The importance of light and air to plants is well known. When
unassisted by these agents, plants lose their colour, and are deprived
of many of their properties. Colour is thus evidently produced by the
absorption of carbonic acid gas: and the colouring matter may be
detected by a powerful microscope, lodged in the cellular substance of
the leaf. How this colour is formed, and why it assumes different
tints in different plants, are, however, questions which it is at
present impossible to decide. The secretions of plants depend upon
light, and their flavour and nutritious qualities are materially
altered by their exclusion from it. The importance of this knowledge
to a practical horticulturist is proved by the fact, that sea-kale, so
well known as a wholesome and palatable vegetable, is not eatable in
its original state; and that any part of the cultivated plant, if
accidentally left exposed to the action of the air and light, becomes
tough, and so strong in flavour as to be extremely unpleasant to the
taste. Celery, also, in its native state, is poisonous; and it is only
the parts that are blanched that are perfectly fitted for the table.
Though colour is generally supposed to depend principally on the
plant's being exposed to the light, some portion of colouring matter
appears to be occasionally absorbed by the root. This colouring
substance is, however, never a deep green. Red and yellow, as may be
seen in forced rhubarb, &c., are the most common hues. Succulent
plants are less susceptible of the influence of light than any others.
As they are always natives of hot countries, nature, to prevent the
danger they would be exposed to from excessive evaporation, has
provided them with leaves almost destitute of pores; and the moisture
they absorb by their roots thus remains for the nourishment of the
plant. It is for this reason that cactuses, mesembryanthemums, and
other plants of a similar description, require very little water when
kept in pots. Scarcely any carbon is found in plants grown in the
dark. Many experiments have been tried to show the stimulus afforded
to vegetation by light; trees of the same species and variety have
been planted in the same garden and the same soil, but against walls
with different aspects, and differently situated with regard to shade.
The effect has been, not only a difference in the growth and
appearance of the tree, but also in the size, colour, and flavour of
the fruit which it produced. The contrast between plants grown in
hot-houses with wooden sash frames, and those grown in hot-houses with
iron sash-frames, has been found equally striking; the difference of
light between the two kinds of houses being as seven to twenty-seven,
or, sometimes, as three to twenty-three. Light is required at an early
period of vegetation; but, as its properties are to give strength and
flavour, it must be admitted with caution, as it is sometimes
injurious. Too much light renders the skin of fruits tough, and will
make cucumbers bitter. Bérard of Montpelier found that the ripening of
fruits is merely the turning the acid which they contain into sugar,
by exposure to the light; and that too much light and heat, before
they have attained their proper size, will bring on premature
ripening, and make them insipid.
_Lindley's Lectures, reported in the Gardeners' Magazine._
* * * * *