200 BIOLOGY: GENERAL AND MEDICAL 



thers before the stigma, when the flower must be ferti- 

 lized by pollen from other flowers younger than itself, 

 or by protogyny, or maturation of the stigma before the 

 anthers, when it must be fertilized by pollen from flowers 

 older than itself. In either case, the conjugating ele- 

 ments must come from different flowers, though these 

 may be upon the same plant. 



In other flowers an inequality in the length and 

 position of the stamens and pistils prevents self-pollination 

 and consequent self-fertilization. 



Some plants develop two sets of flowers, male and 

 female, respectively, and other plants are of distinct 

 sexes, certain individuals producing only male, and 

 others only female flowers. 



In a few cases the pollen from a flower when dropped 

 upon the stigma of the same flower fails to grow into the 

 usual filaments, or, doing so and descending to the egg 

 cell, fails to conjoin and fertilize it, or does so only when 

 no pollen from a different flower finds its way to the 

 same stigma. In some cases, when pollen, from the 

 same flower and from a different flower arrive at the 

 same time, upon the same stigma, the exogenous pollen 

 appears to outgrow the autogenous pollen and more 

 quickly finds its way to the egg cells and effects fertiliza- 

 tion. It is probably by this means that cross-fertiliza- 

 tion is effected in those cases in which the pollen from 

 various flowers with sexual organs maturing at the 

 same time is freely distributed by the wind. The same 

 means of securing the advantage of cross-fertilization 

 is probably adopted in those cases in which the flowers 

 are entomophilous and visited by insects that not only 

 sprinkle pollen belonging to the flower upon its stigma, 

 but also bring it pollen from other and remote flowers. 



Thus the phaneroganic vegetation displays a pro- 

 nounced disposition to secure cross-fertilization, though 

 its importance seems to vary widely in different cases. 



The animal kingdom, however, shows a much more 

 restricted sexual development. Hermaphroditism, or the 



