FILLING OF CAVITIES 103 



always at least a potential cause of the tree's 

 destruction. 



It is no wonder, then, that heroic measures are 

 often resorted to in order to stop the spread of 

 a cavity. For, as the essential reason why they 

 must be treated is that all cavities grow, so the es- 

 sential principle of their treatment is to stop their 

 growth. And, conversely, the degree to which 

 a treatment is successful is the degree to which the 

 growth of the cavity has been stopped. Although 

 the essential idea in the treatment of cavities is to 

 stop their growth, other principles are involved, 

 which, though not primary, are by no means with- 

 out importance. These are that the treatment 

 must be such as to facilitate the healing over of 

 the interruptions in the continuity of the bark 

 which accompany wounds and cavities, and, in so 

 far as possible, it should be such as to restore to 

 the tree the strength which has been taken from it 

 by the destruction of its tissues. The first of these 

 is in one way a corollary to the principle first 

 stated, that of stopping growth, because the heal- 

 ing over of a wound is the surest way to prevent 

 reinfection. 



The ways in which these principles are put into 

 practice will appear later, but it may be well to lay 

 down at this point a skeleton program of the dif- 

 ferent processes which are normally included in a 

 complete cavity treatment: 



