M. BECQUEEEL, ON THE CLIMATIC EFFECTS OF FORESTS. 321 



ture in the air and in the tree are in the ratio of 3.80 to 0.81 ; that is to 

 say, they are 4.7 times greater in the air than in the tree, instead of 

 being 5.89 times, as found at Geneva. This difference is evidently due 

 to the poor couductability of the wood, which does not allow the changes 

 of temperature in the air to be conveyed rapidly into the tree. We find 

 in fact that very decided changes, but of short duration, are noticed 

 in the air which cannot be appreciated in the tree. 



The leaves and young green branches of the trees and the low plants 

 that cover the meadows are in similar conditions as to warming or 

 cooling, and produce like effects in radiation. It is therefore only in 

 branches of a certain size, and in the trunks, that we can study the in- 

 fluence that the temperature of vegetables properly exerts upon the 

 temperature of the ambient air. We may practically consider a green 

 stem as a body covered with an envelope that possesses great emissive 

 and absorbent power, by virtue of which its temperature is lowered 

 or raised continually from the effect of celestial radiation on solar heat; 

 but when the parenchymatous tissue is replaced by a cortical one, the 

 wood within being damp and a poor conductor of heat, either in a trans- 

 verse or a longitudinal direction, the movement of the heat then ope- 

 rates but slowly, and we no longer observe in the interior those rapid 

 changes of temperature noticed in the young branches. We see, there- 

 fore, that the variations are much less in the trunk of a tree of certain 

 volume than in the air. If the temperature of the air varies within ex- 

 tended limits but of short duration, the thermal condition of the tree is 

 but little affected. If the changes, however, are moderate in extent, 

 and of long period, the tree finally acquires the temperature of the air. 



Every vegetable needs a certain degree of heat to enable its organs 

 to act. If the temperature rises gradually, the parts dilate, and evap- 

 oration and the circulation of the sap are accelerated ; if the tempera- 

 ture is lowered the opposite effects are produced. On the other hand, 

 alternations of heat and cold give a new activity to vegetation. But the 

 great variations of temperature within the tropics, between day and 

 night, in the portions of the air that envelop the trees, become likewise 

 manifested within the interior of the trees, and aflbrd conditions that 

 eminently favor forest vegetation. 



The atmosphere is, therefore, the source from whence all vegetables 

 derive the heat which their being requires for developing and accom- 

 plishing all the phases of their existence. The mean temperature of a 

 place, and the daily variations and extremes ot temperature in the aio?, 

 are, therefore, the caloric elements that we are principally to take into 

 account in the phenomena of vegetable life, and in researches relating 

 to this vegetable life, and to the thermal influences of forests, and effects 

 of woodlands generally upon the climate. Whatever heat may be gene- 

 rated in the tissues where the transformation of the sap takes place 

 does not materially affect their temperature — at least it is not apprecia- 

 ble by our instrnments, and whatever it may be it is dissipated. We 

 have undertaken several series of observations upon temperature in dif- 

 ferent localities, within the woods and without, and to a certain distance 

 from them, in order to determine the influence that the forests exercise 

 upon the mean temperature. The results obtained will be made the 

 subject of another memoir. 



We now come to remark that vegetables possess within themselves 

 the power of resisting, for a certain time, an extreme reduction of temper^ 

 ature without suffering organic lesions, as we have proved in a series of 

 experiments, that leave no room for doubt upon this point. We have, 

 from this, been led to think that there exists in vegetable organisms aa 

 21 F 



