ZOOLOGICAL AND BOTANICAL ASSOCIATION. 51 



whcro insignificant streams now trickle through a waste of rocks and 

 shingle ? The shifting of the fluids of a plant is another disturbing 

 source in the determination of this question, as these fluids generally 

 rise from a soil always differing in temperaturo from the surrounding 

 air. This, however, cannot be the whole source of this difference, espe- 

 cially in surface-growing plants, in arid and sandy places, where it is 

 remarkable that they are very often succulent," and always cooler than 

 the soil or air. 



Let us look now to the chemical (say vital) changes which go on in 

 growing plants ; and, if a vera causa exists, we are bound to give every 

 credit to it ; and I think it can be shown that a very considerable 

 amount of cooling influence arises — that is, the specific heat of the re- 

 sultants produced is greater than their components — purely from che- 

 mical causes. Although this has not as yet received proper scientific 

 attention, it has attracted the observation of many travellers. Sir E. 

 Tennent in his work on Ceylon, says — " that sufficient admiration has 

 hardly been given to the marvellous power thus displayed by the vege- 

 table world, in adjusting its own temperature, notwithstanding atmos- 

 pheric fluctuations — a faculty in the manifestation of which it appears 

 to present a counterpart to that exhibited by the animal economy, in 

 regulating its heat. So uniform is the exercise of the latter faculty in 

 man, and the higher animals, that there is barely the difference of 3° 

 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 the power of producing cold, analo- 

 gous to that exhibited by animals in producing heat ; and of this bene- 

 ficial arrangement man enjoys the benefit, in the luxurious coolness of 

 the fruits which nature lavishes on the tropics." Again, he says : — 

 " The faculty of maintaining a temperature below that of the surround- 

 ing air, can only be accounted for by referring it to the mechanical pro- 

 cess of imbibing a continuous supply of fresh moisture from the soil, tho 

 active transpiration of which imparts coolness to every portion of the 

 tree and its fruit." " Dr. Hooker, in the valley of the Ganges, found 

 the fresh milky juice of the Mundar Calotropis to be 72°, while the 

 damp sand of the bed of the river where it grew was from 90° to 104°." 



It is, I believe, a received doctrine with chemists, that action and 



