The Nervous Mechanism of Plants

The nerve-strand of sub-petiole (i) extends along the petiole to the central or pulvinar end. The wave of excitation in nerve (i) might have passed laterally into nerve (2) in the petiole itself, without taking the longer journey involved in reaching the pulvinus and then returning. Is there any evidence that the impulse actually takes the longer path ? It is to be found in the fact that, under normal conditions, the reflected impulse is initiated after the arrival of the afferent impulse at the pulvinus. There is a definite sequence of events : first the initiation of peripheral excitation by the stimulation of the sub-petiole ; then the conduction of the afferent impulse to the pulvinus, signalled by the fall of the leaf ; and finally the arrival of the reflected efferent impulse at the periphery. The average time for the double journey up and down the petiole (obtained from a large number of experiments) comes to about 24 seconds, (see Table IX, p. 182) : had the afferent impulse crossed over immediately after reaching the petiole, the total time would have been but a few seconds only. The reason why the impulse travels through the. longer distance is that, of

the two alternative paths, it follows the line of least resistance. The nerve-strands in the petiole are, as previously explained, remote from each other ; while in the piilvinus they are brought into the closest contiguity. The advantage of the shorter path across the petiole is more than counterbalanced by the great resistance to be overcome in passing across the area of imperfectly conducting tissue that intervenes between the nerve-strands. Hence, under ordinary conditions, excitation passes from one nerve to another only at the central or pulvinar end.

Two alternative theories may be proposed to account for the passage of excitation at the central end from one nerve to another. The transmission may either be regarded as having been effected through the intervention of the excited pulvinus, or be ascribed to nervous overflow or irradiation from the end of one nerve to that of another. With regard to the first view, since the reflected impulse is normally initiated after the fall of the leaf, it may be thought that the excitation is transmitted from nerve (i) to nerve {2) via the excited pulvinus. It has however been shown (Experiment 19) that, while an excitatory impulse can travel from the nerve to the contractile tissue, it cannot proceed in the opposite direction from the contractile tissue to the nerve. The following experiment, showing the persistence of the reflection of impulse even after abolition of contractility in the pulvinus, proves that the reaction of the motor cells has nothing to do with the efferent impulse.

Experiment 87. Persistence of reflected impulse after abolition of contractility of the motor cells, — I have explained that the contractile power of the pulvinus is temporarily abolished by the absorption of an excessive quantity of water. When the pulvinus has thus become immotile, an afferent impulse which reaches it cannot bring about the fall of the leaf, but it is nevertheless reflected as an efferent impulse. It is thus clear that there is no causal relation between the contraction of the pulvinus and the reflection of the impulse. The efferent impulse must therefore be due to a reflex arc. The irradiation from one nerve-end to another takes place whether the pulvinus contracts or not.

Since all the various motile organs of Mimosa pudica are employed in the detection of the excitatory impulse, it is necessary to say a few words about their relative sensibility. A very feeble stimulus causes the closure of the leaflets, the latent period of the pulvinules being only a very small fraction of a second : their sensibility is, moreover, not readily affected by adverse conditions. The excitability of the main pulvinus is not so great : it is affected by adverse conditions, such as diminished intensity of daylight, which cause depression of its sensibility. On very damp days during the rains, the turgor of the organ becomes excessive, and it is then relatively insensitive. The depression of sensibility of the pulvinus so induced may cause such a prolongation of its latent period that the leaflets exhibit closure even before the fall of the leaf. All these aberrations are avoided, by choosing a vigorous plant and a bright and warm day for the observation of normal effects. The secondary pulvinus of the sub-petiole is the least sensitive of all. Thus an impulse of moderate intensity which excites the main pulvinus at the centre and the sensitive leaflets at the periphery, fails to cause any movement of the secondary pulvinus which is interposed between the two ends ; it is only under stimulus of considerable intensity that this organ exhibits any responsive movement. This relates to the autumn season, when the majority of these experiments were carried out. The excitability

of the secondary pulvinus is enhanced in summer, specially on warm and bright days. I will presently indicate the artificial means by which it is possible to increase its sensibility. Reflected Impulses on Stimulation of the SEVERAL Sub-petioles I go on to describe the reflexes produced on stimulation of the second, third and fourth sub-petioles. The characteristic results described below are obtainable under different modes of stimulation. Thermal stimulation may be employed, but this stimulus cannot be maintained constant in successive experiments. For quantitative investigation it is therefore preferable to stimulate by an induction-current. The characteristic effects of stimulation of the different sub-petioles may, of course, be observed by using a fresh specimen in each case ; but it is far more interesting to obtain all the characteristic effects with an identical leaf. This is possible when the specimen is in such vigorous condition that it does not exhibit fatigue during the course of a somewhat prolonged investigation. It is also necessary to choose conditions such that light and temperature remain unchanged. On a bright day these remain fairly constant at about noon, when the temperature is 30°-3i° C. (August). On a cloudless day the experiments were carried out in a glass-house facing north. On cloudy days the plant was taken inside the laboratory and exposed to light of constant intensity given by an incandescent electric lamp of 500 candle-power.

Experiment 88. Stimulation hy induction-cufrent. — The intensity of stimulus employed was shghtly above the minimal, and was maintained constant for all the experiments in the series. The duration of stimulation was one second. The transmission-time was slightly shortened under a stronger intensity of stimulus, and increased when the petiole was longer. In the present series, the intensity of stimulus was constant, and the length of petiole along which the impulse had to travel was practically the same throughout.

Fig. 78. Reflected impulses initiated by stimulation of the several sub-petioles. A. Moderate stimulation of sub-petiole (i) gives rise to an efferent impulse which causes fall of leaf (not shown in the figure) . The afferent impulse is converted into an efferent after reflection at centre, which causes response of leaflets on sub-petiole (2). Afferent impulse represented by full arrow and efferent by dotted arrow. B. Diagram illustrating the relation of the four nerve-ends in the

pulvinus, and the responses of the leaflets to a single reflected impulse on stimulation of sub-petioles (i) and (4), and to two reflected impulses on stimulation of sub-petioles (2) and (3). Experiment 89. Effect of the stimulation of sub- -petiole (i). — Stimulation of sub-petiole (i) gave rise to a single reflected impulse, which caused the closure of the leaflets of only sub-petiole (2). The transmission-time of the afferent impulse was 18 seconds, while that of the efferent was 2 seconds.

Experiment 90. Effect of the stimulation of subpetiole (2). — The afferent impulse reached the pulvinus after 23 seconds, and the reflected impulse was not single, but double. These two impulses caused closure of the leaflets of the sub-petioles (i) and (3), 4 seconds after reflection at the pulvinus. The velocity of the efferent was, as in the last case, considerably higher than that of the afferent impulse. Experiment 91. Effect of the stimulation of subpetiole (3). — ^The reflected impulse in this case also was double ; one of the two efferent impulses caused response in sub-petiole (2) and the other in sub-petiole (4). The transmission-time for the afferent impulse was 20 seconds, while that for the efferent was only 3 seconds. The velocity of the out-going impulse was here more than six times greater than that of the in-going.

Table IX. — Transmission-Times op Afferent and Efferent Impulses Experiment 92, Effect of the stimulation of stibpetiole (4). — The afferent impulse after reflection gave rise to a single efferent impulse as in the case of stimulation of sub-petiole (i). The efferent impulse caused the closure ; of the leaflets of only sub-petiole (3). The transmission- I time for the afferent impulse was 24 seconds, that for the Reference to fig. 78 will facilitate a clearer understanding of the principal results.

It is now possible to offer a full explanation of the characteristic effects brought about by peripheral stimulation of the several sub-petioles (c/. fig. 78). The ends of the different nerves, as previously explained, are brought into the closest contiguity in the pulvinus. The following relation is thereby established between afferent and efferent impulses in the four sub-petioles : — ^ {a) The nerve-end (i) is in close contiguity to that of (2), The afferent impulse caused by stimulation of sub-petiole (i) is transferred at the centre from nerve-end (i) to nerve-end (2). The efferent impulse therefore causes response of the leaflets on sub-petiole (2).

(6) The nerve-end (2) is intermediate between the nerveends (i) and (3). The afferent impulse originated in subpetiole (2) is irradiated into nerves (i) and (3) ; there are therefore two efferent impulses causing response in subpetioles (i) and (3). (c) The nerve-end (3) occupies an intermediate position between the nerve-ends (2) and (4). The afferent impulse in (3) therefore gives rise to two reflected impulses which cause response in sub-petioles (2) and (4).

{d) Finally the nerve-end (4) to the extreme right is contiguous only to nerve-end (3). The reflected impulse is therefore single ; the efferent impulse conducted by nerve (3) causes response of only sub-petiole (3). The important conclusion established by the experiments described above is the existence of a reflex arc in the pulvinus. Equally significant is the fact that the efferent impulse is far quicker than the afferent. The transmission-time of the latter, through the average length of the petiole {30 mm.) and the short length of sub-petiole (2 mm.), was about 21 seconds. But the mean value of velocity of transmission in the petiole has been found to be about i6mm. per second (Chapter VI.). Hence the transmission-time (21 seconds) of the afferent impulse would appear to be abnormally high ; it should have been about 2 seconds or so, since the distance of transmission is about 32 mm. The anomaly, I shall show, is only apparent and not real. The point is that in the determination of the normal velocity of transmission in the petiole, the stimulus was apphed at a certain distance from the pulvinus on the petiole itself. But in the determination of the transmissiontime of the afferent impulse, the stimulus was applied on the sub-petiole ; the excitation had therefore to traverse the additional length of the sub-petiole, the conductivity of which is about ten times less than that of the petiole. The intensity of excitation is moreover considerably weakened during passage through the sub-petiole. This reduction in intensity, together with the added distance of sub-petiole to be traversed, explains the comparatively long time necessary for the transmission of the afferent impulse.

I have succeeded in shortening the transmission-time of the afferent impulse, by eliminating the intervening length of sub-petiole as in the following experiments. Experiment 93. — ^The excitability of the pulvinus of the sub-petiole of Mimosa is considerably increased under favourable conditions of season and warmth in summer, when it readily responds to excitation by a lateral movement, as also by desiccation (c/. p. 69). By the application of glycerine the excitability of the secondary pulvini of the first and second sub-petioles was considerably increased. In effecting stimulation, the proximal electrode of the secondary coil was placed on the secondary pulvinus of the first subpetiole, the second electrode on the middle of it. The

impulse now travelled through the length of the petiole, and caused the faU of the leaf ; the reflected impulse travelled outwards and caused the lateral movement of the secondary pulvinus of the second sub-petiole. The loss of time due to transmission through the short length of the sub-petiole was thus eliminated. The results of three typical experiments are given below. The afferent transmission-time is represented by that of the efferent transmission by ; V, is the velocity of the afferent, and

The results described above show: flrst, that when the intermediate length of the sub-petiole is eliminated, the transmission-time of the afferent impulse is reduced to a value corresponding to that of the velocity of impulse in the petiole; and secondly, that the efferent impulse travels at least three times more quickly than the afferent. In reality it is even quicker ; it was however impossible, by mere ocular observation, to carry quantitative measurement to a very high degree of accuracy, requiring as it does measurement of fractions of a second.

This very promising method of time-measurement by the indications of mechanical response of the secondary petiole is unfortunately not suitable for prolonged experiments ; for long application of glycerine is apt to produce loss of excitability in the secondary pulvinus. The definite nerve-connection between the nerve-end in each quadrant of the pulvinus at the centre and the corresponding sub-petiole at the periphery is demonstrated by localised central stimulation.

Feeble peripheral stimulation gives rise to the response characteristic of the corresponding quadrant of the pulvinus : the excitation remains localised. But when the intensity of peripheral stimulation is adequate, the ingoing or afferent impulse reaching the pulvinus becomes reflected along a new path, as an efferent impulse. A Reflex Arc is formed at the centre. The velocity of the efferent impulse is, in all cases, greater than that of the afferent impulse.

Experiments are described which show that the Reflex Arc is localised at the central end. Stimulation of subpetiole (i) or sub-petiole (4) gives rise to a single reflected impulse; stimulation of sub-petiole (2) or sub-petiole (3) gives rise in each case to two reflected impulses. These characteristic effects are explained by the mutual relation of the several nerve-ends in the pulvinus. In an animal nerve conduction takes place in both directions ; hence it might be inferred that the same nervefibre may conduct sensory impulse from the periphery to the centre and motor impulse from the centre to the muscle. It is, however, held that this is not the case, but that in a single nerve-trunk there are two kinds of fibres isolated from each other, one of which subserves the afferent and the other the efferent function.

In the petiole of Mimosa also, both the sensory and motor impulses are apparently conducted by the same nerve. For stimulation of sub-petiole (i) gives rise to an afferent or sensory impulse along nerve (i) which, after reflection at the centre, becomes an efferent or motor impulse along nerve (2). Further, the sub-petioles (i) and (2) are interchangeable; external stimulus may be applied to sub-petiole (2) instead of to (i), when nerve (2) carries the sensory while nerve (i) conducts the motor impulse. It would thus appear, at first sight, that while in the animal great specialisation has been effected in separating the sensory and motor elements in the same nervetrunk, no such differentiation had been reached in the nerve of the plant.

A detailed consideration of the results, however, brings out certain significant differences between the afferent and efferent impulses, which suggest that they may be transmitted by ^stinct channels. By afferent or sensory impulse is meant the particular one generated hy an external stimulus, which impulse is converted into efferent or motor in the reflex arc at the centre. The efferent impulse may therefore be distinguished as the reflected impulse. Now there is a marked diiference in the velocity of the two impulses by which they may be discriminated from one another: the efferent impulse is six to seven times quicker than the afferent. To take a concrete example : sub-petiole (i) is stimulated ; the transmission-time to the centre along nerve (i) is i8 seconds ; but when this impulse becomes efferent after reflection, it travels along nerve (2) in the course of 3 seconds. Sub-petiole (2) is now stimulated; the afferent impulse in nerve (2) takes 23 seconds to travel to the centre, in place of the 3 seconds taken by the efferent impulse to travel through the same distance. Hence there may be two^ conducting elements in the same nerve-strand, one of which conducts the efferent or motor impulse and 'the other the afferent or sensory impulse, the velocity of the former being about six times greater than that of the latter.

The existence of two distinct conducting nerves in the same vascular bundle is strongly supported by the following evidence : (1) Microscopical examination (figs. 9, ii, 12) shows two separate phloems, one external, the other internal, to the xylem. (2) Investigation with the Electric Probe demonstrated two distinct nervous impulses, one conducted by the outer and the other by the inner phloem (fig. 50). The proof of the existence of separate conducting elements would be complete if it could be shown that in the same vascular bundle the external phloem conducted the slower and the internal phloem the quicker impulse. That this is the case is demonstrated by the following experiments.

Experiment 94. I stimulated in succession the external and the internal phloem of nerve (3) situated in the upper quadrant of the pulvinus ; nerve (3), it will be remembered, termmates m sub-petiole (3) at the periphery. The transmitted impulse generated at the centre was, as usual, manifested by the closure of the first pair of sensitive leaflets. On the application of a superficial scratch to stimulate the upper phloem, the transmission-time to the third subpetiole was found to be 22 seconds. On recovery of the leaflets, a second stimulation was effected by thrusting the pin deeper in, so as to reach the inner phloem ; the velocity was now found to be greatly enhanced, the transmission-time being reduced to 3 seconds. The inner phloem is therefore the conducting tissue for the more rapid impulse. I carried out ten experiments with different specimens, numbers I. to X., and obtained very uniform results, as shown in the following table:

Table X.— The different Rates of Transmission by the Outer and the Inner Phloem. It is very remarkable that the average times of the afferent and efferent impulses due to central stimulation given in the above table should so closely agree with those obtained by peripheral stimulation (Table IX.). The results of numerous experiments carried out by different methods show that the velocity in the inner phloem is about six to seven times greater than in the outer. The results of different lines of investigation all tend to indicate that the afferent or sensory impulse is conducted by the external, and the efferent or motor impulse by the internal phloem.

In the diagram (fig. 79, a) the internal phloem is shaded dark to distinguish it from the external phloem in light outline. Stimulation of the external phloem by scratchstimulus causes a slow outgoing impulse Se, inducing response of the pulvinules of the sensitive leaflets on the sub-petiole, A deeper prick stimulates not only the external, but also the internal phloem. The quicker impulse i*eaches the pulvinule and causes an earlier response. The thick line

Fig. 79. Diagramniaticrepresentationof transmission of impulses initiated by stimulation of external and internal phloem. (a) Stimulation of outer phloem (shaded light) by superficial prick causes a slow outgoing impulse Se; deeper prick stimulates inner phloem (shaded black) and gives rise to a quicker impulse Si ; the outer phloem-strand is represented by a thin line and the inner strand by a thick line ; the lengths of the arrows represent the different velocities.

(b) Slow afferent impulse on stimulation of sub-petiole (i) is conducted along external phloem of nerve-strand (i) ; it passes into the inner phloem and then crosses over to inner phloem of nerve-strand (2), Quickened efferent impulse transmitted along inner phloem causes response in sub-petiole (2). represents the inner nerve, and the quicker rate of conduction through it is represented by a longer aiTow. From the results of experiments already given, it will be seen that the descriptive terms, centripetal or afferent, and centrifugal or efferent, are to be regarded as relative

and not absolute. For the same stimulus which applied at the periphery gives rise to a centripetal impulse/causes a centrifugal impulse when applied at the centre. The real difference lies in the conducting elements by which the two different impulses are propagated. The external phloem, which is the conductor for the slower impulse, is the ‘ sensory ’ nerve ; whereas the inner phloem, which conducts the more intense and quicker impulse, is the ‘ motor ’ nerve.

The localisation of the external sensory and the internal motor nerves, affords a clearer insight into the transformation of the afferent impulse from the periphery into the efferent impulse at the centre. The inner phloems of the vascular bundles point inwards and are therefore in closest contiguity in the pulvinus. Fixing attention on the first sub-petiole, an external stimulus, under natural conditions, excites the outer conducting sensory nerve : this gives rise to a relatively slow afferent impulse in the external phloem which, reaching the pulvinus, passes inwards and thence to the inner phloem of nerve (2) which lies nearest, as shown in figure 79, b. This transference of excitation into the inner conductor involves an enhancement of speed characteristic of the efferent or motor impulse. The transformation of the afferent into efferent impulse in the reflex arc at the centre does not merely connote a reversal in the direction of propagation ; for there is a marked disproportion between the afferent and the efferent impulses : it would appear as if a discharge of energy took place at the centre, by which the latter became far more intense than the feeble afferent impulse that provoked it.