The Motor Mechanism of Plants

problem was solved by balancing the living against the dead tissue. Experiment 141. — A living fruit of Carissa suspended from the right pan of the balance was counterpoised by a dead fruit of the same size suspended from the left pan, the second specimen having been previously killed by boiling water. Both were immersed in the same heating bath after exact balance had been obtained by placing fragments of aluminium in one or the other pan. The two specimens

on opposite sides of the balance were thus subjected to the same physical conditions. The record (fig. 135) so obtained exhibits, therefore, the effect of the purely physiological ^variation of the living tissue. The first part of the curve shows a loss in weight which was slight ; in other cases the loss was a little more pronounced. At the critical death-point, in this case %t 570, there is an rbrupt inversion of the curve indicating a sudden increase in weight. In every case the death-spasm is violent in its abruptness and intensity.

I will now endeavour to explain the variation in weight induced by change of temperature, and to give the reason of the similarity between the thermo-mechanical curve and that of the variation in weight. In regard to the apparent weight of the organ measured by the balance, it should be remembered that it is the real weight W minus the weight of the volume of water displaced by the organ. Now, a physiological expansion of the organ must obviously increase the volume of water displaced by it, and thus cause an apparent diminution in weight as recorded by the balance. Conversely, an abrupt contrac¬ tion and diminution in volume of the organ involves a reduction of the volume of water displaced, which causes a sudden increase in its apparent w'eight. The increasing loss in weight up to the critical temperature is therefore due to the physiological expansion of the organ ; the sudden increase in weight and inversion of he curve are, on the other hand, due to the spasmodic death-contraction of the organ as a whole.1

I he thermo-mechanical curve and that of weight-varia¬ tion, recording the effect of rise of temperature to the d .ath-point, resemble each other because they represent the same physiological changes, expansion and contraction. But whereas the method of thermo-mechanical curve is especially applicable in the case of anisotropic organs, the method of weight-variation described in the present chapter extends the scope of inquiry to all organs and establishes a wider generalisation.

l he curve obtained by the Recording Balance shows that a living tissue heated in a water-bath exhibits, during rise of temperature, a continuous diminution in weight till at thJ / iieKdeath-record occasionally exhibits a diminution in weight when thedeath-spa.n, .s attended bv a considerable expulsion 0? sapTom the the critica] temperature of about 6o° C. there is produced a sudden increase in weight. A tissue which had been previously killed exhibits no such variation in weight.

The effect of physical factors is eliminated by balancing a dead organ against a living one ; so the curve thus obtained exhibits the effect of purely physiological variation. The curve of variation in weight under changes of tem¬ perature is in every way similar to the thermo-mechanical curve. Both exhibit a sudden inversion of the curve at the fatal temperature. The change in weight is shown to be due to physiological expansion or contraction. 1 he sudden increase in weight at the critical temperature is due to abrupt volumetric con¬ traction which is the spasm of death.

1 he movements induced by external stimulation have been fully described in previous chapters. There are, however, other movements of a pulsatory nature which are apparently not induced by any external stimulation and are therefore described as automatic or spontaneous. The manifestations of these automatic activities aire some¬ times on a macroscopic scale, the pulsations being readily visible. Ihey are exhibited in a striking manner by the heart of the animal. Similar activities are also exhibited by plants, notably by the leaflets of Desmodium gyruns . There are, in addition, * other pulsatory activities of great functional importance to the life of the plant, which being on a microscopic, scale have not hitherto been suspected. The method of detection and record of these imperceptible pulsations will be described in a later chapter.

Desmodium gyrans, or the Telegraph Plant, grows wild in the Gangetic plain, it is a Papilionaceous plant, with trifoliate leaves, of which the terminal leaflet is large and the two lateral ones vert? small (fig. 136,'. Each of these has a small pulvinule, the periodic contraction and expansion of which cause rnythmic down-and-up movements. Foi experimental purposes it is more convenient to take a detached petiole carrying the pulsating leaflets. The amputation is often followed by an arrest of pulsation, due to the shock-etfect of operation. -The pulsations are however, revived after a suitable period of rest.

Fig. 137. U-tube support for the leaf, and the plant-chamber. The petiole bearing the leaflets is mounted in a U-tube. A stopcock allows the water to be replaced by any required solution poured through the funnel-end of the tube, so that the effect of different chemical solutions on the rhythmic activity can be easily studied. A light chamber with mica windows is made to enclose the specimen. The temperature in the chamber can be raised to any desired degree by means of an electric current sent through a spiral of heating wire ; or be lowered by sending a stream of cooled air thro ugh it . V arious gases and vapours can be similarly passed through it

The two small lateral leaflets exhibit auto¬ matic movements. lever is connected, by means of a cocoon-thread and a drop of shellac- varnish, to a point about the middle of the leaflet. As the pull exerted by the leaflet is very feeble, the writing- lever has to be made extremely light. The record is Fig. 138. Photograph of the Oscillating Recorder, reduced to one-fourth the natural size. taken with an Oscillating Recorder, one type of which is seen in fig. 138. The recording-plate is made to move to- and-fro by an eccentric so tha+ a dotted record is traced on the moving smoked-glass plate, the interspaces indicating definite intervals of time.

followed by a slower diastolic up-movement, there being an intervening pause at the turning-points. The record of the pulsation of the Desmodium leaflet is remarkably similar to that of the heart-beat of the animal, as summarised below : nature during either systole or diastole (cf. fig. 209). They often exhibit periodic groupings. large, and then that of the next one is very small, as if a large discharge caused, fatigue for the next pulsation. The uniformity of pulsation of the Desmodium leaflet under favourable conditions is clearly sho\yn in the con¬ tinuous record for 4 hours given in fig. 139- The pulvinule

is sensitive to light ; it should therefore be adjusted with its length perpendicular to the window so that its two flanks are equally illuminated. Stronger illumination of one flank gives rise to elliptical movement of the leaflet. Ihe record of a single pulsation, magnified 2} times, is shown in fig. 140, the successive dots being at intervals of 1 second. The period of a complete pulsation was 102 seconds, of which the down-movement (represented by

I JG. 140. Record of a single pulsation of Desinodium taken on a faster moving plate. .Successive dots at intervals of 1 second. the up-curve) was accomplished in 41 seconds, and the up- movement in 61 seconds. The leaflet attained its maximum rate at the fourteenth second of its actual downward journey, the maximum rate being 0-9 nun., and the average rate 0-44 mm. per second. The maximum rate of the up-move¬ ment was, on the other hand, 0-56 mm. and the average 0-3 mm. per second.

is determined, by its energy-content . derived ultimately from environmental stimuli. The power of contraction has been shown to be dependent on the tonic condition. The possibility of repeated contraction exhibited by the rhythmic tissue of the Desmodium-leaflet is similarly dependent on previous storage of energy, as demonstrated by the following experiments. 1 Experiment 143. Stoppage of pulsation by run-down of stored energy. — -The cut specimen, whose record in a lighted room for 4 hours is given in fig. 139, was removed to a dark room, and its record continued for the next 10 hours. The amplitude of pulsation was found to exhibit

a continuous decline, and ultimately to come to a stop at the tenth hour in darkness (fig. 141), the tonic condition having by this time fallen below par. This arrest is not always indicative of the permanent abolition of irritability at death ; it is often an instance of suspended activity due to run-down of energy, resulting in a subtonic condition of the tissue. Since there is a possibility of revival of pulsatory activity by fresh stimulation, I therefore undertook to investigate :

for revival of pulsation in tissues in different degrees of subtonicity, and In the case of a tissue immediately after the arrest of pulsation, its tonic level will have fallen but slightly below par. If, on the other hand, a long time Ims been allowed to elapse after the stoppage of pulsation, the tissue will have been rendered relatively more subtonic. The following experiments relate to the intensity and duration of stimu¬ lation required to revive pulsation (1) in a slightly and (2) in a more pronouncedly subtonic tissue.

Experiment 144. Action of stimulus on slightly subtonic tissue. — Light was applied for 2 seconds on a leaflet which had come to a state of standstill only half an hour previously. This gave rise to three recurrent responses, after which the multiple activity came to a stop. A second application of light for 2 seconds gave a similar result. Experiment 145. Action of stimulus on pronouncedly subtonic tissue. — 1 next took a specimen which had been in a state of standstill fcr 5 hours ; its tonic level had there¬ fore become considerably lowered. Stimulus of light for 2 seconds was now found to be quite ineffective. It was only after the application of light for 30 seconds that the stimulus became effective, giving rise to a single response. The light was applied a second time, but for the longer duration of 60 seconds. The response now consisted of a large pulsation followed by two smaller ones.

Relation of the Persistence of Revived Pulsation to the Quantity of Absorbed Stimulus l he persistence of the revived activity will next be shown to depend on the intensity and on the duration of stimulation, that is to say, on the quantity of stimulus that impinges on the organ. The experiments were carried out with Desmodium-leaflets which had just come to a state of standstill. I studied the effect of both electric and photic stimulation. Experiment 146. Effect of electric shock. — A single electric shock of moderate intensity gave rise to a single

response ; a repetition of a similar stimulation caused once more a single response (fig. 142). Experiment 147. Effect of con¬ tinuous i nduction- shock. — The electric stimulation was in this case stronger and the duration of application was prolonged to 5 seconds. The effect was a long- continued pulsatory response of the leaflet originally at a standstill (fig. I43>. The persistence of revived rhythmic activity, therefore, depends on two factors, on intensity and on

Fig. 142. Response of Desmodium-Ieafiet in a state of standstill.. Fig. 143. Prolonged revival of pulsation under moderate electric % stimulation for 5 seconds. duration ; that is to say, on the quantity of impinging stimulus. Experiment 14S. Effect of increased duration of stimulus of light. — -Similar effects were obtained under stimulus of light. The application of strong light for 5 minutes gave rise, in the present case, to a single pulsation of the leaflet, previously at standstill. The next application of light of the same intensity for 10 minutes gave rise to four pulsa- | tions, two during and two after the application. Lighf was . next applied for 45 minutes, and the rhythmic activity then persisted for nearly an hour after the withdrawal of light (fig. 144). m

1. The pulsating organ, after isolation from the stimu¬ lating agencies of its environment, comes to a state of standstill by the depletion of something that had been conserved. 2. In this state of standstill, stimulation revives the pulsatory activity, which becomes increasingly persistent Fig. 144. Effect of photic stimulation in renewing pulsation of Desmodium gy ans originally at standstill. Successive exposures to light for 5, 10, and 45 minutes. A portion

with the intensity and duration of incident stimulation. There must, therefore, be a causal relation between stimuia* tion and pulsatory activity. • 3. It is clear that the tissue, in consequence of stimu¬ lation, has regained its power of automatic pulsation which it had lost in the subtonic condition. In other words, previous stimulation has contributed sufficient energy to the protoplasm to restore its capacity for performance of its normal function.

I he question next arises whether rhythmic activity was suddenly perfected in certain plants like Desmodium, or whether there were intermediate steps in the evolution. In regard to this, l have been able to discover a connecting link between ordinary response, in which a single stimulation gives rise to a single response, and automatic movement of a rhythmic character. The phenomenon which bridges over the gap is that of multiple response. It has already been shown that rhythmic tissue which has come to a state of standstill responds to feeble stimulation by a single response, and to strong stimulation by a series of multiple responses (cf ■ Experiments 145, 146, T47). Similarly, tissues of many plants respond to feeble stimulation by a single response,

Fig. 1 4 5. Multiple response in Biophytum to a moderately strong and to stronger stimulation b}^ repeated responses. This will be clearly understood from the following typical experi¬ ments carried out with Biophytum sensitivum. Phenomenon of multiple response. — To a feeble electric stimulation applied to the petiole of Biophytum, the leaflet answered by only a single response. Experiment 149. M ultiple response under strong electric stimulation. — Stronger stimulation gave rise to a series of multiple responses (fig. 145).

Experiment 150. Effect strong thermal stimulation. — As many as sixteen multiple responses were caused by a strong thermal stimulation. Experiment 151. Effect 0 j chemical stimulation. Strong chemical stimulation gave rise to as many as seven multiple responses, the average period being 30 seconds (fig. 146). Multiple mechanical response under strong stimulation was also obtained with /ther sensitive plants like Averrhoa and Mimosa. I next attempted to discover if multiple

F"ig. 147. Multiple electric response of Biophytum under thermal response were given by ail tissues under strong stimulation, the excitation being detected by means of an electric response of galvanometric negativity. Experiment 152. Multiple electric response.— The em¬ ployment of the electric method demonstrated that every plant, both sensitive and ordinary, gave multiple excitatory response under moderately strong stimulation. Fig. 147 shows the multiple electric responses given by the petiole of Biophytum under thermal stimulations applied at intervals of 5 minutes. Each stimulation is seen to have given rise to from five to eight multiple responses, the average period of each pulsation being 30 seconds.

In the multiple mechanical responses which have been described, the stimulus was applied at a distance from the motile organ, the recurrent responses being due to the trans¬ mitted multiple excitations. The question arises whether the motile tissue is itself multiple-responding. Experiment 153.— The leaflet of Biophytum was directly stimulated by light from an arc-lamp. This gave rise to multiple responses (fig. 148) which persisted for a time even after the stoppage of the stimulus of incident light.

The next point is to discover the intermediate stages of transformation from single to multiple response. Experiment 154. Response on ‘ all or none principle.— The minimally effective intensity of electric stimulation for response of Biophytum was o-i unit. After taking the record under this minimal intensity, a second response to stimulation ten times as strong, i.e. 1 unit, was recorded. It was found that both the minimal and maximal stimuli induced practically the same effect. I he leaflet responds either to its fullest extent or not at all.

What then became of the excess of energy of the maximal stimulus ? It is not necessary to suppose that in every instance the whole energy of the impinging stimulus is expended in the resulting movement ; some portion of it may be wasted as heat, while another may be utilised to do the internal wrork of raising the tonic level of the tissue. The excess of energy derived from the incident stimulus may also find external expression in the form of responsive move¬ ment. To take a physical illustration, the energy stored up in a compressed spring, on release gives rise to long- continued and rhythmic oscillations. Similarly, the stored energy in a living tissue finds external expression in re¬ peated responses, as illustrated by the following experiment.

Experiment 155. — Successive responses of .Biophytum w'ere recorded at interva 3 of 3 minutes to stimuli increasing from 0*1 to 2 units. Owing to incomplete recovery in eacli response, the base-line was displaced upwards. The ampli¬ tudes of successive responses, up to 1 unit, were nearly the same, though the stimuli were increasing. On the application of the fourth stimulus of intensity 2, the response became multiple (fig. 149). The repeated responses are therefore due to the excess of energy of the strong incident stimulus that was held latent in the tissue.