c acid eliminated from all parts of the plants.

Although the action occurring during the night is the reverse of that
which takes place during the day, it is in no degree to be attributed to
a re-oxidation of the carbon which had been deposited in the tissues of
the plant. It appears, on the contrary, to be a purely mechanical, and
not a chemical process. During the night the sap continues to circulate
through the vessels of the plant, and moisture, carrying with it
carbonic acid in solution, is absorbed by the roots; but when it reaches
the leaves, where the sun's light would have caused its decomposition
during the day, it is again exhaled unchanged. The oxygen absorbed
during the night must, however, take part in some chemical processes,
for if it were merely mechanical, the absorption would not be confined
to that gas alone, but would be participated in by the other
constituents of the air. Moreover, the amount of absorption varies
greatly in different plants--being scarcely appreciable in some, and
very abundant in others. Plants containing volatile oils, which are
readily converted into resins by the action of oxygen, or those
containing tannin or other readily oxidizable substances, take up the
largest quantity. This is remarkably illustrated by an experiment in
which the leaves of the Agave americana, after twenty-four hours'
exposure in the dark, were found to have absorbed only 0.3 of their
volume of oxygen, while those of the fir, in which volatile oil is
abundant, had taken up twice, and those of the oak, containing tannin,
eighteen times as much oxygen.

In the flowers, both by day and night, there is a constant absorption of
oxygen, and evolution of carbonic acid. In fact, an active oxidation is
going on, attended by the evolution of heat, which, in the _Arum
maculatum_ and some other plants, is so great as to raise the
temperature of the flower 10 deg. or 12 deg. above that of the surrounding air.

_Decomposition of Water in the Plant._--In addition to the function
which water performs in the plant, as the solvent of the different
substances which form its nutriment, and hence as the medium through
which they pass into its organs, it serves also as a direct food,
undergoing decomposition, and yielding hydrogen to the organic
substances. Its constituents, along with those of the carbonic acid
absorbed, undergo a variety of transformations, and form the principal
part of the non-nitrogenous constituents.