ECT. V.j THE ROOT. 211 



the veil, remove much of the mystery with which, 

 to ordinary observers, the subject appears to be 

 enveloped. An investigation of this kind is as 

 useful to the physician as to the botanist and agri- 

 culturist; for, many of the exciting causes of 

 diseases, particularly of epidemics, are to be 

 looked for in the nature of the soil and other local 

 circumstances connected with the situations where 

 they originate. 



The fact cannot be too often repeated and im- 

 pressed on your minds, that plants are living 

 beings, possessed of powers which enable them 

 to convert into their own material substance, 

 matters of a nature apparently very different from 

 it. Without keeping this in view, we should be 

 forced to look for all the different productions of 

 plants ready formed in the soil where they grow, 

 and to suppose that these are simply taken up by 

 their roots, and deposited in the different parts of 

 the plant : an idea too incongruous to be admitted. 

 On the contrary, they do not even take up those 

 principles which are most abundant in the soil 

 where they grow; bat select peculiar parts of 

 them, although these are not found, in general, 

 forming in their uncombined state any part of the 

 vegetable frame. Linnaeus himself, however, I 

 believe, and many others, have imagined, that 

 every soil held in it something which is peculiarly 

 the proper food of every kind of plant that can be 



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