SHADE TREES. 237 



plant, one-eighth inch hi diameter and 5 inches high, was instantly killed 

 when treated with a current of 20 milliamperes, and currents of 2 and 3 

 milliamperes of thirty to sixty seconds' duration accomplished the same 

 result. In all the tomato plants, considerable heat was developed, and 

 their death was caused by the generation of heat developed by the current. 

 The electrodes in these tests were about one-half inch apart. If the elec- 

 trodes had been farther apart, no perceptible effect would have been 

 observed. 



When trees with a more or less thick bark are drenched with rain the 

 conditions are quite different. A large maple tree which was in circuit 

 with a feed wire (500 volts) and rail of an electric road when dry gave a 

 current equal to 70 milliamperes (one-fourteenth ampere) with the elec- 

 trodes placed vertically 1 foot apart. These connections were left on the 

 tree for several months. The observations were made on dry days and 

 no appreciable amount of heat developed with this current. During 

 periods of wet weather considerable heat always developed, especially at 

 the positive electrode, but not enough to melt the soft solder which con- 

 nected the wires with the electrodes. 



Examination of the tree ten months later showed that a portion of the 

 tissues near the electrodes had been killed. After removing the dead bark 

 an oval space 6 by 11 inches was found to be dead about the positive 

 electrode and a space about 14- by 3 inches near the negative electrode. 

 The burned area about the positive electrode was about 95 per cent, 

 greater than that occurring about the negative electrode; in each case 

 it extended about twice as far above and below the point of contact as 

 out to the sides of the electrodes, thus showing a tendency of the current 

 to spread laterally as well as vertically, but more largely vertically. The 

 immediate area around the electrodes was more affected than that 

 farther remote. There was an area of tissue about 5 inches long 

 between the large and smaU oval burning that was uninjured, showing 

 that burning was confined about the electrodes. The current traversing 

 the film of water on the bark between the electrodes was not sufficient to 

 destroy the tissues at that point. 



If a miUiammeter had been placed in the circuit when the tree was wet 

 a greatly increased current would have been detected, since the current in 

 this case traversed the less resistant film of moisture on the bark. But 

 the electrical resistance of the vital layer under such conditions would 

 remain practically the same as when the tree was dry, or it would show 

 only such variation as might be induced by an increase in temperature. 

 The burning and injury in this case resulted from the heating of the film 

 of moisture, which became so intensely heated that the vital tissue was 

 destroyed, especially near the point of insertion of the electrodes. The 

 more the film became heated the greater was the lessening of the resistance 

 and increase of the current. 



Practically all of the burning of trees from either alternating or direct 



