The Motor Mechanism of Plants

The petiole was held by a clamp so that the more ex¬ citable half of the pulvinus ‘remained in the normal lower position. A short silk string tied to the strip of epidermis projecting from the upper half of the pulvinus, was attached to the short arm of a recording-lever (fig. 68). The response curve was traced on an Oscillating Recorder, the successive dots of which were at intervals of 2 minutes. Method of stimulation. — The terminals of the secondary of an induction-coil were connected by flexible silver tinsel, one electrode to the end of the strip of epidermis E, and the

lower half of pulvinus facing downwards ; E, strip of epidermis tied to string s attached to short arm of recording-lever r. The pulvinus is enclosed in a small chamber. other to the petiole. Induction-shocks of definite intensity and duration could thus be passed through the pulvinus at regular intervals. The plant-chamber. — This was made very light and com¬ pact, the size of the chamber being a cube 5x5x5 c.cm. The sides of the chamber consisted of thin sheets of mica The strip of epidermis E passed through a narrow aperture for attachment to the recorder. Moist tlotting-paper maintained the interior of the chamber in a proper humid condition. A small piece of cloth moistened with highly

dilute glycerine was applied against the cut end of the pulvinus to prevent drying of the tissue. After carrying out all necessary precautions, I tried if stimulation would cause a down-curvature of the pulvinus by the greater contraction of its more excitable lower half But che recording-lever showed no indication of such a movement. The pulvinus had evidently lost its contractility in consequence of the drastic treatment to which it had been subjected, the severe shock of the operation. It then

occurred to me that the lost power of contraction might possibly be restored after a suitable period of rest. The surmise was justified by results of the following experi¬ ments. Experiment 63. Uniform response under stimulation. — After allowing an hour’s rest, the contractility of the isolated pulvinus was found to have become restored. I reproduce a series of responses recorded at intervals of 28 minutes (fig. 69) which show the extraordinarily uniform character of the responses.

The responsive contraction and curvature of the pulvinus was recorded by the recording-lever as an up-curve. The intensity of stimulus was 3 units and the duration of stimula- tion was o - 2 second. The contraction was very rapid and the maximum contraction was observed to be completed, as in an intact plant, in the course of less than 2 seconds. On account of the rapidity of movement of the recording-Iever during the phase of contraction, the record appears as a scratch. The recovery was slow, as indicated by successive dots made at intervals of 2 minutes. It will be noticed that the period of complete recovery here was about 28 minutes, instead of 15 minutes as in intact plants. The following con¬ siderations offer an explanation of this difference. It has been explained how the expansive recovery of the con¬ tracted pulvinus is hastened by the active supplv of sap in the intact plant. In the isolated pulvinus, on the other hand, expansion depends upon the relatively slow absorption of water from the moist cloth applied against the cut end of the pulvinus.

The most important lesson learnt from the experiment is that the lost irritability of the pulvinus due to rough handling or even to the severe shock of wound, is restored after a suitable period of rest. The irritability of the pulvinus is thus practically inde¬ pendent of the plant. The isolated pulvinus, on account of the smallness of its s*ze, offers exceptional advantages for many investigations ; several researches, in fact, have been rendered possible which were impracticable even with the petiole-pulvinus preparation.

1 I he reliability of this method in general investigations on irritability will be understood from the following accounts of experiments on the action of different gases and vapours, and also of different drugs in solution. Gases and vapours can be easily introduced into or removed from the plant- chamber by means of a pair of inlet and outlet pipes. As regards the action of chemical solutions, the liquid drugs had hitherto been applied indirectly at the cut end of the stem, relying on the slow rise of sap to carry it to the pulvinus. In the present case the application is direct and there was

no delay in inducing the characteristic reaction. 'I he solu¬ tion is applied, by means of a pipette, to the strip of epidermis projecting above the chamber. The liquid trickles down and is quickly absorbed by the exposed cut end of the pulvinus. The depressing action of strong application of this gas has already been described [of. fig. 43). Experiment 64. — The record (fig. 70) exhibits the effect nlr r'rmtirmprl Tvnlina tinn nt dilute C Oo ! the result is a

Note preliminary stimulation followed by depression , normal excitability restored after removal of gas (Mimosa). preliminary stimulation followed by depression ; substitu- I tion of fresh air was followed by restoration of normal ^ excitability. Experiment 65. — Two drops of chloroform were cropped 1 into the plant-chamber, and the rising vapour, at first very k dilute, gradually increased in strength, lhe stimulating j effect *of the minute dose is seen in an enhancement of the I response ; continued action of the stronger do^e induced, however, depression (fig. 71) ; when liquid chloroform was *

applied at the cut end of the pulvinus, the response became permanently abolished (cf. fig. 46). Fig. 71. Effect of Chloroform on pulvinar response (Mimosa). With regard to the demonstration of the action of a stimulating reagent, it may be said, in general, that a tissue in a slightly depressed condition exhibits the effect in a very striking manner : for then the responsive contraction of the highly excitable tissue is the maximum possible and cannot therefore be exceeded.

I have carried out a large number of experiments on the action of various drugs in solution, of which a few typical examples are given below. Experiment 66. — After taking three normal responses, very dilute solution of camphor was applied at the cut end of the pulvinus. this is seen to have induced a very great increase in the amplitude of the contractile response (tig. 72). The specific action of camphor was si marked that I was led to investigate its action on the contractile response of

Fig. 72. Stimulating action of Camphor on pulvinar response (Mimosa). % Experiment 67.— The myogram of frog’s sartorms was taken on a fast-moving drum under minimally effective stimulation, which was kept constant for the succeeding experiment. After the first normal response a second was taken after application of dilute camphor solution, fhe amplitude of response is seen to have undergone a marked increase (fig. 73). This identical effect of a specific drug on the contractile tissues of both animal and plant is highly significant.

Experiment 68. — A 4 per cent, solution of caffein caused an enhancement of excitability, but continued appli- Fig. 74. Stimulating action of Caffein on pulvinar response. Continued application caused depression (Mimosa). cation induced depression (lig. 74). It should be remem¬ bered in this connexion that both the amount of the dose and the duration of the application are matters of importance ; for even a stimulating agent in large quantities is apt to exert a depressing effect.

Experiment 69.— A 2 per cent, solution of this well-known cardiac stimulant was found to exert a stimulating action on the contractile response of the pulvinus (fig. 75). Experiment 70. — A small dose of strychnine was found to induce a great enhancement of excitability. 1 his was Fig. 76. Enhancement of amplitude of puivinar response under minute dose of Strychnine (Mimosa). shown by the fact that a sub-minimal stimulus which was ineffective proved to be fully effective after the application

of strychnine. Feeble contraction was, moreover, greatly increased under the action of a minute dose of this drug Experiment 71. — A similar enhancement of contractile response also occurred in frog’s muscle under the action of a minute dose of strychnine (fig. 77). In previous chapters it has been shown how essentially similar are the contractile responses of plant and animal, and how similarly they aie modified by parallel variations of external conditions. J11 both, individually ineffective

Fig. 77. Effect of minute dose of Strychnine in enhancing the contractility of Frog's muscle. stimuli become effective on repetition. They both exhibit ‘ staircase ’ increase of response, and decline under fatigue. Temperature nas a similar effect on both. The presence of some ‘ active substance ' enhances the rapidity of reaction of both. And in both, finally, the response is modified in an identical manner by the specific action of drugs. In these circumstances the pulvinus of Mimosa may justifiably be regarded as its ‘ muscular ' tissue.

The pulvinus of Mimosa loses its power of contraction immediately after isolation of the leaf from the plant. This is due to the paralysing effect of the shock of operation ; the moto-excitability is, however, restored after a suitable period of rest. Under uniform external conditions the responses remain uniform for a considerable length of time. The isolated pulvinus can therefore be employed With success for all investigations on the moto-excitability of the plant. In certain respects it offers greater facilities for investigation.

Th.e specific actions of various drugs are found to be essentially the same in plant and in animal. All life-movements are ultimately due to contraction of the smallest unit of life, the individual cell, an aggregation of which constitutes the living organism. It is the additive effect of the infinitesimal contraction of an almost countless number of active cells in the lower half of the pulvinus of Mimosa that brings about the strikingly impulsive fall of the leaf. By the tedious process of counting the cells in stained sections of the pulvinus, the number of active cells was found to exceed a hundred thousand.

The question arises : Would it be possible to measure the extraordinarily minute contraction of a definite number of these ceils under stimulation, and thus to obtain a clearer idea of the cellular mechanism ? The use of the microscope for the observation of cellular contraction in the interior of an opaque tissue is obviously impossible. E'ren if it were possible to make the pulvinus transparent, the con¬ traction of an individual cell would be beyond the power of the microscope to detect. I nevertheless hoped to be able to observe the total contraction of a row of cells in a micro¬ scopic section of the pulvinus about 2 cells in thickness, but the severe shock of operation was found to have effec¬ tively killed the cells. I tried to overcome the difficulty by benumbing the tissue before making the necessary section ; but attempts to keep the section alive have hitherto proved to be unsuccessful. With greater experience, it may perhaps be possible to overcome the difficulty.

The problem of measurement of the responsive con traction of a row of cells was, however, solved by a new method which will be described in this chapter. The following facts have been established in regard to contraction of the active cells in the pulvinus of Mimosa : mainly of two masses of cortex separated by a thin vascular strand. The active contractile cells can be sharply discriminated from the inactive by means of selective staining (cf. fig. 35).

is relatively unexcitable, its contraction even under moderate stimulation being practically negligible compared with that of the more excitable lower half (cf. Experiment 12). traction may be regarded as practically confined to the lower half of the organ. Let us visualise a vertical row of actively contractile cells placed between two rods of german silver, the upper and shorter being fixed, while the lower and longer rod is movable, functioning as a magnifying primary lever. A short length of the primary lever towards the left is flattened into a spring and thus acts as a flexible hinge H. The two diametric points of contact are at C ('/ ; the spring also serves to keep the lever pressed against the lowermost cell. On stimulation the vertical length of the row of cells becomes contracted and shortened. The upper rod being fixed and immovable, the shortening of the row of cells makes the lower lever move upwards, thus indicating the diametric contraction of the row of cells. The actual shortening is

I return from the theoretical to the practical method of experimentation on a pulvinus. For facility of manipu¬ lation it is more convenient to employ a cut stem bearing a leaf, in place of an entire plant, the cut end being placed in a small vessel of water. The shock-effect of the operation is, as previously stated, minimised by previously benumbing the stem with ice at the place of section. The normal excitability of the specimen is fully restored in the course of about 2 hours.

In the diagrammatic representation of the apparatus a vertical section of the pulvinus is indicated between the fixed upper rod and the movable lever below, the diametric points cl contact being C C/. The contraction of the vertical layer of active cells, as already explained, is indicated by a very slight, and almost imperceptible up- mo vcment of the primary lever, which magnifies 30 times. Fur the 1 magnification was necessary and was produced by linking the end of the first lever to the short arm of a secondary recording-lever R, which magnifies 150 times (fig. 79) : the total magnification is therefore 4500 times. With this arrangement the amplitude of response under strong stimulation sometimes proved to be too great for reproduction ; .the magnification had then to be suitably reduced to either 2000 or 1000 times.

The' over-weight of the longer arm of the recording-lever keeps the contact point of the first lever C always pressed against the pulvinus with a moderate pressure. Diametric contraction of the pulvinus under stimulation causes an up-movement of the first, and a down-movement of the second lever, the record being a down-curve. For con¬ venience of inspection the record is printed upside down, so that the magnified contractile response would appear as an

Vertical section of pulvinus shown in figure of which l is tlie row of more effective contractile cells in lower half of puivinus. Movement of primary lever l, further magnified by recording- up-curvt, while the down-curve exhibits the expansive movement of slow recovery. It is convenient to give the instrument a shorter name than that of Recorder of Cellular Contraction. It records not only the contraction but also the concomitant variation of pressure. In normal condition the cells charged with sap are tense and their aggregate pressure may be described as the sap-pressure. Stimulation causes contraction and expulsion of sap frcm the cells with resulting diminution of pressure. During recovery there is an absorption of sap. with expansion and a gradual restoration of the original pressure. Tire instrument therefore records not merely

contraction or expansion, but also the corresponding diminu¬ tion or increase of pressure. I will therefore designate the apparatus as the Cell-Sphygmograpli, of which there are three types varying in sensitiveness. For the present purpose a magnification of 1000 to 5000 times is quite sufficient. The apparatus and general experimental arrangement is shown in tig. 80, reproduced from a photograph. The Leaf of Mimosa held rigid b}' clamp c. Pulvinus placed between a h.ied rod and a movable lever l, attached *0 short arm of recording-lever r.

record is taken on an Oscillating Recorder, the successive dots being at intervals of 5 seconds. The plant, after being suitably mounted on the apparatus, is allowed a period oi rest of i to 2 hours, after which the normal excitability is fully restored. An induction shock sent through the pulvinus would cause the fall of the leaf, a down- movement of ? the first lever and a very large up-movement of the recording- lever. But the fall of the leaf indicates merely the force of contraction and not the contraction itself. For the measure- ^ ment of the contraction of the vertical row of cells, the two opposite contacts of the pulvinus with the two sphygmo- graphic rods must always be maintained unbroken. The j fall of the leaf would, however, break the contact with the upper rod to prevent this the leaf is restrained from falling, by being held rigid by a clamp. This is found in no way to interfere with the fundamental contraction of the cells under stimulation. The fall of the leaf is a remote ^ consequence of contraction ; hence restraint imposed on the leaf does not interfere with the primary reaction. 1 his is fully confirmed by the results of experiments to be J presently described.

I draw attention to the characteristic difference in the sign of the recorded response of the downward pull exerted by the falling leaf, and that of the iundamental cellular con¬ traction. In the first case the unrestrained leaf would cause ; b'r its fall a down-movement of the primary lever ; in tae second case the diametric contraction of active cells of the ’ pulvinus of the restrained leaf causes an up-movement of v the lever. The maximum cellular contraction occurred in the course | of 2 seconds, which is also the period for the maximum fall j|j of the free leaf. The period of expansive recovery of the \ contracted cells was about 15 minutes or so. The sphygmo- I graphic record of cellular contraction and subsequent . l expansion {see fig. 81) is thus essentially similar to the I phytographic record of the responsive fall and subsequent |

erection of the leaf. There are thus two independent ways of determining the fundamental excitatory reaction, namely by the Sphygmograph and by the Phytograph. 1 he cause of similarity between the two responses is found in the fact that in the one the contraction is recorded directly, and in the other indirectly by the fall of the leaf, which is a remote consequence of the cellular contraction. The results of experiment and the quantitative values given below are the answers to the above questions.

I applied successfully feeble, moderate, and strong stimulation ; in every case a marked contractile response was recorded, the amplitude of response increasing with increased intensity of stimulation. The record of response under stimulus of feeble intensity taken under a magnifica¬ tion of 4500 times was too large to be contained in a page. I therefore reproduce (fig. 81) a typical record obtained under the low magnification of 2000 times, the stimulus being of moderate intensity.