236 MASS. EXPERIMENT STATION BULLETIN 170. 



from the low voltage (110-volt) lines, but the lines of higher potential 

 found on streets constitute a source of danger to trees. The liigher the 

 electrical potential the more dangerous the wires become to trees, for, 

 owing to the lessened effectiveness of the ordinary insulation, more leak- 

 age occurs and consequently greater opportunity for burning. 



The effects of alternating currents on trees are local, producing injury 

 only near the point of contact with the wire. Such contact results in 

 death of that part of the tree, and if it is a leader or large limb it usually 

 has to be sacrificed. In no case, to our knowledge, has an alternating 

 current caused the death of a tree, although it may burn or disfigure the 

 tree so badly that it amounts to practically the same thing. It is doubtful 

 whether the current from a fairly high potential wire would kill a large 

 tree under any circumstances. It is different in the case of small plants, 

 as has been frequently demonstrated in the laboratory, although the cur- 

 rent must produce heat enough to kill the protoplasm. The close rela- 

 tionship between the maximum temperature required to kill a plant and 

 that induced by electrical current indicates that the collapse of the plant 

 tissue in such cases is probably due to the heat rather than to ar>y specific 

 electrical shock. It is possible to pass the same current through larger 

 plants where heat is not generated without causing any collapse of the 

 tissue. The ordinary house circuit wires are perfectly harmless to trees, 

 and it seems strange that a judge could render a verdict to the effect that 

 an ordinary insulated 1 10- volt house circuit was responsible for the death 

 of a tree whose terminal branches were located 3 feet from it. There is 

 only one court record of which we know where such a judgment has been 

 given. 



Very high-tension line wires are not provided with insulation and are 

 known to affect the atmosphere surrounding them to a considerable extent. 

 Any increase in the electrical potential of the atmosphere if not too high 

 would favorably affect vegetation in general.^ 



General Effects of Direct Currents. 



Most of the direct currents affecting trees are those used for operating 

 electric railroads. Trolley feeders may be at 500 to 550 volts. Ordinarily 

 the burning from direct currents is similar to that produced by the alter- 

 nating current in being largely local or confined mainly to the point of 

 contact with the wires. The feed wires cause no burning except when the 

 tree is moist, in which case grounding takes place. 



The strength of current which will kill one plant will produce not the 

 slightest effect on another; in other words, the maximum current for 

 each individual varies materially. Small, tender plants possess a maxi- 

 mum much below that of woody plants. A young, succulent tomato 



1 There is evidently much difference in plants in this respect. A crop of radishes showed a 

 gain of 57 per cent, when subjected to an average atmospheric potential of 167 volts, whereas an 

 electrical potential equal to 500 or 1 ,000 volts is beyond the stimulation zone for some plants (16th 

 Ann. Rept. Mass. Agr. Exp. Sta. (Hatch), 1904, p. 31). 



