The Motor Mechanism of Plants

Experiment 80. — A detached leaf of Sunflower, with the cut end of the petiole wrapped in moist cloth, was held on a stand in a horizontal position. The leaf was attached to a recording-lever, and the response to an electric shock sent through the length of the petiole was recorded in the usual manner. The petiole responded by a down-movement which, though less pronounced than that of a pulvinus. was very definite. This demonstration can be made very easily with the Optical Lever. The comparatively feeble movement of petiolar response is due to the relatively small mass of the cortex as compared with that of the woody tissue which otters resistance to the movement. Since the petiole responds like the pulvinus, it may be regarded as an elon¬ gated pulvinoid.

In all anisotropic organs diffuse stimulation induces a marked responsive down-movement which may be described as one-sided or dorsi-ventral, whereas radial organs exhibit no such movement. Is the radial organ, then, non-con- tractile and insensitive in spite of the fact already de¬ monstrated that cortical tissues undergo contraction on stimulation ? I will show that, in spite of appearances to the contrary, a radial organ such as a young stem does undergo contraction which can be demonstrated (i) under diffuse and (2) under unilateral stimulation.

It should be remembered that a radial organ is almost equally excitable all round, so that the contractile move¬ ment of one side is counteracted by that of the diametri¬ cally opposite side. Hence diffuse stimulation does not cause a response of the stem by a lateral movement or bending to one side or the other. Nevertheless a responsive move¬ ment can be demonstrated of the nature of a longitudinal contraction or shortening of the length of the organ.

Experiment 81. Longitudinal contraction of radial organ under diffuse stimulation . — This can be clearly demonstrated under moderately high magnification of about a thousand times. As an illustrative example I will describe the response of the flower-stalk of Crinum. The specimen was growing at a slow rate, as shown in the record, where the growth-elongation is represented by the down-curve. An electric shock of moderate intensity caused the growth- elongation to be suddenly converted into longitudinal con-

traction, shown by the up-curve. The maximum contraction was attained in the course of 4 minutes ; on the cessation of stimulus the organ ex¬ hibited recovery, the original length being regained n minutes after the reception of the shock (fig. 86). Re¬ peated s t i m u 1 a t i o n s gave successive responses which were uniform. The response of a growing organ is thus essentially similar to .that of a pulvinated organ. When the stimulus is feeble the incipient contraction is shown

rate of growth. Even when the growth has come to a state of standstill the tissue still exhibits contraction under stimulus. Contractility only disappears when the tissue is very old. Fig. 86. Longitudinal contrac tion of radial growing organ under electric shock (Crinum). Vertical lines below indicate in¬ tervals of i minute. (Magnifi¬ cation iooo times.) Elongation shown by down curve, con n ac¬ tion by up-curve. The pulvinus of Mimosa responds by bending in one direction or the other when the upper or the lowrer half is subjected to local stimulation. The responsive con¬ traction is manifested by the induced concavity of the stimulated side. Does an ordinary stem exhibit similar response by bending, the stimulated side becoming con¬ cave ?

The radial stem cannot, for reasons already given, respond to diffuse stimulation by bending to one side. In order to obtain bending or curvature, it is essential that only one side of the organ should be subjected to local stimulation. Experiment 8 2. Bending of the steni under unilateral stimulation. — Local unilateral stimulation was effected by means of a constant current, as already described. By means of a pin thrust into the cortex, a minimally effective electric current was applied for a short time on the right Hank of the stem of a young Bean-plant, which was made the kathode, the anode being at a distant indifferent point on the leaf (fig. 87). Local stimulation of the right flank of the organ

Fig. 87. Diagram showing connexions for studying the unilateral k, key ; r, right, flank of the stem which becomes concave on local stimulation. Parallel effect induced on the left flank. g, smoked - glass plate (Phaseolus). induced at kathode-make a contraction and concavity of that side, as evidenced by the response inscribed by the recording- lever attached to the upper end or the stem. The latent period was 20 seconds ; after the attainment of maximum bending, there was recovery on the cessation of stimulation (fig. 88). When the stimulus was transferred to the left flank of the stem the movement of response became reversed by the induced contraction and concavity of that side. The responses of the right and left flanks were

essentially similar, showing that the contractility of the cortex on opposite sides of the stein is practical iy the same. This demonstration is of importance in the explanation of Fig. 88. Response of the stem of Phaseolus to unilateral stimulation by polar action of electric current. Cessation of stimulation, at arrow within circle, is followed by tropic curvature in plants under various modes of unilateral stimulation. Other plants, such as Balsam, Helianthus, Hibiscus, and Vinca, gave similar results.

An intermediate instance to fill up the gap between radial and pronouncedly anisotropic organs, is the transient anisotropy which can be induced in a radial organ. When a radial stem is held horizontal so as to be subjected to the stimulus of gravity, there is at first, no immediate induction of physiological differentiation, as indicated by the absence of unilateral response to diffuse stimulation. But after a while the stem exhibits a geotropic curvature, the upper side becoming contracted and concave, while the lower side becomes expanded and convex. This is attended by induced physiological anisotropy, as demonstrated by the following experiments.

Experiment 83. Response of geotropically curved organ. — When the upwardly curved stem, still held in its original position, is subjected to diffuse electric stimulation, the re¬ sponse is a down-movement (fig. 89, a). The organ, originally isotropic, having become anisotropic, the expanded and convex lower side has become the more excitable. The upper side, being already contracted, is less capable of exhibiting any further contraction. This induction of physiological anisotropy is, however, temporary, unlike the permanent anisotropy of dorsi-ventral organs. Che induced physiological anisotropy can be made

Fig. 89. Diagrammatic representation of responsive movements of geotropically curved organs under diffuse electric stimulation. c, geotropically curved stem held inverted. Response by up- movement shown in dotted outline, b. no response after disappearance of anisotropy ; a, response by down-movement. (Illustrations to be followed from right to left.) to disappear, or even to undergo reversal, by holding the curved organ in an inverted position as in fig. 89, c, the convex and more excitable side being uppermost.

Experiment 84. — The stem then exhibited the following changes under geotropic action : it became straight like b, and, after a further interval, the curvature became reversed upwards as in a (fig. 89). Experiment 85. Response of curved organ held in inverted position. — Under diffuse electric stimulation the greater con¬ traction of the more excitable convex upper side gave rise to an up-response (fig. 89, c) ; stronger stimulation gave rise to a response of greater amplitude. The maximum contrac¬ tion was attained in the course of about 2 minutes, and on

the cessation of stimulus the recovery became completed in about 15 minutes (fig. 90, c). Experiment 86. Response on disappearance of aniso¬ tropy. — When the organ became straight (see fig. 89.. b), dif¬ fuse electric stimulation induced no responsive movement either up or down (fig. 90, b). The anisotropy previously induced had thus disappeared at this particular phase. Experiment 87. Response on reversal of anisotropy. — After a furt her interval, as already stated, the original con¬ vex upper side became concave and the lower side convex

Fig. -90. Response of geotropicaily curved organ under diffuse electric stimulation, at three phases of geotropic curvature. c, response by an up-movement in an up-curved organ held inverted. The two records are for feeble and moderate stimulation, b, no response after disappearance of aniso¬ tropy. a, response of up-curved organ by down-movement. as in fig. 89, a. Diffuse stimulation now caused a down- response by the greater contraction of the lower convex side of the organ (fig. 90, a). The changing physiological differentiation in an originally radial organ is thus manifested by a definite sequence of transformation from an up-response through intermediated zero to a down-response.

Returning to the response of the cortical cells in the pulvinus or in the stem under stimulation (c/. Experiments 75. 76), it has been shown that this can be detected and recorded by the Sphvgmograph or by the Phytograph On stimulation there is a sudden expulsion of sap from the contracted cells ; on the cessation of stimulation, recovery takes place by absorption of sap and re-expansion of the contracted cells. These characteristic responses, indicative of contraction or expansion due to loss or supply of sap, occur in all the different organs ; in the pulvinus, in the leaf-joint, and in the stem. There are various ways in which the withdrawal or supply of water can be effected, and the study cf the charac¬ teristic responses of different organs gives a clearer and more comprehensive insight into the cellular mechanism. In place of the sudden removal of sap from the cortical cells on stimulation, a slower withdrawal can be effected by drought or by plasmolysis. The increasing shrinkage or contraction due to loss of water from the cortical cells can be easily demonstrated by the phytographic record of the responsive fall of the leaf of Mimosa or of any ordinary plant. Supply of water by irrigation gives rise, on the other hand, to expansion of the cortical ceils, as shown by the conse¬ quent erectile response of the leaf.

Experiment 88— A young Mimosa plant, carefully trans¬ planted with the root embedded in soil, was placed in a linen bag. The specimen was securely held by a clamp and one of the leaves attached to the recorder. The plant was in a state of drought ; a vessel full of water was now raised from below at the moment marked by vertical arrow so that the linen bag containing the roots was in water. This caused an erectile movement of the leaf within 10 seconds of the application of water (fig. 91) ; this must have been due to the ascent of sap, which entering the pulvinus caused an expansion and the up-movement of the leaf. In order to ascertain the effect of withdrawal of water, a quick change

was made at the horizontal arrow by substituting a 5 per cent solution of KNCh for the water of the vessel in which the roots were immersed. The effect of plasmolysis was made evident, not only by the arrest of the erectile movement, Fig. 91. Response of the Mimosa pulvinus to irrigaticn and to Increased turgor by application of water at point marked with vertical arrow induced erectile movement. Diminution of turgor by application of KN03 solution at the point marked with the horizontal arrow, brought about the fall of the leaf. Successive dots at intervals of 10 seconds. (The down-curve represents up-movement and vice versa.)

but by a revetted movement of fall of the leaf, which occurred in the course of 40 seconds. In order to demonstrate the universality of the phenomenon I repeated the experiment with an ordinary plant (Impatiens). Experiment 89. — ! he cut end of a shoot of Impatiens was placed in cold water, the leaf soon attaining its normal outspread position. The record of this state of turgor- balance is seen in the horizontal curve (fig. 92). Application of warm water W at the cut end of the stem caused an

erectile response of the leaf ; this must have been due to the enhancement of the rate of ascent of sap caused by the application of warm water, with the result that excess of water was forced into the leaf-joint. It may be stated here, in anticipation of what will be described in greater detail in a Fig. 92. Response of leaf-joint of Impatiens to alternate supply, withdrawal, and renewed supply of water. Warm water applied at w induced erectile response, which raised leaf above normal position ; piasmolysis by KNOa caused a down- response, and reapplication of warm water at arrow within circle induced once more the erection of the leaf. (In this figure, the up-movement is exhibited by an up-curve, t lie fail of the leaf by a down-curve.)

subsequent chapter, that the rate of ascent of sap is modified by physiological agents, warmth causing an enhancement, while cold causes a depression of the rate. While the leaf was performing the erectile movement under the action of warm water, KN03 solution was sub¬ stituted at the cut end of the stem. Piasmolysis now caused a rapid down-movement, indicative of contraction, fn order to demonstrate the unfailing efficiency of the cellular mechanism, fresh warm water was substituted for the plasmolytic solution at the horizontal arrow. The ascent of sap became once more renewed and the previous response by fall was converted into one of erection, the leaf recovering its original position.

It has been explained that the fundamental contraction or expansion of the cortical cells can be recorded by two Fig. 93. Response of sRm, of leaf-joint, and of sensitive pulvinus of Mimosa to drought, irrigation, and plasmolytic with¬ drawal of water. Record of : a, stem of Antirrhinum (by Cell-Sphygmograph) ; b, leaf-joint of Impatiens (by Phytograph) ; c, pulvinus of Mimosa (by Phytograph). The initial up-curve represents contraction under drought ; the down-curve, expansion under irrigation ; and the second up-curve, contraction under plasmolysis.

different methods : by the Phytograph or by the Sphyg- mograph. 1 reproduce a series of records obtained with the three typical organs, the stem, the leaf -joint, and the pul¬ vinus, under similar variations. The reeord of the cellular contraction or expansion of the stem was obtained with the Cell-Sphygmograph ; the Phytograph recorded the move¬ ment of the leaf of the ordinary plant and of that of Mimosa. Taking the three records in order, the diametric contrac¬ tion of the cortex of the stem under drought is shown by the up-curve at the beginning oi the record (fig. 93, a). I nder irrigation at the vertical line, the contraction is reversed into expansion exhibited by the down-curve, the applica¬ tion of the plasmoiytic solution of KNOs at the horizontal arrow reversed the response once more, this time from expansion to contraction (fig. 93, a). The responses of the leaf-joint of the ordinary plant (fig. 93, b) and of thepulvinus of the sensitive plant Mimosa (fig. 93, c) under drought, irrigation, and plasmolysis, are precisely similar to those oi the stem.

The withdrawal of water and contraction under either drought or plasmolysis is a relatively slow process ; a Fig. 94. Responses (a) of stem, (b) of leaf-joint, and (r) ot puhinus of Mimosa to drought, irrigation, and stimulation. In all cases the record indicates the four successive phases : (1) np- curve, the slow contraction under drought ; (2) down-curve, the expansion after irrigation; (3) up-curve, the sudden contrac¬ tion after stimulation at s; (4) down-cui ve, the expansive recovery.

similar, though a far more rapid reaction, as previously stated, is produced by the action of a stimulus. The response to stimulation is, moreover, similar in the cortical tissues of the stem, in the leaf-joint, and in the sensitive Experiment 90. — In the series of records given in fig, 94, drought caused diametric contraction of the stem and con¬ tractile fall of the leaf of an ordinary plant and of Mimosa, shown by the up-curve, irrigation at the vertical ime caused expansion of the stem and erection of the leaf, recorded as a down-curve. Stimulus applied at S produced in all cases a contractile response (up-curve) which was practically instantaneous and which was far more intense than the contraction induced under drought. Owing to the excessively rapid rate of contraction of the pulvinus of Mimosa the dot-record is prolonged into a dash during contraction. Recovery occurred in all cases, the expansive movement, of recovery being shown by the down-curve.

The sensitiveness of the cortex of ordinary plants is exceptionally high, response by contraction occurring under an intensity of stimulation which is below the threshold of human perception. Diametric contraction under stimulation is exhibited not only by herbaceous but also by woody stems. In the condition of subtonicity the response is abnormal positive, gradually converted into normal negative under continuous stimulation. The modifying influence of tonicity is similar in ‘ sensitive ’ and in ordinary plants.

Contractility disappears after death caused by intense electric stimulation. The cortical tissue is continuous in the stem, in the pulvinaied or non-pulvinated leaf-joint, and in the petiole. The movements of these organs under changing conditions are due to responsive expansion or contraction of the cortical cells. The response of dorsi- ventral organs to stimulation is a down-movement due to the greater contraction of the lower half of the organ. The leaf- joint of ordinary plants,

as also the petiole, responds like the pulvinus of Mimosa, the difference being one of degree and not of kind. The differential excitability of the two halves of a dorsi- ventral organ has been determined by their response to local stimulation due to the polar action of a constant current. The sensitivity and contractility of the upper half are practically negligible compared with those of the lower half. Radial organs, such as young stems, are also sensitive and react to diffuse stimulation by a shortening of their length.

A radial organ subjected to the stimulus of gravity gradually becomes, curved. The organ originally isotropic is now anisotropic, the convex side being the more excitable. Diffuse stimulation causes response by the contraction of the more excitable side of the curved organ. The pulvinus of Mimosa responds to local stimulation by bending, the stimulated side exhibiting a concave curva¬ ture ; stimulation of the upper half of the pulvinus induces an up-movement, that of the lower half, a down-movement.

A young radial stem exhibits similar contraction and bending on unilateral application of stimulus, the stimulated side becoming concave. The cellular mechanism functions similarly in every part of the cortical system, not only in sensitive but also in ordinary plants. Withdrawal of water under drought or by plasinolysis induces similar shrinkage or contraction in the stem and in the leaf- joint of ordinary plants as in the puivinus of the sensitive Mimosa. Supply of water, on the other hand, induces expansive response in all these organs.