Plant Response as a Means of Physiological Investigation

I obtained similar responses by subjecting the lower pulvinoid, that, namely, at the junction of the petiole with the stem, to similar stimulus, which was now applied on the adjacent stem. In the preceding experiment the Optic Lever was attached to the lamina, but in this case, in order to avoid possible complications from the responses of two pulvinoids in succession, I tied a thin stiff wire to the intervening portion of the petiole, and the Optic Lever was attached to this wire, instead of to the lamina. By this means the response of the lower pulvinoid alone was recorded.

Response of Ordinary Leaf (Artocarpus) Note preliminary erectile twitch. Excitatory reaction in older tissues.— One very important and hitherto undecided question, which is answered by this experiment, is as to whether all tissues, even the old, give contractile response by expulsion of water, or by negative turgidity-variatiom In the pulvinus such an effect is made evident by the differential contractile movement produced. In young tissues it will be shown that we have effects exactly similar. But in older tissues no such movement is observable. Of this fact, two alternative explanations are possible: (i) there may be no excitatory expulsion of water in old tissue ; (2) such expulsion may occur, while movement is at the same time rendered impossible, by the inflexible condition of the tissue. We have already seen how, whenever there is negative turgidityvariation due to excitation, there is also a simultaneous galvanometric negativity. Conversely, induced galvanometric negativity may be taken as an indication of the excitatory expulsion of water. And from this indication the conclusion is arrived at that, even in relatively old tissues, this excitatory expulsion occurs, since these tissues on excitation exhibit galvanometric negativity.

I was desirous, however, to obtain additional proof ot this excitatory expulsion of water in the older tissues, and from the preliminary positive twitch seen in the mechanical response (fig. 32) the existence of such action is clearly proved. For the preliminary erectile twitch observed in the leaf, when the rigid part of the petiole — or of the stem at a distance beyond the petiolar junction — is excited, can only be explained by a pulse of increased pressure towards the motile area, initiated by water expelled from the stimulated point. Independent experiments, carried out by other methods, also tend to show that, though the power of excitatory contraction undergoes diminution with age, it does not, generally speaking, disappear entirely.

We saw in Biophytum that when stimulus was applied at a distance, the hydrostatic disturbance produced the preliminary erectile twitch. But no such effect was observed when the pulvinus was directly stimulated, the result now being the true excitatory fall alone. In the case of Artocarpus I obtained the same result. Direct stimulation was here applied, at the motile region, by electric induction shocks, the electrical connections being made on two sides of the pulvinoid, by means of thin flexible spirals of tinsel which did not offer any obstruction to the free movement of the responding leaf. The response thus obtained was normal, and unattended by any preliminary positive twitch.

The motile responsiveness of Artocarpus, then, is seen to follow that of Biophytum or Mimosa, even in details. An instance of this is found in the fact that in all three cases abnormal increase of turgidity is unfavourable to the manifestation of mechanical response. Thus, just after the rainy season, the Artocarpus is highly turgid, and response not easily obtainable. But in November, when the turgor has become moderate, its motile indications are once more made conspicuous.

If the motile region be extended, or diffuse, the responsive action when the stimulus is applied at any single point becomes very complicated. For we have, first, the preliminary hydrostatic pulse, travelling along the whole length of the motile organ, and producing a somewhat long-continued erectile effect, which is, therefore, unlike the quickly exhausted erectile twitch that took place during the short passage of the hydrostatic pulse through the restricted pulvinoid. And, secondly, follows, in the wake of this, a wave of negative turgidity-variation concomitant to the passage of true excitation. Such complicated effects may be avoided, however, by applying electrical stimulus simultaneously throughout the whole area. The electrical form of stimulation has therefore certain advantages, but it is apt to bring on quick fatigue of the tissue.

Localisation of motile areas. — 1 shall now deal with some further difficult points, in connection with the motile response of the leaf. And first comes the question of localising the motile area itself. Next comes that of the possibility of excitation reaching the motile organ from a distance, through certain specific tissues, and the determination of the nature of such tissues. By using the experimental methods which I have described, it is possible to determine these important points. But, before doing this, it will be well to enter upon the theoretical considerations in connection with the subject.

As regards the first point, it has been shown that any flexible anisotropic organ is capable of lateral response owing to differential action. In the leaf, when young, the power of movement extends throughout the length of the petiole and its prolongation, the midrib. But when older, as has been said, the motile power becomes localised at the pulvinoids or pulvini. This conclusion is verified by experiment. The young petiole is found motile throughout its length, but when older the existence of certain specialised areas is made evident by the fact of the strong response obtained, when stimulus is applied on such areas, and its rapid diminution on application at gradually increasing distances. For example, in the case of Artocarpus leaf, when this distance from the pulvinoid is increased to 1 cm. the normal response to moderate stimulus disappears altogether. In this connection it is well to bear in mind the fact, which will be fully demonstrated later, that the distance to which the effect of stimulus is transmitted depends not only on the conductivity of the tissue, but also on the intensity of the stimulus. A moderate intensity of stimulus is only effective in producing motile indications when the application is on, or very near, the pulvinoid. We thus see, as regards motile organs, that there is a strict continuity between those cases in which the property of motility is diffused through a large area, and others in which it is contracted to one or more definite points, this last form culminating in the maximum flexibility of pulvini proper.

The petiole of the leaf of Biophytum itself gives an»excellent example of a diffuse pulvinoid. It is not provided with any specialised pulvinus, such as that of Mimosa, or of its own leaflets. But if this petiole be subjected to any form of stimulus, it responds by a depression of the leaf as a whole. On the cessation of stimulus there is the usual recovery. In a subsequent chapter I shall describe in detail the responses of the petiole of Biophytum to different forms of stimulation.

Conducting* properties of various tissues. — With regard to the next point, this is to say, the transmission of excitation to a distance, it will be shown in Chapter XX. that the transmission of the excitatory state is brought about by the propagation of protoplasmic changes from point to point We may therefore expect that tissues in which there is more or less uninterrupted protoplasmic continuity will be those which will, other things being equal, show the greatest power of transmitting stimulus. Even in such cases it will be understood that the transmission becomes enfeebled with distance. Now the tissues in which this continuity is most uninterrupted are the fibro-vascular elements. Hence, stimulus applied on the petiole, or its prolongation, will reach the motile organ, with the greater intensity, the nearer the point of application is to the motile region. We should expect, on the other hand, that indifferent tissues, like leaf parenchyma proper, would prove to be, owing to the more or less complete cellular partitions, incapable of transmitting stimulus to a distance. These considerations I have been able to verify experimentally by means of electric response. I shall here, however, describe experiments in which the same conclusions are established by the method of mechanical response.

Moderately strong electrical stimulus from an induction coil was applied on the petiole of Artocarpus at points increasingly far from the laminal pulvinoid ; these produced motile responses which diminished rapidly with distance, as was described in the last experiment. But when such stimulus was applied on the lamina, at a point relatively near the pulvinoid, there was no response. This shows that the lamina is not to any extent the perceptive organ, and that stimulus received on such an area does not cause movement of the leaf as a whole.

It will thus be seen that the petiole when young is both the motile and the transmitting organ. With increasing age, certain areas become inflexible, and the power of motility is narrowed to the still remaining points of flexibility— the pulvinoids. But the inflexible petiolar portion still retains the power of transmitting stimulus. Using the language of Animal Physiology, we might say, therefore, that a young petiole is, functionally, nerve and muscle combined. Later there is a differentiation into motile pulvinoids, corresponding to muscle, and the rest corresponding to conducting nerve. The pulvinoid, however, has still the power of transmitting stimulus.

To serve the purpose of a concrete example, I have taken as a typical case the Artocarpus leaf. I shall now, however, show that responsive motile effects are obtainable from the leaves of plants in general. In the record given (fig. 32) I applied only moderate stimulus, in order that the magnified response might be brought within the limited space of record. From this, it might be supposed that the extent of responsive movement in ordinary leaves is relatively much smaller than what is obtained with the sensitive Mimosa. But this is by no means the case. By using stronger stimulus a response is obtained, in the case of many ordinary leaves, which in its extent is strictly comparable with that of Mimosa. Of this, I shall here proceed to give examples. But, before doing so, we must remind ourselves that the promptitude of mechanical response is a matter of the delicacy of poise of the motile apparatus. In the case of Philanthus, which is provided with distinct pulvini, we found that response took place very sluggishly, and was evoked only by relatively strong stimulus. The difference between the response in this case and in that of Mimosa is, however, not of kind, but of degree.

Response of ordinary leaves comparable with that of Mimosa. — When we come to the response of ordinary leaves, we find, under strong stimulation, considerable extents of movement, even rivalling those of Mimosa, but taking place with a comparative sluggishness which, generally speaking, is not so great as that of Philanthus. In the following experiments various leaves were excited by strong electric shocks, sent through the entire length of the petiole, and responsive movements were, as a rule, observed, the leaves in these instances falling downwards from their normal more or less horizontal position. Of these I shall give only three instances. A particular leaf of Bryophyllum calcynum, which is unprovided with a pulvinoid, was 5 cm. in length. After tetanic shocks of three minutes, the leaf fell, the distance traversed by the tip being 2*1 cm. A leaf of Canna indica, again, unprovided with any pulvinoid and not too young, was 20 cm. in length> On the application of tetanic shocks for one minute, the leaf fell, the distance traversed by the tip being as much as 14 cm. The third instance is a leaf of Ficus religiosa, 14 cm. in length. After three minutes of continuous stimulation, the leaf fell, the tip passing through a distance of 12 cm. It will thus be seen that the movement caused by stimulation in ordinary leaves is quite considerable.

It may, however, be objected that the fall of the leaf is really due simply to its weight, acting on the petiole when rendered flaccid. Even in this case the flaccidity would be directly due to stimulation, and the movement of the Mimosa leaf itself must be regarded as being of a similar nature. But in order to prove that the movement of the leaf is mainly due to the active differential contraction of the upper and lower halves, and in order to eliminate completely the effect of weight, I arranged the experiments as follows : The plant was held with the leaf directed vertically downwards. The movement due to the greater contraction of the lower half of the organ, in consequence of the true excitatory effect, ought now to raise the leaf against the force of gravity. And this, as will be seen from the records (fig. 33), was found to be the case.

In order to compare the angular movements of some ordinary leaves, and their time-relations, with those of Mimosa, when subjected to strong electric shocks, I took records of such movements in these different cases on the same revolving drum. And with the purpose of making the angular movements, in the -various instances of long and short leaves, comparable, I rendered the virtual length the same in all cases, by attaching longer or shorter indicators, as was necessary, and the movement of the tip of the indicator or its projection was then recorded on the drum. The longest leaf experimented on in this series was Mimosa, having a length of 5 cm. In this case no additional indicator was attached ; and in other cases the indicator-lengths which were added made each leaf have a virtual length of 5 cm.

Fatigue-reversals in ordinary leaves, as in Mimosa. — In comparing the response-records of various specimens, it is necessary to understand one peculiarity which is observed in certain responses under long-continued stimulation. In order to do this, I must anticipate a certain characteristic of Mimosa response, which will be more fully described in Chapter IX. It is found that the leaf of Mimosa gives immediate response to stimulus, by a fall or contraction of the lower half. But when the stimulus is long continued, the leaf rises gradually to its original position, or even, sometimes, beyond this. This position is, however, only apparently normal, being really a posture of fatigue. For whereas, while the leaf is fresh, it responds to stimulus by depression, it is now, though it occupies the same position, entirely insusceptible of excitatory depression. In exhibiting the effect of long-continued stimulation on ordinary leaves, I shall be able to show that they resemble Mimosa, not only in the extent of their responses, but also, in certain cases, even with regard to this specific characteristic.

In the records given (fig. 33) the first curve is of Mimosa leaf. It should be remembered that the experiments in all these cases were carried out with the leaves held perpendicularly downwards, the excitatory movement being produced by strong and continuous tetanic shocks. The excitatory effect recorded, therefore, is due to the differential contraction of the two halves of the motile organ, lifting the leaf against gravity. In the specimen of Mimosa [a) the maximum responsive deviation of 250 was reached in a little more than one second, after which occurred the peculiar reversal already referred to. This, which was due to fatigue, was completed in seven and a half seconds. The next curve {&) was obtained from the young leaf of Citrus decumana. It will be observed that here we have a practical duplication of the curve of Mimosa, the only difference lying in the longer period necessary for the completion of its different parts. The responsive deviation in this case is 300, that is to

Fig. 33. Response of Leaves of Ordinary Plants to Electric Stimulation The ordinate represents the angular movement in degrees. say, even greater than that of Mimosa. This was attained in eleven seconds, and reversal completed in twenty five seconds. The extent of these responsive movements, being due to differences of excitability as between the two halves, will, it is easy to understand, be modified by the age of the leaf. This fact is illustrated by the next curve (V), obtained with an older leaf of the same plant. Here the responsive movement takes place through only io°, and there is only a partial reversal.

The instances given may be considered as typical, the responses obtained with specimens of other plants being simply variations of these. For example, while some leaves under continuous stimulation exhibit complete, and some partial, reversals, there are again other species of plants whose leaves do not exhibit any reversal at all. Still others, again, exhibit periodic reversals. The effects produced are thus seen to depend on the species of the plant, on the age of the leaf, and on the intensity and duration of the stimulus.

It will thus be seen that an anisotropic organ like the petiole responds to stimulus by differential contraction of its upper and lower halves, and that the mechanics of such movements in the case of ordinary leaves are precisely the same as in those of the leaves of sensitive plants such as Mimosa. In the next chapter I hope to show that even radial organs are not insensitive, but exhibit responsive contractile movements. The motile organs of ordinary leaves, owing to dorsiventral differentiation, give mechanical response by differential contraction, in a manner precisely the same as does the motile organ of Mimosa.

In a young leaf, the petiole and its prolongation act as a diffuse pulvinoid ; later, the motile area becomes restricted to certain points, generally speaking, the junctions of petiole and stem, and petiole and lamina. There is no sharp line of demarcation between pulvinoids and pulvini proper. Even in relatively old and rigid tissues, response to stimulus is by excitatory expulsion of water. Responsive movements take place in ordinary leaves, when the stimulus is applied at or near the motile organ.

The lamina is not, generally speaking, the perceptive organ. The effect of stimulus remains in this case localised, and is not transmitted to a distance. Organs like the petiole, which contain fibro-vascular elements, are capable of conducting stimulus to a distance. Under continuous stimulation there may be reversal due to fatigue in ordinary leaves as in Mimosa. In some cases, periodic reversals are also observed. Absence of lateral response movements in radial organs due to mutually antagonistic effects of equal contractions of diametrically opposite sides — Lateral response in radial stem of Walnut under unilateral stimulation — Also in pistil of Musa — Diffuse stimulation of radial organ causes longitudinal contraction — The • Kunchangraph ' — Longitudinal contraction of stamens of Cynerecc not unique — Similar longitudinal responses obtained with stems, roots, tendrils, petioles, stamens, and styles of ordinary plants — Also in fungi— Responsive contraction in Passiflora, comparable in extent with that in Cynerea — Longitudinal response in plants modified by the physiological variations due to age, season, and chemical agencies.

We have now considered the phenomenon of responsive lateral curvature in plant-organs such as leaves, and have seen that it is brought about by the differential contraction of two unequally excited halves. But in the case of radial organs, under diffuse stimulus, such responsive movements are not usually noticeable, hence these organs have generally been regarded as insensitive. It is, however, conceivable, as already said, that this absence of responsive movement might be due to the occurrence of simultaneous contractions in diametrically opposite sides, which would balance each other, and thus permit of no lateral movement. The correctness of this supposition might be tested in either of two ways. We might, for instance, apply a unilateral instead of a diffuse stimulus, and so bring about a lateral contraction. Or we might use a diffuse stimulus, and look for the contraction in length of the specimen as a whole.

Experiments showing equal and opposite reactions of radial organs. — If we apply local and unilateral stimulus, we may expect contraction of the acted side alone. The stimulated side should thus become concave. I have been able to verify this by experimenting with, amongst others, the radial stem of a young Walnut plant ; stimulus was applied in a region about 4 cm. from the tip, this being a plastic area, just below the zone of growth. There are practical difficulties in the application of unilateral stimulus, which might easily be overcome by the use of photic stimulus. In the present chapter, however, I intend to deal with forms of stimulation other than that of light. In the experiment on Walnut, therefore, I made use of the stimulus of electric shocks from an induction coil. The electrical connections were made with one side only of the radial stem. Small quantities of kaolin paste, moistened with normal saline solution, were placed on the stem at two points, at a distance of 2 cm. one above the other, and these were connected with the terminals of the induction coil by means of flexible spirals of tinsel. Electrical excitation was now produced by tetanising shocks for the requisite length of time. With regard to this, it must be pointed out that excitation given by such means cannot be strictly confined to one side of the organ alone, since the current will pass through the further side also ; but as most of it will take the shortest path, excitation will be relatively greater on the side of the electric connections. The responses were recorded as usual, by attaching the stem to the Optic Lever.

Strong electrical shocks being now applied during a period of five seconds on one side of the stem, a responsive concavity of that side was produced, the amplitude of the response being eight divisions. The effect attained its maximum in forty seconds, after which the recovery was partially completed, in six minutes. On now exciting the opposite side of the same stem, a response which was practically the same — i.e. 7*5 divisions — was obtained, the curvature being now in the opposite direction. Hence it is clear from these two experiments, that when a radial organ is simultaneously excited on all sides, there can be no lateral

curvature, owing to the fact that opposite reactions neutralise each other. I succeeded in obtaining a singularly perfect demonstration of the response of a radial organ to local and unilateral stimulation, by paying special attention to the selection of the specimen and mode of stimulation. 1 took a pistil of Musa paradisiaca, in which I was able to localise with great distinctness the zone of growth, in this particular case of not greater extent than 2 mm. The stimulation, which was thermal, was effected by placing on one side of the selected area, but not in actual contact with

it, the outer point of a small V-shaped piece of platinum wire which could be put in circuit with a battery. An exactly similar arrangement was made on the other side of the organ, at the diametrically opposite point. By sending a heating current from the battery through either of the platinum pieces for a definite length of time, the tissue in that region is stimulated locally by thermal radiation. In this manner two opposite points of the tissue may be alternately subjected to equal stimulation. The accompanying figure (fig. 34) gives the record of such an experiment. The up-responses here represent the concavity produced by stimulating on one side, say the right, and the down-responses the same on the left. It will thus be seen that equal and opposite responses were obtained, equal alternate stimulation of the two sides. The recovery, it will be noticed, is not complete, the curvature caused by

Fig. 34. Alternate opposite-directioned Re- sponses obtained by the successive Unilateral Stimulations of opposite sides of Pistil of r, responses of right side, upwards ; L, responses of left side, downwards. stimulus being partially fixed by growth. The responses, moreover, afford some slight indication of fatigue. But though there is thus no lateral movement, owing to the antagonistic character of the simultaneous longitudinal contractions on all sides, we might nevertheless hope to detect some responsive contraction in the length of the organ as a whole. And perhaps it will be well to consider at this point what would be the most favourable condition for the exhibition of such contraction. Taking the parallel instance of contractile animal tissue, namely muscle, we can easily see that if this were attached throughout its length to a rigid structure such as bone, contractile movement would have been an impossibility. Similarly, contractile vegetable tissues when attached to hard elements, such as wood, must be prevented from exhibiting contractile movements. The vegetable tissues, therefore, which ought to be best fitted to exhibit this effect, will be comparatively deficient in hardened fibro-vascular elements, and will consist largely of prosenchymatous cells, relatively longer in one direction, and very elastic and highly extended by turgidity. The diminution of this turgidity by stimulus might then be expected to produce a relatively large degree of longitudinal contraction.1

Excitatory contractions in Cynereae. — This shortening in length is most strikingly exhibited by the filaments of the stamens of Cynerece. This subject has been specially studied by Pfeffer, and the account which I give here is epitomised from Sachs' ' Physiology of Plants.' 2 The filaments of the stamens of Centaurea jacea are free from each other, but the anthers cohere, forming a tube. The stamens are, in the unexcited state, strongly curved convexly outwards. If now the filaments are all excited simultaneously, say by mechanical touch, there is produced a downward withdrawal of the