2 5 6 



DISEASES OF TREES 



The cortex of the lower edge of the wound forms a pivot 

 round which the sinking branch turns, and, although the effects 

 may not be immediately visible, still the crushing and tearing 

 at that point kills the cambium for an inch or two back from 

 the edge of the wound. Of course, in such a case, the new 

 growth namely, the callus is not formed at the edge of the 

 wound, but at a considerable distance from it, where it is covered 

 by the cortex (Fig. 148). The result is that the cortex, which was 

 originally in intimate contact with the wood, becomes detached, 



FIG. 147. A branch- wound on 

 an oak which has been half 

 occluded by a callus. 



FIG. 148. The lower edge 

 of a branch-wound one 

 year after being formed. 

 The cortex, a, that has 

 been crushed during the 

 sinking of the branch, 

 dies as far as 6, at which 

 point the formation of 

 callus, <", begins, and the 

 cortex is gradually separ- 

 ated from the wood. 

 Natural size. 



so that a cavity is formed beneath the wound between the wood 

 and the dead tissues. This cavity acts like a gutter to catch the 

 rain-water that flows over the surface of the wound, as well as 

 all the organisms that it may contain. This forms a specially 

 suitable place for the germination of the spores of parasitic 

 fungi, and it is from here that water containing the soluble 

 products of decomposition finds its way by means of the 

 medullary rays into the interior of the wood. This cavity is 

 a gutter in every sense of the term, and at the same time the 

 point of attack for fungi. Even although the surface of the 

 wound may have been coated with tar immediately after 

 pruning, this spot remains unprotected, and indeed it is only 



