CROSS-FERTILIZA TION. 81 



dioecious. In some of the maples there are 

 staminate and pistillate and j^erfect flowers 

 on the same plant ; they are polygamous. 



The pollen grains vary widely in size and 

 form. Some of the forms are shown in Fig. 

 67, which represents respectively the pollen 



Fig. 67. (After Gray.) 



of the musk-plant, wild cucumber (Echino- 

 cystis), mallow, lily, chicory, pine, and even- 

 ing primrose. 



Cross-Fertilizatio7i. 

 How is the pollen transferred from the 

 anther to the stigma ? In the perfect flow- 

 ers, where the stamens and pistils are placed 

 almost in contact, we can readily imagine 

 how such transfer could take place, but how 

 is it performed in the monoecious and dioe- 

 cious plants ? And if we were to examine 

 critically the perfect flowers, we should find 

 that even there this transfer is not a simple 



