rbon and water, the change
was very simply explained by supposing that the carbon was oxidised and
converted into carbonic acid and its water eliminated. But this
hypothesis is incapable of explaining all the phenomena observed; for
woody fibre, which is one of the chief constituents of the young plant,
contains more carbon than the starch and sugar from which it must have
been produced, and we are, therefore, forced to admit that the action
must be more complicated. There is every reason to believe that the
nitrogenous constituents of the seed are most abundantly oxidized, for
they are remarkably prone to change; but the action of the air is not
confined to them, and it appears most probable that all the substances
take part in the decomposition, and the process of germination may, in
some respects, be compared to decay or putrefaction, which, like it, is
attended by the absorption of oxygen and evolution of carbonic acid; but
while in the latter case the residual substances remain in a useless
state, in the former they at once become part of a new organism.

_Changes occurring during the After-growth of the Plant._--When the
plant has developed its roots and leaves, and exhausted the store of
materials laid up for it in the seed, it begins to derive its
subsistence from the surrounding air, and to absorb carbonic acid,
water, ammonia, and nitric acid, and to decompose and convert them into
the different constituents of its tissues. These changes take place
slowly at first, and more rapidly as the organs fitted for the
elaboration of its food are developed. The roots and the leaves are
equally active in performing this duty, the former absorbing the mineral
matters along with the carbonic acid, ammonia, nitric acid, and moisture
in the soil, or the manure added to it; the latter gathering the gaseous
substances existing in the air. Each of these undergoes a series of
changes claiming our consideration.

_Decomposition of Carbonic Acid._--Carbonic acid, which appears to be
absorbed with equal readiness by the roots, leaves, and stems, undergoes
immediate decomposition, its carbon being retained, and its oxygen, in
whole or in part, evolved into the air. This decomposition occurs only
under the action of the sun's rays, and has been found to be
proportionate to the amount of light to which the plant is exposed. It
takes place only in the green parts of plants, for though the roots
absorb carbonic acid, they cannot decom