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. 
