The Motor Mechanism of Plants

Thermal stimulation thus induces effects which are precisely similar to those obtained under other modes of stimulation, such as electric or chemical. The law of peristaltic propagation is now established on a wide and hrm basis, including as it does the reactions of both animal and plant. I he experiments described in this chapter profoundly modify the conceptions h. herto held in regard to the move¬ ment of the sap in the stem. This has been regarded as fundamentally dependent upon tne absorption of water at 0 xe end and transpiration of it at the other. The persistence ol the movement of sap in the absence of either accession or locs of water shows the groundlessness of tnis hypothesis. It proves that the propulsive 'activity is by no means con¬ fined to the terminal organs but exists in the active, cortex throughout the length of the stem. Propulsion may be in

a dormant state, or the sap may be at standstill in the con¬ dition of balance. Diverse modes of stimulation, electric, chemical, or thermal, awaken the dormant tissue or activate it above the normal. A movement of the sap is thus initiated in a quiescent tissue, and the direction of its propagation can be expressed by the general statement that the sap flows Fig. 242. Effects of alternate thermal stimulation and depression. Stimulation by heat sT applied simultaneously below and above

contact, with resulting increase of pressure at contact (up- curve). Simultaneous depression by cold d above and below from the stimulated to the indiff rent or depressed region. The normal ascent of sap is only an instance of the effect of differential stimulation, the direction of flow being from the root of tne plant, activated by hydrostatic distension due to absorption of water, to the apex of the stem depressed by partial drougm caused by the transpiration from the leaves. The sap-flow becomes reversed if the plant be irrigated at tiie upper instead of the lower end.

The conception that the sap moves only in an upward direction has stood in the way 01 the recognition of the im¬ portant part played by the circulation of sap in establishing hydraulic means of communication between distant organs. 'The plant is a multicellular organism, and hence necessity arises for intercommunication and interaction between more or less distant organs. This is accomplished in two different ways : by transfer of -matter and by trans¬ mission of motion. The first is exemplified by the hydraulic convection of liquids carrying chemical substances in solu¬ tion, such as occurs in the circulation of sap ; the second by the conduction of excitatory change along nerves.’ 1‘

The intercommunication between distant organs by transference of liquid is thus brought about by differential stimulation. The cellular mechanism for directing the flow of sap is highly sensitive, and is automatically adjusted for subserving the well-being of the plant. A local depression or stimulation starts the alert machinery into action, making the sap rush towards the depressed, or away from the over- stimulated, region. The peristaltic or anti peristaltic waves of cellular contraction, which cause the movement of sap upwards or downwards, are in constant operation

In the animal, chemical substances, termed hormones, produced in certain organs, , are carried in the circulating blood to the various o gans or parts of the body, in which their regulatory action becomes manifested. It is undoubted that hormones are likewise produced in plants, and that these chemical substances, } oduced in cue organ, are conveyed to distant organs in the sap, distributed by cellular activity. The distant members of the plant-body are thus linked together in an organised unity.

Investigations on the propulsion of sap were under¬ taken with bare sealed steins, che factors of absorption and transpiration being completely eliminated. Propulsion, in these circumstances, was in a state of standstill. Stimulation initiated the movement of sap, the direction of which was from the more to che less activated region, i he sap could be made to flow not onl^ upwards, but down¬ wards as well. J'he responsive movement of sap occurred under different modes of stimulation, mechanical, electric, chemical, and thermal Response was abolished after the poisoning of the tissue, proving its physiological character.

The results obtained with the sealed stem with its ac¬ tivity at standstill are exactly parallel to those obtained with the quiescent stomach. In both, peristaltic and antiperi- staltic waves ..re generated by stimulation, the direction of propagation being always from the more to the less activated region. The peristaltic wave in both is about 4 times more intense than the antiperistaltic wave. The law of the propagation of the peristaltic wave is established on a wide and fnm basis, including reactions of both animal and plant.

The plant is constituted an organised unity by inter¬ communication and interaction between distant organs. This is accomplished ouickly by transmission of motion as in the nervous impulse, and slowly :>y the transfer of liquid carrying chemical substances in so.' ition, such as occurs in the circulation of sap. Unicellular >rganisms exhibit contractility, rhythmicity, and conductivity, which are different aspects of a funda-, mental protoplasmic! reaction. These characteristics of the unicellular organism persist when cell-complexes and tissue- systems are formed in multicellular organisms. In the associated physiological division of labour, each particular tissue becomes specialised to discharge a given function in a more efficient manner.

The exhibition of contractility and irritability, obvious in the case of the locomotory animal, is so much less so in the sessile plant that, beyond the exceptional sensitivity .and movements of sensitive plants, it has been tacitly assumed that plants in general are neither excitable nor capable of movement. This is the matter specially dealt with in several chapters of this book. Beginning with the sensitives, overwhelming evidence has been adduced that their sensitiveness to stimulation, the conduction of excitation, and the consequent responsive movement, are accompanied by the physiological signs, notably the negative electromotive variation, which have

long been known to accompany these phenomena in the animal body. L>ut the mat ter nas been earned further ] no generalisation can be based on sensitive?, alone. Observation jnust be extended to include the generality of plants which make no obvious motile response to stimulation from without. These plants cannot execute responsive movements on account of their anatomical structure. It has, however, been shown bv conclusive experiments that these apparently insensitive and immotile ‘ ordinary ’ plants respond to stimulation by the infallible sign of negative electromotive variation.

1 hough ordinary plants do not exhibit such conspicuous movements as do the sensitives, they do nevertheless respond to stimulation by movement ; but a movement so slight that it can only be detected by specia1 sensitive apparatus • vhich at the same time magnifies the movement sufficiently to make record of it possible. the apparent absence of mechanical response in ordinary plants is principally due to the inflexibility of the wood with which the living tissue is connected. Contractile movement has, however, been recorded in the diametric contraction of the stem under electric stimulation, due to the moto- excitability of the cortical tissue. For this demonstration I hrst determined the diametric contraction of the pulvinus of Mimosa ; afterwards, employing the same method, I measured the contraction of the cortex of its stem. Finally, the diametric contraction of the stem in ordinary plants was determined.

Measurement of the diametric contraction of an active cell of the pulvinus of Mimosa. — By means of the Cell-Sphygmo- graph the amount of the contraction of a single cell of the pulvinus has been determined. Tht leaf was kept restrained from movement, a condition approx, mating to the immotiJi.ty of ordinary plants. The average diameter of an active cell is 0*024 ram- and the diametric contraction under feeble electric stimulation is 0*70 /*, the micron p being a millionth part of a metre. The amount of contraction under feeble stimulation is 3 *2 per cent., increased to 13 *3 per cent, under strong stimulation (p. 137}-.

Diametric contraction oj cortex in the stem of Mimosa. — The cortical cells in tne stem also underpo contraction. Each cell is 0-014 mm. in diameter, and the diametric con¬ traction under feeble stimulation is o *29 /a, the amount being 2 per cent. (p. 140), and of the same order as that of the active cell in the puivinus. Diametric contraction of stem of ordinary plants. — Similar results were obtained with the cortex of ordinary plants, which exhibit marked contraction under stimulation. It was an unexpected revelation to find that the sensitivity of even the ordinary conex is often exceptionally high, response by contraction taking place under an. intensity of electric stimulation which is below the threshold of human perception (p. 145). The plant thus registers impressions of extra¬ ordinarily feeble stimulations.

The responsive movements of ordinary plants differ from those of sensitive plants in degree only, just as the movements of the sensitive plants differ from those of animals in degree but not in kind. it has been possible to trace a continuity of response in radial and in pronouncedly anisotropic organs such as the puivinus of Mjmosa, in which excitation is followed by the impulsive and very striking movement of the fall of the leaf. Radial ■ organ. — A radial organ such as a stern subjected to diffuse stimulation does not exhibit any markedly visible movement as does the leaf of Mimosa. The difference is apparent and not real. In a radial organ the excitability is equal on ail sxdes ; hence the response cannot be one of bending to one side or the other. Nevertheless, contraction is induced, and is manifested as a shortening of the length of the organ (p. 151).

1 ropic curvature of radial organs. — A bending movement is,, however, produced in a radial organ under unilateral stimulation, the stimulated side becoming contracted and concave (p. 152;. J his fact offers a satisfactory explanation of the tropic curvature induced under diverse inodes of unilateral stimulation. § Transient anisotropy. — Between the radial and the pronouncedly anisotropic is the intermediate type of organ, which exhibits transient anisotropy. Under geotropic action, a horizontally laid radial organ becomes curved and aniso¬ tropic, the curvature and induced anisotropy being reversed when it is rotated through 1S00 round the horizontal axis.

I he convex .side of the organ is found to be the more excitable ; hence diffuse stimulation causes a flattening of the curvature. The changing physiological differentiation in an originally radial organ is manifested by a definite sequence of trans¬ formation from an up-response througn intermediate zero to a down-response (p. 155). the tissue which is active in all the forms of movement described is the cortex ; it is continuous, both structurally and functionally, throughout the body of the plant, not only in the sensitive but alsa in the ordinary plant. Drought or withdrawal of Water under plasmolysis induces similar shrinkage or contraction in the stem and in the leaf-joint of ordinary plants as it does in the pulvinus of Mimosa. Supply of water, on the other hand, induces expansive response in all these organs. In contrast with the slow contraction by removal of water under drought or plas¬ molysis, is the sudden contraction induced by external stimulation (p. 160).

Having shown the continuity of contractile reaction in the cortical tissue, the detailed study of the phenomenon of contraction is facilitated by the 1 ‘suits of the experiments carried out with Mimosa p'tdica. The mechanical response of Mimosa was obtained not cnly with the intact plant, but also with the petiole-pul vinus preparation, which functions like the nerve-ana-muscle of the animal (p. 79). The method of experimentation was further simplified by the discovery that the irritability persisted in the isolated pulvinus as it does in the isolated muscle of the frog (p. 119).

The fact that the fall of the leaf of Mimosa under stimula¬ tion is not due to the passive yielding of the flaccid pulvinus, but to active contraction, is demonstrated by the vigorous contractile response exhibited by the plant held in an inverted position, the leaf being now lifted against the force of gravity The rapidity of contraction. — The presence of an f active ' substance apparently confers on the motor organ its power of rapid contraction. The highly contractile cells of the pulvinus are demarcated from others by the differential action of stains. The distribution of the stainable active substance affords a measure of the rapidity of contraction of the motor organ. In the semi-active Neptunia, the active cells are more sparselv distributed than in Mimosa. The relatively inactive pulvinus of Phaseolus contains no trace of the active substance (p. 64).

Modifying effect of ionic- condition on response. — Stimula¬ tion does not always cause a depletion or run-duwn of energy (symbolised as the D-reaction) ; under certain, conditions it causes an accession or storage of energy (A-ieaction), as when the leaf of Mimosa in a subtonic condition exhibits an erectile response under stimulation. The relative intensity of the two reactions *s modified in a definite manner according to the tonic condition of the tissue. When the tonic levei is above par , the D-reaction is predominant (D> A) ; but when the tonic level is below par , the accession of energy is the more pronou ’iced ( A > D) . Stimulation effects both internal

and external work, the relative values of which undergo pro¬ gressive change. The ' trigger action ' is only a particular event in the tonic cycle (pp. 51 57). Diurnal variation of yioto-excitability . — The excitability of Mimosa does not remain constant, but undergoes charac1 ■ t eristic vaiiation at different hours. In the spring, the excitability attains its maxinnyn* value at midday and remains constant for several hours. There is then a con¬ tinuous fall of excitability till the minimum is reached at about 8 in the morning, when the plant is practically insensi¬ tive. The excitability is then gradually restored in a staii- case manner, till the maximum is reached once more at 12 noon. The diurnal variation is primarily due to the hourly variation of temperature, the effect being modified in a less degree by variation of light (p. 97).

Common characteristics of contractile response in plant and animal . — Stimulation individually ineffective becomes effec¬ tive after repetition (p. 46}. In a slightly subtonic condition the tissue exhibits a staircase response under successive equal stimulations (p. 52). A fatigue-relaxation occurs under continuous stimulation (p. 48). There is an inhibition of response when the contractile tissue, pulvinus or muscle, becomes water-logged by excessive absorption of water (p. 70). The, amplitude of response is determined by the number of cells that undergo contraction (p. 40). Con¬ tractility in both cases is temporarily abolished at a thermo¬ metric minimum ; the amplitude of response is maximum at an optimum temperature, and there is a decline of excitability above the optimum.

Effect of drugs. — The action of anaesthetics and drugs is, in general, remarkably similar in plant ana animal tissues. Vapour of ether exerts a moderate narcotic action, inducing a temporary aboliticn of excitability which is restored on readmission of fresh air (p. 75). Chloroform is a stronger narcotic, excessive application of which causes death (p. 76). The action of BaCl2 on the pulvinu ;, like that on the animal muscle, is a prolongation of the period of recovery and a characteristic double-coc traction. The response becomes normal after a short period of tetanisation (p. 87). Acid and alkali are antagonistic in their reactions (p. 88). Dilute solution of camphor enhances the excitability of the pulvinus, as it does that of the muscle (p. 124). "The effect of strych¬ nine is similar in plant and animal, a minute dose inducing a great, increase of excitability (p. 127), while a larger close

abolishes it. A minute dose of a toxic agent often exerts a stimulating action. Polar action of constant current in excitation. — It has been supposed that the laws of polar action on non-fibriliated protoplasm are different from those on highly differentiated animal muscle. My experiments on the polar action of electric current prove, on the contrary that the reactions of the undifferentiated protoplasm of the plant-body are identical with those of the animal muscle. The Laws of Polar Excitation for Plants under feeble and moderate current are :

not at breaK. The anode excites neithef at make nor at break. excites at make and not at break. The anode excites at bieak and not at make (p. iro). Two other independent methods have been devised to detect and record e ccitatory reaction in plants, by electro¬ motive variation ana by change of electric resistance. Electromotive response. — All plants and their different organs respond to stimulation by an electric response of galvanometric negativity. The electric response of a motile organ persists even when it is restrained from executing any movement (p. 167). The amplitude of response, within limits, increases with the intensity of stimulation (p. 173). Uniform stimuli, applied at suitable intervals, induce uniform responses (p. 174). Fatigue is induced by shortening the period of rest, protoplasmic recovery being then incomplete (P‘ I75)- There is an arrest of response at a minimum temperature, the amplitude of response being maximum at an optimum temperature": There is a fatal tempera¬ ture at which the electric response undergoes permanent

abolition (p. 175). Strong vapour of chloroform induces rapid depression and abolition of response (p. 176). riiotic stimulation of the cell-protoplasm of a green leaf induces the catabolic id-reaction indicated by a negative ' iec trie variation, as under other modes of stimulation ; light, however, acts at the same time upon the chloroplasts, inducing anabolism in the form of photosynthesis, indicated °y ^positive electric variation. , The negative variation, generally speaking, masks the positive, which can be detected a^ an after-effect upon the withdrawal of light, as also by the phenomenon of ' overshooting * (p. 182).. The electric response of actively assimilating Hydrilla is positive, indicating the predominance of anabolism (p. 1S4).

Response by variation of electric resistance.— Mechanical nr electric stimulation induces a diminution of the electric resistance of the tissue (p. 188). The physiological character of the response's demonstrated by the action of chloroform, which produces a rapid diminution of tire amplitude of response (p. 109). The effect of the stimulus of light is demonstrated by the very sensitive Quadrant Method (p. 194). Increasing intensity of light is found to induce a correspond¬ ing diminution oi resistance (p. 106). Changes of external conditions affect alike the different modes of response, bv mechanical movement, by electromotive variation, and by change of electric resistance. As further examples of this may be cited the effect of feeble stimulation on a subtonic tissue, and the effect of strong stimulation on an excitable tissue. Feeble stimulation induces positive or erectile response of the leaf of Mimosa Q 51), positive electro¬ motive response of the pulvinus of the same plont. (p. 166), and a positive response by increase of electric resistance of various tissues (p. 192) . Strong stimulation induces multiple response in all cases (pp. 193, 238, 239). Response by mechanical movement, by electromotive change, and by variation resistance are,* therefore, different expressions of the fundamental re action mduced by external stimulation.

An intense excitation is shown to be induced in plant- tissues at die critical moment of death. Mechanical spasm of death. — When various plants, sensitive or ordinary, are subjected to a gradual rise of temperature, a sudden excitatory contractile spasm occurs at the critical temperature of about 6o° C. (p. 202). Electric spasm at fatal temperature. — -An abrupt electro¬ motive variation of galvanometric negativity also occurs at the critical point of 6o° C. A sudden diminution of electric resistance is also found to take place at the critical death temperature (p. 210).

Death-record by apparent variation of weight. — Various organs of plants exhibit a sudden increase of apparent weight at the critical temperature, due to abrupt volumetric ’ contraction, which is the spasm of death (p. 227). Transmission of death- excitation. — The excitation at death is conducted to a distance, causing excitatory fall of the indicating leaves of sensitive plants (p. 212). This is equally true of excitation due to fatal temperature or to the action of poisonous solutions. The duration of applica¬ tion of poison for producing death-excitation is found to depend on the strength of the dose and- on the virulence of the poison (p. 215).

Automatic movements, like those of animals, are mani¬ fested by plants, notrbly by the leaflets of Desmodnim gyrans. Pas this automatic movement been suddenly evolved, or is there an intermediate link between it and the ordinary responsive movement ? It has been shown that this intermediate link is to be found in the multiple 1 esponse under strong or long-continued stimulation, ex¬ hibited by Biop by turn and other plants (pp. 237-242). It has also been shown that there is no strict line of demarca¬ tion between the phenomena of multiple response and of