226 SCIENCE REMAKING THE WORLD 



Varieties are known to have progressively lost their 

 vigour and entirely run out in certain regions. We 

 know now that many cases of degeneracy are due to 

 slow diseases and not to old age. Symptoms that were 

 once attributed to senility are now recognized as 

 typical symptoms of "Mosaic" and other virus diseases. 

 An extreme view holds that a given variety would re- 

 main vigorous and live indefinitely if grown continu- 

 ously under favourable conditions. According to this 

 view any degeneracy of a variety not due to apparent 

 disease simple means that it is unable to cope with the 

 adversities of its environment. This is undoubtedly 

 often the case because varieties have been grown for 

 many years in some places without loss of vigour, but 

 gradually degenerate in another region. Potatoes for the 

 spring planting in the southern states are usually im- 

 ported from the north, as these varieties degenerate 

 when the seed stock is home-grown. Even in the 

 more northern states it is often necessary to obtain the 

 seed tubers of some varieties from other regions in 

 order to maintain the normal vigour of the crop and 

 certain desired characters of the tubers. Lack of 

 adaptation to environment can be argued against any 

 claims for an inherent tendency to old age in potato 

 varieties, so we may still ask the question, does our 

 giant potato plant naturally grow old ? 



SEED STOCK. Modern potato research is showing a 

 very decided trend toward studies of the factors in- 

 fluencing the vitality of the seed stock and of practical 

 methods for maintaining the normal vigour of the 

 variety. The potato plant is grown from a specialized 



