102 LY THRUM. [CHAP. 



yellow in the shorter ones ; while the filaments are 

 pink in the long stamens, uncoloured in the shorter 

 ones. 



Mr. Darwin has also proved by experiment that 

 this species does not set its seeds, if the visits of 

 insects are prevented ; in a state of nature, however, 

 the plant is much frequented by bees, humble bees, 



FIG. 77. Lythrum sahcnria. 



and flies; which always alight on the upper side of 

 the flowers on the stamens and pistil. Mr. Darwin 

 has shown that perfect fertility can only be obtained 

 by fertilising each form with pollen from pistils of 

 the corresponding length. 



Thus the long-styled form is naturally fertilised by 

 oollen from the long stamens of the two other forms *, 



