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 Nave 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 tke 
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 
