The Nervous Mechanism of Plants

From the results of electrical method of exploration, it is found that there are two conducting phloems in each bundle, one external and the other internal to the xylem. The bundle is therefore bi-collateral. Microscopic examination confirms the existence of the two phloems, which stain deep violet with haematoxyhn. Tubular cells, which are characteristic of conducting tissue, are the main constituents of both the outer and the inner phloems. The anatomical characteristics of the conducting tissue are the same in the stem as in the petiole. There are two main bundles containing conducting phloem on opposite sides of the stem, which run vertically up and down. The unilateral propagation of excitation under moderate stimulus is due to this particular nerve-distribution.

Microscopic examination shows that the nerve-strands of the two bundles meet at the apex. Hence it is possible for the ascending impulse on one side to cross over at the apex and to be conducted downwards on the other. Anatomical examination shows that there is a continuity of nerve-connection between the stem and the leaf. Hence stimulation of the stem gives rise to a centrifugal impulse which causes the fall of leaves and the closure of leaflets ; conversely, strong stimulation of a sub-petiole gives rise

to a centripetal impulse which, reaching the stem, causes fall of its leaves. In addition to the two main bundles in the stem there are intermediate ones. There is no continuity, but contiguity, of the phloems in a lateral direction. A block or resistance to the lateral spread of excitation therefore exists. This block can, however, be overcome by a strong stimulus, when the excitation becomes outspread, causing fall of leaves on both sides of the stem.

There are reasons for the belief that in the animal there is no protoplasmic continuity across the nerve-junction where neurone joins neurone, the s^arating membrane being known as a synapse. The effect of discontinuity is most strikingly exhibited at the neuro-muscular junction where the intervention of the end-plate between the nerve and the muscle gives rise to a valve-action, on account of which excitation passes only in one direction. The conduction is therefore irreciprocal ; for while there is no obstacle to prevent the passing of the excitatory impulse from the nerve into the muscle, the excitation of the muscle does not pass backwards into the nerve. ^

Can a similar condition be discovered in Mimosa at the junction of the conducting nervous strands with the motor organ, the pulvinus ? Taking the case of one of these strands which terminates centrally in the lower quadrant of the pulvinus, while at the periphery it is led to the second sub-petiole bearing the sensitive leaflets, stimulation of the second sub-petiole gives rise to an impulse which is propagated along the particular conducting strand in the petiole from the periphery to the centre. The impulse conducted across the junction between the conducting and the motor tissue causes contraction of the lower half

^ ' There is a membrane intervening between the nerve-ending and the muscle-fibre supplied by it, as also between one neurone and the fibre connecting it with another neurone. The membrane is called by Sherrington synaptic membrane ” '--'Bd,y'\hs~-Principle of General Physiologv, of the pulvinus and the resulting fall of the leaf. The impulse can thus pass easily from the conducting nerve to the contractile tissue. If now the contractile tissue of the pulvinus were directly stimulated, would the excitation pass in the opposite direction, from the excited lower half of the pulvinus to the nerve-end imbedded in it ? If this occuiTed, the fact would be demonstrated by a centrifugal impulse, which on reaching the second sub-petiole would cause successive closure of its leaflets.

Experiment ig. Effect of stimulation of the contractile cells, — The contractile cells of the pulvinus can be stimulated without irritating the imbedded nerve. For this purpose the petiole is held within two clamping jaws, with padding of soft cottonwool. The lower half of the pulvinus is now scratched superficially with a pin. The result of this is the contraction of the pulvinus, as evidenced by the fall of the leaf as soon as the jaws are slightly opened. The object of slightly opening the jaws is to prevent the impulsive fall of the leaf which might cause diffuse stimulation. With these precautions it was invariably found in these experiments that stimulation of the pulvinus is not followed by closure of the leaflets of any one of the sub-petioles ; hence no conduction of excitation takes place from the contractile motor cells to the nerve. The conduction at the junction of the nervous and contractile tissues in the pulvinus of Mimosa is therefore irreciprocal as at the neuromuscular junction in the animal.

The interposition of synapses in the path of nervous impulse in the animal also renders the conduction irreciprocal or unequal in the two directions. Two other modiflcations occur in the velocity of the impulse due to the presence of a synapse or of a series of synapses. ' If the passage (of the impulse) be too often repeated, phenomena of fatigue are produced and there is an increase of the block at each synapse. If, however, the stimulus be not excessive and the reaction not too frequently evoked the effect of passage of an impulse is to diminish the

resistance, so that a second application of stimulus evokes the reaction more easily.’ ^ The above-mentioned characteristic tests may be applied, in determining the presence of a synapsoidal membrane in the conducting path in Mimosa, to ascertain The conducting tissue of Mimosa has been shown (see fig. 9) to consist essentially of elongated tubular cells, the transverse septa of which are mostly not perforated. The septa may therefore be regarded as synapsoids, if it is found that the transmission of excitation across them exhibits the characteristics already described.

This is demonstrated by the fact that the facility for conduction of excitation in the plant-nerve is unequal in the two directions. This statement holds good not only for Mimosa pudica but also for other sensitive plants. The intensity of stimulus employed was minimal. Experiment 20. Stem of Mimosa pudica. — ^The intensity of stimulus unilaterally applied was gradually increased till it became minimally effective. Excitation was now found to be transmitted upwards and not downwards. Conduction under minimal stimulus is therefore irreciprocal.

E3q)eriment 21. Petiole ofM. Spegazzinii.-—i:h.Q stimulus of equi-alternating induction-shocks was applied below the pair of sub-petioles numbered 4 (fig. 13). The stimulus became minimally effective at intensity i ; transmission took place only in the centrifugal direction, as shown by the successive fall of the two pairs of sub-petioles 4 and 3, the leaflets of which also underwent closure. The distance of propagation of impulse was 37 mm., the transmissiontime being 22 seconds ; the velocity of transmission was

Fig. 1 3 . Transmission of Excitation under Minimal and Maximal Stimuli. Induction-shock 'applied by electrodes E, E'. Under minimal stimulus, transmission occurs only in ttxe centrifugal direction. Under strong stimulus, excitation transmitted in both directions, the velocity in centrifugal direction being much greater than in the centripetal. (M . Spegmzinii ,) therefore 1-7 mm. per second. There was no evidence of transmission in the centripetal direction.

Experiment 22. Averrhoa Caramhola. — ^The two electrodes for excitation by polar action of a constant current were applied midway on the petiole bearing numerous sensitive leaflets. Application of an E.M.F. of 6 volts was found to be minimally effective, giving rise to an impulse which was propagated in an outward or centrifugal direction, the three pairs of the leaflets to the right exhibiting closure in serial succession. There was no evidence of transmission in an inward or centripetal direction, the leaflets to the left remaining unexcited.

Experiment 23. Biophytum sensitivum. — ^The two electrodes of an induction-coil were applied midway on the petiole. As the secondary coil was gradually brought nearer the primary, a position was found for the minimally effective stimulus. On account of the partial block to the transmission of impulse in an ingoing or centripetal direction, the excitatory impulse was propagated only in the centrifugal direction. The typical experiments carried out with diverse sensitive plants described above, prove that conduction is irreciprocal. They also indicate the existence of synapsoidal membranes which, by their valve-like action, permit propagation of impulse in one direction only.

It has been shown (Experiment 18) that the block or resistance to the passage of an impulse may be overcome by increasing the intensity of the stimulus. Excitation by unilateral stimulus which, when of moderate intensity, was only conducted lengthwise, was transmitted crosswise when the stimulus was stronger. It might therefore be expected that a hitherto ineffective transmission downwards in stems and inwards in petioles might become effective under increased intensity of stimulus. Irreciprocal conduction would thus be gradually transfonned into conduction in both directions. Even after the partial forcing of the block, the valve-action of the synapsoid might stfll be exhibited by preferential conduction, in which case the velocity would be higher in the centrifugal direction than in the centripetal.. These anticipations have been

fulfilled in the results described below, in which the intensity of stimulus was increased from minimal to sub-maximal. Stem of Mimosa pudica. — I have shown (Experiment i) that under a sufficiently strong stimulus conduction takes place both upwards and downwards. But the velocity in the upward direction was the greater ; for while the transmission upwards through 37 mm. took place in 10 seconds, the transmission in the downward direction through 28 mm. required 20 seconds. The velocity in the former case was 3-7 mm. per second, while in the latter it was 1-4 mm. per second. Hence the velocity in the preferential upward direction was about 2-7 times that in the opposite direction.

The intemodes below are older than those above, and the observed difference might have been partially due to the diminished conductivity of the older tissues. It is therefore desirable to observe the velocity in the two directions in the same internode. In Experiment 3, carried out with a highly excitable specimen, a moderately strong unilateral stimulus initiated an impulse which was at first conducted upwards; it then crossed to the opposite side and was propagated downwards along the same intemodes. A large number of experiments carried out in this manner showed that the rate of propagation upwards was from 3 to 3 - 5 times quicker than that in the downward direction.

The s5mapsoidal block is more and more effectively forced under increasing intensity of stimulus. Thus, while under minimal stimulus, the conduction is irreciprocal, under sub-maximal stimulus a partial forcing of the block occurs, the conduction in the preferential direction being considerably quicker than in the opposite. Under maximal stimulus the forcing of the block is more complete, and the two velocities then tend to become nearly equal. Experiment 24. Petiole of M. Spegazzinii. — In Experiment 21 a stimulus-intensity of i caused the fall of the outer sub-petioles 4 and 3 (fig. 13)- Using the same specimen, the stimulus was increased to 1-5; this gave rise to a stronger impulse which caused the fall not only

of sub-petioles 4 and 3 but also of sub-petioles 2 and i. The excitation under minimal stimulus had not been at all transmitted inwards ; but under the stronger stimulus of 1-5 the impulse also travelled in a centripetal direction and caused the fall of the sub-petiole 5, below the point of application of stimulus. But this occurred five seconds after the fall of the outermost pair of sub-petioles. The rate of conduction was therefore much quicker in the centrifugal direction than in the centripetal.

Experiment 25. Petiole of Averrhoa. — ^The intensity of stimulus was increased slightly above the value for irreciprocal conduction. The centrifugal impulse caused closure of all the leaflets to the right, but the centripetal impulse reached only one pair of leaflets to the left. Even in this case of transmission in both directions, the velocity in the centrifugal direction was about three times the greater. Experiment 26. Biophytim. — Results in every way similar to the above were obtained with this plant, which is far more sensitive than Averrhoa. When the intensity of stimulus was slightly increased above the minimal, the impulse was transmitted in both directions ; but w'hile all the leaflets to the right (centrifugal) closed, only a few of the leaflets to the left (centripetal) underwent closure. A still stronger stimulus caused a more effective conduction in both directions, but the centrifugal velocity was one and a half times greater than the centripetal.

Irreciprocal conduction and conduction in a preferential direction both indicate the existence in the conducting cells of synapsoidal membranes across which the excitation is transmitted with greater or less facility. Their existence is, as previously stated, also indicated by the fatigue of conduction under excessive stimulation, and by ‘ Bahnung ’ or Facilitation as an after-effect of moderate stimulation. I proceed to give an account of the effects of excessive and moderate stimulation on the velocity of conduction which I was able to determine with the highest

degree of accuracy by the automatic method fully described in the next chapter. The experiments were carried out with the petiole of Mimosa pudica. Fatigue was produced by the previous application of an excessively strong stimulus. It is to be noted that the experiments were carried out with vigorous specimens in which the normal velocity was high. Experiment 27.— In a particular specimen the normal velocity was found to be 18-7 mm. per second. In order to observe the after-effect of excessive stimulation, the end of the petiole beyond the point of application of the testing stimulus was then cut off ; after recovery, the record of velocity of transmission under the test-stimulus was taken once more. It was found that the excessive wound-stimulus had induced a depression of the conducting power, the velocity being reduced from the normal 18-7 to 10*7 mm. per second.

In another experiment the normal rate of 30 mni. was reduced by fatigue to 19 mm. per second. In highly excitable specimens of Mimosa, strong and long continued stimulation induces, as stated above, a depression or fatigue in the rate of conduction. But in less excitable specimens the after-effect of stimulus is to confer an enhanced power of conduction, as if the passage of impulse removed some resistance or block that existed before. Experiment 28.-— -The conducting power of the specimen was so low that the impulse due to the test-stimulus applied on the petiole, at a distance of 15 mm. from the pulvinus, failed to be transmitted. On application of a much stronger stimulus the excitatory impulse was found to be effectively

transmitted with a considerable speed. After a period of rest the original ineffective stimulus was applied once more. The impulse was now found to be transmitted, the velocity being as high as 25 mm. per second. This enhanced conducting power began slowly to decline, and after an hour the velocity was reduced to 4 mm. per second. The application of a strong stimulus was once more found to confer enhanced conducting power, the velocity under the test-stimulus being increased from 4 to 25 mm. per second. The conducting fath thus becomes canalised by stimulation.

Experiment 29. Petiole of M. Sfegazzinii. — A similar effect was strikingly demonstrated by this specimen, which was only moderately excitable and conducting. A stimulus of intensity of 1-5 from an induction-coil, applied belowsub-petiole 3, failed to initiate any excitatory impulse. Stimulus of intensity 2 was similarly ineffective ; and it was only under an intensity of 2 - 5 that the impulse was effectively transmitted, causing the fall of sub-petioles 3, 2,_ and I (see fig. 13). After this the formerly ineffective stimulus of I *5 was applied once more, and w-as now found to be effectively transmitted, causing the fall of all the sub-petioles.

Conduction of excitation takes place by means of the tubular cells in the phloem. The transverse septum between any two of the cells acts as a synapsoidal membrane. The characteristics of conduction across the synapsoidal membrane m the plant are similar to tho.se across the synaptic membrane in the animal. ^ Conduction is irreciprocal at the neuro-muscular junction. It is also irreciprocal at the junction of the plant-nerve TOth the contractile tissue of the pulvinus.

nerve is irreciprocal, excitation being easily transmitted in one direction and not transmitted in the other. Under sub-maximal stimulus there is a partial forcing of the block, with the result of transmission in both directions : the irreciprocal conduction becomes transformed into preferential conduction, the velocity being greater in one direction than in the opposite. Fatigue in conduction takes place after excessive and long-continued stimulation.

The phenomenon of ‘ Bahnung ’ or Facilitation occurs as an after-effect of short-lived stimulation. A previously ineffective stimulus now becomes effective. Stimulus thus canalises its own conducting path. The characteristics of nerve-conduction in the plant are similar to those in the animal. The results of the qualitative experiments described in the last chapters are sufficiently convincing to prove the nervous character of the transmission of impulse in plants. But for quantitative research it is necessary to devise a method for the determination of its velocity of as high a degree of accuracy as that attained in the determination of the velocity of nervous impulse in animals. For this purpose two conditions are essential : (i) a device for stimulation such that the intensity may be kept constant or be varied in a graduated manner ; (2) an automatic recorder by which a time-interval, shorter than a hundredth of a second, may be measured with the utmost accuracy.

The excitatory impulse may be initiated in various ways, of which scratch-stimulation is an example. Section of the petiole is also very effective, though a hydro-mechanical disturbance is produced along with the excitatory impulse. The application of a hot glowing point causes an intense excitation which is transmitted to a great distance. Stimulation by a drop of hydrochloric acid is equally effective. The above modes of stimulation may be employed for purposes more or less qualitative. They cannot, however, be repeated with equal intensity in successive experiments, nor can they be gradually increased from minimal to maximal.

The most satisfactory mode of stimulation of the plant to obtain mechanical response is the electrical. It can be readily effected by electric shocks from an induction-coil, the effective intensity of which is determined jointly by the intensity of the shock and the duration of its application. This latter can be very accurately adjusted by means of a Plunger actuated by clockwork, which completes the electric circuit of the primary coil. The Plunger dips into a deep cup of mercury ; the duration of the closure of the current may be adjusted from half a second to five seconds. This is done by raising or lowering the mercury cup by a rack and pinion. It is better to use only a moderate intensity of induction-current, the effectiveness of which can be increased by prolonging the duration of application ; under these conditions the excitability of the tissue remains unchanged for a comparatively lengthy period.