Plant Response as a Means of Physiological Investigation

Receptivity versus motile excitability. — At the beginning of the present chapter, I drew attention to the necessity of discriminating between the functions of receptivity and motile excitability. It is only by carefully distinguishing these that we can possibly come to an understanding of certain apparent contradictions. Let us suppose that stimulus is applied on a motile organ, say the pulvinus of Mimosa. In this particular case, the areas of receptivity and motile excitability are coincident. By the reception of stimulus the motile machinery is eventually set in motion. The mobility of the superficial particles will thus determine the receptivity and •the inner mechanism of the organ, the motile excitability. The motile excitability is measured by the amplitude of response. Receptivity, on the other hand, may be partially discriminated by (i) the length of the latent period, and (2) the value of the minimally effective stimulus.

When a tissue is cooled, say to y° C. or lower, its receptivity and motile excitability both undergo diminution. Hence the latent period is prolonged (p. 268), and the stimulus which was formerly effective becomes ineffective. In such a case, where the two factors conspire, it is difficult to distinguish between the relative effects of receptivity and motile excitability. But when, on the other hand, the temperature is raised, say to 350 C, the amplitude of contractile response, by which we are in the habit of gauging the motile excitability, is generally speaking diminished (fig. 79). Hence we are apt to infer that excitability in general is decreased at 350 C.

But if we test this question by means of the minimally effective stimulus, we arrive at a very different conclusion. For example, taking a specimen of Biophytum at 300 C, I found that the minimally effective stimulus was given by a condenser charged to twenty-two volts, whereas when the temperature was raised to 350 C. the minimally effective timulus was a charge of fourteen volts. It is clear from this that the excitability at 350 C. is higher than at 300 C.1 Hence we arrive at two conclusions directly opposed to each other.

This apparent anomaly completely disappears, however, in the light of the distinction between the receptive and motile excitabilities ; for it was said that it was the mobility of the superficial particles which determined the receptivity, and this is evidently enhanced by rise of temperature. The amplitude of mechanical response, however, by which e measure the motile excitability, is not solely dependent on molecular mobility. This mechanical response is, as we have seen, brought about by diminution of turgor, and any agent which produced increase of turgor would act antagonistically, and thus diminish the motile expression of excitation. For example, we have seen that a pulvinus of Mimosa, when highly turgid, failed to show any motile response, though excited (p. 49). Now, it will be shown (p. 400), that rise of temperature has the effect of increasing turgor. Hence the diminution of mechanical response with increasing temperature does not indicate diminution of excitability in general, but rather the setting in of an antagonistic force, whose influence will be to increase the" force of recovery from molecular distortion. It should be mentioned, however, that there is a limit to the enhancement of excitability by rise of temperature ; for the molecular disturbance caused by heat will when excessive be detrimental to response.

Excitability versus conductivity. — The same considerations which have thus enabled us to distinguish between receptivity and motile excitability, will also enable us to see the difference between motile excitability and conductivity. We have seen, for example, that at 350 C. the conductivity in a given specimen of Biophytum was almost three times as great as at 300 C. in spite of the fact that, as just explained, contractile response is considerably diminished at high

1 It will be found in Chapter XXX. that growth, which is a phenomenon of excitatory response, is, in the case of many plants, at its maximum at or near temperatures. This distinction between the effects of conductivity and of excitability is especially important, since by its means we are enabled to explain certain facts apparently anomalous, which seem at first sight to lend support to the hydro-mechanical theory of excitation. I have shown that, under normal conditions, the intensity of excitation must exceed a certain value before it can be manifested as mechanical response. I have also shown that under unfavourable circumstances, motile excitability is abolished earlier than conductivity. An excited tissue may thus conduct stimulus, without itself exhibiting any motile indication. Numerous examples of such a state of things may be cited. It must be borne in mind that the mechanical indication of the state of excitation can be afforded by a pulvinated organ, only when there is some difference of excitability as between its upper and lower halves. If this difference of excitability be in any manner reduced or diminished, there will be a failure of the mechanical response. In old leaves of Biophytunu for example, not only is the general excitability diminished, but the differential excitability also has disappeared. Hence, excitation of such leaves gives rise to no local excitatory response of the leaflets. But that the leaf is still nevertheless excitable, and can transmit that state of excitation, is shown by the fact that on stimulating it strongly, the leaflets of younger leaves at a distance are, after a time, seen to be depressed in serial succession. This proves that, though unable itself to give the motile indication, the leaf was capable of receiving and transmitting the state of excitation. Similarly, it may be shown that a tissue whose motile excitability is temporarily abolished, by, say, the application of ether, may, nevertheless, be the conductor of stimulation.

In order to demonstrate this, let us take a plant of Biophytum, and expose some of the leaflets of a particular leaf to ether-vapour. Strong stimulation of that portion of the petiole which bears them, will now fail to induce movement of the leaflet in the etherised region ; but the excitation is found to be conducted through the anaesthetised area, and to produce responsive depression, not only of the leaflets beyond, but also of those of other leaves.

This experiment is important in its relation to the theory of the mode of transmission of excitation. I have already adduced conclusive proofs that the conduction of stimulus is dependent, not on the mere mechanical transmission of hydrostatic disturbance, but on the propagation of protoplasmic changes. Strong support has been lent to the hydro-mechanical theory by a classical experiment in which the pulvinus of a leaf of Mimosa was chloroformed. On then strongly exciting the leaflets of this leaf, the excitation was found to be conducted across the anaesthetised pulvinus and to produce depression of leaves beyond. At first sight it was natural to suppose that, as the motile excitability of the pulvinus was abolished by chloroform, the conductivity must also have been abolished. It was therefore inferred that, unlike the conduction of stimulus in animal tissues, where such transmission takes place by the propagation of protoplasmic changes, the conduction of excitation in the plant was purely mechanical. It will be seen, however, that the assumption on which this conclusion is based — that conduction must necessarily be abolished, with the abolition of motor excitability — has been invalidated by the experiments which I have just described.

In the present chapter, then, it has been shown that those agencies which, like cold, anaesthetics, and fatigue, diminish molecular mobility, also diminish the excitability and conductivity of the plant-tissue. I shall in the next chapter describe a series of experiments on the profound excitatory changes, of opposite character, which are induced in the experimental tissue, by the passage of an electrical current, the nature of such changes being dependent on the question whether the current enters or leaves the tissue at a given point. It must be added that this series of observations will be found to offer a further disproof of the hydro-mechanical theory of conduction of stimulus.

Motile excitability is temporarily abolished by anaesthetics. Strong application of cold produces a temporary abolition of motile excitability. Moderate application of cold prolongs the latent period. Similarly, fatigue produces a diminution or abolition of motile excitability ; and this is restored, after a sufficient period of rest. Conductivity, similarly, undergoes diminution as the effect of cold, anaesthetics, and fatigue. Receptive excitability, again, undergoes diminution or abolition by the action of similar agencies.

Conductivity may persist even after the abolition of motile excitability. Hence a strong stimulus may be conducted through a region which exhibits no motile excitability. The anode acts as a block to the transmission of stimulus — Opposite effect of kathode — Experiments on Biophytum, showing variations of conductivity by anode and kathode respectively — Experiments on Mimosa, showing increase of motile excitability at or near the kathode, and diminution of motile excitability at or near the anode — Curious ' development ' of response, near the kathode.

We have seen in the last chapter that on account of the diminished molecular mobility caused by physical and chemical agents, the response underwent a diminution. It was also seen that this reduction of molecular mobility found expression in the diminution of conductivity and excitability. External agents, like cold and ether, produce a temporary reduction of mobility, after which there is a revival to the original condition on the removal of the depressing agents. But certain other agents, such as poisons, produce permanent immobility, from which there is no recovery of response. The tissue is then said to be ' killed.'

Returning now to the molecular model, described in the last chapter, we see that while stimulus causes molecular upset, yet, at the same time, the force which restores the molecule to its equilibrium position, or, in other words, that which determines its stability, resists such an upset. Let us then first imagine the molecular model to be under the moderate directive action of the earth's magnetism. The stability of the individual molecule will thus be neither too great nor too small, and we shall call this, for convenience, the normal stability. This stability may further be increased by increasing the external directive force with the help of an auxiliary magnet, arranged in a suitable manner. Or it may

be decreased, below the normal, by the action of an external magnet which reduces the earth's directive force. On now obtaining responses to a uniform disturbing force, under these three conditions of normal, increased, and diminished stability, we shall find that while in the first case we get moderate response, in the second the response is very much diminished (and may even disappear entirely, when the stability is very great), and in the third it becomes exalted.

An-electrotonus and kat-electrotonus. — I shall now proceed to show the opposite effects of the anode and kathode on molecular responsiveness, during the passage of an electrical current through a plant-tissue. This change, induced by an electrical current, is known as electrotonus, and the effect due to the kathode is distinguished as katelectrotonus, while that due to the anode is known as anelectrotonus. It is probable that here, also, the variation of sensibility is brought about by the variation of molecular mobility, and that this is induced by an increase or diminution in the conditions of stability, as in the model. These opposite variations of the susceptibility to excitation, due to the anode and kathode respectively, will be demonstrated by the changes which they induce in the conductivity and excitability of the tissue.

In the chapter on the Excitatory Polar Effects of Currents, the intensity of the E.M.F. used was such that the excitation caused by the kathode was visibly manifested in the motile effects to which it gave rise. In the present chapter, however, we shall have to deal with latent excitatory effects, the E.M.F. used not being sufficient to give rise to any immediate external reaction. In the cases referred to, again, the distinctive action of the anode could not be demonstrated, inasmuch as under ordinary conditions it could not give rise to any motile indication. It will now, however, be shown that the effect of the anode is one of depression, or the opposite of that of the kathode. In studying variations of conductivity we have to remember that when the conductivity of a tissue is great, the state of excitation is transmitted

The progressive wave of excitation, initiated at x , stopped by anode, one pair of leaflets to its left. either with greater velocity or to a greater distance ; but if conductivity be in any way diminished, the distance to which the excitatory disturbance will be transmitted, will be correspondingly reduced. The anodic block. — In order to demonstrate the depressing action of the anode, I took a leaf of Biophytum, and sent a current through portions of it, entering at A, the anode, and leaving at K, or kathode (fig. 99). The E.M.F. used was two volts, and was thus insufficient to cause responsive action. In this and the following experiments, it will be understood, unless the contrary is stated, that the intensity of the electrotonic currents was not such as to create any direct action stimulus was now applied at x, found to be stopped at a distance of one pair of leaflets to the left of A. This shows that the depressing effect of the anode acts as a block to the passage of stimulus, and that such depressing action extends to some distance beyond the anode itself.

Experiments showing differences of anode and kathode. — In order to show that the kathode acts differently from the anode, not offering a block, but rather facilitating the passage of stimulation, I performed another experiment on a leaf similar to the last. In that case, the anode was near the point of application of stimulus. I now made the nearer electrode kathode. On next applying the usual stimulus, the excitatory wave passed on through the kathodic area, producing successive

Fig. 100. Experiment showing the Transmission of Excitatory Wave through Kathodic Area, and its Stoppage by the Anode fall of leaflets, and was only stopped by the depressing action of the anode, which this time extended to a distance of two pairs of leaflets to the left of A (fig. ioo). The next experiment was devised to show the opposite effects of anode and kathode simultaneously. For this, the xg|W)(ifl wave-systems were found V) W ^ T^ > to start from the excited

Opposite Effects of Anode and Kathode That towards K not only on Transmission of Excitation . , „ • i Excitation is transmitted through great beyond it, causing the distances in the kathodic region, but depression of all the leaflimited in the anodic. . lets, six pairs in number, on that side. But the excitatory wave that travelled towards A passed through only two pairs of leaflets, and was stopped at a point one pair to the left of the anode (fig. ioi).

Electrotonic variation of motile excitability. — We have seen that protoplasmic excitability finds expression in, among other things, the conductivity and motile response of the tissue. We have seen also how the former, that is to say, the conductivity, is modified in opposite ways by the influence of the anode and kathode. I shall now proceed to describe experiments in which the opposite character of the effects at anode and kathode is still more strikingly demonstrated by the exaltation at kathode, and depression at anode, of the motile excitability.

I used two pairs of electrodes, the first pair, KA, for the purpose of producing stimulation ; and the second pair, K'a', in order to produce variation of excitability, through electrotonus (fig. 102) ; or vice versa. The first pair was applied on the stem, the kathode K being in contact with the pulvinus of the lateral leaf, k'a' were applied on the petiole of that leaf. The plant was very sensitive, and in order that there should be no responsive fall, by the direct and unaided

excitatory action of K, the current due to an E.M.F. of two volts was reduced, by separating A from K, and thus interposing a greater resistance. A distance was thus found — i.e. 10 cm. — such that, on completing the AK circuit, the excitation was not sufficient to produce response of the leaf. The AK circuit was now opened, and adjustments made with the second circuit a'k', with an E.M.F. of two volts, in such a way that, on completing that circuit alone, there was no response of the leaf. Owing to the shorter length of petiole available — i.e. 3 cm. — the current could not be reduced to the requisite amount by simply increasing the interpolar distance. An external resistance had therefore to be added, in order to attain the desired condition. Thus either circuit, acting alone, was ineffective.

Kat-electrotonic increase of excitability. — Now, in order to show the increase of excitability in the pulvinus at K, as induced by the neighbourhood of kathode K', we first complete the a'k' circuit. This, as has been said, is ineffective. But now, on making the AK circuit, its previously ineffective stimulus becomes effective, and the leaf responds. From this it will be seen that during the passage of a current through the a'k' circuit, a point in the neighbourhood of the kathode K', that is to say, the pulvinus, is rendered more excitable.

This experiment may be varied by first making the AK, which is now the electrotonic, circuit, and then completing a'k' Fig. 102. Diagrammatic Representation of Electrical Connections in Mimosa to Exhibit Variation of Motile Excitability, induced by Anode and Kathode In the first of these experiments, the a'k' circuit is electrotonic and the AK circuit excitatory. In the second and third experiments the AK circuit is made electrotonic, and a'k' excitatory. In the third experiment, A and K are reversed.

for the purpose of stimulation. It is then found that the hitherto ineffective stimulus of a'k' is thus rendered effective. An-electrotonic depression of excitability. — The depressing action of the anode has been already demonstrated in the case of Biophytum (p. 234). The following experiment exhibits the same effect in a different manner in the case of Mimosa. In this instance, I used an E.M.F. of four volts in each of the two circuits AK and a'k'. When each circuit was made separately, the leaf responded by depression. At make, then, of one of the circuits the leaf responds, but as the stimulus is only effective at make, the leaf recovers during the continuation of the current. After this, on the second circuit being completed, the excitement at make again caused response.

The experiment was now modified in the following way. The AK circuit was reversed, the pulvinus becoming anode. The excitation of the distant kathode, however, was still strong enough to cause response of the leaf. The current was kept on till the leaf recovered. On now making the A'k' circuit, the leaf did not respond. Thus the stimulus of A'k' at make, which was formerly effective, now became ineffective, by the depressing action of A. Developing action of kathode. — Another experiment, showing the latent excitatory action of the kathode, is very

striking. This experiment, however, is somewhat difficult, as it requires a very delicate adjustment of the stimulus. The specimen used was a leaf of Biophytum. A current insufficient to produce any direct excitation was kept flowing through the circuit AK (fig. 101). The point of special difficulty was to apply a stimulus of exactly subminimal intensity at x , so as not to excite the adjacent leaflet. I have sometimes succeeded in obtaining this condition-

A subminimal stimulus applied at x -, ineffective to produce excitation of nearer leaflets, becomes effective in the neighbourhood of kathode. The effect of this imperceptible stimulus, then, which passed through the nearer pair of leaflets, without giving any sigh of its presence, became suddenly ' developed ' on reaching the further pair of leaflets, R (fig. 103), which were rendered more excitable by the neighbourhood of the kathode. These peculiar variations of excitability, induced by the action of the anode and kathode, as well as those caused by other physical and chemical agencies, are exactly similar to what are observed in animal tissues under the same influences. They bring out, further, the essential unity of physiological response, as seen in the highly differentiated protoplasm of the animal and the undifferentiated protoplasm of plant tissue.

Motile excitability is diminished at or near the anode, so that previously effective stimulus becomes ineffective. Motile excitability is exalted at or near the kathode. Stimulus previously ineffective here becomes effective. Difficulties in accurate determination of velocity of transmission, due to unknown variations of excitability arising from injury, and variations of conductivity through fatigue — A perfect method of obtaining accurate and consistent results — Relative advantages of studying conduction in plants as compared with animals — Determinations of velocity of transmission in centripetal and centrifugal directions— Preferential conductivity in centrifugal direction— Diminution of conductivity and excitability by fatigue — Within a certain critical interval, organ ' refractory ' to further stimulus — Increased velocity of transmission with increasing stimulus — Measurement of diminution of conductivity by cold — Fibrovascular elements the best conducting channels — Conductivity lengthwise greater than crosswise - Electric mode of determination of velocity of transmission — Indifferent parenchymatous tissues do not transmit stimulation — Comparative tables showing velocity of transmission in various plant and animal tissues.