10 MASS. EXPERIMENT STATION BULLETIN 156. 



cally charged, and when wet it was impossible for linemen to work on 

 them. Under these conditions the insulation was much less etfleient, 

 and even wooden sleeves imbedded in coal tar and rubber proved of 

 small use in jDreventing leakage, but othemvise there was little or no 

 trouble from burning. 



We were able to examine only a few of these trees, most of them 

 having been removed at the time of our observations; but a large per- 

 centage showed a characteristic burning at the base and the bark was 

 burned off in some instances to quite an extent. One limb that had 

 been in contact with the negative feed wire was found dead, but the 

 tissue at the base of the trunk was normal. Dr. J. W. Tourney, director 

 of the Yale Forestry School, who examined many of these trees, found 

 a disintegration of the wire where it came into contact with the limbs, 

 apparently due to electrolytic action, and chemical analysis showed the 

 presence of copper and zinc in the tissues of the wood that had been 

 in contact with the negative or overhead wire. Dr. Tourney believes 

 that in such cases the disintegration of the copper wire and the absorp- 

 tion of the copjDer by the tissue were responsible for the death of the 

 limbs. If true, this entirely new state of affairs would indicate that 

 the electrical injuiy from direct currents not only arises from heat but 

 also from the electrical disintegration of metals, which may poison the 

 tissues. These observations demonstrate thar we have a variety of con- 

 ditions to deal with in considering the effect of direct current electricity 

 on trees, and these phenomena may be summarized as follows : — 



Burning and injury to plant tissue are much more noticeable at 

 points with a positive potential ^ than at points with a negative po- 

 tential. 



When the rail is at a positive potential the overhead wire, which 

 touches some part of the tree, is negative, and the bark and soil are 

 saturated with moisture, and a circuit is formed by means of this sur- 

 face moisture. 



The moisture conditions and the electrical resistance, etc., at the base 

 of the tree are different from those above, therefore a larger area of 

 tissue is affected by the positively charged rail. 



As the bark becomes heated through the film of water, the electrical 

 resistance is reduced and the current increased to such an extent that 

 the vital layer is destroyed. 



The actual cunent passing through the inner tissues mus*^ neces- 

 sarily be insignificant, and when there is a film of water on the bark, 

 probably no current passes through the cambium; furthermore, the 

 moist soil between the rail and the trunk of the tree becomes a better 



1 Positive electro-static charges have a more stimulating effect on plants than negative charges, 

 and retardation of growth and injury to the cells are more pronounced. The phenomena asso- 

 ciated with the positive and negative galvanotropic bendings of roots may be explained in this 

 way (24th Ann. Rept. Mass. Agr. Exp. Station, Pt. I., p. 144, 1912). 



