TREATMENT OF WOUNDS 49 



should be facilitated by lightly scraping their ap- 

 proaching surfaces in spring. In extreme cases, 

 if unsightly lips have been formed which offer no 

 hope of ever growing together, the entire ridge 

 can be sawed off and the wood covered with a 

 strip of sheet iron. In most instances it will be 

 enough if a dressing (but not an unsightly one) 

 is kept over the crack in order to keep out fungus 

 spores. The cracks open in very cold weather, 

 and that is the best time to put in fillings or to ap- 

 ply dressings. Scraping the bark increases the 

 liability of a tree to suffer from frost cracks. 



Lightning and other electrical phenomena af- 

 fect the trees in many different ways. Lightning 

 often smashes a tree all to pieces. Usually, how- 

 ever, it breaks a few branches out of the top and 

 then passes down the trunk to the ground. As 

 the moister parts of the tree are the best con- 

 ductors, the electricity almost invariably takes its 

 course down the cambium and the wet sapwood 

 just below it. The course is usually rather nar- 

 row, oftenest three or four inches wide, though 

 sometimes there are two or more such courses 

 down the trunk. The wood offers sufficient re- 

 sistance to the electricity to produce a high degree 

 of heat. This heat instantly vaporizes the sap 

 and it is the pressure of the steam thus produced 

 which rips the long ribbons of bark and splinters 

 of wood out of the trunk. The only thing to do 



