288 PHYSIOLOGY AT -THE FARM. 



has been so much debated is, Whether these organic matters in 

 the soil, which manifestly are the source of food to plants, 

 undergo complete decomposition before being absorbed, so 

 as to supply mere mineral carbonic acid to the radicles; or 

 whether they are taken into the plant while still possessed 

 of an organic character, and are decomposed to supply food 

 actually within the plant ? The determination of the question 

 in this particular form does not, perhaps, possess the im- 

 portance usually attached to it ; for if the carbonic acid 

 derived from the organic matters of the soil, whether set free 

 in the soil before the absorption, or set free after the absorp- 

 tion of these organic matters within the structure of the 

 plant, be decomposed solely in the leaves by the influence of 

 light, so that its carbon is then only fixed in the plant, the 

 question is virtually settled ; that is to say, the carbonic 

 acid, whatever be its source, contributes to the process of 

 vegetation only under the influence of light, and while it has 

 been reduced to a purely mineral state. To be set free in the 

 plant from organic matters previously received by the radicles 

 is not to nourish ; to nourish, it must be carried to the leaves, 

 and there, being decomposed by light, and its oxygen set free, 

 its carbon is ready, like that immediately derived from the 

 carbonic acid of the atmosphere, to become incorporated with 

 the plant. 



If in the process of vegetation all the carbon incorporated 

 with the structure of plants be derived from mineral carbonic 

 acid, it is easy to understand that the whole soil of the earth, 

 in so far as it contains carbon, has been in process of time 

 accumulated from the carbonic acid of the atmosphere that 

 is to say, that the first beginnings of soil were from the 

 decay of such vegetables as mosses and lichens growing 

 on moist rocks, and that by the successive creation of plants, 

 as the soil increased under the decay of the previous vegeta- 



