An extract of Lycopersicon leaf tissue applied to the base of an excised Lycopersicon leaf causes a variation potential to spread through the leaf. This spread of an electrical wave is closely dependent on the spread of the extract via the transpiration stream, and the amplitude of the variation potential depends on the concentration of extract applied. Extract from 5–10 mg fresh leaves diluted to 1 ml elicits threshold response, and saturation is accomplished with a concentration about 40 times greater. Evidently, the active factor or factors are effective at very low concentrations. A variety of substances cause electrical disturbances when applied to the excised leaf, but plant extract is the only tested material which under the given conditions desensitizes the leaf to an application of saturating extract 5 min after the initial application.The active factor or set of factors has been extracted from plants in several families and appears to be closely related to the substance implicated by Ricca in 19...