452 THE ANIMALS AND MAN 



take place; pollination being the bringing and applying 

 of ripe pollen-grains to the ripe surface of the stigma. How 

 fertilization then takes place is succinctly explained by 

 fig. 235 and its caption, which are copied from Stevens 

 (''Introduction to Botany," Boston, 1902). 



The advantage of cross-pollination, as first experimentally 

 proved by Darwin and since then confirmed by other ex- 

 perimenters and by hosts of plant breeders, lies in the fact 

 that the seeds from cross-pollinated plants usually produce 

 stronger plants than are produced by the seeds of self- 

 pollinated plants. To effect cross-pollination plants have 

 developed two kinds of means; first, means to attract insects 

 and humming-birds, and to insure cross-pollination as the 

 result of their visits; second, means to prevent self-pollina- 

 tion. Also insects that visit flowers have developed certain 

 peculiarities of structure and habit which correspond to the 

 special character of the flowers chiefly visited by them. 

 These reciprocal modifications of flowers and insects have 

 gone so far in some cases that certain plants and certain 

 kinds of insects cannot exist apart from each other. Many 

 flowers are not fertile when fertilized by their own pollen 

 and these may have no other possible means of getting 

 pollen from other plants except by insect visits. 



The principal means which have been developed to avoid 

 self-fertilization are the following: first, having flowers 

 either only staminate or pistillate; these different flowers 

 may occur on the same plant individual or only on separate 

 individuals; second, having both pistils and stamens on 

 each flower but with the pollen not ripe at the same time as 

 the stigma; third, having the stamens and pistils in the same 

 flower of different length or having them so situated that 

 the pollen will be unlikely to fall on the stigmas. 



The principal means for insuring cross-pollination are: 

 first, the secretion of nectar to feed insects; second, the 

 development of odor, color, pattern and shape to guide 



