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 been 

 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 some 

 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 been 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 



