^05 
CHAPTER IX. 
OF THE FOOD OF PLANTS MANURE. 
The principal part of the food of plants is derived from the 
earth, and is introduced into their system through the roots. 
The latter are, however, incapable of absorbing anything 
solid ; fluid and gaseous matter only can pass through their 
spongelets. It is, perhaps, exclusively in the form of water 
that the nutritive matter of the soil is received by roots ; not, 
however, of pure water, which in fact does not exist in nature, 
but of water holding various solid matters in solution, the 
most remarkable and abundant of which are silex, lime, and 
many of its salts, several other earths, oxyde of iron, and 
copper. 
These substances, however, although they undoubtedly each 
perform their allotted part in the economy of vegetation, — ■ 
consolidating the tissue, hardening the cuticle, or assisting 
in depriving a plant of organs which become unhealthy and 
worn out, — cannot be altogether considered as nutritive mat- 
ter. There are, perhaps, only two forms of matter, which can 
properly be called nutritive ; the one is carbon, the other water. 
Soil in its natural state is filled with the remains of organic 
bodies, which decompose and become converted into carbo- 
nic acid. In proportion to the abundance of these is soil 
fertile. The carbonic acid, thus incessantly forming below 
the surface of the earth, enters freely into the roots ; combining 
with water and such other principles as may already have 
been formed there, it ascends the stem, apparently decompos- 
ing to a certain extent as it passes along, and giving its oxygen 
to the spiral vessels, which convey it into other parts of the 
system; when it reaches the leaves, it liberates its oxygen 
completely, and leaves its carbon to combine with the 
tissue of vegetation, or to enter into new proportions with 
