[Illustration: FIG. 6.--(_a_) EXPERIMENT FOR EXHIBITING ELECTRIC RESPONSE IN PLANTS BY METHOD OF NEGATIVE VARIATION. (_b_) RESPONSES IN LEAF-STALK OF TURNIP TO STIMULI OF TWO SUCCESSIVE TAPS, THE SECOND BEING STRONGER. A and B contacts are about 2 cm. apart, B being injured. Plant is stimulated by a tap between A and B. Stimulus acts on both A and B, but owing to injury of B, effect at A is stronger and a negative variation due to differential action occurs.] A sharp tap was now given to the stalk, and a sudden diminution, or negative variation, of current occurred, the resting potential difference being decreased by .026 volt. A second and stronger tap produced a second response, causing a greater diminution of P.D. by .047 volt (fig. 6, _b_). The accompanying figure is a photographic record of another set of response-curves (fig. 7). The first three responses are for a given intensity of stimulus, and the next six in response to stimulus nearly twice as strong. It will be noticed that fatigue is exhibited in these responses. Other experiments will be described in the next chapter which show conclusively that the response was not due to any accidental circumstance but was a direct result of stimulation. But I shall first discuss the experimental arrangements and method of obtaining these graphic records. [Illustration: FIG. 7.--RECORD OF RESPONSES IN PLANT (LEAF-STALK OF CAULIFLOWER) BY METHOD OF NEGATIVE VARIATION The first three records are for stimulus intensity 1; the next six are for intensity twice as strong; the successive responses exhibit fatigue. The vertical line to the left represents .1 volt. The record is to be read from right to left.]