88 ORIGIN OF THE 



and agriculturists since their youth ; it has, doubtless, 

 only acquired of late years the frightful significance, 

 which seems to threaten the well-being of nations, 

 since the causes, which formerly brought it locally 

 into existence, have spread over whole districts and 

 countries. The writings of HALES bring to our cen- 

 tury from a preceding one the consoling certainty 

 (and this is especially important), that the cause of 

 it is not this decay is not to be looked for in a degeneration 

 generato" f the plant, but depends on the combination of cer- 

 ut * am conditions accidentally coincident; and that 



tnese > when they are well ascertained and kept in 

 causes* 1 y i ew > enable the agriculturist, if not to annihilate* 



at least to diminish, their hurtful influence. 



The potato- The potato-plant obviously belongs to the same 



of a those ne class of plants as the hop-plant, namely, to that class 



ferg C mosf" wm 'ch is most seriously injured by the stagnation 



fr r ese S d P f their juices in consequence of suppressed trans- 



tk>n P ra ~ piration. According to KNIGHT, the tubers are not 



formed by swelling of the proper roots, but by the 



development of a kind of underground stalks or 



runners. He found that when the tubers under 



ground were suppressed, tubers were formed on the 



stalks above ground ; and it is conceivable that every 



external cause which exerts a hurtful influence on 



the healthy condition of the leaves and stalks, must 



Character act in like manner on the tubers. In the districts 



weather in which were most severely visited by the so-called 



i847, a when potato disease in 1846, damp, cold, rainy weather 



followed a series of very hot days ; and in 1847, cold 



vailed. 



