Trees, considered as organisms, produce in themselves, or in the air, a certain amount of heat, by absorbing and condensing atmospheric vapor, and they exert an opposite influence by absorbing water and exhaling it in the form of vapor; but there is still another mode by which their living processes may warm the air around them, independently of the thermometric effects of condensation and evaporation. The vital heat of a dozen persons raises the temperature of a room. If trees possess a specific temperature of their own, an organic power of generating heat, like that with which the warm-blooded animals are gifted, though by a different process, a certain amount of weight is to be ascribed to this element, in estimating the action of the forest upon atmospheric temperature. "Observation shows," says Meguscher, "that the wood of a living tree maintains a temperature of +12 deg. or 13 deg. Cent. [= 54 deg., 56 deg. Fahr.] when the temperature of the air stands at 3 deg., 7 deg., and 8 deg. [=37 deg., 46 deg., 47 deg. F.] above zero, and that the internal warmth of the tree does not rise and fall in proportion to that of the atmosphere. So long as the latter is below 18 deg. [= 67 deg. Fahr.], that of the tree is always the highest; but if the temperature of the air rises to 18 deg., that of the vegetable growth is the lowest. Since, then, trees maintain at all seasons a constant mean temperature of 12 deg. [= 54 deg. Fahr.], it is easy to see why the air in contact with the forest must be warmer in winter, cooler in summer, than in situations where it is deprived of that influence."[149] Boussingault remarks: "In many flowers there has been observed a very considerable evolution of heat, at the approach of fecundation. In certain _arums_ the temperature rises to 40 deg. or 50 deg. Cent. [= 104 deg. or 122 deg. Fahr.]. It is very probable that this phenomenon is general, and varies only in the intensity with which it is manifested."[150] If we suppose the fecundation of the flowers of forest trees to be attended with a tenth only of this calorific power, they could not fail to exert an important influence on the warmth of the atmospheric strata in contact with them.