174 EXPERIMENT STATION. [Jan. 



Morse, whereas the average resistance for the north side of the 

 tree runs about 5 per cent, more than that for the south side. 

 The relationship of temperature to resistance manifests itself 

 throughout; the higher temperature giving rise to a low, and 

 conversely a low temperature giving rise to a high resistance. 



The sun strikes the tree on the east, south and west sides, and 

 each side is exposed for the same length of time ; but the angle 

 of the sun is variable as it strikes the tree's surface. In the 

 earlj morning and late afternoon the sun's rays are more or 

 less at right angles to the tree trunk, whereas at noon the angle 

 is more oblique. The surface of a tree is not a good reflector of 

 light and heat, and in the early morning and late afternoon, 

 when the rays are more at right angles to the surface of the 

 trunk, there is less loss of light and heat by reflection. Assum- 

 ing that the light intensity is uniform throughout the day, and 

 that the temperature is the same, we would expect to find fairly 

 uniform resistances for the east, south and west sides of the 

 tree. This, however, is not the case, as the temperature is sel- 

 dom uniform, neither are the light conditions, as is shown by the 

 flow of sap. 



The north side of the tree gives the highest average resistance, 

 followed by the west, south and east sides. From the point of 

 view of influence of temperature this might be expected, espe- 

 cially during seasons when there is considerable diiference be- 

 tween the night and day temperatures, and very likely for long 

 periods of observations thermometers placed in trees would 

 demonstrate this. Electrical resistances taken in the afternoon 

 usually run lower than those taken in the morning on all sides 

 of the tree, which results from a general increase in the temper- 

 ature of the surrounding air and of the tree occurring in the 

 daytime. The electrical resistance is less in the warm than in 

 the cold months, and less on warm than cold days. In the morn- 

 ing the sun afl'ects the east side of the tree most markedly, and 

 in the afternoon the west side. 



In experimenting with cut branches of maple trees we did not 

 find, however, that this held true. The resistances obtained 

 from branches placed out of doors when it was cold were in no 

 wise different from those taken from the same branch when 

 placed in the laboratory, where it was warm, or even when 



