322 M. BECQUEREL, ON THE CLIMATIC EFFECTS OF FORESTS. 



independent cause of conductability, which operates against this cooling 

 to a degree below the Ireezing-point, and preserves them, for a certain 

 time, against the disastrous eftects of severe cold. This action varies 

 according to the diameter of the tree, and probably according to the 

 species to which it belongs. 



In northern regions, the temperature of plants, as compared with that 

 of the air, is more remarkable. M. Bourgeaud, in 58° of latitude, in 

 places where the temperature goes down to the congealiug-poiut of 

 mercury, or to — 40=^ (C.), observed tlie following facts : 



1. The Populus hahamifera and Abies alba were observed, during the 

 eight months from November, 1857 to June, 1858, at nine o'clock in the 

 morning — the time of day when he supposed the temperature to be at 

 about the average of the day — the mean temperatures of the air and of 

 the poplar were the same; agreeing in this, with the observations of 

 which we have above spoken, and going to establish the principle, by 

 virtue of which the temperature of the plants tends constantly to become 

 that of the ambient air, notwithstanding the causes tending constantly 

 to increase or diminish this effect. 



2. The monthly temperatures presented little diff'erence in the trees 

 and in the air, although they differed very greatly in their maxima and 

 minima. In the month of January, for example, the maximum and min- 

 imum of the air were +6° and —^iP.Q (C), while in the poplar they 

 were — 2o.2 and — 21P.7. 



3. During the eight months of observation the mean temperature of 

 the soil, at 0.913 and at 0.G09 meters depth, was twice greater than in 

 the air. 



The thaw commonly comes in May ; spring begins at once, and very 

 soon afterward the summer is come. The vegetation is so rapid, that 

 cereals sown in this mouth are harvested toward the end of July f the 

 blossoms on the poplars appear when the temperature of the air is at 

 + 130.4:7 (0.), while the frost is still in the ground to a depth of 0^.609 

 to 0".913 (23.62 to 35.85 inches). The leaves appear on the iirst days in 

 June, when the roots are still in a soil at the freezing-temperature, the 

 effects being the same as are produced when we bury in a warm bed of 

 soil the tops of vines while their roots are still in the open air. The 

 buds and even the leaves begin to develop while it freezes from — 8° to 

 — 10° (C). We have here a new evidence of the influence of the temper- 

 ature of the air upon the trees, even while the roots are in the frozen 

 soil, in promoting and developing vegetation. 



The Fopulus balsamifera and the Abies alba, as well as other species, 

 are exposed to a cold of — 40° (C). without suff'ering in their organiza- 

 tion ; yet the roots of these trees find themselves in strata of earth that 

 are not sensibly touched by the frost, thus affording a new proof that 

 there exists a certain resistance to the cold at the extreme minima of 

 the air, — 31o.G (C), while in the poplar it is but — 29^.7, and that the 

 temperature in a tree may be twice as great as in the d]pei\ air. 



Having shown the relations that exist between the temperature of the 

 air and its variations and those of plants, there remains to be shown 

 what is the temperature of the air above that of trees of the Iirst mag- 

 nitude, such as the horse-chestnut, 21.25 meters high, at the top of which 

 is placed one of the poles of an electric thermometer in contact with 

 the leaves. It is shown, from multiplied observations, that the tem- 

 perature of the air over the horse-chestnut depondschietiy on the thermal 

 condition of the leaves and branches, which more or less warm or cool 

 the ambient air, according as they are more or less exposed at the time 

 to the solar rays or to nocturnal radiation. 



