an
electromagnetic tapper may be used.

[Illustration: FIG. 11.--THE TORSIONAL VIBRATOR
Plant P is securely held by a vice V. The two ends are clamped by
holders C C'. By means of handles H H', torsional vibration may be
imparted to either the end A or end B of the plant. The end view
(_b_) shows how the amplitude of vibration is predetermined by means
of movable stops S S'.]

#Vibrational stimulus.#--I find that torsional vibration affords another
very effective method of stimulation (fig. 11). The plant-stalk may be
fixed in a vice (V), the free ends being held in tubes (C C'), provided
with three clamping jaws. A rapid torsional vibration[9] may now be
imparted to the stalk by means of the handle (H). The amplitude of
vibration, which determines the intensity of stimulus, can be accurately
measured by the graduated circle. The amplitude of vibration may be
predetermined by means of the sliding stops (S S').

#Intensity of stimulus dependent on amplitude of vibration.#--I shall now
describe an experiment which shows that torsional vibration is as
effective as stimulation by taps, and that its stimulating intensity
increases, length of stalk being constant, with amplitude of vibration.
It is of course obvious that if the length of the specimen be doubled,
the vibration, in order to produce the same effect, must be through
twice the angle. I took a leaf-stalk of turnip and fixed it in the
torsional vibrator. I then took record of responses to two successive
taps, the intensity of one being nearly double that of the other. Having
done this, I applied to the same stalk two successive torsional
vibrations of 45 deg. and 67 deg. respectively. These successive responses to
taps and torsional vibrations are given in fig. 12, and from them it
will be seen that these two modes of stimulation may be used
indifferently, with equal effect. The vibrational method has the
advantage over tapping, that, while with the latter the stimulus is
somewhat localised, with vibration the tissue subjected to stimulus is
uniformly stimulated throughout its length.

[Illustration: FIG. 12.--RESPONSE IN PLANT TO MECHANICAL TAP OR VIBRATION
The end B is injured. A tap was given between A and B and this gave the
response-curve _a_. A stronger tap gave the response _b_. By means of
the handle H, a torsional vibration of 45 deg. was now imparted, this
gave the response _c_. Vibration through 67 deg. gave _d_.]

#Effectiv