104 DISEASES OF CROP-PLANTS 



no power to regenerate its covering, only the cambium can 

 produce new material to cover it. The process can be followed 

 on trees from which branches have been cleanly removed. From 

 beneath the cut edges of the bark around the margin of the 

 wound a lateral outgrowth of callus from the cambium creeps 

 in to cover gradually the exposed wood. If the surface is 

 smooth enough and the wood lasts long enough without rotting, 

 this ingrowing callus meets and unites in a more or less central 

 point or line, and restores the continuity of the cambium, so that 

 new wood and new bark are formed over the wound. If on the 

 other hand the exposed wood rots away, leaving a cavity, the 

 foundation for this process is lacking, the callus curls round on 

 itself at the margin of the wound, and an open passage is left to 

 the interior of the tree. 



When a branch is cut through or broken some distance from 

 its base, there is a tendency, especially if the branch is situated 

 low on the stem where it is in the shade of the leaves, for the 

 stub so left to be thrown out of the circulation, so to speak, and 

 die back to its base on the parent branch or stem. So long as 

 the dead stub remains, the callus which forms around its base 

 cannot cover its site. If it is left to rot away it usually leaves 

 a cavity which is also difficult to heal. 



If a vertical stem is cut through, the upward growth is usually 

 taken on by a shoot or shoots arising below the cut, and a process 

 of healing similar to that just described for a lateral wound 

 begins. 



If the new shoots arise close to the cut, so that no dead 

 stump blocks the progress of the callus, the wound will cover, 

 subject to the same condition as before, that the wood resists 

 decay for a sufficient time. For thick stems the required time 

 may be many years. Frequently in such a case the covering 

 remains incomplete, the wood rots and the new stem or stems 

 sooner or later break down owing to the incompleteness of their 

 structure on the inner side. 



The Treatment of Wounds. 



On the facts thus outlined are based the principles of tree 

 surgery, which are simple enough, but are commonly neglected. 

 They may be stated as foUows : — 



1. When a branch is removed it should be cut close to the 

 base and the resulting wound shaped into as close conformity 

 with the form of the parent branch or stem as is possible without 

 making the wound unduly large. The edges of a saw cut should 

 be rounded off, especially the projecting lip usually left on the 

 lower side. 



2. The live bark should extend, cleanly cut and not bruised 

 or torn, close up to the edge of the wound. If there is dead bark, 

 it should be cut back until eliminated, and the wood shaped 

 accordingly. 



