The Motor Mechanism of Plants

Vertical marks below record indicate intervals of o*3 minute. Fig. 14. Response of leaf of Neptunia. Successive dots are at intervals of 0*5 minute in the contractile portion, and 1 minute in the recovery portion of curve. minimum stimulus for excitatory contraction of the leaf is 2 units, and the maximum contraction is attained in 3 minutes in a moderately vigorous and in 1 minute in an exceptionally vigorous specimen (fig. 14). Experiment 5* — There are numerous other pulvinated leaves, such as those of Phaseolus and of Iiyythvinci wdicu, which are usually regarded as insensitive to mechanical or electric shocks^ This supposition is, however, erroneous, since a responsive movement, under a moderately strong stimulus, can be clearly demonstrated by means of high magnification. The response of Erythrina is seen to be

Fig. 15. Slow and feeble response c£ Erythrina. Stimulus applied at vertical line. Successive dots are at intervals of * 30 seconds. very similar to that of Mimosa, the difference consisting in the great sluggishness of the reaction, the maximum con¬ tractile fall .occurring only after 8 minutes (fig. 15). Though the pulvini of different plants are anatomically very similar, yet there are characteristic differences in the rate of their reaction, some being active, others semi-active, and still others practically inactive. On wiiat does this differing rate of reaction depend ? This problem will be discussed in a subsequent chapter.

The vital activity of the plant at "any moment can be gauged by its contractile response to a definite testing stimulus. The plant-tissue is stimulated by the same agents that excite the animal tissue. For quantitative investigations, devices have been perfected for thermal and electric stimula¬ tion of the plant, such that the intensity of the stimulus can be kept constant or increased in a graduated manner. The pulvinus of Mimosa is far more sensitive than had been previously supposed. Quantitative tests show that in an optimum condition it gives a responsive fall to an intensity of electric stimulus, below the threshold of human perception.

Accurate records of responsive movement have been secured by the Optical Lever, which gives a magnification more than a thousand times. Records have also been ob¬ tained with different types of mechanical recorders. The error arising from friction is eliminated by the intermittent contact in the record., taken with the Tapping and the Oscillating Recorders. The dotted curve gives the time- relations of the response : the phytogram is its own chrono¬ gram. With the Resonant Recorder time-intervals as short as o-ooi second can be accurately measured.

Contractility is a general characteristic of living tissues. In pulvinatecl leaves response by a movement of fall is effected by the greater contraction of the more e: citable lower half of the pulvinus. Many pulvini, usually regarded as quite insensitive, also respond, though by a feeble and relatively sluggish movement. In regard to the rapidity of contractile response, Mimosa is highly active, Neptunia semi-active, and Erythrina is relatively inactive.

As has been previously explained, the two halves of the pulvinus nVciy be conveniently distinguished as upper and lower- that is, above and below the vascular cylinder. The lower half is known to be highly sensitive, whilst the upper half of the organ is commonly supposed to be quite in¬ sensitive- The downward movement of the leaf after stimulation has been considered to be brought about passively by the flaccid pulvinus yielding to the weight of the leaf, whilst Haberlandt "regards the, movement of fall as being accentuated by the expansion of the upper half of the pulvinus. I will, however, show (i) that the upper half of the pulvinus is also sensitive, though feebly, and responds to stimulus by contraction;

(2) that the fall of the leaf is due to the active and pre¬ dominant contraction of the lower half of the pulvinus ; (3) that the factor of active contraction is so g^eat that the weight of the leaf has little effect in hastening the fall , and (4) that the fall of the leaf is not helped by any expansion of the upper half of the pulvinus. 4 , I will first demonstrate that the upper half of the pulvinus is excitable. Experiment* 6. — The upper half of the pulvinus was locally * stimulated by a narrow beam of light from an arc-lamp acting from above, fins gave rise to a relatively feebR up-movement due to contraction of the upper half

of the organ. A magnificat icn of 30 times was necessary to obtain a record of moderate size. The latent period for initiation of response w as 20 seconds. Owing to physio¬ logical inertia, the response persisted for a certain length ol time, after which there was a recovery (fig. 16). Under stronger or long-continued stimulation the excitation be¬ comes transmitted across the pulvinus to the more excitabk lower half, causing an energetic down-movemdht.

Fig. 16. Effect of local stimulation of upper half of pulvinus of Mimosa by light. Application of light at arrow, its cessation at arrow within circle. Erectile movement shown by down- curve. halves of the organ are sensitive, and that the resultant fall on diffuse stimulation is due to the predominant contraction of the lower half. It should be mentioned here that the minimum intensity of stimulus necessary for the contraction of the upper is considerably stronger than for .hat of the lower half (cf. Experiment 12, p. 37).

lhere is an additional purely mechanical factor, the unequal pliability of the organ in the two directions, that enfeebles the up-response. I he unequal movement upwards or downwards under the action of the same force is demon¬ strate! by the experimental method illustrated in fig. 1/. 1 lie petiole is* subjected to an equal pull, upwards and downwards, and the' resulting displacement of the long aluminium index is read on the circular scale. A thread is tied to the petiole, the ends of which are attached to two

small aluminium pans of equal weight : the upper portion passes over a frictionless pulley. Experiment 7. — In the first series of observations suc¬ cessive weights of 0-25 grm. .are carefully added to the upper pan, causing an upward tension. The increasing angular displacement reached a hmit of 210 under a weight of 1 5 grm. ; when this was increased to 2 grm. no further displacement was produced. On the removal oi the weight the leaf regained almost its former positior. The experi¬ ment was repeated by successive additions of 0-25 grm. to the lower pan, causing an increasing downward pull. The angular displacement was now very much greater ; undei a weight of 2 grm. the downward displacement was * in contrast to the limiting upward displacement 01 2± . The following table gives a detailed account of the results :

Table I. — Showing the Difference between Upward and Downward Displacement, under equal weights. It should be mentioned here that the downward sinking of the leaf brought about by the careful addition of a weight is in no way connected with the excitatory fall, for the removal of the weight is followed bv an immediate recovery of its original position. The curves (fig. 18) show the great difference between the upward and dowir /ard displacements. The movement under the same force is thus considerably less in an upward direction. The feebleness of the erectile response to local stimulation may therefore be regarded as the joint effect of the low excitability of the upper half and of the unequal pliability of the organ. A cylinder of lignifted sclcrenchyma surrounds the vascular bundles in the petiole, the rigidity of which is in a great measure due to this protective cylinder. In the pulvinus itself there is no such lignification of the cylinder ; were it lignifted, the very marked responsive movement of the pulvinus would be an impossibility. There is another impjrtant

factor which also rerders the movement unequal in the two direction . Schwendener has shown that the lower side of the pulvifius, in contrast to the upper, is more Conspicuously provided with furrows like the folds of the skm in a human finger. I obtained very interesting results Eig. iS. Curves showing angular displacement under increasing weights which pulled the leaf upwards (lower curve) and downwards (upper curve). in making longitudinal sections of the pulyinus under two different conditions. In making the sections, the cut made by the razor acts as an intense stimulus, causing contraction and concave curvature of the lower half ; the furrows now become very conspicuous. But if the pulvinus be previously benumbed by freezing, its contractility disappears, and the subsequent sections exhibit no furrows. These furrows.

under normal conditions, permit of considerable curvature of the pulvinus in a downward direction wit tout any com¬ pression or deformation of the tissue. The fact that the excitatory fall of the leaf is due not to passive yielding of the flaccid pulvinus but to active con¬ traction is clearly demonstrated by the following experiment. Experiment 8. — Response to an electric shock was first taken of the leaf of a potted Mimosa (fig. 19, a). It was then

Movement due to contraction of lower half represented in botli by an up-curve, which on account of its rapidity is seen as a scratch. In a it is the fall of the leaf that is represented by the up-curv§ ; in b it is the erectile movement. held in an inverted position and response to similar electric shock obtained once more. The response was now an erectile movement, lifting the weight of the leaf against the force of gravity. The record was taken with the Oscillating Recorder, the successive dots being at inter¬ vals of 10 seconds. The maximum contraction, causing maximum erectile movement, occurred in the course of 2 seconds, which is also the average value for the

maximum fall of the leaf in I lie normal position. Com¬ parison of the two records (a) and (b) (fig. 19) shows the essential similarity of response in the normal and in the inverted positions. This proves that the normal fall of the leaf under stimulation is due not to flaccidity but to active contraction. The factor of active contraction is so great that the rapidity of the resp msive movement of the leaf is but little a'fected by its weight. I describe experiments carried

out with one and the same plant which fully justify this con¬ clusion. Experiment 9. — The records of re¬ sponse were taken under the following conditions : (1) when the leaf was helped during fall by its weight ; (2) when the action of weight was practically elimi¬ nated ; and (3) when the fall had to be executed against an equivalent weight. The mechanical share of the leaf in the movement of fall may be regarded as mainly due to the weight of the four sub-petioles and their leaflets acting at the end of the main petiole. In the following series of experiments the sub-petioles were cut off ajid 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

n curve ot response without weight ; w, with weight helping ; a, weight opposing. no weight attached to the petiole, (2) with 0*5 grm. attached to its end; and (3) with 0-5 grm. attached to the left arm of the lever at an equal distance from the fulcrum. In the first case the fall due to the excitatory contraction will have little weight to help it ; in the second ease it will be helped by a weight equivalent to that of the sub-petioles ; and in the third case the fall will be opposed by an equivalent weight. The records (fig. 20) show that in these three cases there was practically no difference in the time taken by the leaf to complete its fall, the period being about 1 *b second.

Having proved that the weight of the leaf has little effect in the rapidity of the fall, I will next consider the conten¬ tion that the movement of fall is effectively helped by the supposed expansion of the upper half of the pulvinus previously in a state of compression. The influence of this expansive force can be tested by taking two records — one * under normal conditions, and the other after the removal of the upper half of the pulvinus. Experiment 10. Response before and after removal of the upber half. — The record of response of an intact leaf was first taken on a fast-moving plate. The upper half of the pulvinus was then removed and a second record obtained, under stimulus of the same intensity as before. Pfeffer was unable to obtain any response after the removal oi the upper half ; his failure, 1 find, is due to the loss of irritability caused by the intense shock of operation. I have been able to reduce the sh jck-effect to a minimum by local application of cold, which temporarily benumbed the tissue. The normal excitability was found to be restored about 3 hours after the operation. Comparison of the two records before and after the operation showed that there was practically no difference in the rapidity of the fall, the time required for maximum fall being in both cases 1 - I second (fig. 21). The

upper half of the pulvinus has therefore, no effect in hastening the fall of the leaf. Experiment n. Effect of removal of lower half of the pulvinus. — A record was taken of the response after removal of the lower half. 1 he shock-effect caused by the amputation of the lower half was. found to be very great, and it required a long period of rest before the upper half regained its excitability. Stimulation by electric shock induced con¬ traction of the upper half of the organ and caused an erection of the leaf. This is an additional proof of the fact previously

Upper record gives normal response before amputation, and the lower, response after amputation. Successive dots at intervals ofo-i second. Maximum fall after i 1 second in both . demonstrated that the upper half is also excitable, and that the direction of its responsive movement is opposite to that of the lower half. The characteristic of the response of the upper half is that not only is its sensitivity extremely feeble, but that its rate of reaction is very slow. The intensity of stimulus required for initiating response of the upper half is con¬ siderably greater than that for the lower half. Again, while the maximum contraction of the upper half was attained in the course of about 40 seconds, the lower half, as previously stated, exhibited its maximum contraction in the course of about 1 i second. Taking these various facts into con- sideration, the excitability of the upper half may be regarded as at least 80 times less than that of the lower.

1 have devised an independent method for comparison of the excitability of the two halves. It will be shown in Chapter IX that local excitation can be induced in a tissue by the application of a constant current, and that this occurs under a feeble current at the kathode, that is to say, at the point where the current leaves the tissue. Experiment 12. — Two electric connexions were made on the upper and lower halves of the pulvinus. Ey means of a reversing key the upper and the lower halves, were sue- cessively made the kathode. It was thus found :

of the lower half was insufficient to induce excita¬ tion of the upper half, proving the Comparatively feeble excitability of the upper half ; reached when both the upper and lower halves underwent contraction ; the response of the former was a sluggish and relatively feeble up-movement, while that of the latter was a very quick and more energetic down-movement. 1 reproduce the two responses thus obtained, the up-movement ol the upper half being represented by a down-curve, tlie down-movement ot the lower half being, on the other hand, represented by an up-curve (fig 22). The two curves clearly demonstrate the relative excitability and rate of reaction of the two halves of the organ.

The results given prove (1) that the upper half of the pulvinus does not contribute to the fall of the leaf ; (2) that both the upper and lower halves of the organ are con¬ tractile, the excitability of the lower being considerably greater than that of the upper half ; (3) that the con¬ traction of the upper half is practical1 y negligible compared with that of the lower half ; (4) that under medium stimula¬ tion the lower half alone contracts, the upper exhibiting

little or no contrac¬ tion ; (5) that under maximum stimula¬ tion both the upper - and the lower halves contract and that, instead of the upper half helping the fall, it offers a feeble re¬ sistance to the fall of the leaf effected by the predominant con¬ traction of the lower half of the pulvinus. Fig. 22. Curves showing differential excita- fully described ill a hility of upper (lower record) and lower , half (upper record) of pulvinus of Mimosa. subsequent chapter

with the lower half of the pulvinus, it was found that strong This is an additional proof of the antagonistic reactions of the two halves of the organ. The upper half, therefore, does not help in any way in the responsive fall of the leaf. Characteristic Responses of Highly Excitable and of Subtonic Pulvinus In higlily excitable specimens the contractile fall of the leaf is maximum, whether the stimulus is moderate or strong. The response is here on the ‘ all or none ’ principle. But in less excitable or subtonic specimens a stronger stimulus is found to give rise to a larger amplitude of response (hg. 23). W hat can be the reason of thh difference between

Louver iecord in response to stimulus of 1 unit, and upper to the response of a highly excitable and that of a subtonic specimen ? It would appear that in a highly excitable specimen all the cells contract to their utmost, even under minimally effective stimulation ; a greater contraction cannot, there¬ fore, be produced under a stronger stimulation. In a sub- tome specimen, on the contrary, increase in the intensity of stimulus up to the limit results in an increase of amplitude of response. This may possibly indicate that a relatively larger number of cells undergo contraction in a subtonic tissue under a stronger stimulus. This hypothesis is sup¬ ported by the results of the following experiment.

Experiment 14. In attempting to modify the number of cells that undergo contraction, I applied the discovery that the contract ilk y of the cells becomes effectively • abolished on .application of dilute solution of potassium chloride. \\ hen the solution is externally applied on the Fig. 24. Effect of the application of KCl solution on th§ puh inus. Uppermost record, normal response ; the intermediate record shows diminished contraction after absorption of KCl for 3 minutes; lowest record shows abolition of contraction afte*- absorption for 18 minutes (Mimosa).

pulvinus it is gradually absorbed, and the outer layer of cells « is naturally the first to be thrown out of operation ; with longer duration of application the solution penetrates more deeply and renders an increasing number of cells ineffective. The effect of the duration of application of the solution in causing decrease in the total contraction is shown in fig. 24. Ihe upper record is the response of the specimen, taken on a last-moving plate, before c the application of KCl, the amplitude of normal response being 40 mm. After ieeovery, 1 per cent, solution of KCl was applied to the pulvinus. and

the record of response. was taken after an interval of 3 minutes. Even within this short period the amplitude of response was found to have been reduced from the normal 40 mm. to 20 mm., the reduction being nearly half. 4 third record was taken after a further period of 15 minutes. The solution had by this time penetrated throughout the pulvinus. and produced the total abolition of response (fig. 24). The amplitude of response is thus seen to be dependent on the number of cells that undergo contraction.