PLANT RESPONSE--EFFECT OF ANÆSTHETICS AND POISONS
Effect of anæsthetics, a test of vital character of response--Effect of
chloroform--Effect of chloral--Effect of formalin--Method in which
response is unaffected by variation of resistance--Advantage of
block method--Effect of dose.
The most important test by which vital phenomena are distinguished is
the influence on response of narcotics and poisons. For example, a nerve
when narcotised by chloroform exhibits a diminishing response as the
action of the anæsthetic proceeds. (See below, fig. 43.) Similarly,
various poisons have the effect of permanently abolishing all response.
Thus a nerve is killed by strong alkalis and strong acids. I have
already shown how plants which previously gave strong response did not,
after application of an anæsthetic or poison, give any response at all.
In these cases it was the last stage only that could be observed. But it
appeared important to be able to trace the growing effect of
anæsthetisation or poisoning throughout the process. There were,
however, two conditions which it at first appeared difficult to meet.
First it was necessary to find a specimen which would normally exhibit
no fatigue, and give rise for a long time to a uniform series of
response. The immediate changes made in the response, in consequence of
the application of chemical reagents, could then be demonstrated in a
striking manner. And with a little trouble, specimens can be secured in
which perfect regularity of response is found. The record given in
fig. 16, obtained with a specimen of radish, shows how possible it is to
secure plants in which response is absolutely regular. I subjected this
to uniform stimulation at intervals of one minute, during half an hour,
without detecting the least variation in the responses. But it is of
course easier to find others in which the responses as a whole may be
taken as regular, though there may be slight rhythmic fluctuations. And
even in these cases the effect of reagents is too marked and sudden to
escape notice.
[Illustration: FIG. 43.--EFFECT OF CHLOROFORM ON NERVE RESPONSE (WALLER)]
For the obtaining of constant and strong response I found the best
materials to be carrot and radish, selected individuals from which gave
most satisfactory results. The carrots were at their best in August and
September, after which their sensitiveness rapidly declined. Later,
being obliged to seek for other specimens, I came upon radish, which
gave good results in the early part of November; but the setting-in of
the frost had a prejudicial effect on its responsiveness. Less perfect
than these, but still serviceable, are the leaf-stalks of turnip and
cauliflower. In these the successive responses as a whole may be
regarded as regular, though a curious alternation is sometimes noticed,
which, however, has a regularity of its own.
My second misgiving was as to whether the action of reagents would be
sufficiently rapid to display itself within the time limit of a
photographic record. This would of course depend in turn upon the
rapidity with which the tissues of the plant could absorb the reagent
and be affected by it. It was a surprise to me to find that, with good
specimens, the effect was manifested in the course of so short a time as
a minute or so.
#Effect of chloroform.#--In studying the effect of chemical reagents in
plants, the method is precisely similar to that employed with nerve;
that is to say, where vapour of chloroform is used, it is blown into the
plant chamber. In cases of liquid reagents, they are applied on the
points of contact A and B and their close neighbourhood. The mode of
experiment was (1) to obtain a series of normal responses to uniform
stimuli, applied at regular intervals of time, say one minute, the
record being taken the while on a photographic plate. (2) Without
interrupting this procedure, the anæsthetic agent, vapour of chloroform,
was blown into the closed chamber containing the plant. It will be seen
how rapidly chloroform produces depression of response (fig. 44), and
how the effect grows with time. In these experiments with plants, the
same curious shifting of the zero line is sometimes noticed as in nerve
when subjected similarly to the action of reagents. This is a point of
minor importance, the essential point to be noticed being that the
responses are rapidly reduced.
[Illustration: FIG. 44.--EFFECT OF CHLOROFORM ON RESPONSES OF CARROT
Stimuli of 25° vibration at intervals of one minute.]
#Effects of chloral and formalin.#--I give below (figs. 45, 46) two sets
of records, one for the reagent chloral and the other for formalin. The
reagents were applied in the form of a solution on the tissue at the two
leading contacts, and the contiguous surface. The rhythmic fluctuation
in the normal response shown in fig. 45 is interesting. The abrupt
decline, within a minute of the application of chloral, is also
extremely well marked.
[Illustration: FIG. 45.--ACTION OF CHLORAL HYDRATE ON THE RESPONSES OF
LEAF-STALK OF CAULIFLOWER
Vibration of 25° at intervals of one minute.]
[Illustration: FIG. 46.--ACTION OF FORMALIN (RADISH)]
#Response unaffected by variation of resistance.#--In order to bring out
clearly the main phenomena, I have postponed till now the consideration
of a point of some difficulty. To determine the influence of a reagent
in modifying the excitability of the tissue, we rely upon its effect in
exalting or depressing the responsive E.M. variation. We read this
effect by means of galvanometric deflections. And if the resistance of
the circuit remained constant, then an increase of galvanometer
deflection would accurately indicate a heightened or depressed E.M.
response, due to greater or less excitability of tissue caused by the
reagent. But, by the introduction of the chemical reagent, the
resistance of the tissue may undergo change, and owing to this cause,
modification of response as read by the galvanometer may be produced
without any E.M. variation. The observed variation of response may thus
be partly owing to some unknown change of resistance, as well as to that
of the E.M. variation in response to stimulus.
We may however discriminate as to how much of the observed change is due
to variation of resistance by comparing the deflections produced in the
galvanometer by the action of a definite small E.M.F. before and after
the introduction of the reagent. If the deflections be the same in both
cases, we know that the resistance has not varied. If there have been
any change, the variation of deflection will show the amount, and we can
make allowance accordingly.
I have however adopted another method, by which all necessity of
correction is obviated, and the galvanometric deflections simply give
E.M. variations, unaffected by any change in the resistance of the
tissue. This is done by interposing a very large and constant resistance
in the external circuit and thereby making other resistances negligible.
An example will make this point clear. Taking a carrot as the vegetable
tissue, I found its resistance plus the resistance of the
non-polarisable electrode equal to 20,000 ohms. The introduction of a
chemical reagent reduced it to 19,000 ohms. The resistance of the
galvanometer is equal to 1,000 ohms. The high external resistance was
1,000,000 ohms. The variation of resistance produced in the circuit
would therefore be 1,000 in (1,000,000+19,000+1,000) or one part in
1,020. Therefore the variation of galvanometric deflection due to change
of resistance would be less than one part in a thousand (cf. fig. 49).
#The advantage of the block method.#--In these investigations I have used
the block method, instead of that of negative variation, and I may here
draw attention to the advantages which it offers. In the method of
negative variation, one contact being injured, the chemical reagents act
on injured and uninjured unequally, and it is conceivable that by this
unequal action the resting difference of potential may be altered. But
the intensity of response in the method of injury depends on this
resting difference. It is thus hypothetically possible that on the
method of negative variation there might be changes in the responses
caused by variation of the resting difference, and not necessarily due
to the stimulating or depressing effect of the reagent on the tissue.
But by the block method the two contacts are made with uninjured
surfaces, and the effect of reagents on both is similar. Thus no
advantage is given to one contact over the other. The changes now
detected in response are therefore due to no adventitious circumstance,
but to the reagent itself. If further verification be desired as to the
effect of the reagent, we can obtain it by alternate stimulation of the
A and B ends. Both ends will then show the given change. I give below a
record of responses given by two ends of leaf-stalk of turnip,
stimulated alternately in the manner described. The stalk used was
slightly conical, and owing to this difference between the A and B ends
the responses given by one end were slightly different from those given
by the other, though the stimuli were equal. A few drops of 10 per cent.
solution of NaOH was applied to both the ends. It will be seen how
quickly this reagent abolished the response of both ends (fig. 47).
[Illustration: FIG. 47.--ABOLITION OF RESPONSE AT BOTH A AND B ENDS BY
THE ACTION OF NaOH
Stimuli of 30° vibration were applied at intervals of one minute to A
and B alternately. Response was completely abolished twenty-four
minutes after application of NaOH.]
#Effect of dose.#--It is sometimes found that while a reagent acts as a
poison when given in large quantities, it may act as a stimulant in
small doses. Of the two following records fig. 48 shows the slight
stimulating effect of very dilute KOH, and fig. 49 exhibits nearly
complete abolition of response by the action of the same reagent when
given in stronger doses.
[Illustration: FIG. 48.--STIMULATING ACTION OF VERY DILUTE KOH]
So we see that, judged by the final criterion of the effect produced by
anæsthetics and poisons, the plant response fulfils the test of vital
phenomenon. In previous chapters we have found that in the matter of
response by negative variation, of the presence or absence of fatigue,
of the relation between stimulus and response, of modification of
response by high and low temperatures, and even in the matter of
occasional abnormal variations such as positive response in a modified
tissue, they were strictly correspondent to similar phenomena in animal
tissues. The remaining test, of the influence of chemical reagents,
having now been applied, a complete parallelism may be held to have been
established between plant response on the one hand, and that of animal
tissue on the other.
[Illustration: FIG. 49.--NEARLY COMPLETE ABOLITION OF RESPONSE BY
STRONG KOH
The two vertical lines are galvanometer deflections due to ·1 volt,
before and after the application of reagent. It will be noticed that
the total resistance remains unchanged.]