PLANT RESPONSE--ON THE EFFECTS OF SINGLE STIMULUS AND OF SUPERPOSED
STIMULI
Effect of single stimulus--Superposition of stimuli--Additive
effect--Staircase effect--Fatigue--No fatigue when sufficient
interval between stimuli--Apparent fatigue when stimulation
frequency is increased--Fatigue under continuous stimulation.
#Effect of single stimulus.#--In a muscle a single stimulus gives rise to
a single twitch which may be recorded either mechanically or
electrically. If there is no fatigue, the successive responses to
uniform stimuli are exactly similar. Muscle when strongly stimulated
often exhibits fatigue, and successive responses therefore become
feebler and feebler. In nerves, however, there is practically no fatigue
and successive records are alike. Similarly, in plants, we shall find
some exhibiting marked fatigue and others very little.
#Superposition of stimuli.#--If instead of a single stimulus a succession
of stimuli be superposed, it happens that a second shock is received
before recovery from the first has taken place. Individual effects will
then become more or less fused. When the frequency is sufficiently
increased, the intermittent effects are fused, and we find an almost
unbroken curve. When for example the muscle attains its maximum
contraction (corresponding to the frequency and strength of stimuli) it
is thrown into a state of complete _tetanus_, in which it appears to be
held rigid. If the rapidity be not sufficient for this, we have the
jagged curve of incomplete tetanus. If there is not much fatigue, the
upper part of the tetanic curve is approximately horizontal, but in
cases where fatigue sets in quickly, the fact is shown by the rapid
decline of the curve. With regard to all these points we find strict
parallels in plant response. In cases where there is no fatigue, the
successive responses are identical (fig. 16). With superposition of
stimuli we have fusion of effects, analogous to the tetanus of muscle
(fig. 17). And lastly, the influence of fatigue in plants is to produce
a modification of response-curve exactly similar to that of muscle (see
below). One effect of superposition of stimuli may be mentioned here.
[Illustration: FIG. 16.--UNIFORM RESPONSES (RADISH)]
[Illustration: FIG. 17.--FUSION OF EFFECT OF RAPIDLY SUCCEEDING STIMULI
(_a_) in muscle; (_b_) in carrot.]
#Additive effect.#--It is found in animal responses that there is a
minimum intensity of stimulus, below which no response can be evoked.
But even a sub-minimal stimulus will, though singly ineffective, become
effective by the summation of several. In plants, too, we obtain a
similar effect, i.e. the summation of single ineffective stimuli
produces effective response (fig. 18).
[Illustration: FIG. 18.--ADDITIVE EFFECT
(_a_) A single stimulus of 3° vibration produced little or no effect,
but the same stimulus when rapidly superposed thirty times,
produced the large effect (_b_). (Leaf-stalk of turnip.)]
#Staircase effect.#--Animal tissues sometimes exhibit what is known as the
'staircase effect,' that is to say, the heights of successive responses
are gradually increased, though the stimuli are maintained constant.
This is exhibited typically by cardiac muscle, though it is not unknown
even in nerve. The cause is obscure, but it seems to depend on the
condition of the tissue. It appears as if the molecular sluggishness of
tissue were in these cases only gradually removed under stimulation, and
the increased effects were due to increased molecular mobility. Whatever
be the explanation, I have sometimes observed the same staircase effect
in plants (fig. 19).
[Illustration: FIG. 19.--'STAIRCASE EFFECT' IN PLANT]
#Fatigue.#--It is assumed that in living substances like muscle, fatigue
is caused by the break down or dissimilation of tissue by stimulus. And
till this waste is repaired by the process of building-up or
assimilation, the functional activity of the tissue will remain below
par. There may also be an accumulation of the products of
dissimilation--'the fatigue stuffs'--and these latter may act as poisons
or chemical depressants.
In an animal it is supposed that the nutritive blood supply performs the
two-fold task of bringing material for assimilation and removing the
fatigue products, thus causing the disappearance of fatigue. This
explanation, however, is shown to be insufficient by the fact that an
excised bloodless muscle recovers from fatigue after a short period of
rest. It is obvious that here the fatigue has been removed by means
other than that of renewed assimilation and removal of fatigue products
by the circulating blood. It may therefore be instructive to study
certain phases of fatigue exhibited under simpler conditions in
vegetable tissue, where the constructive processes are in abeyance, and
there is no active circulation for the removal of fatigue products.
It has been said before that the E.M. variation caused by stimulus is
the concomitant of a disturbance of the molecules of the responsive
tissues from their normal equilibrium, and that the curve of recovery
exhibits the restoration of the tissue to equilibrium.
#No fatigue when sufficient interval between successive stimuli.#--We may
thus gather from a study of the response-curve some indication of the
molecular distortion experienced by the excited tissue. Let us first
take the case of an experiment whose record is given in fig. 20, _a_.
It will be seen from that curve that one minute after the application of
stimulus there is a complete recovery of the tissue; the molecular
condition is exactly the same at the end of recovery as in the beginning
of stimulation. The second and succeeding response-curves therefore are
exactly similar to the first, _provided a sufficient interval has been
allowed in each case for complete recovery_. There is, in such a case,
no diminution in intensity of response, that is to say, no fatigue.
We have an exactly parallel case in muscles. _'In muscle with normal
circulation and nutrition there is always an interval between each pair
of stimuli, in which the height of twitch does not diminish even after
protracted excitation, and no fatigue appears.'_[10]
[Illustration: FIG. 20.--RECORD SHOWING DIMINUTION OF RESPONSE WHEN
SUFFICIENT TIME IS NOT ALLOWED FOR FULL RECOVERY
In (_a_) stimuli were applied at intervals of one minute; in (_b_) the
intervals were reduced to half a minute; this caused a diminution of
response. In (_c_) the original rhythm is restored, and the response
is found to be enhanced. (Radish.)]
#Apparent fatigue when stimulation frequency increased.#--If the rhythm of
stimulation frequency be now changed, and made quicker, certain
remarkable modifications will appear in the response-curves. In fig. 20,
the first part shows the responses at one minute interval, by which time
the individual recovery was complete.
The rhythm was now changed to intervals of half a minute, instead of
one, while the stimuli were maintained at the same intensity as before.
It will be noticed (fig. 20, _b_) that these responses appear much
feebler than the first set, in spite of the equality of stimulus. An
inspection of the figure may perhaps throw some light on the subject. It
will be seen that when greater frequency of stimulation was introduced,
the tissue had not yet had time to effect complete recovery from
previous strain. The molecular swing towards equilibrium had not yet
abated, when the new stimulus, with its opposing impulse, was received.
There is thus a diminution of height in the resultant response. The
original rhythm of one minute was now restored, and the succeeding
curves (fig. 20, _c_) at once show increased response. An analogous
instance may be cited in the case of muscle response, where 'the height
of twitch diminishes more rapidly in proportion as the excitation
interval is shorter.'[11]
[Illustration: FIG. 21.--FATIGUE IN CELERY
Vibration of 30° at intervals of half a minute.]
From what has just been said it would appear that one of the causes of
diminution of response, or fatigue, is the residual strain. This is
clearly seen in fig. 21, in a record which I obtained with celery-stalk.
It will be noticed there that, owing to the imperfect molecular recovery
during the time allowed, the succeeding heights of the responses have
undergone a continuous diminution. Fig. 22 gives a photographic record
of fatigue in the leaf-stalk of cauliflower.
[Illustration: FIG. 22.--FATIGUE IN LEAF-STALK OF CAULIFLOWER
Stimulus: 30° vibration at intervals of one minute.]
It is evident that residual strain, other things being equal, will be
greater if the stimuli have been excessive. This is well seen in
fig. 23, where the set of first three curves A is for stimulus intensity
of 45° vibration, and the second set B, with an augmented response, for
stimulus intensity of 90° vibration. On reverting in C to stimulus
intensity of 45°, the responses are seen to have undergone a great
diminution as compared with the first set A. Here is seen marked
fatigue, the result of overstrain from excessive stimulation.
[Illustration: FIG. 23.--EFFECT OF OVERSTRAIN IN PRODUCING FATIGUE
Successive stimuli applied at intervals of one minute. The intensity of
stimulus in C is the same as that of A, but response is feebler
owing to previous over-stimulation. Fatigue is to a great extent
removed after fifteen minutes' rest, and the responses in D are
stronger than those in C. The vertical line between arrows
represents ·05 volt. (Turnip leaf-stalk.)]
If this fatigue be really due to residual strain effect, then, as strain
disappears with time, we may expect the responses to regain their former
height after a period of rest. In order to verify this, therefore, I
renewed the stimulation (at intensity 45°) after fifteen minutes. It
will at once be seen from record D how far the fatigue had been removed.
One peculiarity that will be noticed in these curves is that, owing to
the presence of comparatively little residual strain, the first response
of each set is relatively large. The succeeding responses are
approximately equal where the residual strains are similar. The first
response of A shows this because it had had long previous rest. The
first of B shows it because we are there passing for the first time to
increased stimulation. The first of C does _not_ show it, because there
is now a strong residual strain. D again shows it because the strain has
been removed by fifteen minutes' rest.
#Fatigue under continuous stimulation.#--The effect of fatigue is
exhibited in marked degree when a tissue is subjected to continuous
stimulation. In cases where there is marked fatigue, as for instance in
certain muscles, the top of the tetanic curve undergoes rapid decline. A
similar effect is obtained also with plants (fig. 24).
[Illustration: FIG. 24.--RAPID FATIGUE UNDER CONTINUOUS STIMULATION IN
(_a_) MUSCLE; (_b_) IN LEAF-STALK OF CELERY]
The effect of rest in producing molecular recovery, and hence in the
removal of fatigue, is well illustrated in the following set of
photographic records (fig. 25). The first shows the curve obtained with
a fresh plant. The effect is seen to be very large. Two minutes were
allowed for recovery, and then stimulation was repeated during another
two minutes. The response in this case is seen to be decidedly smaller.
A third case is somewhat similar to the second. A period of rest of five
minutes was now allowed, and the curve obtained subsequently, owing to
partial removal of residual strain, is found to exhibit greater
response.
[Illustration: FIG. 25.--EFFECT OF CONTINUOUS VIBRATION (THROUGH 50°) IN
CARROT
In the first three records, two minutes' stimulation is followed by two
minutes' recovery. The last record was taken after the specimen had
a rest of five minutes. The response, owing to removal of fatigue by
rest, is stronger.]
The results thus arrived at, under the simple conditions of vegetable
life, free as they are from all possible complications and
uncertainties, may perhaps throw some light on the obscure phenomena of
fatigue in animal tissues.
FOOTNOTES:
[10] Biedermann, _Electro-physiology_, p. 86.
[11] Biedermann, _loc. cit._