SHADE TREES. 233 



ELECTRICAL INJURIES. 



The increase in electric railroads, electric lighting systems and tele- 

 phone lines, whose wires are usually located near the tree belts of our 

 cities and towns, has made necessary a lamentable amount of disfiguring 

 pruning. When strung too close to trees, wires also often cause serious 

 injury by burning, sometimes mechanical injury is done, and lightning 

 discharges will cause harm when guy wires are attached to trees. 



Both the alternating and direct currents are used. They produce 

 different physiological effects on plant life, the alternating current ap- 

 parently being less injurious than the direct; and when either is used at a 

 certain amperage it acts as a stimulus to the plant, and growth and devel- 

 opment are accelerated. 



There are minimum, optimum and maximum currents affecting plants. 

 The minimum represents that strength of current which just perceptibly 

 acts as a stimulus, and is very insignificant. The optimum is that pro- 

 ducing the greatest stimulus about .2 milliampere and the maximum, 

 that causing death. The maximum current necessary to cause death is 

 very variable. The direct current has a less stimulating effect than the 

 alternating, and on account of its electrolyzing effect is capable of causing 

 more injury to vegetable life than the alternating current. 



Most of the injury to trees from trolley or electric light currents is 

 local, i.e., the injury takes place at or near the point of contact of the 

 wire with the tree. This injury is done in wet weather when the tree is 

 covered with a film of water, which provides favorable conditions for 

 leakage, the current traversing the film of water on the tree to the ground. 

 The result of contact of a wire with a limb under these conditions is a 

 grounding of the current and burning of the limb, due to "arcing." The 

 vital layer and wood become injured at the point of contact, resulting in 

 an ugly sear and sometimes the destruction of the limb or leader. In a 

 large number of tests made by the aid of sensitive instruments with guy 

 wire and other connections of wires to trees we have never found any 

 leakage during fair weather or when the surface of the tree is dry. Since 

 the amount of current that can be passed through a tree depends upon 

 the resistance and electromotive force, we shall consider this resistance. 



As might be expected, there is considerable difference in the electrical 

 resistance of various trees as well as of the different tissues found in trees. 

 The heartwood, sapwood, cambium, bark and sieve tubes possess quite 

 different properties and functions, and their electrical resistance would 

 naturally vary to a large extent. The living cells containing protoplasm, 

 such as are found in the cambium, present the least resistance, as shown 

 by various observations on lightning discharges. The minute burned 

 channel caused by comparatively insignificant lightning discharges follows 

 down the cambium, indicating that this is the line of least resistance. 

 Moreover, by driving electrodes into a tree to different depths and meas- 

 uring the resistance it can be shown that the least resistance occurs in the 

 region of the cambium. 



