DE CANDOLLE’S EXPLANATION, 151 
stance which might be explained on the suppo- 
sition that the living tree actually possessed a 
power and source of internal heat. John Hunter 
seemed to prove it still more satisfactorily; for he 
caused a hole to be bored into the trunk of a tree, 
and placing a thermometer in it, he found that 
it stood two or three degrees higher than one 
exposed to the air. 
M. de Candolle has offered an ingenious ex- 
planation of these singular facts. After mention- 
ing the experiments of some other authors upon 
the subject, in which it was found that thermo- 
meters buried in the earth at the foot of the tree, 
indicated the same temperature as the trunk of 
the tree, he comes to the conclusion that it is 
because under the surface of the earth the water 
in the soil is actually warmer than the surface, as 
it is well-known to be in Winter, and because 
this water rises into the tree through the roots, 
communicating its warmth to the wood through 
which it passes, that therefore the interior of a 
tree 1s warmer than the wintry air around it. 
In other words, he believes that trees have no 
source of internal heat of their own, but derive 
their warmth from the soil in which they grow. 
