TREE TEMPERATURES. 



73 



in the soil. In ¥\ix. 4.) is given ii grapliical representation of the mean 

 monthly temperature, for the tree (unbroken line), for the air (broken 

 line), and for the soil aib the depth of 4:^ feet (dotted line) The straight 

 lines across the diagram horizontally represent the mean annual tem- 

 peratures for these three positions respectively. It appears that the 

 mean temperature of the tree is lower than that of the air (by 0o.3), 

 and still more so than that of the soil at the depth of 4 feet ())y — l^.T). 

 The results are not easy to interpret. A part of the difference is 

 probably due to a retardation of diurnal nuixima and minima in the 

 tree. M. Becquerel found this retardation very marked, but his obser- 

 vations were of such a kind that they can not be satisfactorily tabu- 

 lated. He found that in small branches the summer diurnal maximum 

 was retarded by two or three hours, while for larger ones it might be 



In a horse-chestnut. In the air. -/;( soil four and a quarter feet below surface. 



¥m. 43.— Tree temperature — Geneva ob.servatious. 



twice this. This being the case, the daily observations would not fall 

 at the same i)arts of the daily curve of temperature, and whether the 

 correction to be api»lied to bring them to a comparative condition 

 should be i»ositive, negative, or zero would depeiul on the hours used 

 and on the retiirdation in the tree. The differences shown in Fig. 43 

 are, therefore, not necessarily the true differences, and with the proper 

 corrections many of the irregularities of the curve would ])robably dis- 

 appear. The curve, however, indicates that the tree is relatively 

 warmer in early spring and late sumn^er and the opposite between 

 these dates, and also that Ihe sunimei' maximum is retarded as com- 

 X)ared with that of the air. 



The trunk of the tree is so constructed as to be a very poor conduc- 



