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 vitahty 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 

 growdng 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 



(a) 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, a is the main axis, 

 B the branch. Everywhere to the outside is a layer of cork 

 c c', 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 ^ 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 c.l. This layer 

 is put in usually, not at the base of the branch, but about 

 1 cm. above it. 



