FOOD OF PLANTS, AND HOW THEY TAKE IT. 109 
in the air), in the form of carbonic acid, ammonia, and water, 
which diffuse themselves in the atmosphere. The plant takes 
possession of these substances, and forms from them (accom¬ 
panied by an incessant increase of the oxygen of the atmo¬ 
sphere) some compounds that are rich in carbon and hydrogen, 
but devoid of nitrogen, such as starch, gum, sugar, and the 
various fatty matters, and others that are rich in nitrogen, 
namely, albumen, fibrine, and casein. These compounds are 
for the service of the animal, who builds up its corporeal 
frame from the latter, and burns the former in the respiratory 
process, for maintenance of the necessary heat. This theory 
stands now firm and unshakeable upon the facts which have 
been brought forward, and the naturalist is perfectly correct 
when he says, that man, through the mediation of plants, in 
the first instance, lives upon air. Or, we may express it in 
this way : the plant collects matters from the atmosphere, 
and compounds from them the food of man ; but life itself 
is only a process of combustion, of which decomposition is 
the final conclusion. Through this combustion all the con¬ 
stituents return back into the air, and only a small quantity 
of ashes remains to the earth, from whence it came. But 
from these slow, invisible flames rises a new-born phoenix, 
the immortal soul, into regions where our science has no 
longer any value/’* 
Humus is however a most valuable element of soils, being 
carbon in a state of minute division ; as carbon it is almost 
indestructible; as carbon it does not appear to be taken up 
by plants; its important role is to absorb water from the at¬ 
mosphere, and to absorb and retain for the use of plants—to 
which it readily yields them—the carbonic acid and the 
ammonia from the air; in this power it appears to have no 
equal. Humus, therefore, always contains water and those 
gaseous matters which are the food of plants, and whenever 
it yields up a portion to growing vegetation, it absorbs fresh 
supplies from the atmosphere. This humus is therefore not the 
food of plants, as was formerly supposed, but it is the food-bearer, 
the storehouse ox pantry, holding the gaseous food of plants. 
Other elements are needed by plants as food, however— 
mineral elements—that do not come from the atmosphere, 
but from the soil itself; hence we find a flora peculiar to 
certain regions, and some plants that are never found in 
other soils than those peculiarly adapted to them. So also 
with our manures; they must be selected in accordance with 
the peculiar wants of the crops to which they are to be 
applied, and with the particular composition of the soil 
upon which they are to be sown. This is the basis of agri- 
* Schleiden, ‘The Plant/ p. 152. 
XXXIV, 
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