182 MASS. EXPERIMENT STATION BULLETIN 170. 



FiG. 51. — Chestnut tree cavity resulting 

 from stump growth, with cavity covering 

 of cement about 6 inches thick. (See 

 Fig. 52.) 



has a tendency to displace the tacks; consequently the metal covering 



becomes loose and valueless in a short time. Metal is inclined to deteri- 

 orate in a few years, and cannot 

 accommodate itself to much 

 movement in the tree unless it is 

 used in sections and imbricated 

 or overlapped like shingles. 



Sometimes metal is used to 

 cover cement-filled cavities, but 

 this is of no particular value, and 

 does not improve the appearance 

 of the tree. The principal pur- 

 pose in using it over cement is to 

 cover the cracks, and when used 

 in connection with iron bands 

 over the surface it is supposed 

 to help hold the cement in place. 

 In some cases where metal is used 

 in this way it is lapped over on 

 the bark 4 or 5 inches, but this 

 destroys the underlying tissues 

 and arrests their future develop- 

 ment, thus defeating one of the 



main objects of treating cavities, — i.e., encouraging and directing the 



healing tissue or callus formation. 

 Elastic Cement. — Elastic cement, such as is employed by slaters, has 



been used for some years in tree repair 



work, and was recommended for this pur- 

 pose by the Massachusetts Forestry Associ- 

 ation about 1900. Its principal value in 



tree repair work consists in its elastic 



properties and its adaptability to places 



where there is considerable movement. It 



is too expensive for use in large cavities, 



costing from 4 to 15 cents per pound, but 



it has been employed to some extent for 



filling small spaces and also as a thin 



covering for cavity openings. In the latter 



case wire mesh is strung across the cavity 



opening, the wire mesh being re-enforced 



with iron and shaped to conform to the 



outer contour of the tree; and the elastic 



cement is plastered on the mesh. (See 



Figs. 54 and 55.) 



This method of treating cavities has been especially recommended by 



Mr. L. F. Prouty, associated with the city forestry department, Spring- 



Fi«i. 52. — illustrating cross-sec- 

 tion of the cement surface 

 •covering to cavity shown in 

 Fig. 51. W, wire stapled to 

 sides of cavity; I, iron reen- 

 forcing; C, cement. 



