The Motor Mechanism of Plants

I will next show how the effect of external stimulation is modified by the tonic condition of the pulsating tissue, describing first the effect on the heart. Effect of strong stimulation on vigorous heart-beat . — Intense stimulation has an inhibitory effect on vigorous pulsation. The effect of stimulation on the heart in a subtonic condi¬ tion is, however, very different ; it enhances the activity Fig. 160. Revival of Frog’s heart-beat by feeble and by strong

Feeble stimulation gave rise to a single response seen at the beginning of record ; subsequent series of multiple responses are due to stronger stimulation. •when the pulsations are feeble and renews the pulsation when the heart is in a state of standstill. Experiment 165. Effect of moderate stimulation on heart at standstill. I ne heart after isolation gradually came to a state of standstill. Stimulation was then found to revive the p ilsation. 1 he heart can, moreovc", be stopped by StanniuT ligature. A single moderate stimulation then produces a single pulsation.

Experiment 106. Effect of stronger stimulation . — -The effect of moderately strong stimulation on the heart brought to standstill by Stannius’ ligature is shown in the series of multiple responses (fig. 160). Parallel effects were observed wita the leaflet of Desmodium (cf. Experiments 146, 147). Experiment 167. Effect of strong direct stimulation on vigorous pulsation of Desmodium. — The normal pulsation becomes inhibited by strong electric stimulation ; there is

Fig. i6i. Effect of strong direct electric stimulation at s : temporary arrest of Desmodium pulsation. . often a revival on the cessation of stimulation, the amplitude of pulsation being even greater than the normal (tig. 161). Experiment 168. Effect of strong indirect stimulation on Desmodium pulsation. — The petiole carrying the leaflet was Fig 162. Effect of strong transmitted excitat on in depressing pulsatory activity in a vigorous specimen.

Note the gradual removal of inhibitory effect on cessation of subjected to strong electric stimulation ; this had an inhibi¬ tory effect, as shown by the diminution of the amplitude of ^ pulsation (fig. 162). It is known that certain chemical agents induce charac¬ teristic modifications of the cardiac activity. Experiments described below show that their effects on the pulsatory activity of Desmodium leaflet are essentially similar. Experiment 174. Stimulating action of Spiritus Atfr- moniae Arcmaticus on heart-beat — The effect is clearly seen

in a pair of records given in fig. 168. The normal heart- beat was extremelv feeble. But application of the drug caused a great increase in the amplitude of pulsation. Experiment 175. Stimulating action of Spirit. Ammon. Arom. on Desmodium pulsation. —The isolated pulvinule was immersed in water and its normal record taken ; a few drops of the drug were then added to the water. This caused a marked increase in the amplitude of pulsation (fig. 169).

Experiment 176. Effect of Potassium Bromide solution on Desmodium pulsation. — Being under the impression that KBr is a depressant, I applied a dilute solution on the pul¬ sating pulvinule. To my great surprise I found that instead of causing depression it induced enhancement of activity Fig. 1 70. Stimulating action of a minute dose kbr on Desmodium pulsation ; larger dose KBr causes depression. as indicated by an increase in the amplitude of pulsation. The depressing action was produced only after continued application or with a stronger dose (fig. 170). \

cooled or warmed water can be passed through the vessel, and records taken at different temperatures. I also wished, in this connexion, to determine the effect of the temperatures to which the plant had been habituated at different seasons of the year. In winter the minimum temperature in Calcutta is about 15 0 C.f while in summer it is 220 C. Experiment 171. Determination of the thermo-metric minimum in a winter-specimen. — The record wTas commenced

Fig. 165. Effect of cooling in arresting the pulsatory activity ot ISiote that the thermometric minimum was at or about 14*5° C. at 150, and cooled water passed through the bath, which reached the temperature of 8°. Long before this, pulsation had become arrested. The bath was tneu allowed to reach the temperature of the room, and the pulsation began to e revhed at. 14° (fig. 165). The thermometric minimum of the particular winter-specimen may therefore be taken as the mean ^ of 15 and 14 , i.e, 14*5°, in contrast with 17*5° which is the minimum for a summer-specimen.

Experiment 172. Effect of variation of temperature from 23 to 400.— The automatic record given by the plant (fig. 166) gives a vivid representation of the variation of pulsatory activity. At 230 the amplitude of pulsation was very large, the period of a single pulsation being 4-75 minutes. At 28° the period of each pulsation had become shortened to 2-4 minutes, which means an increase of frequency of pulsation. The amplitude had decreased, but Fig. 166. Effect of rise of temperature in increasing frequency

Pulsation temporarily arrested at 40° C. and revived at 37" C. Successive dots at intervals of 5 seconds. this must not be taken as an indication of depression of activity ; it is due to the quickening of the rate which did not allow time for complete expansion or contraction. At 38° thel period was further shortened to 1-25 minute ; at qov the amplitude was so reduced that the pulsation appeared to be arrested. That the pulsation did not undergo any permanent arrest is clear from the fact that on lowering the tempera-j ture to 370 there was an immediate revival of pulsation.

I give below a table showing the effect of variation of temperature on the period and frequency of pulsation. Table XV. — Effect of Variation of Temperature on Pulsation of Desmodium Leaflet. Accurate determination of the effect of variation of tem¬ perature on the frog’s heart in situ is, as previously stated, a matter of extreme difficulty. This has been overcome, in the majority of cases, by the following experimental device. I have explained that the effect oi external agencies on the heart can be studied by subjecting the sinus alone to changing external conditions.

Experiment 173.— A stream of normal saline at different temperatures is projected on the sinus and on a sensitive thermometer placed on its side. The variation of tem¬ perature of the sinus can thus be ascertained with great accuracy. 1 he record of the heart-beat was taken at the attainment of a steady temperature. The experiment was commenced at o° C., at which the pulsation was arrested. It became feebly renewed at 5“ C. and more pronouncedly revived at 10 C. The period of each pulsation became shorter with the rise of temperature. At 50 C. the period was 3 seconds ; at io° C., 2-8 seconds; and at 15° C., 2-1 seconds. Rise of temperature thus caused a shortening uf the period, that is, increased its frequency, ihus the frequency of pulsation

Records to be read from below upwards for or to 1 50, and downwards for 220 to 450. Note small amplitude of pulsation at 45 \ increased on fall c;f temperature to 430 C. though with reduced amplitude. W hen the temperature was lowered to 43° the amplitude became immediately increased (hg. 167). A somewhat parallel effect was observed with Desmodium (cf. hg. 166). The effect of stimulation in reviving pulsation of the Desmodium leaflet has already been given in the previous chapter (cf. figs. 143, 144). I describe below the effects of direct and indirect stimulation on feeble pulsation.

Fig. 163. The feeble automatic pulsation in dark enhanced after Experiment 169. Effect of direct stimulation on feeble pulsation of Desmodium. — -The specimen was kept for a time in the dark with resulting enfeeblement of activity due to lowering of its tonic condition. Direct exposure to the Fig. 164. Transmitted effect of moderate a d indirect electric stimulation enhancing the pulsatory activity of Desmodium in a subtonic condition. stimulus of light caused a great increase of pulsatory activity (fig. 163).

Experiment 170. Effect of indirect stimulation on feeble pulsation. -T he leaflet was in a subtonic condition and its pulsatory activity was feeble. Electric stimulation of moderate intensity was applied on the petiole bearing the leaflets. The transmitted excitation caused an enhance¬ ment of the pulsatory activity (fig. 164). V The important result established is that while stimulation inhibits pulsation in a vigorous specimen , it revives or enhances it in a subtonic specimen . Two opposite effects are thus produced which depend on the tonic condition of the tissue.

I have in a previous work 1 described the effect of varia¬ tion of temperature on sumn'ier-spedimens of Desmodium. The results were that the pulsation of the leaflet became slowed down and finally arrested at a thermometric minimum •which was about 17-5° C. Rise of temperature, on the other hand, increased the frequency of pulsation ; at a tem¬ perature of about 42 0 the frequency was increased, but the amplitude became so small that the pulsations appeared to have come to a stop. This was by no means due to coagulation of protoplasm and death of the plant; for subsequent lowering of temperature by a few degrees caused an immediate revival of pulsatory activity.

The method previously employed for raising the tem¬ perature of the leaflet was to enclose it in a moist chamber, the temperature of which was gradually lowered by blowing in cooled air, or raised by electric heating. But it was impossible to be certain whether the tissue in the interior of the semi-conducting plant acquired the temperature of the chamber. The only certain way of ensuring definite thermal variation of the tissue is to immerse the specimen in a water-bath (cf. fig. 156), the temperature of which can be accurately adjusted.

The method of obtaining a record of the pulvinule immersed in water has the advantage that a stream of Experiment 1 77. Effect of dilute Potassium Bromide on cardiac activity . — ihe result described above was so unex¬ pected that I investigated the action of a minute dose of bromide on the heart-beat. On applying it on the sinus, Fig. 1 71. Opposite effects of minute and strong doses of KBr on N, normal ; kbr, stimulating action of a minute dose ; KBr, depression produced by a. large dose (Frog).

the pulsation showed a great increase of amplitude ; a stronger dose, however, produced a marked depression (fig. 171). It is very interesting to find that KBr, which is universally regarded as a depressant, causes stimulation in a sufficiently minute dose. A most interesting experiment is that on the f antago¬ nistic action ’ of drugs. Thus Muscarin and Pilocarpin are known to arrest the heart at diastole. Atropin, however, acts as an antidote, reviving the arrested activity (fig. 172).

Experiment 178. Antagonistic action of Atropin and Pilocarpin on Desmodinm pulsation —It is very remarkable Fig. 172. Antagonistic action of Muscarin and Atropin m arresting and reviving the pulsatory activity of the heart (Frog). that the two drugs should exert similar antagonistic effects on the pulsation of Desmodium. The arrest produced by Pilocarpin was revived by the action of Atropin (tig. 173). The establishment of this identity of reaction in the plant and animal has led to the discovery of the stimulating action of extracts from various Indian plants the physiological properties of which had not hitherto been suspected. An example of this is given on p. 272.

tiG. 174* KevivaJ of depressed activit}r of Desmodium by extract of an Indian plant, Abroma. N, normal ; KBr, depression due to application of KBr solution ; Abr, revival of activity by extract of Abroma. Fig. 175. Parallel effects on pulsating activity of Frog's heart. N’ normal ; M3r, deprt sed activity under Potassium Bromide • Abr, revived activity under Abroma extract. Experiment 179. Effect of Abroma augusta on Des - modium pulsation . — The pulsation of Desmodium was depressed under the action of KBr ; application of extract of Abroma removed the depression (fig. 174)- fed

to experiment on the action of Abroma on the animal heart. The upper record exhibits normal pulsation of the frog's heart ; the middle record shows depression induced by KBr ; the lowermost exhibits the revival of activity induced by Abroma (fig. 175). Variations of external conditions induce similar modi¬ fications' in the pulsating activity ol the animal and of the plant. A diminution of internal tension causes depression or arrest of the heart-beat, the pulsation being revived by suitable increase of intra-cardiac pressure. The pulsation of Desmodium is likewise arrested by a diminution of internal hydrostatic pressure, the rhythmic activity being renewed on restoration of the normal pressure.

A supply of oxygen is necessary for maintenance of both cardiac and Desmodium pulsation. Carbon dioxide has an asphyxiating action,, arresting pulsation. .jj The pulsation of the heart, as well as that of the Desmodium leaflet, undergoes arrest in the condition oi subtonicity. In this state of standstill, the pulsatory activity becomes revived on external stimulation. fl Stimulation inhibits pulsation in vigorous specimens, while in subtonic specimens it enhances or revives it. lwc| opposite effects are thus produced which depend on die tonic condition of the tissue.

Variation of temperature induces characteristic mod r ca¬ tions of rhythmic activity, which are similar in the animal and in the plant. Arrest occurs below a thermometrj minimum, the pulsations becoming revived as soon as thfr temperature is raised above the critical point. With fur chef! rise of temperature, the frequency of pulsation, within limits, undergoes an increase. Chemical agents induce reactions which are very similar in the animal and plant.

Stimulating agents like Spiritus Ammoniac Aromaticus enhance the rhythmic activity in both animal and plant. Potassium bromide, which is universally regarded as a depressant, causes enhancement of activity in both animal and plant when in minute dose ; a stronger dose induces the well-known depression in both. The effect of one drug is often antagonised by that of another. Muscarin and Pilocarpin cause arrest of the heart, whereas A tropin, by its physiological antagonism, revives the activity. Effects exactly parallel are induced in the pulsating leaflet of Desmodium.

1 he characteristic effects of different drugs on plant and animal are so very similar that the physiologica1 action of extracts of various Indian plants on the activity of the animal heart has been discovered by their action on the pulsation of Desmodium. The results of the experiments described above prove that the rhythmic mechanism in plants is essentially similar to that of animals. Experiments were described in the two previous chap¬ ters proving that plants possess an automatic rhythmic mechanism similar to that of animals. Further, continuity between ordinary response and automatic movement was established through the intermediate link ot multiple re¬ sponse. Broadly speaking, two types of rhythmic activity were described as occurring in plants, namely :

during external stimulation and for a short time afterwards. * maintained, seemingly without any external stimu¬ lation. Prolonged isolation from the action of environmental stimuli, however, brings the pulsa¬ tion to a state of standstill, to be revived once more on fresh exposure to stimulation. 1 Various degrees of automatic activity are observed in plants, some being extremely quick and regular, others being less so. Is there also in the rhythmic tissue of the animal a similar gradation of automaticity, and is thel difference dependent on the special physiological function that the tissue is called upon to subserve ?

The heart is essentially a tubular organ,1 each section of which is rhythmic, and in which excitatory contraction proceeds onward from point to point as a peristaltic wave. The rate of propagation of the wave in the animal heart is about 10 to 15 mm. per second, very much slower than the excitatory impulse in nerve. There are, however, certain resemblances in the propagation of the two impulses. In the cardiac tissue, the propagation occurs by sequence of contraction from cell to cell, the passage of excitation being manifested by the visiHc propagation of a contractile wave. The evolutionary process appears to have been carried further in the nervous tissue, the quicker rate of conduction of im¬ pulse being secured at the expense of contractility. The nervous impulse is invisible, and its propagation can only be detected by the electromotive change that accompanies it. There are two independent methods of intercommunica¬ tion between more or less distant organs : the relatively slow propagation of a visible peristaltic wave in cardiac and other tissues, and the rapid transmission of an invisible impulse in the nervous tissue.

Is there any other organ in the animal in which similar peristaltic activity for propulsion has been developed, though to a less perfect degree ? It is well known that the alimen¬ tary canal possesses a peristaltic mechanism, and I will show that it is essentially similar to that of the heart. Since the In the most primitive form, the vertebrate heart is composed of a simple tube, in which a contraction starts at the venous end and is pro- pags ted in a wave-like manner along the tube to the arterial end In the