on Vegetables, with reference to their Temperature. 13 



during coldness is directly as the greatness of the drying power, 

 and inversely as the approximation of the dew-point to the at- 

 mospheric temperature. 



§ 3. The sensible heat of plants is directly as the atmo- 

 spheric temperature and the chemical action going on in their 

 cells, and inversely as the radiation, evaporation and con- 

 duction together, tyc. (p. 1 .) 



We have introduced this postulate rather to give complete- 

 ness to the subject than to enter into any lengthened examina- 

 tion. That it is true, can be readily shown by a few references 

 to the foregoing tables ; the proofs drawn may be conveniently 

 ranged under three heads : — 



1st. The temperature of plants varies nearly with the at- 

 mosphere, the greatest difference measured being about 5° 

 Fahrenheit. 



2ndly. The parts in which the greatest exhibitions of tem- 

 perature above the air have been witnessed are the seat of ac- 

 tive chemical and organic action, as the ovaries, male spadix, 

 midrib of leaves, &c, the stem being seldom above or below 

 the external temperature. 



3rdly. Roots and subterrene stems are of the same tempe- 

 rature as the soil, and generally below the atmosphere, in con- 

 sequence of evaporation taking place from the earth. This 

 diminished temperature in the plant must depend partly upon 

 conduction. That vegetables also lose heat by radiation, is 

 shown by the copious deposit of dew seen upon their leaves 

 after a clear chilly night. 



§ 4". A review of the subject, with some remarks on apparent 



anomalies. 



Since the preceding experiments were made there has been 

 published in the Journal de Chimie, an article on vegetable 

 heat by M. Dutrochet*. He inclosed a dead and living 

 plant in an atmosphere saturated with moisture, and examined 

 their temperature with Breschet's physiological pair. The 

 result of his experiments brought him to the conclusion, that 

 living plants possessed a temperature that exceeded the atmo- 

 spheric temperature by one-third centigrade as a maximum. 

 Van Beck has since repeated the experiments of M. Dutro- 

 chet and arrived at an opposite conclusion, viz. that the living 

 plant betrayed two-thirds centigrade as a maximum below the 

 dead plant. 



Independently of the discordance in these measures, we 

 cannot understand how a plant can be said to possess a spe- 



* The author did not see the original paper, but an extract in the Edin- 

 burgh Philosophical Journal of Professor Jameson, 1840. 



