Plant Response as a Means of Physiological Investigation

wire, heated by electrical current, is placed in the neighbourhood of the point to be stimulated. The intensity of stimulus in this case is regulated by varying either the strength of the heating current, or the distance of the heating wire from the point to be excited. All the different forms of stimulation will be shown to produce the same results. Responsive results of : i. Longitudinal transmission of effect of stimulus from tip : (a) Moderate unilateral stimulation.— Using the thermal mode of stimulation applied at T (fig. 212, a), I obtained positive electric-variation at A, the record of which is given in fig. 213. A similar result was obtained

Fig. 213. Record of Positive Electrical Variation, indicating Positive Turgidity- Variation (represented by Down Curve), induced in Growing Region by Moderate Stimulation on same side of Tip. Timemarks = minutes Fig. 214. Record showing Galvanometric Positivity subsequently Neutralised under Transmission of True Excitatory Effect, due to Continuance of Moderate Stimulation of the Tip with the mechanical stimulation of a pin-prick. The same was again observed on effecting stimulation by dilute acid.

(b) Effect of stronger unilateral stimulation. — I next produced a somewhat stronger thermal stimulation by suitably increasing the heating current. This gave rise to the electrical indication of the preliminary positive ttirgidity-variation, as the immediate effect. But the long-continued action of the stimulus caused the transmission of the true excitatory, which, reaching A, caused the neutralisation of the previous effect (fig. 214). With another specimen, I next applied to the tip a still stronger unilateral stimulus. This caused a brief positive electrical and turgidity variation, followed by a reversed response of

galvanometric negativity, thus proving that the strong excitatory effect transmitted to the organ had not only neutralised, but also reversed, the previous effect (fig. 215). 2. Direct unilateral stimulation of growing region : Moderately strong stimulus. — When stimulus is applied near the responding organ, say at C (fig. 212, a), there is always Fig. 215. Record showing Neutralisation and Reversal of Electrical Response at Responding Region, under Strong Stimulation of Tip

This indicates that the first positive turgidity - variation, due to indirect stimulation, is converted into negative turgidity-variation of transmitted true excitation. Fig. 216. Record showing Negative Electrical Re- sponse represented by Up Curve, indicating Negative Turgidity - Variation due to Direct Stimulation produced at A a negative electrical, indicating a negative turgidity, variation (fig. 216). Thus we have obtained, using the same moderate stimulus, two opposite effects, of positive and negative turgidity-variations, according as the point of application was at the distant tip or in the vicinity ofthe responding organ. It is evident, moreover, that there is a continuity between these two extreme effects, for, as we gradually shift the point of application from the distant point nearer to the responding organ we observe corresponding intermediate changes, from pure positive, through the neutral, due to equal positive and negative effects, to pure negative.

Moderate stimulation. — We have seen that the direct application of stimulus at a point has the local effect of a negative turgid ity-variation. It is of great theoretical importance ! that the effect of this stimulus on the diametrically opposite point should be clearly demonstrated. As conductivity has already been shown to be very feeble across a tissue (p. 250), we might expect that it would be the indirect effect, that is to say the positive turgidity-variation, which on stimulation of A would reach the diametrically opposite, or distal, point b. The experiment is carried out, by making galvanometric connections with B and L, and applying stimulus at A (fig. 212, b\

On now applying moderate stimulus at A, we obtain a positive electrical response, indicating positive turgidity-variation, at B (fig. 217) — a result precisely the same as was obtained by stimulating the distant tip. The effect, then, of stimulating any point is to induce a negative turgidity-variation of the point itself, and a positive turgidity-variation of the diametrically opposite point. (b) Stronger stimulation. — When stronger stimulus is applied, however, at A, the true excitatory effect is gradually transmitted across the tissue, and we obtain at B the neutralisation of the preliminary positive effect, as in fig. 214. And, lastly, when the point A is very strongly stimulated, the responsive effect on the diametrically opposite point B is a transient positive, followed by a strong negative variation, as in fig. 215.

We have thus studied the separate effects produced at A and B by stimulation of a point near A. The differential Fig. 217. Record showing Positive Electrical Variation indicating Positive Turgidity -Variation of Distal Point, under Moderate Stimulation of Proximal effect as between A and B can be inferred by the algebraical summation of these separate effects. Or it can be directlyobtained by making electrical connections with the diametrically opposite points A and B, as in fig. 212, c, and applying gradually increasing stimulus at C near A.

It will be remembered that these opposite effects of direct and indirect stimulation have received independent demonstration, in both mechanical and growth responses. In Biophytum and in Artocarpus the positive turgidityvariation was proved to be the indirect effect of stimulation, by the erectile responses of the motile organs (pp. 24, 420). In the case of growthresponse, again, the indirect effect of stimulation, with the concomitant positive turgidity-variation, was shown in the increased rate of growth (p. 430). The effect of direct stimulation in inducing a negative turgidityvariation, again, was exhibited in Biophytum and Artocarpus, by the responsive depression of the motile organs. In the case of growth-response, it was exhibited by contraction and concomitant retardation of growth.

Mechanical response inferred from observed electrical response. — From the results of the electrical investigation on these turgidity-variations, induced by direct and indirect effects of stimulus, as just described, we are led to conclude that : 1. (a) Moderate unilateral stimulation of the tip gives rise by longitudinal transmission to the indirect effect of stimulus, namely a positive turgidity-variation, on the same side of the distant responding-organ. This will give rise to acceleration of growth and convexity of that side, and by the consequent responsive movement the tip will be carried aivay from the source of stimulation.

(b) When the unilateral stimulation of the tip is a little stronger, the indfrect effect of stimulus is neutralised by the subsequently transmitted true excitatory effect, and there is no resultant action. Very strong unilateral stimulation of the tip gives rise by longitudinal transmission to the direct effect of stimulus, namely a negative turgidity-variation, on the same side of the distant responding organ. This will give rise to retardation of growth and consequent concavity of that side, the tip being carried, by the responsive movement, towards the source of stimulation.

2. (a) Stimulation of a point at or near the responding region of growth will induce a negative turgid ityvariation as the direct effect of stimulus, and at the diametrically opposite point a positive turgidity-variation as the indirect effect of stimulus. This will give rise to concavity of the proximal with convexity of the distal sides. The mechanical effects of negative turgidityvariation on one side and positive turgidity-variation on the other side are thus additive. In this way, the concavity of the proximal and convexity of the distal conspire to bring about the resultant curvature.

(J?) When the stimulus applied at A is stronger, the effect is a negative turgidity-variation at A, and later, owing to the transmission of stimulus across the tissue, the positive is succeeded by a negative turgidity-variation of B. The resultant effect obtained by algebraical summation thus tends to become zero. When the stimulus at A, however, is still stronger and of longer continuance, the negative response of A will be found to undergo a gradual diminution owing to fatigue, while the transmitted effect at the diametrically opposite point undergoes an increase. Under these circumstances the responsive negative change at B will predominate over that at A. The final result may thus be a relative negativity of B, that is to say an effect opposite to that seen

Fig. 218. The Relative Electrical and Turgidity Variations of two Diametrically Opposite Points, A and B, when Strong and Long-continued Stimulus is applied near A (cf. fig. 212, c). We see here (1), in the up curve, the negative variation of the proximal, followed by (2) neutralisation, followed by (3) reversal, that is to say, relative negativity of B. Note the multiple response which here makes its appearance. in case (a). These different phases of the effect induced by long-continued application of strong stimulus at A — namely the relative negativity of A, followed by neutralisation, followed by reversal or relative negativity of B — are well seen in the record given in fig. 218. Translated into terms of the resulting mechanical response, this would mean : (1) a movement of the organ towards the stimulus ; (2) neutralisation of this movement ; and (3) a pronounced movement away. I give here a tabular statement which shows at a glance the various electrical effects and the corresponding mechanical responses which are theoretically to be inferred from them, the experimental verification of these inferences being given in the next chapter.

Tabular Statement of Electrical Effects and Inferred Mechanical Responses I. Unilateral, of tip, on Positive electrical and j Convexity of A, and side A : positive turgidity vari- j movement of tip away Positive and subsequent negative effects neutralise each other. Positive twitch, followed by strong negative electrical and turgidity variations, of same side, A. Negative variation of side acted upon, and positive variation of diametrically opposite side.

Transient twitch away from, followed by strong movement towards, stimulus. Positive and subsequent negative effects neutralise each other. Positive twitch, followed by strong negative electrical and turgidity variations of the opposite side, B. Resultant mechanical response opposite to that in 2 (a) ; i.e. movement away from j the stimulus. The verification of these theoretical inferences will be given in full in the following chapter. From electrical investigation we find that the responsive peculiarities of the tip of the root are not characteristic of that organ alone, but of the tip of the shoot also.

The different effect between stimulation applied to the tip and to the growing region of an organ lies in the fact that from the former — it not being a good conductor of excitation — only the indirect effect, that is to say the positive turgidity - variation, is transmitted to the responding organ. In the latter case, however, direct excitation gives rise to negative turgidityvariation. Moderate unilateral stimulation of the tip, therefore, gives rise on the same side of the responsive region to increased turgidity, which, translated into mechanical response, means a negative curvature or movement away from stimulus.

Stronger and long-continued stimulation of the tip causes transmission of the direct effect. This will gradually neutralise or even reverse the first effect, the positive turgidityvariation giving place to negative. These events translated into terms of mechanical response mean a change from negative to positive response, or movement towards stimulus. Direct unilateral stimulation of the responsive region causes negative turgidity-variation of the proximal and positive turgidity-variation of the distal. The mechanical expression of these will be a movement towards stimulus.

By strong and long-continued action the stimulus ceases to be unilateral and becomes internally diffused. The excitation thus reaches the distal side. The difference of turgidityvariation on the proximal and distal is thus gradually abolished, or even reversed. The corresponding mechanical response will be a neutralisation or reversal into negative, that is to say a movement away from the stimulus. Scope of the investigation : I. Mechanical response to unilateral stimulation of the tips of shoot and root : (a) Moderate stimulus — (b) Stronger stimulation— 2. Effect of unilateral stimulus, applied at the responding growing region : (a) Moderate stimulus — (b) Strong or long-continued stimulus — Experiments on the direct and indirect effects of stimulus on Mimosa : (a) Direct stimulation— (b) Indirect stimulation, longitudinal transmission — (c) Indirect stimulation, transverse transmission — The curious response of an Aroid — Table showing responsive effects common to pulvini, pulvinoids, and growing organs — Laws of responsive growth-curvature.

We were able, at the end of the last chapter, to pass in review the theoretical conclusions to which we had been led by the electrical mode of investigation, as to the responsive movements which might be expected to follow on the unilateral application of stimulus to the tip and the growing region respectively. I shall now proceed to submit these theoretical conclusions to experimental verification, by taking records of the mechanical movements actually induced.

It will be understood here that my object is (i) to show that the peculiar response given by the tip of the root is characteristic of the tip of the shoot also ; and (2) to demonstrate the effect of unilateral stimulation on the growing region. We have therefore to study the effects which are induced at the growing region by the action of stimuli of different intensities, according as they are applied unilaterally at the distant tip, or, locally, on the growing region itself. As a specimen of the shoot-tip, we may employ either the tip of a stem or that of an unopened flower-bud. This latter, composed mainly as it is of indifferently conducting elements,

has all the characteristics of the tip of the organ, while the peduncle below often represents the area of quickest growth, and functions as the responding region. Another advantage of the unopened flower-bud, again, lies in the fact that its upper end is not covered over with appendages like that of the stem. The unopened buds, with peduncles, of Crocus, then, will be found suitable for this investigation. As specimens of the roots, again, the long straight water-roots of Bindweed are very suitable for these experiments.

The records of the responsive movements are taken by means of the recording microscope fully described in a subsequent chapter. The quiescence of the organ, before the application of stimulus, is tested by the fact that the record is a horizontal line. The occurrence of an up curve in the record represents responsive movement towards, while a down curve means movement away from, the stimulus. In this investigation, as in the preceding, various forms of stimulation have been employed. Mechanical stimulus is applied by friction of emery-paper. The jar produced by this causes a temporary disturbance of the image in the field of view of the microscope ; but this soon subsides, and the excitatory movement commences after a short latent period, increasing steadily until the effect of stimulus is exhausted. The chemical form of stimulation has the advantage of producing no mechanical jar. The stimulation produced by light, which is the most perfect, will be described in the subsequent chapter on heliotropism. Thermal stimulation is effected by holding a platinum wire, heated by the electrical current, in more or less proximity to the point in the tissue which is to be excited.

1. Mechanical response to unilateral stimulation of the tips of shoot and root : (a)- Moderate stimulus.— Applying a single mechanical stimulus of emery-paper friction, of moderate intensity, unilaterally to the bud of Crocus, a movement was induced in the responding organ, which carried the tip away from the source of stimulation. This movement persisted for four minutes, after the application of this single stimulus. I performed a similar experiment on the tip of the root of Bindweed, the response being precisely the same. The rate of movement was in this case somewhat slower, but persisted for eight minutes. The application unilaterally of dilute sulphuric acid brought about exactly similar results in both cases. The unilateral application of thermal stimulus of moderate intensity, again, to the tip, induced movement away

from the source of stimulation in both shoot and root, as will be seen from the record given below (fig. 219). (b) Stronger stimulation. — A somewhat stronger stimulation of the same character caused a movement away, followed by movement towards, the source of stimulus, the resultant effect being neutral. We now pass on to the effects induced by still stronger stimulation. I have already explained how, when very strong stimulus is applied unilaterally at the tip of an organ, its first and transitory effect is to induce a positive turgidityvariation on the same side of the growing region. From this we inferred the occurrence of a convexity on that side, which would carry the tip away from the source of stimulus. The direct effect of stimulus next reaches the responding region, reversing the first effects and causing a negative turgidityvariation, which would, pari passu, induce a concavity, and carry the tip towards the source of stimulation.

In carrying out the experimental verification of these mechanical movements on a bud of Crocus, I found that an application of strong sulphuric acid on one side of the bud caused it to move first away from, and then very energetically towards, the direction in which the application was made. I next tried to determine the effect of a strong unilateral application of thermal stimulus on the root-tip of Bindweed. Fig. 219. Mechanical Re- sponses of Shoot, s, and of Root, R, to Unilateral Stimulus applied at the Tip

In this and following records the down curve indicates negative movement or away from source of stimulus. The up curve indicates positive or movement towards source of stimulus. The time-marks represent minutes (cf. fig. 213). Here, too, after a transient movement away, there was an energetic movement towards the stimulating heated wire (fig. 220). 2. Effect of unilateral stimulus applied at the responding growing region.— I shall now show that when stimulus is applied near the growing region, it induces effects which are opposite to those resulting from stimulation of the tip.

Fig. 220. Mechanical Response of Root of Bindweed to very strong Unilateral Stimulation applied at the Tip This causes a preliminary negative, followed by a positive, movement, that is to say towards the source of stimulus (cf. fig. 215). Fig. 221. Mechanical Re- sponses of Peduncle of Crocus, s, and Root of Bindweed, R, to Unilateral Thermal Stimulation at the Growing Region The responses are positive and towards the source of stimulus (cf. fig. 216).

(a) Moderate stimulus. — When moderate stimulation of any kind is applied unilaterally in the growing region, the consequent negative turgidity-variation of the side directly excited makes it concave ; and a positive turgidity-variation due to the indirect effect of stimulation occurs at the distal side, making that side convex. Thus the induced concavity of the proximal, and convexity of the distal, both conspire to cause a movement of the organ towards the source of stimulation. This is seen in the following records obtained with the peduncle of Crocus and the root of Bindweed, the stimulus used having been thermal (fig. 221).

(b) Strong or longcontinued stimulus. — I have already explained that with moderate stimulation the negative turgidity-variation and concomitant contraction of the proximal, and the positive turgidity-variation and concomitant expansion of the distal, conspire to induce a positive responsive curvature ; but when the stimulus is strong or longcontinued the excitation is conducted across the tissue to the distal side, which, now contracting, antagonises and reverses the action of the proximal. We have seen this exemplified in the electrical response given in fig. 218, where the first positive electrical response of the distal, indicating positive turgidity-variation, was afterwards neutralised and converted into negative by the transverse conduction of excitation.