The Motor Mechanism of Plants

l'here is another complicating factor : in vigorous green leaves the negative excitatory response is brought about by the D-ieaction associated with dissimilation and break¬ down of complex chemical compounds. Light, however, also causes in green leaves the opposite assimilatory process A (photosynthesis) indicated by a positive electric response. The resultant response is therefore A — D (cf. p. 50). For obtaining the response, a vigorous leaf is taken and pinned on a block of paraffined wood. Two pieces of thin muslin in connexion \yith non-polarisable electrodes are spread over two areas of the leaf A and B ; when these pieces of muslin are moistened with normal saline solution, they become practically transparent.

Experiment 102. — When light from an arc-lamp is thrown on A, that area becomes galvanometrically negative and the direction of the responsive current is in the direction of GAB. Light thrown on B (A being shaded) causes a response in the opposite direction (left illustration, hg, 106). Experiment 103. — The fact that the electromotive response under light is the same as that under any other form of stimulation, such as the mechanical, is demonstrated as follows : the moist piece of cloth 011 A is rubbed against the surface of the leaf by means of a glass rod ; or the leaf may be struck with a glass hammer. In both these cases A becomes galvanometrically negative, the direction of the current of response being the same as when A is stimulated by light.

Having given a simple demonstration of the fundamental reaction, I now describe the photo-electric cell made of the two halves of a leaf. In the experiment described above the resistance of the circuit is very great, first, on account of the high resistance of the two non -polar isable electrodes, and secondly, because of the resistance offered by the leaf. The non-polarisable electrode, moreover, is a source of much trouble ; an attempt was therefore made to discard it and employ means for diminishing the resistance of the circuit. For the following experiments I employed the leaf of Musa sapientum, which is divided into two longitudinal halves by a slit along the thick midrib. The two half-leaves thus obtained, measuring about 10 x io cm., are hung parallel to and separate from each other in a rectangular glass vessel filled with normal saline ; the distance between the two is 3 cm . A gold wire is thrust through the length of each of the half-midribs ; the wires serve as the electrodes of the photo -voltaic cell, leading to the galvanometer G. The glass vessel is placed inside a rectangular wooden chamber with hinged doors on two opposite sides, by which the two half-leaves, A and B, can be alternately exposed to light (fig. 106). When the doors are closed and A and B are in darkness, they are practically iso-electric, there being no indication of current by the galvanometer. Exposure of A to light gives rise to a difference of potential between the two half-leaves, A becoming galvanornetrically negative, the resulting galvanometric deflection being in one direction. Exposure of B gives rise to a responsive deflection in the opposite direction.

1 he two half-leaves serve as the two plates in a voltaic ceil ; but unlike an ordinary voltaic cell with plates of different metals, the two plates of the ‘ vegetable cell 1 are the two halves of the same leaf, the electromotive force being generated by the excitatory action of light on one of them. The advantages of this method of obtaining an electromotive response are ; (i) that the troublesome employment , of non-polarisable electrodes with their high resistance is dispensed with; (2) that the area of the surface of the leaf exposed to light is con¬ siderably increased ; (3) that the electric resistance of the circuit is greatly decreased, since the interposed resistance is that of normal saline about 3 cm. thick with a broad section of 100 square cm. ; and (4) that alternate and opposite responses may be obtained by successive exposures of the two leaf-plates to the parallel beam of an arc-lamp, this being easily secured by turning the rectangular plant- chamber upon a revolving base.

Experiment 104. — The photo-voltaic cell thus con¬ structed is stimulated by light from an arc-lamp which passes through a trough of alum-solution for the absorption of the heat-rays. Successive exposures are made for 10 seconds and records obtained on a moving photographic plate. The normal responses are uniform, exhibiting induced galvano- metric negativity shown by the up-curves. On the cessation of light there is a complete recovery ; in fact, the recovery shows an overshooting towards galvanometric positivity from which it returns to almost the original zero position (see fig. 108), The reason for this will be presently given.

Experiment 105. — The amplitude of response is increased with increasing intensity of light ; it is also increased with longer, duration of exposure. Thus, keeping the intensity constant, responses to increasing durations of light, of 5 seconds, ..10. seconds, and .15 seconds, are recorded. The amplitude of response is seen to undergo an increase with the increased duration of exposure (fig. 107). But this increase does not go oh indefinitely ; for the continuous

action oi light causes a maximum negative response, beyond which a decline sets in. There must, therefore, be an opposing element which tends to neutralise the normal excitatory I) -effect. It has been stated that the positive A -effect due to light is often masked by the D-effect. I have, however, succeeded in unmasking A by the stoppage of light. When the light ^ is stopped, the A -effect is often found to be more persistent 4 than the D, the result being a transient overshooting of the f response in the positive direction.

Experiment 106. Positive after-effect. — In the records of the electric response of Musa to light, the up-curves indicate galvanometric negativity during the continuance of light. On the stoppage of light the unmasking of A is seen in the positive after-effect. For the recovery does not stop at the Fig. 108. The negative electric response of Musa to light, up- curve exhibiting the predominant D-reaction. Note the unmasking of a in the positive after-effect (down-curve).

109. The ‘ overshooting’ of the response of Musa in the positive direction on the cessation of light. I rst half of record shows neutralisation under continuous light ; down-curve in the second half exhibits the unmasking of the positive a on stoppage of illumination. zero base-line, but goes beyond it towards the positive direction and then returns to zero (fig. 108). The phenomenon of * overshooting * after neutralisation . — I have succeeded in demonstrating the A-effect by another method. Under the continuous action of light the negative

response undergoes a decline almost to neutralisation. This is due to the joint effects of fatigue and of the increasing positive reaction which neutralises the negative. On the stoppage of light, the A-effect, hitherto masked, exhibits itself by an overshooting of the response in the positive direction (tig. 109). The Hydrilla plant readily absorbs C02 from water, and thy anabolic activity A is quite evident from the rapid rate of evolution of oxygen during photosynthesis. The excita¬ tory D-process, no doubt, is also in operation, but I hoped that in very active specimens the anabolic A would be so pronounced as not to be completely masked by the catabolic D. My anticipations were fully verified, as in the following experiments.

Two middle portions of Hydrilla stems, bearing leaves, were employed as the two plates of the photo-electric cell, electric connexion with the galvanometer being made by gold wire thrust through tiie interior of the stems. After a suitable period of rest the normal activity of the plant was found to be restored. The photo-electric cell was filled with tank-water containing a sufficient amount of C02. Alternate exposure of the two plants to sunlight (suitably 1 effected by a mirror) caused photosynthesis, evidenced by the evolu¬ tion of oxygen. The photo-electric cell was enclosed in a dark box provided with a photographic shutter for giving the necessary exposure.

Experiment 107. — I give a record (fig. no) of the electric response thus obtained* the duration of exposure to light was 1 minute, and a very large down-response occurred, indicating galvanometric positivity of the exposed plants. The electric response disappeared after recovery on the cessation of light.. The record here given shows the pre- - dominant A-effect. The responses of Musa and of actively assimilating ' Hydrilla to light arc seen to exhibit characteristic differ-

ences on account of the relative predominance of the D- or the A-effect. In Musa, D is predominant, A being exhibited either as a positive after-effect or by the overshooting of the response in the positive direction. In an actively assimi¬ lating Hydrilla plant, on the other hand, A is predominant and the resultant response is positive. the electric response to exposure to light is most con¬ veniently obtained by the device of the photo-electric cell in which the two plates are leaves or entire plants.

The electric response of a leaf to light exhibits an alge¬ braic summation ; photic stimulation of the cell-protoplasm induces catabolism I), which is indicated by a negative electric variation : at the same time light acts upon the chloroplasts inducing anabolism A in the form of photo¬ synthesis, associated with a positive electric variation. Generally the negative variation masks the positive : but the positive can be detected as an after-effect upon the withdrawal of light. The A-effect is also unmasked by the phenomenon of ‘ overshooting ’

The electric response of the actively assimilating Hydrilla plant is positive, indicative of the predominant anabolism A. « Two independent methods have been described for the determination of physiological changes induced in the plant, namely, those of mechanical and of electromotive response. I will now describe a third method, successfully employed in my previous work, viz. that of resistance- variation.1 This method has been considerably improved and extended for my recent investigations. The specimen plant is made the fourth aim of a Wheatstone Bridge, and the normal^ resistance determined. This is very high, of the order of some hundred thousands of ohms. W hen the tissue is sub¬ jected to stimulation its resistance undergoes variation.

The inquiry relates to the effects of mechanical, electric, j and photic stimulation on the electric resistance of the tissue. The stem of one of the various plants used is mounted as in the diagram (fig. hi). It is clamped in the middle, and its two lengths, P and Q, form the two arms of the Wheatstone Bridge. The electric contacts with the stem are made by means of two platinum pins which are thrust into it. The ratio arms of the bridge, R and S, are made by a rheostat R S with a sliding contact. There is balance when P S - O R ; successive completion of the battery and

of the galvanometer circuits by means of a double contact- key (not shown in the figure) now causes no deflection of the galvanometer. But if the portion ol the plant P when stimulated undergoes a diminution of resistance the fact is demonstrated by a resulting deflection, say, to the right ; a deflection to the left indicates, on the other hand, an increase of the resistance of P. Fig. hi. Experimental method for obtaining response to mechanical stimulation by resistivity variation.

P and q are lengths of plant which form two arms of the bridge, of which p alone is subjected to torsional vibration by means of the revolving eccentric e, worked by clockwork c. The two other arms of the bridge, r and s, are formed by the rheostat with sliding contact. Balance is easily secured in the following manner : the sliding contact of the rheostat is at first placed in the middle position, and the resistance of 0 is gradually7 diminished by moving the platinum contact inwards from the extreme right. After obtaining an approximate balance, the plant is allowed a period of rest for about 15 minutes, after which the irritation caused by the prick of the platinum wires has disappeared. Complete balance is then obtained by the careful adjustment of the sliding contact of the rheostat.

The mechanical stimulation is produced by the length of stem P being subjected to rapid torsional vibration by Jf means of the revolving eccentric E, actuated by the clock¬ work C. The clockwork is released by pressing a button which results in 20 rapid torsional vibrations being imparted to P. 1 Experiment 108. — A series of uniform responses to a constant stimulus applied at intervals of 10 minutes is given Fig. 1 12. Record showing uniform responses to mechanical stimulation by diminution of resistance (up-curve) followed by recovery (down-curve) ( Caloiropis gigantea).

in hg. 112, the response (up-curve) indicating a diminution of resistance . The experimental specimen was a stem of Caloiropis gigantea. Other plants also give similar results, though some are more sensitive than others. 1 he sensitive¬ ness is further modified by age, by season, and by the physiological condition of the tissue. 1 he responsive varia¬ tion under stimulus may sometimes amount to 10 per cent, of the normal resistance. The physiological character of the response is demon¬ strated by the action of anaesthetics.

Experiment 109. — The effect of chloroform in inducing depression of response is seen in fig. 113. The first record shows the normal response under stimulation ; chloroform Fig. 1 1 3. Effect of Chloroform in inducingdepression of response was applied at the point marked with an arrow, the sub¬ sequent responses being obtained at intervals of 10 minutes. The amplitude of response is seen to undergo a rapid diminu¬ tion under the continued action of the anaesthetic. Under a moderate dose of the anaesthetic the preliminary effect is an enhancement of response, followed by depression.

The electric method of stimulation has the great advan¬ tage of easy graduation of its intensity. The experimental method is illustrated by fig. 114 ; two platinum pins are thrust about 5 cm. apart into the stem of a plant growing in a pot. Records are taken of the variation of resistance induced by stimulation in this particular portion of the Fig. 1 14. Method of resistance- variation in response to stimula¬ tion. Pressure on the tilting key k t causes stimulation o£ the plant by induction-shock from coil s, the galvanometer circuit being cut off at the same time (see text).

plant, which forms the fourth arm of the Wheatstone Bridge. An electric shock from an induction-coil is passed through the length of the plant, between the apex and the root, thus causing uniform stimulation of the intermediate portion of The electric stimulation is direct, so the prevention oi the leakage of shock-current into the galvanometer circuit presents certain difficulties. These are, however, com¬ pletely removed by cutting off the galvanometer connexion with the plant during the passage of the induction-shock. The shock-circuit S must, moreover, be cut off from the plant during the determination of the resistance and its induced variation ; otherwise the coil S would act as a shunt.

The method of procedure is as follows : the key K is closed and Kx and K2 opened. Exact balance is obtained by the sliding contact by which the ratio of the two arms of the bridge P and O can be varied. The balanced condition is shown when the galvanometer deflection is reduced to zero. The plant is then cut off from the galvanometer and put into the shock-circuit ; this is done by opening the key K and closing Ki ; K2 is simultaneously closed so as to short- circuit the galvanometer. The electric shock is thus allowed to pass through the plant for half a second, after which Ki and I\2 are opened and K closed. The variation of resistance induced by stimulation causes an upset cf the previous balance of resistance, with a resulting deflection of the galvanometer spot oi light.

In practice, the successive manipulations of making, breaking, and remaking the connexions are performed almost automatically by a momentary pressure on the tilting key T, and release of the pressure. The relative position of the three keys and the sequence of their action will be understood from the illustration given at right corner oi flg. 1 14. In the following investigations, intact or cut specimens of various plants have been successfully employed ; among these may be mentioned 'seedlings of Helianthus annuus and of Impatiens, also the climbing stem of Ipomoea pulchella and Porana paniculate,. The excitability depends on age, season, and the previous history of the plant. Under, favourable circumstances very pronounced response is obtained with an electric shock so feeble as to be below human perception.

specimen is allowed a period of rest for the complete sub¬ sidence of irritation caused by manipulation. The charac¬ teristic responses of variation of electric resistance are then obtained under sub-minimal, moderate, and strong stimula¬ tion. Experiment no. — The intensity of the stimulus is con¬ tinuously increased by the approach of the secondary coil to the primary till the amplitude of the resulting response Fig. 11-. Effect of electric stimulation of moderate intensity. Response by diminution of resistance shown by the up-cur\ e.

Fic i :r6 Effect of feeble electric stimulation. Response by in¬ crease of resistance exhibited by the down-curve (Helianthus). is about 3 centimetres. The duration oi electric Mi mu lation is only half a second, and successive stimulations oi equal intensity are applied at intervals of 5 to 10 minutes. Fig. 115 shows (1) that the response is a diminution 0 resistance as indicated b}' the up-cuxve ; (2) that the recovery is complete on the cessation of stimulation ; and (3) that the amplitudes of successive responses are equal under uniform stimulation. If successive stimulations are at short intervals, the protoplasmic recovery is then incomplete, and a sign of fatigue is exhibited by a diminution in the amplitude of successive responses. Another interesting phenomenon sometimes observed is the occurrence of alternating fatigue, that is to say, a large response followed by a small one, and this in recurrent series.

Experiment in. — I have already referred to the fact that the sign of mechanical response to a feeble stimulation is opposite to that to a moderate stimulus ; this is specially the case when the tissue is in a subtonic condition (p. 51). It is very remarkable that the method of resistance- variation also shows that with a sub-minimal stimulus the sign of the response is the opposite to that with a stronger stimulus, namely, an increase of resistance instead of diminution (fig. 116). A parallel effect was observed in the case of electromotive response (p. 166).

It will be shown in a later chapter that sensitive plants exhibit a series of multiple responses under a single strong stimulation. Similar results are obtained by the method of elec a emotive variation, both sensitive and ordinary plants giving a series of multiple electric responses under strong stimulation. Experiment 112.— By the application of strong stimu¬ lation, I obtained multiple response, the resistance of the tissue undergoing a recurrent variation.

In order to obtain response by resistance-variation to piiotic stimulation, I have devised the highly sensitive Quadrant Method, the principle of which will be understood from the diagram given in fig. 117. The specimen is a leaf- / blade of Tropaeolum, the four quadrants of which, P, Q, R, S, J serve as the four arms of a Wheatstone Bridge. I he diagonal connexions are made with the battery and the galvanometer | respectively. Three contacts with the leaf are fixed, and