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CHAPTER XXI. 



ON' rilB DISEASES OP PLANTS, PARTICULARLY AS ILLUSTRA- 

 TIVE 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, especial- 

 ly the more succulent kinds, of which a very curious ac- 

 count, concerning the Cactus coccinellifer^ Indian Fig, 

 or Nopal, extremely to our present purpose, 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 twenty years since, through the 

 Spanish settlements in South America, 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 Span- 

 iards, that he succeeded in conveying not only the liv- 

 ing insects, but the bulky plant necessary for their sus- 

 tenance, notwithstanding severe edicts to the contrary. 

 He had attended previously to the management of the 

 Nopal, and made his remarks crn the diseases to which it 

 is liable. Of these the Gangrene is extremely frequent 

 in the true Nopal of Mexico, beginning by a black spoty 

 which spreads till the whole leaf or branch rots off, or 

 the shrub dies. But the same kind of plant is often af- 

 fected with a much more serious disease, called by Thie- 



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