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 (b) 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 (b) it grows round through the wood by the route GH. 

 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. 1 



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. 



