nvex, 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).

[Illustration: FIG. 32.--INCREASED RESPONSE WITH INCREASING VIBRATIONAL
STIMULI (CAULIFLOWER-STALK)
Stimuli applied at intervals of three minutes. Vertical line = .1 volt.]

(_B_) 1. The vibrational stimulus was increased from 2.5 deg. to 5 deg. to 7.5 deg.
to 10 deg. to 12.5 deg. in amplitude. It will be observed how the intensity of
response tends to approach a limit (fig. 32).

TABLE SHOWING THE INCREASED E.M. VARIATION
PRODUCED BY INCREASING STIMULUS

+----------------------------------------+
| Angle of Vibration | E.M.F |
+----------------------------------------+
| 2.5 deg. | .044 volt |
| 5 deg. | .075 volt |
| 7.5 deg. | .090 volt |
| 10 deg. | .100 volt |
| 12.5 deg. | .106 volt |
+----------------------+-----------------+

2. The next figure shows how little variation is produced with low value
of s