AS ILLUSTRATED BY THE POTATO MUKEALV. 183 



an express intimation that the condition of a plant as an object 

 of cultivation and its disposition to disease are perfectly iden- 

 tical. Both are alteratious of cliemical action in the plant, pro- 

 duced through the conditions under which it grows. All our 

 cultivated plants, with very few exceptions, may be regarded as 

 diseased ; that is, as deviations from the normal process of for- 

 mation of the species, and it is only the egotism of man which 

 thiniis otJierwise of them, inasmuch as he finds his profit in these 

 diseases as in tlie artificial enlargement of the liver in the Stras- 

 burg goose. The greater part, indeed, if not all the peculiar 

 internal diseases to which plants as well as men are subject, arise 

 from an improper, deficient, or more frequently too abundant, 

 nourishment. In order, however, not to deviate too widely from 

 the common mode of speech, I shall distinguish that condition of 

 cultivated plants in wliich they deviate from the normal form 

 which they exhibit when wild, in consequence of too abundant 

 nourisliment as a general inward tendency to disease. 



This tendency, however, becomes specific when they belong 

 naturally to a very light or sandy soil, as oats or potatoes, and 

 are now cultivated on heavy ground, or if, in consequence of tlie 

 plan of cultivation which was once imiversal, they are grown in 

 the first year in wliich the land is manured, as wheat, rye, 

 potatoes ; or finally, if the climate in which they are cultivated 

 deviates greatly from their natural place of growth, as is the cast 

 in wheat, maize, and potatoes. Under these circumstances very 

 slight prejudicial influences are requisite, as for instance wet, 

 cold, or extreme heat at an improper time, to cause the appear- 

 ance of disease. 



The only point which man has in his power is the avoidance 

 of unsuitable soil and site in the cultivation of a plant, which 

 will, however, be exercised by every intelligent agriculturist. 

 Climate is beyond our power, and we cannot increase the general 

 disposition to disease without at the same time hazarding the 

 total loss of any particular species. The outer appearances of 

 the diseases of vegetables are well known, and their specification 

 belongs to the pathology of plants. Tiie inward phenomena, as 

 far as they liave at present been examined, possess something 

 general, which deserves a place here, since it points decidedly to 

 the fact that the general tendency to disease is due to the influ- 

 ence of artificial culture. 



As the physiology of plants turns on the investigation of the 

 life of a single cell, if it is to be worked out with any degree of 



every country the most fearful difficulties arise, -when a harvest fails on which 

 dependence was placed. The loss is however entirely free from evil 

 conse(iueuces where it has been calculated and provided for beforehand. 



