108 M. Dutrochet on the Temperature of Vegetables. 



supplies with rapidity what is lost by the evaporation of the 

 leaves. This sap, in passing from the roots into the trunk, 

 carries along with it the temperature of the ground. This 

 temperature is modified in the trunk by the temperature of 

 the surrounding atmosphere, and often by the direct action of 

 the solar rays, so that a different temperature is found to 

 exist in the trunk of the same tree according to the height 

 where it is observed at the same time. The trunk retains for 

 several hours this acquired heat after it has ceased to exist in 

 the atmosphere, and the sap which traverses it in its ascent to 

 the branches, conveys to them the temperature which it tliere 

 acquires. Here, then, is a source of error which it is impossible 

 to avoid in the investigation of the heat proper to the trunk 

 and the branches of trees. It was not, therefore, to these 

 that I directed my researches. Thinking that the heat proper 

 to plants, if it existed, ought to be found more easily in the 

 soft parts, where life is active, than in the hard portions where 

 life possesses less activity, it was to the former that I purposed 

 to devote my special attention. 



The Sorel apparatus, intended for procui'ing a constant tem- 

 perature, to which one of the two soldered points of the ther- 

 mo-electric circle is submitted, cannot be employed in this 

 kind of investigation. M. Becquerel formed the happy idea 

 of supplying its place by inserting one soldered point of the 

 circuit in a living branch, and the other in a dead branch of 

 the same tree ; both branches having the same dimensions. 

 It was evident that these two branches, on account of their 

 equality, ought to partake simultaneously of the variations of 

 the temperature of the surrounding atmosphere ; so that, if 

 the living branch had a heat proper to itself, it ought to add 

 it to the heat transmitted from without, and to manifest its 

 excess of heat above that of the dead branch by a deviation of 

 the magnetic needle of the multiplicator. I hastened to fol- 

 low this plan of experimenting. My first experiment was 

 made on a young branch of Campanula medium which I cut, 

 and which, having its interior extremity plunged in a vessel 

 full of water, was carried to my room. One of the soldered 

 points was placed in its interior, and the other in a branch of 

 the same plant which had been dead and dried since the pre- 

 ceding year, and which was of the same size. The constant 



