RBPKODUCTIVE ORGANS OF PLANTS. 321 



service. The insect, in exploring a flower for nectar, 

 leaves upon its stigma pollen taken from the flower last 

 visited, and in emerging renews its burden of pollen to 

 bestow it in turn upon the stigma of a third flower. 



Cross-fertilization is doubtless often effected by insects 

 in case of flowers which are in all respects adapted for 

 self-fertilization, while flowers that casual exatoination 

 would pronounce self-fertile are in fact of themselves 

 sterile. The flowers of rye open singly, the long stamens 

 shortly mature and discharge their pollen, which falls on 

 the stigmas of flowers standing lower in the same head, 

 or on neighboring heads. According to Eimepare, the 

 individual rye-flower can fertilize neither itself nor the 

 different flowers of an ear, nor can the different ears of 

 one and the same plant pollinate one another with suc- 

 cess, although no mechanical hindrance exists. (Sachs, 

 Physiology of Plants, p. 790. ) 



Results of Self-Fertilization and Cross-Fertili- 

 zation. — Sprengel, one of the early students of Plant- 

 Eeproduction, wrote in 1793, " Ifature appears to be 

 unwilling that any flower shall be fertilized by its own 

 pollen." Extensive observation indicates decidedly 

 that, cross-fertilization is far more general than self- 

 fertilization, especially among the higher plants. Dar- 

 win has shown that, in many cases, the pollen of a flower 

 is incapable of fertilizing its own ovules, and that the 

 pollen from another flower of the same plant is scarcely 

 more potent. In these cases the pollen from a flower 

 borne by another plant of the same kind is potent, and 

 the more so the more unlike the two plants are. 



In Darwin's trials on the reproduction of the Morning 

 Glory, Ipomea purpurea, carried out through ten gener- 

 ations, the average height of 73 self-fertilized plants was 

 616 inches, while that of the same number of crossed 

 plants was 85.8 inches, or in the ratio of 77 to 100. 

 The relative number of seeds produced by the self-fertil- 

 21 



