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PHYSIOLOGY. 
BOOK II. 
CHAPTER XI. 
or 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, and oxides of iron 
and copper. 
These substances, however, although they may each 
perform their allotted part in the economy of vegetation, 
consolidating the tissue, hardening the epidermis, or assisting 
in depriving a plant of organs which become unhealthy 
and worn out, cannot be altogether considered as nutritive 
matter. There are, perhaps, only three forms of matter 
which can properly be called nutritive ; carbon, water, and 
nitrogen. 
Soil in its natural state is filled with the remains of organic 
bodies, which decompose, and yield nitrogen, or become con- 
verted into carbonic acid. In proportion to the abundance of 
these is soil fertile. Nitrogen, and the carbonic acid incessantly 
forming below the surface of the earth, enter freely into the 
roots ; combining with water and such other principles as may 
already have been formed there, they ascend the stem, the car- 
bonic acid decomposing to a certain extent as it passes along, 
and giving, apparently, 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 
