PLANTS UNDER MECHANICAL STIMULUS. 291 
Table showing the increased Electromotive Variation produced 
by increasing stimulus. 
| 7 | 
_ Amplitude of Vibration. | E.M. Variation. | 
| 
i 
| 
| 2°75 “O44 volt | 
| 5° 075, 
| 75 090 
| 10 100 ,, | 
| 12°5 | 106. ,, | 
| | 
From numerous other records obtained, I find that if a curve 
be drawn with the electric responses as ordinates and the 
amplitudes of vibration as abscisse, the first part of such a curve 
is slightly convex to the abscissa, then it is straight and 
ascending, and in the last part concave. 
In some cases, as the intensity of stimulus is gradually 
increased from a low value, there would-at first be apparently no 
response ; but when a critical value is reached a marimum 
response would suddenly oecur, which would be but little 
exceeded when the stimulus was further inereased. All these 
remind us of the various types of response in animal tissue. 
Effects of Superposition of Stimuli. 
Additive effect.—There is apparently little or no response 
when the stimulus is feeble. But even a subminimal stimulus 
will, though singly ineffective, become effective by the summation 
of several. This is shown in fig. 14, where the first record a is the 
response to a feeble stimulus, and the second @ is the response to 
the same stimulus repeated in quick succession, thirty times. 
Fusion of partial effects—Similar additive effects are seen 
with stimulus of ordinary intensity. If the frequency of stimu- 
lation is sufficiently rapid, the individual effects will become 
fused. A maximum effect is thus produced depending on the 
frequency and intensity of individual stimuli. Further con- 
tinuation of stimulation adds nothing to the effect. If there 
is no fatigue, the top of the response-curve remains approxi- 
mately horizontal. But there would be a decline if the specimen 
