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air, — is necessary for the generation of 

 carbonic acid from the humus in the soil ; 

 and that from carbonic acid plants assimi- 

 late their carbon (which, loosely speaking, 

 the whole of the tree may be said to 

 consist of), by decomposing the carbonic 

 acid, and giving off the oxygen. I should 

 consider this as the cause why roots keep 

 within the reach of atmospheric aera- 

 tion, since the main article of the food of 

 trees is found in that district ; though 

 Liebig follows Priestly, Senebier, and De 

 Saussure in thinking that after the first 

 infancy of the plant, that is, after the de- 

 velopment of leaves, it is indebted to the 

 atmosphere only for the supply of carbonic 

 acid. But can we doubt that the chief 

 growth of plants (I speak of dicotyle- 

 donous forest trees) is from constituents 

 absorbed from the soil, not from the 

 atmosphere, when we see the perpetual 

 difference of growth of the same plants in 

 the different soils of the same parish ; that 



