234 MASS. EXPERIMENT STATION BULLETIN 170. 



The electrical resistance may average throughout the year about 

 25,000 ohms in 10 feet of the trunk of a large maple tree, but in cold weather 

 it often exceeds 100,000 ohms. The lowest resistance in all cases corre- 

 sponds to periods of high temperatures, and the highest to periods of the 

 lowest temperature. The difference shown by the various sides of the 

 tree is also related to temperature. The resistance of the sapwood is very 

 much greater, and probably that of the heartwood is even higher than 

 that of the sapwood. 



In determining the electrical resistance it is necessary to know the path 

 or course of the current, and the only manner in which the resistance of 

 different tissues can be determined accurately is by isolating the tissues. 

 By girdling a tree and scraping the trunk down to the solid wood we can 

 get the resistance of the wood. Mr. G. H. Chapman^ found the resistance 

 of a freshly cut rock maple stem, 1| inches in diameter, to be 70,000 ohms 

 with the bark on, but 150,000 ohms when the bark was removed. The 

 electrodes were 1 foot apart. Next to the cambium the phloem has the 

 least resistance, followed by the sapwood. The outer bark appears to 

 offer the most resistance, but when wet the resistance may be decreased, 

 owing to the less resistant film of moisture on the bark. The resistance 

 obtained from an elm tree in summer, with the electrodes 10 feet apart 

 and in contact with the cambium, was 10,698 olims, whereas when the 

 electrodes were inserted into the middle of the cortex or phloem we ob- 

 tained 11,300 olmis' resistance. When driven one-quarter inch into the 

 wood, and some of the exterior tissues surrounding the electrodes re- 

 moved, the resistance was 98,700 ohms. The outer bark gave 198,800 

 ohms' resistance, but when the electrodes were inserted slightly deeper 

 into the bark we obtained 109,900 ohms. It should be pointed out, 

 however, that the data given here do not represent the actual resistances 

 of the various tissues, but they indicate that there exist very material 

 differences in the electrical resistance of the tissues in trees. The resist- 

 ance obtained for the cambium, however, may be taken as fairly repre- 

 sentative, as shown by the use of other methods, such as the employment 

 of relatively high potentials and current measurements. 



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, includ- 

 ing 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 to 165,000 ohms) than that shown by 

 trees at corresponding temperatures. 



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

 in resistance, and it might be expected that the various plastic substances 

 would influence it also. 



The cambium ring is very insignificant in size, and even on a large tree 

 the total area is small. In aU probability it is the protoplasm itself which 



1 Mass. Agr. Exp. Sta. Rept. 24, Pt. I., 1912; also Bulletins 91 and 156. 



