s
well as directly injurious.

There are nevertheless some considerations which render the admission of
air, when regulated and applied with discretion, an operation of
importance to the health of plants: it is productive of beneficial
effects in carrying off the noxious vapours, which may although unseen,
and guarded against, still float in the atmosphere; and there can be
little doubt that another beneficial influence which it exercises, results
from the motion which is produced by a body of air changing its position,
which probably promotes circulation, and increases the excitability of the
plants.

Since therefore a change of the volume of the atmosphere in plant houses,
is productive of benefit, and the admission of a large body of cold air,
is at the same time so decidedly objectionable, it is important, that in
endeavouring to secure the benefits of the practice, the injuries which
are liable to result, should if possible be avoided. The regulation for
the admistion of air, which is described in the second chapter of this
treatise, may be regarded as being of some importance in this respect, as
well as in the provision which it includes, of supplying the heated air,
with a due proportion of moisture.

Physiologists tell us, that plants derive a considerable proportion of
their food, directly from the atmosphere, by a process similar to the
inhaling of animals; and that the substances thus derived, are carbonic
acid, ammonia, and water, which contain the elements of organic matter in
considerable proportions. The influence of the atmosphere is exerted
beneficially, by its constituents entering into combinations with other
matters, which are taken into the system by the roots, and spread out and
exposed in the leaves: this exposure has so far the effect of altering the
character of the substance carried up from the roots, that it is no longer
a body of crude juice, but is undergoing a process of elaboration, and is
being assimilated with the superincumbent tissue of the plant. There seems
to be no reason why those particular gaseous bodies which plants
appropriate to themselves from the atmosphere, should not to a great
extent be supplied to them artificially, at such periods as it may be
necessary, or desirable, to accelerate their growth, and induce a more
perfect and mature developement. It has been already stated, that the most
important of these aeriform bodies, are nitrogen, which plants derive from
ammoni