172 GENERAL BOTANY 



are depressed at the same time by a visiting bee. The anthers 

 in the locust remain within the keel during insect visitation 

 and finally wither away and disappear. When an insect visits 

 a locust flower, it alights upon the keel petals and the wings, 

 as in other flowers of the pea family described above. The 

 weight of the bee depresses the keel and wing petals, thus pro- 

 jecting the stigma and the pollen-laden stylar brush against the 

 hairy portion of the insect's abdomen. By this act the abdomen 

 is touched first by the stigmatic surface and later by the pollen 

 of the stylar brush. The stigmatic surface is thus protected 

 from its own pollen, while the insect's body is well dusted 

 with pollen grains. When the insect visits another flower, he 

 is quite certain to dust the stigma of the second blossom with 

 the pollen of the first. If a succession of flowers is visited, as 

 is nearly always the case, the above operation is repeated over 

 and over again, thus insuring abundant crosses both on the 

 same and on different plants. 



On account of the large number of inflorescences on the same 

 tree in the locust, bees are more likely to pollinate flowers on 

 the same tree than on two different trees. Where groups of 

 locusts grow in proximity, however, the more effective cross- 

 pollinations between flowers on separate plants is almost certain 

 to be accomplished. 



The common red clover (Trifolium pratense') is another 

 example, selected to illustrate a definite mechanism in a flower 

 of the pea tribe for insuring against self-pollination by insects. 

 Darwin states that one hundred flower heads on plants of 

 red clover, protected by a net, did not produce a single seed, 

 while one hundred heads on plants growing outside, which 

 were visited by bees, yielded approximately 2720 seeds. This 

 species is thus largely, if not entirely, dependent upon either 

 close-pollination or cross-pollination for the production of seed, 

 and thus for self-preservation and dissemination. The general 

 structure of the flower of the red clover is similar to that of 

 the garden pea and locust already described, and the parts are 

 similarly arranged to facilitate the dusting of the abdomen of 

 visiting insects with pollen. 



