RELATION OF THE DISEASE TO EXTERNAL CAUSES. 49 



when they arrived at London, that they were only applica- 

 ble for manure. 



(188.) To preserve potatoes, especially when they are 

 suspected to have a predisposition to the gangrene, it is 

 almost essential that they should be kept in a cold place. 



(189.) An improper temperature may then cause death 

 in the potatoe, though it is not to be assumed that it has 

 produced the present malady. Excessive heat, under 

 certain circumstances, materially influences the progress 

 of the disease, and hastens the death of the plant and its 

 consequent putrefaction. 



LIGHT. 



(190.) Every plant requires a certain amount of light ; 

 many plants are killed by a strong light, and others will 

 die in the shade. 



(191.) Absence of light has been assumed as a cause 

 of the disease, but the examination of the last three years 

 shows that it cannot be assigned either to a deficiency or 

 superabundance of this agent. 



(192.) The action of light upon the tubers is supposed 

 to mature them, and to render them especially fit for plant- 

 ing. Potatoes when exposed to the sun's light become 

 green ; in fact, they become above-ground, instead of 

 under-ground stems. 



(193.) Potatoes required for keeping should be protected 

 from the influence of light, being thus less prone to grow 

 or sprout, as it is termed. In fact, light favors all changes. 

 Absence of light may cause the death of the potatoe, but it 

 cannot be said to have produced the present epidemic. 



3 



