either is utilized at a certain strength it accelerates growth and devel- 

 opment, or, in other words, it stimulates the plant. 



MO R MX 



—I 1 1 1- 



A CCELERATION -RETARDATION- -DEATH- 



I^~!g: 3. Diagram s/ioii'i/ig I'ange of electric current 

 affecting plajits. M — miiii>uu//i. O — optivnini, or cur- 

 rent producing greatest stinin/ns. AfX — niaxinnini, or 

 death current. J\ to MX— retardation current. 



There is a minimum, optimum and maximum current. The mini- 

 mum represents that strength of current which just perceptibly acts 

 as a stimulus. The optimum that producing the greatest stimulus 

 and the maximum that causing death. Between the optimum and 

 the maximum there is a strength of current that causes retardation, 

 thisjbeing represented between r and mx in figure 3. 



The direct current stimulates less than the alternating, and on 

 account of its polarizing effect, appears to cause more injury to veg- 

 etable life than the same strength of alternating current. Most of 

 the injurious electrical effects to trees arising from trolley or electric 

 light currents are, as a rule, local : that is, the current causes an 

 injury at or near the point of contact of the wire to the tree. This 

 injury is produced in moist or wet weather, when the tree is cov- 

 ered with a tihn of water. This 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 to a limb, under these 

 conditions, causes a grounding of the current, and a burning of the 

 limb of the tree due to arcing. The vital layer of the limb may 

 become partially or entirely killed at the point of contact which may 

 result in an ugly scar or greatly disfigure the tree. In a large num- 

 ber of tests made by the aid of sensitive instruments upon the con- 

 nections of feed wires to trees by means of guy wires we have never 

 found any leakage during fair weather, although such may occur in 

 wet weather, especially when the voltage is considerable. Since the 

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

 the resistance and voltage it will be well to consider the resistance 

 exhibited by some trees. 



*NoTE. — It is not an uncommon thing during wet weather to observe cases of electrical 

 shock produced by touching trunks of trees where leakage occurs. The writer has observed 

 apple trees which were accidentally charged, by being in contact with uninsulated feed 

 wires through guys, where the fruit was perfectly safe. 



