THEORY OF LIEBIG. 145 



When the soil is stirred, it facilitates the introduction of oxy- 

 gen, which also hastens the process. How then does the de- 

 caying woody fibre act ? Simply by yielding carbonic acid : 

 and when any substance, as stagnant water, or the composi- 

 tion of the soil, arrests the process of decay by excluding the 

 air, then the carbonic acid is not yielded to the roots of 

 plants, and the leaves turn yellow and fall off. 



7. Finally, that carbonic acid is the only source of the 

 carbon, appears from the fact, that in the " chemical trans- 

 formations " which take place in the process of assimilation, 

 the eifete, or excrementitious matters, which are thrown out 

 by the roots, contain a quantity of carbon, nearly or quite 

 equal to that which the humus yields in the form of carbonic 

 acid. This matter becomes humus, and is converted into 

 carbonic acid again, in the process of decay. Now when 

 these excretions are added to the carbon, derived from the roots 

 of plants and from their leaves, the quantity annually returned 

 to the soil, must be greater than that which is taken from it. 

 " A soil receives more carbon in this form, than its decaying 

 humus had lost in the form of carbonic acid." — L. 



From all these facts, it appears evident that humus does 

 not nourish plants by being taken up and assimilated in its 

 unaltered state, but by furnishing a slow and lasting source 

 of carbonic acid. It should be remarked in this connection, 

 that it is only during the period of youth, that plants use 

 even the carbonic acid derived from the humus of the soil. 

 As soon as their organs are sufficiently enlarged, they de- 

 rive their carbon wholly from the atmosphere. 



We have been thus particular in giving the leading argu- 

 ments by which this theory is advocated, because of the bold- 

 ness and novelty of the views which it contains, and also be- 

 cause it points out the most important source of the carbon of 

 plants ; and although it is not true, as we shall show, that 

 carbonic acid is the only source of the carbon, this theory 

 will aid us in determining what portion is derived from that 

 source. 



