19L2.] PUBLIC DOCUMENT — No. 31. 163 



The flow of sap, as is well known, is influenced by various con- 

 ditions, a very important one being night temi:)eratnre, as well 

 as the conditions which prevail during the day. Temperature 

 records were taken for the same period, but there was little or no 

 direct relation between the temperature of this period and the 

 sap flow. In all cases the air temperature was at freezing or 

 below this point during the night, while in the daytime it 

 ranges from 43 to 57°. 



Jones, Edson and Morse ^ found that the maximum yield of 

 sap occurred quite generally between the hours of 9 A.isr. and 

 12 M. They also maintain that on a typical sap day the tree 

 will yield more sap and sugar on a southern exposure than on 

 any other, while on a cloudy day, when all the sides of the trees 

 are subject to a uniform temperature, there is little or no difter- 

 ence in the sap flow as regards the cardinal points of the com- 

 pass. It is known that the percentage of sugar varies in the 

 tissues of a tree from day to day, and it is doubtful whether 

 this variation in the chemical composition of the sap, or even 

 the amount of flow, would affect resistance even if our observa- 

 tions were confined to the woody tissues alone. This opinion is 

 based on laboratory experiments. 



Electrical Resistance of Different Tissues. 

 It might be expected that there would be found considerable 

 difl'erence in the electrical resistance of various trees, as well as 

 of the different tissues found in trees. The heartwood, sapwood, 

 cambium, bark and sieve tubes possess quite different properties 

 and functions, and their electrical resistance would naturally 

 vary to a large extent. The living cells containing protoplasm, 

 such as are found in the cambium, present the least resistance, 

 as would seem from various observations on lightning dis- 

 charges. The minute burned channel found in trees caused by 

 comparatively insignificant lightning discharges follows down 

 the cambium, indicating that this is the line of least resistance. 

 Moreover, by driving electrodes into a tree to different depths 

 and measuring the resistance it can be shown that the least re- 

 sistance occurs in the region of the cambium. 



' The Maple Sap Flow, by C. H. Jones, A. W. Edson and W. J. Morse, Vt.Agr. Exp.Sta., Bui. 

 No. 103, December, 1903. 



