THE LARCH CANKER 69 
of incipient cankers at the bases of their branches. Blackened 
bark and slight exudation of resin show that the fungus has 
penetrated to the cortex of the trunk, but, though they may 
develop into normal cankers, these spots frequently dis- 
appear as the tree grows older, and the fungus, held from 
further penetration by a cork layer, has died from lack of 
nutriment. 
The greater susceptibility of weakly-growing trees may 
thus be associated with their enfeebled power of forming 
cork layers. The surprising rarity of canker on other 
conifers, such as spruce or Scots pine, which have beeu 
shown experimentally to be susceptible to infection, may 
be due to their possessing this power in a higher degree. 
In particular, isolated cankers have not infrequently been. 
found on the Japanese larch, but in general this tree is 
surprisingly free from the disease, even when grown in close 
association with cankered European larch. It has yet to 
be demonstrated that Japanese larch has a more pro- 
nounced faculty for making cork layers than the European 
larch, but it is probably along these lines that the explana- 
tion for the comparative immunity of the former tree should 
be sought. And as it is now considered that nearly every 
species is composed of a number of ‘races’ which differ 
only in small particulars, it may be expected that somc 
races of European larch are better able to make such cork 
layers than others, and the selection of such races may be 
the ultimate means of growing larch without canker. Selec- 
tion of tree races is a slow affair, but it is clearly advisable, 
when collecting seed, to choose those trees which have 
grown free from canker, in the hope that their progeny 
may share their immunity. 
Methods of prevention. The treatment of larch which 
has been recommended at various times for the prevention 
of canker has always becn based on the current views as 
to the causes which lead to the disease. Thus Hartig 
insisted that every kind of wound must be, as far as possible, 
avoided, and he is particularly emphatic that branches 
must never be removed until they are dead. Somerville 
