THE HANDLING OF THE PLANTS 



147 



opening, and filled on the inside with concrete, the brick serving 

 the purpose of a retaining wall to hold the concrete in place. 

 Concrete used for the main filling is usually 

 made in the proportion of one part good 

 Portland cement, two parts sand, and four 

 parts crushed stone, the consistency of the 

 mixture being such that it may be poured 

 into the cavity and require little or no tamp- 

 ing to make the mass solid. (Fig. 160.) 



'^Fillings thus made are considered by 

 expert tree surgeons to be a permanent pre- 

 ventive of decay. The outside of the filling 

 is always coated with a thin covering of 

 concrete, consisting of one part cement to 

 two parts fine sand. Cavities resulting from 

 freezing, and which, though large on the in- 

 side, show only a long narrow crack on the 

 outside, are most easily filled by placing a 

 form against the entire length of the opening, 

 having a space at the top through which the 

 cement may be poured (Fig. 161). Another 

 method of retaining the concrete is to re- 

 inforce it from the outside by driving rows 

 of spikes along the inner surface of either 

 side of the cavity and lacing a stout wire 

 across the face of the cavity. For best re- 

 sults, all fillings must come flush with the 

 inner bark when finished. During the first IBIf ('ilnfij 

 year, this growing tissue will spread over the 

 outer edge of the filling, thus forming an her- 

 metically sealed cavity. In the course of / ' 

 time, the outside of small or narrow openings I6i. A wound, made 

 should be completely covered with tissue, ^^/TnTluedTitb 

 which buries the filhng from view. cement. 



