THEORY OF THE DISEASE. 113 



a far more destructive influence upon the cultivated, than 

 upon the wild plant. 



(448.) In plants which are propagated by the mere 

 extension of the individual, as the potatoe is, the injury is 

 to a great extent continued ; because, when the relation 

 between the leaf and root, between the fluid or sap and the 

 cellular tissue or solid parts, becomes deranged, the plant 

 will propagate the same diseased relation, and consequent- 

 ly reproduce the same injury. 



(449.) The propagation of the injury from set to plant 

 gives rise to all those phenomena which I have already 

 described when treating of the potatoe gangrene, and is 

 nothing more than a propagation of the mischief from one 

 part of the plant to another. 



(450.) In all human probability, the disease, under 

 favorable circumstances, if the cause be not continued, will 

 at last wear itself out and cease ; but we have not at 

 present had sufficient experience to ascertain this fact with 

 certainty. Of course, cessation of the disease will almost 

 absolutely require that the vastator shall not again renew 

 its attacks upon the plant. 



(451.) There is an exquisite relation existing between 

 the effects upon the plant and the welfare of the animal. 

 This creature cannot well live upon a very vigorous plant, 

 because it would be drowned by the water transpired at 

 night. Hence it generally commences upon leaves which 

 have in a great degree lost their vigor. On placing 

 insects upon the new leaves of very vigorous plants, I 

 have observed that the creature has always been obliged 

 to leave them. It commences upon the larger and nearly 

 exhausted leaves ; from these it passes to others, and so on 

 till the entire foliage is affected. 



