1912.] PUBLIC DOCUMENT — No. 31. 165 



It is quite evident from our observations on the resistance of 

 trees that the cambium gives the least resistance, the phloem 

 next, and it is not at all unlikely that in some trees there may 

 be some variation in this respect. 



The resistance given by small tree trunks and woody stems, 

 even for small distances, is quite large. About 4 feet of a young- 

 pear tree, with a maximum diameter of stem equal to 1 inch, 

 gave a resistance of about 300,000 ohms, and the resistance 

 given by a tobacco plant in which the distance between the elec- 

 trodes was only 14 inches, was much higher (110,000 to 165,- 



000 ohms) than that shown by trees. In the case of the pear 

 tree, which was in a large box, filled with soil, one of the elec- 

 trodes (metal plate) was in contact with the small roots, the 

 other being in contact with the apex of the plant. 



The presence of water and various salts undoubtedly plays a 

 role in resistance, and it might be expected that the various 

 plastic substances in the plant would influence resistance. 



The path of a current in a tree, as already stated, follows the 

 line of least resistance, but this line may not necessarily be a 

 straight one between one electrode and another. Although in 

 many lightning strokes a straight line is generally followed, we 

 have seen instances where the whole cambium zone was involved, 

 and when the tissue in a tree is twisted the discharge will follow 

 the tissue. A lightning discharge may therefore completely 

 circle a tree trunk, passing from the apex of the tree to the 

 ground. In earth discharges the path follows up the trunk and • 

 is generally diverted to the branches, often causing them to 

 s\Ait. When heavy lightning discharges occur and the tissues 

 of the tree become shattered, as is often the case, the line of least 

 resistance seems to be an unimportant factor, and in this respect 

 the electric discharges resemble an avalanche in their behavior. 

 In some of our experiments, where trees were connected with 

 wires carrying relatively high currents and the electrodes were 



1 foot apart vertically, all of the injury was done by burning 

 on one side of the tree in close proximity to the electrodes, but 

 even here the burning of the tissue covered an area of more 

 than 1 foot in width on the trunk. Burning under these condi- 

 tions, however, occurred only when the bark of the tree was 

 moist, and was not caused by a decrease in resistance in the tis- 



