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In previous work on the electric polarity in the Douglas fir LUND (1, 2, 3) attention was limited to the phenomena occurring in the main axis and lateral branches of the tree. The measurements of P. D. which incidentally were made upon the apex of the main stem and the first apical whorl of lateral branches showed a small or at times even a reversed polarity LUND (2, tables 1 and 2). Since most of these measurements were necessarily made out of doors under conditions which involved mechanical stimulation of the tips by slight wind movements, and also involved a method of taking readings of the P. D.'s by gently bending the tips of the apex and branches into cups, it seemed desirable to make a separate study of the electric polarity of the extreme apical region of the main axis and lateral branches under more uniform laboratory conditions. The present measurements were made upon actively growing freshly cut tips whose cut ends were immersed in water. Electrode contacts were made as usual with narrow strips of loose cotton, carefully placed around the axis between the leaves, and saturated with tap water. Frequent washing of the electrode contacts and measurements of the electrodes showed that the electrodes and contacts never varied more than one millivolt during the readings. This fact should be remembered when considering the curves of figures 1, 2, 3, and 4 presented here. All measurements were made with the potentiometer and consequently we are not concerned with possible variations in electrical resistance. Many tips of lateral branches and main axes were studied with care, but all gave the same type of result. In view of this fact the details of only two typical experiments will be given, one upon the apex of a lateral branch and the other one on the apex of the main stem.

External polarity potentials in the apex of the douglas fir before and after mechanical stimulation