370 x 
that of the atmosphere, by a difference varying from 
one to four degrees *, 
6S2. These varying and contradictory results op- 
pose the belief that the tree possesses any natural 
power of steadily maintaining a temperature higher 
than that of the surrounding air, and lead to the sup- 
position that its fluctuating condition, in this respect, 
proceeds entirely from accidental causes. Variations 
in atmospheric temperature must be supposed to in- 
fluence that of the tree ; but the rapidity with which 2 
changes occur in the former would not, in an equal 
degree, affect the latter. Thus, in summer, the tem- 
perature of the tree rose slowly, when that of the at- 
mosphere was rapidly changing; and when the latter 
was as high as 57*, the heat of the tree was 
generally less. On the contrary, when the atmos- 
phere fell below 57, the tree was a few degrees 
warmer ; but if the cold continued a few days, both 
the tree and the atmosphere came down to the same 
temperature, even in April when vegetation was ac- 
tively going on. These facts seem to prove, that the 
tree possesses no internal power of producing heat ; 
but that its temperature follows that of the surround- 
ing air, subject to such variation as arises from a , 
difference in its conducting power, and other acciden- 
tal circumstances. 
683. But whatever doubts may exist as to the na- 
tural temperature of trees, the facts already adduced 
(17O.), concerning the heat exhibited by certain 
plants, during the process of fecundation, are too 
* Phil. Trans, 1/78, p. 46. 
