PROPAGATION OF PLANTS. 215 



variation amono- plants seem to he : ( i ) differ- 

 ence in food supply ; (2 ) climatic conditions ; 

 (3) sexual reproduction. 



None of these causes would be of any avail 

 were it not for the fact that selection preserves 

 and accumulates all variations which are benefi- 

 cial, and discards those which are detrimental to 

 the orocinism. 



(i) Difference in Food SurrLV. — Every one 

 has noticed in different fields of grain, or even 

 in the same field, that in some portion the plants 

 were sickly and stunted, while in others they 

 were strong- and well developed. One of the 

 many conditions which may cause tJiis valuation 

 (in development) is a difference in the supply 

 of proper food. 



This lack of the necessary constituents for the 

 growth of this particular plant may be due to 

 the exhaustion of these elements by former 

 crops, or to the poorness or thinness of the soil. 

 (See Chapter VII.) 



(2) Climatic Conditions. — Variation in cli- 

 mate tends to modify the structure and habits 

 of plants, their fruitfulness, and the color and 

 flavor of their fruit. On approaching colder 

 climates plants become smaller and more thickly 

 set with leaves, as is illustrated by the same 

 species growing at the base and at the summit 

 of a mountc.in (Figs. 53, 54), as the spruce and 

 fir of the Rockies. "^ 



