246 



EVOLUTION OF PLANTS 



time, by further modifications of their descendants, 

 other adaptations for cross-pollination would arise. 



There are many genera, especially among the lower 

 Dicotyledons, which exhibit in a most interesting way 

 all gradations between inconspicuous self-pollinated 

 flowers, and showy ones dependent upon insects. This 



is shown, for example, 



A /-,$7\ . in the genus Ranun- 



culus, which includes 

 the various species of 

 buttercup. The in- 

 conspicuous R. abor- 

 tivus is entirely in- 

 dependent of insect 

 aid, while such showy 

 species as R. acris or 

 R. Californicus are 

 visited freely by in- 

 sects, although they 

 are probably not en- 

 tirely dependent upon 

 them to insure fertili- 

 zation. Similar vari- 

 ation is found in the 

 genus Geranium. 



In the simplest of these " entomophilous " or insect- 

 fertilized flowers, such as the buttercup or anemone 

 (Fig. 55, A), the flower is wide open, with the entirely 

 separate parts arranged radially, and often indefinite 

 in number. We find in such generalized flowers that 

 the variety of insects visiting them is large, and they 

 are seldom incapable of self-pollination in case insect 



FIG. 55. A, flower of Anemone coronuria, 

 the petals absent, but replaced by the 

 showy sepals, s ; B, inflorescence of the 

 dogwood (Cornus florida), the incon- 

 spicuous flowers, .#, surrounded by four 

 showy bracts, &; C, the "calla-lily" 

 (Richardia), with the central spike of 

 small flowers enclosed by the large 

 white "spathe," sp. 



