_The Properties of Light with regard to Vegetation._
The illuminating power of light will come under our consideration
hereafter. Its agency, with regard to organic life, is too
important not to be noticed, though this must be done briefly.
Light appears to be as necessary to the health of plants as air
of moisture. A plant may, indeed, grow without it, but it does
not appear that a species could be so continued. Under such a
privation, the parts which are usually green, assume a white
colour, as is the case with vegetables grown in a cellar, or
protected by a covering for the sake of producing this very effect;
thus, celery, is in this manner blanched, or _etiolated_.
The part of the process of vegetable life for which light is
especially essential, appears to be the functions of the leaves;
these are affected by this agent in a very remarkable manner.
The moisture which plants imbibe is, by their vital energies,
carried to their leaves; and is then brought in contact with the
atmosphere, which, besides other ingredients, contains, in general,
a portion of carbonic acid. _So long as light is present_, the
leaf decomposes the carbonic acid, appropriates the carbon to the
formation of its own proper juices, and returns the disengaged
oxygen into the atmosphere; thus restoring the atmospheric air to
a condition in which it is more fitted than it was before for the
support of animal life. The plant thus prepares the support of
life for other creatures at the same time that it absorbs its own.
The greenness of those members which affect that colour, and the
disengagement of oxygen, are the indications that its vital powers
are in healthful action: as soon as we remove light from the plant,
these indications cease: it has no longer power to imbibe carbon
and disengage oxygen, but on the contrary, it gives back some of
the carbon already obtained, and robs the atmosphere of oxygen for
the purpose of reconverting this into carbonic acid.
It cannot well be conceived that such effects of light on
vegetables, as we have described, should occur, if that agent, of
whatever nature it is, and those organs, had not been adapted to
each other. But the subject is here introduced that the reader may
the more readily receive the conviction of combining purpose which
must arise, on finding that an agent possessing these very peculiar
chemical properties, is employed to produce also those effects of
illumination, vision, &c., which form the most obvious portion of
the properties of light.