BODY AND BRANCH DISEASES AND INJURIES 61 



due to electrical injury as to the severe and unscientific pruning 

 along pole-lines. 



Two types of injury due to current electricity are recognized. 

 Alternating and direct -currents of low voltage, carried in in- 

 sulated wires, cause but little damage, except when local burn- 

 ing occurs at places where the insulation is rubbed away by 

 contact with the tree. The resistance of the bark and wood 

 is so great that the amount of the low voltage current which 

 passes down the tree and into the ground is not suflBcient to 

 raise the temperature sufficiently to kill living cells. The 

 damage is due rather to the combination of mechanical and 

 burning injury at the point of contact, resulting in rough open 

 wounds which become weak points in the limb and centers of 

 infection for wood-rot fungi. 



Direct current electricity causes more damage than does the 

 alternating current. High voltage, uninsulated feed and 

 trolley wires carrying direct current, when the charge is posi- 

 tive, cause local burning but do not kill the tree. The short 

 circuiting is more complete in wet weather, when the tree is 

 covered with a film of water. In cases in which the rails of 

 electric railroads carry the positive charge, and the trolley and 

 feed wires the negative, the effect of contact of the wires with 

 a tree is more serious and often results in death. The dif- 

 ference in effect is due to the root surface being imbedded in 

 the soil-water which is a good conductor and the high voltage 

 positive current is transmitted to the roots of the tree which 

 expose a large surface. For this reason, more current enters 

 the tree than in the case in which the wires carry the positive 

 charge, and severe burning of the living tissues occurs in the 

 roots and at the base of the trunk, often killing the tree. 



Lightning acts in the same way as current electricity, except 

 that a large variety of other effects may also occur. The usual 

 type of lightning-injury is a groove plowed in the bark and wood 

 down the trunk. This groove is straight when the wood- 



