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 shghtly 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, 6). 
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, 6, 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, 
