HOW PLANTS ARE INFLUENCED 



331 



"natural selection," and finds a close parallel in artifi- 

 cial selection as practiced by man in adapting plants 

 to his needs. All our farm and garden plants have 

 been brought to their present condition largely by 

 artificial selection. 1 



We believe that all the various kinds of plants 

 and animals have reached their present condition by 

 evolution and that evolution is still going on, though 

 at a compara- 

 tively slow rate. 



An examina- 

 tion of plants 

 which live in dry 

 situations shows 

 that they have the 

 following charac- 

 teristics. 2 



(1) Reduced 

 surface, obtained 

 by suppression of 

 leaves (complete 

 or nearly so) . This is accompanied by thickening of 

 stems in the Cacti (Fig. 182), some of which assume 

 a spherical form, which gives a minimum surface. The 

 Switch Plants, such as the Gorse (Fig. 198, a), Cytisus 

 (Fig. 183), etc., assume the form of green, rod-like 

 switches. 



182. A spherical Cactus; a form which presents the 

 minimum surface for evaporation with a maximum 

 bulk for storage. 



1 See Chapter X. 



2 For experiments on this subject, see ^ages 215-218. 



