68 THE LARCH CANKER 



As he points out, finer larch trees are found in mixed 

 woods than generally occur in pure plantations, even when 

 these are free from canker. This is partly due to the fact 

 that ' when mixed sparingly with deep-rooting or compact- 

 rooting trees, the larch-roots can spread without meeting 

 much opposition, while their more rapid stem-growth gives 

 them a lead over other species from the first '. Also the soil 

 under mixed woods is maintained in a state more generally 

 suitable to tree-growth than in pure coniferous plantations. 



One result which we gain from the considerations of 

 contributory causes is that the conditions which are favour- 

 able to the restriction of canker are in general identical 

 with those that encourage the active growth of the tree. 

 Keep the trees growing vigorously and canker is not so 

 likely to become epidemic. The explanation of this may be 

 sought in the nature of the struggle between the parasite 

 and the host. Given an uninterrupted tissue of living cells, 

 the fungus can push forward, killing the cells in advance of 

 it by its secretions and itself growing into them. But the 

 tree resists this progress by cork layers, which we may 

 liken to a series of trench systems, each holding up the 

 enemy for a time, and perhaps entirely preventing his 

 advance. When the fungus grows down from a branch, 

 the first trench it has to capture is the cork layer normally 

 made at the base of a dying branch. If this is passed, fresh 

 cork layers are made in the tissues of the tree, and it often 

 happens that the first of these prevents the canker from 

 spreading farther. If this fails, more and more cork layers 

 are formed, and so the struggle progresses purely offensive 

 on the one hand, and purely defensive on the other. For- 

 tunately the advance of the fungus through living tissue is 

 very slow, especially in a tree which is healthy and fast- 

 growing, so that such a tree has time to make a fresh cork 

 layer (or, to revert to the simile of war, it can complete new 

 earthworks before they are reached by the advancing 

 enemy), and the better and more thoroughly this layer is 

 made the less chance the fungus has of passing it. Not 

 infrequently trees which are growing strongly show a number 



