phyll, or green colouring
matter. This decomposition of carbonic acid, and the fixation of the
carbon by the plant with the formation of starch, takes place only under
the influence of sunlight. During the night a reflex action takes place,
which is commonly known as _respiration_, and which is exactly analogous
to animal respiration.[21] The rate at which the fixation of carbon
takes place depends on the strength of the sun's rays. It seems to take
place very rapidly under a strong tropical sun.[22] The action of
sunlight on the absorption of carbon has been studied by a number of
observers, among others by Sachs, Draper, Cloez, Gratiolet, Caillet,
Prillieux, Lommel, &c.

_Action of Light on Plant-growth._

Experiments made by several observers, more especially Pfeffer, have
shown that the yellow rays of the solar spectrum are the most potent in
inducing this decomposition.

Some interesting experiments have been carried out by different
observers on the possibility of growing plants under the influence of
artificial light. While it would seem that the light from oil-lamps or
gaslight is unable to promote growth, except in very exceptional cases,
the electric light, or other strong artificial light, seems to be
capable of taking the place of sunlight. Heinrich was the first to show
that sunlight could be replaced by the magnesium light.

Experiments with the electric light have been carried out by
Herve-Mangon in France and Dr Siemens in England. The plants grown under
the influence of the electric light were observed to be of a lighter
green colour than those grown under normal conditions, thus indicating a
feebler growth; in fact, Siemens was of the opinion that the electric
light was about half as effective as daylight.[23]

These experiments are interesting from an industrial point of view; for
it is conceivable that at some distant time electricity might be called
to the aid of the agriculturist.

_Source of Plants' Oxygen._

With regard to the source of the oxygen, which, next to carbon, is the
element most largely present in the plant's substance--amounting to,
roughly speaking, about 40 per cent--all evidence seems to indicate that
it is chiefly derived from water, which is also the source of the
plant's hydrogen. In addition to water, carbonic acid and nitric acid
may also furnish small quantities. It has been pretty conclusively
proved that the atmospheric oxygen, while necessary to plant-growth,