1912.] PUBLIC DOCUMENT — No. 31. 107 



small branches or woody stemmed plants may possibly bo due to 

 the presence of less conductive tissue, whereas in a tree tlu; con- 

 ductive zone, if we include the phloem, is larger. 



It is known that there are minute currents of electricity in 

 plants, but we have never noticed their effects on our galvanom- 

 eters nor have we detected them by the use of a milliammeter. 

 Trees frequently become charged with electricity, and sparks 

 are given off from the apices of the leaves. Vegetation in gen- 

 eral responds quickly to electrical stimulation, and trees un- 

 doubtedly play an important part in equalizing the differences 

 in electrical potential between the atmosphere and earth. In 

 this respect conifers appear to behave differently from decidu- 

 ous trees, and in our experiments we have found that the atmos- 

 pheric electrical potential under thick conifers was the same as 

 that which characterizes the earth. 



Relationship of Electrical Resistance to Other 



Factors. 



We had little or no opportunity to observe the effects of wdnds, 

 if such exist, on electrical resistance. Most of our records were 

 taken while the tree was in a dormant condition. In some cases 

 the trees were well protected from the winds. It is known that 

 transpiration is increased by wind, and the movements of water 

 in the tissues of the tree are accelerated. I^o relationship, 

 however, between the wind and electrical resistance has been 

 noted by us in comparing the records of the local meteorological 

 station with our data, neither was there any specific relationship 

 observable between barometer pressure and electrical resistance. 

 A careful study of the humidity conditions, also, did not seem 

 to affect the electrical resistance so far as we could observe. 

 Aside from the temperature effects coincident with light inten- 

 sity no special changes in resistance were observable except such 

 as would naturally follow from the variations in temperature. 



Influence of Temperature on Resistance. 

 The most important factor which we have observed as influ- 

 encing the electrical resistance of trees is temperature. The 

 effects of temperature on various metals give rise to an increased 



