TEMPERATURE DIFFERENCES BETWEEN TREES AND AIR. 75 



]\—() lor soil t(Mni)eratiives. Tlie tree, like tlie soil, is warmer in 

 wiiitiT and cooler in siunnier llian tlie mean value for W—0. It is 

 also noteworthy that for the smaller trunks and braucLes the varia- 

 tions from air temperature, aud the variations from its own mean, are 

 smaller, exactly as would be the case with any inorganic body. 



In short, the variations of temperatures correspond to the varia- 

 tions of temperature in inorcanic bodies of similar character and 

 similarly i^laced. In fact, Bravais, in winter observations at Bossekoj), 

 La])laud, fouud the temperatures in a live and dead tree to be practi- 

 cally the same. 



It only remains to discuss the average lower temperature in trees. 

 In Dr. Ebermayer's tables, this reduction of temperature at the base 

 of trees as compared with air is 2^.2, while at the crown, with an aver- 

 age diameter of limb two-thirds as great, it was 1°.2. This reduction 

 may be due to the temperatures of the water taken from the ground. 



When the sap tiows most freely— iu spring and early summer— the soil 

 is cooler than the air aud the water drawn from the soil must have its 

 temperature. The tree is so thoroughly protected from the conduction 

 of heat that this temperature must change but slowly as the sap rises. 

 It must give a lower temperature to the interior of the tree and this 

 lowers the annual mean. At the same time, for reasons to be given in 

 the next section, the return sap is probably also cooled to some degree, 

 so that all the water circulating in the tree is, in the warm season at 

 least, cooler than the air. 



The foliage seems especially arranged for the exchange of heat. Its 

 surface is very large for its mass, and it is so exposed to the open air, 

 to the sky, and to the sun's rays that it is adapted to receive and part 

 with heat easily and quickly. The temperature of leaves considered 

 as purely inorganic bodies should, therefore, be lower than that of 

 the air at minimum, and higher at maximum. There seem to be no 

 systemati(; observations on the temperatures of foliage, but botanists 

 have taken a good many occasional observations which confirm the 

 statement just made. The temi)erature of the leaves falls decidedly 

 lower than that of the air on ch^ar summer nights, the ditt'ereuce being 

 cenerallv several degrees and sometimes readiing eight or ten. On 

 the other hand this temperature rises in the sunshine to several, and 

 occasionally to many degrees of temperature higher than the air 

 around; in other words the maximum temperature of leaves iu sun- 

 shine is higher than that of the air, while the minimum temi)erature iu 

 clear nights is lower than that of the air, A leafy branch with the cut 

 end in water and exposed to the sunshine has been found to be cooler 

 than one not in water, suggestiug that the average teuiperature of 

 foliage is on the whole lower than that of the air about it. 



