104 M. Dutrochet on the Temperature of Vegetables. 



Hence it follows, that vegetables, in their natural state, have 

 a temperature always lower than that of the atmosphere, and 

 thus appear to produce cold. Among the cooling causes of 

 vegetables, the vaporization of the sap is most conspicuoiis. 

 In proof of this, it is only necessary to place the vegetable 

 in an atmosphere completely saturated with water. This I 

 have done by means of a particular apparatus. It is a large 

 glass vessel, closed with a cork, and in which is a little 

 water. The two soldered points of the thermo-electric cir- 

 cuit are situated in the interior of this vessel. One of these 

 is occupied by a vegetable killed by immersion in hot water ; 

 the other by a vegetable exactly similar, but alive. The 

 needles are covered with gum lac, to preserve them from 

 oxidation and the action of the acids. The two vegetables, 

 in the interior of which are the two soldered points, are thus 

 placed in the same condition with respect to the external 

 causes capable of modifying their temperature ; the dead one 

 takes the temperature of the surrounding atmosphere ; that 

 which is alive takes the same temperature, but in addition, it 

 then manifests the heat possessed by it in virtue of its prin- 

 ciple of life, and which, in its natural state, was absorbed by 

 the vaporization of the sap. The maximum of this heat is \ of 

 a centesimal degree ; generally, however, it is only ^, or even 

 1^0 ^^ A' of a degree. This I have observed in the young 

 stems of vegetables whose pith is still gi'een, in flower-buds, 

 and in leaves sufficiently thick to admit the needle, such as 

 the leaves of the house-leek {Sempervivum tectoruni). The 

 heat peculiar to yoimg stems and leaves, disappears during the 

 night, or artificial darkness, and reappears under the influ- 

 ence of light sufficientl}' long continued. The peculiar heat 

 of flower-buds continues during the night. At first, I be- 

 lieved this heat disappeared during darkness, but afterwards 

 discovered my error. The heat peculiar to vegetables is in- 

 creased in proportion to the elevation of the external tempe- 

 rature ; to have marked results, therefore, it is necessary to 

 make these experiments above -f- 59° Fahr. ; they are still 

 more appreciable above -f- 68° Fahr. The highest elevation 

 of the heat peculiar to vegetables, corresponds to that hour of 

 the day when the external heat and the light are both at their 



