PT. rv. 



PRUNING AND THINNING. 



225 



dark mould. Into this the tree itself often strikes 

 roots, which descend between and through the rotten 

 bark which I have mentioned to its very base. Then 

 comes the miraculous force of turgescence, acting in 

 the true line of cleavage of the tree, and the twin 

 leaders are rent from each other to as great a certainty 

 as the granite is split by the wetted bolt of wood. I 

 can show a root thus formed on an elm after (as I 

 beheve) it had been the cause of 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 

 experiment which I have mentioned, p. 98 ? 



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 accidental 

 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 com- 

 munication between the upward course of the sap in 

 the wood, and the downward course in the bark — is a 

 hving leaf or bud. 



Q 



