72 Diagnosing of Diseases 



bole there may not be any more consequences than accom- 

 pany any ordinary or frost-split wound which may be healed 

 by callusing, but when it follows a spiral course, killing 

 cambium on all sides of the tree, the result is like that of 

 girdling. 



\'ery lately it has been discovered and experimentally 

 demonstrated ' that electric shocks of low tension occurring 

 during thunder-storms, especially in winter, are the frequent 

 cause of the phenomenon described above as stag head, the 

 leader and upper portions of the crown for from five to ten 

 feet or more being killed while the rest of the tree remains 

 uninjured. 



E\-ery tree owner should look out for the wire stringer, 

 who not only disfigures the tree by chopping off branches, 

 regardless of consequences, but introduces the danger of 

 electrocution. Trees and branches, beating against elec- 

 tric wires in winter storms, wear off the insulation and thus 

 establish short circuits, which under certain weather condi- 

 tions as, for example, in a thunder-storm, may bring about 

 the total destruction of a long-cherished old tree. 



According to the latest investigations '^ there is a difference 

 in the effects of direct currents which are used chiefly in 

 operating electric railroads, and of alternating currents of 

 the electric light plants, which, although carrying a higher 

 current, appear to be less disastrous. 



In moist or wet weather, when the tree is co\Tred \\ith a 

 film of water, the current causes, at the point of contact, an 

 injury to the tree, which provides favorable conditions for 

 leakage through the film of water, grounding the current and 

 burning the hmb, partially or entirely killing the cambium 



' See Forestry Quarterly, Feb., 1904. 



■^"Injuries to Shade trees from Electricity." Bulletin No. 91, Mass. 

 Agr. College. 



