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.
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