240 MASS. EXPERIMENT STATION BULLETIN 170. 



true, this entirelj^ new state of affairs would indicate that the electrical 

 injury from direct currents arises not only from heat but also from the 

 electrical disintegration of metals, which may poison the tissues. These 

 observations demonstrate that we have a variety of conditions 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 prominent at points 

 with a positive potential^ than at points with a negative potential. 



When the rail is at a positive potential and the overhead wire which toiiches 

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

 moisture, a circuit is formed by means of this surface 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 theftlm of water, the electrical resistance 

 is reduced and the current increased to such an extent that the vital layer is 

 destroyed. 



The actual current passing through the inner tissues must necessarily 

 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 conductor 

 for the current than the roots. The actual injury, therefore, is done by 

 the current traversing the film of water rather than any of the inner 

 tissues. The maximum heat and the areas most affected are near the 

 base of the trunk. 



In regard to the possibility of injury to large trees by direct currents 

 passing directlj^ through them, experiments show that what holds true 

 for alternating currents is true also to a great extent of direct currents. 

 However, it would require a voltage much higher than that furnished by 

 most electrical railways at the present time. 



It might be possible for direct currents to affect trees without causing 

 any perceptible burning. If, for example, a tree were subjected to a suffi- 

 cient strength of current, there might occur a disintegration of the cell 

 contents, causing the tissues to become abnormal and finally to die, but 

 the electrical resistance of trees is so great that a quite high potential 

 would be necessary. If the potential of the electric railway systems were 

 greatly increased it is possible that some injury might result to trees even 

 under ordinary conditions. 



Probably the amount of ground leakage occurring through imperfect 

 rail connections would not cause any perceptible injury to trees; nor 

 is there any direct evidence that lightning arresters when placed near 

 trees cause any injury by discharges. However, the guy wires used by 



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 

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

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



