PLANT RESPONSE--ON DIPHASIC VARIATION
Diphasic variation--Positive after-effect and positive response--Radial
E.M. variation.
When a plant is stimulated at any point, a molecular disturbance--the
excitatory wave--is propagated outwards from the point of its
initiation.
#Diphasic variation.#--This wave of molecular disturbance is attended by a
wave of electrical disturbance. (Usually speaking, the electrical
relation between disturbed and less disturbed is that of copper to
zinc.) It takes some time for a disturbance to travel from one point to
another, and its intensity may undergo a diminution as it recedes
further from its point of origin. Suppose a disturbance originated at C;
if two points are taken near each other, as A and B, the disturbance
will reach them almost at the same time, and with the same intensity.
The electric disturbance will be the same in both. The effect produced
at A and B will balance each other and there will be no resultant
current.
By killing or otherwise reducing the sensibility of B as is done in the
method of injury, there is no response at B, and we obtain the
unbalanced response, due to disturbance at A; the same effect is
obtained by putting a clamp between A and B, so that the disturbance
may not reach B. But we may get response even without injury or block.
If we have the contacts at A and B, and if we give a tap _nearer_ A than
B (fig. 26, _a_), then we have (1) the disturbance reaching A earlier
than B. (2) The disturbance reaching A is much stronger than at B. The
disturbance at B may be so comparatively feeble as to be negligible.
It will thus be seen that we might obtain responses even without injury
or block, in cases where the disturbance is enfeebled in reaching a
distant point. In such a case on giving a tap near A a responsive
current would be produced in one direction, and in the opposite
direction when the tap is given near B (fig. 26, _b_). Theoretically,
then, we might find a neutral point between A and B, so that, on
originating the disturbance there, the waves of disturbance would reach
A and B at the same instant and with the same intensity. If, further,
the rate of recovery be the same for both points, then the electric
disturbances produced at A and B will continue to balance each other,
and the galvanometer will show no current. On taking a cylindrical root
of radish I have sometimes succeeded in finding a neutral point, which,
being disturbed, did not give rise to any resultant current. But
disturbing a point to the right or to the left gave rise to opposite
currents.
It is, however, difficult to obtain an absolutely cylindrical specimen,
as it always tapers in one direction. The conductivity towards the tip
of the root is not exactly the same as that in the ascending direction.
It is therefore difficult to fix an absolutely neutral point, but a
point may be found which approaches this very nearly, and on stimulating
the stalk near this, a very interesting diphasic variation has been
observed. In a specimen of cauliflower-stalk, (1) stimulus was applied
very much nearer A than B (the feeble disturbance reaching B was
negligible). The resulting response was upward and the recovery took
place in about sixty seconds.
[Illustration: FIG. 26.--DIPHASIC VARIATION]
(2) Stimulus was next applied near B. The resulting response was now
downward (fig. 26, _b_).
(3) The stimulus was now applied near the approximately neutral point N.
In this case, owing to a slight difference in the rates of propagation
in the two directions, a very interesting diphasic variation was
produced (fig. 26, _c_). From the record it will be seen that the
disturbance arrived earlier at A than at B. This produced an upward
response. But during the subsidence of the disturbance at A, the wave
reached B. The effect of this was to produce a current in the opposite
direction. This apparently hastened the recovery of A (from 60 seconds
to 12 seconds). The excitation of A now disappeared, and the second
phase of response, that due to excitation of B, was fully displayed.
#Positive after-effect.#--If we regard the response due to excitation of A
as negative, the later effect on B would appear as a subsequent positive
variation.
In the response of nerve, for example, where contacts are made at two
surfaces, injured and uninjured, there is sometimes observed, first a
negative variation, and then a positive after-effect. This may sometimes
at least be due to the proximal uninjured contact first giving the usual
negative variation, and the more distant contact of injury giving rise,
later, to the opposite, that is to say, apparently positive, response.
There is always a chance of an after-effect due to this cause, unless
(1) the injured end be completely killed and rendered quite
irresponsive, or (2) there be an effective block between A and B, so
that the disturbance due to stimulus can only act on one, and not on the
other.
I have found cases where, even when there was a perfect block, a
positive after-effect occurred. It would thus appear that if molecular
distortion from stimulus give rise to a negative variation, then during
the process of molecular recovery there may be over-shooting of the
equilibrium position, which may be exhibited as a positive variation.
#Positive variation.#--The responses given by muscle or nerve are,
normally speaking, negative. But that of retina is positive. The sign of
response, however, is apt to be reversed if there be any molecular
modification of the tissue from changes of external circumstances. Thus
it is often found that nerve in a stale condition gives positive,
instead of the normal negative variation, and stale retina often gives
negative, instead of the usual positive.
[Illustration: FIG. 27.--ABNORMAL POSITIVE RESPONSES IN STALE LEAF-STALK
OF TURNIP CONVERTED INTO NORMAL NEGATIVE UNDER STRONG STIMULATION[12]
The relative intensities of stimuli in the two cases are in the ratio
of 1:7.]
Curiously enough, I have on many occasions found exactly parallel
instances in the response of plants. Plants when fresh, as stated, give
negative responses as a rule. But when somewhat faded they sometimes
give rise to positive response. Again, just as in the modified nerve the
abnormal positive response gives place to the normal negative under
strong and long-continued stimulation, so also in the modified plant the
abnormal positive response passes into negative (fig. 27) under strong
stimulation. I was able in some cases to trace this process of gradual
reversal, by continuously increasing the intensity of stimulus. It was
then found that as the stimulus was increased, the positive at a certain
point underwent a reversal into the normal negative response (fig. 28).
[Illustration: FIG. 28. ABNORMAL POSITIVE PASSING INTO NORMAL NEGATIVE
IN A STALE SPECIMEN OF LEAF-STALK OF CAULIFLOWER
Stimulus was gradually increased from 1 to 10, by means of
spring-tapper. When the stimulus intensity was 10, the response
became reversed into normal negative. (Parts of 8 and 9 are out of
the plate.)]
The plant thus gives a reversed response under abnormal conditions of
staleness. I have sometimes found similar reversal of response when the
plant is subjected to the abnormal conditions of excessively high or low
temperature.
#Radial E.M. variation.#--We have seen that a current of response flows in
the plant from the relatively more to the relatively less excited. A
theoretically important experiment is the following: A thick stem of
plant stalk was taken and a hole bored so as to make one contact with
the interior of the tissue, the other being on the surface. After a
while the current of injury was found to disappear. On exciting the stem
by taps or torsional vibration, a responsive current was observed which
flowed inwards from the more disturbed outer surface to the shielded
core inside (fig. 29). Hence it is seen that when a wave of disturbance
is propagated along the plant, there is a concomitant wave of radial
E.M. variation. The swaying of a tree by the wind would thus appear to
give rise to a radial E.M.F.
[Illustration: FIG. 29.--RADIAL E.M. VARIATION]
FOOTNOTES:
[12] For general purposes it is immaterial whether the responses are
recorded up or down. For convenience of inspection they are in general
recorded _up_. But in cases where it is necessary to discriminate the
sign of response, positive response will be recorded up, and negative
down.