now PLA.XTS ABE IXFLCEXCED 



"natural selectiou," 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 .' 



"We believe that all the various kinds of plants 

 and animals have reached their present condition by 

 evohition 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- 

 teristic s.- 



(1) Eeduced 

 surface, obtained 

 by suppression (;f 

 leaves (complete ^'■'^^ '""^ storage. 



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. 



1 See Chapter X. 



2 For experiment on this subject, see pages 215-218. 



1^2. A spherical Cactus; a form Tvhicli presents the 

 mimmuni surface for eraporatiou with a maximum 



