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°, whilst the damp sand in the bed of the river where it grew was from 90° to 104°.
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 instance of those with hard skins, such as the pomegranate, or with a tough leathery coating, like the mango, the evaporation might be imagined to be less than in those of a soft and spongy texture. But all share alike in the general coolness of the plant, so long as circulation supplies fluid for evaporation; and the moment this resource is cut off by the separation of the fruit from the tree, the supply of moisture failing, the process of refrigeration is arrested, and the charm of agreeable freshness gone.