238 



VARIATION AND ITS RESULTS 



peaches may be borne on nectarine trees; russet apples 

 are sometimes borne on Greening apple trees; white roses 

 are sometimes found on red-flowered plants. 



403. Frequently a plant begins a new kind of varia- 

 tion long after germination, even after it has become well 



established. It is on 

 this fact that success- 

 ful agriculture de- 

 pends, for the farmer 

 makes his plants 

 better by giving them 

 better nutrition and 

 care: and betterment 

 (like deterioration) is 

 only a variation as 

 compared with the 

 average plant. Plants 

 that start to all 

 appearances equal 

 may end unequal: 

 some may be tail and 



vigorous, others may be weak, others may be dwarf: some 



will be worth harvesting and some will not. 



404. The Causes of Variations. — Variations are induced 

 by several, and perhaps many, causes. One class of origin 

 lies in the environment, and another lies in the tendencies 

 derived from parents. Of the environmental causes of vari- 

 ation, the chief is probably food-supply. Good agriculture 

 consists largely in increasing the food-supply for plants — by 

 giving each plant abundant room, keeping out competing 

 plants, tilling the soil, adding fertilizers. (Fig. 406.) Another 

 strong environmental factor is climate (Chapter XXIX). 

 It is very difficult to determine the exact reasons for any 

 variation. There is much difference of opinion respecting 

 the causes of variation in general, The extent of variation 



406. Variation. — Big and little redroot pigweeds 

 of the same kind. 



