RELATION OF THE DISEASE TO EXTERNAL CAUSES. 47 



of nature, become more or less affected with the malady, 

 and die either locally or generally. 



(179.) The above statements also show that early kinds, 

 which are ripe before August, are generally less affected 

 by the disease. 



CHAPTER VII. 



RELATION OF THE DISEASE TO EXTERNAL CAUSES, TEMPERA- 

 TURE, LIGHT, ELECTRICITY, ETC. 



Gangrene not the result of any internal cause (180). Effect of tern- . 

 perature upon plants generally (181). Potatoe thrives in various 

 climates (182). Cold (184). Influence of temperature (185) ; 

 upon the decay of tubers (187). Effect of light upon plants 

 (190). Effects of light upon tubers (193). Influence of electri- 

 city (194). Influence of atmospheric moisture on plants (195). 

 Excessive moisture a cause of disease (196) ; its influence on the 

 disease (197); effect on the tubers (199); other atmospheric 

 agents (200). Effect of winds (201). 



(180.) As we cannot explain the gangrene by the sup- 

 position of its origin from internal causes, and as, on the 

 contrary, facts forbid us to assign any such causes for the 

 malady, we are led in the next place to inquire into the 

 effects of external agents upon the plant. 



(181.) Every plant' requires a certain temperature, or 

 rather a certain range of temperature, which is indispensa- 

 ble to its existence. The plant of the frigid region soon 

 grows itself to death in the torrid region, and the plants of 

 the torrid regions soon cease to grow in the arctic. 



