose sexual plant was much smaller than its 

 asexual plant, but it has been carried much farther in 

 the pine. 



d. In the development of a pollen tube,- -which, carries the 

 male gamete to the egg. This has made it unnecessary 

 for the male gamete to form special organs for moving 

 itself about. 



e. In the formation of a seed, containing an embryo which 

 has stopped growing for a time. The distribution of 

 mosses and ferns is brought about by the scattering of 



