174 PHYSIOLOGICAL BOTANY. PART II. 



to account for these facts without the necessity of as- 

 cribing them to the results of any physiological action. 

 The roots penetrate the earth to a depth where the soil 

 is always warmer than the atmosphere in winter and 

 cooler in summer, and the moisture which they imbibe 

 will consequently partake of this influence. Hence it 

 has been observed, that the internal temperature of trees 

 is about the same as the soil at one-half the depth to 

 which their roots penetrate. The maintenance of an 

 internal temperature distinct from the external is as- 

 sisted by the nature of the wood itself, which is a bad 

 conductor of heat; and also by the property which it 

 possesses of conducting heat better in a longitudinal 

 than in a transverse direction. As an example, we may 

 mention that the milk of the cocoa-nut is kept cool 

 during the hottest part of the day by the thick fibrous 

 coating of the pericarp, which is a very bad conductor 

 of heat. 



