CHAP. III.] TEMPERATURE OF FRUIT. 121 



of which it appears to present a counterpart to that ex- 

 hibited by the animal ceconomy in regulating its own heat. 

 So uniform is the exercise of the latter faculty in man and 

 the higher animals, that there is barely a difference of 

 three degrees between the warmth of the body in the 

 utmost endurable 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 in the luxurious 

 chillness of the fruit that nature lavishes on the 

 tropics, man enjoys the benefit of this beneficent ar- 

 rangement. 



The peculiar organisation by whiqh this result is ob- 

 tained is not free from obscurity, but in all probability 

 the means of adjusting the temperature of plants is 

 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 de- 

 gree as to be almost inappreciable, except at the period 

 of germination, when it probably arises from the con- 

 sumption 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 con- 

 sequent check of evaporation. 



On the other hand, the faculty of maintaining a tem- 

 perature 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 cool- 

 ness 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 



