THE LARCH CANKER 65 
canker should be more frequent in lowlands than in the 
larch’s natural home in the Tyrol. Hartig adduced three 
reasons to account for this difference. The first is that 
when the larch is growing in its native home, where natural 
regeneration takes place freely, young trees are always 
springing up among the older ones, and forests are com- 
posed of trees of all ages. But canker, as an epidemic, is 
nearly confined to plantations under twenty years of age, 
and trees which are older than this are generally free from 
attack. So in the Tyrol the canker fungus can only pick 
out here and there the younger trees from among a much 
larger number of older ones. (This leaves out of account 
the frequency of canker on side branches of older trees 
which I have particularly noticed in the Tyrol.) The second 
reason is that in the mountains the trees are not often 
surrounded by damp stagnant air, which is conducive to 
the formation of fructifications-of the fungus and to the 
germination of the spores. Lastly, the early spring in the 
lowlands causes a premature renewal of vigour. The twigs 
become full of sap and the needles begin to appear. But 
too often this is followed by May frosts, which cut back the 
shoots and make a number of frost-wounds on the young 
twigs, which reduces the vitality of the trees. In the Alps 
summer development does not begin so early, so that the 
trees are less likely to be cut back by frosts. 
All these causes are no doubt operative to a limited 
extent, but their importance must not be over-cstimated. 
Indeed, larch in its native habitat is by no means free from 
canker, and as high as 5,000 ft. in the south Tyrol I have 
found parts of trees almost riddled with the disease, though 
not so much on the main trunks as on the side branches. 
Boden (1904) also comments on the fact that trees may be 
cankered as high as 7,200 ft. in the Alps, whereas the 
disease is often absent from plantations in the plains. In 
parts of the Tyrol, which I have seen, the larches do not grow 
in dense formation, but are sufficiently scattered to allow 
the full development of the side branches. Of course many 
branches must die, but not, as a rulc, till they have grown 
1888 PF 
