SHADE TREES. 241 



electric systems are a source of danger from lightning, and we have ob- 

 served cases where large limb.s have been destroyed and the trunks of the 

 trees badly lacerated by electrical discharges from these wires. 



On the whole, the cases of death to trees from electricity are by no 

 means so numerous as is generally believed. Because a large number of 



FIG. 100. Showing electrolysis of gas pipes. (After A. A. 

 Knudson, " Corrosion of Metals by Electrolysis.") 



trees near electric roads, etc., often look sickly it must not be concluded 

 that electricity is always the cause. In cities and towns, where most of 

 these unhealthy specimens are found, there are innumerable destructive 

 factors for trees to contend with. It is quite essential in diagnosis work, 

 therefore, that all of these factors be taken into consideration before a 

 definite opinion in regard to the cause of any abnormal condition is formed. 



Electrolysis. 



Direct current electricity is frequently responsible for electrolysis of 

 gas and water mains, and lead coverings of underground telegraph circuits 

 are often affected. The trouble is often so serious that the iron gas and 

 water pipes become corroded and eaten with holes in a few weeks or 

 months, causing leakage. When gas mains are affected by electrolysis, 

 the gas escapes and permeates the soil, so that electricity sometimes be- 

 comes a primary and gas a secondary factor in the death of trees. 



The phenomena associated with electrolysis are often complex and 

 difficult to do away with entirely, according to expert electricians, but 

 much of the trouble can be eliminated by proper bonding of the rails of 

 electric roads and the grounding of different systems. 



Electrolysis is more common in wet than in dry soils. Cases are on 

 record where severe electrolysis has taken place 700 or more feet from the 

 source of the leakage. It more often becomes troublesome in cities where 

 numerous railways and public-service corporations of all kinds make use 

 of the streets. We have observed cases where plants have been stimulated 

 and their growth increased by escaping electricity in the soil. 



Lightning. 



The common effects of lightning strokes on trees are so well known that 

 it is not necessary to dwell upon them here; but lightning does not always 

 strike a tree in the same way, and the peculiar effects sometimes produced 

 are often interesting. Very powerful discharges of lightning act somewhat 



