Life Movements in Plants

■ I have in the present Paper shown.that a parallel series of reactions is seen in the response, of growing organs. In vigorously growing specimens the action of stimulus is negative, *L«e., incipient contraction, diminution of turgor, : and : retardation of the rate .of growth. But in sub-tonic specimens/ with enfeebled rate of growth, the effect of turgor, and acceleration of the rate of growth. Continuous stimulation also raises the sub-tonic growing tissue to a condition at par, converting the response from abnormal positive to normal negative.

It was also explained that every stimulus gave rise to dual reactions, positive and negative, and that in a highly excitable tissue the positive is masked by the predominant negative. The positive, or A-effect, is generally described as a “ building up ” process. By choosing a sub-tonic specimen, I have been able to unmask the positive, A. In the case, of sub-tonic growing organs the positive, A, is Literally a building up process, giving rise to an accelera¬ tion of growth.

From these facts and others given previously it will be seen that the abnormal response^ of acceleration of growth under stimulus is by no means accidental or fortuitous but is a definite expression of an universal reaction, characteristically exhibited by all tissues in a condition of sub-tonicity. A given plant-tissue may exist in widely different conditions of tonicity. Let us take two extreme condi¬ tions, the optimum and the minimum. The tonic level will be at its lowest at the minimum, where growth will be at a standstill. The range between the optimum and minimum will be very extended ; hence strong and long continued stimulation will be necessary to raise the tissue from the tonic minimum to the optimum level. There are innumerable grades of tonicity between the optimum and minimum. Within this wide range the characteristic response will be the abnormal positive. As we approach the optimum, the range for positive response will

become circumscribed, and the intensity and duration of stimulus necessary to convert the positive to negative will be .feebler and shorter. It will be very seldom that a plant is likely to. be found at the optimum. Hence plants in general rnay be expected to give a feeble positive response under sub-minimal stimulus. These considerations le.d me to look for the positive response under sub-minimal stimulation; the tracings which I have obtained with my highly sensitive Cresco- grapli and other recorders show that my anticipations have been justified.

Positive response under sub-minimal stimulus: Experi¬ ment 89 .—In normal specimens, light of strong intensify in¬ duces a retardation of growth. When the source of light is' placed at a distance,' the intensity of light undergoes great diminution. Under the action of such feeble stimulus I .obtained an acceleration of growth even in specimens which may be regarded as moderately vigor¬ ous (Fig. 83). Similar acceleration .of growth was also Fig. S3—Acceleration of growth under sub-minimal light stimulus. Record on moving plate ; stimulus applied at 5th dot, and subsequent, erection of curve exhibits acceleration of growth. Last part of curve shows recovery of normal growth on cessation of siimulus.

obtained under feeble electric stimulation. The response is reversed to norma! negative by increasing the intensity or duration of stimulus. Very feeble stimulus'thus .in¬ duces an acceleration and strong stimulus a retardation of growth. I have frequently obtained positive mechanical and electrical responses under sub-minimal stimulation. As chemical substances often act as stimulating agents, the opposite effects of the same drug in . small and large doses may perhaps prove to be a parallel phenomenon.

It has been shown that stimulus induces simultane¬ ously both A- and D-effects, with the attendant positive and negative responsive reactions, alike in pulvinated and in growing organs. A tissue, in an optimum condition, exhi¬ bits only the resultant negative response ; the comparatively feeble positive is imperceptible, being masked by the pre¬ dominant negative; but with the decline of its tone exci¬ tability diminishes, with it the D-effect, '-and we get the A-effect unmasked, resulting response then becomes diphasic. Iu extreme sub-tonic condition, it exhibits only the positive. The sequence is reversed when we begin with a tissue in a state of extreme sub-tonicity, which first exhibits only the positive. Successive stimulations continually exalt the tonic condition, the subsequent responses becoming, diphasic, and, with the attainment of optimum tone, a resultant negative response, As a further verification of the simultaneous existence of both A-and D-effects, it has been shown that in ordinary tonic condition a swb-minimal stimulus gives rise only to- positive response; this becomes convert¬ ed into normal negative under mo derate, stimulation.

I have described the action of stimulus on tissues in which, on account of sub-tonicity, growth has become en¬ feebled. I shall next take "up the question of effect of stimulus on tissues in which growth, on account of extreme sub-tonicity, has been brought to a state of standstill. The modifying influence of tonic condition on response is similar in pulvinated and growing organs. The motile organ of Mimosa in a condition of sub-toni¬ city, exhibits a positive response, by expansion, increase of turgor and erection of the leaf. Continuous stimulation converts the- abnormal positive to normal negative.

In sub-tonic growing organs stimulus likewise induces & positive response, by expansion, increase of turgor and •acceleration of the rate of growth. Continuous stimulation converts the abnormal acceleration to normal retardation. Sub-minimal stimulus tends to induce even in normal tissues, an acceleration of rate of growth. Stimulus "i moderate intensity induces in the same tissue the normal retardation of growth.. The, autonomous activity of growth is ultimately derived from energy supplied by the environment. The internal activity may fall below par with consequent diminution or even arrest of growth ; this condition of the tissue I have designated as sub-tonic. The inert plant can only be stirred up to a state of activity by stimulus from outside; and we saw that under the action of stimulus the rate of growth of a sub-tonic tissue was enhanced.

As the general question of depression of autonomous acti¬ vity and its restoration by the action of stimulus is of. much theoretical importance, I shall describe experiments carried out on a different form of autonomous activity, seen in spontaneous pulsation of the lateral leaflets of Desmodium gyrans. Under favourable conditions of light and warmth these leaflets execute vigorous movements, the period of a single pulse varying from one to two minutes. As the energy for this activity is ultimately derived from the environment, it is clear that isolation from the aetion of favourable environment will bring about a gradual depletion of energy with concomitant decline and ultimate cessation of spontaneous movement. For this we may keep the plant in semi-darkness; we may further hasten the rundown process by isolating the leaflet from the parent plant. A leaflet immersed in water was .kept in a dimly lighted room ; it was attached by a

cocoon thread to the recording lever of an Oscillating Recorder to be fully described in the next Paper. The pulsation continued even in this isolated condition for abodt 48 hours, after which the spontaneous movement came to a stop. Further experiments showed that the arrest of pulsation was not indicative of mortality but of ‘ latent life in a state of suspense, to be stirred up again by shock stimulus into throbbing activity P,c 84 —Renewal of autonomous activity in Du,nodim at standstill

by a Ttio„ of light. Up-curve represents up-movement. The horizontal lines below standstill. A narrow pencil of light from e ee nc arc was first thrown on the lamina in which the presence o chlorophyll rendered photo-synthetic action possmle. ^ Thi had no effect on the renewal of pulsation. ® u e the pulvinule. This preferential effect on pulvmule snowed that the renewal of activity was due not to photo-syn¬ thesis but to the stimulating action of light. The pulsa¬ tion was also restored by chemical stimulants, such as

As regards the action of light, the pulsation continued tor. a time* even on the cessation of light. This per¬ sistence of autonomous activity increases with the intensity and duration of incident stimulus, that is to say with the amount .of incident energy. In the present case a duration of live minutes’ exposure gave rise to .a single pulsation, after which the movement of the leaflet came to a stop. The next application lasted for ten minutes and this gave, rise to four ■ pulsations, two during application, 'and two after cessation of light. The next application was for forty-five minutes, and’ the pulsation persisted for nearly an hour after the cessation of light. The experiments on sub-tonic specimens show clearly that the energy supplied by the environment ' becomes as it were latent in ‘the'.plant, increasing its potentiality for work.

■ The ; renewal , of autonomous activity in a sub-tonic tissue by the action of. external. stimulus, will b© found‘in every way parallel 'to.'the renewal "of-growth in a sub-tonic organ. I find that application of electric stimulus renews growth in specimens where, on account of extreme sub-tonicity growth has come to a state of standstill. The resumption of growth in grass haulms under the stimulus of gravity is a phenomenon probably connected with the above. The causes which bring - about cessation of growth in a mature organ are uoknown; that , there is a potentiality of growth '©veil in a fully grown grass haulm is evidenced by the fact of its renewed growth under fresh 'stimulation. That this- is not an exceptional phenomenon appears from the record which- 1 obtained with a fully grown style of

Datura alba. I subjected it to periodic stimulation, and obtained from it a series of contractile responses. After recovery from stimulus it regained its normal length which remained constant for some time as seen in the horizontal base-line. But as a result of successive stimulations, the mature style resumed its growth with increasing acceleration. This is seen in the recovery overshooting its former hori¬ zontal limit (Fig. 85). Fig. 85.—Record of responses of a mature style in which growth had come to a stop. Up-curve shows contraction uncter stimulus. Renewal of growth at sixth response, after which growth-elongation is shown by the trend of the base-line downwards.

From the investigations that have been described in this and in the previous Papers an insight is obtained into the complexity of response arising from various factors* It has been shown that the sign of response is modified by the intensity of stimulus, by its point of application, and by the tonic condition of the responding tissue. Tfie funda¬ mental reactions have been found to be essentially the same in pulvinated, in growing and non-growing organs. The

results described enable ns . to enunciate general Laws of Effects of Direct and Indirect stimulus on tissues - In normal and in sab-tonic condition. 1 have referred'to the fact previously demonstrated, that while Direct stimulus induces contraction and retardation of growth, ^moderate rise of temperature induces the ' opposite effect of expansion and acceleration of growth. Further demonstration of the antagonistic effects of stimulus and warmth will, be given in the next Paper.

' ‘The autonomous activity- of pulsating leaflet of JDmmo Mum gyram . comes . to a stop under depletion of internal energy. A cat leaf isolated .from the plant maintains the rhythmic activity of its leaflets for about 48 hours, after which there is an, arrest of movement. ■ In this state of sub-tonicity the arrested autonomous activity is revived under the action of various stimuli. Thus the incidence of light on the pulvinule initiates pulsa¬ tory movements, which persists for a time even on the cessation of stimulus. This persistence of autonomous, activ¬ ity increases with the intensity and duration of stimulus to which the leaflet had been subjected.

The arrested autonomous activity of growth may often be revived by the action of stimulus._ Thus the ^arrested growth in a mature style or Datuva ctlbct was renewed by electric stimulation. In the preceding Paper I have shown the essential simi¬ larity of effect of stimulus on autonomous activity of the De&modyum leaflet, and of the growing organ. It was shown how stimulus revived the pulsatory activity of Desmodium leaflet in a state of standstill, in the same way as it re¬ newed the arrested growth-activity.

The investigation of this subject was rendered possible by the successful device of my Oscillating Recorder. A very light glass fibre was used for the construction of the lever, which was. supported on jewel hearings. The short arm of the lever was 2 cm. in length, and the long arm 8 cm. This gave a magnification of 4 times. But it is quite easy to increase the magnification to 10 times or more. The pull exerted by the pulsating leaflet is extremely slight, and the relatively heavy lever made of steel wire used in the Resonant Recorder is not well-suited for our purpose. The pulsation of the leaflet is relatively slow, being once in two minutes or so. The; intermittent contact of ten times in a second, given by the Resonant Recorder, is therefore too quick, in the Oscillating Recorder the in- termittencO' was, therefore, reduced to once, in a second, or onc^in five seconds, the recording plate itself being made to move to-and-fro at ibis rate. . The carrier of the plate- holder slides backwards and iorward® on ball bearings ; a

wheel in the clockwork connected with an eccentric is released periodically, at intervals which may be varied® between one and five seconds. y the action of the eccen¬ tric, the plate carrier approaches the writing lever with diminishing speed till the movement is zero at the contact. This contrivance is essential, since any sudden shock of the plate against the lever is apt to give rise to after-vibra¬ tions of the writer. The plate carrier is quickly ‘with¬ drawn after the production of a dot on the smoked glass plate by contact with the writing lever.

The clockwork is governed by a revolving fan which can be gradually opened out by a regulating screw. The speed can thus be adjusted within wide limits, and main¬ tained constant and at any desired speed. A second set of wheels connected with the clockwork moves the plate-holder in a lateral direction. A series of records may thus be taken for fifteen minutes, half an hour, or an hour. The record obtained in this way is very perfect. Not only is the effect of an external agent shown by variation in the amplitude and frequency of pulsations, but the change of speed in any phase of the pulse becomes auto¬ matically recorded.

The whole plant can not be conveniently manipulated for different investigations. It is, however, possible using the precautions described below to use the detached petiole carrying the pulsating leaflets. The terminal large leaf may also be removed. The necessary amputation is often followed by an arrest of pulsation* But as in the case of isolated heart in a state of standstill, the movement of the leaflet may be revived by the application of internal hydrostatic pressure. Under these conditions, the rhythmic pulsations may easily be maintained uniform for many hours.

The petiole carrying the leaflet is mounted water-tight in the short arm of an U-t.ube filled with water; for pro¬ ducing internal hydrostatic pressure in the plant the height of water in the longer arm is suitably raised* The U-tube holding the specimen may be adjusted up and down, and laterally. A hinged support a!so allows the specimen to be placed at any inclination* The movement of the leaflet, it ■ is to' be remembered, does not always take place in a vertical direction. .The object of the mechanical, adjustments is to place the specimen,at sack an angle that its np and down movements when in a straight line should be vertical, or have its long axis vertical when the movement is elliptical. It is important that the specimen should be illuminated equally from all sides; for one-sided illumina? tion causes a bending .over of the leaflet towards light* ;

The pulvinule of the leaflet acts like the pulvinus of Mimosa, that is to say, the leaflet undergoes a sudden fail to down position by the contraction of the more effective lower half of the pulvinule; the 1 up * position denotes re¬ covery and expansion of the more effective half. The up-and-down movements of the leaflet correspond to the diastolic and .systolic movements of the animal heart. There is, indeed, as I have shown' e!sewhere # a very close resemblance between the activities of rhythmic tissue in. the plant, and in the animal.

Experiment* 92*— For the study of .effect of light on Dm - medium, 1 first obtained record in darkness. A horizontal beam of divergent light from an arc lamp placed at a dis¬ tance of 200 cm. was made to act diffusely on the leaf from ^ all"sides. This was done by mtans of tbifee inclined■ mir¬ rors, the, first throwing the light vertically downwards/the second vertically upwards, and the third horizontally for- ward from the side opposite the lantern. The effect of * light is seen demonstrated in Fig. S6.

Fig. 80.— Effect of light on pulsation of Ihsmoditun leaflet. Duration of application of light is represented by the horizontal line. Up-curve represents diastolic expansion and down-curve systolic contraction. Note contractile effect of light in diminution of amplitude and r-duetion of diastolic limi of Light was applied at the second pulsation. It will be seen that light retards or arrests the autonomous activity. On the cessation of light the normal activity was found to be gradually restored. It is of much interest to note here the similarity of action of light on autonomous acti¬ vity of the leaflet of Desmodium and of a growing organ. In both, we find that while in the sub-tonic 'condition of the tissue the effect of light is to enhance or renew the autonomous activity of growth and pulsation, in normal condition the effect is to retard it.

Inspection of the record exhibits another very interest¬ ing characteristic. We saw that light retarded' growth by inducing an incipient contraction. In the Besmodium leaflet the contractile reaction of light is exhibited by the characteristic modification of its pulsations. The duration' of .application of light is represented by the horizontal line. In Fig. 86 the up-curve represents up-movement of diastolic expansion, and the down-curve of systolic con¬ traction, The contractile reaction of light is seen to counteract the normal expansion, with diminution of diasto¬ lic limit of pulsation.

It has been shown that while rise of temperature up to an optimum enhanced the rate of growth, the effect of light was to retard it. Hence the effects of light and warmth are antagonistic. Effect of rise of temperature on pulse-record ; Experi¬ ment 93 .—In studying the effect of rise of temperature on the pulsation of leaflets of Desmodium , we discover similar p £ G. 87._Effect of rise of temperature on pulsation of leaflet of Desmodium gyrans. Horizontal line represents the duration of gradual . rise of temperature from 30 5 C. to 35°C. Note the expansive effect of rise of temperature..in reduc¬ tion of systolic limit of pulsation. ^

antagonistic reactions'' of light and warmth. The leaflet was placed in a plant-chamber with an. electric arrange¬ ment for gradual rise of temperature. : The; first two records were taken in the normal temperature of the roomy which was 30°C. The 'temperature was gradually raised to 35° 0, the record being taken all the time, ‘ It will be. seen (Fig. 87) that the effect of warmth is dia¬ metrically opposite ' to that of light. The record in Fig. 86 exhibited the - contractile effect of light by re¬ ducing the diastolic limit of expansion. In the present case the expansive reaction of warmth is exhibited by the reduction of systolic limit of contraction. The temperature of the plant chamber was now al’owed to return to 30 3 0*, and we observe the gradual restoration of normal systolic limit of contraction.