The Motor Mechanism of Plants

Further, in all cases stimulation causes sudden contrac¬ tion of tiie cortical cells, involving rapid expulsion of sap. It has been explained in previous chapters that the supposed difference between sensitive and ordinary plants is quite arbitrary, and that any conspicuous movement of the tissue of ordinary plants is prevented by the physical restraint imposed by its connexion with the rigid wood. It was therefore necessary to apply some method independent of movement, to the study of the universal sensitiveness of plants. Accordingly I had recourse to the method of observing excitatory change by means of the concomitant variation either of electric potential or of electric resistance. In the present chapter I give and discuss the record of the response of plants as obtained by the method of electro¬ motive variation. It is known that the state of excitation in an animal tissue can be detected by the induced electric variation, the excited point A being galvanometricaliy negative in relation to the unexcited point B. In regard to plants it had been thought that it is the sensitives alone that are excitable, exhibiting response to stimulus by mechanical and electric reactions. I have, however, de¬ monstrated that not sensitive plants alone, but all plants and all their organs are excitable, the state of excitation being detected by the concomitant change of galva nometric negativity.1

In -order to show that the mechanical and electric responses are but two different expressions of the same excitatory reaction, a leaf of Mimosa was attached to the Optical Lever for the mechanical response ; suitable electric connexions were made at the same time with a reflecting galvanometer for the electric response. Stimulation was found to give rise to both mechanical and electric responses, which occurred practically at the same time. 1 he electric I response was due to the induced galvanometric negativity 1; of the more excited point.

Since the excitability of the lower half of the pulvinus f is very much greater than that of the upper half, either ft direct or indirect stimulation induces a greater excitatory J reaction of galvanometric negativity at the lower side. 1 he 1 electric connexions are therefore made as follows : the first contact is made with the more excitable lower half of the ; | pulvinus, and the second with the relatively unexcitable upper half. It is still better to make the second contact with a distant indifferent point on the stem. 'I

The electric contacts are made by thrusting fine platinum wires into the tissue, iiie irritation caused by the sligln : wound passes away in a short time. This direct contact is 4 iar better than the complicated method of electrolytic con- •' tact by means of non-polarisable electrodes, whxh is not only unnecessary but often harmful ; for unless great precautions | are taken the zinc sulphate solution leaks, and coming in contact with the plant abolishes its excitability. 1 he ^ amalgamated zinc rods, moreover, are not absolutely iso- , electric ; platinum wires, on the other hand, can be made iso-electric after annealing. The direct method oi contact has the advantage of lowering the resistance to a minimum.

The object of the non-polarisable electrodes is to reduce the , counter E.M.T, caused by the passage of the current of * response but as this current is extremely feeble there is hardly any counter. E.M.E. induced by it. I uniform stimulation in successive experiments, and of increas¬ ing the intensity of stimulation in a graduated manner. This can be easily secured by the employment of electric shocks from an induction coil. The oscillatory induction shock may cause, however, a serious complication in case of leakage of the shock-current into the galvanometer. This difficulty was completely removed by the interposition of a

T'J . <J5. Method of obtaining electric response of the pulvinus The choking-coil c prevents leakage of shock-current from secondary coil s into the galvanometer circuit. magnetic choking-coil, which prevents the rapidly alternating current from entering the galvanometer circuit (lig. 95). An important condition for obtaining the normal electric response is the maintenance of the plant in a favourable tonic condition by keeping it exposed to the diffuse light of the sky. As the galvanographic records ..re taken by photography, the wires attached to the plant are led to the galvanograph in a photographic room

Experiment 91. Effect of moderate stimulation. — Electric stimulation of uniform intensity was applied at regular intervals and the resulting response recorded. The photo¬ graphic plate was moving at a slow rate, hence the records of response and recovery are almost superposed ; it will be Fig. 96. Uniform response of galvanometric negativity under uniform stimulations of moderate intensity (Mimosa). noted that the amplitude of successive responses under uni¬ form stimulation is the same (fig. 96). The response is that of galvanometric negativity given by the more excitable lower half of the pulvinus. This corresponds with the responsive fall of the leaf effected by the relatively greater contraction of the lower half, ihe electric lecoveiy from excitation occurs in the course of 3 to 4 minutes.

Experiment 92. Effect oj feeble stimulation . rhis was found to induce a response which is of opposite sign to that of the normal, namely, of galvanometric positivity indicative of expansion instead of contraction (fig. 97). A parallel effect was obtained in the mechanical response of Mimosa (cf. Experiment 20). If the stimulus be gradually increased from feeble to strong, the response changes from positive to normal negative at a critical intensity of stimulation. The critical value is found to depend on the tonic condition of the tissue. In a highly excitable specimen the critical point is low ; it is relatively high when the tissue falls into a sub tonic condition. It is therefore easy to obtain with subtonic specimens positive response under feeble stimula¬ tion, as in the following.

Experiment 93. Positive response of subtonic tissue. — The plant is kept in diffuse light, and the minimum intensity of stimulus which invariably gives negative response is determined. A cover is next placed over the plant so as to maintain it in darkness for about an hour, thus inducing a condition of subtonicity. Application of the stimulus which previously induced a negative response is now found to bring about a positive response. The piant is then ex¬ posed to light, to improve its tonic condition. The response is now found once more to be the normal negative.

Experiment 93 a. Unmasking of positive response by physiological block. — Application of cold (or anaesthetics) on the petiole in the path of the impulse causes a block of the passage of excitation I), but allowrs the passage of impulse A (cf. p. 145), and thus unmasks the positive electric response. Experiment 94. Electric response of physically restrained leaf. — It has been shown (Experiment 72) that contraction of the cells of the pulvinus occurs even when the leaf is restrained from executing the movement of fad. Similarly, the normal electric response of galvanometric negativity persists even when the leaf is held in a fixed position. The electric response is therefore due to the fundamental excitatory reaction and not to the movement of the leaf.

Experiment 95 . Electric response of water-logged pulvinus. It has been shown that the pulvinus becomes immobile after excessive absorption of water. This may be explained on the supposition that in an over-inflated condition the expulsion of sap is prevented, thus causing an abolition of contractile response (cf. Experiment 34). lhe question now arises whether this absence of mechanical response is due to the abolition of irritability, or merely to physical restraint imposed by over-turgid tissue. In testing this question 1 took a pulvinus of Mimosa rendered mechanically irre¬ sponsive by excessive absorption of water. Stimulation gave, nevertheless, the normal response of galvanometric negativity, proving that the tissue was still iiiritable though unable to manifest it externally by mechanical movement.

The next point to be settled is whether or not ordinary plants also respond to stimulation by electromotive change of gilvanometiic negativity, proving that they also are fully excjtable. After taking suitable precautions I succeeded in obtaining electromotive response of ordinary plants to stimulation caused by an induction-shock. Mechanical stimulation, however, removes all complications which might arise from the leakage of induction current into the receiving circuit. I will now describe a method of mechanical stimu¬ lation which has been rendered very efficient for the present investigation. Associated with it is the Method of I dock, which will be found to have removed many experimental difficulties.

The specimen A B, which may be a stem or a root, is tightly clamped at the middle by a vice V. The free ends of the specimen are held in tubes ((_ (/) provided with three clamping jaws. A or B can be subjected to torsion by means of the handles H or H7 (hg. 98). lhe midpoints A and B are connected with a reflecting galvanometer of the D’ Arson val type of sufficient sensitivity to produce a deflection of 1 mm. at a distance of a metre by a current of io‘ 9 ampere.

If the end A be slowly twisted, say to the right, through 50, the physical distortion is found to produce no electric varia¬ tion ; but a rapid torsion produces an electric response to the right, indicating negativity at A, the responsive current flowing from B to A through the galvanometer. It is the suddenness of the disturbance that constitutes the stimulus ; the response disappears even if the specimen is kept twisted to the right. If a sudden twist to the left be now given (bringing the specimen to the original untwisted position), the responsive electric deflection to the right is produced as before. The plant is next subjected to a rapidly alternating torsion to the right and left through 5° ; the response is now found to be nearly doubled. The rapid twist and untwist, designated as torsional vibration, is thus found to be a very effective method of stimulation ; the intensity of which, within limits, is found to increase with the angle of torsion. The amplitude of torsion for successive uniform stimulation is predetermined by movable stops S S', diagrammaticaliy shown in the figure to the right. If next B be subjected to sudden to-and-fro vibration, the responsive current flows in the opposite direction, B becoming negative (fig. 98).

Advantage of the Method of Bloch. — The method employed in obtaining electric response in animal tissue is to render one of the two contacts, say B, insensitive by injury, the injured point being negative. Diffuse stimulation, inducing negativity at the uninjured A, causes a negative variation of the current of injury. Then the current of injury gradually subsides, with the result of disappearance of response by negative variation. By the method of injury, one end is made initially abnormal ; whereas by the method of block,

ou the other hand, the tissue throughout is in a normal condition. There is, again, the further advantage that any set of results obtained by stimulating A can be verified by Plant p is securely held by a vice v which serves as a Mock. The two ends are clamped by holders c c'. By means of handles h h', torsional vibration may be imparted to either ■ the end a or the end b of the plant. The end view (6) shows how the amplitude of torsion is predetermined by means of movable stops s s'.

corroborative reversal experiments by stimulating B. The excitation of A and B remains localised, the transmission of excitation from one end to the other being prevented by the interposed mechanical block. Experiment 96. — The amplitude of the response can, as already stated, be increased by increasing the angle of the torsional vibration. This is, however, liable to produce fatigue ; but it is possible to obtain enlarged response by the additive effect of repeated feeble stimulations, each of which is individually ineffective. Thus a single stimulation by a torsion of 30 produced little or no effect ; but when it was repeated 30 times it evoked a large response, the amplitude of which was 40 mm. The advantage of this is that with small angles there is little physical distortion pro¬ duced in the tissue. The mechanical stimulation is effected by automatic means ; one *.nd of the plant-organ is fixed in a torsional clamp, and rapid alternating torsions are given to that end by means of a clockwork ( see fig. hi). Suitable

adjustments are provided for regulating the angle of torsion. After the clock is fully wound a press-button releases it, with the result that a definite number of torsional vibrations are produced in rapid succession. The complete torsional apparatus and the plant- chamber are shown in fig. 9 9. Moistened cotton threads Temperature is regulate! by the electric heating coil r. VTapour or gas can be blown into the chamber through the side tube. in connexion with non-polarisable electrodes make secure electric contacts with A and B. It is, however, preferable, as previously explained, to make direct platinum contacts with the two points. The air in the chamber is kept in a humid condition by means of moistened blotting-paper.

For experimenting on the effects of temperature there is an electric heating coil, R, inside the chamber, for raising the temperature to any desired degree. For the study of the effects of different gases on the excitability of the plant there are inlet and outlet tubes which enable a stream of the required gas or vapour to be circulated through the chamber. Response Recorder. — I devised the following simple form of recorder for obtaining tracings of the electric response. The curves are obtained directly by tracing the excursion

of the galvanometer spot of light thrown down on the revolv¬ ing drum by a fixed mirror inclined at 45°. The responsive deflection of the galvanometer is followed by moving the carrier which holds the recording-pen (fig. 100). The return of the spot, to the original position is similarly followed, and the complete curve exhibits response and recovery. The ordinate represents the electromotive variation, and the abscissa the time. Photographic record of response can be obtained by wrapping a sensitive film round the drum. The following photographic records attest the reliability of m the experimental method.

Experiment 97. — The stimuli of torsional vibration were increased from 2*5° to 12-5° by steps of 2*5° at a time. Fig. ioi. Increasing amplitude of electric response to increas¬ ing torsional stimulation ; the vertical line to the right represents o • r volt. Stimulation applied at intervals of 3 minutes (Cauliflower-stalk). Fig. 101 shows that the intensity of response increases and approaches a limit. The following table gives the absolute values of the responsive electromotive variations.

Table XI. — Showing the Increased Electromotive Variation Induced under Increased Intensity of Stimulation. Such electric response to mechanical stimulation is obtainable with all plants and with every plant- organ, the following table contains ? list of specimens which give a fairly large electromotive response, which may sometimes be as high as o-i volt. Horse Chestnut ( Aesculus R 1 ppocastanum) Turnip ( Brassica Napus) Experiment 98. — The responses are uniform under uniform stimulation, provided sufficient time is allowed for

Fig. 102. Uniform electric responses under torsional stimulation (Carrot). the completion of protoplasmic recovery. The uniform electric responses obtained with Carrot are given in fig. 102. Experiment 99.-— Fatigue, however, occurs under shorter periods of rest. In the first three responses (fig. 103) the | stimulation was at intervals of 1 minute, by which time ' the recovery was complete ; the successive responses are therefore large. The rhythm of stimulation was now changed to intervals of half a minute instead of a whole, while the

stimuli were maintained at the same intensity as before. The responses were then much diminished on account of fatigue. The original minute-rhythm was now restored, with the restoration of the original amplitude of response. Experiment 100. — The electric response is reduced or abolished on lowering the temperature. The tropical P'ig. 104. Abolition oi electric response by steam (Carrot). I he first two responses (left) took place at a temperature of 1 70 C. Steam was then admitted. At the end of 5 minutes response ceased, the Carro4 was dead. The vertical line (right) represents o*i volt.

Eucharis Lily is particularly sensitive to the action of cold. When the temperature is lowered to 20 C. the electric response is temporarily abolished, the excitability being || restored on return to the normal temperature. The response | j is at its maximum at a certain optimum temperature, ft is , permanently abolished when the plant is exposed to tool high a temperature ; fig. 104 exhibits the effect of introduc- | tion of steam into the plant-chamber. There was at first a transitory augmentation of excitability. But this cpiicklyl disappeared, and response was permanently abolished withH the death of the plant (fig. 104). ■

Experiment 10 1. — The mode of experimentation was first to obtain a series of normal responses to uniform stimuli! applied at regular intervals of time. After this, without interrupting the procedure, the anaesthetic agent, chloroform vapour, was blown into the closed chamber containing the plant. It will be seen how rapidly chloroform produced a depression culminating in abolition of response and death Various poisons wore found to produce permanent aboli¬ tion of response. Among the poisonous agents which abolish the electric response by killing the plant may be mentioned solutions of potassium cyanide and mercuric

Plants and all their different organs are excitable and respond to stimulation by an electric reaction of galvano- metric negativity. A feeble stimulation, individually ineffective, becomes effective on repetition. The amplitude of response, within limits, increases with the intensity of stimulation. Uniform stimuli, applied at suitable intervals, induce uniform responses. Fatigue is induced by shortening the intervening period of rest. The electric response of a subtonic tissue is positive. External stimulation raises the tonic level of the plant and transforms the positive to the negative.

Electric response is arrested at a minimum temperature. Rise of temperature up to an optimum enhances the excita- bihty, as seen in the increased amplitude of response, there is a fatal temperature at which the response undergoes permanent abolition. Vapour of chloroform in strong doses induces a rapid depression and abolition of the electric response. Under all variations of external conditions, the records of mechanical and electric responses show similar modifica¬ tions. These rerponses are, therefore, both expressions of one and the same protoplasmic variation.

1 HE electric response of plants described in the previous chapter w as obtained under mechanical and electric stimula¬ tion which caused excitation of the tissues as a whole. The stimulus of light impinging from outside has to traverse the epidermis before it can reach the living protoplasm in the interior. In my previous works it has been shown that the normal response of leaves to light is one of galvanometric negativity, as it is under other modes of stimulation. Some observers have, however, obtained in green leaves a response of galvanometric positivity, from which the conclusion has been drawn that the response to light is of an opposite character to that induced by other modes of stimulation.

1 can, however, show that the normal response of the leaf to photic stimulation is that of galvanometric negativity , the positive reaction being manifested only under certain specific conditions. These are, first, the age of the specinn n ; a highly excitable young specimen gives normal negative electric response ; but the excitability is greatly enfeebled when the tissue is either too young or too old. In these circumstances the response tends to become the abnormal , positive.