THE LARCH CANKER 67 
of recently cleared coniferous woods, the superficial layer 
is poor in nutriment ; and if the site is one where grass or 
heather has recently been growing, the roots of these plants 
will prevent a free interchange of gases between the soil 
and the air. Larch is peculiarly sensitive to the condition 
of the surface, for being in its youth a surface-rooter it 
cannot range through the deeper layers in search of the 
nutriment which it requires. For the same reason the sub- 
soil is of secondary importance, i.e. for the normal growth 
of the tree. (The nature of the subsoil is an all-important 
factor in susceptibility to heart-rot.) So long as this is 
well drained a good thick surface-soil will grow satisfactory 
larch, and if the trees are growing vigorously there is a good 
chance of their remaining free from canker. Perhaps the 
safest situation is a mountain-side, where the porous, well- 
drained, gravelly soil derived from the rocks above provides 
all the factors favourable to larch culture. 
Only in comparatively few cases can we say that a soil 
will or will not grow healthy larch until it has been tried. 
I have seen healthy woods grown on a clay subsoil and 
plantations riddled with canker on sand. Mitchie (1885) 
says that soil which is suitable for barley will generally 
prove successful with larch. 
Conditions of culture. Pure larch-plantations are more 
liable to attack than trees which are mixed sparingly with 
hardwoods, such as hornbeam or beech. This is usually 
explained by supposing that the hardwoods act as a kind 
of screen, protecting the larch from fungal spores which are 
blown through the woods. Such a theory, however, cannot 
be entertained when, as is often the case even in mixed 
woods, the dead lateral branches of the larch become 
covered with the fructifications of Dasyscypha. For this 
shows that the spores have found out the larch trees, and 
the number of fructifications made on the dead branches 
would be sufficient to ensure infection of the trees, were 
they susceptible. Probably Forbes is right in accounting 
for the comparative immunity in mixed woods in quite 
another way. 
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