10 



BURSA BURSA-PASTORIS AND BURSA HEEGERI 



The flowers are adapted to both cross- and self-fertilization, all the evi- 

 dence now at hand indicating- that the latter method is normally by far the 

 more efficient. Leaving out of account 2 of the 21 original cultures which 

 showed by their constitution that their parents were hybrids, less than 1 

 per cent of the plants raised from seed collected in nature showed evidence 

 of being the result of cross-fertilization between different biotypes . Crosses 

 between different flowers of the same plant and between different indi- 

 viduals of the same biotype may take place somewhat more frequently than 

 crosses between the flowers of plants belonging to different biotypes, 

 though nothing in the resultant offspring gives any clue to the frequency 

 of such crosses. 



Fig. 1. — Enlarged sections of buds and flower of Bu?-sa bursa pastoris, show- 

 ing three stages in anthesis. A. Exposure of the stigmatic surface for the 

 reception of foreian pollen. B. Anthers opening in contact with the stigma, 

 thus insuring self-pollination. C. The flower fully open, allowing the access 

 of the visiting insects to the pollen. All magnified 20 diameters. 



The conditions which favor cross-fertilization are : (a) Slight prote- 

 rogyn}', which allows the stigina to receive foreign pollen some hours before 

 the anthers of the same flowers dehisce. The distal portion of the globular 

 or disk-shaped stigma is exposed between the tips of the unopened sepals and 

 petals (fig. 1 , a) . Qy) Although the fully developed inflorescence is a typical 

 raceme, the flowers and buds are arranged in a nearly flat-topped corymb 

 having the flowers at the circumference, giving this part of the inflores- 

 cence a condition quite analog'ous to that of the head of the Compositae, in 

 which the whole inflorescence appears to act the part of a single flower in 

 the attraction of insects. Small insects, particularh- flies and small bees, 

 visit the flowers freely. These rest upon the top of the inflorescence as a 

 whole while visiting the several individual open flowers about the circum- 

 ference. In these fully open flowers forming the exterior circle of the inflo- 

 rescence the anthers have opened, while in the second circle the summits 

 of the stigmas are exposed on the same general level as the rest of the 

 corymb. As the insect walks about over the top of the inflorescence, the 



