EXCITATION IN PLANT.
THE METHOD OF EXPERIMENT.
I may here say a few words of the manner in which the period of
transmission can be found from the record given by my Resonant
Recorder, fully described in my previous paper. The writer attached to
the recording lever of this instrument is maintained by electromagnetic
means in a state of to-and-fro vibration. The record thus consists
of a series of dots made by the tapping writer, which is tuned to
vibrate at a definite rate, say, 10 times per second. In a particular
case whose record is given in Curve 1 (Fig. 46), indirect stimulus
of electric shock was applied at a distance of 15 mm. from the
responding pulvinus. There are 15 intervening dots between the
moment of application of stimulus and the beginning of response; the
time-interval is therefore 1.5 seconds. The latent period of the
motile pulvinus is obtained from a record of direct stimulation; the
average value of this in summer is 0.1 second. Hence the true period
of transmission is 1.4 seconds for a distance of 15 mm. The velocity
determined in this particular case is therefore 10.7 mm. per second.
Precaution has to be taken against another source of disturbance,
namely, the excitation caused by the sudden commencement or the
cessation of the constant current. I have shown elsewhere[O] that the
sudden initiation or cessation of the current induces an excitatory
reaction in the plant-tissue similar to that in the animal tissue. This
difficulty is removed by the introduction of a sliding potentiometer,
which allows the applied electromotive force to be gradually increased
from zero to the maximum or decreased from the maximum to zero.
[O] BOSE--‘Plant Response’ (1906); ‘Irritability of Plants’ (1913).
[Illustration: FIG. 45.--Complete apparatus for investigation of
the variation of conducting power in _Mimosa_. A, storage cell; S,
potentiometer slide, which, by alternate movement to right or left,
continuously increases or decreases the applied E. M. F.; K, switch key
for putting current “on” and “off” without variation of resistance; E,
E′, electrodes of induction coil for stimulation; C, choking coil; G,
micro-ammeter.]
The experimental arrangement is diagrammatically shown in Fig. 45.
After attaching the petiole to the recording lever, indirect stimulus
is applied, generally speaking, at a distance of 15 mm. from the
responding pulvinus. Stimulus of electric shock is applied in the
usual manner, by means of a sliding induction coil. The intensity of
the induction shock is adjusted by gradually changing the distance
between the secondary and the primary, till a minimally effective
stimulus is found. In the study of the effect of direction of constant
current on conductivity, non-polarisable electrodes make suitable
electric connections, one with the stem and the other with the tip of
a sub-petiole at a distance from each other of about 95 mm. The point
of stimulation and the responding pulvinus are thus situated at a
considerable distance from the anode or the cathode, in the indifferent
region in which there is no polar variation of excitability. By
means of a Pohl’s commutator or reverser, the constant current can be
maintained either “with” or “against” the direction of transmission of
excitation. The transmission in the former case is “down-hill,” and
in the latter case “up-hill.” Electrical connections are so arranged
that when the commutator is tilted to the right, the transmission is
down-hill, when tilted to the left, up-hill.
The electrical resistance offered by the 95 mm. length of stem and
petiole will be from two to three million ohms. The intensity of the
constant current flowing through the plant can be read by unplugging
the key which short-circuits the micro-ammeter G. The choking coil
C prevents the alternating induction current from flowing into the
polarising circuit and causing direct stimulation of the pulvinus.
Before describing the experimental results, it is as well to enter
briefly into the question of the external indication by which the
conducting power may be gauged. Change of conductivity may be
expected to give rise to a variation in the rate of propagation
or to a variation in the magnitude of the excitatory impulse that
is transmitted. Thus we have several methods at our disposal for
determining the induced variation of conductivity. In the first place
the variation of conductivity may be measured by the induced change
in the velocity of transmission of excitation. In the second place,
the transmitted effect of a sub-maximal stimulus will give rise to
enhanced or diminished amplitude of mechanical response, depending on
the increase or decrease of conductivity brought about by the directive
action of the current. And, finally, the enhancement or depression
of conductivity may be demonstrated by the ineffectively transmitted
stimulus becoming effective, or the effectively transmitted stimulus
becoming ineffective.
_Exclusion of the factor of Excitability._--The object of the enquiry
being the pure effect of variation of conductivity, we have to assure
ourselves that under the particular conditions of the experiment the
complicating factor of polar variation of excitability is eliminated.
It is to be remembered that excitatory transmission in _Mimosa_ takes
place by means of a certain conducting strand of tissue which runs
through the stem and the petiole. In the experiment to be described,
the constant current enters by the tip of the petiole and leaves by
the stem, or _vice versâ_, the length of the intrapolar region being
95 mm. The point of application of stimulus on the petiole is 40 mm.
from the electrode at the tip of the leaf. The responding pulvinus is
also at the same distance from the electrode on the stem. The point of
stimulation and region of response are thus at the relatively great
distance of 40 mm. from either the anode or the cathode, and may
therefore be regarded as situated in the indifferent region. This is
found to be verified in actual experiments.
EFFECTS OF DIRECTION OF CURRENT ON VELOCITY OF TRANSMISSION.
A very convincing method of demonstrating the influence of electric
current on conductivity consists in the determination of changes
induced in the velocity of transmission by the directive action of the
current. For this purpose we have to find out the true time required
by the excitation to travel through a given length of the conducting
tissue (1) in the absence of the current, (2) ‘against’ and (3) ‘with’
the direction of the current. The true time is obtained by subtracting
the latent period of the pulvinus from the observed interval between
the stimulus and response. Now the latent period may not remain
constant, but undergo change under the action of the polarising
current. It has been shown that the excitability of the pulvinus does
not undergo any change when it is situated in the middle or indifferent
region. The following results show that under parallel conditions the
latent period also remains unaffected:--
TABLE V.--SHOWING THE EFFECT OF ELECTRIC CURRENT ON THE LATENT PERIOD.
+------------------------------------------------+-------+-------+
| Specimens | I. | II. |
|------------------------------------------------+-------|-------|
| | sec. | sec. |
| Latent period under normal condition | 0.10 | 0.09 |
| " " current from right to left | 0.11 | 0.10 |
| " " current from left to right | 0.09 | 0.09 |
+------------------------------------------------+-------+-------+
The results of experiments with two different specimens given above
show that a current applied under the given conditions has practically
no effect on the latent period, the slight variation being of the order
of one-hundredth part of a second. This is quite negligible when the
total period observed for transmission is, as in the following cases,
equal to nearly 2 seconds.
_Induced changes in the Velocity of Transmission._--Having found that
the average value of the latent period in summer is 0.1 second, we
next proceed to determine the influence of the direction of current on
velocity.
_Experiment 41._--As a rule, stimulus of induction shock was applied
in this and in the following experiments on the petiole at a distance
of 15 mm. from the responding pulvinus. The recording writer was
tuned to 10 vibrations per second; the space between two succeeding
dots, therefore, represents a time-interval of 0.1 second. The middle
record, N in Fig. 46, is the normal. There are 17 spaces between the
application of stimulus and the beginning of response. The total time
is therefore 1.7 seconds, and by subtracting from it the latent period
of 0.1 second we obtain the true time, 1.6 seconds. The normal velocity
is found by dividing the distance 15 mm. by the true interval 1.6
seconds. Thus V = 15/1.6 = 9.4 mm. per second. We shall next consider
the effect of current in modifying the normal velocity. The uppermost
record (1) in Fig. 46 was taken under the action of an ‘up-hill,’ or
‘against’ current of the intensity of 1.4 microampères. It will be seen
that the time interval is reduced from 1.7 seconds to 1.4 seconds;
making allowance for the latent period, the velocity of transmission
under ‘up-hill’ current V_{1} = 15/1.3 = 11.5 mm. per second. In the
lowest record (3) we note the effect of ‘down-hill’ current, the
time-interval between stimulus and response being prolonged to 1.95
seconds and the velocity reduced to 8.1 mm. per second. The conclusion
arrived at from this mechanical mode of investigation is thus identical
with that derived from the electric method of conductivity balance
referred to previously.
[Illustration: FIG. 46.--Record showing enhancement of velocity of
transmission “up-hill” or against the current (uppermost curve) and
retardation of velocity “down-hill” or with the current (lowest curve).
N, normal record in the absence of current; ← indicates “up-hill” and →
“down-hill” transmission.]
That is to say, _the passage of a feeble current modifies conductivity
for excitation in a selective manner. Conductivity is enhanced_
against_, and diminished_ with_, the direction of the current._
The minimum current which induces a perceptible change of conductivity
varies somewhat in different specimens. The average value of this
minimal current in autumn is 1.4 microampères. The effect of even a
feebler current may be detected by employing a test stimulus which is
barely effective.
TABLE VI.--SHOWING EFFECTS OF UP-HILL AND DOWN-HILL CURRENTS OF FEEBLE
INTENSITY ON PERIOD OF TRANSMISSION THROUGH 15 MM.
+-------+-------------+---------------------+---------------------+
|Number.|Intensity of | Period for | Period for |
| | current in | up-hill | down-hill |
| |microampères.| transmission. | transmission. |
+-------+-------------+---------------------+---------------------+
| 1 | 1.4 |14 tenths of a second|16 tenths of a second|
| 2 | 1.4 |13 " " |15 " " |
| 3 | 1.6 |19 " " |Arrest. |
| 4 | 1.7 |12 " " |14 tenths of a second|
+-------+-------------+---------------------+---------------------+
Having demonstrated the effect of direction of current on the velocity
of transmission, I shall next describe other methods by which induced
variations of conductivity may be exhibited.
DETERMINATION OF VARIATION OF CONDUCTIVITY BY METHOD OF MINIMAL
STIMULUS AND RESPONSE.
In this method we employ a minimal stimulus, the transmitted effect of
which under normal conditions gives rise to a feeble response. If the
passage of a current in a given direction enhances conductivity, then
the intensity of transmitted excitation will also be enhanced; the
minimal response will tend to become maximal. Or excitation which had
hitherto been ineffectively transmitted will now become effectively
transmitted. Conversely, depression of conductivity will result in a
diminution or abolition of response. We may use a single break-shock
of sufficient intensity as the test stimulus. It is, however,
better to employ the additive effect of a definite number of feeble
make-and-break shocks.
We may again employ additive effect of a definite number of induction
shocks, the alternating elements of which are exactly equal and
opposite. This is secured by causing rapid reversals of the primary
current by means of a rotating commutator. The successive induction
shocks of the secondary coil can thus be rendered exactly equal and
opposite.
_Experiment 42._--Working in this way, it is found that the transmitted
excitation against the direction of current becomes effective or
enhanced under ‘up-hill’ current. A current, flowing with the direction
of transmission, on the other hand, diminishes the intensity of
transmitted excitation or blocks it altogether.
Henceforth it would be convenient to distinguish currents in the
two directions: the current in the direction of transmission will
be distinguished as _Homodromous_, and against the direction of
transmission as _Heterodromous_.
AFTER-EFFECTS OF HOMODROMOUS AND HETERODROMOUS CURRENTS.
The passage of a current through a conducting tissue in a given
direction causes, as we have seen, an enhanced conductivity in an
opposite direction. We may suppose this to be brought about by a
particular molecular arrangement induced by the current, which
assisted the propagation of the excitatory disturbance in a selected
direction. On the cessation of this inducing force, there may be a
rebound and a temporary reversal of previous molecular arrangement,
with concomitant reversal of the conductivity variation. The immediate
after-effect of a current flowing in a particular direction on
conductivity is likely to be a transient change, the sign of which
would be opposite to that of the direct effect. The after-effect of a
heterodromous current may thus be a temporary depression, that of a
homodromous current, a temporary enhancement of conductivity.
[Illustration: FIG. 47.--Direct and after-effect of heterodromous and
homodromous currents. First two records, N, N, normal. ↓, enhanced
transmission under heterodromous current; ⇣ arrest of conduction is an
after-effect of heterodromous current. Next record ↑ shows arrest under
homodromous current. Last record ⇡ shows enhancement of conduction
greater than normal, as an after-effect of homodromous current. (Dotted
arrow indicates the after-effect on cessation of a given current. ↑
homodromous and ↓ heterodromous current.)]
_Experiment 43._--This inference will be found fully justified in the
following experiment:--The first two responses are normal, after which
the heterodromous current gave rise to an enhanced response. The
depressing after-effect of a heterodromous current rendered the next
response ineffective. The following record taken during the passage
of the homodromous current exhibited an abolition of response due to
induced depression of conductivity. Finally, the after-effect of the
homodromous current is seen to be a response larger than the normal
(Fig. 47). These experiments show that the after-effect of cessation of
a current in a given direction is a transient conductivity variation,
of which the sign is opposite to that induced by the continuation of
the current.