420 



LESSOJVS WITS PLANTS 



will do it. More heat, less heat, more food, less 

 food, more water, training, growing under glass or 

 in the shade or in the sun,— these and other factors 



which the horticultur- 

 ist has at his control 

 cause, or at least 

 bring out, differences 

 in plants ; or he can 

 ^lll produce differences by 

 crossing (Obs. xlvi). 

 The variation is the 

 start ; selection does 

 the rest. 



535. The wild dah- 

 lia and a cultivated 

 variety are shown in 

 Fig. 430. How was 

 it done? The wild 

 kind was cultivated. 

 It got better care and' 

 more food than it 

 had in its native 

 Mexican home, and 

 all the conditions of its life were changed. It 

 began to vary. Just what factor or set of factors 

 caused these variations we shall never know ; but 

 one variation produced an abnormal number of 

 rays (probably because of more food supply and 



Fig. 430. 



Forms of the wild and cultivated dahlia, 

 Half natural size. 



