Researches on Irritability of Plants

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. 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. If instead of giving the full period of rest necessary for complete protoplasmic recovery, the period of rest be Fic. 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 I5 minutes each. The intervals between succes- sive stimulations were now shortened to Io 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. Under certain conditions we obtain an exhibition of continuously growing fatigue. We have seen that when the plant is intensely excited, it takes a longer time for complete protoplasmic recovery. The specimen whose responses are given in fig. 32 happened to be in an optimum condition. A maximum ex- citation was here induced, even under a moderate stimu- lus. The normal interval of I5 minutes, which was found in the previous case to be sufficient for complete proto- plasmic recovery, here proved to be insufficient. Hence we have the exhibition of a growing fatigue seen in the diminishing heights of succes- sive responses.

Another very curious type Fic. 34.—Fatigue under shortened period of rest. First three of ecae guise sometimes met uniform responses obtained at with, is that of alternating intervals of15 minutes. The fatigue. Here, while the first second three, under shortened : : period of rest of 10 minutes, response 1S very lat Se, the exhibit fatigue. On returning second is correspondingly to interval of 15 minutes, the longer or shorter time (fig. 36). After several such alterna- tions, 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 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 Fic. 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 diminish- ing 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 succes- sive 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 :—

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. E The periodic variation seen in the above cases some- times 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.

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 parti- cular response it would be well to discuss certain pheno- mena 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 better- ing 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

Fic. 37.—Preliminary staircase Fic. 38.—Staircase response followed by fatigue in the followed by fatigue in response of frog’s muscle- 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 excita- bility. 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 pre- ceding 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.!

This after-effect of a _ single stimulus in inducing a_ second contraction is significant as show- ing the possibility of holding inci- dent stimulus latent for a time, to find expression later. It heralds the phenomenon of Multiple Re- sponse, which we shall consider in a subsequent chapter. The curious phenomenon of alternation sometimes observed in ? a highly excitable specimen has parry eee oe meee already been noticed (fig. 36). modifying tonicity, and The characteristic peculiarity ob- producing _ staircase served there was a large response ae 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 alterna- tion 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

1 ‘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, 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.

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 after- wards 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-relaxa- tion 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 Fic. 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. Fic. 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 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 ex- citability 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 shifting 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 them- selves 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 maxi- mum 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 con- tinuous 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

relaxation, not at first sight very different from the response- curve due toa single stimulus. There would nevertheless be an actual difference, inasmuch as the resulting contraction under tetanisation would, on account of additive effect, be greater than that caused by a single stimulus. After the ap- parent recovery, due to fatigue- reversal under tetanisation, how- ever, the excitability, as already shown, is temporarily abolished ; whereas after the normal re- covery from a single stimulus, Fic. 42.—Different phases

consideration of which led us to these conclusions, was that of a plant which was in a somewhat sub-tonic con- dition. Had the plant been in the optimum condition to start with, then following the same line of reasoning we should expect that the curve of tetanisation would be modified in a definite way. Referring back to fig. 35, which gives successive records of a highly excitable specimen, we find in this instance that the Very” airst stimulus evoked the maximum response, and that the subsequent responses ex-

Fic. 43.—Fatigue-reversal in M7z- cag mosa. Lower record shows hibited fatigue. There response under single stimulus ; is not here, to begin upper figure exhibits response ; : under continuous stimulation. with, any staircase effect, 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.

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 suffi- cient intervening periods of rest, the responses are found to be uniform. The responses exhibit fatigue under conditions of incom- plete 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 explic- able on the common characteristics of response in plant and animal tissues. In both, contraction is reversed to relaxation under fatigue. Induced change of excitability under sudden variation of light— Abolition of excitability by absorption of water — Restoration of excitability by application of glycerin—Stimulating, depressing, and toxic agents—Phenomenon of accommodation—Stimulating action of ozone — Effects of: carbonic-acid gas, vapour of alcohol, ether, carbon disulphide, coal gas, chloroform, ammonia, sulphuretted hydrogen, laughing-gas, nitrogen dioxide, and sulphur dioxide.

In order to investigate the effects of various gases in modi- fying the excitability of Mzmosa, a series of responses, more or less uniform, is first obtained under uniform stimuli, at intervals of 15 minutes. The given gas is now introduced into the plant-chamber, and another series of responses are once more obtained by the action of the same stimuli as before. The variation of amplitude of responses then gives an indication of the excitatory or depressing action of the agent.

In carrying out the experimental investigation in this manner, we proceed on the assumption that the stimuli applied are invariable, and that the external conditions are maintained constant, with the sole exception of the change induced by the introduction of the given gas. In order to complete a single investigation a period of nearly two hours is often necessary, which is the time required to take eight responses at intervals of 15 minutes. Of these, the first two give the normal responses, the next four the modified