HOW PLANTS ARE INFLUENCED 355 



more cold than others (just as some kinds stand more 

 water, light or alkali than others). Individual plants 

 vary considerably in this respect : after a frost which 

 kills most plants of a particular species it is often pos- 

 sible to find one or two individuals which have sur- 

 vived, not because of a more sheltered position but by 

 reason of their greater hardiness. By preserving the 

 seed of such plants and selecting seed from the hardi- 

 est plants year after year, hardy varieties may be 

 obtained. 



Of equal importance with hardiness in securing va- 

 rieties for cold regions is the ability to ripen quickly 

 (earliness). This may also be obtained by selection, 

 as has happened in the case of Wheat in the United 

 States; as a result the Wheat belt has moved steadily 

 northward, especially in the last fifteen years. We 

 have a record of one case in which a winter Wheat was 

 in three years converted into a summer Wheat in this 

 manner: conversely, in three years a summer Wheat 

 was converted into a winter Wheat. The same thing 

 is even more strikingly true of Corn, which on the 

 plains of Santa Fe in South America requires six 

 months to ripen, while in the Rainy Lake district of 

 Lake Superior it ripens in two months and a half. 

 This great change has all been accomplished by man. 



It has been found repeatedly in experiments that 

 cuttings (of the same kind of plant) taken from a 

 northern region and placed side by side with those of 



