Life Movements in Plants

Fig, 32.—Effect of -amputation of upper half of pulvinus. Upper record gives normal response before amputation, and the lower, response after amputation (Successive decs at intervals "of 0*1 sec. 1. Apex-time IT sec., in both. difference between them is. in £he exhibition of slight dimi¬ nution, of excitability due-'to operation.' But,, as. regards the latent .period and the. quickness of attaining maximum fall, there is no difference between the two records before and after the amputation of the upper half. The upper part of. the. pulvinus is thus seen practically to have little influence in hastening the" fall,

Experiment 25 .—The shock-effect caused by the amputation of the lower half was found to be very great, and it requir¬ ed a long period of rest before the upper half regained its excitability. The excitatory reaction of the upper half is by contraction, and the response is, therefore, the lift¬ ing of the petiole. Thus, in an intact specimen, excitation causes antagonistic reactions of the two halves. But the sensibility of the upper’ half is very feeble and the rate of its contractile movement, relatively speaking, very slow. The record of the response of the upper half of the pulvinus, seen in Fig. 33, was taken with an Oscillating Recorder, where the successive dots are at intervals of 1 sec.: the

.Fig. 38. —Response after amputation of lower half of pulvinus. (Successive dots at intervals of a second; vertical lines mark minutes.) Apex-time. 40 secs.. t agnification employed was about live times greater than recording the response of the lower half (Fig. 32), The intensity ■ of ! stimulus to evoke response had also to be considerably increased. Taking into account the factors of magnification and the intensity of stimulus for effective res¬ ponse, the lower half I find to be about 80 rimes more sensitive than the upper. Thus,.. under feeble. stixhulus the upper half exerts practically no ; antagonistic reaction. The

excitatory response of the upper half is also seen to he very sluggish. Experiment 26.— It is obvious that the mechanical moment exerted by the weight of the leaf must help its responsive fall under excitation. But the relative import¬ ance of the factors of active contraction of the lower half of the pulvinus and of the weight, in the rate of the responsive down-movement, still remains to be detetpaiaed. A. satisfactory way of solving the problem would lie in tfie study of the characteristics of response-records taken under three different conditions: (1) When the leaf is helped in its fall by its weight; (2) when the action'of the weight is eliminated; and (3) when the fall has to be executed against an equivalent weight. An approximation to these conditions was made in the following manner. We may regard the mechanical moment to be principally due to the weight of the four sub-petioles applied at the end of the main petiole. In a given case these sub-petioles were cut off, and their weight found to be 0-5 grm. The main petiole was now attached to the right arm of the lever, and three successive records were taken: (1) With' no weight attached to the petiole; (2) with 0-5 grm. attached to its end ; and Q’>J with 0*5 grm. attached to the left arm of the lever at an equal distance from the ful¬ crum. In the first case, the fall due to the excitatory contraction wi.ll practically have little weight to help it; in the second cage, "t will Ije helped by a weight equivalent to those of the sub-petioles with their attached leaflets ; and in the third case, the fall will be opposed by an equivalent weight. We find that in these three cases there is very

little difference in the time taken by the leaf to complete the fall (Fig. 34).' • Fig. 34—Effect of weight on rapidity of fall. N, without action of weight W, with weight helping: and A, with weight opposing. upper half of the pulvinns makes practically no difference in the period of fall; it is now seen that the weight exerts comparatively little effect. We are thus led to conclude that in determining the rapidity of fall, the factors of expansive force of the upper half of the pulvinus and the weight of the leaf are negligible compared to the active force of contraction exerted by the lower half of the palvinus.

In connection with this subject it need bfhrdly be said that the various experiments which I had previously carried oat with the intact plant can also be repeated with the isolated preparation. I will only give here accounts of experimeiite--.I'w-liick .-are...entirely The chemical solution may be applied directly to the pulvinus, or it may be absorbed through the cut end, the absorption being hastened by hydrostatic pressure. The normal record is taken after observing precautions which have already been mentioned. The reaction of a given chemical agent is demonstrated by the changed character the record. The effect of the drug is found to.depend not merely on its chemical nature, but also on the dose. There is another very important factor—that of the tonic condition of the tissue—which is found to modify the result. The influence of this will be realised from the account of an experiment to be given presently, where an

effects on two specimens, one of which was in a normal, and the other in a sub-tonic, condition. The experiments described below relate to reactions of specimens in a normal condition. Hydrogen Peroxide: Experiment 27.- -This reagent in dilute solution exerts a stimulating action. Normal records, were taken after long-continued application of water on tfie pulvinus. The peroxide, as supplied by Messrs* Parke Davis & Co., was diluted to 1 per cent., and applied to the pulvinus; this gave rise to an enhancement of response. Re-application of water reduced the amplitude to the old normal value (Pig. 35).

Barium Chloride .* Experiment 28 , —The action of t|-s agent is very characteristic, inducing great sluggishness in recovery. The preparation had ! been kept in 1-per cent, solution of this substance for two hours. After this the first response to a given test-stimulus was taken; the response was only moderate, and the recovery incomplete. The sluggishness was so great that the next stimulation, represented 'by a thick dot (Pig.. 36), was ineffective. Tetanising electric shock at T, not only ‘ broughj about

§1I%IG. 36.-— Incomplete recovery under tbe action of BaC! 3 and transient response, but* removed lor the time being the induced sluggishness. This is seen in the. next two .records^ . which were taken under'the old test-stimulus. There is now ah enhanced response and a complete recovery. Beneficial of stimulus. This is seen in the next two records which were, taken after two hoars. The amplitude of response was not only diminished, but the recovery also was incomplete.

Antagonistic actions of Alkali and Acid: Experiment 29 .—Alkali and acid are known to exert antagonistic actions on the spontaneous beat of the heart; dilute solu- * tion of NaOH arrests the beat of the heart in systolic contraction, while dilute lactic acid arrests the beat in diastolic, expansion. I have found identical antagonistic re¬ actions in the pulsating tissue of Desmodium ggrans , the FK3-. 37.—Antagonistic action of alkali and acid. Arrest of response in contraction under NaOH (A)j restoration and final arrest in expansion under actic acid ( jss ).

telegraph plant. It is very interesting to ■ find that these agents also exert their characteristic effects on . the response of Mimosa in a,, manner which is precisely; the same. This is seen * illustrated in Fig. 37,' .where ' the application of NaOH arrested, the response in .a contracted state; after this, the antagonistic effect of dilute lactic acid is seen first, in its power ■ of restoring the excitabilityits conti- lined application, however, causes a second arrest, but this time in a state of relaxed expansion.

CuSO i Solution.— This agent acts as a poison, causing a gradual diminution of amplitude of response, culminating in actual arrest at death. Certain poisons, again, exhibit another striking symptom at the moment of death, an account of which will be given in a separate paper. With Mimosa, after each excitation the recovery becomes complete after a resting period of about 15 min. With this interval of rest the successive responses for a given stimulus are equal, and are at their maximum.

Experiment SO .-When the resting interval is dimi¬ nished the recovery becomes incomplete, and there is a Pie. S8.—“Fatigue” Maced by shortening intervening'period of rest. consequent diminution of amplitude of response. There fgh thus an increased fatigue with diminished period of rest. This is illustrated *in Fig. 38, where the first two responses are at intervals of 15 min.; the resting interval was then reduced to 10 min., the response undergoing, a marked diminution. Conversely,- by increasing the resting interval, first to 12 and then to 15 min., the extent of fatigue was reduced and then abolished.

Experiment 81. —From the above experiment it would appear that since the incompleteness of recovery induces fatigue, hastening of recovery would remove it. With this idea I tried various methods for quickening the recovery of the excited leaf. The application of a constant electric current' was found to have the desired effect. Two elec¬ trodes for introduction of current were applied, one on the stem and the other on the petiole, at some distance from the pulvinus. In-order'to avoid the excitatory effect of Sadden application, the applied current should. be in-

FlO. $9.—Action of constant current in removal of fatigue by hastening f^oopry; N, carve of response in fatigued specimen$ 0,' after passage of current creased gradually; this was secured by means of a poten¬ tiometer slide. In my experiment a current having an intensity of 1*4 micro-ampere was found to be effective. Responses at intervals of 10 min., as we have seen, exhibit marked fatigue. Two responses were recorded on a fast- moving' plate, N before, and C after, the application of the current. It will be seen (Fig. 39) how the application of current has, by hastening the recovery, enhanced the amplitude of response and brought about a diminution of fatigue. In connection with this, I may state that the tonic condition is, in general, improved as an after-effect of the passage of current. This is seen in some cases by a slight increase in excitability; in others, where the res¬ ponses had been irregular, the previous passage of a cur¬ rent tends to make the responses more uniform.

In taking continuous records of responses I was struck by the marked change o§ excitability exhibited fey the intact plant under variation of light. Thus the appearance of a cloud was quickly followed fey an induced depression, and its disappearance by an equally quick restoration of excit¬ ability. This may be explained on the theory that certain explosive chemical compounds are built up by the photo- synthetic processes .in green leaves, < and that the intensity „jg£ response depends on . the presence of these compounds. Bit. the building up of a chemical compound must neces- be , a' slow process, and it is difficult on the above

Experiment. i2 «—In order to find out whether photo-syn¬ thesis had any effect on -excitability, I placed an intact plant in a* dark room and-obtained from it a long series of responses under uniform test-stimulus. While .this was being done the green leaflets were alternately subjected fcO strong light and to darkness, care being taken that the pulvinus was shaded all the time. The alternate action of light and darkness on leaflets induced no variation in the uniformity of response. This shows that the observed variation of excitability in Mimosa under the alternate action of light and darkness is not attributable to the photo-synthetic processes.

I next took a petiole-pulvinus preparation from which the sub-petioles bearing the leaflets had been cut off, and placed it in a room illuminated by diffused daylight The normal responses were taken, the temperature of the room being 30 ? 0. The room was darkened by pulling down the blinds, and records were continued in darkness. The temperature of the. room remained unchanged at 30° 0. It will be seen from records given in Fig. 40, that in PiO. 40.—Stimulating action of light, and depressing action of darkness. Horizontal line feelow represents period of darkness.:

darkness there is a great depression of excitability. Blinds w4 . re next pulled up and the records now obtained exhibit the normal excitability under light. The sky had by this time become brighter, and this accounts for the slight enhancement of excitability. This experiment proves conclusively that light has a direct stimulating action on the pul "inns, independent of photo-synthesis. On isolation of a petiole-pulvinus preparation, the shock of operation is found to paralyse its sensibility. After suit¬ able mounting the excitability is restored, and remains practically uniform for nearly 24 hours. After this a depres¬ sion sets in, the rate of fail of excitability becomes rapid 40 hours after the operation, sensibility being dually abolish- ed after the fiftieth hour.

Experiments carried out on bin- ed-c: oi weighi, and the influence of selective amputation of the upper arm lower halves of the puiVinus, show that in determining the rapidity of fail of leaf, the assumed factor ut the expansile force of the upper half of the paivinns and the weight o, the leaf are negligible compared to the force ot active con¬ traction exerted by the lower half of the pnlvinus. The -excitability of the lower half is eighty times greater than

Chemical agents induce characteristic changes in excita¬ bility. Hvdrogen peroxide acts as a sumulanu arium chloride renders the recovery incomplete: bat tetanisauon temporarily removes the induced sluggishness. Acids and also Bose and D as-‘PhysiologicaHavestigations with Fetioie-Palvinu* alkalis induce antagonistic reactions, abolition of excitability with alkali taking place in a contracted, and with acid in an expanded condition of the pulvinus.

The responses exhibit fatigue when the period of rest is diminished. The passage of constant current is found to remove the fatigue. Response is enhanced on exposure to light, and dim¬ inished in darkness. Light is shown to exert a direct stimulating action on the pulvinus, independent of photo¬ synthesis. The plant Mimosa offers the best material for invest¬ igation on conduction of excitation. With regard to this question the prevailing opinion had been that in plants like Mimosa , there is merely a transmission of hydro¬ mechanical disturbance and no transmission of true ex¬ citation comparable with the animal nerve. I have, however, been able to show that the transmission in the plant

is not a mechanical phenomenon, but a propagation of excitatory protoplasmic change. This has been proved ^ by the arrest of conduction by the application of various physiological blocks. Thus local application of increasing- cold retards, and finally abolishes the conducting power. The conducting tissue becomes paralysed for a time as an after-effect of application of cold; the lost conducting power may, however, be quickly restored by tetanising electric shocks. The conducting power of an animal nerve *■ la arrested by an electrotonic block, the conductivity being restored on the cessation of the current. I have succeeded in inducing similar electrotonic block of conduction m Mimosa. Conductivity of a selective portion of petiole may also be permanently abolished by local action, of poisonous solution of potassium cyanide

*B0SE-“ An Automatic Method for the Investigation oj Velodty ofTrans- mission of Excitation in ‘Fhil. Trans. Having thus established the physiological character .of the transmitted impulse in plants I shall now proceed to give some *of the principal results of my earlier and recant investigations on the effects of various agencies on conduction of excitation in plants. Apart from any question of hydro-mechanical trans¬ mission, it is important to distinguish two different .modes of transmission of excitation. In a motile tissue contrac¬ tion of a cell causes a physical deformation and stimula¬ tion of the neighbouring cell. Examples of this are furnished by the cardiac muscle of the animal, the pul- vinus of Mimosa^ and ‘ the stamen of Berber is • This mode of propagation may better be described as a connection of excitation.

different process e£ transmission of protoplasmic change. The conducting tissue in this case does not itself exhibit any visible change of form. In the plant ■ the necessary condition for transmission of excitation to a distance is that the conducting tissue should. be possessed of proto¬ plasmic .continuity in a greater or less degree. This condition is fulfilled by vascular bundles. There being greater facility of transmission along the bundles than across them,‘the velocity in the longitudinal direction is very much greater than in the transverse.

For accurate determination of velocity of transmission the testing stimulus should be' quantitative, and capable of repetition. Abnormal high velocity has been observed lft Mimosa by applying crude and drastic methods of stimu¬ lation, .by a transverse cut or a bum. This is apt ,to give rise to a very strong hydro-dynamic disturbance, which travelling with great speed, delivers a mechanical blow on the responding pulvinus. Such hydro-dynamic trans¬ mission is $ot the same as physiological conduction.

■ In the primary petiole of Mimosa the highest velocity under electric stimulation I find to be about mm. per second. This velocity is considerably lower than the velocity in the nerve of higher animals, but higher than in the lower animals. As an example ot the latter, mention may be made of the velocity of 10 mm. per second jn the nerve of Anodon and 1 mm. per second in the nerve of Eledone . Experiment S3 .— 1 The conduction o£ excitatory impulse takes place in both directions. This can be demonstrated by taking a petiole of Biophytum sensitivum or of Averrhoa carambola. These petioles are provided with a series of motile leaflets. Stimulation at the middle point of the petiole gives rise to two waves of excitation, one of which travels towards the central axis of the plant, and the other away from it. The centrifugal velocity is greater than the centripetal as will be seen frbm the following results:

Variation of temperature has a marked effect on the velocity of transmission of excitation. Lowering of tem¬ perature diminishes the velocity, culminating in an arrest. Rise of temperature, on the other hand, enhances the velocity. This enhancement is considerable m specimens in which the normal velocity is low, bat in plants m optimum condition, the velocity being already hi|h, cannot be further enhanced. The following tabular statement gives results of effects of temperature on velocity- of trans¬ mission in Mimosa and Biopsy turn ;

TABLE IV.—EFFECT OF TEMPERATURE ON VELOCITY OF TRANSMISSION. The velocity of transmission is very much lower in winter than in summer. In the petiole of Mimosa, . the velocity in summer is as high as 30 mm, per second;; in winter it is reduced to about 4 mm. The lowering of velocity in winter is partly due to the prevailing low fceip* peratiire and also to the depressed state of • physiological activity.' In a Mimosa plant, different leaves will be, found of different age. Of these the youngest will . be at the top* Lower down, we obtain a fully grown young leaf, and near the base, leaves which are very old. The investigation deals with the effect of age on the conducting power of the petiole.

Comparison of conducting power in different leaves: Experiment 34* —Selecting three leaves from the same plant we apply an identical electric stimulus at points 2. cm. from the three responding puivini. The electric connec- t{ons are so made that the same tetanising shock is applied ■ on. the three petioles, very young, tally grown, and very ; old. The secondary coil is gradually pushed in till the leaves exhibit responsive fall. The fully grown leaf was the first to respond, the velocity of transmission being 23 mm. per second. The secondary coil had to be pushed nearer the primary through 6 cm. before excitation could be effectively transmitted through the young petiole; for the oldest leaf still stronger stimulus was necessary, since in this case the secondary had to be pushed through an additional distance of 4-cm. Tor effective transmission of excitation.