76 



TECHNICAL PROPEllTIKS UE WuOD. 



unsound knobs are common in broaJ-leuvcd trees, and are fre- 

 quently due to the breakage of a branch. 



If, on the contrary, the cut of the pruning be close along the 

 stem, and thus lying in the direct course of the sap, the process 

 of occlusion is greatly facilitated ; once a cushion of callus 

 (fig. 19) is formed, it closes-in each year on the wound, and 

 covers it over more or less rapidly, according to the rate of 

 growth of the tree and the size of the wound (fig. 20). It is 

 certain, however, that wood so exposed for several years must dry 



Fig. 19 Fia. 20. 



and shrink, and become at least slightly decomposed, whilst all 

 this time it is exposed to the entrance of spores of fungi, which 

 may cause rapid decay, extending to the heartwood of the tree. 

 [Broillard recommends that in the case of oak, no pruning should 

 be done after any heartwood has formed in a branch. — Tr.] 

 The turpentine which flows from such wounds in conifers, and 

 hardens into resin on their surface, protects them to a certain 

 extent from fungi ; in the case of broad-leaved trees snch 

 protection should be afforded by smearing the wound with coal- 

 tar. All closed wounds exceeding two inches in diameter 

 should be looked on with suspicion. 



