The Nervous Mechanism of Plants

hand, offers Special advantages in such investigation, since its response is practically instantaneous. The only drawback is that the sensitivity of the apparatus is very much lower than that of a highly sensitive galvanometer. The electromotive variation in some plants is, however, sufficiently high to be recorded even by the Einthoven galvanometer. The monophasic record of the midrib obtained with the Einthoven galvanometer is reproduced in fig. 42. The time-marks below indicate intervals of tenths of a second.

The maximum excitation was attained, in this case, in the course of 0 • 8 second, the recovery being completed in the further period of 4*3 seconds. The results here given prove that the characteristics of transmitted impulse, as ascertained from the mechanical response of motile sensitive plants, find an exact parallel in the electric response of ordinary non-motile plants. They are, in fact, common to all plants. A wider generalisation on the effects of Direct and In- direct Stimulation can now be established :

Fig. 42.— Time-relations of Monophasic Re- sponse of Midrib obtained by Einthoven Stringgalvanometer. (1) The effect of all forms of Direct Stimulation IS DIMINUTION OF TURGOR, CONTRACTION, Under direct stimulation all plants and their different organs exhibit an excitatory reaction of galvanometric negativity. The electric response disappears on the death of the tissue. An impulse transmitted in a non-conducting or semiconducting tissue induces an electric change of galvanometric positivity.

The electric response of the midrib to transmitted impulse is negative. Moderate stimulation enhances the conducting power, with resulting staircase-increase in successive responses. The conduction is irreciprocal. Under minimal stimulus conduction takes place only in a centrifugal direction : it is only under strong stimulus that conduction takes place in both directions. When two electric connections are made on the midrib, the transmitted impulse gives rise to a diphasic response ; this is due to the arrival of the excitatory wave first at the proximal contact, and later at the distal contact.

The maximum excitation at one of the contacts is attained in the course of about o-8 second; it disappears in the further course of 4-3 seconds. The electric responses of ordinary plants to direct and indirect stimulation are parallel to the mechanical responses of motile sensitive plants. Though the characteristic responses to indirect stimulation were shown in the last chapter to be universal in all plants, it is desirable to repeat the experiments with the sensitive Mimosa : for with this plant it is possible to obtain in one

Fig. 43. — Electric Connections for Response to Indirect Stiniulation in Mimosa. a, Electric connection for response of lower half of pulvinns to b, for Monophasic response of pulvinus to stimulus applied on c, for Diphasic response of petiole to stimulus applied to petiole experiment a record of both the mechanical and the elecMc responses, so as to demonstrate the practical identity of the two reactions. Thus, on application of sub-maximal stimulus to the sub -petiole, it was found that the mechanical and electrical responses occurred almost simultaneously. The very slight difference between the two is accounted for by the delay in starting the motile mechanism of the pulvinus into action.

The electrical connections for obtaining the electric response are given diagrammatically in fig. 43. A thin platinum wire is thrust about i mm. into the pulvinus, the second contact being made at a distant indifferent point on the surface of the stem, which is not reached by the excitatory impulse. The effect of stimulation transmitted across the semi-conducting pulvinus can be obtained with similar electric connections, stimulus being applied locaUy on the upper surface of the pulvinus. To record the velocity of the nervous impulse in the conducting strand of the petiole, two fine platinum wires are thrust in to make the necessary electric connections with the recording galvanometer.

It was ascertained in Experiment 48 that the mechanical response to feeble indirect stimulation was positive, which was transformed into negative under stimulus of a stronger intensity. Experiment 59. Electric response to feeble stimulus. — Records of response to feeble stimulus applied on the distant sub-petiole are given in fig. 44 ; the responses are seen to be positive, that is to say, the electric change is one of galvanometric positivity.

Experiment 60. Electric response to strong stimulus. — The experiment was repeated with the same specimen, the only difference being that the intensity of the stimulus was made sub-maximal. The response is seen to be an electric change of galvanometric negativity (fig. 45). Fig. 45. — Response of Galvanometric Negativity to stronger Stimulus. It may be thought that the electric variation might be due to the mechanical movement of the falling leaf which would occur under excitation. That this is not the case is proved by holding the leaf in a fixed position, when the electric response is found to be the same as before. The mechanical and electric responses are therefore independent manifestations of a common excitatory reaction.

A muscle immersed in distilled water loses its power of contraction. Application of water to the pulvinus of Mimosa also renders it mechanically irresponsive. I have shown that the conductivity of the embedded nerve is not modified by the abolition of motile excitability. The question now arises whether the excitatory process is still in operation in the water-logged pulvinus, though it does not exhibit it by actual contraction. The problem was solved by the following experiment.

Experiment 6i. — ^The pulvinus of Mimosa was made mechanically irresponsive by application of water. In this condition, stimulation, as already stated, is not followed by the normal fall of the leaf. An electric record of response was then obtained, which showed that the mechanically insensitive leaf gave the normal electric response of galvanometric negativity, proving that the tissue was still irritable though unable to manifest it outwardly by mechanical movement. This fact helps to explain the occurrence of excitatory electric response in ordinary plants in which mechanical response is practically absent.

Determination of Velocity of Excitatory Impulse BY Diphasic Response I next attempted the difficult task of recording the extremely short interval of the passage of the excitatory impulse from A to B in the conducting tissue itself. Two fine platinum wires made the necessary electric connections with the conducting phloem (fig. 43, c), stimulus being applied at a point not very distant from A. In this method, A is first rendered galvanometrically negative ; the e.Ycitation then reaches B, rendering the point negative, and causing a reversal of the electric response. The interval between the two reactions is extremely short, specially when the two points A and B are separated by a comparatively short distance. The record of the diphasic re.sponse cannot therefore be obtained by an ordinary

galvanometer ; it can only be secured, as previously explained, by the stringgalvanometer of Einthoven. Before describing the results obtained with the plant-nerve, I reproduce the record of diphasic response of the olfactory nerve of the Pike obtained by Garten with the Capillary Electrometer (fig. 46) . The proximal point on the nerve first became negative ; the later arrival of the impulse at the second contact is seen as a reversed response due to the negativity induced at that point. ■

Experiment 62. — I now describe the diphasic response of the nerve of Mimosa obtained with the Einthoven stringgalvanometer. The two points of contact were at a distance of 7 mm. from each other. The arrival of the excitatory wave at A is signalled by an up-curve denoting galvanometric negativity at that point. The arrival of the wave at B produces a reversal of the curve due to the electric negativity at that point (fig. 47). The time-interval between the responses A and B is' '25 second ; as the distance traversed by the impulse is 7 mm., the velocity

Fig. 46. — ^Diphasic Response in Olfactory Nerve of Pike as shown by the Capillary Electrometer (after Garten), Fig. 47. — Diphasic Electric Response of Nerve of Mimosa ; with Einthoven Galvanometer (see text). I reproduce another record of the diphasic response of the conducting nerve of Mimosa ; the sensitiveness of the of transmission of excitation is 28 mm. per second/ which is practically the same as the value obtained by the mechanical method. In summer this varies in different specimens from about 15 to 30 mm. per second.

Fig. 47A. A second record of the diphasic response of nerve of Mimosa : obtained after fitting a finer string to the galvanometer. string-galvanometer was increased, with resulting enhancement of the amplitude of response (fig. 47A). The distance between the two points of contact was 10 mm. and the time interval between responses of A and B was 0-5 second (timemarks not shown in the figure). This gives 20 mm. per second as the velocity of transmission of excitation.

It is impossible to isolate the nervous tissue in Mimosa without injury ; hence we are confronted with the problem of localising in situ the particular tissue which serves as the conductor of excitation. The problem was ultimately solved by the invention of the Electric Probe. Fig. 48.-— The Electric Probe for localisation of Nervous Tissue in the petiole of Mimosa. v, the probe in circuit with the galvanometer, g ; s, the screw-head, by the rotation of which the probe is forced into the petiole;

The principle of the method wUl be readily understood if the petiole be thought of as a cable along which electric messages are being transmitted. The conducting strand is here inside a non-conducting sheath. The embedded conductor can be localised, and the transmitted messages picked up, by gradually thrusting in the Electric Probe, which is insulated except at the extreme tip. A galvanometer included in the circuit of the probe indicates the messages that are being transmitted as soon as the tip of the Probe comes in contact with the conducting strand. The depth of insertion attained can be read on a suitable scale, and thus the position of the conductor can be determined.

The exact position of the conducting nerve embedded in the petiole of Mimosa can be localised in a similar manner. Excitation of the sub~ petiole gives rise to an excitatory impulse which travels in a centrifugal direction, the impulse in the nerve being detected by a change to galvanometric negativity. Of all the tissues the conducting nerve will be most intensely excited by the transmitted impulse, and the induced electrical change of this particular tissue will be maximum.

Excitation will no doubt be irradiated, but this will show a rapid diminution in the more external tissues. If the stimulus be moderate or feeble the irradiation will be slight. The experimental procedure is as follows : The probe is thrust in perpendicularly to the diameter of the petiole from above (fig. 48). The intrusion of the probe is by steps, say, of 0*05 mm. at a time. The slight wound produced by the insertion of the tip of the probe causes excitation, which subsides completely in the course of about fifteen minutes. The third sub-petiole in connection with the upper nerve is periodically stimulated by uniform stimuli.

Fig. 49. — Transverse Section of the Upper Vascular Bundle containing Conducting Nerve. Fjg, ^o. — ^The Galvanometric Record of Transmitted Excitation in different tissues of the Petiole. The first is the positive response of the epidermis; the second is the feeble negative response of the cortex ; the third, fourth and the fifth are the enhanced responses in the outer phloem ; the sixth shows absence of excitation in the xylem; the seventh is the enhanced response in the inner phloem; the eighth is the diminished response in the pith.

In the records given above (fig. 50) the electric response of the epidermis was + 12 divisions of the galvanometer. I have shown elsewhere that the epidermis, which is a non-conductor, gives either a zero or a positive response, in contradistinction to the normal negative response of conducting tissue. The probe at a depth of o-i mm. encountered the cortex, when the response on account of irradiation was but slight, being — 17 divisions. It next arrived at the region of the phloem which extends through 0 *15 mm., the average depth being 0 -2 mm. The response in this region underwent a sudden enhancement, as seen in the three responses — 61, — 65 and — 40 divisions. The xylem, which was at a depth of 0-3 mm., showed little or no response, proving that it was practically a non-conductor.

Fig. 51. — Curve showing the different intensities of Transmitted Excitation in the different tissues. E, epidermis; c, cortex; p, outer phloem; x, xylem; inner phloem ; o, pith. When the probe reached a depth of o mm. it encountered the inner phloem, and the response underwent a second enhancement of — 56 divisions. The probe reached the pith at a depth of 0-4 mm., and then the response underwent a diminution to — 26 divisions. In cases where the stimulation of the sub-petiole is feeble, the irradiation effects are greatly diminished ; the excitatory transmission is then found only in the two phloems.

It wiU be seen that in all cases the phloem is invariably found to be the best channel for conduction of excitation. and that there are two conducting phloems, one external and the other internal. The curve (fig. 51), plotted from the mean values of ten different experiments, illustrates this in a striking manner. In the diagram of the transverse section usually given in text-books, there is in each bundle a single phloem-strand outside the xylem. The existence of two electrical maxima drew my attention to the possibility of there being two phloems in the bundle. As stated in a previous chapter, differential staining clearly brings out the fact that the phloem is not single but double (p. 35) .

It was shown (p. 91) that the pulvinar tissue of Mimosa is a semi-conductor. It was also shown that local application of the stimulus of light on the upper surface of the pul vinus induced an increase of turgor and expansion, manifested by a preliminary positive or erectile response. The excitatory impulse subsequently reached the more excitable lower half and produced an abrupt negative response, the fall of the leaf (c/. fig. 35). Such is the mechanical response.

Experiment 64.— I will now describe the corresponding electrical response. The electric connections will be understood from fig. 43 (a) in which the contact A is made with the lower half of the pulvinus, B being made with a distant indifferent point on the surface of the stem. Stimulation of the upper surface without mechanical disturbance was effected by application of a small drop of dilute hydrochloric acid. The electric response Fig. 52. — The Electric Response of the lower half of the pulvinus to impulse transmitted from the upper half.

Note preliminary positive (up -curve) followed by the more intense excitatory negative. Timemarks, one-tenth of a second. given by the Eiiithoven string-galvanometer is reprodticed in fig. 52. It will be seen that there is an tip-response indicating the brief galvanometric positivity at A due to the preliminary positive impulse in a semi-conductor. The excitatory impulse then reached A, and caused the more intense down-response of galvanometric negativity at that point. When the stimulation of the upper surface is intensified by the application of a more concentrated solution of hydrochloric acid, the velocity of the excitatory impulse is greatly

increased, with the result that the more intense negative response masks the feeble preliminary positive. This proves once more that the transmitt ed impu] se is double in every case, and that in highly conducting tissues the preliminary positive impulse becomes obliterated by the predominant negative which overpowers it. One of the difficulties in obtaining a preliminary positive mechanical response of Mimosa to indirect stimulation of the conducting petiole is the inertia of the heavy leaf ; two rapidly succeeding impulses of opposite sign cause a resultant movement which is the predominant negative. The two impulses positive and negative can, however, be recorded electrically by the Einthoven galvanometer, the inertia of which is negligible. Excitation by scratch-stimulus was effected on the lower side of the petiole, at a distance of 2 cm. from the pulvinus. The proximal electric connection was made with the pulvinus, and the distal with the indifferent surface of the stem. The record (fig. 52A) shows

Fig. 52 a. Electric record of positive and negative responses to indirect stimulation of the petiole of Mimosa : positive, downcurve. that the interval between the feeble positive and the stronger negative was only o • 05 second. A mechanical record of the preliminary positive impulse would, under the circumstances, have been an impossibility. The result given by the electric record is of importance, proving that two impulses (A and D reactions) are generated even in a conducting tissue, though the mechanical response fails to show the positive.

Preliminary Positive Impulse in Conducting Tissues of the Animal The investigations on response in plants which I have been carrying out for the last twenty-five years have proved that the reactions in the plant are essentially similar to those in the animal. An apparent discrepancy arises from the fact that no indication of a preliminary positive response to the impulse transmitted along animal tissues bad hitherto been noted. I will now show that a preliminary positive response is given by animal tissiles, which is often masked by the predominant negative. The positive can be unmasked by the employment of a semiconducting tissue, and obtaining the record by an Einthoven galvanometer. The result is shown in the following Einthoven-galvanometer record (fig. 53) obtained by Orbeli and Brucke of the transmitted impulses in the semi-conducting ureter of the animal. The proximal electrode at which the wave first arrived became positive (up-curve) ; it then became strongly negative, due to the later arrival of the excitatory wave. The method is monophasic, the