394 I'RIXCIPLES AND PRACTICE OF PRUNING 



The edges of cement fillings in the crotches of limbs are es- 

 pecially difficult to keep water-tight. Besides bolting the cavity and 

 guying the limbs above it, the crevices at the edges of such cement 

 linings must be made as nearly waterproof as thick tar or asphalt 

 can make them. 



After the cement filling has become thoroughly dry, the outer face 

 may be painted with coal tar or paint, especially around the edges 

 where cracks are likely to appear. This should not be done for 

 several weeks after the cement has been put into the cavity. 



301. Defects in cement work.— Although fillings made with ce- 

 ment mixtures (cement mortar and concrete) have many, and often 

 serious defects, this material is so cheap and so easily handled that 

 no other at present is so generally used for the purpose in this 

 country. The most serious defects in cement mixtures are directly 

 due to the hardness and rigiditjr after the cement has become dry. 

 This inflexiVjility results in cracks appearing across the cement of 

 long fillings (where not put in in sections or blocks, as recommended 

 here) as the tree sways back and forth in the wind (Fig. 303 A). 

 Rods for re-enforcing the concrete are often placed in large cavities 

 to be filled in one block (Fig. 303 D). 



During a cold period in winter, particularly one that has been 

 preceded by warm weather, the wood of an unbolted cavity may 

 draw away from the cement, often leaving a comparatively wide 

 crack (Fig. 303 A). Sometimes, by the contraction of the wood on a 

 cold day, the tree itself will split above or below the filling, or even 

 through the cement when the cavity has been nailed but not bolted. 

 This cracking may be prevented to some extent by having nailed 

 cavities with a vertical partition of tarred paper extending through- 

 out the length of the filling. 



On the other hand, the cement filling forms a surface over which 

 new wood and bark can form during the growing season, and if the 

 decayed and diseased matter in the cavity is entirely removed be- 

 fore the cement is used, it very largely, if not entirely, checks 

 further decay. If cracks appear in the cement, or the wood draws 

 away from the cement, or the work is not properly done, decay 

 organisms may again gain entrance at the edge of the cement and 

 cause further trouble. 



302. Open cavities. — Tn a tree which is not considered of sufficient 

 value to warrant cleaning and lilling the decayed areas, these may be 

 excavated, sterilized and waterproofed (Fi.g. 303 E). In this condi- 

 tion they may often be safely left for years if the waterproof covering 

 is renewed as soon as cracks or blisters appear. Cavities treated in 

 this way are prolialjly as safe as ordinary cemented cavities and 

 ha\e the ad\antage of easy inspection from time to time. Shallow 

 cavities in \aluable trees may be very satisfactorily treated in this 

 manner. The new wo( d and liark produced by the cambium along 

 the mar.gins will form an inwardly rolled edge (Fig. 306 A), as there 

 is no cement across the cavity. 



