PLANT RESPONSE 55 



which are given, it will be seen that the slope of a 

 curve which shows the relation of stimulus to response 

 will at first be slight, the curve will then ascend rapidly, 

 and at high values of stimulus tend to become horizontal. 

 The curve as a whole becomes, first slightly convex 

 to the abscissa, then straight and ascending, and lastly 

 concave. A far more pronounced convexity in the first 

 part is shown in some cases, especially when the 

 specimen is stale. This is due to the fact that under 

 these circumstances response is apt to begin with an 

 actual reversal of sign, the plant under feebler than 

 a certain critical intensity of stimulus giving positive, 

 instead of the normal negative* response (fig. 33, b). 



Diminution of response with excessively strong 

 stimulus. It is found that in animal tissues there is 

 sometimes an actual diminution of response with ex- 

 cessive increase of stimulus. Thus Waller finds, in 

 working with retina, that as the intensity of light 

 stimulus is gradually increased, the response at first 

 increases, and then sometimes undergoes a diminution. 

 This phenomenon is unfortunately complicated by 

 fatigue, itself regarded as obscure. It is therefore 

 difficult to say whether the diminution of response is 

 due to fatigue or to some reversing action of an 

 excessively strong stimulus. 



From fig. 33, b, above, it is seen that there was an 

 actual reversal of response in the lower portion of the 

 curve. It is therefore not improbable that there may 

 be more than one point of reversal. 



In physical phenomena we are, however, acquainted 

 with numerous instances of reversals. For example, 



