SHADE TREES. 



181 



some advance in cavity cement work, it does not solve the problem of 

 treating cavities. In many cases of sectional work it is an absolute fail- 

 ure. This is true especially when there is too much swaying or when 

 the tree cannot stand the 

 load, or when there is too 

 much crushing force, as 

 in narrow cavities. All 

 concrete work on trees 

 is better adapted to cav- 

 ities located near the 

 ground or below the sur- 

 face than to high cavities 

 where swaying constitutes 

 an important factor, and 

 where an increase in the 

 load which a tree is 

 obliged to carry is ob- 

 jectionable. 



Concrete Coverings for 

 the Cavity Opening. 

 Concrete may be used to 

 advantage as a covering 

 for cavity openings to 

 form a surface for direct- 

 ing the healing tissue. 

 With this method the 

 interior of the cavity is 

 left unfilled, and if the 

 cement is properly re- 

 enforced with iron the 

 scheme is practicable and 

 possesses many advan- 

 tages. The writer has 

 treated some large cav- 

 ities by this method, and it has proved as satisfactory as solid fillings. 

 Considerable cement is also saved. (See Figs. 51 and 52.) 



Metal Coverings. Metal was much used formerly, and is to some 

 extent to-day, to cover the openings of cavities, and some very creditable 

 work has been done in this line. For this purpose tin or zinc is cut and 

 shaped to meet the requirements of the cavity opening, and after some 

 of the bark has been cut away the metal is securely fastened to the sap 

 wood with tacks. With this method of treating cavities the usual clean- 

 ing and disinfecting are done, but the cavity itself is left unfilled. 



The principle underlying this method is good, but metal has not proved 

 a durable covering, nor are its physical properties suitable to work of 

 this nature. It is affected too greatly by changes in temperature, which 



FIG. 50. Concrete filling built in sections. fFrom 

 "Tree Talk.") 



