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) 15 "

(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 236

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 a.s.sumed 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 a.s.sumption. 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 a.s.sumption 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.

CHAPTER V

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 _teta.n.u.s_, in which it appears to be held rigid. If the rapidity be not sufficient for this, we have the jagged curve of incomplete teta.n.u.s. 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, a.n.a.logous to the teta.n.u.s 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.

[Ill.u.s.tration: FIG. 16.--UNIFORM RESPONSES (RADISH)]

[Ill.u.s.tration: 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).

[Ill.u.s.tration: 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).

[Ill.u.s.tration: FIG. 19.--'STAIRCASE EFFECT' IN PLANT]

#Fatigue.#--It is a.s.sumed 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 a.s.similation, the functional activity of the tissue will remain below par. There may also be an acc.u.mulation 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 a.s.similation 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 a.s.similation 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]

[Ill.u.s.tration: 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 a.n.a.logous 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]

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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 pa.s.sing 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.