The circuit for measuring potentials was essentially a bridge network using an FP54 electrometer tube (5) in vacuo with an input resistance of 1.16 x 1011 ohms. With the voltages of the plants used the current was below 10~12 amperes. Contact with the plants was made by means of felt brushes moistened with a KCI solution flowing from test tubes into which were immersed Ag?AgCl electrodes. The sensitivity of the apparatus was about 0.1 mv./div. Tomato plants were chosen for this study because of their relative homogeneity. Two groups of 30 plants each were cultivated; one group in sixinch standard flower pots, the other in four-inch pots. Fifteen of the plants in the six-inch together with 10 of the plants in the four-inch pots were maintained as control. All plants were inoculated ; those in the experimental groups with a strain of Phytomonas tumefaciens which produces a hairy tumor, and those in the control groups with distilled water. The latter action was taken so that effects due to external injury would be the same for all plants. The injections were made with a hypodermic needle at a depth of about 1 mm. below the surface. Precautions were taken to be able to locate the exact spot of the inoculation. The plants were protected with bandages at the point of insertion of the needle to prevent further infection. The plants were approximately 50 cm. in height at the time of inoculation, and about 1 cm. in diameter.