66 THE LARCH CANKER 
for many years, and probably snow accounts for the fall of 
most of them. At any rate the dense screen of small lateral 
dry branches so characteristic of our own woods is generally 
absent in the Alps; and since, as has been shown above, 
dead branches are one of the most important sources of 
canker, the absence of such branches in the Alps is a note- 
worthy factor in the suppression of the fungus. 
In the case of British plantations it is questionable 
whether the altitude at which larch is planted is ever great 
enough materially to lessen the attack of the canker fungus. 
Trees may be as badly attacked at a height of 1,000 ft. as 
at sea-level, and where a mountain-side is immune from the 
disease this may be accounted for by the favourable soil 
conditions that such a slope provides. 
Though altitude has little effect in restricting canker, low 
hollows seem to encourage it very markedly. Such hollows 
are peculiarly liable to late frosts, which have an adverse 
effect on the general health of the trees and render them 
more liable to infection, and frost-wounds may admit the 
fungus in some cases. Also the atmosphere is there more 
humid, and conditions are favourable for the formation of 
fructifications of the fungus. 
No direct relation can be traced between the frequency 
of canker and the rainfall or the site exposure. 
Soil. Though it is very difficult to determine exactly the 
soil conditions which favour canker and the reverse, never- 
theless the edaphic factor seems to be of more importance 
than the climatic. Even in flat country neighbouring plots 
of larch may be very variously affected, and since this cannot 
be attributed to climatic differences, it seems a just inference 
that soil is responsible. On the whole we may say that 
soil conditions which favour larch-growing in other respects 
are generally least conducive to canker ; a good, deep, well- 
aerated soil is better than one which is peaty or rendered 
shallow by a pan near the surface. Forbes (1904) lays 
great stress on the fact that young trees are often notched 
or slitted into the upper six inches of soil quite unsuited to 
their requirements. If it is composed of dry peat or débris 
