GENERAL SUMMARY 187 
Some soils are unsuitahle for tree growth owing to the 
presence of a pan or hard stony layer, which sometimes 
forms in sandy soils. When a pan is near the surface it 
has to be broken through, in planting, to allow free develop- 
ment for the roots. When at a depth of 12 in. or more it 
may not. be discovered during planting operations, but it 
is none the less detrimental to tree growth, as it interferes 
with the normal flow of water. In wet weather it allows 
water to stagnate round the tree roots, and in times of 
drought it prevents water rising by capillary attraction 
from the lower levels where moisture is still present. One 
of the worst cases of canker that I have met with can only 
be adequately explained by the trees being weakened 
through shortage of water caused by such a pan in dry 
weather. 
It has been found that the correct choice of site, soil, 
and mixture does much to restrict attack by the, canker 
fungus. This is not because under these circumstances the 
canker fungus is less common, for, wherever larch is grown, 
it is present on the dead branches in sufficient profusion to 
infect all the trees. It is the direct outcome of the greater 
vigour of the trees. The larch normally protects itself 
against the canker, first by its original layer of cork, which 
covers the whole tree and extends across the phloem and 
cortex of newly dead branches, and, if the fungus has passed 
this, by new layers of cork which are put in to prevent its 
further penetration. If the tree is vigorous these layers 
are made quickly and made thick, so that the fungus has 
much more difficulty in reaching the cambium. By such 
means the tree throws off attacks and cankers are pre- 
vented; but if the trees are weakly the fungus has things 
too much its own way, and has little difficulty in reaching 
the cambium and forming cankers. 
It has been shown that the most frequent way in which 
the main stems of larch trees become infected with canker 
is by the fungus, which lives saprophytically on the branches, 
growing down, past the cork layers at the branch-bases, into 
the trunk. It is suggested that when larch woods are 
