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 



