INTERRELATIONS OF PLANTS AND ANIMALS 45 



or pistils. Again, in some cases, imperfect flowers having stamens 

 only are alone found on one plant, while those flowers having 

 pistils only are found on another plant of the same kind. In such 

 flowers, cross-pollination must of necessity follow. Many of our 

 common trees are examples. 



Other Cases. - The stamens and pistil ripen at different times 

 in some flowers. The " Lady Washington' geranium, a common 





A B 



The flower of ' Lady Washington " geranium, in which stamens and pistil ripen 

 at different times, thus insuring cross-pollination. A, flower with ripe 

 stamens; B, flower with stamens withered and rip3 pistil. 



house plant, shows this condition. Here also cross-pollination must 

 take place if seeds are to be formed. 



Summary. - - If we now collect our observations upon flowers 

 with a view to making a summary of the different devices flowers 

 have assumed to prevent self-pollination and to secure cross- 

 pollination, we find that they are as follows : - 



(1) The stamens and pistils may be found in separate flowers, 

 either on the same or on different plants. 



(2) The stamens may produce pollen before the pistil is ready to 

 receive it, or vice versa. 



(3) The stamens and pistils may be so placed with reference to each 

 other that pollination can be brought about only by outside assistance. 



