124 PRACTICAL TREE REPAIR 



As regards the encouragement of calluses, the im- 

 portant thing is to keep in mind the facts and 

 rules concerning the cambium and the growth of 

 calluses which have been explained in the chapter 

 on the nature of tree growth. The mouth of the 

 cavity should be kept as narrow as possible, the 

 sides fairly straight and parallel to the strong- 

 growing ridges of the trunk, and the ends of the 



Cross sections of four incorrect fillings. In the first, the exca- 

 vation is not correctly shaped for retaining the filling; in the 

 second, the filling is brought out so far that the cambium 

 cannot spread over it; in the third, the filling bulges too 

 much; in the last, the wood at each side of the opening is 

 cut too thin 



opening should be tapered. Some otherwise skil- 

 ful operators cut the top of the opening off square. 

 That is a mistake, for if the tree is growing very 

 slowly a triangular-shaped piece of bark just 

 above the filling either dies or makes very slight 

 growth, thus forming an ideal haven for boring 

 insects. To avoid the possibility of the cam- 

 bium's drying out, there must be a reasonable 

 thickness of sap-wood beneath it, which means 

 that very thin lips should be cut back to a point 

 where the wood is an inch, say, in thickness. In 



