Plant Response as a Means of Physiological Investigation

In the case of responsive growth-curvature we obtain results precisely similar. Long-continued unilateral stimulation is here often found to neutralise the first or normal effect Or if, again, the unilateral stimulation be very strong, the proximal side is liable to become fatigued, and the response of the distal to transmitted stimulus being thus predominant, a responsive movement occurs which is reversed or negative, that is to say, away from the source of stimulation. Examples of this will be seen in Chapter XLII. I shall also presently give a demonstration of similar preliminary effects with subsequent transverse conduction, giving rise to reversed effects, in the case of the motile response of Mimosa.

We have thus seen that growth-curvature is induced by unequal variations of turgidity, on the diametrically opposite sides of the growing region. We have seen that the effect of indirect stimulation is a positive turgidity-variation. When a feebly conducting tissue is unilaterally subjected to moderate stimulation, the direct excitatory effect cannot be transmitted far, and it is the indirect effect which reaches the responding region, R (fig. 222), and induces convexity. Hence we obtain the typical examples of this effect by stimulating the tip, T, of either root or shoot. The sensibility of these regions is itself in no way different from that of any other portion of

the plant-tissue, for they respond to direct external stimulation by contraction. But their power of transmitting stimulus is relatively feeble. For this reason, under ordinary circumstances, they transmit only the indirect effect of stimulus, and it is only when the unilateral stimulus is very strong that the direct excitatory effect is transmitted, inducing the opposite to the usual result, in the responsive concavity of the same side of the growing region.

The fact that the sensitiveness of the tip is not fundamentally different from that of the growing region may be demonstrated by applying stimulus to a given point D in the growing region, and observing the responsive effect induced at R, diametrically opposite. The power of the tissue to conduct stimulus transversely being feeble, the result is in this case the same as in that of the ordinary longitudinal transmission from the tip ; that is to say, it is the indirect effect that reaches the diametrically opposite point, R, and induces convexity there, this effect being aided, as it happens, by the concavity of the proximal side, due to the direct effect of stimulation. Here again, as before, a stronger or long-continued stimulus may later transmit the direct effect, and neutralise or reverse this first responsive curvature. A third case arises when unilateral stimulus is applied at L, lower down on the stem, at some distance from the responding region, and if this be sufficiently feeble, it will be the indirect effect which will reach the same side of the responding region, and produce convexity there. Stronger or longcontinued stimulation will in this case, as before, neutralise or reverse the first effect.

FlG. 222. Diagram showing the various Responsive Effects induced at the Growing Region, r When moderate stimulus is applied unilaterally at the tip, T, or distant point, L, it is the indirect effect that reaches R, and produces convexity. The same convexity of R is induced by stimulation of the transverse point, D; but here the induced curvature is aided by the concavity of the directly excited D. I have been at some pains to make these examples of the direct and indirect effects of stimulus clear ; for all the complex curvatures of growth, which appear at first sight so anomalous, are ultimately resolvable into these. And since the subject is so important, I shall add still another demonstration, which is capable of easy repetition, and will be found to be striking and convincing.

Experiments on the direct and indirect effects of stimulus on Mimosa.— We have now seen that all growthcurvatures may be analysed ^J_ into, (i) that contraction, with turgidity-variation that constitutes the direct, or transmitted direct, effect of stimulus ; and (2) that expansion, with consequent convexity, that is concomitant to the positive turgidity-variation, which constitutes the indirect effect of stimulus. Now, the negative turgidity-variation is exhibited in the case of the motile leaf of Mimosa by depression, and the positive turgidityvariation by erection.

I shall now explain the experimental arrangements by which the plant itself may be made to record these opposite effects. The indicating leaf is attached to the short arm of a long writing lever. This lever consists of the quill of a long tail-feather of a peacock. Its short arm, 1 cm. in length, is tied by a thread to the petiole of the leaf. A fine needle is passed through the quill, and rests on frictionless supports which may be glass tubes. The longer arm of the lever, 10 cm. in length, has a piece of bent aluminium, with a sharp point, tied to the end,

Fig. 223. Experimental Arrangement for obtaining Records on Smoked Drum of Responses given to Direct and In- direct Stimulation by Leaf of Mimosa Thermal stimulator at s produces direct stimulation, and consequent fall of leaf. Moderate stimulation, at a distant point, s,,, gives rise to indirect effect of erection. to serve as a writer. This writing-point is, by the elasticity of the feather, pressed lightly against the smoked-paper surface of a vertical revolving drum (fig. 223).

When the leaf is under no stimulation, either direct or transmitted, the record is a horizontal line. But true excitation produces a depression of the leaf, causing an up curve. The erectile response, on the other hand, which is due to indirect stimulation, produces a down curve. The records given in the following figures are accurate reproductions of some which were taken in this way (fig. 224). (a) Direct stimulation. — Stimulus is applied by the close proximity of a V-shaped platinum wire, heated electrically. Its intensity is varied by varying either the distance of the stimu-, lating wire or the intensity of the heating current. When stimulation is now applied directly at S, that is to say, near the responding organ, response takes place by a negative turgidity-variation, producing a fall of the leaf. This is seen in the up curve (fig. 224, a).

{b) Indirect stimulation, longitudinal transmission. — Stimulus of moderate intensity is now applied lower down on the same side of the stem, at S„. This is observed first to induce the positive turgidityvariation, causing erectile response, which is due to the indirect effect of stimulus, and is shown in the preliminary down twitch of the curve (fig. 2 24, b). Later, the d irect effect is trans- (a) record of responsive fall when stimulus applied near the responding organ (cf. fig. 22 1 ) ; (b) response when stimulus is applied on same side, but at greater distance, s/;. A preliminary erectile response is here followed by the true excitatory depression. This is due to the indirect effect first transmitted being succeeded by the direct. Had the stimulus applied been feebler, or more distant, there would have been only the first, or indirect erectile effect, similar to fig. 225 (cf. fig. 220).

mitted, causing the fall of the leaf, as shown in the up curve. When the stimulus is feebler, or applied at a still greater distance, the indirect effect alone reaches the organ, and only the erectile response, due to positive turgidity-variation, results, being similar to that shown in the next record (fig. 225). (c) Indirect stimulation, transverse transmission. — We next obtain the very interesting case in which feeble stimulus is applied at the transverse point s4. The record (fig. 225) shows that we have here an erectile response due to the positive turgidity-variation of indirect stimulation. But if this transverse stimulus be made strong or be long continued, the direct effect is transmitted somewhat later, and, in that case, we obtain a fall of the leaf, .preceded by the positive erectile twitch, which is similar to that shown in the previous record (fig. 224, b).

The curious response of an Arisaema. — This fact will explain a very remarkable phenomenon which I have noticed in certain species of Ariscema, that grow on the mountains round Darjeeling, at a height of about 7,000 feet. This plant, before flowering, consists of a long petiole bearing a terminal whorl of leaflets, which are arranged like rays in a strictly horizontal plane. Later, however, the inflorescence, borne on a peduncle enclosed within its spathe, breaks out from one side of this petiole. Unilateral mechanical stimulation is thus undoubtedly brought about, and gives rise to indirect stimulation on the distal side, which, as we have just seen, causes an erectile mechanical response. In the case of this Ariscema, it is a striking fact that immediately after flowering, the most distal leaflet of the whorl — that is to say, the

Fig. 225. Erectile Re- sponse of Leaf of Mimosa due to Transmission of Indirect Effect to Distal Side, when Proximal, sy, is Stimulated If stimulus were stronger, this would be followed by the fall of the leaf due to the later transmission of true excitation. The response would then become like that of fig. 224 (b) (cf. fig. 219). leaflet which is situated in the diametrically opposite line — hitherto horizontal, becomes abruptly vertical (fig. 226). This is the only leaflet which stands out from its fellows, and it is invariably found to be situated on the line diametrically opposite to the flower, such differentiation being induced only after flowering.

All the variations exhibited by diverse forms of response — electrical, mechanical, responsive acceleration or retardation of growth, and growth-curvatures — are only so many Before flowering the leaflets lie in a horizontal plane, as seen in the figure to the left ; but when inflorescence breaks through unilaterally, the indirect stimulation of the distal side causes erection of the diametrically opposite leaflet, as seen in figure to the right.

expressions of these two fundamental phenomena, the effects of direct and indirect stimulation. It is these two variables which, conjoined with stimulus unilateral or diffuse, give us all those manifold effects that at first sight would appear to belong to different classes of phenomena. That such a unity does actually underlie them all may be seen at a glance from the following concise statement of responsive effects, induced in pulvini, pulvinoids, and in growing regions which act as pulvinoids.

Table showing Responsive Effects Common to Pulvini, pljlvinoids, and growing organs Remarks. — It will be remembered that the indirect effect is a secondary consequence of the direct contractile effect of stimulus on the excited point. Thus the motive power is the active contraction of that point. The indirect effect, described as No. 7, is exemplified by the moderate unilateral stimulation of the tip of shoot or root. When this stimulus is stronger, or sufficiently long continued, we have a transmission of the direct effect of stimulus, and case No. 7 is displaced by case No. 5.

1 The induction of concavity, of either upper or lower side of the pulvinus, by local stimulation will be found demonstrated in a subsequent chapter. Confining our attention to the effects induced at the growing region, we arrive at the following laws of growthcurvature. Laws of responsive growth-curvature.— It must be remembered here that the effect of indirect stimulation is to cause an increase in the rate of growth, and that of direct stimulation a retardation of the rate. By a positive effect is meant a responsive mechanical movement towards, and by negative, away from, the source of stimulation.

1 . Unilateral stimulus of moderate intensity, applied at the tip of root or shoot, gives rise to a negative effect, the tip being moved away. 2. Stronger or longcontinued stimulus, applied unilaterally at the tip of root or shoot — being conducted gradually to the growing region — results in a neutralisation of the first negative by a subsequent positive effect. Or there may be a resultant positive, due to the predominance of the transmitted effect.

3. Direct unilateral stimulation of moderate intensity on the growing region causes a positive response or movement towards stimulus. 4. Strong or long-continued unilateral stimulation of the growing region, on account of the transmission of effect to the distal side, may give rise either to neutralisation of the normal, or to a reversed or negative effect, that is to say, to movement away from stimulus. All the mechanical effects induced at the responsive growing region of either root or shoot, by unilateral stimulation, may be summarised as follows, it being understood that positive response means movement towards, and negative, movement away from, stimulus :

(1) Positive response is induced, first, by direct unilateral application of stimulus on the growing region ; second, by the long-continued unilateral application of moderately strong stimulus at the tip. (2) Negative response is induced, first, by the unilateral application of feeble stimulus at the tip ; and second, by longcontinued unilateral application of strong stimulus at the growing region, causing fatigue of the proximal, and transmission of true excitation to the distal, side.

No specific difference as regards their responses between shoot and root — Darwinian curvature — Localisation of geotropic sensibility at the root-tip— Experiments as to whether amputation of root-tip abolishes excitability — The tip of the root the organ of graviperception — The perceptive versus the responding organ — True perceptive region. We have in the course of the last chapter studied in detail the expression in growth-curvatures of the direct and indirect effects of stimulus, and have found them to be, under known conditions, very definite. We have seen that the responsive effects produced at the tips of root and shoot are in no waydifferent from those which have been observed in other parts of the tissue. The specific sensitiveness hitherto so generally attributed to the root-tip is thus found not to be justified. The differences of effect which have been noticed, according as tip or responding region is the point of stimulation, we have seen to be due merely to the transmission of the indirect instead of the direct effect. I shall next proceed to show that the opposite curvatures induced in shoot and root by stimulus of gravitation are really due, not to two distinct sensibilities positive and negative, but to the action of a single stimulus which acts in one case directly, and in the other indirectly.

Darwinian curvature. — It has thus been shown that the responsive curvature manifested by the radicle on stimulation of its tip is not characteristic of roots alone, but, under similar circumstances, of the shoot also. It has not, then, been specially evolved for the advantage of the plant. As proving this, the abrupt conclusion of certain experiments of my own has a peculiar interest. The tip of the root was in these cases subjected to strong unilateral stimulation by the proximity of a heated platinum wire. Instead of responding, however, by movement away, as would have been the case had the advantage of the plant been the primary object of its action, the root-tip gave only a preliminary spasmodic twitch in the negative direction, and then reversed its movement ; the organ now turned towards the heating wire, fell upon it, and was burnt.1 There is thus no protective adaptation here, any more than in the case of the moth which is impelled to throw itself upon the destroying flame. No choice exists for either of these, for, in both alike, the movement is due to the working of the inexorable laws which govern the phenomenon of response. In the case of the plant, that increased turgidity which is the indirect effect of a distant stimulus always induces an increased rate of growth ; but when the stimulus is sufficiently strong or long continued, its direct or true excitatory effect, being transmitted, brings about retardation of growth. And each of the diverse curvatures of growth, induced by the unilateral application of stimulus, forms a particular case of the working out of these two laws, of the direct and indirect effects of stimulation.

Having now arrived at certain definite conclusions as to the mechanism of the responsive curvature induced by unilateral stimulation of the tip of root or shoot, it will be well to pass in rapid review the exhaustive series of experiments on the responsive behaviour of the tip of the radicle, which were carried out by Darwin, more especially as some of the cases which he noted as somewhat exceptional will be found, in view of the investigation which I have described, to be susceptible of very simple explanation. He produced unilateral stimulation in three different ways, first, by attaching minute fragments of cardboard to one side of the

1 The same thing happens, in a still more striking manner, when the wire is placed in front of the growing region, and suddenly heated. The excitatory effect on the proximal side is then so great that the organ at once rushes upon the stimulating wire and is scorched. This occurs before the distal side could be. excited by the transmitted effect of stimulus. root-tip by means of gum or shellac varnish. The moderate and constant irritation which was produced in this way was usually found to induce a convexity on the same side of the growing region. His second method was chemical. He touched one side of the tip with silver nitrate. This also, generally speaking, induced a convexity similar to the last. The third and last method of unilateral stimulation employed by Darwin was a slanting cut, resulting, in the majority of cases, in the same responsive curvature as before. All these cases, it will be understood, are illustrations of the indirect effect of moderate stimulation.

But we have seen — in the second law of responsive growth-curvature — that under the long-continued action of a stimulus sufficiently strong, this first, or negative, effect is neutralised by the subsequent conduction of excitation (p. 52). The degree of such conduction and the power of neutralisation will obviously depend on the conducting power of the particular specimen. Now, it was found by Darwin, in several of his experiments, that the negative effect was followed by neutralisation. This he regarded as somewhat exceptional. We have seen* however, that such a result, under certain circumstances, was to be expected.

We have also seen, in the same law of responsive growthcurvature, that when the transmitted stimulus is strong, it induces a reversed or positive curvature. And in some of Darwin's experiments also, especially those in which he used the strong stimulation of caustic, this was observed. Finally, in some six cases, Darwin found that on applying unilateral stimulation to the tip there followed an induced concavity. With regard to this it need only be said that the sensibility of the tip to direct stimulation is not specifically different from that of any other tissue. All tissues contract, in response to direct stimulation. The difference between the contractile powers of the tip and the growing region is one merely of degree, that of the latter being relatively greater, and for certain reasons, to be described later, the more evident. Visible contraction

of the tip can only be seen under considerable stimulation, or when that part of the tissue is highly excitable. Localisation of geotropic sensibility at the root-tip.— I next turn to Darwin's very important demonstration of geotropic perception as residing in the root-tip. He showed this by extensive researches on the fact observed by Ciesielski, that on the decapitation of the root-tip the geotropic action is found to disappear. Various objections have been urged against this view, which I shall be able to show to be groundless.

Sachs, for instance, has argued that if such sensibility resided in the root-tip, there is no reason why the tip of the shoot should not exhibit the same. This argument is not, however, valid, inasmuch as the statolithic or other elements, which by their weight cause stimulation, may be concentrated locally in the root-tip, and more diffusely distributed in the shoot. And that the general sensibility of the root-tip to other forms of stimulation is not different from that of the shoot-tip I have already fully demonstrated. Francis Darwin has shown, again, that localised graviperceptive areas may occur in organs other than the root. In the seedling of Soi-ghum, for example, he finds this sensibility to reside in the cotyledons.

The second objection that has been raised is that the shock of amputation might either abolish the general excitability or arrest the growth of the root, on which the responsive growthcurvature depends. With regard to the abolition of excitability, it is true that the effect of a strong stimulus, such as that of cut, would persist for some time, yet there is always recovery, after a longer or shorter interval. I have tested the persistence of the fatigue caused by amputation, by the electrical method, and I find that with certain plants, such as Bryophyllum, the recovery from the effect of amputation-shock is very rapid, taking place in the course of about a minute. The longest persistence of the after-effect which I have been able to observe was in the case of Colocasia, where it lasted for a period of

from forty-five to sixty minutes. It is thus unlikely that amputation would permanently abolish the power of renewed response. With regard, next, to the contention that amputation would be liable to arrest that growth of the root on which responsive curvature is supposed to depend, it is to be borne in mind that though growth-activity is a sign of excitability, yet we may have excitability without growth. Hence it is quite possible that responsive curvature might be initiated in a tissue which was not beforehand in a condition of growth by the stimulus of gravity, as indeed was found to be the case with grass haulms.