78 FORESTRY IN SPAIN. 



which almost exclusively determine the temperature of 

 the tree, both in its aerial and its subterranean parts. 

 The influence of the temperature of the soil extends frotn 

 the roots to the trunk, but only to a certain height, which 

 is determined by the temperature of the air which 

 surrounds the tree. 



' The temperature of the roots is imparted by the soil. 

 The origin and nature of the soil and the subsoil, its 

 mechanical and physical properties, the degree of humi- 

 dity, the absence or presence of subterranean water, the 

 superficial covering of the ground, and the varying 

 degrees of sunshine and shade, are so many other factors 

 which influence that by modifying the character and 

 degree of temperature imparted to the roots. With 

 change of temperature in the soil, which in most kinds of 

 land are very slight even in the warm days of summer at 

 a depth of one metre from the surface, the temperature of 

 roots must change in the same proportion. The portion 

 ot these roots situated more than a metre in depth will 

 only be affected by the heat of the ground throughout the 

 year, which varies very little ; while the roots at a less 

 depth experience the daily changes of temperature in the 

 superficial layers. A long, large, deeply-penetrating tap- 

 root will produce in the warm months a diminution, and 

 in the cold months an increase, in the temperature of the 

 superior roots. Humidity exercises a very great influence 

 on the thermometric conditions of the soil and roots. 

 From this it may be inferred that the influence of the 

 heat of the soil is not confined to the roots, but through 

 the ascending sap makes itself to be felt to a certain 

 height in the trunk. To determine by experiment this 

 hypothesis, Hartig made some observations on two oaks 

 of the same magnitude and same age, 200 years. At the 

 height of a metre from the ground, he introduced ther- 

 mometers to three different depths, and noted the ther- 

 mometric condition of both trunks during the winter's 

 lethargy, as well as during the season of active vegetation. 



* The aerial parts of a tree are exposed to the direct 



