Chapter 25

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.

#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).

#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).

#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]

#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]

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.

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.

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).

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.

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._