4 MASS. EXPERIMENT STATION BULLETIN 156. 



deeper into the bark we obtained 109,900 ohms. It must not be under- 

 stood, however, that these readings gave the electrical resistance of 10 

 feet of the various tissues enumerated except in the case of the cam- 

 bium, since if these tissues were isolated the resistance would be much 

 greater in some cases. They show that there is much difference in the 

 resistance of different tissues, but in all cases we obtained merely a 

 resistance of the cambium, together with that of a part of the other 

 tissues which the current had traversed from its various points of 

 entrance to the cambium. 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, in- 

 cluding the root system, 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 electrodes 

 was only 14 inches, was much higher (110,000 ohms to 165,000 ohms) 

 than that shown bj' most trees at corresponding tem^Deratures. 



The water and various salts in the living plant undoubtedly play a 

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

 stances would influence resistance. 



The cambium ring is very insignificant in size, 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 current to pass through a great many cell walls even for compara- 

 tivelj' short distances on the trunk. In case the protoplasm was con- 

 tinuous or there existed continuity, the strands would be so very small 

 that they would undoubtedly offer some resistance. Whatever condi- 

 tions prevail, trees show relatively high electrical 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 

 develoj^ed is enough to do only slight injury. On the other hand, if 

 trees possessed tissue with relatively small electrical resistance they 

 would be much more subject to injuries from burning from lightning 

 strokes, and would be more seriously affected by currents from high- 

 tension wires. The electrical resistance of trees is so high that it is 

 doubtful whether injury ever occurs to them from contact with low- 

 er even high-tension wires except that produced by grounding when 

 the bark is moist. Any escaj^ing current from transmission lines that 

 can be transmitted even through the least resistant tissue is likely to be 

 insignificant. 



