Plant Response as a Means of Physiological Investigation

Response of plagiotropic stems. — In order to obtain a series of responses, made from plagiotropic stems, demonstrating their similarity to those obtained from the pulvini of sensitive plants like Mimosa, we may use specimens of Cucurbita or Convolvulus, selecting the last internode of the stem as the most sensitive. The cut ends of the specimens are placed in water, and the abnormal turgidity thus produced may sometimes at first cause erratic responses ; but after a while these become very regular. Stimulation is produced by thermal shocks, the specimen being held erect, within an inclosing spiral of heating wire, in the manner already described. The responses are now given in the form of lateral movements, which are recorded by the use of a magnetically controlled horizontal recorder, fully described in a subsequent chapter. The responses might easily have been recorded also by the use of the Optic Lever, which was employed in the case of Mimosa. But my object in the present case was to eliminate as far as possible the effect of gravity, and this could not have been done by

holding the specimen horizontally, as would be necessary in using the Optic Lever. In the records obtained with Cucurbita (fig. 39), it will be noticed that the recovery was not very complete, even though an interval of five minutes elapsed between successive stimuli. In the case of Convolvulus (fig. 40), however, the recovery was almost complete in two minutes. But of the two, Cticurbita was the more sensitive, Convolvulus requiring a stimulus about four times as great, in order to produce the

same amplitude of response. Both records show evidence of fatigue. Response by collapse of divergent halves of Allium peduncle. — The effect of the anisotropy thus induced by the unilateral action of light may also be exhibited by taking the hollow petiole of Cucurbita or the peduncle of Allium. Of these two specimens, the latter is the more sensitive, and reacts far more quickly. It will be observed that the outside of such tubular organs growing erect has been long exposed to light, whereas the inside has been protected from it. If we now split the specimen for a few centimetres of its length, then the freed halves, owing to the differences of tension as between the outside and inside, will be found to curve outwards, that is to say, the inner side becomes convex. In the previous experiments, where the two halves were rejoined, anisotropy having been induced, we observed the effect of the relatively greater contraction of one of the two halves. But we are now about to study natural differences of excitability as between the inner and outer surfaces of each half. As the outer surface has already been exposed to the continuous action of stimulus of light,

Tube by sudden Collapse E and e' are connected with induction we should expect the inner or protected side to be relatively the more excitable. The effect of diffuse stimulus should then be to straighten the curled halves, by producing a greater contraction of the more excitable inner side, which is at present convex. The experiment is carried out by dipping the freed ends in a beaker of water, the undivided portion being held in a clamp (fig. 41). Electrical connections are made with the upper part, and with the water in the beaker in which the free ends of the specimen are dipped. After passing a few shocks from an induction coil

through the specimen, the divergent curled portions are seen to fall together, the observed effect being very like the sudden collapse of the divergent leaves of a gold-leaf electroscope. The experiment described becomes very striking when magnified by optical projection. In the response-phenomena of anisotropic organs we meet with instances in which the continuous action of stimulus gives rise to alternate movements up and down. One factor of this obscure phenomenon may be determined by a modification of the experiment just described. Since we have seen that in a typically anisotropic or dorsi-ventral organ the excitabilities of the two halves are different, there must also exist a difference of time-relations as between their responses ; that is to say, the beginning of response, the attainment of the maximum, and so on, will take place earlier in the one half than in the other. The response of the organ as a whole will thus be the resultant of the curves of response of its constituent halves ; and since these latter differ, in amplitude and phase, we are in a position to understand how we may have great variations in the resultant effect. In order to show the difference of phase I shall take a simple case of induced anisotropy. One half of the bifurcated peduncle is cooled by immersion for a time in ice-water. The two halves are now dipped in a vessel of water, as in the last experiment, and electric shocks are passed through the peduncle as a whole. It will now be found that we obtain successive, instead of simultaneous, excitations of the two halves. For the uncooled half responds at once, whereas the cooled half only begins to respond after ten or more seconds. It is thus clear that had the two halves been joined to form a single organ, the observed response would have been a compound of these two constituent responses. The first part of this response would be due to the active contraction of the uncooled half, but, later, the contraction of the cooled half would reverse this first movement.

Response by uncurling'. — Having now studied the anisotropy, and consequent differences of excitability, caused by the unilateral stimulus of light, we shall next proceed to consider the similar effects induced by unilateral mechanical stimulation. We shall find, if we touch a tendril unilaterally, that it responds to this one-sided stimulus by the concavity of the excited side, and we obtain a curvature. After a more or less prolonged contact, this curvature becomes fairly permanent. Thus, by means of unilateral excitation, the originally radial tendril — like the unilaterally excited plagiotropic organ — has been made to become bilateral and anisotropic, and the excited concave surface should now be relatively less excitable than the convex.

A tendril of Passiflora was taken, in which a curvature of half a spiral had been induced by stimulus of unilateral contact. The straight lower end of the tendril was now fixed in a clamp, the hooked end being attached to the Optic Lever. A spiral of tinsel made one of the electrical contacts at the hooked end, the other being made at the clamp. An electrical shock of moderate intensity was now passed through the length of the tendril. From what has been said already, we should expect that diffuse excitation would now produce concavity of the more excitable, or convex, side of the hooked tendril. We should expect, in other words, that the electric stimulation would have the effect of undoing the existing curvature, or straightening out the curved tendril. Such a responsive uncurling would, if it occurred, relax the tension on the Lever, and cause a concomitant movement upwards of the spot of light. On the cessation of stimulus, again provided this have been not too strong, there should be a restoration of the tendril to its original curvature. Greater intensity of stimulus, or stimulus of longer duration, should, other things being equal, produce greater responsive movement. i\nd the recovery from such stronger stimulation would require a relatively longer time.

All these theoretical considerations are found fully verified in the record given below (fig. 42). It will there be seen that a stimulus of moderate electric shock, lasting fifteen seconds, produced a correspondingly moderate response of fourteen divisions, and the recovery was completed in eight minutes. Stimulus of longer duration, that is to say, of twenty seconds, was next applied, and the response was correspondingly greater, that is to say, twenty-one divisions, recovery taking place in the longer period of eleven minutes ; and finally, stimulus was applied for a still longer period, that is to say, thirty seconds, the response, of thirty-five divisions, being now

correspondingly great, and recovery requiring sixteen minutes. Tt will be noticed that in this case the amplitude of response and the period of recovery varied almost in direct proportion with (a) Response to electric shock of fifteen the effective intensity of seconds duration;^) of twenty seconds; the stjmulUs. Thus the (r) ot thirty seconds. Note increase in height of response and lengthening of restoration to the Original period of recovery with increasing: ... c -,-■, •

takes place quickly when the stimulus is feeble ; but the period is prolonged, when stimulus is strong ; or recovery may even be postponed indefinitely, after very strong stimulation. Recovery, when it does occur in such a case, may only be partial, a permanent after-effect being left. The effects here described are obtained most easily by direct stimulation of the organ. But similar results may nevertheless be exhibited by means of transmitted stimulation. To show this, we may take a spiral of Passiflora tendril, and apply strong electric stimulation through two points at its lower end. The transmitted stimulus, reaching the free spiral end, causes response by uncurling, and variation of the twist.

Response by curling. -In the case of the experiments just described, stimulus has been found to produce uncoiling of the spiral. From this, however, it must not be too hastily concluded that similar effects will ensue in every case. The fundamental phenomenon to be kept in mind is the greater contraction of the more excitable side. This might give rise to curling or uncurling, according to individual circumstances. I shall now describe an experiment which illustrates the opposite, or curling, action of this particular form of response.

If we cut a petiole of Cucurbita or a peduncle of Allium corkscrewwise, so as to form a spiral strip, and pass electric shocks through this prepared specimen, the index at its lower end shows very energetic movement, but of coiling. Here we must bear in mind that the inside of the spiral of Passiflora has been formed by the stimulus of contact, and is thus the less excitable. Diffuse stimulation in such a case, therefore, will cause the contraction of the convex surface, with the result of uncoiling. But in this spiral preparation of Allium or Cucurbita, it is the outside which has been long acted on by light, and it is the inner or concave side, therefore, which remains the more excitable. Hence, under stimulus, it is this more excitable inner side which becomes still more concave with the result of coiling (fig. 43).

Writhing movements of excited spiral tendril.— The most striking of this series of results were obtained, however, with long spiral tendrils of Passiflora, which were not too old. Very strong stimuli of electric shocks were sent through the entire length of these spirals, with results so striking and unmistakable as to furnish a final refutation of the popular assumption which distinguishes between animal and vegetable tissues as relatively motile and non-motile. These

Fig. 43. Response by Coiling of spirally-cut Allium Peduncle Through E and e' are passed electrical shocks from an induction coil. spiral tendrils, on receiving electric stimulation, began instantly to uncurl — their free ends, as they did so, sweeping through large arcs — and then straightened themselves out. Startling as this was, however, it was not all. I have already alluded to the phenomena of successive excitations and alternating fatigue, of the different sides of an anisotropic organ, under continuous stimulation. Often, owing to this peculiarity, the tendrils after their first uncoiling action showed, though with less vigour, the movement of recurling. These violent contortions were strongly suggestive of the writhing of a worm under torture.

Though the response to the stimulus of strong electrical shock is the most vigorous, yet this responsive movement of uncoiling can also be obtained by other forms of stimulation, such as the thermal and chemical. For this purpose we may dip the spiral tendril into hot water, or into dilute sulphuric acid. The differential contractile response may then be observed. We have thus traced out in unbroken continuity the various types of mechanical response as seen in plants. To begin' with, we have observed the responsive longitudinal contraction, pure and simple, of a strictly radial organ. Next, in the case of plagiotropic stems, the same longitudinal contraction, but acting differentially, produced lateral movement, the differential action being the result of induced molecular anisotropy and consequent difference in the excitabilities of the two halves, as a result of which, a plagiotropic stem functions as a diffuse pulvinoid. From this we pass to the anatomical anisotropy, which may be observed in the dorsi-ventral petioles of ordinary leaves. Here we find a tendency in the diffuse pulvinoid to become contracted to certain definite areas ; and the responsive movement, in such cases, also, is brought about by differential longitudinal contraction of the upper and lower halves. And, finally, we discover the culminating type of such differentiation for the purpose of motile efficiency, in the pulvinus of the so-called ' sensitive ' plants, where also responsive movement is brought about by differential longitudinal contraction.

It has thus been clearly established that there is no specific sensibility of the dorsi-ventral organ which is in any way distinct from that of radial organs — the responsive lateral movements of leaves being merely a special or differential form of the longitudinal contraction which has thus been found to be widely prevalent. When the two sides of an organ become unequally excitable by reason of molecular differentiation, a resultant lateral response, due to differential longitudinal contraction, is obtained, the more excitable side becoming concave. This molecular differentiation may be induced artificially by unilateral application of cold, or of strong stimulation.

This molecular differentiation occurs under natural conditions in plagiotropic stems, the upper surface being acted on by stimulus of vertical sun-light. In a hollow tubular organ, such as the petiole of Cucurbita, or the peduncle of Allium, the outer surface, which is constantly acted on by light, is found to be less excitable than the inner surface. The spiral formed, by unilateral stimulus of contact, in such tendrils as that of Passiflora is less excitable on the already stimulated, or concave, than on the outer, or convex, side ; diffuse stimulation, causing greater contraction of the more excitable convex side, gives rise here by differential contraction to the responsive movement of uncurling.

Molecular anisotropy culminates in dorsi-ventral inequality, as seen in the petioles or in the pulvini of leaves. Here, too, diffuse stimulation causes lateral response, by inducing concavity of the more excitable half. Dorsi-ventral organs do not possess any specific sensibility different from that of radial organs, the lateral responsive movement, being the result of the differential longitudinal contraction of two unequally excitable halves.

The universal law of responsive movement is : Mechanical response takes place by the concavity of the more excited side. Ineffective stimulus becomes effective by repetition — Two types of response in contractile animal tissues, cardiac and skeletal — Response of cardiac muscle on ' all or none ' principle ; parallel case in Biophytum — In skeletal muscle, increasing stimulus causes increasing response, which tends to reach a limit — Parallel^ results in longitudinal and electrical response of plants — Effect of superposition of stimuli — Tetanus.

The application of stimulus to a tissue initiates a series of events which culminates in the contraction of the excited cells. It is easily seen that a certain minimum intensity of stimulus is necessary in order to bring the excitatory condition of the tissue to the threshold of response. In the case of the electrical stimulation of Biophytum, we found this minimum stimulus-intensity to be of a very definite order. In the production of longitudinal response also a certain minimum amount of stimulus, either electrical or thermal, is necessary in order to evoke response.

Additive effect.— A thermal shock which is singly ineffective, may become effective by repetition. Below is given a record which shows this. A single shock produced by the closure for one second of an electrical circuit, acted on by six volts, was found to be ineffective in inducing mechanical response. But when the same stimulus was repeated six times in succession, it gave rise to a moderately large response (fig. 44). This additive effect I also find in the electric response of plants (fig. 45). And it is well known in the case of animal tissues.

In the case of contractile animal tissues, again, we have two distinct types of response. The first is that of cardiac muscle, which is said to be on the ' all or none ' principle. That is to say, on applying a gradually increasing stimulus, we presently arrive at the threshold of response, where the response becomes at once the maximal possible. In this case, then, the minimal response is also maximal. We shall The line to the left shows that single stimulus produced no mechanical response. The curve to the right shows the effect produced when stimulus had been repeated six times.

(a) A single stimulus of 30 vibration produced little or no effect, but the same stimulus when rapidly superposed thirty times produced the large effect (/>). Leaf stalk of turnip. find, in Chapter XXII., that an exactly similar type of response is afforded by Biophytum, where the minimally effective stimulus suddenly produces maximal effect. Relation between stimulus and response in animal and vegetable. — The second type of contractile response is shown by skeletal muscle. Here, after reaching the threshold of response, increasing stimulus causes increasing response, which, however, tends to reach a limit. Exactly parallel effects are seen in the case of plants, in the longitudinal responses exhibited by different radial organs.

The experimental method by which this is demonstrated is as follows: The intensity of thermal stimulus may be appropriately increased, as has been explained, by increasing the value of the heating current, the circuit being always closed for a certain definite time, say one second. The resistance of the heating coil being kept constant, the thermal effect is proportional to the square of the current. If, then, we use currents which increase as the square root of the natural numbers, the successive thermal effects will be increased in arithmetical progression. The currents to be used are previously adjusted, by means of an ammeter, and

46. Mechanical Responses to Stimuli increasing in Arithmetical Progression suitable external resistances. In this way I have obtained successive responses to thermal stimuli, which increased in arithmetical progression. And we sec from the records in what manner the corresponding responses undergo an increase It will be seen from this figure that with a stimulus represented by unity, the response was 1*5 division. With a stimulus twice as great, the corresponding response was slightly greater than twice as much, that is to say, it was 35. The intensity of response went on increasing, but with

special acceleration when the stimuli were four and five. After the stimulus of six there was a tendency for the response to approach a limit. The subjoined curve (fig. 47) shows the relation between the increasing stimuli and the corresponding responses. Another interesting feature of these response-curves is one which has already been referred to, the prolongation of the period of recovery with increasing stimulus. In the present case, with the stimulus of unit-intensity, the recovery was completed in forty seconds. With an intensity twice as great, it required fifty-six seconds for the restoration of equilibrium. And this increase in the period of recovery

Fig. 47. Curve showing Relation between Stimulus and Response The abscissa represents the stimulus, and ordinate the height of response. Continued progressively, until, with the stimulus-intensity of 21'ght, the time required for restoration to the original condition was as much as 4 minutes 40 seconds, or exactly seven times that necessitated by unit-stimulus. All these peculiarities are observable in the electrical responses of plants, as will be seen from the record in fig. 48. The stimulus applied was vibrational, and was increased in amplitude in successive experiments, from 2'5° to 7-5° to io° to 12-5°. It will be seen that here also as in the case

Fig. 48. Increased Electrical Response with Increasing Vibrational Stimuli (Cauliflower-stalk) Vertical line to right = -I volt. Stimuli applied at intervals of three minutes. of the response of skeletal muscle, and the longitudinal response of plants, the amplitude of response increasing with increasing stimulus tends to approach a limit. These curves also show the increase in the period required for complete recovery, but in a manner different from that of fig. 46. In fig. 46 the time-intervals were suitably increased, to allow of complete recovery. In the case of the electrical responses, however, stimuli were applied at equal intervals of time throughout. This was enough to bring about complete

recovery in the case of the first two responses. But afterwards it was not sufficient, so that recovery in the last three instances was more and more incomplete, as seen by the tilting upwards of the base-line of the responses (fig. 47). Tetanus. — Having now observed the effect produced by single stimuli, we shall proceed to study the effects of similar Record to left shows incomplete tetanus, with moderate frequency 01 stimulation. Record to right shows tetanus more complete, with greater frequency of stimulation (Brodie).

stimuli when superposed. In muscle, we find that when stimuli succeed each other with great rapidity, the effect of the second stimulus becomes superposed on that of the first, which has not had time to disappear. The result is a fusion of effects, more or less complete. With moderate frequency of stimulation we thus obtain incomplete tetanus, which, with : similar tetanic effects with the longitudinal contractions of the pistil of Datura alba (fig. 50). Stimuli were here applied at intervals of ten seconds, which was too short an interval, when compared with the natural period of recovery, lasting about two minutes. Hence we obtained incomplete tetanus. This incomplete tetanus became more complete when the stimulationfrequency was increased, successive stimuli being now applied at intervals of five seconds. It may be noted here that, in the tetanus both of muscle and of plant, the effects of individual stimuli, when rapidly succeeding, become so merged as to appear FlG. 50< Photographic continuous. It is only after the maxi- Record of Genesis of