ITS USE EXPLAINED. 59 



lopment of the fruit. Substances, containing a large 

 proportion of carbon, are excreted by the roots and 

 absorbed by the soil. Through the expulsion of 

 these matters unfitted for nutrition, therefore, the 

 soil receives again the greatest part of the carbon, 

 which it had at first yielded to the young plants as 

 food, in the form of carbonic acid. 



The soluble matter, thus acquired by the soil, is 

 still capable of decay and putrefaction, and by un- 

 dergoing these processes furnishes renewed sources 

 of nutrition to another generation of plants ; it be- 

 comes humus. The leaves of trees, which fall in 

 the forest in autumn, and the old roots of grass in 

 the meadow, are likewise converted into humus by 

 the same influence : a soil receives more carbon 

 in this form than its decaying humus had lost as 

 carbonic acid. 



Plants do not exhaust the carbon of a soil, in the 

 normal condition of their growth; on the contrary, 

 they add to its quantity. But if it is true that 

 plants give back more carbon to a soil than they 

 take from it, it is evident that their growth must 

 depend upon the reception of nourishment from 

 the atmosphere. The influence of humus upon 

 vegetation is explained by the foregoing facts, in 

 the most clear and satisfactory manner. 



Humus does not nourish plants, by being taken 

 up and assimilated in its unaltered state, but by 

 presenting a slow and lasting source of carbonic 

 acid which is absorbed by the roots, and is the 



