5i8 



General Botany 



Insects and flowers. The increasing importance of the angio- 

 sperms among plants of the Tertiary was paralleled among 



U.S. Forest Service 



Fig. 324. Black willow {Salix nigra): C, vegetative branch; A, branch with spike of 

 pistillate flowers ; B, spike of staminate flowers. 



animals by the increasing number and diversity of insects. 

 This is of botanical interest because flowers and insects seem to 

 have reacted on each other ; and there have come to be numbers 

 of insects that are dependent upon certain plants, and likewise 

 many plants whose pollination is effected only by certain insects. 

 Like the gymnosperms, the early angiosperms seem to have been 

 mostly wind pollinated, while the later and more specialized 

 angiosperms are largely pollinated by insects. 



The life history. The details of the life history of the angio- 

 sperms are somewhat variable in different orders and families. 

 We have already given a general account of floral structures, 

 pollination, fertflization, and seeds in Chapters XXV to XXVIII 

 (pages 232-271), and the first half of the book is concerned with 

 the vegetative structures and processes of the angiosperms. It 

 may be well, however, to repeat the life history in the same terms 

 that we have used to describe those of the preceding plant 



