10 THE POTATO 



ration is normal. The minimum, optimum, and max- 

 imum temperatures have been ascertained for some 

 plants. Young wheat plants will respire at as low a 

 temperature as 28 F., or below freezing-point. The 

 optimum temperature for wheat is about 104 F., 

 while that of potato plants is about 113 F. The 

 maximum for wheat is 113 F., while that for pota- 

 toes is about 131 F-k In other words, the potato 

 respires best at about 113 F., but should the temper- 

 ature go above 131 F., the respiration will be some- 

 what less than before, and the vitality weakened ; 

 hence, after a hot spell, when the temperature exceeds 

 the maximum for respiration, it is noticeable that the 

 potatoes fail and become more susceptible to the blight 

 or other troubles, owing to their impaired constitu- 

 tion. By selection we might procure plants of greater 

 vitality, capable of standing the higher temperatures, 

 which would enable them to be better ' ' disease-resist- 

 ers." Present-day potatoes thrive best in a cool 

 climate. 



Influence of Temperature on Growth. The 

 minimum temperature for germination of potato tubers 

 is about 50 F. ; hence, in the Northern States early 

 planted tubers make little or no growth unless planted 

 shallow, and this is not desirable, except, perhaps, for 

 the earliest varieties. It is better to germinate the 

 tubers in the barn before planting, thus saving time 

 (see Chapter VI., " Sprouting Potatoes ") 



Potato Roots. Generally speaking, far more at- 

 tention has been paid to the stems and leaves of plants 

 than the roots, yet in order to cultivate the soil in a 

 rational manner it is essential to know where the roots 



