Part IV.] PRUNING AND THINNING. 
257 
annually in girthing, the junction at their two 
bases progresses upwards, inclosing the bark of 
each between the two. This double stratum 
of thick bark is killed by mechanical pressure, 
perhaps for a foot or two in height, and rots to 
that extent. This prevents the deposit of any 
new wood on those parts of the inside of either 
leader, and consequently also prevents the per¬ 
fect junction, or anastomosing, of the wood of 
the two leaders. Water lodges in the hollow at 
the fork ; and a frost which is severe enough to 
freeze this water will rend apart the trunk of 
the sturdiest oak to a certainty. Besides this, 
the hollow at the fork becomes a leaf-trap, 
catches any dust which may be driven by wind, 
receives the moss and detritus of the bark 
which are washed down by rain, and forms a 
cupful of fine dark mould. Into this the tree 
itself often strikes roots, which descend between 
and through the rotten bark which I have men¬ 
tioned to its very base. Then comes the mira¬ 
culous 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 believe) it had been the cause of 
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