316 BOTANICAL GAZETTE [april 



806 pollen visits, and 1155 nectar visits of females and workers, 

 show the following percentages of visits to social flowers with 

 exposed pollen: for nectar 41 . 1 ; for pollen 49. 2. Compared with 

 the visits of the females for nectar, the females when collecting 

 pollen make 8. 1 per cent more visits to social flowers. 



There are some large social inflorescences composed of flowers 

 with exposed or only slightly concealed nectar. Long-tongued 

 bees practically avoid them on their nectar visits, but often visit 

 them for pollen. Such are Comus, Hydrangea, and Viburnum. 

 Vitis, with exposed nectar, seems to be an important source of 

 pollen for female bumblebees. The aggregation of flowers in social 

 clusters has been interpreted as an adaptation for gitonogamy, 

 but it occurs about as often in cases where gitonogamy is impossible. 



Finally, the evolution of entomophilous flowers is held to have 

 proceeded in the following manner. The primitive flowers were 

 non-social flowers of class AB, with partly concealed nectar, 

 adapted to short- tongued bees. These have produced flowers with 

 exposed nectar more favorable to flies, and flowers with more 

 concealed nectar still more favorable to bees. A few have become 

 adapted to flesh flies (Asimina), and others to minute flies (Aristo- 

 lochia) . 



The non-social small bee flowers have produced social forms still 

 favoring small bees., but admitting other short-tongued insects. 

 These finally pass into the extreme social forms which have become 

 modified to suit miscellaneous short-tongued insects. 



The non-social small bee flowers have been modified further 

 and developed into non-social long-tongued bee flowers. Some of 

 these have been appropriated by birds and others by Sphingidae, 



and perhaps still others by butterflies. 



The non-social long-tongued bee flowers have also been modified 

 into social forms attracting Lepidoptera and long-tongued Diptera. 

 These are still considered as bee flowers, but some of them may more 

 properly be regarded as adapted to miscellaneous long-tongued 

 insects. The social long-tongued bee flowers also pass into social 

 short-tongued bee flowers, and finally into social flowers adapted to 

 miscellaneous short-tongued insects. 



Carlixville, III* 



