Light Conditions 73 



at point of contact, and drying out the limb above. With 

 ahernating currents the damage seems usually confined to 

 diis local destruction at or near the point of contact as a 

 result of the heat created. With direct currents, in addi- 

 tion to these local burnings, electric effects are sometimes 

 noted at a distance, the escaping current girdling trees at 

 the base, and killing the cambium without the characteristic 

 burning at point of contact or at point of discharge. This 

 appears to hapijen only when the positive current is not as 

 usually carried by the feed-wire, but by the rail, and leak- 

 age through imperfect rail-connection occurs. In an ob- 

 served case the damage w^as done when the trees were four 

 feet from the rail, the moisture conditions of soil and bark 

 being evidently most favorable for electric discharge. 



The only help is, of course, to string the wires outside of 

 the tree line, or at least to prevent contact, keeping in mind 

 that winds will sway the branches and that allowance 

 must be made for this. 



No authentic cases of injury or death, due to the mere 

 effect of the light of arc lamps, are on record. 



Light Conditions. Where trees are planted close to- 

 gether, standing in groups or clumps, there is apt to be an 

 uneven distribution of light, some trees or some parts of 

 trees being deprived by their neighbors of a sufficiency in 

 that respect. The hght is necessary, as we have seen, for 

 the leaf to perform its functions in assimilating food. Defi- 

 ciency in this respect may show itself in etiolation, the leaves 

 becoming small and elongated, and the leaf-stalks and 

 branchlets thin and slender. Finally, when the shade 

 becomes too dense, the shaded portions die. This very 

 important observation is utilized by the fprester in secur- 

 ing branchless boles by the natural trimming that takes 

 place in dense growths. 



