58 THE LARCH CANKER 



the other cases the branch probably died in the autumn and 

 the canker was initiated in the spring. This cannot be 

 proved from my specimens, but since the branches may 

 become infected with Dasyscypha immediately after death, 

 this seems the easiest explanation of the facts. 



It is thus clear that the conception of a canker being 

 initiated at the base of a living branch and killing the 

 branch is erroneous, and in the majority of instances it can 

 be definitely proved that infection takes place a year or 

 more after the branch has died. Thus at the base of a 

 branch, just at the time when it regularly contains mycelium 

 of Dasyscypha calycina, the main trunk becomes infected 

 by the fungus. It is scarcely possible to avoid the con- 

 clusion that the mycelium passes from the dead branch into 

 the living trunk, and that this is commonly the source of 

 infection of those cankers which are seen on the older 

 trunks of the larch. 1 



There is, however, one important obstruction which the 

 fungus has to pass before it can grow from the branch into 

 the main stem. This is a cork layer which is always made 

 across the cortex and phloem of a branch just before death, 

 and which in the majority of cases is a sufficient obstacle 

 to prevent the passage of the fungus. But in many instances, 

 as in that shown in fig. 27, the mycelium occurs on both sides 

 of the cork layer, and must presumably have passed it. There 

 are three possible ways in which this might occur. Either 

 (i) The mycelium obtains entrance to the trunk before 

 the cork layer is completed, or 



1 A similar conclusion has been reached independently by Mr. P. V. 

 Laidlaw on an estate in Northumberland (vide Quart. Journ. Forestry, 

 1914, p. 216). Also, since the publication of my paper in 1915, Mr. A. C. 

 Forbes has pointed out in the Gardener's Chronicle for February 6, 1915, 

 that he suggested as early as November 15, 1902, in the same paper, that 

 the canker fungus might attack a living twig, and, having killed it, grow 

 down to the main stem and cause a canker there. As this article is not 

 referred to in the index for the last six months of 1902, I may be forgiven 

 for not having seen it earlier. The fact that two foresters have reached 

 a similar conclusion by another route lends support to the correctness of 

 the theory. 



