52 ELEMENTARY BOTANY. 



contact with long and short pistils respectively, the pollen of 

 one flower would in each case be applied to the stigma of an- 

 other flower ; or, in other words, cross-pollination would neces- 

 sarily result. It is found, besides, that the pollen grains of the 

 two sets of stamens are of different sizes, and each less active 

 upon its own stigma than upon the stigma of another flower. 

 In some genera three sets of flowers with stamens and pistils 

 of differing lengths exist (trimorphism), evidently designed for 

 intercrossing. 



10. There are other adaptations for cross-pollination besides 

 dichogamy and dimorphism. . An interesting case is furnished 

 by papilionaceous flowers ; for example, the Pea (Figs. 79-81). 



Fig. 80. Pig. 81. 



The ten stamens and single pistil are enclosed within the 

 keel. There are hairs on the style below the stigma, and these 

 loosely retain the pollen which is discharged early by the an- 

 thers, the latter remaining in the keel. When a bee alights 

 on the wings and keel of the flower, they are together pressed 

 downward, and the pistil protrudes in consequence. The stigma 

 strikes the abdomen of the bee, and the style also brushes 

 against it. When the bee visits the next flower, the stigma 

 of that one strikes the abdomen as before, but it has been 

 dusted with pollen from the previous flower, and of course a 

 portion of it is retained by the stigma, thereby effecting cross- 

 pollination. In like manner pollen from that flower is car- 

 ried to the next, and so on. 



