88 Flowers and their Pedigrees. 



mode of growth of the plant, and partly on the 

 amount of available material. Thus, in the allied 

 tormentil the lower leaves have five leaflets, but the 

 upper ones have usually three only. In the spring 

 potentilla, a rare English species, the lower leaves 

 have seven or five, and the upper ones five or three. 

 Again, where a species creeps along the ground, it is 

 apt to have long pinnate leaves with many leaflets, as 

 happens, for example, with silverweed and many 

 similar plants. But where the leaves grow habitually 

 among tall grass or choking wayside weeds, the 

 number of leaflets is very apt to be reduced to three, 

 as happens, for example, with clover and lotus among 

 the peaflower tribe, and with wood-sorrel among the 

 geranium tribe, many of whose allies have long pin- 

 nate leaves with numerous leaflets. Now, the straw- 

 berry and the barren strawberry differ conspicuously 

 in habitat from the other potentillas in the fact that 

 they grow maftily among grass, on banks, or in hedge- 

 row thickets. Hence it suits them best to raise their 

 trefoil leaves on tall stalks above the neighbouring 

 herbage, and thus to get at the light and air which 

 they require for their proper growth. Natural selec- 

 tion has easily brought about this result, because in 

 such situations those potentillas which raised their 

 leaves highest would best survive, while those which 



