SHADE TREES. 



235 



offers the least resistance to the transmission of an electric current; and 

 even if there were no continuity it would be necessary for the current to 

 pass through a great many cell walls even for comparatively short dis- 

 tances on the trunk. In case the protoplasm was continuous or there 

 existed continuity, the strands would be so very small that they would 

 undoubtedly offer high resistance, due to their attenuation. Whatever 

 conditions prevail, trees show relatively high electrical resistances, a 

 feature which is no doubt 

 of some biological impor- 

 tance. The high resistance 

 of trees, moreover, is un- 

 doubtedly a protection in 

 case of lightning strokes, 

 since often the heat devel- 

 oped is enough to do only 

 slight injury. On the 

 other hand, if trees pos- 

 sessed tissue with rela- 

 tively small electrical re- 

 sistance, they would be 

 more seriously affected by 

 currents from high-tension 

 wires. The electrical re- 

 sistance of trees is so high 

 that it is doubtful whether 

 injury ever occurs to them 

 from contact with low or 

 even high-tension wires, 

 except that produced by 

 grounding when the bark 

 is moist. Any escaping 

 current from transmission 

 lines that can be trans- 

 mitted even through the 

 least resistant tissue is 

 likely to be insignificant. 



Fi<;. 98. Showing disfigurement of trees caused by 

 high-tension alternating current wires. 



Effects of Alternating Currents. 



The alternating current systems employed for lighting purposes vary 

 greatly in their potential. Cases of burning from alternating currents are 

 more numerous than those from direct currents because trees are brought 

 into more frequent contact with the wires, and, owing to the higher po- 

 tential, more leakage is likely to occur. The high and low voltage lines 

 may vary from 100 to 100,000 volts. The high-tension systems are in- 

 variably constructed across country, and are naturally not brought into 

 very close proximity to shade trees. No injury to trees whatever occurs 



