DISEASES OF PLANTS. 121 



table forms. These changes are often inappreciable to 

 our senses and powers of observation, and consequently 

 disease may often be set up in plants which come within 

 our care and notice, without our being able to assign the 

 reason or to suggest the remedy. 



Good cultivation consists in adapting the mode of treat- 

 ment to each particular plant ; and without some knowledge 

 of general principles success is seldom attainable. By 

 cultivation plants are led more and more from their 

 natural habits of growth: in some the conditions are so 

 widely different as to interfere greatly with their vital 

 powers; in others, again, they exert a less debilitating 

 influence. At all times, however, a departure from normal 

 conditions engenders a susceptibility to disease, which may 

 exist in every degree, from'a simple constitutional derange- 

 ment up to such an aggravated form as to be fatal to the 

 plant. In the first stages altered conditions may be able 

 to arrest the progress of the malady, and it is here that 

 the observation and knowledge of the cultivator is called 

 into play to apply the proper remedy. In a state of 

 nature, when disease is once set up there is but little pro- 

 bability of amendment, as the plants are confined to the 

 spot on which they grow, and there is 110 power of selec- 

 tion. The air is imbibed by the leaves, whatever be its 

 quality, and the moisture and inorganic matter by the roots, 

 in whatever state they may be, and however they may 

 be deficient in the constituents necessary for the healthy 

 nutrition of the plant; and if these are not suitable to 

 the nature and requirements of the plants, we have at onco 

 the elements of internal disease. 



Diseases from external causes vary very much in their 

 origin, and in their mode of exhibiting themselves. In 

 some places it is not an unusual thing to see the clover 

 fields marked in various places by large circular patches, 

 on which the plant is entirely obliterated, though growing 



VOL. ii. 41 



