166 EXPERIMENT STATION. [Jan. 



sue, but l)y the presence of a film of water, which is a far better 

 conductor, on the bark, which became heated and killed the 

 underlying tissue. In the case of some large trees which we 

 observed and which had been killed by direct currents from trol- 

 ley wires, the tissue was as a rule affected nearly equally around 

 the entire trunk of the tree, although the point of contact was 

 on one side of the tree. In both cases it was a heating of the 

 film of water on the trunk caused by the escaping electric cur- 

 rent which caused the injury. 



The cambium ring is very insignificant in size, practically 

 y^oo to Mooo in diameter, and even on a large tree the total area 

 is small. In all probability it is the protoplasm itself which 

 offers the least resistance to the transmission of an electric cur- 

 rent; and even if there were no continuity it would be necessary 

 for the cnrrent to pass throngh a great many cell walls even 

 for comparatively short distances on the trunk. In case the 

 protoplasm was contiguous or there existed continuity, the 

 strands would be so very small that they would undoubtedly 

 offer some resistance. Whatever conditions prevailed trees 

 showed relatively high electric resistances, a feature which is 

 no doubt of some biological importance as trees are often struck 

 by lightning. The high resistance of trees, therefore, is un- 

 doubtedly a protection in case of lightning strokes, since often 

 the heat developed is enough to do only slight injury. On the 

 other hand, if trees possessed tissue with relatively small elec- 

 trical resistance they would be much more subject to injuries 

 from burning from lightning strokes, and would be more seri- 

 ously affected by currents from high tension wires. The elec- 

 trical resistance of trees is so high that it is doubtful whether 

 injury ever occurs to them from contact with low or even high 

 tension -wires except that produced by grounding Avhen the bark 

 of the tree is moist. Any escaping current which can be trans- 

 mitted even through the least resistant tissue is likely to be 

 insignificant. 



The amount of current necessary to kill a plant depends upon 

 its size, etc. A current equal to .01 amperes may be sufficient 

 to kill a small i)lant, whereas a current ten times as great would 

 cause no perceptible injury to a large tree even wdien passed 

 through the tissue for months. The higher resistance shown by 



