6 MASS. EXPERIMENT STATION BULLETIN 156. 



It has been suggested that arc lights are injurious to trees, although 

 we have never seen any cases of injury. It is well known that electric 

 light is different from sunlight in its effects on plants, and it stimulates 

 photosynthesis in proportion as it resembles sunlight in its rays. Some 

 artificial lights contain rays that may act injuriously on small plants 

 and in other ways modify their development, but even if a tree in close 

 proximity to such a light should die it is no proof that it has been 

 injured by this cause, as there are so many other causes for the death 

 of trees. 



Effects op Direct Currents. 



Most of the direct currents affecting trees are those used for op- 

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

 Ordinarily the burning from direct currents is similar to that produced 

 by the alternating 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. 



We have made a number of exioeriments, using large trees and small 

 herbaceous plants, with direct currents from electric railroads showing 

 the amount of current passing through trees, etc. In a number of in- 

 stances a wire was passed from the tree to the rail or ground, and 

 another wire was connected to a bare feed wire (450 to 500 volts) 

 leading to some other portion of the tree, a milliammeter being placed 

 in the circuit to obtain the actual current. The results were as follows : 

 a young pear tree, 2 feet 8 inches in height, and V-^ inches in diameter 

 at the base, which had been growing one year in a box 14 by 16 by 

 9 inches, and provided with a copper plate in the bottom in direct 

 contact with the roots, showed a current of 2.2 milliamperes (M54 

 ampere) when one electrode leading to the rail was connected with the 

 copper plate, and the other leading to the feed wire joined the tojD of 

 the tree; 161/2 feet of a maple tree 18 inches in diameter gave 25 milli- 

 amjDeres (%o ampere), and 7 feet of the same tree gave a current of 

 45 milliamperes (%2. ampere). Connections made with a poison i\"y 

 {Rhus toxicodendron L.) plant gi'owing on a tree showed in most 

 cases similar results when the electrodes were inserted into the stem 2 

 inches apart. A stem % inch in diameter gave a current equal to 

 4.4 milliamperes {V22- ampere) ; 1/2 inch in diameter, 25 milliamperes 

 (Mo ampere) ; and another of the same size, 50 milliamperes (^20 

 ampere). In the latter case, and some others not included here, the 

 currents went down from 50 milliamperes to nothing almost instantly. 

 From these experiments with ivy it appears that the current burned 

 out the cambium or vital layer of the stem, leaving the dry and highly 

 resistant wood which was unable to transmit a perceptible current. 



In another experiment young sunflowers and tomato plants grown in 

 3-incb pots, with copper plates at the bottom, were treated from a 



