urable vicissitudes of heat and cold; and in vegetables
an equivalent arrangement enables them in winter to keep their
temperature somewhat above that of the surrounding air, and in summer to
reduce it far below it. It would almost seem as if plants possessed a
power of producing cold analogous to that exhibited by animals in
producing heat; and of this beneficent arrangement man enjoys the
benefit in the luxurious coolness of the fruit which nature lavishes on
the tropics.

The peculiar organisation by which this result is obtained is not free
from obscurity, but in all probability the means of adjusting the
temperature of plants is simply dependent on evaporation. As regards the
power possessed by vegetables of generating heat, although it has been
demonstrated to exist, it is in so trifling a degree as to be almost
inappreciable, except at the period of germination, when it probably
arises from the consumption of oxygen in generating the carbonic acid
gas which is then evolved. The faculty of retaining this warmth at night
and at other times may, therefore, be referable mainly to the closing of
the pores, and the consequent check of evaporation.

On the other hand, the faculty of maintaining a temperature below that
of the surrounding air, can only be accounted for by referring it to the
mechanical process of imbibing a continuous supply of fresh moisture
from the soil, the active transpiration of which imparts coolness to
every portion of the tree and its fruit. It requires this combined
operation to produce the desired result; and the extent to which
evaporation can bring down the temperature of the moisture received by
absorption, may be inferred from the fact that Dr. Hooker, when in the
valley of the Ganges, found the fresh milky juice of the Mudar
(_calotropis_) to be but 72 deg., whilst the damp sand in the bed of the
river where it grew was from 90 deg. to 104 deg.

Even in temperate climates this phenomenon is calculated to excite
admiration; but it is still more striking to find the like effect rather
increased than diminished in the tropics, where one would suppose that
the juices, especially of a small and delicate plant, before they could
be cooled by evaporation, would be liable to be heated by the blazing
sun.

A difficulty would also seem to present itself in the instance of fruit,
whose juices, having to undergo a chemical change, their circulation
would be conjectured to be slower; and in the in