258 
PRUNING AND THINNING. 
[Part IV. 
splitting off half the tree. The root is still alive, 
though the soil in which it grew is gone. 
Whence comes the upward sap in the wood of 
this root ? or of those denuded in the experi¬ 
ment which I have mentioned, p. 109 ? 
Early and constant pruning will avoid the 
cause of these fruitful sources of decay in 
timber. 
If the heads of trees are dying in, from acci¬ 
dental blight, or from the destruction of their 
leaves and shoots by a strong south-wester, or 
from frost, &c., in all cases they should be cut 
in, not only to where the boughs are alive, but 
to where they are vigorous, and, if possible, at 
the foot of a living twig or bud. If the dying 
boughs are left on the tree, the sap is wasted by 
going up the boughs, without the power of 
breaking out or returning, consequently the 
roots are starved; for the only power of return— 
that is, the only communication between the 
upward course of the sap in the wood, and the 
downward course in the bark—is a living leaf 
or bud. 
If the dying boughs are cut off, the sap, 
which would have been uselessly expended in 
them, invigorates the present shoots, or bursts 
forth in the form of new shoots, and, in return- 
