ELECTRIC RESPONSE IN PLANTS--METHOD OF NEGATIVE VARIATION
Negative variation--Response recorder--Photographic
recorder--Compensator--Means of graduating intensity of
stimulus--Spring-tapper and torsional vibrator--Intensity of
stimulus dependent on amplitude of vibration--Effectiveness of
stimulus dependent on rapidity also.
I shall first proceed to show that an electric response is evoked in
plants under stimulation.[8]
In experiments for the exhibition of electric response it is preferable
to use a non-electrical form of stimulus, for there is then a certainty
that the observed response is entirely due to reaction from stimulus,
and not, as might be the case with electric stimulus, to mere escape of
stimulating current through the tissue. For this reason, the mechanical
form of stimulation is the most suitable.
I find that all parts of the living plant give electric response to a
greater or less extent. Some, however, give stronger response than
others. In favourable cases, we may have an E.M. variation as high as
·1 volt. It must however be remembered that the response, being a
function of physiological activity of the plant, is liable to undergo
changes at different seasons of the year. Each plant has its particular
season of maximum responsiveness. The leaf-stalk of horse-chestnut, for
example, exhibits fairly strong response in spring and summer, but on
the approach of autumn it undergoes diminution. I give here a list of
specimens which will be found to exhibit fairly good response:
#Root.#--Carrot (_Daucus Carota_), radish (_Raphanus sativus_).
#Stem.#--Geranium (_Pelargonium_), vine (_Vitis vinifera_).
#Leaf-stalk.#--Horse-chestnut (_Æsculus Hippocastanum_), turnip (_Brassica
Napus_), cauliflower (_Brassica oleracea_), celery (_Apium graveolens_),
Eucharis lily (_Eucharis amazonica_).
#Flower-stalk.#--Arum lily (_Richardia africana_).
#Fruit.#--Egg-plant (_Solanum Melongena_).
#Negative variation.#--Taking the leaf-stalk of turnip we kill an area on
its surface, say B, by the application of a few drops of strong potash,
the area at A being left uninjured. A current is now observed to flow,
in the stalk, from the injured B to the uninjured A, as was found to be
the case 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 the spot of light
beyond the recording surface, a potentiometer balancing arrangement may
be used (fig. 9), by which the P.D. due to injury is exactly
compensated; E.M. variations produced by stimulus are then taken in the
usual manner. This compensating arrangement is also helpful, as has been
said before, for calibrating the E.M. value of the deflection.
[Illustration: FIG. 9.--THE COMPENSATOR
A B is a stretched wire with added resistances R and R'. S is a storage
cell. When the key K is turned to the right one scale division =
·001 volt, when turned to the left one scale division = ·01 volt. P
is the plant.]
#Means of graduating the intensity of stimulus.#--One of the necessities
in connection with quantitative measurements is to be certain that the
intensity of successive stimuli is (1) constant, or (2) capable of
gradual increase by known amounts. No two taps given by the hand can be
made exactly alike. I have therefore devised the two following methods
of stimulation, which have been found to act satisfactorily.
[Illustration: FIG. 10.--THE SPRING-TAPPER]
#The spring-tapper.#--This consists (fig. 10) of the spring proper (S),
the attached rod (R) carrying at its end the tapping-head (T). A
projecting rod--the lifter (L)--passes through S R. It is provided with
a screw-thread, by means of which its length, projecting downwards, is
regulated. This fact, as we shall see, is made to determine the height
of the stroke. (C) is a cogwheel. As one of the spokes of the cogwheel
is rotated past (L), the spring is lifted and released, and (T) delivers
a sharp tap. The height of the lift, and therefore the intensity of the
stroke, is measured by means of a graduated scale. We can increase the
intensity of the stroke through a wide range (1) by increasing the
projecting length of the lifter, and (2) by shortening the length of
spring by a sliding catch. We may give isolated single taps or superpose
a series in rapid succession according as the wheel is rotated slow or
fast. The only disadvantage of the tapping method of stimulation is that
in long-continued experiment the point struck is liable to be injured.
The vibrational mode of stimulation to be presently described labours
under no such disadvantage.
#The electric tapper.#--Instead of the simple mechanical tapper, 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° and 67° 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° was now imparted, this
gave the response _c_. Vibration through 67° gave _d_.]
#Effectiveness of stimulus dependent on rapidity also.# In order that
successive stimuli may be equally effective another point has to be
borne in mind. In all cases of stimulation of living tissue it is found
that the effectiveness of a stimulus to arouse response depends on the
rapidity of the onset of the disturbance. It is thus found that the
stimulus of the 'break' induction shock, on a muscle for example, is
more effective, by reason of its greater rapidity, than the 'make'
shock. So also with the torsional vibrations of plants, I find response
depending on the quickness with which the vibration is effected. I give
below records of successive stimuli, given by vibrations through the
same amplitude, but delivered with increasing rapidity (fig. 13).
[Illustration: FIG. 13.--INFLUENCE OF SUDDENNESS ON THE EFFICIENCY OF
STIMULUS
The curves _a_, _b_, _c_, _d_, are responses to vibrations of the same
amplitude, 30°. In _a_ the vibration was very slow; in _b_ it was
less slow; it was rapid in _c_, and very rapid in _d_.]
Thus if we wish to maintain the effective intensity of stimulus constant
we must meet two conditions: (1) The amplitude of vibration must be kept
the same. This is done by means of the graduated circle. (2) The
vibration period must be kept the same. With a little practice, this
requirement is easily fulfilled.
The uniformity of stimulation which is thus attained solves the great
difficulty of obtaining reliable quantitative values, by whose means
alone can rigorous demonstration of the phenomena we are studying become
possible.
FOOTNOTES:
[8] A preliminary account of Electric Response in Plants was given at
the end of my paper on 'Electric Response of Inorganic Substances' read
before the Royal Society on June 6, 1901; also at the Friday Evening
Discourse, Royal Institution, May 10, 1901. A more complete account is
given in my paper on 'Electric Response in Ordinary Plants under
Mechanical Stimulus' read before the Linnean Society March 20, 1902.
I thank the Royal Society and the Linnean Society for permission to
reproduce some of my diagrams published in their _Proceedings_.--J. C. B.
[9] By this is meant a rapid to-and-fro or complete vibration. In order
that successive responses should be uniform it is essential that there
should be no resultant twist, i.e. the plant at the end of vibration
should be in exactly the same condition as at the beginning.