404 FORESTS AND THEIR CLIMATIC INFLUENCE. 



0^.81, tliat is to say, that they were 4.T times greater in the air than in 

 the tree, instead of 5.89 as was realii:ed at Geneva. This difference 

 depends evidently on the bad oondnctibility of the wood, which does not 

 permit the variations of temperature in the air to be rapidly transmitted 

 into the tree ; it is easy to conceive that variations in the air distinctly 

 marked, but of short duration, cannot become appreciable in the tree. 



The leaves and young l)ranclies of trees, and the humbler plants which 

 cover the meadows, existing under the same conditions as regards warm- 

 ing and cooling, produce the same effects of radiation; it is in the boughs 

 of a certain bulk and in the trunks, therefore, that we must study the 

 intiuence exerted by the proper temperature of the plant on the ambient 

 temperature. A green stem should be considered, in fact, as a body 

 covered with an envelope possessing a great emissive and absorbent 

 power, by virtue of which its temperature is lowered or elevated inces- 

 santly through the effect of the radiation into space or of the solar radia- 

 tion ; but when the parenchymatous tissue is rei)laced by a cortical tissue, 

 the lignum which is beneath being humid and a worse conductor in a 

 transverse than longitudinal direction, the movement of heat is then 

 effected very slowly, and brisk changes of temi)erature are no longer 

 observed in the interior as in the case of the young branches. From the 

 above it will be seen that the variations being much less in the stem of 

 a tree of a certain volume than in the air, if the temperature of the air 

 varies even to a wide extent, but the variations are at the same time of 

 brief duration, the caloritic state of the tree is but little affected thereby. 

 In the contrary case, the tree finally assumes an equilibrium of tempera- 

 ture with the air. 



Every vegetable has need of a certain degree of heat in order for its 

 tissues to perform their functions ; w^hen the temperature rises gradually, 

 the parts dilate ; the evaporation and circulation of the sap is acceler- 

 ated ; the lowering of the temperature i^roduces contrary effects. On 

 the other hand, alternations of heat and cold give a new activity to vege- 

 tation ; thus under the tropics, the great variations of temperature during 

 the day and night, in that part of the air which envelops the trees, being 

 equally manifested in the interior of the trees, this state qf things proves 

 eminently favorable to the forest vegetation. 



The atmosphere is the source from which all plants derive the heat 

 of which they have need in order to germinate, develop themselves, and 

 accomplish all the phases of their existence. The mean temperature of 

 a place, the daily variations and the extremes of the temperature of the 

 air, are the calorific elements to be principally taken into consideration 

 in the phenomena of vegetable life and in researches relative to the calo- 

 rific influence of forests and of woods in general upon climate. The 

 heat produced in the tissues in which the transformation of the sap is 

 effected does not act sensibly on the temperature of vegetables ; at least 

 it is not appreciable by our instruments; what they possess is bor- 

 rowed. 



We have ourselves undertaken several series of observations on tem- 

 perature in different localities, both within the woods and without, to a 

 certain distance, in order to ascertain the influence which forests exerr 

 on the mean temperature. The results which we shall obtain will form 

 the subject of another memoir. 



It is proper here to remark that plants possess of themselves the fac- 

 ulty of resisting for a certain time an extreme degree of refrigeration 

 without suffering organic lesion, as we have ascertained in a series of 

 experiments which leave no doubt on that point. We have been thus 

 led to believe that there exists in the organization of vegetables a cause 



