Response in the Living and Non-Living | Page 9

in the animal tissue. The potential difference depends on the
condition of the plant, and the season in which it may have been
gathered. In the experiment here described (fig. 6, a) its value
was ·13 volt.
[Illustration: FIG. 6.--(a) EXPERIMENT FOR EXHIBITING
ELECTRIC RESPONSE IN PLANTS BY METHOD OF NEGATIVE
VARIATION. (b) RESPONSES IN LEAF-STALK OF TURNIP TO
STIMULI OF TWO SUCCESSIVE TAPS, THE SECOND BEING
STRONGER. A and B contacts are about 2 cm. apart, B being injured.
Plant is stimulated by a tap between A and B. Stimulus acts on both A
and B, but owing to injury of B, effect at A is stronger and a negative
variation due to differential action occurs.]
A sharp tap was now given to the stalk, and a sudden diminution, or
negative variation, of current occurred, the resting potential difference
being decreased by ·026 volt. A second and stronger tap produced a
second response, causing a greater diminution of P.D. by ·047 volt
(fig. 6, b). The accompanying figure is a photographic record of
another set of response-curves (fig. 7). The first three responses are for
a given intensity of stimulus, and the next six in response to stimulus
nearly twice as strong. It will be noticed that fatigue is exhibited in
these responses. Other experiments will be described in the next
chapter which show conclusively that the response was not due to any
accidental circumstance but was a direct result of stimulation. But I
shall first discuss the experimental arrangements and method of
obtaining these graphic records.
[Illustration: FIG. 7.--RECORD OF RESPONSES IN PLANT
(LEAF-STALK OF CAULIFLOWER) BY METHOD OF NEGATIVE
VARIATION The first three records are for stimulus intensity 1; the
next six are for intensity twice as strong; the successive responses
exhibit fatigue. The vertical line to the left represents ·1 volt. The

record is to be read from right to left.]
#Response recorder.#--The galvanometer used is a sensitive dead-beat
D'Arsonval. The period of complete swing of the coil under
experimental conditions is about 11 seconds. A current of
10^{-9} ampere produces a deflection of 1 mm. at a distance of 1 metre.
For a quick and accurate method of obtaining the records, I devised the
following form of response recorder. The curves are obtained directly,
by tracing the excursion of the galvanometer spot of light on a
revolving drum (fig. 8). The drum, on which is wrapped the paper for
receiving the record, is driven by clockwork. Different speeds of
revolution can be given to it by adjustment of the clock-governor, or by
changing the size of the driving-wheel. The galvanometer spot is
thrown down on the drum by the inclined mirror M. The galvanometer
deflection takes place at right angles to the motion of the paper. A
stylographic pen attached to a carrier rests on the writing surface. The
carrier slides over a rod parallel to the drum. As has been said before,
the galvanometer deflection takes place parallel to the drum, and as
long as the plant rests unstimulated, the pen, remaining coincident with
the stationary galvanometer spot on the revolving paper, describes a
straight line. If, on stimulation, we trace the resulting excursion of the
spot of light, by moving the carrier which holds the pen, the rising
portion of the response-curve will be obtained. The galvanometer spot
will then return more or less gradually to its original position, and that
part of the curve which is traced during the process constitutes the
recovery. The ordinate in these curves represents the E.M. variation,
and the abscissa the time.
[Illustration: FIG. 8.--RESPONSE RECORDER]
We can calibrate the value of the deflection by applying a known
E.M.F. to the circuit from a compensator, and noting the deflection
which results. The speed of the clock is previously adjusted so that the
recording surface moves exactly through, say, one inch a minute. Of
course this speed can be increased to suit the particular experiment, and
in some it is as high as six inches a minute. In this simple manner very
accurate records may be made. It has the additional advantage that one

is able at once to see whether the specimen is suitable for the purpose
of investigation. A large number of records might be taken by this
means in a comparatively short time.
#Photographic recorder.#--Or the records may be made
photographically. A clockwork arrangement moves a photographic
plate at a known uniform rate, and a curve is traced on the plate by the
moving spot of light. All the records that will be given are accurate
reproductions of those obtained by one of these two methods.
Photographic records are reproduced in white against a black
background.
#Compensator.#--As the responses are on variation of current of injury,
and as the current of injury may be strong, and throw