THE LARCH CANKER 55 



with his foot. If this were so cankers should be especially 

 frequent near the ground, which I have not found to be 

 the case. It is true that in some woods cankers are apt to 

 occur at rather definite levels ; but this level may be near 

 the ground, or 3 ft., 6 ft., or even 30 ft. above it. This 

 probably means that in certain years the meteorological 

 conditions have favoured the formation of cankers, which 

 have then occurred at the level most susceptible to attack. 

 On examination of a plantation in which attack had com- 

 monly occurred near the base one might be tempted to 

 adopt such a theory as that suggested. But my own 

 experience has not led me to attach much importance to 

 this source of infection. 



The second suggestion as to the source of wounds is 

 based on the observation that on such main stems cankers 

 nearly invariably occur at the base of branches which have 

 died. On this account it has been thought that the swaying 

 of branches in the wind, and their depression under snow, 

 may cause cracks in the bark at the branch bases, through 

 which infection may take place. This would further be 

 encouraged by the fact that spores settling on the trunk 

 would be washed down by the rain and often get lodged in 

 the axils of the branches. Now if this were really a serious 

 cause of canker we should expect to find cankers particularly 

 frequent on the edge of plantations, where the side branches 

 grow bigger and thus offer more surface to the wind and 

 snow, and where also the wind is more pronounced and better 

 able to sway the branches. But, as a matter of fact, the 

 reverse is the case, and cankers are much more frequent 

 inside a larch plantation than on its edges. Also it will 

 not be found easy, by any ordinary swaying of a branch, to 

 cause such ruptures as this theory requires. 



In order to test the various theories that have been 

 advanced to account for infection, I have examined a large 

 number of cankers on main stems of larch. Most of the 

 specimens were obtained from Bagley and Tubney woods 

 near Oxford, and in all cases the cankers were taken from 

 otherwise healthy trees which had full crowns and gave no 



