Researches on Irritability of Plants/Chapter 6
CHAPTER VI
VARIOUS TYPES OF RESPONSE
Some of the effects brought about by varying external conditions on the excitability of the plant have now been noted. Certain other variations may, however, be induced in the excitability, in consequence of the after-effect of the stimulus itself, even when the external conditions are maintained constant. We may trace these induced internal changes in the modification of the response-records.
It is clear that we can only be assured of the occurrence of such internal changes from the observed variation of response-record if we have been able in the first place to keep the plant under unvarying external conditions. This, taking certain special precautions as regards light, temperature, and so forth, presents no difficulty. But, in the second place, we have to be specially careful that the testing-stimulus itself shall be absolutely constant in successive experiments. The problem then resolves itself into the successful devising of some arrangement by which records may be taken automatically at definite pre-determined intervals of time. The stimulus of unvarying intensity must also be made to act automatically upon the specimen. Under these conditions any variation that may be observed in the record will be due to changes of excitability induced as an after-effect of the stimulus itself.
Maintaining the stimulus-intensity absolutely constant is not so easy to secure with a single make- or break-shock, since the intensity of such a shock is liable to variation, according to the degree of suddenness with which it is effected; but the total additive value of a group of such shocks may be expected to be fairly constant. For this reason, therefore, tetanising shocks caused by a vibrating interrupter would be preferable, provided the duration of these shocks, depending on the duration of closure of current in the primary circuit, be maintained in successive experiments rigorously equal. Such constancy cannot be arrived at if the closure of the circuit be caried out by hand, or even by metronome. Some special mechanical device must therefore be adopted for this purpose.
It is further necessary, in order to maintain constancy of conditions, that identical periods of recovery should be allowed in successive records. For this the stimulus must be applied at accurate and pre-determined intervals of time. The ideal condition, then, for the final elimination of all uncertainties due to the personal factor, is that the plant attached to the recording apparatus should be automatically excited by a stimulus absolutely constant, make its own responsive records, go through its own period of recovery, and embark on the same cycle over again without assistance at any point on the part of the observer.
These demands have been fully met in the devices and adjustments now to be described, consisting as they do of two chief elements—namely, the Periodic Starter and the Automatic Exciter. By the former the time-interval between successive stimulations is regulated; by the latter the stimulation itself, of a definite duration, is applied.
The Periodic Starter
In the case of Mimosa recovery from excitation is practically completed in a period of about 10 to 20 minutes, according to the season and the condition of the plant. In practice, therefore, we require arrangements by which successive stimulations can be automatically effected at these intervals. As we require a slowly moving plate for the purpose of these records, the plate-carrier is let down by a thread which is wound round a wheel attached to the minute-hand axis of the driving-clock. To the same axis is also screwed one or other of the three separate discs, bearing equidistant projecting rods, either 3, 4, or 6 in
Fig. 30.—Periodic Starter and Automatic Exciter.
number. During one complete revolution, which takes an hour, these rods will press and release a spring at intervals of 20, 15, or 10 minutes, as the case may be (fig. 30).
The Automatic Exciter
If the primary circuit of the induction coil provided with a spring-interrupter be closed for a definite period of time, say ·1 second, then the number of interruptions, with consequent induction-shocks, will also be definite. What is wanted is some contrivance for release, through which the Periodic Starter can close the main circuit for a definite length of time, say ·1 second. It might at first sight appear that this could be secured by an electrical contact made by the revolving radial-rods already referred to, but the period of such contact would be impracticably long—more than 15 seconds.
So continuous a tetanisation would undoubtedly fatigue or even injure the tissue. The contact made by a seconds-hand, again, though sufficiently brief, would have the serious defect that its movements were jerky and would therefore make the duration of contact unequal.
I succeeded in overcoming these difficulties by using a released revolving disc, which could be made to complete an electrical circuit for any definite short period that was required. For this I employed a phonograph motor, an axis of which, carrying a disc, could be adjusted to revolve once in a second.
This disc is usually held stationary by a lever-clutch, and can be started only by the pressure on it of the revolving rod of the Periodic Starter. It is re-arrested after one complete revolution and is not again released till the next rod comes into position, after an interval of 10, 15, or 20 minutes, as the case may be. There is also attached to the disc a sector whose arc is one-tenth the circumference of the circle. This sector, during the revolution, will press against a closing-key, the period of closure being then ·1 second. By increasing or diminishing this arc the time of closure, and with it the duration of the tetanising shock, can be correspondingly changed. It is necessary that the sector should, at the moment of the release, be at the greatest possible distance from the closing-key. By the time it reaches this key it will have acquired a constant and definite velocity. Thus the periods of closure, and consequent duration of the exciting-shock, will be identical in successive experiments.
There is another possible source of variation which must be guarded against. The electrodes of the secondary coil are connected with the plant by means of moistened threads. These threads, in long-continued experiments, may become more or less dried up, the electrical resistance being thus increased in an unknown manner. The intensity 
of the exciting current may, under these conditions, undergo a change. This difficulty has been overcome by a contrivance for keeping the thread uniformly moist. This will be understood from the diagram (fig. 31) of the electrolytic contact-maker: Two small cells are made of cork; the upper cell is filled with very dilute saline solution, a little of which also lies in the bottom of the lower cell. A bent piece of silver-wire coated with a deposit of chloride, and fixed to the horizontal metallic-rod, pricks through two cells and dips into the solutions above and below. The moistened thread coming through a hole near the bottom of the upper chamber makes one loop round that portion of the plant specimen where electrical connection is desired, turns back into the lower cell (which it enters through the open aperture), and dips into the saline solution. It will be seen that apart from capillary action,
owing to the upper chamber being at a higher level, the thread will be kept constantly moist by the slow streaming down of the solution. The current from the coil, again, will have two entries by means of the doubled thread, the resistance being thus halved. The second electrode of the coil is connected with the other contact-point on the plant in a similar manner. The resistance offered by the plant tissue is relatively high, being of the order of a million ohms. The resistance of the electrolytic contacts, on the other hand, need be no higher than a few thousand ohms. By thus making the resistance of the moist contact relatively small, the total resistance of the circuit remains practically the same, especially since we guard against any variation that might 
be induced in the moist thread by drying. The horizontal rod holding the cork of the contact-maker is soldered to a tube which can move up and down a vertical rod. These adjustments enable the point of electrolytic contact to be brought to a level with the point of connection on the specimen. The rod is insulated on ebonite.
All the practical difficulties having thus been eliminated, I shall now proceed to show the various records obtained under this mode of periodic stimulation of uniform intensity. In fig. 32 is given a photograph of the apparatus with its accessories. The recorder is of duplex type, for taking two sets of records at the same time.
Before entering upon the detailed consideration of the results of these experiments, I may say that at the beginning of this investigation my attention was roused by the apparently capricious variations in the responses obtained under conditions which were rigidly uniform.
A long-continued investigation through the different seasons of the year has given me the clue to what at first appeared to be so anomalous. The outward response, it is obvious, is dependent on two factors—the intensity of the impinging stimulus and the capacity for reply possessed by the plant itself. It is easy to see that the second of these factors must be dependent on the vigour of the plant, or in other words, on its tonic condition, which in its turn is modified by the environmental condition. Thus in unfavourable circumstances the plant may fall into an atonic or sluggish condition. The absorption of energy from without, by Whatever form of stimulation, will improve the tonic condition of the plant, with consequent enhancement of excitability.
Taking a plant in a subtonic condition, then, we may expect that any application of stimulus will increase its excitability, a fact which will find expression in a growing amplitude of response. This enhancement of excitability will reach a limit at which the plant will be in an optimum condition. After reaching this climax there may be a reversal, with decline of excitability, a state of things which we associate with fatigue.
It must be remembered that in Nature, according to the conditions of its environment, a plant may be found in any of the three states. One specimen may be found in the pre-optimum or subtonic condition; another may be near the optimum condition, and this we shall designate as the normal; a third may be found in the post-optimum condition predisposed to fatigue. The first and third of these conditions may be distinguished from each other by means of testing blows or stimuli. If the plant be in the former condition, these will evoke responses of increasing amplitude; in the latter, they will show a decline.
These three conditions modify not merely the amplitude of response but also exhibit themselves appropriately in other aspects of protoplasmic excitation. These will be seen in the chapters on the Latent Period, and on the Transmission of Excitation.
Uniform Responses
When—selecting a plant which is neither subtonic nor yet at its optimum—we take a series of responses under uniform stimulation of moderate intensity, allowing sufficient intervals for complete recovery, we obtain uniform responses. This may be accepted as the characteristic effect of a plant in the normal condition.
In fig. 33 is seen a series of such responses taken at intervals of 15 minutes. The ascending portion of each response is here seen to be dotted. This is because of the rapidity of the movement of fall. The successive dots caused by the recorder vibrating ten times per second are widely spaced. In the recovery or down part of the curve, however, as that process is slow, the dots become fused and make a thick continuous line. In the record of the responsive fall, variations of rate of movement may be noticed. At first the speed increases, then very gradually slows down, and the leaf becomes for a time stationary at the apex. These varying rates of fall are seen in the growing and then in the diminishing intervals between the dots.
Fatigue
If instead of giving the full period of rest necessary for complete protoplasmic recovery, the period of rest be
Fig. 33.—Uniform responses of Mimosa; stimuli applied at intervals of 15 minutes.
shortened, we obtain a diminution in the height of response indicative of fatigue. This is well seen in fig. 34. The first three uniform responses here—taken, as it is unnecessary to repeat, under uniform stimulation—were recorded at intervals of 15 minutes each. The intervals between successive stimulations were now shortened to 10 minutes, which at once results in a fatigue-diminution of the height of responses. The second three responses appear crowded together, owing to the shortening of the time allowed for record. The time of recovery, after the third of these responses, was again restored to its first value of 15 minutes, and we see at once the reversion of the response to its original height. A similar exhibition of fatigue is also seen in muscle-records, in the same circumstances of diminished interval of rest.
Another very curious type of response sometimes met with, is that of alternating fatigue. Here, while the first response is very large, the second is correspondingly small, and this alternating sequence is observed for a longer or shorter time (fig. 36). After several such alternations, however, the responses tended to become uniform. An explanation of this interesting variation may be gathered from careful observation of the record. The freshness of the specimen and its high excitability account for the great amplitude of the first response. An intense excitation requires, as we have seen, a correspondingly longer time than does a feeble one for complete recovery. Hence in the present case the second stimulation is seen to have impinged on the organ before complete protoplasmic recovery has had time to take place, during the usual resting-interval of 15 minutes. The consequence of this is the diminished
Fig. 35.—Record showing growing fatigue in Mimosa.
excitatory effect exhibited in the second response. As the excitation in this case was relatively slight, the recovery was very much more complete than in the first. The
Fig. 36.—Periodic fatigue; alternate responses here tend to become uniform.
third response therefore was large, but not so large as the first, when the organ was fresh. This excitation, however, being less than the first, recovery is also somewhat more complete, and the subsequent fatigue is less than after the first response. Therefore the fourth response, though small, is not so small as the second. Thus while the first, third, and odd series of responses are progressively diminishing from a maximum, the even series—second, fourth, and so on—are increasing from a minimum. In this way the difference between the successive responses is tending to disappear, a process which is practically complete in the seventh and eighth, after which uniformity is attained. It is very interesting to note that the sum of heights of each pair of responses is approximately the same for successive pairs, and the height of a response in the uniform series is not appreciably different from the mean of the maximum and minimum of the preceding pairs, as will be seen from the following table:—
Table giving Heights of Successive Responses
| Number. | Height of response. | Mean of successive pair. | ||
| (1).. (2).. |
|
mm. 19 | ||
| (3).. (4).. |
|
mm. 19·2 | ||
| (5).. (6).. |
|
mm. 18 | ||
| (7).. (8).. |
|
mm. 17·2 |
In this adjustment to uniformity we are able to watch a tuning of the organ, as it were, its gradual accommodation to the stimulus impinging upon it. Uniform responses may often be obtained in this way after a preliminary period of variation.
The periodic variation seen in the above cases sometimes finds still more complex expression. This is the case where waning and waxing occur in series instead of simple alternation. That is to say, response may undergo a continuous diminution in a sequence of three or more, to be followed by a corresponding sequence in which the amplitudes wax larger and larger, such serial alternations being repeated.
Staircase Response
We have seen the responses that characterise highly excitable specimens, in which there is an exhibition of growing fatigue. Taking a specimen in the contrasted condition of more or less sub-tonicity, we obtain an equally characteristic effect, which is the antithesis as it were of that which we have been considering. In this, successive responses undergo a gradual enhancement, or what is known in muscle-response—with which it is exactly parallel—as a staircase increase (figs. 37, 38). After attaining a maximum excitability, under successive stimulations, there generally ensues a fatigue-decline.
Before entering on a detailed description of this particular response it would be well to discuss certain phenomena characteristic of a relatively a-tonic condition of the tissue. In a specimen in the normal condition there is a certain amount of tonicity, accompanied by a moderate degree of contraction. When deprived of the invigorating influences of favourable external stimuli the plant becomes sub-tonic, such relative a-tonicity being characterised by relaxation or the absence of normal tonic contraction. Under the action of successive stimuli the tonic condition of the specimen will be improved. The loss of tone, with its consequent relaxation, will gradually give place to a better tone with increasing tonic contraction. Or the same improvement of tone might take the form of a gradually increasing excitability. Hence the gradual bettering the tonic condition, under successive stimulations, may often find two simultaneous expressions. In the first place the growing tone, with its increasing normal tonic contraction, will be seen in the shifting of the base-line upwards. Secondly, it will be exhibited in the growing amplitude of successive responses. These two features will
Fig. 37.—Preliminary staircase followed by fatigue in the response of frog's muscle. (Brodie.)
Fig. 38.—Staircase response followed by fatigue in Mimosa.
both be noticed in the record depicted in fig. 38. Here, as might be expected, in a specimen in sub-tonic condition we find that the first stimulus gives rise to a relatively feeble response. But in consequence of stimulation the tonic condition itself is improved, as demonstrated by the fact that the leaf remains in a slightly more contracted attitude than at the beginning. The next stimulus finds it in a better tonic condition, with accompanying higher excitability. Hence the response is larger. In this way the tonic condition reaches an optimum, with the attainment of highest degree of excitability. Here impinging stimulus has evoked the maximum response.
We see in a general way that in these responses the accession of stimulus has given rise to two kinds of effects, external and internal, whose relative values have been progressively changing. At the beginning a portion of the stimulus was utilised to improve the tonic condition, the complementary portion inducing external response. Hence at the beginning the response was small. At the end of the series, however, where the maximum tonicity has been attained, the whole blow of the stimulus is utilised in giving external response, which now therefore is maximum. After this attainment of maximum excitability the usual fatigue-decline is seen to have taken place.
We must nevertheless be on our guard against drawing too hasty a conclusion, as regards the tonic condition, from the relaxation or contraction seen in the record; we should remember that a relaxed condition is not only indicative of a-tonicity, but may also be brought about by fatigue due to over-stimulation. The changing position of the leaf, owing to daily periodicity, should also be taken into account. Bearing in mind, however, the immediately preceding history of the given plant, the experimenter will not find it difficult to guard himself against wrong inferences.
Being desirous of ascertaining how far the theoretical considerations here advanced would be borne out in extreme cases, I tested a specimen which from appearances was not at all vigorous and likely to be a-tonic. The record it gave at the beginning, of increasing relaxation, probably indicated its growing a-tonicity (fig. 39). That it was lacking in tone at once became evident from the fact that the first stimulus—applied at the point shown by the thick dot—did not evoke any response. But that this nevertheless did cause improved tonicity, is seen from the fact that the former rate of relaxation underwent a diminution, the record tending to become more horizontal. The second stimulus was then effective in evoking a feeble response. The most striking fact, however, is that on the completion of recovery the specimen actually exhibited a growing contraction as an after-effect of stimulus. Thus, while at the beginning a growing condition of a-tonicity gave rise to increasing relaxation, afterwards in consequence of stimulation this state of things became reversed, and we have a growing condition of tonic contraction appearing as the after-effect of stimulus. That the tonic condition in fact became improved is shown by the large response evoked as the immediate effect of the usual stimulation.[1]
The curious phenomenon of alternation sometimes observed in a highly excitable specimen has already been noticed (fig. 36). The characteristic peculiarity observed there was a large response followed by a small one, such alternation continuing for a time. The difference between successive responses, however, vanished after a time. With plants in a sub-tonic condition the phenomenon of alternation is also found occasionally. The characteristics here exhibited (fig. 40) are in sharp contrast to those seen in fig. 36. Here the first response is small, and the second large, and the difference between the pairs in the series goes on increasing. The sum of heights of pairs of successive responses remains however approximately the same. The odd numbers in the series decline continuously (1) 18·5, (3) 16, (5) 11, (7) 6; whereas the responses in the even series grow in amplitude (2) 25·5 (4) 30, (6) 33.
Table giving Heights of Successive Responses
| Number. | Height of response. | Sum. | ||
| (1).. (2).. |
|
mm. 44 | ||
| (3).. (4).. |
|
mm. 46 | ||
| (5).. (6).. |
|
mm. 44 |
Fatigue-reversal under Tetanisation
If the Mimosa leaf be subjected to continuous stimulation it has been found that, after the preliminary fall, it re-erects itself in spite of the stimuli which are still acting upon it. This at first sight would appear to be very perplexing, but the apparent anomaly would however disappear when we recognise the essential unity of response in the plant and the animal. A frog's muscle, under continued tetanising electric shocks, at first exhibits the normal contraction, but afterwards relaxes, in spite of the excitation to which it is being subjected (fig. 41). The difference between the normal relaxation of recovery (expansion) and this fatigue-relaxation induced under continuous stimulation, lies in the fact that in the former case response takes place on renewed stimulation, while in the latter the tissue has become irresponsive, and only after a period of rest can it exhibit excitation. The same phenomena are observed in the case of the contractile organ of Mimosa. Here also, after
Fig. 40.—Alternating response; differences between alternate responses become accentuated.
erection (expansion) under continuous stimulation, the leaf is irresponsive and only renews its excitability after a definite period of rest.
As regards this particular reaction in Mimosa, we are
Fig. 41.—Fatigue-decline in frog's muscle under continuous stimulation. (Brodie.)
in a position to trace out the various phases through which contraction under single stimulus is reversed to expansion under fatigue induced by continuous stimulation. It has to be borne in mind that the effect of continuous stimulation is, after all, the effect of successive stimuli with the resting interval shortened. On referring back to fig. 38 we notice two phases in the response-series: in the first phase the excitability is increasing; in the second phase, it is decreasing. In the first phase again, we notice that there is a residual contraction, the recovery being incomplete. Owing to this, the base-line is gradually shitting upwards. This, coupled with the enhancing excitability and consequent staircase increase in the individual responses, brings about a maximum additive contraction, as will be understood, by joining the tops of these contractile responses. The additive effect of such contractions would be a responsive fall much greater than could have taken place under any single stimulation.
If we were now to repeat this experiment, shortening the intervals between the successive stimuli, we should obtain a somewhat similar result, with the sole difference that the successive component responses would appear nearer each other and with their recoveries still further reduced. The result of this would be slight notches in an ascending curve. Carrying this process to a limit—that is to say, when the successive stimuli follow each other quickly, as in continuous tetanisation—the notches themselves will disappear and we shall have merely an ascending curve.
Turning to the second phase in the response-series, where the excitability has reached a maximum, we find these phenomena reversed. The leaf having attained its maximum limit of fall, its capacity for further contraction is now reduced. In sharp contrast to the first phase of the series, however, successive contractions now grow smaller and smaller, under growing fatigue, while the relaxations tend to become increasingly large. In the extreme case of continuous tetanisation the resulting record in this phase would be one of relaxation, appearing as a down-curve. Thus under tetanisation we should have a response-curve, showing first the normal contraction, followed in the second place by 
response, and that the subsequent responses exhibited fatigue. There is not here, to begin with, any staircase effect, nor are the contractions additive, the initial response being the greatest possible. On increasing the frequency of stimulation we should, after the first maximum response, obtain the phasic variation due to fatigue. The successive contractile responses would thus appear smaller and smaller, their respective recoveries being correspondingly larger and larger. This is clearly seen in fig. 42, where the successive stimulations are applied at intervals of seven minutes.
Thus on subjecting a specimen in an optimum condition to continuous stimulation, we should expect to find that the extent of contraction due to tetanisation was but little different from that due to a single stimulus. This is verified by the following pair of records (fig. 43) showing the response of a plant near optimum condition, under single stimulus and under tetanisation.
Summary
The contractile response of the pulvinus of Mimosa exhibits characteristics similar to those of the response of muscle.
Under normal conditions of the plant, and with sufficient intervening periods of rest, the responses are found to be uniform.
The responses exhibit fatigue under conditions of incomplete recovery.
The excitability of the plant in a sub-tonic condition is enhanced by the action of the stimulus itself. Under such conditions the responses exhibit a staircase increase.
The anomalous erection, after a preliminary fall of the leaf of Mimosa under continuous stimulation, is explicable on the common characteristics of response in plant and animal tissues. In both, contraction is reversed to relaxation under fatigue.
- ↑ When the specimen is extremely sub-tonic the sign of response may even be reversed into abnormal erectile movement. After a period of stimulation, however, the response is converted into normal.