THE SOLVENT ACTION OF ROOTS 57 



is not necessary to assume the existence of an excretion from 

 the roots of the plant of a permanent acid, organic or in- 

 organic, to attack the solid mineral particles of the soil and to 

 bring them into solution for the nutrition of the plant. The 

 growing portions of a plant root are always giving off carbon 

 dioxide, and carbon dioxide, especially in the concentrated 

 solution which must be momentarily formed in the cell wall 

 of the root hairs, has an appreciable solvent effect upon the 

 majority of the minerals composing the soil. This carbon 

 dioxide alone is capable of giving rise to such solutions as are 

 required for the nutrition of the plant. As the direct evidence 

 is also adverse to the idea of an excretion of acid, the principle 

 of not seeking remote causes would lead us to attribute to 

 carbon dioxide, and to carbon dioxide only, the long-recognisec? 

 solvent power of the plant upon the soil. 



