The Motor Mechanism of Plants

After a fall by contraction, the leaf slowly regains its original position by the gradual reabsorption of sap and restoration of the turgidity of the contracted cortical cells of the pulvinus. The most natural explanation of this would seem to be that the necessary fluid is supplied from the cortex of the stem, which will be shown later to be the important channel for the movement of sap. This finds support from the characteristic variation in the rate of recovery from stimulation in an identical Mimosa (1) when in a condition of incipient drought, and (2) when there is active ascent of sap after irrigation. In the first case the period of recovery was more than 20 minutes, while in Hie second case it was shortened to 8 minutes or so.

The plant, like the animal, can be made to perform work in lifting weight by its excitatory contraction. In muscle, the work performed is found to increase to a limit under increased load and tension. Similar results are also obtained with Mimosa. Thus while under a load of too mgrms, the work performed wras 4100 mm. mgrms. ; it was increased to 26,000 mm. mgrms. under a load of 2000 mgrms. The rate of work was 7480 mm. mgrms. per second. In the pul yin us of Mimosa both the upper and the lower halves of the pulvinus are sensitive and contractile, the sensitiveness^ and contractility of the lower half being con¬ siderably greater than those of the upper.

A mechanical factor in the feeble erectile response under ’ocal stimulation of the upper half is the unequal pliability of the organ, which bends more easily downwards thm upwards. Application of the same pull in the two directions produces greater curvature downwards. When the plant is held in an inverted position, stimula¬ tion causes an erectile movement which raises the weight of the leaf against gravity. The normal fall of the leaf is therefore due not to flaccidity of the pulvinus but to active contraction of the lower half of the organ.

This active contraction is so great that the rapidity of the fall is practically the same whether it is helped by the weight of the leaf or opposed by ar equivalent weight. Removal of the upper half of the pulvinus has little or no effect on the rapidity of the fall of the leaf. « After removal of the lower half of the organ the leaf responds i/o stimulation by an erectile movement due to the contraction of the upper half. The sensitivity, the rate and the amount of contraction of the upper half are found to be very much less than those of the lower half. This result finds independent support from the effects of local stimulation of the upper and lower halves of the organ under the polar action of a constant electric current.

The factors of the weight of the leaf and of the action of the upper half of the pulvinus are, therefore, negligible compared with the active force of contraction of the lower half of the organ* Under medium stimulation the lower half alone exhibits contraction, that of . the upper half being practically absent. The erectile recovery of tlie leaf of 'Mimosa is dependent on the rapidity of the flow of sap into the puivinus, for recovery is hastened when t lie rate of ascent of the sap is enhanced.

The amplitude* of the response is determined by the number of the cells that undergo contraction. The work performed by the puivinus is increased under an increased load. The rate of work performed by the puivinus is found to be 7480 mm. mgrms. per second.®. Having described the motor mechanism of Mimosa in the previous chapter, if will be interesting to consider in some detail the characteristics of its responsive movements, as modified by successive stimulations and by the tonic condition of the plant.

(. om pared with other sensitive plants, the contractile reaction is most rapid in Mimosa. Its latent period, that is to say the interval between the impact of stimulus and the initiation of responsive movement, is comparatively short. Experiment 15.— The latent period of a highly excitable Mimosa was determined by the Resonant Recorder, the successive dots being recorded at intervals of .ji ^ of a second. The responsive movement is seen to have occurred at the fifteenth dot, the latent period of the specimen being 0*075 second (tig. 25). In less vigorous specimens it may be as long as 0 • 12 second.

i he maximum rate of contractile movement is attained in the course of about 0*2 second after the reception of the electric shock, ihe maximum contraction and fall of the leaf occur in about 1*5 second, after which gradual recovery takes place. Trom the results of electric investigation given in a lutejy chapter, it would appear that the protoplasmic recovery is complete in the course of about 3 to 4 minutes. Rut tlie mechanical manifestation of this recovery is delayed

by the fact that it takes sometime for the reabsorption of sap to produce expansion of the cortical cells and the full erection of the leaf. The maximum rate of erectile movement is about 260 times slower than the rate of maximum contractile fall. The recovery is completed in 8 to go minutes, depending on Fig. 23. Record of latent period of Mimosa with a 200-vibration recorder. Shock at vertical line. external conditions. Other things being equal, the recovery is protracted after stronger stimulation.

In regard to the stimulation of the pulvinus of Mimosa by induction-shock, a single break-shock, on account of its abruptness, is relatively more effective than a make-shock. A feeble shock, individually ineffective, can, moreover, be made effective by repetition, as demonstrated by the following experiment. Experiment 16. — A single make-and-break shock of intensity 0*5 unit was found in the particular specimen to be below the threshold of response. A reed-interrupter, tuned to vibrate live times per second, was interposed in the circuit of the primary of the induction-coil in order to produce a series of make-and-break shocks till the leaf responded by a fall. The' electric signals below the record show that the singly ineffective stimulus of 0*5 became

effective ui iei fom repetitions (lig, 26). 1 lie experiment I’lc ,26. Additive eifect of stimulus of intensity ^becoming effective on lacing repeated ,j times. oi stimulus reduced to o-i ; the- stimulus had now to be repeated 20 times before it became effective (fig. 27). Ido. 27; Additive effect of stimulus of intenrity o • j , becoming effective on being repeated 20 times Comparing the two results it is found that 0 5 x 4 = C‘i X 20; thus the number of additive stimulations requisite for effective excitation varies, within limits, inversely as the intensity of the stimulation employed.

Experiment iy.— In normal specimens the responses are uniform under successive stimulations of constant intensity. Ihe intervals between the successive stimulations must be long enough to ensure complete recovery. Fig. 28 shows uni form response to stimulation of moderate intensity, applied at intervals of 15 minutes, becomes prolonged, as already stated, under intense stimu¬ lation. Fig. 28. Uniform responses of Mimosa ; stimulus applied at intervals of 15 minutes.

When the recovery is incomplete the responses exhibit fatigue. The three initial responses (tig. 29) were ob¬ tained at intervals of 15 minutes between the stimulations A reduction of the resting-period to 10 minutes resulted in fatigue as shown in the diminu¬ tion of the amplitude of the next three responses. The response was restored to the normal after return to the original resting- period of 15 minutes. First three uniform responses obtained at intervals of 15 minutes. The second three, under shortened period of rest of 10 minutes, exhibit fatigue. On returning to interval of 15 minutes, the last record shows recovery^ from fatigue.

The leaf of Mimosa, when subjected to continuous stimula¬ tion, at first exhibits the normal fall, followed bv' re-erection in spite of the stimulation which is still acting upon it. This appears paradoxical; but the apparent anomaly disappear when we recognise the essential similarity of responsive reaction in plant and anima1. Tt is well known that under continued electric stimulation the frog’s muscle exhibits normal contraction followed by subsequent relaxation. *

Experiment 19. — A reduction of the intervening period of rest has been shown to be followed by fatigue, the power oi contraction ^undergoing marked depression. When the resting interval is greatly shortened the tissue becomes refractory, as it were, to further stimulation. An indication of th s is seen in the record, given in fig. 30, of the effect of stimulation at intervals of 3 minutes ; the responses subse¬ quent to the first stimulation are seen to have been reduced

to a mere twitch. When the resting intervals are further reduced, as under continuous stimulation, the twitch disappears, and the leaf exhibits fatigue-relaxation. There is an additional factor which may contribute to the erectile movement under con- tinuous •stimulation. The upper half of th© pulvinus has been shown to be relatively uaexci table, on Which account it requires a longer period of stimulation to undergo contraction contributing to an erectile movement et the leaf, ibis may Conspire with the fatigue-relaxation of the lower half to produce the erectile movement under continuous stimulation.

Fig. 30. Different phases in the fatigue-reversal in Mimosa. Record of response to stimulation at intervals of 3 minutes (see text). \'. lien Mimosa is in a vigorous condition, the feeblest stimulus precipitates the response. The leaf falls from a higher to a lower level, indicating a run-down or dissipa¬ tion of energy. From this instance of ‘ trigger action ’ the conclusion has been drawn that the energy evolved must always be disproportionately larger than the stimulus that provoked it. The question arises whether this is universally

true and whether under other circumstances the energy of stimulation, instead of causing a run-down, may not actually raise the potential energy of the plant. It should be borne in mind that living tissue is not merely a mass of matter but a higher complex of matter plus energy. The functional activity of the tissue depends on the previous absorption of energy from the environment, among which may be mentioned light, warmth, and chemical stimulants present in or absorbed from the soil. These environmental stimuli raise the tonic level of the plant, on which, as it will be presently seen, depend its diverse physiological activities. It will be shown in a future chapter that the continuous movement of sap is maintained in the plant by pumping action, which must necessarily require expenditure of energy which had been previously stored. Now when the plant is cut off from energy supplied be external stimulation, its tonic level falls below par ; in this condition of subtonicity the movement of-sap is found to become arrested. The lost power can be restored by fresh accession of energy from outside. What has been said of this rhythmical movement is equally true of other activities of the plant.

I will describe experiments regarding the effect of sub¬ tonicity on the contractile reaction of Mimosa. A simple method of obtaining a subtonic specimen is to isolate a branch of Mimosa and keep it in darkness with the cut end placed in water. In these circumstances the-e is a con¬ tinuous lowering of motile excitability of the leaf, analogous to the depression of excitability in an excised muscle. Cut branches of Mimosa pass through various degrees of sub- tonicity ; a specimen that has been detached for, say, 3 hours exhibits but slight depression, whereas others isolated for longer periods occupy correspondingly lower positions in the scale of subtonicity.

I will first describe the response of Mimosa when in a con¬ dition of pronounced subtonicity, the specimen having been in an isolated condition and in darkness for 12 hours. The testing stimulus employed was either photic (Experiments 20, 21) or electric (Experiments 22, 23), both of which give similar results. The stimulus of light has certain ad¬ vantages in causing moderate stimulation ; it is, moreover, the stimulus to which the plant is subjected under natural conditions.

Experiment 20.— A beam of light from a small arc-lamp was thrown by means of suitably inclined mirrors on botli the upper and lower surfaces of the pulvinus so as to cause diffuse stimulation. The response of the subtonic specimen was an erectile moventent (fig. 31), the leaf being raised from a lower to a higher level , indicating an increase of potential energy in the plant. The positive or erectile response of the subtonic specimen indicates an accession of energy in con¬ trast with the normal negative response connoting evolution and run-down of energy. The accession of energy will for convenience be designated as the A-reaction, the run-down of energy being described by the symbol D.

The tonic level of the subtonic tissue had been gradually raised towards the normal by absorption of energy supplied by external stimulation. What would be the character of the response during and after the transformation ? The ques¬ tion is answered by the characteristic change in the response under successive stimulations described below. Experiment 21. — Continuation of the last experiment gave very significant results. Hie first response, as already stated, was purely positive ; the second was diphasic, positive followed by negative ; the third, a slight positive twitch followed by a larger negative ; the fourth was normal and enhanced negative (fig. 31).

How are we to account for this transformation ? It was the lack of stimulation that reduced the isolated specimen to a state of subtonicity with characteristic positive response. The energy that the tissue lacked was supplied by energy of successive stimulations, a portion of which must have been utilised by the tissue to raise its tonicity to the normal level with ifs characteristic negative response. It shows, further, that the relative intensity of t}ie two reactions, the positive A and the negative D, is modified in a definite manner according to the condition of the tissue; when the tonic level is below par the acces¬ sion of energy A under stimula¬ tion is the more pronounced ; when it is above par the depletion of energy,, D becomes accentu¬ ated. Since there is continuity between the positive and nega¬ tive reactions, stimulation may be regarded as inducing both the A and D reactions, the

Fig. 31. Positive, diphasic, and negative response under successive photic stimula¬ tions in a subsonic specimen of Mimosa. There is a significant fact noticeable ir the record (fig. 31) showing the effect of the chang¬ ing tonic condition on the atti¬ tude of the leaf. In the subtonic condition the pulvinus is abnormally leiaxed. But after each stimulation there is a moderate residual contraction, the base-line of the record being thereby raised upwards, indicative of persistent contraction. ..he atonic expansion of the pulvinus is

thus changed into improved tonus manifested by moderate contraction. Experiment 22. — The energy absorbed from the incident stimulus does* not merely end in a transformation of the abnormal positive response into the normal negative, but also increases the functional capacity of the tissue. This is illustrated in the record of the responses, to successive uniform electric stimulations (fig. 32), of a specimen which was in a slightly subtonic condition. The successive responses in this case ex¬ hibit a staircase increase ; the state of subtonicity is noticeable in that the first two responses exhibit a preliminary positive twitch which disappeared later.

A continuous trans¬ formation of the tissue from sub tonic to normal condi¬ tion under stimulation has been traced with a corre¬ sponding sequence of posi- sive responses of subtonic Mimosa. five, diphasic, and stairci se response. In this last case +he change induced is from a feeble to a strong contraction. The question now arises: What further modification of response would there be after the impinging stimulus had raised the tissue to the optimum tonic condition ?

Experiment 23.— -The answer to the above question is to be found in the record (fig. 33) obtained with an intact Mimosa plant which was in a slightly subtonic condition, the stimulus being electric. The first three records in the series exhibit a staircase increase, the third response being exceptionally large ; after this intense excitation, which meant a considerable run-down of energy, the two succeed¬ ing responses exhibit a fatigue decline. The record of response of a frog’s muscle (fig. 34) exhibits a similarity which is remarkable. Here also a series of stimulations at first gives a staircase increase which reaches a maximum.

Fig. 34. Preliminary staircase followed by fatigue in the response of Frog’s muscle (Brodle). The subsequent responses exhibit a decline due to the onset of fatigue. The following consideration offers a probable explanation of the cyclic change. 'The incident stimulus has been shown to give rise to two reactions, positive and negati\re._ the relative values of which undergo variation according to the change of tonic level of the tissue resulting from stimulation. Starting with the tissue in a subtonic condition, the energy «)f incident stimulation is utilised in the internal work of rising the tonic level of the organ ; at this stage there is no

stored energy for expenditure in negative response by con¬ traction. As the tonic level gradually rises, the positive undergoes a diminution while the negative exhibits an augmentation ; on the attainment of optimum tonicity the negative response is at its maximum. The expenditure and run-down energy is now too great for quick recupera¬ tion ; the subsequent responses therefore exhibit fatigue. To recapitulate, the protoplasmic reactions to external Stimulation are the positive A and the negative D. The positive A is connected with the uphill work of storage and of the increase of potential energy of the system ; the negative D is associated with the run-down of energy. The energy-Content at any moment in the tissue is A — D, the algebraical stim of tile work done on the plant in storage, and the work done by the plant in its response. The relative intensity7 of the two reactions, as already stated, is determined by the tonic condition, as summarised below.

The theories of assimilation ~rd dissimilation of Hering and of anabolism and catabolism cf Gaskeil rest on the idea of opposition between these two processes. Objections have been raised against the supposition that the pnenomena are mutually exclusive, or that the increase of anabolism must necessarily result in a decrease of catabolism.1 The experi¬ mental evidence from the response of Mimosa shows that the two processes, instead of being mutually exclusive, rake place at the same time.