[12] For general purposes it is immaterial whether the responses are recorded up or down. For convenience of inspection they are in general recorded _up_. But in cases where it is necessary to discriminate the sign of response, positive response will be recorded up, and negative down. CHAPTER VII PLANT RESPONSE--ON THE RELATION BETWEEN STIMULUS AND RESPONSE Increased response with increasing stimulus--Apparent diminution of response with excessively strong stimulus. As already said, in the living tissue, molecular disturbance induced by stimulus is accompanied by an electric disturbance, which gradually disappears with the return of the disturbed molecules to their position of equilibrium. The greater the molecular distortion produced by the stimulus, the greater is the electric variation produced. The electric response is thus an outward expression of a molecular disturbance produced by an external agency, the stimulus. #Curve of relation between stimulus and response.#--In the curve showing the relation between stimulus and response in nerve and muscle, it is found that the molecular effect as exhibited either by contraction or E.M. variation in muscle, or simply by E.M. variation in nerve, is at first slight. In the second part, there is a rapidly increasing effect with increased stimulus. Finally, a tendency shows itself to approach a limit of response. Thus we find the curve at first slightly convex, then straight and ascending, and lastly, concave to the abscissa (fig. 30). In muscle the limit of response is reached much sooner than in nerve. As will be seen, the range of variation of stimulus in these curves is not very great. When the stimulus is carried beyond moderate limits, the response, owing to fatigue or other causes, may sometimes undergo an actual diminution. [Illustration: FIG. 30.--CURVES SHOWING THE RELATION BETWEEN THE INTENSITY OF STIMULUS AND RESPONSE Abscissae indicate increasing intensity of stimulus. Ordinates indicate magnitude of response. (Waller.)] I have obtained very interesting results, with reference to the relation between stimulus and response, when experimenting with plants. These results are suggestive of various types of response met with in animal tissues. 1. In order to obtain the simplest type of effects, not complicated by secondary phenomena, one has to choose specimens which exhibit little fatigue. Having procured these, I undertook two series of experiments. In the first (_A_) the stimulus was applied by means of the spring-tapper, and in the second (_B_) by torsional vibration. [Illustration: FIG. 31 Taps of increasing strength 1:2:3:4 producing increased response in leaf stalk of turnip.] (_A_) The first stimulus was given by a fall of the lever through _h_, the second through 2 _h_, and so on. The response-curves clearly show increasing effect with increased stimulus (fig. 31).