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CHAPTER XXI. 



o:n5 the diseases of plants, particularly as 

 illustrative of their vital principle. 



The diseases of Vegetables serve in many instances 

 to prove their vitality^ and to illustrate the nature of 

 their constitution. 



Plants are subject to Gangrene or Sphacelus^ espe- 

 cially the more succulent kinds, of which a very 

 curious account, concerning the Cactus cocci?7elli/er, 

 Indian Fig, or Nopal, extremely to our present pur- 

 pose, is given by M. Thiery de Menonville, in his 

 work on the culture of the Nopal as the food of the 

 Cochineal insect. This writer travelled about 30 years 

 since, through the Spanish settlements in South Ame- 

 rica, chiefly noted for the cultivation of this precious 

 insect, on purpose to transport it clandestinely to some 

 of the French islands. Such were the supineness and 

 ignorance of the Spaniards, that he succeeded in con- 

 veying, not only the living insects, but the bulky plant 

 necessary for their sustenance, notwithstanding severe 

 edicts to the contrary. He had attended previously to 

 the management of the Nopal, and made his remarks 

 on the diseases to which it is liable. Of these the 

 Gangrene is extremely frequent in the true Nopal of 

 Mexico, beginning by a black spot, which spreads till 



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