The Nervous Mechanism of Plants

Fig, 25, The various physiological blocks are interposed at b, in the path of conduction. lowering of temperature did not in any way affect either the receptive excitability of the stimulated point, or the motor excitability of the responding pulvinus. In order to ensure this, cold was applied locally at the middle of the petiole. The experiment was carried out in summer, the specimen being moderately, sensitive. A strip of cloth 10 mm. in breadth was wrapped round the petiole midway between the stimulated point and the pulvinus, and the local lowering of temperature was produced by application of cooled water ; excessive lowering was secured by placing powdered ice on the piece of cloth. The stimulus employed was maximal.

intervals of 20 minutes, which is more than sufficient time for complete recovery from previous stimulation. Record (i) gives the time-interval between stimulus and response under normal conditions (fig. 26). There are 20-5 intervening spaces, each representing -i second. The latent period is 1-5 seconds. The true time of transmission is thus 1-9 seconds for 30 mm. ; the transmission-time through 10 mm. is therefore -63 second. The next record was taken when an intervening length of 10 mm. on the petiole was moderately lowered in tempera-

Fig. 26. Effect of Cold ia inducing Retardation and Arrest of Transmission. (i) Normal record; (2) Retardation due to slight cooling; {3) Arrest of conduction brought about by intense cold ; (4) Record of direct stimulation. ture by the application of cold water. This cooling should be commenced immediately after the previous responsive fall of the leaf. This not only gives sufficient time for locahsed cooling of the petiole, but also avoids the excitatory disturbance of the pulvinus caused by a sudden application of cold water to the petiole. During the locahsed cooling of the petiole, the leaf re-erects itself and becomes fully sensitive when the time arrives for the next apphcation of stimulus. Record (2) exhibits the effect of moderate cooling ; the transmission-time is now prolonged by •8 second. On the assumption that the effect of cooling

had remained localised, it is seen that the lowering of temperature had prolonged the time of transmission through the lo mm. of the petiole from -63 second to i’43 seconds. The conductivity had thus been reduced by more than half. In record (3) is seen the effect of a further lowering of temperature by placing small fragments of ice on the strip of cloth. The excitatory impulse initiated by the maximal stimulus of induction-shock had hitherto been unfailingly transmitted. But under the action of intense cold the impulse was arrested. In order to show that the abolition was not due to the depression of the motile excitabilty of the pulvinus, record (4) was taken of the effect of direct stimulation : the record shows that the motile excitability had undergone no change. It is thus clear that the impulse initiated by the stimulus had been arrested by the physiological depression of conductivity, induced locally by the action of cold.

Paralysis of Conductivity and Restoration by Tetanising Shocks Experiment 40.— In connection with this subject I came across the interesting phenomenon of paralysis of conductivity as an after-effect of intense cold. After obtaining the record of the block under local application of cold, the fragments of ice were removed and the cooled portion of the petiole allowed to regain the temperature of the room, which must have been accomplished in the course of 20 minutes. After this, on taking a record of the transmitted effect of stimulation, I found that the block was still persistent. The conducting power of the benumbed tissue is thus paralysed for a period which generally lasts for about 45 minutes.' I have, however, discovered the very suggestive fact that the lost conductivity can very quickly be restored by subjecting the paralysed portion of the petiole to the action of tetanising electric shocks.

I have in a previous chapter described the effect of the electrotonic block in arresting conduction (c/. fig. 7). It was shown that by putting the electrotonic current ‘ on ’ and ‘ off ’ the transmitted excitation could alternately be arrested and allowed to proceed without hindrance. These characteristic effects were manifested in records of the mechanical response of the leaf. I now describe a different method for demonstrating the change of conductivity effected by the electrotonic block, the record being taken on a fast-moving plate.

of 30 mm. from the Riq. ,7. Record of Electrotonic Block, pulvinus. Half-way uppermost record normal ; lowest record bctwCGH tllG point of shows block of transmission ; middle record two polarising electrodes, 5 mm. apart, through which a constant current would be maintained constituting an electrotonic block. The first and uppermost of the three records (fig. 27) was taken without the block : the velocity of transmission was 29 mm. per second. The blocking current was next introduced. In order to prevent the excitation due to sudden make or break of the current, the blocking current, having an E.M.F. of 2 volts, was gradually introduced or withdrawn by the proper manipulation of a potential-slide. During the passage of the blocking current the testing-stimulus of induction-shock was applied, as in the first experiment of the series. The lowest record

shows that there was no response, the transmission of excitation being effectively blocked. In order to show that the block persists only during the passage of the current, the latter was next gradually reduced to zero. On again repeating the stimulation the block was foimd to be no longer operative (middle record) and the excitation was transmitted at practically the same rate as at the beginning. The slight increase in the rate is probably due to facilitation caused by previous stimulation.

Reference has been made to the inconclusive character of Pfeifer’s narcotisation experiment. The ineffectiveness of the block, it was explained, might have been due to the thickness of the petiole preventing free access of the anaesthetic to the conducting elements in the interior. It occurred to me that the physiological block induced by a drug could be made more efiective by the employment of solutions of strongly toxic agents, like copper sulphate or potassium cyanide. The choice of a strong poison was deemed advisable, because the absorption of even a small quantity might then prove effective in inducing a depression culminating in abolition of conduction. The application of chloroform has the drawback that its vapour is always escaping, with resultant stoppage of absorption ; an additional drawback is that the spreading vapour renders the motile organ insensitive. There is no such disadvantage in the employment of a non-volatile poison like copper sulphate or potassimn cyanide in solution, which by local application would affect the conductivity of the petiole without modifying the motile sensibility of the pulvinus. The poisonous solution was applied on a strip of cloth TO mm. wide, wrapped round the petiole, midway between point of stimulation and the pulvinus.

I will now describe experiments on the effect of poison applied on a narrow zone of the petiole of Mimosa. Successive records of transmission-time were taken at intervals of 20 minutes, before and after subjecting an intermediate portion of the petiole to the action of poison. Effective stimulus of induction-shock of intensity of 2 units was applied on the petiole, generally at a distance of 30 mm. from the pulvinus. The first record of the series gives the velocity of normal conduction ; the second and the subsequent records exhibit the progressive action of the toxic agent.

Experiment 42. Copper sulphate solution.— normal record (i) in fig, 28 shows response to have taken place Fig. 28. Ejffect of Copper Sulphate solution in the retardation and final arrest of Conduction. (i) Normal record; (2) Retardation caused by 20 minutes' application ; (3) Arrest caused by application for 40 minutes ; 27 spaces after the application of the stimulus, the interval between the successive dots being -i second. The total time was therefore 2-7 seconds. Subtracting from this the latent period -15 second, then 2-5 seconds is the actual transmission-time through 30 mm. The time of transmission through 10 mm. is therefore ‘83 second. Record (2) of the series shows the effect of application of copper sulphate solution for 20 minutes on a portion of the petiole 10 mm. in breadth. It will be noticed that the transmission-time was prolonged by 10 spaces, i.e. by I second. Assuming that the effect of the poison was

localised, it appears that it had, during 20 minutes’ application, increased the time of transmission through 10 mm. from -83 to 1-83 seconds. By the absorption of a small quantity of poison the conductivity had thus been reduced by more than 50 per cent. Record (3) of the series was taken after a further period of 20 minutes. The transmitted excitation is seen to be completely blocked, as the result of the application of copper sulphate for 40 minutes. In order to show that the absence of response was due not to the abolition of motile excit-

Fig. 29. Abolition of Conductivity by the action of Potassium Cyanide. (i) Normal record ; {2) Arrest of conduction after application for five minutes ; (3) Record showing arrest of impulse* even with strong stimulus ; (4) Record of direct stimulation. ability of the pulvinus, but to the block of conductivity of the petiole, a fourth record was taken under direct stimulation, which proves that the motile excitability of the leaf had remained unimpaired.

Experiment 43. Mercuric chloride solution. — A series of records was taken exhibiting the effect of mercuric chloride solution. Conduction was found to be arrested after a period of application so short as 10 minutes. The record obtained was very similar to that given in fig. 29. Experiment 44.' Potassium cyanide solution. — I also took a series of records exhibiting the effect of strong solution of potassium cyanide. Record (i) gives the normal transmission-time (fig. 29). Record (2) was taken, as before, after allowing 20 minutes for recovery. The poisonous solution was, however, applied 15 minutes after the previous record ; consequently the second record shows the effect of application of potassium cyanide for a period of 5 minutes only. The effect of this poison on the conductivity of the petiole of Mimosa was so great that even with such a short application, the transmission of excitation caused by maximal stimulus of two units was completely blocked. Record (3) was taken with the secondary coil pushed close to the primary, the stimulus-intensity being thus raised to 15 units. Even under this intense stimulation conduction was found to be arrested. Record (4) was obtained under direct stimulation. The response shows that the motility of the pulvinus had undergone no change. It is thus clear that the abolition of response to indirect stimulation was solely due to the abolition of conductivity induced by the action of the poison.

Rise of temperature, within limits, increases the velocity of transmission of excitation in Mimosa; conversely, lowering the temperature diminishes the velocity. The transmission of excitation can be arrested by inducing physiological block in the conducting tissue. Under excessive lowering of temperature, the power of conduction becomes paralysed ; the paralysis persists for a time even after return to a normal temperature. The condition of paralysis can be quickly removed by tetanising induction-shocks.

Conduction is inhibited by the interposition of an electrotonic block. On the stoppage of the blocking current the normal velocity of conduction is restored. Application of poison, at an intermediate region in the path of conduction, at first lowers the velocity : the depression.culmkiates in permanent abolition of conduction in tfe poisoned area. 1 The time required for abolition of conduction depends .on.-thQ TOiSalenc^ of 'the poison employed. The abolition ,ig..much- -more rapid under potassium cyanide than under copper sulphate solution.

When a stimulus is applied directly on the pulvinus of Mimosa, the leaf undergoes an almost instantaneous fall. The latent period, i.e. the interval between the application of stimulus and the resulting response, is about o • i second. Indirect stimulation, i.e. application of stimulus at a distance from the pulvinus, also causes a fall of the leaf ; but a longer interval elapses between the incidence of stimulus and the response, for a certain length of time is required for the transmission of the generated impulse through the intervening conducting tissue.

Conductivity varies widely in different plants and their organs. In thin petioles of Mimosa the velocity has been shown to be as high as 400 mm. per second. In the stem the velocity is considerably less, about 5 mm. per second in the longitudinal direction; but conduction across the stem is a very much slower process. I have shown that a stronger or a more prolonged stimulation is necessary to overcome the resistance offered to the passage of excitation in a transverse direction (Experiment 18). The velocity of conduction in other plants is very much lower. In the petiole of Averrhoa the longitudinal velocity is of the order of I mm. per second.

The mechanism of movement of the leaf depends ultimately upon variation of turgor in the pulvinus. The following experiments demonstrate the characteristic movements produced by changes of turgor of the motile organ. Experiment 45. Effect of increase of turgor. — A young Mimosa plant was carefully removed from the ground and placed, with the soil adhering to the roots, in a linen bag. This was held securely by a clamp, and one of the leaves attached to the recorder. Water was withheld for a day.

Fig. 30. Response of pulvinus of Mimosa pudica to variation of turgor. Increased turgor by application of water at point marked with vertical arrow induced erectile movement. Diminution of turgor by application of KNOg solution at the point marked with the horizontal arrow, brought about the fall of the leaf within 8o seconds. Successive dots at intervals of 5 seconds. (The down-curve represents up-movement and vice versa,) the result being a general loss of turgor throughout the plant. A vessel filled with water was now raised from below so as to moisten the roots. The absorption of water led to an increased turgor of the pulvinus, which caused an erectile movement of the leaf. The distance between the

immersed portion of the plant and the leaf was 20 mm., and the up-movement of the leaf was initiated 10 seconds after the application of water. Experiment 46. Effect of diminution of turgor. — While the leaf was in the process of erection, a change was produced by substituting KNOg solution for the water supplied to the roots. The plasmolytic withdrawal of water gave rise to a wave of diminished turgor, the effect of which became perceptible within 40 seconds, by the responsive fall oi the leaf (fig, 30).

Thus the leaf is capable of two distinct mechanical reactions : (i) an erectile or positive movement indicating expansion and enhancement of turgor, and (2) a fall or negative movement indicating contraction and diminution of turgor. The motile leaf or leaflet is merely an indicatoi which manifests the induced changes of turgor in the pulvinus. Direct stimulation induces a sudden diminution of turgor resulting in a sudden fall of the leaf. A strong indirect stimulus applied to the highly conducting petiole of Mimosa gives rise to an impulse which quickly reaches the pulvinus and, like direct stimulation, causes a fall of the leaf. This excitatory impulse may, since it induces a negative response, be distinguished as negative. When, however, a moderate stimulus is applied to a less actively conducting tissue, such as the semi-conducting petiole of Averrhoa, and at a considerable distance from the leaflets, the impulse transmitted from the point of stimulation causes an erectile movement of the leaflets as it reaches them ; that is to say, the response is not negative but positive, and the impulse inducing it may be distinguished as positive. But when the distance of transmission is somewhat reduced, the response is not single but double; the erectile, positive movement is followed by a fall or negative movement.

I here give a detailed account of a typical experiment carried out with Averrhoa Carambola, which, clearly brings out the characteristic effects of Indirect Stimulation. Experiment 47. — Stimulus of electric shock applied at a point on the long petiole of Averrhoa causes the successive Festive erectile response represented as a down-curve : the more intense negative response of fall shown as an up-curve. (Averrhoa.) Successive dots at intervals of one second. Stimulation indicated by short vertical line.

of I second (fig. 31). The record shows that an identical stimulus causes two responses of the distant leaflet, the positive followed after a considerable interval by the negative. The positive or erectile response occurred 1-5 seconds after stimulation, the recovery being completed in 16 seconds. After a further delay of 26 seconds, during which the responding leaflet remained quiescent, the normal negative response occurred by the excitatory fall of the leaflet. These characteristic results lead to the

conclusion that indirect stimulation gives rise to two impulses and that they travel with different velocities. The movement in response to the positive impulse is slow, whereas that due to the negative is very abrupt, almost ‘ explosive,’ the successive dots in the record being relatively far apart. As regards the velocity of transmission the relation is reversed, the positive being the quicker of the two. In the present case, the velocity of the excitatory negative impulse was i • i mm. per second, as against 33 mm. for the positive impulse.

In the last experiment the stimulus was apphed at the moderate distance of 50 mm. When it was applied at a greater distance the response was positive only, for the reason that, in a tissue whose conducting power is not great, the excitatoiy (negative) impulse is weakened even to extinction in transmission through a long distance, whereas the positive impulse is not weakened to the same extent by long transmission. The negative impulse may thus fail to reach the responding organ, whilst the positive does so, in which case stimulus applied at a distance will give rise only to a positive response. When, however, the ^stance of transmission is reduced, a different result is obtained. The negative now overtakes the positive impulse, and, since it is the more intense of the two, the feeble positive response will be masked by the superposed negative. The separate exhibition of the two responses is therefore only possible when there is a sufficient lag of the negative impulse behind the positive. The lag increases with the increase of distance of transmission and decreases with its diminution. Application of stimulus near the responding organ therefore gives rise only to a negative response, which masks the positive.

From what has been said it wiU be understood that the exhibition of positive response is favoured when the transmitting tissue is semi-conducting and the stimulus feeble. It is thus easier to exhibit the positive effect with the feebly conducting petiole of Averrhoa than with the better conducting petiole of Mimosa. _ It is, however, possible to obtain positive response in Mimosa under suitable conditions. The positive response can be obtained with the stem, in either longitudinal or transverse conduction. The rate of longitudinal transmission in the stem is much slower than it is in the petiole, hence under sufficiently feeble stimulus only a positive response is obtained; undei stronger stimulus it is followed by a negative response. It should be remembered that previous stimulation enhances conductivity, so that ineffective conduction becomes gradually effective ; the response may thus become diphasic, that is, positive folloiyed by negative (Experiment 48) .1

The transverse conductivity of the stem is, as has been shown, very feeble. Hence positive and diphasic responses can be easily obtained when the conduction is in a transverse direction (Experiment 49). Positive Impulse in Stem of Mimosa : Longitudinal Transmission Experiment 48.— The testing stimulus was applied to the stem at a distance of 10 mm. below the responding leaf. The intensity of stimulus in the three successive experiments was increased from i to 5 and then to 8 units. The first and the lowest record of the series (fig. 32) was taken under the stimulus-intensity of i. Under this relatively feeble stimulus, a positive or erectile response, indicative of increase of turgor in the pulvinus, was alone induced, 0-7 second after stimulation; there was no indication whatsoever of the occurrence of a negative response. After the usual interval of 20 minutes the next or middle record was taken under a stronger stimulus of 5 units. The

1 Cf . Plant Response, p. 535 ; Comparative Eledro-Physiohgy, p, 64 : Irriiability of Plants, p. ipS* response was now diphasic, positive followed by negative ; the positive took place o-6 second, and the negative 9 -4 seconds, after the application of stimulus. Finally, when the stimulus-intensity was raised to, 8 units, the positive Lowest record under stimulus-intensity of i, response positive ; middle record under 5, response diphasic ; uppermost record under 8 units, response diphasic with shortened interval,

response took place after the same interval as before, but the negative or excitatory response occurred earlier than in the last case, that is to say, after an interval of 4 • 6 seconds instead of 9*4 seconds. Experiment 49. — I have obtained a positive response to transmitted impulse even in the highly conducting petiole. The stimulus in this case has to be considerably reduced. The difficulty in the detection of the positive response arises from its small amplitude. This drawback is completely removed by the employment of my Optical Lever (' Plant Respornse,' p. 5), which magnifies the responsive movement of the leaf more than a thousand times. The optical lever affords many advantages for demonstration ; the weight of the lever is slight, and there is practically no friction. When a sub-mininial stimulus is