ELECTRIC RESPONSE IN PLANTS--BLOCK METHOD
Method of block--Advantages of block method--Plant response a
physiological phenomenon--Abolition of response by anæsthetics and
poisons--Abolition of response when plant is killed by hot water.
I shall now proceed to describe another and independent method which I
devised for obtaining plant response. It has the advantage of offering
us a complementary means of verifying the results found by the method of
negative variation. As it is also, in itself, for reasons which will be
shown later, a more perfect mode of inquiry, it enables us to
investigate problems which would otherwise have been difficult to
attempt.
When electrolytic contacts are made on the uninjured surfaces of the
stalk at A and B, the two points, being practically similar in every
way, are iso-electric, and little or no current will flow in the
galvanometer. If now the whole stalk be uniformly stimulated, and if
both ends A and B be equally excited at the same moment, it is clear
that there will still be no responsive current, owing to balancing
action at the two ends. This difficulty as regards the obtaining of
response was overcome in the method of negative variation, where the
excitability of one end was depressed by chemical reagents or injury, or
abolished by excessive temperature. On stimulating the stalk there was
produced a greater excitation at A than at B, and a current of action
was then observed to flow in the stalk from the more excited A to the
less excited B (fig. 6).
But we can cause this differential action to become evident by another
means. For example, if we produce a block, by clamping at C between A
and B (fig. 14, _a_), so that the disturbance made at A by tapping or
vibration is prevented from reaching B, we shall then have A thrown into
a relatively greater excitatory condition than B. It will now be found
that a current of action flows in the stalk from A to B, that is to say,
from the excited to the less excited. When the B end is stimulated,
there will be a reverse current (fig. 14, _b_).
[Illustration: FIG. 14.--THE METHOD OF BLOCK
(_a_) The plant is clamped at C, between A and B.
(_b_) Responses obtained by alternately stimulating the two ends.
Stimulation of A produces upward response; of B gives downward
response.]
We have in this method a great advantage over that of negative
variation, for we can always verify any set of results by making
corroborative reversal experiments.
By the method of injury again, one end is made initially abnormal, i.e.
different from the condition which it maintains when intact. Further,
inevitable changes will proceed unequally at the injured and uninjured
ends, and the conditions of the experiment may thus undergo unknown
variations. But by the block method which has just been described,
there is no injury, the plant is normal throughout, and any
physiological change (which in plants will be exceedingly small during
the time of the experiment) will affect it as a whole.
[Illustration: FIG. 15.--RESPONSE IN PLANT (FROM THE STIMULATED A TO
UNSTIMULATED B) COMPLETELY IMMERSED UNDER WATER
The leaf-stalk is clamped securely in the middle with the cork C, inside
the tube T, which is filled with water, the plant being completely
immersed. Moistened threads in connection with the two
non-polarisable electrodes are led to the side tubes _t t'_. One end
of the stalk is held in ebonite forceps and vibrated. A current of
response is found to flow in the stalk from the excited A to the
unexcited B, and outside, through the liquid, from B to A. A portion
of this current, flowing through the side tubes _t t'_, produces
deflection in the galvanometer.]
#Plant response a physiological or vital response.#--I now proceed to a
demonstration of the fact that whatever be the mechanism by which they
are brought about, these plant responses are physiological in their
character. As the investigations described in the next few chapters will
show, they furnish an accurate index of physiological activity. For it
will be found that, other things being equal, whatever tends to exalt or
depress the vitality of the plant tends also to increase or diminish its
electric response. These E.M. effects are well marked, and attain
considerable value, rising sometimes, as has been said before, to as
much as ·1 volt or more. They are proportional to the intensity of
stimulus.
It need hardly be added that special precautions are taken to avoid
shifting of contacts. Variation of contact, however, could not in any
case account for repeated transient responses to repeated stimuli, when
contact is made on iso-electric surfaces. Nor could it in any way
explain the reversible nature of these responses, when A and B are
stimulated alternately. These responses are obtained in the plants even
when completely immersed in water, as in the experimental arrangement
(fig. 15). It will be seen that in this case, where there could be no
possibility of shifting of contact, or variation of surface, there is
still the usual current of response.
I shall describe here a few crucial experiments only, in proof of the
physiological character of electric response. The test applied by
physiologists, in order to discriminate as to the physiological nature
of response, consists in finding out whether the response is diminished
or abolished by the action of anæsthetics, poisons, and excessively high
temperature, which are known to depress or destroy vitality.
I shall therefore apply these same tests to plant responses.
#Effect of anæsthetics and poisons.#--Ordinary anæsthetics, like
chloroform, and poisons, like mercuric chloride, are known to produce a
profound depression or abolish all signs of response in the living
tissue. For the purpose of experiment, I took two groups of stalks, with
leaves attached, exactly similar to each other in every respect. In
order that the leaf-stalks might absorb chloroform I dipped their cut
ends in chloroform-water, a certain amount of which they absorbed, the
process being helped by the transpiration from the leaves. The second
group of stalks was placed simply in water, in order to serve for
control experiment. The narcotic action of chloroform, finally
culminating in death, soon became visually evident. The leaves began to
droop, a peculiar death-discolouration began to spread from the mid rib
along the venation of the leaves. Another peculiarity was also observed.
The aphides feeding on the leaves died even before the appearance of the
discoloured patches, whereas on the leaves of the stalks placed in water
these little creatures maintained their accustomed activity, nor did any
discolouration occur. In order to study the effect of poison, another
set was placed in water containing a small quantity of mercuric
chloride. The leaves here underwent the same change of appearance, and
the aphides met with the same untimely fate, as in the case of those
subjected to the action of chloroform. There was hardly any visible
change in the appearance of the stalks themselves, which were to all
outer seeming as living as ever, indications of death being apparent
only on the leaf surfaces. I give below the results of several sets of
experiments, from which it would appear that whereas there was strong
normal response in the group of stalks kept in water, there was
practically a total abolition of all response in those anæsthetised or
poisoned.
#Experiments on the effect of anæsthetics and poisons.# A batch of ten
leaf-stalks of plane-tree was placed with the cut ends in water, and
leaves in air; an equal number was immersed in chloroform-water; a third
batch was placed in 5 per cent. solution of mercuric chloride.
Similarly a batch of three horse-chestnut leaf-stalks was put in water,
another batch in chloroform-water, and a third batch in mercuric
chloride solution.
I. LEAF-STALK OF PLANE-TREE
The stimulus applied was a single vibration of 90°.
A. After 24 hours in | B. After 24 hours in | C. After 24 hours in
water | chloroform water | mercuric chloride
| |
[All leaves standing up | [Leaves began to | [Leaves began to droop
and fresh--aphides | droop in 1 hour | in 4 hours. Deep
alive] | and bent over in | discolouration along
| 3 hours--aphides | the veins. Aphides
| dead] | dead]
| |
Electric | Electric | Electric
Response | response | response
(1) 21 dns. | (1) 1 dn. | (1) 0 dn.
(2) 31 " | (2) 1 " | (2) ·25 "
(3) 26 " | (3) 2 " | (3) ·25 "
(4) 15 " | (4) 0 " | (4) 0 "
(5) 17 " | (5) 1 " | (5) ·25 "
(6) 23 " | (6) 1·5 " | (6) ·25 "
(7) 30 " | (7) 2 " | (7) 0 "
(8) 27 " | (8) 1 " | (8) ·25 "
(9) 29 " | (9) 1 " | (9) ·25 "
(10) 17 " | (10) ·5 " | (10) ·5 "
------------------------+----------------------+-----------------------
Mean response 23·6 | Mean 1 | Mean ·15
II. LEAF-STALK OF HORSE-CHESTNUT
(1) 15 dns. | (1) ·5 dn. | (1) 0 dn.
(2) 17 " | (2) ·5 " | (2) 0 "
(3) 10 " | (3) 0 " | (3) 0 "
------------------------+----------------------+-----------------------
Mean 14 | Mean ·3 | Mean 0
These results conclusively prove the physiological nature of the
response.
I shall in a succeeding chapter give a continuous series of
response-curves showing how, owing to progressive death from the action
of poison, the responses undergo steady diminution till they are
completely abolished.
#Effect of high temperature.#--It is well known that plants are killed
when subjected to high temperatures. I took a stalk, and, using the
block method, with torsional vibration as the stimulus, obtained strong
responses at both ends A and B. I then immersed the same stalk for a
short time in hot water at about 65° C., and again stimulated it as
before. But at neither A nor B could any response now be evoked. As all
the external conditions were the same in the first and second parts of
this experiment, the only difference being that in one the stalk was
alive and in the other killed, we have here further and conclusive proof
of the physiological character of electric response in plants.
The same facts may be demonstrated in a still more striking manner by
first obtaining two similar but opposite responses in a fresh stalk, at
A and B, and then killing one half, say B, by immersing only that half
of the stalk in hot water. The stalk is replaced in the apparatus, and
it is now found that whereas the A half gives strong response, the end B
gives none.
In the experiments on negative variation, it was tacitly assumed that
the variation is due to a differential action, stimulus producing a
greater excitation at the uninjured than at the injured end. The block
method enables us to test the correctness of this assumption. The B end
of the stalk is injured or killed by a few drops of strong potash, the
other end being uninjured. There is a clamp between A and B. The end A
is stimulated and a strong response is obtained. The end B is now
stimulated, and there is little or no response. The block is now removed
and the plant stimulated throughout its length. Though the stimulus now
acts on both ends, yet, owing to the irresponsive condition of B, there
is a resultant response, which from its direction is found to be due to
the responsive action of A. This would not have been the case if the end
B had been uninjured. We have thus experimentally verified the
assumption that in the same tissue an uninjured portion will be thrown
into a greater excitatory state than an injured, by the action of the
same stimulus.