THE LARCH CANKER 59 
(ii) It grows through the cork layer, or 
(iii) It grows round the cork layer. 
(i) This is impossible, for the formation of the cork layer 
is the last act of the living cortex at the base of the branch. 
The mycelium, however, does not spread till the branch is 
quite dead. So this alternative must be rejected, except 
in the case of trees whose vitality has become very much 
reduced before the death of the branch. 
(ii) This also is apparently impossible. I have examined 
hundreds of cankers but have never found any mycelium 
growing through a cork layer—which in fact forms an 
impassable barrier to the hyphae, except when they attack 
it from the inside. 
(iii) We are thus reduced to the third alternative. And 
since the cork layer is continuous with the peripheral cork 
layer of the tree, it is impossible for the mycelium to get 
round it on the outside. It can thus only get round it on 
the inner or wood side. Here again there are two possibili- 
ties—either 
(2) It grows through the cambium, just outside the 
wood, the cork layer not having been welded 
sufficiently perfectly on to the wood, or 
(b) It grows through the wood. 
To discuss these alternatives the anatomy of the cork 
layer must be described. It will best be understood by 
following the accompanying diagrams (fig. 26). Fig. 26, a, 
shows the general arrangement of the tissues, at the point 
where a branch joins the main stem. 4 is the main axis, 
B the branch. Everywhere to the outside is a layer of cork 
cc’, which becomes wrinkled at the point of junction by the 
continuous thickening of each member. Between c and d 
are the tissues known as the cortex and phloem; at d is 
the cambium, inside which is the wood w. On the death 
of the branch the dead tissue of the latter would come into 
direct contact with the living tissues of the main stem, were 
it not for the interpolation of a cork layer cl. This layer 
is put in usually, not at the base of the branch, but about 
1 cm. above it. 
