CH. VIII.] THE DISEASES OF PLANTS. 297 



If a pelargonium, or any other plant, be placed in a 



highly stimulating heat, and is abundantly supplied 



with root-moisture, it immediately increases its sur- 



face of leaf to elaborate and digest the large amount 



of sap forwarded from the roots. If this amount of 



sap is subsequently suddenly reduced, by lowering the 



temperature and adding water to the soil less freely. 



the increased suiface of leaf is no longer required, 



and it is a law pervading all the vegetable creation, 



that the moment any of the parts of a plant are 



unnecessaiy to it, that moment it begins to decay. 



I placed a plant of the Manuel of Peru, or Heliotrope, 



in a high temperature, and supplied it abundantly 



"\nth water until its leaves were much increased in 



size ; the temperature and moisture were then much 



reduced, and the leaves in a few days were completely 



decayed round their edges and in spots upon their 



surfaces. The extent of leaf was accommodated to the 



amount of sap to be elaborated. 



The tubers of the potato also are liable to the 

 speck, black spot, or diy gangrene ; a disease which 

 I once thought was occasioned by the calcareous 

 earth (lime, or chalk) contained by the soil, but more 

 lengthened observation has con\dnced me of my 

 error ; and having obsen^ed it in all soils, and in 

 seasons characterized by opposite extremes of wet- 

 ness and dryness, I am induced to consider that the 

 disease arises from some defect in the sets employed ; 



