On the Food of Plants. 
517 
up their nourishment, and compare its composition with that of the 
vegetable substances which compose their structure." * 
Go now, and examine plants which groAV entirely in a state of 
nature, quite uncultivated. Take a pine forest growing in a 
barren sand like the Landcs of Bordeaux, and note the rapid 
growth of the timber, and the quantity which can consequently 
be every year cut down and removed, without diminishing the 
total amount. Examine, also, the soil in which the trees grow 
from year to year, and note its constantlij increasiiuj ridinexx in 
" humns^' — in vegetable matter. Far from exhausting the soil in 
this respect, these trees pour out constantly from their rootlets 
matter containing carbon, which, by decay in the soil, becomes 
humus. The dead leaves, and small branches accidentally broken 
by the wind, accumulate beneath, and add to this store of humus ; 
and in the end, when the trees themselves have perished, a soil 
has been prepared capable of supporting all the plants that the 
climate will suffer to live. Whence did these trees obtain their 
carbon ? — The earth did not yield it : it must have been the air. 
To ascribe the origin of the carbon of plants, in a state of 
nature, to the absorption of humus from the soil in which they 
grow, is about as reasonable as to suppose the possibility of a 
race of animals subsisting on their own offspring. That substance 
is a product of the decay of previously existing plants, which must 
have got their carbon from some other source. Its quantity, 
moreover-, increases every year ; and if this latter fact is not true 
with respect to .some cultivated soils, it is easy to see the reason 
in the greatly accelerated destruction of this substance by the 
oxygen of the air, brought about by the constant loosening of the 
soil. Add to this the very slight degree of solubility possessed 
either by humus itself or by the so-called " humate of lime," and 
it is easy to see its total inadequacy to supply even a small part of 
the carbon fixed in a growing plant. 
The result of nearly all the great chemical actions constantly 
going on at the surface of the earth — combustion, the respiration 
of animals, the putrefaction of organic matter — is the production 
of carbonic acid, and that in very large quantities. According to 
an estimate given, a little town of 7000 inhabitants, where wood is 
consumed for fuel, and that sparingly from its high price, more 
than one thousand millions of cubic feet of oxygen are annually 
withdrawn from the atmosphere by this combustion alone, and a 
great part of it consequently returned to it in the shape of car- 
bonic acid ! Add to this the amount produced by respiration. 
* Liebig, Agricultural Chemistry, p. 44. 
