THE LARCH CANKER 61 
where it turns sharply down the stem at F and ends blindly. 
The cork made from this layer becomes welded perfectly 
with the outside cork layer, and is perfectly continuous 
across the cortex and phloem. 
Thus the two alternatives (a) and (6) of p. 59 resolve 
themselves into these: either (a) the mycelium grows 
through the gap between the cork layer and the wood at 
F, or (0) it grows round through the wood by the route Gu. 
I think the course by F is the more probable. The space 
left here, though narrow, is not infrequently just large 
enough. And, though I have never been able to trace 
hyphae right through it, I have seen hyphae at each end of 
it. The mycelium of the fungus can under certain circum- 
stances be found in the wood, but I have never observed it 
growing to any extent up or down the stem except when 
the wood is quite dead ; when it occurs it is usually either 
in the wood just inside a canker, where fungal secretions 
have killed all the living cells, or else in the heart-wood. 
It is conceivable, however, that in some circumstances it 
might grow down through the wood from the branch to 
the main stem, and there attack the phloem.? 
For the present the question must be left open whether 
the mycelium passes from a dead branch to the main axis 
through the wood or just outside it. That it can pass such 
a cork layer is shown by an ordinary canker. For this is 
surrounded each year by a cork layer in every way com- 
parable with that at the base of a branch, yet in the 
majority of cases the mycelium successfully penetrates to 
the other side. . 
It is difficult to obtain experimental proof of this method 
of infection. But it may be somewhat strikingly demon- 
strated by cutting down a larch tree, 15-18 years old, 
which has on it numerous dead branches, bearing the canker 
1 As the trunk grows in thickness, the cortex and phloem at the base 
of a side branch generally gets torn away from the wood, and frequently 
the bark of the trunk becomes corrugated round the branch bases. It is 
likely that the canker fungus may sometimes reach the trunk through 
ruptures caused in this way. 
