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. Berard 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.
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