Plant Response as a Means of Physiological Investigation

By means of the electric mode of investigation detailed in the preceding chapter, I have shown that all vegetable organs, whether of ordinary or of sensitive plants, are excitable. Hence the supposed absence of motile indications, in ordinary plants, is not to be considered as due to want of sensitiveness, but to the lack of proper conditions for mechanical movement. What these conditions are will be described presently. But before entering upon such considerations, I first wish to demonstrate that even in the matter of the pulvinar motility itself there is no abrupt division, but a very gradual transition, from those plants in which it is scarcely perceptible, to others which exhibit it in a marked degree.

Range of sensitiveness in sensitive plants. — Three typical instances of plants possessing extremely sensitive, moderately sensitive, and almost insensitive leaflets, are : (i) Mimosa pudica, (2) Biophytum sensitivum, and (3) Philanthus urinaria. In Philanthus the small leaves borne on pulvini are arranged on two sides of the long twig or petiole, in a manner somewhat resembling the arrangement of leaflets in Biophytum. The plant appears on casual inspection to be

wholly insensitive, ordinary mechanical stimulation having no effect on its leaves. But if we apply strong thermal or electrical stimulus to the end of the twig bearing the leaves, then they begin to close very slowly, in serial succession. But whereas with Biophytum the response begins almost instantaneously, the maximum being reached in less than a second, and complete recovery attained in about four minutes, in the leaf of Philanthus urinaria the latent period, for moderate stimulus, is as long as three minutes, the maximum reached in not less than forty-five minutes, and complete

Fig. 30. Responses of (a) quickly reacting Biophytum, and (b) sluggish Philanthus urinaria, under moderate and (c) under stronger Stimulation recovery may require from two to three hours (fig. 30). It is seen, then, that even with regard to the so-called sensitive plants, there is a wide range of sensitiveness. In some, a slight shock produces quick reaction, in others a very intense stimulation is necessary to initiate response, and the reaction itself is very sluggish.

Anisotropy necessary to lateral response. — We shall now try to understand a little more of the mechanics which cause this responsive curvature. In these pulvinated organs, one thing that is noticeable is that the organ is not isotropic, that is to say, its properties are not the same in all directions. The isotropic condition is seen in the case of radial organs, like cylindrical stems or peduncles. One way of showing this is to try to bend such a stem in all directions, when it will be found that equal forces produce equal bending in any direction. This, however, is not the case with dorsi-ventral organs, such as the pulvinus. This is more pliable in a vertical than in a lateral plane.

The mass of cells which constitute the lower half of the pulvinus, in Mimosa for example, is larger than that of the upper. The lower half is also the more excitable. The leaf remains in a balanced horizontal position, under the action of two opposing forces, the tensions of the opposite halves of the pulvinus, these tensions being modified by the turgidity of the cells. PfefTer and Sachs have shown that under stimulation there is an expulsion of water from these excitable cells. This may be seen if we watch the cut end of a pulvinus very attentively after the disappearance of the excitation due to cut, and during the application of a new stimulus. The application of this stimulus is followed by a visible escape of water from the cut end. A more striking demonstration of this fact will be given in Chapter XXI.

Stimulus, then, induces diminution of the turgidity of the organ, by expulsion of water from the excited tissue, and I shall show by experiment how this negative variation of turgidity, owing to the dorsi-ventral inequality of the organ, causes the depression or fall of the leaf. Response by artificial turgidity-variation. — We may fix air-tight the cut end of a branch of Mimosa bearing leaves in a U-tube filled with water. When this is done, a. quantity of water is sucked up, and owing to this increase of turgidity the leaves will be forced to assume a highly erect or almost vertical position. After several hours this excessive turgidity will disappear, and the leaf will then assume a more or less horizontal position. The other end of the tube may now be connected alternately with a vacuum and with a force-pump, by means of which a diminution or increase of internal pressure may be induced at will. When connected with the

former, or vacuum pump, water is sucked away, or expelled from the plant and its organs ; when, on the contrary, the pressure is increased by connection with the force-pump, water is forced in. From the curve given below (fig. 31), it will be seen that the expulsion of water from the organ actually causes the fall of the leaf, and that the forcing of it back brings about erection. From the processes involved in this artificial response and recovery, we can see clearly how in the true response to stimulation we have a two-fold process of (1) the expulsion of water caused by stimulus, bringing about the depression of the leaf, and (2) the return

The plant was subjected to diminished pressure up to a, and to normal pressure to b, after which the pressure was increased. The effect of diminished pressure, in the depression of the leaf, continues for a while. The ordinate represents movement of tip of leaf in cm., abscissa represents time. of water into the organ, bringing about the restoration of the leaf to its original position, or recovery. And since the lower half of the organ is the more contractile, it is evident that in this lower half there must be relatively greater expulsion and absorption during response and recovery.1

Two possible types of response. — We must bear in mind that the entire response consists of these two alternating processes, and that the recovery, or restoration of turgidity, 1 As a normal type I have taken the pulvinus of Mimosa, the excitability of the lower half of which is greater, and where the responsive movement is down. But there may be other cases. If the excitability of the upper half be relatively the greater, excitation will in such cases cause upward responsive movement. In what follows, unless the contrary be stated, I shall speak of the normal type.

is not a passive but an active process ; for when the tissue is killed it remains flaccid. Throughout the phenomenon of response, the essential factor is the variation of turgidity from, and its return to, the normal. This variation, as generally seen, consists of a fall below, and recovery to, the original level of turgescence. And this, as we have seen, is accompanied by the sequence of the fall and rise of the leaf. But theoretically it should be quite possible to bring about a responsive movement by means of the counter-variation, namely, an increase, followed by diminution of turgescence. In such a case the concomitant movement would be a rise and fall, instead of the opposite. Something of this kind will be observed in studying the daily periodic movements of the Mimosa leaf, where the rise and fall of the leaf will be found to synchronise with alternating increase and diminution of hydrostatic pressure.

Abnormal hydrostatic and true excitatory effects. — Certain effects due to the variation of turgor above and back to the normal, will be seen in growth-responses, to be treated later. For the present we may find an instance in the abnormal erectile twitch, which has already been noted, in certain responses of Biophytum (fig. 17). In that case, as was explained, the pulse of increased pressure, due to the expulsion of water from the distant stimulated point, was the first to reach the motile organ, causing erection. The true excitatory effect reached the same organ later, with the normal effect of depression. The abnormal erectile effect may be produced artificially, say by sudden forcing in of water. But, in the case mentioned, the pulse of increased pressure which brought about this effect was due to stimulation of a distant point. I shall henceforth, as stated before, distinguish the two effects as (a) the direct and (b) the indirect effects of stimulation. When a tissue is directly stimulated, there is produced a negative turgidity-variation ; normal negative mechanical response, or fall of the motile leaf; and normal electric response of galvanometric negativity. The velocity of transmission of this excitation is

relatively slow, and, as has been said, definite, and characteristic of the plant under normal conditions. This velocity varies, as will be seen later, in the case of different plants, from a rate of about *5 to about 15 mm. per second. And this true excitatory response, mechanical or electrical, undergoes appropriate modifications, according to the physiological changes of the tissues, and is abolished at death. The hydrostatic effect, on the other hand, may be seen, in the form of a preliminary twitch, when the specimen is indirectly stimulated — that is to say, when the stimulus is applied at a distance from the point where the responsive effect is observed. The hydrostatic effect gives rise to positive turgidity-variation ; abnormal positive or * up ' mechanical response ; and abnorma 1 gal vanometric positivity. The velocity of transmission of this hydrostatic disturbance is relatively very great, being about several hundreds of mm. per second.

The conditions of exhibition of excitation by lateral response. — We shall next consider the question of that division of plants into sensitive and ordinary which has led to the impression that only the former are excitable. And, first, we shall study the conditions which are favourable to the exhibition of the motile effect, according to which it is customary to estimate the sensitiveness of the plant. We have seen that in plants like Mimosa it is the difference in excitability between the two halves of the motile organ which makes it possible for it to exhibit the state of excitation by means of lateral movement. If, then, through any circumstance, this difference of excitability as between the two halves of the organ be diminished or abolished, a plant which is undoubtedly sensitive will appear insensitive, as judged by the mechanical test. The reductio ad absurdum is reached when the same plant is sensitive and insensitive at the same time.

As an instance of this, we may take the plant Biophytum. In consequence of age, the differential excitability of the pulvini of the leaflets disappears. Hence, when an old leaf is excited, its leaflets give no motile indication, and we are apt to consider it as insensitive. But on applying the test of electrical response, we discover that, though there is no mechanical indication, excitation is nevertheless present. Again, if the stimulus be sufficiently strong, the wave of excitation will pass through the old leaf, without producing any visible effect, and on reaching the younger will be manifested by the conspicuous motile response of their leaflets.

Again, we have seen that one of the conditions for the production of the responsive movement was the expulsion of water from the excited tissue. Hence, if this expulsion of water be in any way impeded, mechanical response may not take place. This may be seen in the following experiment : The cut end of a Mimosa stem is placed in water. A large amount of water is now found to be absorbed, and an abnormal turgidity is produced in the tissue, in consequence of which the leaves are erected almost vertically. If stimulus be now applied, there is no responsive movement owing to the difficulty of the expulsion of water from the gorged tissue. But the specimen is found to exhibit its state of excitation by electrical response, thus proving that not its sensitiveness, but its power of manifesting it mechanically, has been arrested.

We must bear in mind that, in these cases of differential response, the efficiency of the motile apparatus depends upon a delicacy of poise as between the two halves of the organ, which is capable of being easily upset, under the action of stimulus. This poise is determined by the antagonistic tissue-tensions of the two halves, and this again must be modified by the distribution of water, or the relative turgor-variations, in the two halves. Any deviationfrom the normal distribution of turgidity might, therefore, be expected to affect the exhibition of the motile effect. Thus, early in the morning, owing to excess of turgor-tension, the leaflets of Biophytum show hardly any response, and their motility disappears altogether, when the turgor is raised still higher, on wet days. But later in the day, when

the periodic turgor-tension characteristic of the morning has passed off, the plant exhibits normal mechanical responses. We have again seen that when the plant Mimosa is placed for a time in a dark room, and an abnormal condition of turgor — as seen in the erection of its leaves — is induced, the strongest blow will often produce no mechanical response. It is thus clear that the fulfilment of certain conditions is necessary, in order that a plant may exhibit its state of excitation, by mechanical response. The absence of this response is therefore no proof of the insensitiveness of the plant. Having thus shown that sensitive plants so called may under certain conditions fail to give motile indications, we shall in the next chapter see whether, on the other hand, ordinary plants- -commonly assumed to be insensitive — may not be found to exhibit mechanical response to excitation ; such mechanical response having been hitherto overlooked, either in consequence of our own imperfect observation, or from the fact that in radial organs excitatory reactions would be likely to have a multiradial character, which would cause them to balance each other.

Explanation of absence of lateral response in radial organs. — Taking first, then, the case of a radial stem, that is to say, one whose properties are the same in all directions, we shall find that a single stimulus applied simultaneously on all sides — in other words, a diffuse stimulus — would, even if it produced responsive contraction, result in no visible lateral movement like that seen in Mimosa leaf. This would be due to the fact that the contractions at various diametrically opposite points would be antagonistic, and balance each other. Though lateral movement would thus not take place in a radial organ, yet it is possible under favourable circumstances, as will be shown in the next chapter, to obtain longitudinal contraction as seen in muscle. Lateral movement in response to a diffuse stimulus can therefore take place only when there is some difference of excitability as between two opposite halves of an organ. The movement will then be

brought about by the greater contraction and resultant concavity of the more excitable. Differential contraction effect in Mimosa magnified by the petiolar index.— We are impressed with the magnitude of the responsive movement of the Mimosa leaf. It is worth while to remember, however, that the fundamental differential contraction of the organ by which this is brought about is very inconspicuous. If the petiole be amputated, we shall hardly notice the responsive action of the pulvinus. But the petiole, with its attached secondary petioles, acts as a long index, by which the curvature produced at the pulvinus on excitation is highly magnified. Many plantmovements, which now pass unnoticed, would have arrested our attention had there been in their case any such magnifying index.

An anisotropic organ is unequally excitable on its two differentiated sides. In a dorsi-ventral organ, like the pulvinus of Mimosa, lateral response is brought about by the differential contraction of the two halves. In such dorsi-ventral organs, owing to the differentiation of the two halves, an increase of turgidity causes erection, and a diminution the depression, of the leaf. Hence two opposite kinds of responses are possible, (a) that which is usually seen, due to the negative variation of turgidity, followed by recovery to the normal ; and (J?) the counter-variation, that is to say, a variation of turgidity above the normal-- causing abnormal erectile response — and recovery.

The ordinary mechanical response of dorsi-ventral organs being dependent on (a) the difference in excitability of the two halves, and (b) on the expulsion of water from the excited organ, it follows that any condition which diminisnes or abolishes this difference, or prevents the expulsion of water, will render the mechanical response impossible. A plant may thus be sensitive, and yet fail to exhibit mechanical response. Hence the absence of mechanical response is no indication of the plant's insensibility.

In a radial organ under diffuse stimulation there can be no lateral response, owing to the balanced and mutually antagonistic character of the contractions produced. The excitatory differential contraction in Mimosa would be inconspicuous but for the magnification produced by the long petiolar index. Pulvinoid and pulvinus -Demonstration of mechanical response in ordinary leaves— Response of Artocarpus similar to that of Biophytiim — Response to stimulus, even in old tissues, by expulsion of water — Localisation of motile organ in ordinary leaves — Conducting properties of various tissues — Lamina is not the perceptive organ — Response in ordinary leaves, though sluggish, yet comparable in extent to that of Mimosa — Peculiar phenomenon of fatigue-reversal seen in Mimosa observed also in ordinary plants — Periodic reversals.

We have seen, in the course of the last chapter, that even a sensitive plant will fail, under certain conditions, to give anymechanical indication of its state of excitation. We shall now proceed to determine whether, on the other hand, motile response may not be detected in the case of ordinary plants. It has already been said that the popular division of plants into sensitive and ordinary is purely arbitrary. It is the erroneous impression consequent on the use of these terms which is responsible for the fact that inquirers have accepted without hesitation the assumption on which the classification rests. Had such not been the case, it must long ago have been discovered that the leaves of even ordinary plants respond to stimulus by mechanical movements, in precisely the same manner as do those of sensitive plants.

We have seen that the condition necessary for the production of lateral responsive movement is, that the organ be anisotropic ; that is to say, there must be a differentiation as between the upper and lower halves. The petioles of ordinary leaves are obviously anisotropic, their upper and lower halves being quite unlike. We might, therefore, expect to find in them the exhibition of differential response. There is, however, some difficulty in detecting these responsive movements of ordinary leaves in an unmistakable manner, inasmuch as flexibility is not so great in ordinary petioles as in those which are provided with a pulvinus. Moderate stimulus therefore causes in these relatively smaller movements, and unless some form of stimulation can be used which brings about no mechanical disturbance of the plant itself, it is impossible to discriminate the true responsive movement. I have, however, described modes of stimulation, by the electro-thermic stimulator and by electric shocks, by means of which this difficulty is overcome. By the use of the Optic Lever, further, a magnified record of the responsive movement and its time-relations may be obtained.

Pulvini proper and pulvinoids. — We have seen that the responsive effect is caused by the turgidity-variation due to stimulus. Such motile indications can occur with facility only in tissues which are not yet hardened. We may therefore expect to find motile effects throughout the anisotropic petiole and its prolongations, especially in young leaves. After a certain time, in many instances, it is only portions of the petiole, such as those at the junctions of the petiole with the stem and lamina, that remain flexible. These flexible points are sometimes rather swollen or cushion-like, and may be seen — though in a much less developed condition than in Mimosa — in the leaves of many ordinary plants. But such areas, in ordinary leaves, are usually regarded as non-motile, and therefore functionally distinct from the true pulvini in sensitive plants. I shall therefore, for the sake of convenience, distinguish between pulvini proper and these pulvinoids. But I shall show presently that, contrary to the usual belief, these pulvinoids also are fully sensitive. Functionally, then, we have in young leaves a diffuse pulvinoid, which is capable of mechanical motility, throughout the length of the petiole.

Later, we find the pulvinoid localised at one or two points only, such as the stem . and laminal junctions, the motile function persisting here for some considerable period. And finally, in the markedly motile pulvini of the so-called 4 sensitive ' plants, we have the property of motility manifested throughout a still greater length of time. It will thus be seen that we can scarcely draw any sharp line of demarcation between pulvini proper and the pulvinoids of ordinary plants. And when the leaves are old, both alike cease to be motile.

Mechanical response of Artocarpus. — I shall now describe the method by which I have obtained records of these mechanical responses in ordinary leaves. For this purpose I took a pot-grown specimen of Artocarpus integrifolia, or Jack-fruit plant, the leaves of which are stiff and, as far as the eye can judge, singularly inappropriate for the exhibition of motile effects, and selected for my investigation the third leaf from the top of a stem, this being neither too young nor too old. I have said that in an anisotropic petiole the response ought to take place by the induced concavity of the more excitable half, whether upper or lower. In this case it was impossible to know from inspection which was the more excitable. But I have described, in the last chapter, an electrical method by which differences of excitability, arising from molecular or anatomical differentiation, can be distinguished. It was there explained that the electrical current of response flows from the relatively more to the relatively less excitable. And on repeating the electric experiment, in the present case, I found the responsive current to flow from below upwards. This proved that in Artocarpus the lower surface of the petiole was, as in the case of Mimosa, the more excitable of the two. Hence, if we should obtain the mechanical response from this apparently non-sensitive leaf, we might expect that it would be downwards. In order to produce stimulation, I used the electro-thermic stimulator, in the manner already described. The stimulus was first applied at a point on the petiole 3 mm. from the laminal

junction. From the record given below (fig. 32), it will be seen that the responses are similar, even in minute details, to those obtained with sensitive Biophytum leaflets. It will be seen that we have first the abnormal erectile twitch, due to transmitted hydrostatic disturbance, which takes place almost instantaneously. The true responsive effect is found to follow, after an interval of four seconds. From this we obtain the velocity of transmission as 75 mm.

per second, that is to say, about one-third the rate found in Biophytum. A somewhat incomplete recovery took place in the course of ten minutes. The successive responses are found to be nearly uniform ; but if only shorter intervening periods of rest be allowed, they exhibit marked fatigue. That the motile region is somewhere near the junction of the lamina with the petiole, was proved by the fact that on bringing the stimulator nearer to, or further away from, this point, the responses underwent corresponding increase or diminution.