THE REPAIR AND REPLACING OF TREES 223 



insure the firmer retention of the filling in the tree the cav- 

 ities are studded with nails. When a tree is weak because 

 of a large cavity, steel braces are sometimes inserted hori- 

 zontally and vertically for reenforcement. 



Small cavities are filled with a mixture of two parts of 

 sand to one part of Portland cement. Larger openings, sev- 

 eral cubic feet in volume, are filled by using broken stone 

 and brick with the concrete. In this way it is easier to 

 build up the filling, and its strengthening power is increased. 

 After the filling has had time to stiffen, but not to become 

 perfectly hard, it is finished with cement, a trowel being 

 used to shape it according to the contour of the tree. 



An extremely important point to remember is that the 

 filling must not be brought up flush with the outside of the 

 bark of the tree ; but must stop at a depth equal to the thick- 

 ness of the bark and the filling shaped to follow the contour 

 of the tree, as shown by the line B C B, Fig. 33. This 

 precaution must be taken to enable the healing callus to 

 overgrow the filling. If not very large, the tissue may in 

 time completely heal over the cement and bury it, just the 

 same as the stub left when a limb is removed. (Plate 44, 

 Fig. 3.) 



It is a good plan, when the cavity is ready for filling, to 

 cut a strip of bark about one-half inch wide all the way 

 around the periphery of the opening, as shown in Plate 44, 

 Fig. 4. The cement can be brought up flush with the wood. 

 The healing callus will start to roll over the wood (Plate 44, 

 Fig. 5) , and then over the cement, hermetically sealing the 

 cavity. The edges of the wound only, or the surface of the 

 entire filling may be painted or covered with coal-tar. 



When the hollow trunks of trees are filled with concrete, 

 they are immensely strengthened and are not in danger of 



