180 



THE GYPSY MOTH. 



time of the year when the bark peels most readily. In ex- 

 planation of this, we cannot do better than to quote Prof. R. 

 Hartig of the University of Munich, whose pathological work 

 is a model of thoroughness and scientific accuracy : — 



At the time when the cambium is active, it is quite impossible 

 to prevent the cortex being loosened, the friction of the saw being 

 sufficient to account for it. Bat the main cause is to be traced to 

 the fact that, in order to prevent the cortex being torn off, a cut is 

 first of all made underneath, and during the sinking of the branch 

 the lower edge of the wound is subjected to severe pressure. 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 from the edge of the wound. 

 Of course in such a case the new growth — namel}^, the callus — is 

 not formed at the edge of the wound, but at a considerable dis- 

 tance from it, where it is covered by the cortex. The result is 

 that the cortex, which was originally in intimate contact with the 

 wood, becomes detached, 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 w^ell as all the organisms that it may contain. This 

 forms a specially suitable place for the germination of the spores 

 or 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 w'ood. 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 formed after the cortex 

 has been separated from the wood by the advancing callus. It is 

 in fact the Achilles' heel of the branch wound. In pruning, the 

 main object must be to prevent its formation ; but this is possible 

 only if pruning be confined to autumn and winter, when growth 

 is at a stand-still, and when the cortex is less liable to be detached 

 from the wood. If one also takes the precaution to support the 

 branch during sawing, and at the moment of separation to push it 

 clear of the w-ound, danger is reduced to the minimum.* 



* " The Diseases of Trees," by Prof. R. Hartig, pages 255-257. English edition by 

 Dr. H. Marshall Ward, London, 1894. 



