The Nervous Mechanism of Plants

A Physico-Physiological Study. With 406 Illustrations and Classified List of 321 new Experiments. 8vo, i6s. net. By Patrick Gedoes, late i^ofessor of Botany, University College, Dundee, St. Andrews University, and Professor of Sociology and Civics, University of Bombay. With Portraits and Illustrations. Svo, i6j. net. My investigations on plant-response date from the discovery of the electric response of non-living matter, such as metals, to stimulus, published in 1900 by the International Congress of Science, Paris. The response, like that of living matter, was shown to exhibit fatigue under continuous stimulation, enhancement under chemical stimulants, and permanent abolition under poisons. These results indicated that the response of the more complex and unstable living matter is ultimately the expression of physico-chemical reactions. I next tried to find whether ordinary plants, meaning those usually regarded as insensitive, exhibit the characteristic electric response already known in ' sensitive ' plants. Ordinary plants were regarded at the time as inexcitable, because they did not respond to stimulation by an obvious movement. In my Friday Evening Discourse before the Royal Institution in May 1901, I was, however, able to show that every plant, and even each organ of every plant, is excitable, and responds to stimulus by electric response of galvanometric negativity, the response being abolished at the death of the plant. A more detailed account of the results was published in my work on ' Response of the Living and Non-Living '

My next investigation was directed towards obtaining evidence of responsive mechanical movement in these plants rendered conspicuous by various devices of magnification (‘ Plant Response/ 1906). The effects of various environmental stimuli on different plant-organs were thus demonstrated by automatic records given by the plant. The most important fact established in plant-response was the nervous character of the impulse transmitted to a distance. My discovery of the excitatory polar action of an electric current and its transmission to a distance, proved that the conduction of excitation in the plant is fundamentally the same as that in the nerve of the animal. Further corroboration was found in the arrest of transmission by the application of electrotonic and other physiological blocks in the path of conduction.

In my ' Comparative Electro-Physiology ' (1907) I employed the independent method of electric response and was able fully to confirm and extend the results which I had obtained by the method of mechanical response. Experiments are described showing that the response of isolated plant-nerve is indistinguishable from that of the animal nerve throughout a long series of parallel variations of condition. So complete, indeed, is the similarity, that the discovery of a responsive characteristic in a given case has proved a sure guide to its observation in the other. As an example of this may be mentioned the control of the ner'S’ous impulse in the animal by a homodromous or a heterodromous electric current, the discovery being due to the success which attended my attempt to control the nervous impulse in the plant by the directive action of electric currents. Accurate measurement of the normal velocity of transmission of nervous impulse, and its induced variations, has been rendered possible by my device of Resonant Recorder, described in ‘ Irritability of Plants ’ (1913), by wEich timeintervals as short as .005 second can be automatically recorded.

The great advance in animal physioiogj^ has been due to very sensitive and accurate methods in quantitative measurement, on which alone can any sound theory be based. In plant-physiology, unfortunately, no such methods had been previously available; this accounts for the unfounded speculations that had paralysed advance of knowledge in plant-physiology. As an instance of this may be mentioned the far-fetched theory recently advanced that the transmission of excitation in the sensitive plant, Mimosa pudica, is due to the movement of sap in the transpirationcurrent. Simultaneous measurements of the rate of transmission of excitation and of the ascent of sap show that the former is far greater than the latter. In Mimosa the velocity of nervous impulse in thin petioles is as high as 400 mm. per second, while the movement of sap is about 200 times slower. No demonstration of the unfounded character of the transpiration-current theory could be more simple and convincing than the observation of the effect of application of a drop of acid to the tip of the uppermost leaf of Mimosa described in Chapter II (p. 19) .

My recent discovery of the transformation of the afferent or sensory into an efferent or motor impulse in the reflex arc in the pul vinus of Mimosa will, it is hoped, materially advance our knowledge of nervous impulse in general. I have, in the present work, given not only a connected account of my previous results which came out in scattered publications, but also a mass of new material that has been accumulated since the discovery of separate nerves for the conduction of sensory and motor impulses.

The results of the investigations which I have carried out for the last quarter of a century establish the generalisation that the physiological mechanism of the plant is identical with that of the animal. For there is hardly any phenomenon of irritability observed in the animal which is not also discoverable in the plant. In the multicellular animal organism as higher complexity was attained, it was accompanied by the gradual evolution of a nervous system, by which the different organs are put in intimate connection with each other and their various activities co-ordinated for ensuring the common good of the organism. Such connecting nervous links had not been suspected in the plant, commonly regarded as distinctly lower in the scale of evolution. The researches described in the present work show that not only has a nervous system been evolved in

the plant, but that it 'has reached a very high degree of perfection, aS marked by the reflex arc in which a sensory becomes transformed into a motor impulse. The characteristics of the two impulses, and the definitely distinct channels for. their conduction, can be studied with greater certainty and accuracy in the plant than in the animal. And it may be confidently expected that the broader outlook of the unity of physiological mechanism in all life will lead to a great advance in the physiological investigation of the irritability of all Hving tissues.

The wide interest that has been roused in the new methods of investigation and their results, has been a matter of much gratification to me. I take this opportunity of tendering my best thanks to Prof. E. Pringsheim for preparing the German edition of the ‘Physiology of the Ascent of Sap,’ and also to Messrs. Gauthier- Villars for publishing a French edition of most of my works. My acknowledgments are also due to my research assistants and scholars for the very efficient help rendered by them.

Modes of intercommimication and interaction beween distant organs — Translocation of matter and transmission of motion-— Slow movement of iiiiid and rapid propagation of protoplasmic excitation — Receptor, Conductor and Effector — Modes of stimulation—-Effect of niiilatera.1 stimulus of moderate intensity — Conduction only on the stimulated side— Simultaneous conduction upwards and downwards — Effect of unilateral stimulus of stronger intensity-conversion of ascending into descending impulse after crossing over at the apex— Conduction between stem and leaves — Summary . . . ' , . . . .. , . i

Hydro-mechanical theory* of Pfeifer and Haberlandt— Supposed initiation of impulse only after escape of sap from deep wound— Question of xylem as the conductor of stimulus— Haberlandt finds the phloem to be the conductor — Effects of narcotisation and of scalding— Complications of Wound-stimulus — Kiihne's experiment—Transpiration-current theory^ — The wounding of the wood— Generation of impulse without wound and escape of sap — Supposed transmission across water-gap — Excretion of hypothetical stimulant — Simultaneous determination of velocity of transmitted excitation and of rate of transport of chemical stimulant — Transmitted impulse independent of movement of ■ sap— Summary . • . ^ ^ ... .■ • a. 8

I^Iodes of stimulation without physical disturbance — Polar excitatory effect of constant current — Feeble current excites only at kathodemake — Excitation under moderate current at kathode-make and at > anode-break — Tabular statement of eEect of polar excitation on various plants — Laws of Polar Excitation for Plants — Polar excitation by feeble current undetected b}^ human tongue — Transmission of polar excitation in stem of Mimosa — Arrest of impulse in animal nerve bj^- electrotonic block — Similar arrest of excitatory impulse in Mimosa — Summary 22

Conducting nerve in plants — Its localisation by Electric Probe — Four conducting nerves in the petiole — Thickness of different tissues in petiole — Practical impossibility of selective removal of different layers — ^Two phloem-strands, external and internal — Differentiation of phloems from adjacent tissues by doublestaining — Tubular cells characteristic of conducting tissue — Ex- planation of the results of Pfeffer on conduction through chloroformed region — Conducting tissue in the stem — Nervc-connection between stem and leaves — Explanation of unilateral propagation of excitation — Forcing of lateral block by stronger stimulus — Diffuse excitation under strong stimulus — Summary . . *32

Synaptic membrane at neurone-j unctions in animal ner^'c — Irreciprocal conduction at neuro-muscular junction — Passage of impulse in Mimosa from nerve-end to contractile pulvinus, but no conduction in opposite direction — Modification of conduction due to interposition of synaptic membrane observed : (i) in irreciprocal conduction, (2) in fatigue of conduction under excessive stimulation, and (3) in Facilitation of conduction as after-effect of moderate stimulus — Irreciprocal conduction in plants — Continuity between^ irreciprocal and preferential conduction— Fatigue of conduction under long-continued stimulation — Balmuog or Facilitation — Summary

Quantitative stimulation-— The Resonant Recorder— E:iinnnation of friction by intermittentcontact — Successive dots in record measure time-intervals as short as thousandth of a second— Determination ■ of the latent |>eriod— -Ordinary ■ method - of ■ determination ' of velocity — -The Differential method*---“Exceptionaliy high velocity in IMn petioles — The inffuence of tonic condition on conductivity— After-effect of stimulation on transmission— -Summary 54

Effect of desiccation — Effect of variation of temperature on velocity — Physiological block — Block of conduction by local cooling — Effect of tetanising electric shocks in rapid restoration of paralysed conductivity — Electrotonic block — Block by local application of poison — Summary . . , . . • . . * 69 Meclianical response — Dual character of transmitted impulse- — Quicker rate of the positive impulse — Effect of distance on transmission— The masking of the positive by the predominant negative — Positive impulse in stem of Mimosa with longitudinal transmission — Positive impulse with transverse transmission — Effects of direct and indirect stimulation on growth— -Heliotropic curvature under unilateral stimulation — Response of puivinus of Mimosa to unilateral stimulation — Propagation of excitation in contractile cells of puivinus — Diphasic response of puivinus, positive followed by negative — Laws of Effects of Direct and Indirect Stimulation — Summary 81

Electric response to direct stimulation — Response abolished under narcotics — Characteristics of transmitted impulse — Positive response in non-conducting or semi-conducting tissues — Diphasic and Monophasic response — Response by negative variation of current of injury — Monophasic response by method of natural block — Stimulation by ttiermai shock — ^The petiole-laminar preparation — Effect of Facilitation in enhancement of conduction, and staircase-increase of response — ^Time-relations of Monophasic response^ — La:ws of Direct and Indirect Stimulation — Summary 95

Positive electric response to feeble indirect stimulus — Conversion of positive to excitatory negative under stronger stimulus^ — ^The excitatory and contractile processes — Electric determination of velocity of impulse— Electric localisation of nervous tissue in Mimosa — ^The Electric Probe— Two conducting phloems in each bundle, one external and the other internal to the xylein — Impulse transmitted across the semi-conducting pulvinus — Preliminary positive impulse in conducting tissues of the animal — Internal and external work of stimulus — Protoplasmic irritability and its diverse manifestations — Parallel effects of stimulation "on contractility, conductivity, and rhythmicity — The leaf as a catchmentbasin for stimulus — Summary . . . . . « .105

The isolated plant-nerve — Experimental method for recording the electric response of nerve — Indefatigabilit}^ of nerve — Enhancement of normal response after tetanisation — Positive response of sub-tonic nerve — Abnormal positive response converted into normal negative after tetanisation — Gradual transition from positive to negative through intermediate diphasic — Parallel effects in plant and animal nerve indicate similarity of physiological mechanism — Receptivity, Conductivitv and Kesponsuity — The Conductivity Balance — Simultaneous determination o'f variations of receptivity, conductivity and responsivity — Effect of dilute solution of NaaCOa on conductivity-— 'Variation of conduction during passage of constant current — Conductivity enhanced in an electrical up-hill, and depressed in an electrical down-hill direction — Suminar}^

Experimental arrangement for determination of effect of constant current on conduction of excitation— Variation of conductivity gauged (i) by change of velocity ; (2) by variation in amplitude of response ; and (3) by ineffective stimulus becoming effective- ' ■ Effect on the latent period— Change induced in velocity of transmission — ^The method of minimal stimulus — After-effect of homodromous and heterodromouscurrents— Influence of direction of constant current on conductivity of animal nerve— Induced variation of velocity of transmission- — Induced vari<ition of intensity of transmitted excitation— After-effect of lielerodromous and homodromous currents on animal ner\’e— Laws of variation of nervous conduction under the action of constant Current-Summary

Complex^ nature of the pulvinus — Anatomy of Petiole-pulvinar junction (Mimosa)— Four distinct eifectors— The Torsional Recorder— The Quadrants of the pnivinns — Response to lateral stimulation — Law of Torsional Response — ^The Torsional Balance —Complex movements under vertical light (Cassia) — Summary . 151 Direct and indirect effect of light on leaf-adjustment— General description of dia-heliotropic phenomena — Mechanical response due to differential excitability — Response to stimulation of upper and lower halves of the organ — Mechanism of heliotropic curvature — Receptor, Conductor, and Effector — Definite innervation of the motile organ — Characteristic leaf-movements on stimulation of the several sub-petioles— Adjustment of leaf in space by transmitted nervous excitation — Summary . . , .160

Demonstration of nerve-connection between centre and periphery by method of central stimulation — ^Transmissiontime — Reflection of afferent as efferent impulse — Localisation of the reflex arc — Irradiation of excitation — Persistence of reflected impulse after abolition of pulvinar contractility — Relative sensibility of motile organs of Mimosa — Reflected impulses on stimulation of the several sub-petioles — Relation between the afferent and the efferent impulses — Greater velocity of transmission in petiole than in sub-petiole — Enhancement of excitability of motile organ on application of glycerine — Summary 175

Separate sensory and motor nerve-fibres in the animal — Significant difference in the velocity of transmission of afferent and efferent impulses in Mimosa — The motor impulse is much more rapid than the sensory — Proofs of separate conducting elements in the same bundle — Excitation of inner phloem conducted at a quicker rate than that of external phloem — Sensory impulse conducted by the outer, and motor impulse by the inner — Greater irradiation of excitation under stronger stimulus — ^The* lost time ' in reflex — Effect of strychnine in abolishing block — Summary . .187

2. Diagram of centripetal impulse in petiole of Mimosa Spegaz- 9. Transverse and longitudinal sections of a single vascular bundle 19. Record of determination of velocity of transmission . . . 20. Record of determination of velocity by differential method 22. Record of after-effect of intense stimulation on velocity . 23. Record of action of glycerine in increasing velocity . . . 24. Record of effect of rising temperature in increasing velocity 25. Diagram of method of physiological block with leaf of M.

26. Record of effect of cold in retarding and arresting transmission 29. Record of abolition of conductivity by potassium cyanide 30. Record of response of pulvinus of M. pudica to turgor-variation 31. Record of dual response under indirect stimulation . , . 33. Record of erectile response of leaf of M, pudica to indirect stimu- 34. Record of effect of direct and indirect stimulation on rate of 35. Record of effect of light acting on upper half of pulvinus of

36. Record of effect of chloroform on electric response of Carrot . 37. Diagram of electric investigation of non-conducting tissue (leaf) 97 38. Record of positive electric response of non-conducting tissue . 97 39. Diagram of diphasic electric response of animal nerve . , 98 41. Record of electric response of midrib to indirect stimulation . loi 42. Record of time-relations of monophasic response of midrib . 103 43. Diagram of electric connections for response of M'. pudica to

44. Record of positive response of Mimosa to feeble stimulus . 106 45. Record of negative response of Mimosa to stronger stimulus . 106 46. Record of diphasic response of olfactory nerve of Pike . . 108 47 and 47A. Record of diphasic response of nerve in Mimosa . . 109 48. Localisation of nerve in M. pudica by the Electric Probe , no 49. Transverse section of vascular bundle in petiole of Mimosa . . m 50. Records of excitation in the different tissues of the petiole . 112

52 and 52A. Records of electric response of lower half of pulviiius of Mimosa to impulse transmitted from the upper half , 1 14, 1 15 53. Record of electric changes accompanying contraction of the 54. Distribution of vascular tissue in stem of Papaya . . . 121 56. Transverse and longitudinal sections of vascular strand of Fern 124 57. Record of effect of tetanisation on response of nerve of Fern .127 5k Record of three types of response in nerve of Fern . . . 127

59 and 60. Records of effect of tetanisation on response of nerve of 62. Record of effect of Na^ COg on conductivity of nerve of Fern , 132 63. Apparatus for investigation of conductivity-variation in 65. Record of direct and after-effect of homo- and hetero-dromous 66. Experimental arrangement for study of the effect of constant 67. Record of effect of constant current on velocity of transmission 68. Record of effect of heterodromous current on transmission of

69. Record of effect of homodromous current on transmission of 76. Record of responses of leaf of Mimosa to stimulation of the 77. Diagram of the innervation of pulvinus and petiole of Mimosa 78. Diagram illustrating reflexion of impulses in leaf of Mimosa . 79. Diagram illustrating transmission of impulses initiated by So.® Curve of relation between duration of application of strychnine and lost time . . . . . In the body of a multicellular organism necessity arises for intercommunication and interaction between the more or less distant organs. I have shown elsewhere^ that this is accomplished in the plant, as in the animal, in two different ways — by translocation of matter, and by transmission of motion. The first is effected by the slow movement of fluids carrying chemical substances in solution, such as occurs in the circulation of sap : the second, by the rapid propagation of protoplasmic excitation, such as the nervous impulse in the animal. Great confusion has arisen in plant-physiology from want of discrimination between the two types of transmission, physical convection and physiological conduction.

Confining our attention first to the true excitatory impulse, it is conducted in the animal by a definite tissue known as nerve, the main function of which is the rapid transmission of excitation to a distance. When the animal nerve is pinched, or irritated in any way, an invisible molecular disturbance is propagated along it, and the transmitted impulse, impinging on the terminal motor organ, produces the well-known muscular twitch. The communicating system may, for convenience, be distinguished into three parts : the point where the stimulus impinges is the Receptor ; the channel for the transmission of excitation is the Conductor ; and the terminal motor organ which serves as an indicator is the Effector.