77 



parts of this by transpiration in the 

 air. I do not perceive what should cause 

 roots to transpire when surrounded by 

 moisture, or if they do, they must return, 

 like the dog to his vomit, and again absorb 

 their own transpirations. 



If it were owing to the poisonous ex- 

 cretions of the roots that the same crops 

 cannot be taken year after year from the 

 same land, this cause would apply equally 

 to all lands ; but some soils are known 

 which return the same crops incessantly. 



It is perhaps possible that the reason 

 why each plant appears to have its 

 favourite soil is, that it finds there in the 

 greatest abundance the particular inor- 

 ganic matters adapted to its peculiar struc- 

 ture ; that the reason why particular 

 plants will not grow in particular soils is, 

 the absence of the particular inorganic 

 matters adapted to their peculiar consti- 

 tution ; and that the reason why parti- 

 cular plants cease to grow on particular 



