GENERAL METHODS 137 



brace it so that it cannot twist, and the raw con- 

 crete is just as weak under torsion as it is under 

 tension. As an inevitable result minute fractures 

 are produced in the filling, which forever destroy 

 its tensile strength. It may be said that the con- 

 crete at least retains its compressive strength, and 

 that it acts as a buttress to the tree. An architect 

 might answer that a shallow buttress offers but 

 little resistance to a horizontal stress, and a num- 

 ber of instances have come to the writer's notice 

 in which the filling seemed to act as a fulcrum in- 

 stead of a buttress, and actually helped to over- 

 turn the tree. Furthermore, trees are twisted off 

 more often than they are blown straight over, and 

 a concrete filling cannot offer much resistance to a 

 twisting force. 



Air these considerations compel the conclusion 

 that the idea that a concrete filling appreciably 

 strengthens a tree is a mistaken one. What, then, 

 is the true function of the filling? That function 

 is threefold. The filling keeps insects, fungi, and 

 water from gaining access to the heart of the tree ; 

 it forms a surface across which the calluses can 

 grow, a growth which aids the tree physically and 

 physiologically; and it probably has some value 

 in preventing the collapse under strain of a hollow 

 trunk, a value due entirely to its power of resist- 

 ing compression. 



The effect of this change in the purpose of fill- 



