54 THE LARCH CANKER 
as regards canker, with others which were sprayed with an 
insecticide. 
Somerville’s suggestion that Chermes may partially 
inhibit transpiration and so increase the growth of the 
fungus after infection is based on Hartig’s theory of the 
connexion between mycelial growth and transpiration, 
which has a very insecure foundation. At present the 
evidence is not conclusive that Chermes does in any way 
encourage canker (vide Gardener’s Chronicle, 1896, p. 435). 
4, Other wounds may be caused by men or animals 
breaking off twigs, and perhaps by one twig rubbing against 
another. 
We thus see that, if the fungus were able to enter the 
tree through any sort of wound, it would have many oppor- 
tunities of infecting young twigs, and it is remarkable that 
cankers are not more frequent than they are on side branches. 
But branch cankers are of small practical importance to the 
forester. What he must try to prevent are cankers on the 
main stems of the trees. 
(ii) Now the factors which bring about these important 
cankers are of a different nature, because such cankers are 
usually found to have been initiated when the portion of the 
stem is three or more years old. This is seen by observing 
‘the number of annual rings which are left intact in a trans- 
verse section of a canker. Cankers dating from the first 
or second year of growth are extremely rare on trunks, and 
probably when infections occur at this early age the leader 
is killed by the fungus and its place is taken by a side shoot. 
But the predisposing factors that we have considered 
are not applicable to the main stems after three years of 
growth. The periderm layer is then sufficiently strong to 
prevent damage by frost (except in extreme cases), hail, or 
Chermes, and the central axis is not likely to be affected by 
abrasions caused by the rubbing of other branches. We have 
thus to seek fresh causes for the infection of the main axis. 
Two sources of wounds have been suggested. The first 
is that in planting out the young trees the labourer may 
graze the bark of the trees when pressing down the earth 
