GROWTH OF TREES. 207 



should produce its effects first upon the lower parts of 

 the tree : whereas the twig that has the warmest si- 

 tuation, is always the first to come into action. In a 

 mulberry tree, upon which we look out while writing, 

 the leaves upon the upper part of the south-west side, 

 which is freely exposed to the sun, are nearly ex- 

 panded, while those upon the lower part of the same 

 side, which is shaded by another tree, are farther back- 

 ward. The experiments made upon the birch tree, by 

 Dr. Walker, in which sap flowed from the lower inci- 

 sions when the temperature was just above 46, and 

 not from the cut points of the twigs, till the thermome- 

 ter was 1 higher, are not proofs of an ascent of the 

 sap from the roots, and an oscillation of it in the ves- 

 sels of the tree, according to the temperature, in the 

 same manner as the fluid oscillates in the tube of a 

 thermometer. The trunk of a tree may, from various 

 causes, be warmer than the twigs, and it is always less 

 injured by the winter; the bark and wood, of which 

 there is more in the bole of a tree than in the branches, 

 prevent it from being so much affected by the frost ; 

 and twigs, without having the powers of vegetation ab- 

 solutely destroyed, may have the expansion of their 

 buds very much retarded. Of three evergreens, the 

 one a common laurel, the other a holly, and the third 

 an aucuba japonica, which are of the same height and 

 equally exposed to the weather, the twigs of the top of 

 the laurel were injured, but not killed, by the cold of 

 last winter, and those of the others were not touched. 

 The leaves that were first shed, were those of the in- 

 jured twigs of the laurel, though the dead edges exfo- 

 liated before the leaves were dropped, which was a 



