The Motor Mechanism of Plants

The question that naturally arises is : Why should the outflow of energy in the form of responsive movement occur in a pulsatory manner ? In explaining this I will describe certain important characteristics of rhythmic tissues, such as those of the heart of the animal and of the pulsating pulvinule of the Desmodium leaflet One such characteristic is the relatively sluggish contraction of the rhythmic tissue. 1 hus the period of contraction of the cardiac tissue is considerably longer than that of a voluntary muscle. Similarly, while the period of contraction of the

pulvinus of Mimosa is about 1*5 second, that of the rhythmic pulvinule of Desmodium is very much longer, i.e. about 45 seconds. Fig. 149. Response of Biophytum leaflet to stimulations of o*i, 0*5, i, and 2 minutes. relations of a single pulsation of Desmodium, I applied an induction- shock to the quiescent leaflet and took the record Fig. 150. Single response of Desmodium leaflet. Successive dots at intervals of 1 second. of response on a fast-moving plate. The frequency of oscilla¬ tion of the recording -plate was once in a second, hence successive dots represent that interval of time. The com¬ plete response, consisting of the contractile fall of the leaflet

and subsequent recovery, was accomplished in the course of 2 minutes and 45 seconds ; the leaflet attained its maximum systolic contraction, shown by the up-curve, 45 seconds after the application of the stimulus. The top of the response curve appears flat, indicating a persistent contraction from which the recovery is relatively slow. The period of diastolic expansion was about 120 seconds (fig. 150). That the characteristics of the response of the rhythmic tissue of Desmodium are similar to those of the rhythmic cardiac tissue of the animal is demonstrated by the following

F - 15 1. Stimulus applied at systolic phase A produced no effect Au extra pulsation was induced by stimulation during diastole 13. [Heart of Frog.] Fig. 152. Extra pulsation of Desmodium under stimulation f at experiments. In the cycle of the heart-beat there is a refractory period during which the tissue takes no account of the stimulus, or is refractory to it ; this refractory period lasts throughout the period of systolic contraction ; the irritability begins to be restored during diastole, the maximum excitability being attained after the completion of diastolic recovery. The restoration of excitability is shown by an extra contraction produced by stimulation applied at any period of diastole (fig. 15 1).

The pulsation of Desmodium exhibits similar charac¬ ter! stjcs as shown by the following experiment. Experiment 157. — The application of electric stimulus during contractile systole was found to have practically no effect on the pulsation. But the application of an induction- shock during diastolic expansion not merely arrested the expansion but caused contraction and an extra pulsation (fig. 152). 1 The response of a rhythmic tissue to continuous external or internal stimulation is not a continuous contraction, but repeated contractions of a pulsatory nature. The explana¬ tion of this is td be found in the ' refractory period ' of the tissue. After each discharge, the excitability disappears for a time, to become slowly restored for the second and ; subsequent responses. There is thus an oscillatory variation in the excitability of the tissue. A physical model will help in visualising the process of rhythmic response. Imagine a reservoir into which flows a constant supply of water ; the elastic overflow-pipe is constricted by a compressing spring. O11 the far end of the pipe abuts the flat end of the indicating lever. Constant supply of water raises the level in the reservoir ; when the pressure of the water-column becomes sufficiently great, the spring which keeps the elastic tube constricted gives jway and there is an impulsive dis¬ charge of water, producing a responsive movement of the indicating lever. The yielding spring closes again, and the tube becomes once more constricted with cessation of out¬ flow. On account of the oscillating mechanism, the outflow and consequent mechanical response are periodic, though the supply is constant. It is clear that a rhythmic series of effects, such as pulsatory responses, need not have a periodic antecedent cause. The stored energy is discharged in a rhythmic manner by the periodic change of excitability ^

The energy which expresses itself in pulsatory move- >| merit is ultimately derived from external scurces of stimu¬ lation. The persistence of pulsatory activity after the cessation of external stimulation depends on the capacity for storage. Certain tissues have but small capacity and the after-effect is short-lived ; in other cases, the capacity for storage is large, and the pulsatory activity persists for a very considerable length of time. The plant is exposed under natural conditions to the action of various stimuli, supplied by its environment. It is subjected to variation of temperature, to the action of light, to the mechanical stimulus of air-currents, and to the action of various chemical agents present in it or absorbed by it. From the joint action of these sources of stimulation, the tonic condition is raised above the optimum with result¬ ing excitatory overflow. Thus, on a warm and sunny day, a particular leaflet of Biophytum may have been moved by a puff of wind, or by the alighting of a small insect, or by the accidental touch of an adjacent blade of grass. The excitation thus initiated in the super-tonic tissue is enough to set all the leaflets of the plant quivering in an extra¬ ordinarily lively manner. The origin of the impulse can be traced in some cases, but not in others which therefore appear as spontaneous. In Mimosa, growing in the open, but under glass, I have on many occasions noticed seemingly spontaneous activity by which the leaflets underwent a rippling closure, followed by the fall of the leaf. The effect was observed when the plant was in a highly excitable condition.

The results described prove that there is no rigid line 0.! demarcation between multiple and automatic response. An ordinary responding plant, like Biophytum, which gives a single response to a feeble stimulus, and multiple responses to a moderate stimulus, passes into an automatically pulsating condition by absorption of excess of energy from without. Desmodium, with its pronounced auto¬ matic activity, becomes reduced to the condition of an ordinarily responding plant by depletion of its store of 1 energy ; the tissue in this subtonic condition gives a 1 single response to a single moderate stimulus, and multiple | responses to a strong stimulus. Biophytum is thus equiva- J lent to a Desmodium brought to a state of standstill by J depletion of its store of energy; and Desmodium to a J Biophytum with an excess of stored energy. j

The automatic rhythmic activity of the leaflet of Desmodium comes to an end when its store of energy is 1 exhausted after isolation from the stimuli of its normal 1 environment. In this state of standstill, response occurs under fresh stimulation ; a feeble stimulus gives rise to a single response, j while a strong stimulus causes a series of multiple responses. An intermediate link between ordinary and automatic response is found in the multiple response exhibited by k Biophytum, Averrhoa, and other plants. These give a single response to a feeble, and a recurrent series of responses j to a strong stimulus.

Multiple responses, mechanic or electric, are induced t by various modes of stimulation, such as electric shock, | light, thermal shock, and chemical excitation. .j The rhythmic character of the response is brought about j by a periodic variation of excitability, due to the refractory | period of the tissue. There is no strict line of demarcation between the phenomena of multiple and of automatic response. Under 1 favourable conditions for absorption of energy from without, an ordinarily responding plant like Biophytum becomes converted into an apparently automatically moving plant ^ like Desmodium. Conversely, under unfavourable circrnn- \ stances brought about by isolation from the stimuli of iU normal environment, the leaflets of an automatically 1 responding plant like Desmodium come to a state of |

standstill ; it then becomes converted into an ordinarily responding plant like Biophytum. In this state of standstill the Desmodium leaflets respond to stimulus in exactly the same way as do those of Bio¬ phytum. To feeble stimulation they respond by a single pulsation ; strong stimulation causes a series of multiple responses in both. The duration of multiple activity depends on the amount of energy absorbed from external stimulation. The per¬ sistence of this after-effect depends on the greater or lesser capacity of the tissue for storage of energy.

Automatic or spontaneous pulsation is tnerefore not self- originated, but is really due to the previous absorption of energy from the stimuli of the environment. The automatic pulsation of the. leaflet of Desmodhm gyrans has been described in the previous chapter. The question which will now be discussed is whether the rhythmic mechanisms in the plant and the animal are essentially , similar or widely different. For successful investigation of i the subject it was necessary to devise recording appliances | of great sensitiveness and accuracy, and to adopt new * methods of experimentation.

The oscillating device for recording the pulsation of the j leaflet of Desmodium has already been described ; the error due to friction is completely remo\ed by the method of i the dotted record ; the record itself, moreover gives the time-relations of the different phases of the pulsatiou. Ihe record of the pulsation of the animal heart by the lever- j recorder, the cardiograph, labours under the serious dis- j advantage of the continuous frictional contact, a source of 9 error in the accurate record of the amplitude and time- relations of the heart-beat. The drawback of continuous contact is eliminated in my Resonant Cardiograph ( tig. I53)> *1 which records the pulsations with great precision by a series j of periodic dots, the cardiogram being also its own chrono- , gram. The great advantage of the intermittent over the ; continuous contact is illustrated by two records of pulsation of an identical heart, in which the lower was taken with

continuous, and the upper with intermittent contact (fig. 154). The continuous-contact record is irregular on account of the slight variation of the friction at different parts of the recording surface ; the intermittent record, on the other hand, exhibits a very remarkable regularity. The phasic alternations of the heart-beat are thus recorded by the Resonant Cardiograph with unprecedented accuracy. The systolic contraction and its persistence, the diastolic

r, the lever-recorder set in resonant vibration by the intermittent current produced by the vibrating reed v. f is the Frog chamber. expansion and the subsequent pause, and variations of these under external agencies, can thus be determined in a quanti¬ tative manner. Experiment 158. Cardiogram of different animals.— As an example of the advantage of the automatic registra¬ tion of extremely short intervals of time in diverse investiga¬ tions, ! reproduce the cardiograms of different animals : those of the tortoise, of the frog/ and of a fish (Ophiocephalus) (ftg- *55) • The resonant writing-lever was tuned to vibrate 20 times in a second, the magnification employed being about

8 times. The time-relation is indicated by the intervals be¬ tween successive dots which are second apart. The record shows the auricular contraction preceding the ventricular. The period of a complete cycle is longest in the tortoise, the total period being 34 dot -intervals, or 1*7 second. The record of the frog’s heart gave 14 dot-intervals or o • 7 second, Fig. 1 S4. Irregular record of lieart-beat of Frog due ,0 unequal friction in the continuous contact (lower record c), com¬ pared with the regular record with intermittent contact

while in the fish the period was 16 dot-intervals or 0-8 second. In the tortoise, after the commencement of the less pronounced systolic contraction of the miricle, the peri¬ staltic contractile wave reached the ventricle in the course of o-6 second, whereas in the frog and in the fish the inter¬ val was only 0-2 second, or one-third that in the toitoise. Other records give very striking visual demonstration ot the relative activities at different phases of the pulsation. When the activity is very great, the markings are wide

apart, and the dots are lengthened into dashes ; with the slowing down of activity, the dots become reduced in Fig. 155. Characteristic cardiograms of Tortoise, Frog, and Fish. The dot-intervJs represent 3*05 second. size and are drawn close together. Under the action of depressing agents, the post-diastolic pause becomes greatly prolonged. 1 now turn to the various difficulties that are en¬ countered in the accurate investigation of the effects of various agents on cardiac activity, and the new method by v hich they have, to a great extent, been removed. In the study of induced variation of cardiac activity, the nearest approach to the normal condition is secured when the heart instead of being detached is left in situ. But the

application of external agents on the mass of the heart now becomes extremely difficult. For example, in the study of thermal reactions, it is impossible to vary the temperature ] of the mass of the heart as a whole with any degree of ‘ accuracy. Similar difficulty is encountered in the study of the action of drugs injected into the circulatory system. The responsive variation is not merely delayed but also modified by the size of the animal. I therefore sought to discover an alternative method kj which would, in a large number of cases, demonstrate in • i the course of a few seconds the specific -reaction of a particu¬ lar drug or the* physiological antagonism produced by a i suitable antidote. Various attempts, made in this direction led me to realise the important part played by the sinus f. in the cardiac rhythm. The different parts of the heart exhibit a descending scale of automatic activity, that of the \ sinus being the highest, whereas it is lowest in the ventricle.

It thus happens that the normal sequence of beat is always sinus, auricle, and ventricle, the highly automatic sinus giving the signal, as it were, for the rest. Any variation induced in the activity of the sinus is thus recorded by the auricle- ventricle, which may be regarded as a convenient indicator. The results given in the course of this chapter will show how many difficulties of investigation have been effectively removed by the local application of various agents on the small area of the sinus.

Before proceeding further, brief reference should be made to the rival theories of the origin of the rhythmic activity, whether nervous or muscular, of pulsating tissues. o The neurogenic theory assumes that the rhythmic impulses are generated by ganglion cells. This theory of the ganglionic origin of cardiac rhythm has been adversely^ affected by the researches of Gaskell and of Fngelmann 1 : Ij chick begins to beat at a time when it is quite devoid ol nerve cells, which only extend to it at a later date.

is slow, like that in a muscle, and not quick as in a nerve. The cardiac rhythm is therefore myogenic and not neuro¬ genic, The normal rhythm can, however, be modified by transmitted nervous impulses. It may similarly be asked concerning the rhythmic pulsation of the Desmodium leaflet : Is it due to periodic nervous impulses coining from the conducting tissue in the petiole, or to automatic contraction of its motor or muscular tissue ? In order to demonstrate the independent rhythmic power of its motor organ, the pulvinule was isolated for investigation, i he pulvinule is excessively small, being 2 mm. in length and 0-3 mm. in diameter. It had to be magnified 4 times to be shown clearly (right-hand lower corner of fig. 156). Would the pulvinule continue to pulsate after isolation * and if so, would it be possible to record movements of an organ so minute and slender ? In pursuit of this investigation I utilised the leaflet l as a con¬ venient handle, which was- clamped within a silver spring s coated with solid paraffin. The pulvinule p was held hori¬ zontal with the lower side downwards. A cocoon-thread attached the tree end of the pulvinule to the recording- lever, the inscription being made on a moving oscillating plate of smoked glass. I he wTiole arrangement was made extremely delicate and sensitive, though I entertained little hope oi success in detecting any pulsation of the isolated

The isolated pulvinule, magnified 4 times, is seen in the lower right-hand figure : /, leaflet ; 5, silver spring ; p, pulvinule. experimentation with the isolated pulvinule offered unique advantages yin several investigations to be presently described. I return to the question raised at the beginning of this chapter, whether the rhythmic mechanism is essentially the same in the animal and in the plant. What are the various features by which all automatic activities are distinguished ? a 1 hese have been studied in the case of the rhythmic : ctivity of the animal heart. The various characteristics of the automatic pulsation of the cardiac tissue are : W

(1) The heart-beat is depressed or comes to a stop when the internal tension or pressure is low, the pulsation being revived by a suitable increase of intra-cardiac pressure. complex organic substance, a supply of oxygen is necessary for the catabolic process of evolving energy for the maintenance of pulsator}7 activity. activity. Rise of temperature increases the fre¬ quency of pulsation ; lowering of temperature, on the other hand, slows down and finally arrests the pulsation at a thermometric minimum.

manner by the specific action of various anaes¬ thetics and drugs. These different criteria will be employed to estimate the characteristics of rhythmic pulsations, first of the pulvinar tissue of Desmodium and later of that in the interior of the It has been stated that a certain amount of internal tension or pressure is necessary for the maintenance of the Fig. 157. Effects of drought and irrigation on pulsation of Desmodium. The first series exhibits normal pulsation ; the second, arrest under drought ; the third, revival caused by irrigation.

rhythmic pulsation of the animal heart, thus the quiescent heart of the snail is made to pulsate by subjecting it to an increased intra- cardiac pressure. A similar effect has been observed in the plant. Experiment 159. — A petiole of Desm odium carrying the two leaflets was suitably mounted in a U-tube filled with water and the normal record taken by the Oscillating Recorder. On ceasing the supply of water the pulsations underwent arrest on account of diminished internal pressure caused by drought ; reapplication of water caused increased turgor and pressure with revival of pulsation (fig. 157).

Experiment 160. — The isolated pulvinule of Desmodium in air gave pulsations which after persisting for a time came to a stop on account of the drying up c f the pulvinule. Another pulvinule was kept immersed in water ; its pulsations how¬ ever, came to a stop after a while. 1 he question arises : To what was this arrest of pulsation due ? It may be thought that one of the probable factors in producing arrest was the abnormal condition in which the pulvinule was placed, namely, immersion in watnr, There was, in reality, another and more important factor, namely, de privation of oxygen, the amount contained in water being liable to exhaustion after a short time. The importance of oxygen will be realised from the following experiment.

Experiment 161. Effect of supply of oxygen on arrested pulsation.— The airest of pulsation of the pulvinule immersed in water is exhibited by the short horizontal portion at the beginning of the record (fig. 15S). I now added * oxygenated water to the water in which the pulvinule ■ had come to a state of standstill. The result was an im¬ mediate revival of pulsation. I his affords conclusive proof of the importance of oxygen in the maintenance of rhythmic activity. A simple way of supply of oxygen is

to add a few drops of highly diluted hydrogen peroxide to the water in which the pulvinule is immersed. Experiment 162. Asphyxiating action of C02. — In continuation of the last experiment, CC)2 water (so-called Fig. 1 58. Effects of oxygen and carbonic acid gas in reviving and arresting pulsation of Desmodium leaflet. The horizontal record l, at beginning, shows arrest of pulsation from lack of oxygen, supply of whith revived it in b. Sub¬ stitution of carbonic acid water arrested the pulsation in c. Fresh application of oxygenated water revived pulsation at d.

soda-water) was substituted for water containing oxygen, this produced a quick arrest of pulsation. Water contain¬ ing oxygen was next applied, with resulting revival of pulsation ( sec last two records, iig. 158). The consumption of oxygen by the pulsating heart is demonstrated by the well-known expeiiment showing that the blood contains less oxygen after passing through the heart than when it entered it. The following experiments which I devised will prove interesting. In repeating with the heart of the frog experiments parallel to those with Desmodium (Experiment 162), 1 first observed the effect of CO2 and afterwards that of oxygen.

Evriprimprit T fil A ftpr.f f)f ROn Otl fvOP S h&(tVt-h6CLt . — Fig. 1 59. Arrest of normal heart-beat of Frog under the action of C02, and its subsequent revival by oxygen, shown in three records. CO2 water was next applied on the sinus by means of a pipette ; this produced an arrest of pulsation in the course of about 30 seconds. Vigorous specimens necessi¬ tated a longer application for bringing about complete | arrest. Experiment 164. Revival of arrested pulsation by supply of oxygen.— The middle record of fig. 159 shows the arrest jj under C02. Application of oxygenated water on the sinus v resulted in the complete revival of pulsatory activity as shown in the third and lowermost series.