CROSS POLLINATION 65 



brush off some of this pollen upon the parts of the flower with 

 which it comes in contact. Because of its position and because 

 of the character of the stigmatic surface, the stigma will be 

 most likely to receive and retain some of this pollen, and thus 

 pollination will be accomplished. 



142. The question now arises, is not pollination by wind or 

 by insects as likely to result in self-fertilization as in cross- 

 fertilization? The study of further facts will lead us to answer 

 this question emphatically in the negative. The facts are these : 

 In the case of anemophilous plants the flowers are either dioe- 

 cious, monoecious, or dichogamous. In the first case, of course, 

 self-fertilization cannot occur. In the case of monoecious 

 plants the staminate and pistillate flowers are not on the same 

 level, consequently the pollen floating horizontally on the wind 

 is not likely to fall upon any of the pistillate flowers of the same 

 twig. If the flowers are hermaphrodite they are also usually 

 dichogamous, which means that either the andrcecium or the 

 gyncecium matures first and hence the pollen cannot fertilize 

 an ovule of the same flower. 



143. Entomophilous hermaphrodite flowers are usually 

 either dichogamous, dimorphic or else by movements of stamen 

 and pistil a result is brought about which is practically equiva- 

 lent to that attained by dichogamy. In the case of dimorphic 

 flowers there are two kinds of flowers which differ with respect 

 to the length of the style and stamens, and the position of the 

 stigma in one form of flower corresponds to the position of the 

 anther in the other. The result of this is that when an insect 

 visits the flower it receives pollen on that part of its body with 

 which the stigma comes in contact when the insect visits a 

 flower of the other type. 



144. Self-fertilization is also known to occur in many herma- 

 phrodite flowers. However, it takes place, usually, only after 

 the methods for securing cross-fertilization have been employed 

 by the flower; the result being to insure fertilization in case 



5 



