RELATION BETWEEN STIMULUS AND RESPONSE 95 



we presently arrive at the threshold of response, where the 

 response becomes at once the maximal possible. In this 

 case, then, the minimal response is also maximal. We shall 



b 



Fig. 44. Ineffective Stimulus made 

 Effective by Repetition 



The line to the left shows that single 

 stimulus produced no mechanical 

 response. The curve to the right 

 shows the effect produced when 

 stimulus had been repeated six times. 



Fig. 45. Additive Effect in Electrical 

 Response 



(a) A single stimulus of 3 vibration pro- 

 duced little or no effect, but the same 

 stimulus when rapidly superposed 

 thirty times produced the large 

 effect (/>). Leaf stalk of turnip. 



find, in Chapter XXII., that an exactly similar type of 

 response is afforded by Biophytum, where the minimally 

 effective stimulus suddenly produces maximal effect. 



Relation between stimulus and response in animal 

 and vegetable. — The second type of contractile response is 

 shown by skeletal muscle. Here, after reaching the threshold 

 of response, increasing stimulus causes increasing response, 

 which, however, tends to reach a limit. Exactly parallel 

 effects are seen in the case of plants, in the longitudinal 

 responses exhibited by different radial organs. 



The experimental method by which this is demonstrated 

 is as follows: The intensity of thermal stimulus may be 

 appropriately increased, as has been explained, by increasing 

 the value of the heating current, the circuit being always 

 closed for a certain definite time, say one second. The 

 resistance of the heating coil being kept constant, the 

 thermal effect is proportional to the square of the current. 

 If, then, we use currents which increase as the square root of 

 the natural numbers, the successive thermal effects will be 

 increased in arithmetical progression. The currents to be 

 used are previously adjusted, by means of an ammeter, and 



