Plant Response as a Means of Physiological Investigation

As the E.M.F. was progressively increased, however, we found in this and other plants a tendency towards the reversal of these normal polar effects. During the first, or Ay stage of this reversal, the excitatory value of the kathode was seen to undergo a diminution, and the anode, which normally had a depressing influence, was observed to have its property reversed, and to produce excitation. The result during this stage, therefore, was the exhibition of excitation at both kathode and anode at make.

With still higher E.M.F. the B stage was reached, and here there was a complete reversal of the normal polar effects. It was then found that the anode produced excitation at make, and the kathode at break. This reversal of polar effects under a high E.M.F. was further demonstrated by means of Death-response in plants, and Glow-response in animals. We have also seen that in consequence of progressive molecular change induced by fatigue, the normal polar effect tended to be reversed, and we have been able to trace the successive stages of such a reversal, in experiments on the plant Mimosa and on the firefly.

And, finally, specimens are occasionally found which, owing to molecular modifications of their tissues— modifications that a knowledge of their previous history could Under high E.M.F. the normal polar excitation tends to be reversed. In the A stage, both the anode and kathode excite at make, and either kathode or anode at break ; in the B btage — that is, with excessively high E.M.F. — it is the anode which excites at make, and the kathode at break.

The firefly under excitation exhibits glow-response. Under moderate E.M.F. it shows normal polar effects. Under a high E.M.F. it, like the plant, exhibits a reversal of these polar effects. Under fatigue, or other tissuemodification, normal polar effects tend to undergo reversal. Receptive excitability, conductivity, and motile excitability — Molecular model — Modification of motile excitability : {a) by anaesthetics— (b) by cold — (c) by fatigue— Variation of conductivity : (a) by cold — (b) by rise of temperature — (r) by fatigue — {(/) by anaesthetics — Variation of receptive excitability by ether — Conductivity versus excitability — Abolition of motile excitability without abolition of conductivity — Hydro-mechanical theory of transmission of stimulus untenable.

HITHERTO we have been concerned mainly with the peculiarities of the responding organ, by which the state of excitation is outwardly manifested. Very often, the responding organ is not directly stimulated, but a distant point is acted on by stimulus, and the state of excitation is transmitted through the intervening distance, by the conducting power of the tissue. In the actual life of a plant it is frequently the case that the stimulus impinging on a receptive area is transmitted along conducting channels, and is manifested, on reaching some responsive organ. The whole cycle of events is something like a telegraphic circuit, in which the message taken at a transmitting station is sent to a distance along conducting wires, and produces a signal at the distant or responding station.

In our experiments, for example, on Biophytum, as given in the previous chapter, the stimulus was applied at the petiole, and was conducted along certain channels. On reaching the specialised motile organ — the pulvinus — this transmitted stimulus caused a responsive depression of the leaflet. The petiole, during conduction of stimulus, was excited, but there was no conspicuous external evidence of this, because, first, the contractility of the tissue was relatively feeble, and

second, the differential excitability, on which responsive curvature depends, was also slight. It is only at the pulvinated organ that the state of excitation is conspicuously exhibited by motile response. In the case of the plant, therefore, we have to study the excitable property of the receptive area, the conducting property of the transmitting tissue, and that property of the responding organ by which the excitatory effect is outwardly manifested. It will be convenient to distinguish the excitability at the point of application of stimulus from that of the motor region, by using a specific term for the former. I shall therefore designate it as receptive excitability, or merely as receptivity, whereas the excitability of the motor region will be described simply as excitability. At the point of application, the stimulus comes from outside, and produces internal changes. In the motile region, the internal excitatory disturbances are manifested outwards. In the physiological study of excitation, some confusion is apt to arise from the failure to discriminate between these three factors. And this confusion becomes greater in those cases in which the area of motile excitability coincides with that of receptivity.

Molecular model. — The state of excitation being ultimately due to molecular upset from the position of equilibrium, we can understand that such a disturbance is propagated from molecule to molecule, till it reaches the responding organ. We may, perhaps, be enabled to visualise this better by means of a mechanical model. The individual molecules in our model should hold a position of stable equilibrium. When disturbed from this stable poise, they should return automatically to the equilibrium position ; and further, the derangement of one molecule should cause a subsequent disturbance of the next, and this disturbance should be transmitted from point to point.

These conditions are realised in the case of the following model, which consists of a row of small suspended spheres of cork, within each of which is placed a magnetic needle. Each sphere is now in stable equilibrium, under the directive action of the earth, and the mutual action of the needles ; hence the north pole of each needle, represented by the arrow-head, points to the north, which is, say, to the left. The disturbance of any individual sphere, say E, brings about the disturbance of its neighbour, and, owing to the mutual magnetic action between contiguous north and south poles, a derangement initiated in this way is transmitted onwards. Such a disturbance may be initiated by means, for instance, of

Fig. 95. Molecular Model Exhibiting (a) Excitability at the Receptive Area ; (b) Conductivity of Intervening Region ; and {c) Mechanical Response of Terminal Responder Disturbance is initiated at the sphere connected with E, by the magnetic action of the electro-magnet seen to the right. This disturbance is conducted by the intervening spheres and reaches the terminal responder, r. Molecular viscosity is increased by immersion of attached dampers in viscous fluid.

a small electro-magnet, placed at right angles to the molecular magnet in E. This electro-magnet is magnetised for a short time by the tapping of a key, which closes an electric current, causing a rotation of the sphere E. The intensity of this disturbing force, the stimulus, may be increased at will, by appropriate exaltation of the strength of the magnetising current (fig. 95). In such a row of molecules, then, that to the extreme right, E, is the point at which we shall initiate molecular disturbance. That is to say, it corresponds to the receptive

point. The intermediate row, C, is the conductor of disturbance ; and the last molecule, R, which may be provided with an index, or a reflecting mirror, by means of which the disturbance can be made conspicuous, represents the motile responder. We shall next observe how the extent of the distortion of each of the molecules from the position of equilibrium by a given force — that is to say, the amplitude of its response — is modified by the factor of molecular mobility. Under the action of certain agencies the freedom of molecular movement may be retarded, by variation of elasticity or of viscosity. We may, with our model, imitate the resultant molecular sluggishness, by means of dampers, which are seen in the diagram, attached to each sphere. The extent of damping is capable of increase by immersion of the damper in a viscous fluid. The response-curve of this particular sphere may now be taken by the usual method of a reflected spot of light. The curves thus obtained will show, firstly, that, the disturbing force remaining the same, diminished molecular mobility is attended by diminution of amplitude of response ; secondly, that this diminution may become so marked that visible response may disappear ; thirdly, that though, with a given moderate disturbance, response may thus be in abeyance, yet it may be restored if the disturbing force be made sufficiently strong ; and fourthly, that the sluggishness thus induced may also be exhibited by delay in the initiation of response, that is to say, by the prolongation of the latent period.

From such considerations, it is clear that if an agency which reduces molecular mobility be applied on the receptive area, then, inasmuch as the initiation of excitation is prevented, there will be no response exhibited by the motile organ, although the conducting power of the intervening tissue, and the motility of the responding organ, remain unchanged. Again, if the intervening conducting tissue be subjected to loss of molecular mobility by any means, the power of conduction will be either very much retarded, or abolished, the receptivity and excitability of the terminal points

remaining unaffected. And, finally, the excitability of the motor region may be depressed by certain agencies, and the stimulation, initiated at the receptive point, and transmitted through the intervening conducting channels, will nevertheless fail to find expression. We shall next proceed to demonstrate experimentally the influence of various agencies on the receptivity, on the conductivity, and on the excitability of the tissue. Variation of motile excitability : (a) Under anesthetics. — First we shall take the variation of excitability in the motor region. Let us then select a leaf of Biophytum and apply ether to the two terminal pairs of leaflets beyond D. Thermal stimulus is then applied at x , by touching with a

hot wire (fig. 96). As the 5£=^7^7 receptivity of the point of application, and the conductivity of the intervening Fig. 96. Effect of Ether in the Abolition . remam unimnaired Ether is applied to the two pairs of leaflets the excitatory disturbance- to the right of d ; stimulus is applied proceeds in the normal fact seen by the successive depressions of the leaflets. Owing, however, to the abolition of their excitability, the last two pairs remain unaffected.

A similar loss of excitability, due to the action of ether, may be demonstrated in Mimosa. On taking a stem provided with three motile leaves, A, B, and C, the pulvinus of B is touched with ether, and thermal stimulus is applied between A and B. The excitation is transmitted in both directions, up and down, as seen by the fall of the leaves A and C. But the intermediate leaf B fails to respond, showing that its excitability has been abolished by the ether.

{b) By effect of cold. — The prolonged application of cold, also, will produce, as would be expected, molecular sluggishness, with consequent loss of motor excitability. This may be shown by touching the small pulvinus of a leaflet of Biophytum with icewater. If stimulus now be applied on the petiole, it will be found that this particular leaflet will not respond. This loss of excitability will, however, be temporary, disappearing as the leaflet returns to its ordinary temperature, when it will be found to respond as usual.

A moderate application of cold does not altogether abolish the response, but the molecular sluggishness induced is shown in the prolongation of the latent period of response. It was found, for example, in an experiment on Biophytum that the latent period was sometimes prolonged by several seconds (p. 268). (c) By effect of fatigue. — We have already seen (p. 113) how the motile excitability of the plant-tissue is diminished by fatigue, as shown in the diminution of successive responses, when the intervening periods of rest are not sufficient for complete recovery. We have seen, too, that under strong and long-continued excitation the motile excitability is abolished ; and that it can be restored after the lapse of a sufficiently long resting period.

Variation of conductivity. — We shall next examine how the transmission of stimulus from point to point is affected by various external agencies. And, first, we shall refer back to the mechanical model (fig. 95). We there* saw how the sluggishness, induced in the intermediate molecules by plunging the dampers to a greater or less depth in a viscous liquid, retarded the transmission of disturbance through them. When this induced sluggishness is slight, the propagation will merely be slowed below the normal ; but when the sluggishness induced is great, the disturbance will not reach the responder R.

(a) By effect of cold. — We shall now proceed to investigate the effect of induced molecular sluggishness on the conductivity of a plant-tissue ; and for this purpose we shall first observe the influence of cold. In an experiment on Biophytum, I found that the normal velocity of transmission, depending on the conductivity, was 37 mm. per second ; but on subjecting the tissue to moderate cold, the velocity of transmission was reduced to 1*3 mm. per second, or nearly to one-third of its original value (p. 249) ; a still greater application of cold produces a temporary abolition of conductivity. This may be shown by touching

a given portion, E, of the petiole with ice, when moderate stimulus applied below such a point will not be transmitted across the lethargic area, and the motile leaflets beyond will Fig. 97. Experimental Demonstration remain unaffected. The nor- Cold or ether applied at s ; stimulus ever, be restored when the at x cannot be transmitted across e, tissue regains the temperature sphere, and a second similar application of stimulus will then be found to be conducted to the motile leaflets, producing successive depressions

(b) By rise of temperature. — We have seen how, in consequence of the molecular sluggishness induced by cold, the conductivity of the tissue is lowered. A rise of temperature might therefore be expected, by increasing molecular mobility, to enhance the conducting power. That this is the case is shown in detail in Chapter XX. In a leaf of Biophytum, for instance, it was found that a velocity of 37 mm. per second at 300 C. was increased at 350 C. to 7*4 mm., and at 370 C. to 9*1 mm. per second. Thus, by a rise of temperature of from 300 C. to 370 C. the conductivity of the tissue was increased to nearly three times its initial value.

(c) By effect of fatigue. — We have already seen (p. in) that motile response, and the transmission of excitation, are both alike expressions of the protoplasmic changes induced by stimulus... We there saw also that just as fatigue of motile excitability was exhibited by diminished motile response, so too a diminished speed of transmission exhibits fatigue of conductivity. An experiment will be described later (p. 245), which shows that in that case, under moderate fatigue, conductivity was diminished by 18 per cent, of its normal value.

The following experiments give us a further and striking demonstration of the diminution or abolition of conductivity under fatigue. If we take a leaf of Mimosa^ and excite it, by snipping off a terminal leaflet, borne on one of the four subpetioles, the stimulus, transmitted along the narrow conducting channel of that sub-petiole, and passing through the large channel of the petiole, will, on reaching the pulvinus, cause the fall of the leaf. After a suitable period of rest, the leaf will re-erect itself. If now the operation be several times repeated, by stimulating the same sub-petiole, it will be found eventually that the leaf no longer responds. That this is due to the fatigue in conductivity of the sub-petiole may be proved, by snipping a leaflet off a second subpetiole, which will be found to conduct the stimulus, and produce depression of the leaf, as did the first sub-petiole when fresh. It will be noticed here that the excitation which abolished the conductivity of the first sub-petiole, did not abolish that of the main petiole. This is due to the fact that the somewhat enfeebled stimulus on reaching the petiole is spread over a larger channel, and therefore the strain-effect which it produces there is relatively much less.

(d) By effect of ancesthetics. — We shall now study the effect of anaesthetics on conductivity. This may be shown by the local application of ether to the petiole, in the intermediate portion of a Biophytum leaf, beyond, say, the first three pairs of leaflets. Stimulus applied below this area will be conducted to it, as seen by the fall of intervening leaflets, but its further passage will be blocked, and neither the leaflets of the etherised area, nor those beyond, will show response. That this abolition of conductivity, however, is only temporary, is seen when the stimulus is repeated after blowing off the ether vapour. All the leaflets, from first to last, will now be found to respond. If etherisation, however, be carried too far, the abolition of conductivity persists for a long time,

and its restoration may not take place for one or more hours. An interesting experiment, on the abolition of conductivity under ether, was performed with a specimen of Biophytum having eight leaves of fairly equal sensitiveness. Of these, four, taken alternately, had ether applied on those portions of their petioles which were next to the stem. On now applying strong thermal stimulus on the stem, the state Fig. 98. Diagrammatic Representation of Experiment on Biophytum

Ether was applied on the alternate petioles marked 1, 2, 3, 4. Stimulus at x is prevented from acting on the leaflets of these leaves. The same diagram also represents the subsequent experiment on variation of receptive excitability. Ether is applied at E instead of on the petioles. Stimulus applied at E now produces no excitation. of excitation radiated to all the leaves. But the passage of stimulus through the four etherised petioles was blocked, and no effect was produced on their leaflets. The leaflets of the non-etherised leaves, however, promptly responded, falling one after another from the centre outwards (fig. 98).

Variation of receptivity by anaesthetics. — Lastly, we shall inquire into the variation of excitability at the point of application of stimulus, that is to say, into the modification of the plant's receptivity, under the action of an external agent. It is to be borne in mind that stimulus coming from without directly affects the outer layer of the tissue, and the excitation may then proceed inwards and in lateral directions, by conduction. The effect of ether in diminishing receptive excitability may be demonstrated by taking, as in the last case, a specimen of Biophytum. We first test the specimen by applying a moderate stimulus on the stem at E. The excitation thus initiated at the receptive area is transmitted to the leaves, and causes depression of their leaflets. When these have recovered, ether is applied locally on the area E. On now repeating the stimulation, we find that none of the leaflets respond. Since the conductivity of the intervening tissue and the excitability of the motile organs have remained unaffected, it is clear that the failure to respond is in this case due to the depression of receptive excitability by ether.

A tissue, however, whose superficial excitability is depressed in this way, may still retain the power of conduction. This is shown by applying stimulus on the stem, as in the last experiment, but at x , below the etherised ring E. The stimulus is now shown to be transmitted, by the fall of the motile leaflets. The explanation of this difference probably lies in the fact that the molecular torpidity induced by the etherisation does not extend very deep, unless it has been excessive and long-continued. In that case, the internal layer of the tissue, remaining unaffected, would serve as the channel of conduction. This view is supported by the fact which I have noticed, that it is much easier to produce a complete block to the passage of stimulation, when a relatively thin tissue, such as the petiole of a leaf, is etherised. It is much more difficult, on the other hand, to do this with a thick stem.

We saw from the molecular model (fig. 94) that though when the molecules were sluggish no response could be obtained to moderate stimulus, yet when the stimulus was very strong response could be brought about. Similarly, in experimenting on plants, I have found it possible, by careful graduation of etherisation, to arrange matters in such a way that while moderate intensity of stimulus, applied on the etherised area, failed to evoke a responsive movement of the distant leaflet, a powerful stimulus was able to do so.