The Project Gutenberg EBook of Response in the Living and Non-Living, by Jagadis Chunder Bose This eBook is for the use of anyone anywhere at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this eBook or online at www.gutenberg.org Title: Response in the Living and Non-Living Author: Jagadis Chunder Bose Release Date: August 3, 2006 [EBook #18986] Language: English Character set encoding: ISO-8859-1 *** START OF THIS PROJECT GUTENBERG EBOOK RESPONSE IN LIVING AND NON-LIVING *** Produced by Bryan Ness, Laura Wisewell and the Online Distributed Proofreading Team at http://www.pgdp.net RESPONSE IN THE LIVING AND NON-LIVING BY JAGADIS CHUNDER BOSE, M.A.(CANTAB.), D.Sc.(LOND.) PROFESSOR, PRESIDENCY COLLEGE, CALCUTTA WITH ILLUSTRATIONS LONGMANS, GREEN, AND CO. 39 PATERNOSTER ROW, LONDON NEW YORK AND BOMBAY 1902 All rights reserved _'The real is one: wise men call it variously'_ RIG VEDA _To my Countrymen_ _This Work is Dedicated_ PREFACE I have in the present work put in a connected and a more complete form results, some of which have been published in the following Papers: 'De la Généralité des Phénomènes Moléculaires produits par l'Electricité sur la matière Inorganique et sur la matière Vivante.' (_Travaux du Congrès International de Physique._ Paris, 1900.) 'On the Similarity of Effect of Electrical Stimulus on Inorganic and Living Substances.' (_Report, Bradford Meeting British Association, 1900.--Electrician._) 'Response of Inorganic Matter to Stimulus.' (Friday Evening Discourse, Royal Institution, May 1901.) 'On Electric Response of Inorganic Substances. Preliminary Notice.' (Royal Society, June 1901.) 'On Electric Response of Ordinary Plants under Mechanical Stimulus.' (_Journal Linnean Society_, 1902.) 'Sur la Réponse Electrique dans les Métaux, les Tissus Animaux et Végétaux.' (Société de Physique, Paris, 1902.) 'On the Electro-Motive Wave accompanying Mechanical Disturbance in Metals in contact with Electrolyte.' (_Proceedings Royal Society_, vol. 70.) 'On the Strain Theory of Vision and of Photographic Action.' (_Journal Royal Photographic Society_, vol. xxvi.) These investigations were commenced in India, and I take this opportunity to express my grateful acknowledgments to the Managers of the Royal Institution, for the facilities offered me to complete them at the Davy-Faraday Laboratory. J. C. BOSE. DAVY-FARADAY LABORATORY, ROYAL INSTITUTION, LONDON: _May 1902._ CONTENTS CHAPTER I THE MECHANICAL RESPONSE OF LIVING SUBSTANCES PAGE Mechanical response--Different kinds of stimuli--Myograph --Characteristics of response-curve: period, amplitude, form--Modification of response-curves 1 CHAPTER II ELECTRIC RESPONSE Conditions for obtaining electric response--Method of injury--Current of injury--Injured end, cuproid: uninjured, zincoid--Current of response in nerve from more excited to less excited--Difficulties of present nomenclature--Electric recorder--Two types of response, positive and negative--Universal applicability of electric mode of response--Electric response a measure of physiological activity--Electric response in plants 5 CHAPTER III ELECTRIC RESPONSE IN PLANTS--METHOD OF NEGATIVE VARIATION Negative variation--Response recorder--Photographic recorder--Compensator--Means of graduating intensity of stimulus--Spring-tapper and torsional vibrator--Intensity of stimulus dependent on amplitude of vibration--Effectiveness of stimulus dependent on rapidity also 17 CHAPTER IV ELECTRIC RESPONSE IN PLANTS--BLOCK METHOD Method of block--Advantages of block method--Plant response a physiological phenomenon--Abolition of response by anæsthetics and poisons--Abolition of response when plant is killed by hot water 27 CHAPTER V PLANT RESPONSE--ON THE EFFECTS OF SINGLE STIMULUS AND OF SUPERPOSED STIMULI Effect of single stimulus--Superposition of stimuli--Additive effect--Staircase effect--Fatigue--No fatigue when sufficient interval between stimuli--Apparent fatigue when stimulation frequency is increased--Fatigue under continuous stimulation 35 CHAPTER VI PLANT RESPONSE--ON DIPHASIC VARIATION Diphasic variation--Positive after-effect and positive response--Radial E.M. variation 44 CHAPTER VII PLANT RESPONSE--ON THE RELATION BETWEEN STIMULUS AND RESPONSE Increased response with increasing stimulus--Apparent diminution of response with excessively strong stimulus 51 CHAPTER VIII PLANT RESPONSE--ON THE INFLUENCE OF TEMPERATURE Effect of very low temperature--Influence of high temperature--Determination of death-point--Increased response as after-effect of temperature variation--Death of plant and abolition of response by the action of steam 59 CHAPTER IX PLANT RESPONSE--EFFECT OF ANÆSTHETICS AND POISONS Effect of anæsthetics, a test of vital character of response--Effect of chloroform--Effect of chloral--Effect of formalin--Method in which response is unaffected by variation of resistance--Advantage of block method--Effect of dose 71 CHAPTER X RESPONSE IN METALS Is response found in inorganic substances?--Experiment on tin, block method--Anomalies of existing terminology--Response by method of depression--Response by method of exaltation 81 CHAPTER XI INORGANIC RESPONSE--MODIFIED APPARATUS TO EXHIBIT RESPONSE IN METALS Conditions of obtaining quantitative measurements--Modification of the block method--Vibration cell--Application of stimulus--Graduation of the intensity of stimulus--Considerations showing that electric response is due to molecular disturbance--Test experiment--Molecular voltaic cell 91 CHAPTER XII INORGANIC RESPONSE--METHOD OF ENSURING CONSISTENT RESULTS Preparation of wire--Effect of single stimulus 100 CHAPTER XIII INORGANIC RESPONSE--MOLECULAR MOBILITY: ITS INFLUENCE ON RESPONSE Effects of molecular inertia--Prolongation of period of recovery by overstrain--Molecular model--Reduction of molecular sluggishness attended by quickened recovery and heightened response--Effect of temperature--Modification of latent period and period of recovery by the action of chemical reagents--Diphasic variation 104 CHAPTER XIV INORGANIC RESPONSE--FATIGUE, STAIRCASE, AND MODIFIED RESPONSE Fatigue in metals--Fatigue under continuous stimulation--Staircase effect--Reversed responses due to molecular modification in nerve and in metal, and their transformation into normal after continuous stimulation--Increased response after continuous stimulation 118 CHAPTER XV INORGANIC RESPONSE--RELATION BETWEEN STIMULUS AND RESPONSE--SUPERPOSITION OF STIMULI Relation between stimulus and response--Magnetic analogue--Increase of response with increasing stimulus--Threshold of response--Superposition of stimuli--Hysteresis 131 CHAPTER XVI INORGANIC RESPONSE--EFFECT OF CHEMICAL REAGENTS Action of chemical reagents--Action of stimulants on metals--Action of depressants on metals--Effect of 'poisons' on metals--Opposite effect of large and small doses 139 CHAPTER XVII ON THE STIMULUS OF LIGHT AND RETINAL CURRENTS Visual impulse: (1) chemical theory; (2) electrical theory--Retinal currents--Normal response positive--Inorganic response under stimulus of light--Typical experiment on the electrical effect induced by light 148 CHAPTER XVIII INORGANIC RESPONSE--INFLUENCE OF VARIOUS CONDITIONS ON THE RESPONSE TO STIMULUS OF LIGHT Effect of temperature--Effect of increasing length of exposure--Relation between intensity of light and magnitude of response--After-oscillation--Abnormal effects: (1) preliminary negative twitch; (2) reversal of response; (3) transient positive twitch on cessation of light; (4) decline and reversal--Résumé 158 CHAPTER XIX VISUAL ANALOGUES Effect of light of short duration--After-oscillation--Positive and negative after-images--Binocular alternation of vision--Period of alternation modified by physical condition--After-images and their revival--Unconscious visual impression. 170 CHAPTER XX GENERAL SURVEY AND CONCLUSION 181 INDEX 193 ILLUSTRATIONS FIG. PAGE 1. MECHANICAL LEVER RECORDER 3 2. ELECTRIC METHOD OF DETECTING NERVE RESPONSE 6 3. DIAGRAM SHOWING INJURED END OF NERVE CORRESPONDS TO 8 COPPER IN A VOLTAIC ELEMENT 4. ELECTRIC RECORDER 11 5. SIMULTANEOUS RECORD OF MECHANICAL AND ELECTRICAL 13 RESPONSES 6. NEGATIVE VARIATION IN PLANTS 19 7. PHOTOGRAPHIC RECORD OF NEGATIVE VARIATION IN PLANTS 20 8. RESPONSE RECORDER 21 9. THE COMPENSATOR 22 10. THE SPRING-TAPPER 23 11. THE TORSIONAL VIBRATOR 24 12. RESPONSE IN PLANT TO MECHANICAL TAP OR VIBRATION 25 13. INFLUENCE OF SUDDENNESS ON THE EFFICIENCY OF 26 STIMULUS 14. THE METHOD OF BLOCK 28 15. RESPONSE IN PLANT COMPLETELY IMMERSED UNDER WATER 29 16. UNIFORM RESPONSES IN PLANT 36 17. FUSION OF EFFECT UNDER RAPIDLY SUCCEEDING STIMULI IN 36 MUSCLE AND IN PLANT 18. ADDITIVE EFFECT OF SINGLY INEFFECTIVE STIMULI ON 37 PLANT 19. 'STAIRCASE EFFECT' IN PLANT 37 20. APPEARANCE OF FATIGUE IN PLANT UNDER SHORTENED 39 PERIOD OF REST 21. FATIGUE IN CELERY 40 22. FATIGUE IN CAULIFLOWER-STALK 41 23. FATIGUE FROM PREVIOUS OVERSTRAIN 41 24. FATIGUE UNDER CONTINUOUS STIMULATION IN CELERY 42 25. EFFECT OF REST IN REMOVAL OF FATIGUE IN PLANT 43 26. DIPHASIC VARIATION IN PLANT 46 27, 28. ABNORMAL POSITIVE RESPONSES IN STALE PLANT 48, 49 TRANSFORMED INTO NORMAL NEGATIVE UNDER STRONG STIMULATION 29. RADIAL E.M. VARIATION 50 30. CURVES SHOWING THE RELATION BETWEEN INTENSITY OF 52 STIMULUS AND RESPONSE IN MUSCLE AND NERVE 31. INCREASING RESPONSES TO INCREASING STIMULI (TAPS) IN 52 PLANTS 32. INCREASING RESPONSES TO INCREASING VIBRATIONAL 53 STIMULI IN PLANTS 33. RESPONSES TO INCREASING STIMULI IN FRESH AND STALE 54 SPECIMENS OF PLANTS 34. APPARENT DIMINUTION OF RESPONSE CAUSED BY FATIGUE 57 UNDER STRONG STIMULATION 35. DIMINUTION OF RESPONSE IN EUCHARIS LILY AT LOW 61 TEMPERATURE 36. RECORDS SHOWING THE DIFFERENCE IN THE EFFECTS OF LOW 62 TEMPERATURE ON IVY, HOLLY, AND EUCHARIS LILY 37. PLANT CHAMBER FOR STUDYING THE EFFECT OF TEMPERATURE 64 AND ANÆSTHETICS 38. EFFECT OF HIGH TEMPERATURE ON PLANT RESPONSE 64 39. AFTER-EFFECT ON THE RESPONSE DUE TO TEMPERATURE 66 VARIATION 40. RECORDS OF RESPONSES IN EUCHARIS LILY DURING RISE 67 AND FALL OF TEMPERATURE 41. CURVE SHOWING VARIATION OF SENSITIVENESS DURING A 68 CYCLE OF TEMPERATURE VARIATION 42. RECORD OF EFFECT OF STEAM IN ABOLITION OF RESPONSE 69 AT DEATH OF PLANT 43. EFFECT OF CHLOROFORM ON NERVE RESPONSE 72 44. EFFECT OF CHLOROFORM ON THE RESPONSES OF CARROT 74 45. ACTION OF CHLORAL HYDRATE ON PLANT RESPONSES 75 46. ACTION OF FORMALIN ON RADISH 75 47. ACTION OF SODIUM HYDRATE IN ABOLISHING THE RESPONSE 78 IN PLANT 48. STIMULATING ACTION OF POISON IN SMALL DOSES IN 79 PLANTS 49. THE POISONOUS EFFECT OF STRONGER DOSE OF KOH 79 50. BLOCK METHOD FOR OBTAINING RESPONSE IN TIN 83 51. RESPONSE TO MECHANICAL STIMULATION IN A Zn-Cu COUPLE 85 52. ELECTRIC RESPONSE IN METAL BY THE METHOD OF RELATIVE 88 DEPRESSION (NEGATIVE VARIATION) 53. METHOD OF RELATIVE EXALTATION 89 54. VARIOUS CASES OF POSITIVE AND NEGATIVE VARIATION 90 55. MODIFICATIONS OF THE BLOCK METHOD FOR EXHIBITING 93 ELECTRIC RESPONSE IN METALS 56. EQUAL AND OPPOSITE RESPONSES GIVEN BY TWO ENDS OF 95 THE WIRE 57. TOP VIEW OF THE VIBRATION CELL 96 58. INFLUENCE OF ANNEALING IN THE ENHANCEMENT OF 101 RESPONSE IN METALS 59. UNIFORM ELECTRIC RESPONSES IN METALS 102 60. PERSISTENCE OF AFTER-EFFECT 105 61. PROLONGATION OF PERIOD OF RECOVERY AFTER OVERSTRAIN 106 62. MOLECULAR MODEL 107 63, 64. EFFECTS OF REMOVAL OF MOLECULAR SLUGGISHNESS IN 109, 110 QUICKENED RECOVERY AND HEIGHTENED RESPONSE IN METALS 65. EFFECT OF TEMPERATURE ON RESPONSE IN METALS 111 66. DIPHASIC VARIATION IN METALS 113 67. NEGATIVE, DIPHASIC, AND POSITIVE RESULTANT RESPONSE 115 IN METALS 68. CONTINUOUS TRANSFORMATION FROM NEGATIVE TO POSITIVE 116 THROUGH INTERMEDIATE DIPHASIC RESPONSE 69. FATIGUE IN MUSCLE 118 70. FATIGUE IN PLATINUM 118 71. FATIGUE IN TIN 119 72. APPEARANCE OF FATIGUE DUE TO SHORTENING THE PERIOD 120 OF RECOVERY 73. FATIGUE IN METAL UNDER CONTINUOUS STIMULATION 121 74. 'STAIRCASE' RESPONSE IN MUSCLE AND IN METAL 122 75. ABNORMAL RESPONSE IN NERVE CONVERTED INTO NORMAL 124 UNDER CONTINUED STIMULATION 76, 77. ABNORMAL RESPONSE IN TIN AND PLATINUM CONVERTED INTO 125 NORMAL UNDER CONTINUED STIMULATION 78. GRADUAL TRANSITION FROM ABNORMAL TO NORMAL RESPONSE 126 IN PLATINUM 79. INCREASE OF RESPONSE IN NERVE AFTER CONTINUOUS 127 STIMULATION 80, 81. RESPONSE IN TIN AND PLATINUM ENHANCED AFTER 127, 128 CONTINUOUS STIMULATION 82. MAGNETIC ANALOGUE 132 83, 84. RECORDS OF RESPONSES TO INCREASING STIMULI IN TIN 134, 135 85. INEFFECTIVE STIMULUS BECOMING EFFECTIVE BY 135 SUPERPOSITION 86. INCOMPLETE AND COMPLETE FUSION OF EFFECTS 136 87. CYCLIC CURVE FOR MAXIMUM EFFECTS SHOWING HYSTERESIS 137 88. ACTION OF POISON IN ABOLISHING RESPONSE IN NERVE 139 89. ACTION OF STIMULANT ON TIN 141 90. ACTION OF STIMULANT ON PLATINUM 142 91. DEPRESSING EFFECT OF KBr ON TIN 143 92. ABOLITION OF RESPONSE IN METALS BY 'POISON' 143 93. 'MOLECULAR ARREST' BY THE ACTION OF 'POISON' 145 94. OPPOSITE EFFECTS OF SMALL AND LARGE DOSES ON THE 146 RESPONSE IN METALS 95. RETINAL RESPONSE TO LIGHT 150 96. RESPONSE OF SENSITIVE CELL TO LIGHT 152 97. TYPICAL EXPERIMENT ON THE E.M. VARIATION PRODUCED BY 154 LIGHT 98. MODIFICATION OF THE PHOTO-SENSITIVE CELL 155 99. RESPONSES IN FROG'S RETINA 156 100. RESPONSES IN SENSITIVE PHOTO-CELL 157 101. EFFECT OF TEMPERATURE ON THE RESPONSE TO LIGHT 159 STIMULUS 102. EFFECT OF DURATION OF EXPOSURE ON THE RESPONSE 159 103. RESPONSES OF SENSITIVE CELL TO INCREASING 161 INTENSITIES OF LIGHT 104. RELATION BETWEEN THE INTENSITY OF LIGHT AND 162 MAGNITUDE OF RESPONSE 105. AFTER-OSCILLATION 163 106. TRANSIENT POSITIVE INCREASE OF RESPONSE IN THE 164 FROG'S RETINA ON THE CESSATION OF LIGHT 107. TRANSIENT POSITIVE INCREASE OF RESPONSE IN THE 165 SENSITIVE CELL 108. DECLINE UNDER THE CONTINUOUS ACTION OF LIGHT 166 109. CERTAIN AFTER-EFFECTS OF LIGHT 168 110. AFTER-EFFECT OF LIGHT OF SHORT DURATION 172 111. STEREOSCOPIC DESIGN FOR THE EXHIBITION OF BINOCULAR 176 ALTERNATION OF VISION 112. UNIFORM RESPONSES IN NERVE, PLANT, AND METAL 184 113. FATIGUE IN MUSCLE, PLANT, AND METAL 185 114. 'STAIRCASE' EFFECT IN MUSCLE, PLANT, AND METAL 186 115. INCREASE OF RESPONSE AFTER CONTINUOUS STIMULATION IN 186 NERVE AND METAL 116. MODIFIED ABNORMAL RESPONSE IN NERVE AND METAL 187 TRANSFORMED INTO NORMAL RESPONSE AFTER CONTINUOUS STIMULATION 117. ACTION OF THE SAME 'POISON' IN THE ABOLITION OF 189 RESPONSE IN NERVE, PLANT, AND METAL RESPONSE IN THE LIVING AND NON-LIVING CHAPTER I THE MECHANICAL RESPONSE OF LIVING SUBSTANCES Mechanical response--Different kinds of stimuli--Myograph--Characteristics of response-curve: period, amplitude, form--Modification of response-curves. One of the most striking effects of external disturbance on certain types of living substance is a visible change of form. Thus, a piece of muscle when pinched contracts. The external disturbance which produced this change is called the stimulus. The body which is thus capable of responding is said to be irritable or excitable. A stimulus thus produces a state of excitability which may sometimes be expressed by change of form. #Mechanical response to different kinds of stimuli.#--This reaction under stimulus is seen even in the lowest organisms; in some of the amoeboid rhizopods, for instance. These lumpy protoplasmic bodies, usually elongated while creeping, if mechanically jarred, contract into a spherical form. If, instead of mechanical disturbance, we apply salt solution, they again contract, in the same way as before. Similar effects are produced by sudden illumination, or by rise of temperature, or by electric shock. A living substance may thus be put into an excitatory state by either mechanical, chemical, thermal, electrical, or light stimulus. Not only does the point stimulated show the effect of stimulus, but that effect may sometimes be conducted even to a considerable distance. This power of conducting stimulus, though common to all living substances, is present in very different degrees. While in some forms of animal tissue irritation spreads, at a very slow rate, only to points in close neighbourhood, in other forms, as for example in nerves, conduction is very rapid and reaches far. The visible mode of response by change of form may perhaps be best studied in a piece of muscle. When this is pinched, or an electrical shock is sent through it, it becomes shorter and broader. A responsive twitch is thus produced. The excitatory state then disappears, and the muscle is seen to relax into its normal form. #Mechanical lever recorder.#--In the case of contraction of muscle, the effect is very quick, the twitch takes place in too short a time for detailed observation by ordinary means. A myographic apparatus is therefore used, by means of which the changes in the muscle are self-recorded. Thus we obtain a history of its change and recovery from the change. The muscle is connected to one end of a writing lever. When the muscle contracts, the tracing point is pulled up in one direction, say to the right. The extent of this pull depends on the amount of contraction. A band of paper or a revolving drum-surface moves at a uniform speed at right angles to the direction of motion of the writing lever. When the muscle recovers from the stimulus, it relaxes into its original form, and the writing point traces the recovery as it moves now to the left, regaining its first position. A curve is thus described, the rising portion of which is due to contraction, and the falling portion to relaxation or recovery. The ordinate of the curve represents the intensity of response, and the abscissa the time (fig. 1). [Illustration: FIG. 1.--MECHANICAL LEVER RECORDER The muscle M with the attached bone is securely held at one end, the other end being connected with the writing lever. Under the action of stimulus the contracting muscle pulls the lever and moves the tracing point to the right over the travelling recording surface P. When the muscle recovers from contraction, the tracing point returns to its original position. See on P the record of muscle curve.] #Characteristics of the response-curve: (1) Period, (2) Amplitude, (3) Form.#--Just as a wave of sound is characterised by its (1) period, (2) amplitude, and (3) form, so may these response-curves be distinguished from each other. As regards the period, there is an enormous variation, corresponding to the functional activity of the muscle. For instance, in tortoise it may be as high as a second, whereas in the wing-muscles of many insects it is as small as 1/300 part of a second. 'It is probable that a continuous graduated scale might, as suggested by Hermann, be drawn up in the animal kingdom, from the excessively rapid contraction of insects to those of tortoises and hibernating dormice.'[1] Differences in form and amplitude of curve are well illustrated by various muscles of the tortoise. The curve for the muscle of the neck, used for rapid withdrawal of the head on approach of danger, is quite different from that of the pectoral muscle of the same animal, used for its sluggish movements. Again, progressive changes in the same muscle are well seen in the modifications of form which consecutive muscle-curves gradually undergo. In a dying muscle, for example, the amplitude of succeeding curves is continuously diminished, and the curves themselves are elongated. Numerous illustrations will be seen later, of the effect, in changing the form of the curve, of the increased excitation or depression produced by various agencies. Thus these response records give us a means of studying the effect of stimulus, and the modification of response, under varying external conditions, advantage being taken of the mechanical contraction produced in the tissue by the stimulus. But there are other kinds of tissue where the excitation produced by stimulus is not exhibited in a visible form. In order to study these we have to use an altogether independent method, the method of electric response. FOOTNOTES: [1] Biedermann, _Electro-physiology_, p. 59. CHAPTER II ELECTRIC RESPONSE Conditions for obtaining electric response--Method of injury--Current of injury--Injured end, cuproid: uninjured, zincoid--Current of response in nerve from more excited to less excited--Difficulties of present nomenclature--Electric recorder--Two types of response, positive and negative--Universal applicability of electric mode of response--Electric response a measure of physiological activity--Electric response in plants. Unlike muscle, a length of nerve, when mechanically or electrically excited, does not undergo any visible change. That it is thrown into an excitatory state, and that it conducts the excitatory disturbance, is shown however by the contraction produced in an attached piece of muscle, which serves as an indicator. But the excitatory effect produced in the nerve by stimulus can also be detected by an electrical method. If an isolated piece of nerve be taken and two contacts be made on its surface by means of non-polarisable electrodes at A and B, connection being made with a galvanometer, no current will be observed, as both A and B are in the same physico-chemical condition. The two points, that is to say, are iso-electric. If now the nerve be excited by stimulus, similar disturbances will be evoked at both A and B. If, further, these disturbances, reaching A and B almost simultaneously, cause any electrical change, then, similar changes taking place at both points, and there being thus no relative difference between the two, the galvanometer will still indicate no current. This null-effect is due to the balancing action of B as against A. (See fig. 2, _a_.) #Conditions for obtaining electric response.#--If then we wish to detect the response by means of the galvanometer, one means of doing so will lie in the abolition of this balance, which may be accomplished by making one of the two points, say B, more or less permanently irresponsive. In that case, stimulus will cause greater electrical disturbance at the more responsive point, say A, and this will be shown by the galvanometer as a current of response. To make B less responsive we may injure it by means of a cross-sectional cut, a burn, or the action of strong chemical reagents. [Illustration: FIG. 2.--ELECTRIC METHOD OF DETECTING NERVE RESPONSE (_a_) Iso-electric contacts; no current in the galvanometer. (_b_) The end B injured; current of injury from B to A: stimulation gives rise to an action current from A to B. (_c_) Non-polarisable electrode.] #Current of injury.#--We shall revert to the subject of electric response; meanwhile it is necessary to say a few words regarding the electric disturbance caused by the injury itself. Since the physico-chemical conditions of the uninjured A and the injured B are now no longer the same, it follows that their electric conditions have also become different. They are no longer iso-electric. There is thus a more or less permanent or resting difference of electric potential between them. A current--the current of injury--is found to flow _in the nerve_, from the injured to the uninjured, and in the galvanometer, through the electrolytic contacts from the uninjured to the injured. As long as there is no further disturbance this current of injury remains approximately constant, and is therefore sometimes known as 'the current of rest' (fig. 2, _b_). A piece of living tissue, unequally injured at the two ends, is thus seen to act like a voltaic element, comparable to a copper and zinc couple. As some confusion has arisen, on the question of whether the injured end is like the zinc or copper in such a combination, it will perhaps be well to enter upon this subject in detail. If we take two rods, of zinc and copper respectively, in metallic contact, and further, if the points A and B are connected by a strip of cloth _s_ moistened with salt solution, it will be seen that we have a complete voltaic element. A current will now flow from B to A in the metal (fig. 3, _a_) and from A to B through the electrolyte _s_. Or instead of connecting A and B by a single strip of cloth _s_, we may connect them by two strips _s s'_, leading to non-polarisable electrodes E E'. The current will then be found just the same as before, i.e. from B to A in the metallic part, and from A through _s s'_ to B, the wire W being interposed, as it were, in the electrolytic part of the circuit. If now a galvanometer be interposed at O, the current will flow from B to A through the galvanometer, i.e. from right to left. But if we interpose the galvanometer in the electrolytic part of the circuit, that is to say, at W, the same current will appear to flow in the opposite direction. In fig. 3, _c_, the galvanometer is so interposed, and in this case it is to be noticed that when the current in the galvanometer flows from left to right, the metal connected to the left is zinc. Compare fig. 3, _d_, where A B is a piece of nerve of which the B end is injured. The current in the galvanometer through the non-polarisable electrode is from left to right. The uninjured end is therefore comparable to the zinc in a voltaic cell (is zincoid), the injured being copper-like or cuproid.[2] [Illustration: FIG. 3.--DIAGRAM SHOWING THE CORRESPONDENCE BETWEEN INJURED (B) AND UNINJURED (A) CONTACTS IN NERVE, AND Cu AND Zn IN A VOLTAIC ELEMENT Comparison of (_c_) and (_d_) will show that the injured end of B in (_d_) corresponds with the Cu in (_c_).] If the electrical condition of, say, zinc in the voltaic couple (fig. 3, _c_) undergo any change (and I shall show later that this can be caused by molecular disturbance), then the existing difference of potential between A and B will also undergo variation. If for example the electrical condition of A approach that of B, the potential difference will undergo a diminution, and the current hitherto flowing in the circuit will, as a consequence, display a diminution, or _negative_ variation. #Action current.#--We have seen that a current of injury--sometimes known as 'current of rest'--flows in a nerve from the injured to the uninjured, and that the injured B is then less excitable than the uninjured A. If now the nerve be excited, there being a greater effect produced at A, the existing difference of potential may thus be reduced, with a consequent diminution of the current of injury. During stimulation, therefore, a nerve exhibits a negative variation. We may express this in a different way by saying that a 'current of action' was produced in response to stimulus, and acted in an opposite direction to the current of injury (fig. 2, _b_). The action current in the nerve _is from the relatively more excited to the relatively less excited_. #Difficulties of present nomenclature.#--We shall deal later with a method by which a responsive current of action is obtained without any antecedent current of injury. 'Negative variation' has then no meaning. Or, again, a current of injury may sometimes undergo a change of direction (see note, p. 12). In view of these considerations it is necessary to have at our disposal other forms of expression by which the direction of the current of response can still be designated. Keeping in touch with the old phraseology, we might then call a current 'negative' that flowed from the more excited to the less excited. Or, bearing in mind the fact that an uninjured contact acts as the zinc in a voltaic couple, we might call it 'zincoid,' and the injured contact 'cuproid.' Stimulation of the uninjured end, approximating it to the condition of the injured, might then be said to induce a cuproid change. The electric change produced in a normal nerve by stimulation may therefore be expressed by saying that there has been a negative variation, or that there was a current of action from the more excited to the less excited, or that stimulation has produced a cuproid change. The excitation, or molecular disturbance, produced by a stimulus has thus a concomitant electrical expression. As the excitatory state disappears with the return of the excitable tissue to its original condition, the current of action will gradually disappear.[3] The movement of the galvanometer needle during excitation of the tissue thus indicates a molecular upset by the stimulus; and the gradual creeping back of the galvanometer deflection exhibits a molecular recovery. This transitory electrical variation constitutes the 'response,' and its intensity varies according to that of the stimulus. #Electric recorder.#--We have thus a method of obtaining curves of response electrically. After all, it is not essentially very different from the mechanical method. In this case we use a magnetic lever (fig. 4, _a_), the needle of the galvanometer, which is deflected by the electromagnetic pull of the current, generated under the action of stimulus, just as the mechanical lever was deflected by the mechanical pull of the muscle contracting under stimulus. The accompanying diagram (fig. 4, _b_) shows how, under the action of stimulus, the current of rest undergoes a transitory diminution, and how on the cessation of stimulus there is gradual recovery of the tissue, as exhibited in the return of the galvanometer needle to its original position. [Illustration: FIG. 4.--ELECTRIC RECORDER (_a_) M muscle; A uninjured, B injured ends. E E' non-polarising electrodes connecting A and B with galvanometer G. Stimulus produces 'negative variation' of current of rest. Index connected with galvanometer needle records curve on travelling paper (in practice, moving galvanometer spot of light traces curve on photographic plate). Rising part of curve shows effect of stimulus; descending part, recovery. (_b_) O is the zero position of the galvanometer; injury produces a deflection A B; stimulus diminishes this deflection to C; C D is the recovery.] #Two types of response--positive and negative.#--It may here be added that though stimulus in general produces a diminution of current of rest, or a negative variation (e.g. muscles and nerves), yet, in certain cases, there is an increase, or positive variation. This is seen in the response of the retina to light. Again, a tissue which normally gives a negative variation may undergo molecular changes, after which it gives a positive variation. Thus Dr. Waller finds that whereas fresh nerve always gives negative variation, stale nerve sometimes gives positive; and that retina, which when fresh gives positive, when stale, exhibits negative variation. The following is a tabular statement of the two types of response: I. _Negative variation._--Action current from more excited to less excited--cuproid change in the excited--e.g. fresh muscle and nerve, stale retina. II. _Positive variation._--Action current from less excited to more excited--zincoid change in the excited--e.g. stale nerve, fresh retina.[4] From this it will be seen that it is the fact of the electrical response of living substances to stimulus that is of essential importance, the sign _plus_ or _minus_ being a minor consideration. #Universal applicability of the electrical mode of response.#--This mode of obtaining electrical response is applicable to all living tissues, and in cases like that of muscle, where mechanical response is also available, it is found that the electrical and mechanical records are practically identical. The two response-curves seen in the accompanying diagram (fig. 5), and taken from the same muscle by the two methods simultaneously, clearly exhibit this. Thus we see that electrical response can not only take the place of the mechanical record, but has the further advantage of being applicable in cases where the latter cannot be used. #Electrical response: A measure of physiological activity.#--These electrical changes are regarded as physiological, or characteristic of living tissue, for any conditions which enhance physiological activity also, _pari passu_, increase their intensity. Again, when the tissue is killed by poison, electrical response disappears, the tissue passing into an irresponsive condition. Anæsthetics, like chloroform, gradually diminish, and finally altogether abolish, electrical response. [Illustration: FIG. 5.--SIMULTANEOUS RECORD OF THE MECHANICAL (M) AND (E) ELECTRICAL RESPONSES OF THE MUSCLE OF FROG. (WALLER.)] From these observed facts--that living tissue gives response while a tissue that has been killed does not--it is concluded that the phenomenon of response is peculiar to living organisms.[5] The response phenomena that we have been studying are therefore considered as due to some unknown, super-physical 'vital' force and are thus relegated to a region beyond physical inquiry. It may, however, be that this limitation is not justified, and surely, at least until we have explored the whole range of physical action, it cannot be asserted definitely that a particular class of phenomena is by its very nature outside that category. #Electric response in plants.#--But before we proceed to the inquiry as to whether these responses are or are not due to some physical property of matter, and are to be met with even in inorganic substances, it will perhaps be advisable to see whether they are not paralleled by phenomena in the transitional world of plants. We shall thus pass from a study of response in highly complex animal tissues to those given under simpler vital conditions. Electric response has been found by Munck, Burdon-Sanderson, and others to occur in sensitive plants. But it would be interesting to know whether these responses were confined to plants which exhibit such remarkable mechanical movements, and whether they could not also be obtained from ordinary plants where visible movements are completely absent. In this connection, Kunkel observed electrical changes in association with the injury or flexion of stems of ordinary plants.[6] My own attempt, however, was directed, not towards the obtaining of a mere qualitative response, but rather to the determination of whether throughout the whole range of response phenomena a parallelism between animal and vegetable could be detected. That is to say, I desired to know, with regard to plants, what was the relation between intensity of stimulus and the corresponding response; what were the effects of superposition of stimuli; whether fatigue was present, and in what manner it influenced response; what were the effects of extremes of temperature on the response; and, lastly, if chemical reagents could exercise any influence in the modification of plant response, as stimulating, anæsthetic, and poisonous drugs have been found to do with nerve and muscle. If it could be proved that the electric response served as a faithful index of the physiological activity of plants, it would then be possible successfully to attack many problems in plant physiology, the solution of which at present offers many experimental difficulties. With animal tissues, experiments have to be carried on under many great and unavoidable difficulties. The isolated tissue, for example, is subject to unknown changes inseparable from the rapid approach of death. Plants, however, offer a great advantage in this respect, for they maintain their vitality unimpaired during a very great length of time. In animal tissues, again, the vital conditions themselves are highly complex. Those essential factors which modify response can, therefore, be better determined under the simpler conditions which obtain in vegetable life. In the succeeding chapters it will be shown that the response phenomena are exhibited not only by plants but by inorganic substances as well, and that the responses are modified by various conditions in exactly the same manner as those of animal tissues. In order to show how striking are these similarities, I shall for comparison place side by side the responses of animal tissues and those I have obtained with plants and inorganic substances. For the electric response in animal tissues, I shall take the latest and most complete examples from the records made by Dr. Waller. But before we can obtain satisfactory and conclusive results regarding plant response, many experimental difficulties will have to be surmounted. I shall now describe how this has been accomplished.[7] FOOTNOTES: [2] In some physiological text-books much wrong inference has been made, based on the supposition that the injured end is zinc-like. [3] 'The exciting cause is able to produce a particular molecular rearrangement in the nerve; this constitutes the state of excitation and is accompanied by local electrical changes as an ascertained physical concomitant.' 'The excitatory state evoked by stimulus manifests itself in nerve fibres by E.M. changes, and as far as our present knowledge goes by these only. The conception of such an excitable living tissue as nerve implies that of a molecular state which is in stable equilibrium. This equilibrium can be readily upset by an external agency, the stimulus, but the term "stable" expresses the fact that a change in any direction must be succeeded by one of opposite character, this being the return of the living structure to its previous state. Thus the electrical manifestation of the excitatory state is one whose duration depends upon the time during which the external agent is able to upset and retain in a new poise the living equilibrium, and if this is extremely brief, then the recoil of the tissue causes such manifestation to be itself of very short duration.'--_Text-book of Physiology_, ed. by Schäfer, ii. 453. [4] I shall here mention briefly one complication that might arise from regarding the current of injury as the current of reference, and designating the response current either positive or negative in relation to it. If this current of injury remained always invariable in direction--that is to say, from the injured to the uninjured--there would be no source of uncertainty. But it is often found, for example in the retina, that the current of injury undergoes a reversal, or is reversed from the beginning. That is to say, the direction is now from the uninjured to the injured, instead of the opposite. Confusion is thus very apt to arise. No such misunderstanding can however occur if we call the current of response towards the more excited _positive_, and towards the less excited _negative_. [5] 'The Electrical Sign of Life.... An isolated muscle gives sign of life by contracting when stimulated.... An ordinary nerve, normally connected with its terminal organs, gives sign of life by means of muscle, which by direct or reflex path is set in motion when the nerve trunk is stimulated. But such nerve separated from its natural termini, isolated from the rest of the organism, gives no sign of life when excited, either in the shape of chemical or of thermic changes, and it is only by means of an electrical change that we can ascertain whether or no it is alive.... The most general and most delicate sign of life is then the electrical response.'--Waller, in _Brain_, pp. 3 and 4. Spring 1900. [6] Kunkel thought the electric disturbance to be due to movement of water through the tissue. It will be shown that this explanation is inadequate. [7] My assistant Mr. J. Bull has rendered me very efficient help in these experiments. CHAPTER III ELECTRIC RESPONSE IN PLANTS--METHOD OF NEGATIVE VARIATION Negative variation--Response recorder--Photographic recorder--Compensator--Means of graduating intensity of stimulus--Spring-tapper and torsional vibrator--Intensity of stimulus dependent on amplitude of vibration--Effectiveness of stimulus dependent on rapidity also. I shall first proceed to show that an electric response is evoked in plants under stimulation.[8] In experiments for the exhibition of electric response it is preferable to use a non-electrical form of stimulus, for there is then a certainty that the observed response is entirely due to reaction from stimulus, and not, as might be the case with electric stimulus, to mere escape of stimulating current through the tissue. For this reason, the mechanical form of stimulation is the most suitable. I find that all parts of the living plant give electric response to a greater or less extent. Some, however, give stronger response than others. In favourable cases, we may have an E.M. variation as high as ·1 volt. It must however be remembered that the response, being a function of physiological activity of the plant, is liable to undergo changes at different seasons of the year. Each plant has its particular season of maximum responsiveness. The leaf-stalk of horse-chestnut, for example, exhibits fairly strong response in spring and summer, but on the approach of autumn it undergoes diminution. I give here a list of specimens which will be found to exhibit fairly good response: #Root.#--Carrot (_Daucus Carota_), radish (_Raphanus sativus_). #Stem.#--Geranium (_Pelargonium_), vine (_Vitis vinifera_). #Leaf-stalk.#--Horse-chestnut (_Æsculus Hippocastanum_), turnip (_Brassica Napus_), cauliflower (_Brassica oleracea_), celery (_Apium graveolens_), Eucharis lily (_Eucharis amazonica_). #Flower-stalk.#--Arum lily (_Richardia africana_). #Fruit.#--Egg-plant (_Solanum Melongena_). #Negative variation.#--Taking the leaf-stalk of turnip we kill an area on its surface, say B, by the application of a few drops of strong potash, the area at A being left uninjured. A current is now observed to flow, in the stalk, from the injured B to the uninjured A, as was found to be the case in the animal tissue. The potential difference depends on the condition of the plant, and the season in which it may have been gathered. In the experiment here described (fig. 6, _a_) its value was ·13 volt. [Illustration: FIG. 6.--(_a_) EXPERIMENT FOR EXHIBITING ELECTRIC RESPONSE IN PLANTS BY METHOD OF NEGATIVE VARIATION. (_b_) RESPONSES IN LEAF-STALK OF TURNIP TO STIMULI OF TWO SUCCESSIVE TAPS, THE SECOND BEING STRONGER. A and B contacts are about 2 cm. apart, B being injured. Plant is stimulated by a tap between A and B. Stimulus acts on both A and B, but owing to injury of B, effect at A is stronger and a negative variation due to differential action occurs.] A sharp tap was now given to the stalk, and a sudden diminution, or negative variation, of current occurred, the resting potential difference being decreased by ·026 volt. A second and stronger tap produced a second response, causing a greater diminution of P.D. by ·047 volt (fig. 6, _b_). The accompanying figure is a photographic record of another set of response-curves (fig. 7). The first three responses are for a given intensity of stimulus, and the next six in response to stimulus nearly twice as strong. It will be noticed that fatigue is exhibited in these responses. Other experiments will be described in the next chapter which show conclusively that the response was not due to any accidental circumstance but was a direct result of stimulation. But I shall first discuss the experimental arrangements and method of obtaining these graphic records. [Illustration: FIG. 7.--RECORD OF RESPONSES IN PLANT (LEAF-STALK OF CAULIFLOWER) BY METHOD OF NEGATIVE VARIATION The first three records are for stimulus intensity 1; the next six are for intensity twice as strong; the successive responses exhibit fatigue. The vertical line to the left represents ·1 volt. The record is to be read from right to left.] #Response recorder.#--The galvanometer used is a sensitive dead-beat D'Arsonval. The period of complete swing of the coil under experimental conditions is about 11 seconds. A current of 10^{-9} ampere produces a deflection of 1 mm. at a distance of 1 metre. For a quick and accurate method of obtaining the records, I devised the following form of response recorder. The curves are obtained directly, by tracing the excursion of the galvanometer spot of light on a revolving drum (fig. 8). The drum, on which is wrapped the paper for receiving the record, is driven by clockwork. Different speeds of revolution can be given to it by adjustment of the clock-governor, or by changing the size of the driving-wheel. The galvanometer spot is thrown down on the drum by the inclined mirror M. The galvanometer deflection takes place at right angles to the motion of the paper. A stylographic pen attached to a carrier rests on the writing surface. The carrier slides over a rod parallel to the drum. As has been said before, the galvanometer deflection takes place parallel to the drum, and as long as the plant rests unstimulated, the pen, remaining coincident with the stationary galvanometer spot on the revolving paper, describes a straight line. If, on stimulation, we trace the resulting excursion of the spot of light, by moving the carrier which holds the pen, the rising portion of the response-curve will be obtained. The galvanometer spot will then return more or less gradually to its original position, and that part of the curve which is traced during the process constitutes the recovery. The ordinate in these curves represents the E.M. variation, and the abscissa the time. [Illustration: FIG. 8.--RESPONSE RECORDER] We can calibrate the value of the deflection by applying a known E.M.F. to the circuit from a compensator, and noting the deflection which results. The speed of the clock is previously adjusted so that the recording surface moves exactly through, say, one inch a minute. Of course this speed can be increased to suit the particular experiment, and in some it is as high as six inches a minute. In this simple manner very accurate records may be made. It has the additional advantage that one is able at once to see whether the specimen is suitable for the purpose of investigation. A large number of records might be taken by this means in a comparatively short time. #Photographic recorder.#--Or the records may be made photographically. A clockwork arrangement moves a photographic plate at a known uniform rate, and a curve is traced on the plate by the moving spot of light. All the records that will be given are accurate reproductions of those obtained by one of these two methods. Photographic records are reproduced in white against a black background. #Compensator.#--As the responses are on _variation_ of current of injury, and as the current of injury may be strong, and throw the spot of light beyond the recording surface, a potentiometer balancing arrangement may be used (fig. 9), by which the P.D. due to injury is exactly compensated; E.M. variations produced by stimulus are then taken in the usual manner. This compensating arrangement is also helpful, as has been said before, for calibrating the E.M. value of the deflection. [Illustration: FIG. 9.--THE COMPENSATOR A B is a stretched wire with added resistances R and R'. S is a storage cell. When the key K is turned to the right one scale division = ·001 volt, when turned to the left one scale division = ·01 volt. P is the plant.] #Means of graduating the intensity of stimulus.#--One of the necessities in connection with quantitative measurements is to be certain that the intensity of successive stimuli is (1) constant, or (2) capable of gradual increase by known amounts. No two taps given by the hand can be made exactly alike. I have therefore devised the two following methods of stimulation, which have been found to act satisfactorily. [Illustration: FIG. 10.--THE SPRING-TAPPER] #The spring-tapper.#--This consists (fig. 10) of the spring proper (S), the attached rod (R) carrying at its end the tapping-head (T). A projecting rod--the lifter (L)--passes through S R. It is provided with a screw-thread, by means of which its length, projecting downwards, is regulated. This fact, as we shall see, is made to determine the height of the stroke. (C) is a cogwheel. As one of the spokes of the cogwheel is rotated past (L), the spring is lifted and released, and (T) delivers a sharp tap. The height of the lift, and therefore the intensity of the stroke, is measured by means of a graduated scale. We can increase the intensity of the stroke through a wide range (1) by increasing the projecting length of the lifter, and (2) by shortening the length of spring by a sliding catch. We may give isolated single taps or superpose a series in rapid succession according as the wheel is rotated slow or fast. The only disadvantage of the tapping method of stimulation is that in long-continued experiment the point struck is liable to be injured. The vibrational mode of stimulation to be presently described labours under no such disadvantage. #The electric tapper.#--Instead of the simple mechanical tapper, an electromagnetic tapper may be used. [Illustration: FIG. 11.--THE TORSIONAL VIBRATOR Plant P is securely held by a vice V. The two ends are clamped by holders C C'. By means of handles H H', torsional vibration may be imparted to either the end A or end B of the plant. The end view (_b_) shows how the amplitude of vibration is predetermined by means of movable stops S S'.] #Vibrational stimulus.#--I find that torsional vibration affords another very effective method of stimulation (fig. 11). The plant-stalk may be fixed in a vice (V), the free ends being held in tubes (C C'), provided with three clamping jaws. A rapid torsional vibration[9] may now be imparted to the stalk by means of the handle (H). The amplitude of vibration, which determines the intensity of stimulus, can be accurately measured by the graduated circle. The amplitude of vibration may be predetermined by means of the sliding stops (S S'). #Intensity of stimulus dependent on amplitude of vibration.#--I shall now describe an experiment which shows that torsional vibration is as effective as stimulation by taps, and that its stimulating intensity increases, length of stalk being constant, with amplitude of vibration. It is of course obvious that if the length of the specimen be doubled, the vibration, in order to produce the same effect, must be through twice the angle. I took a leaf-stalk of turnip and fixed it in the torsional vibrator. I then took record of responses to two successive taps, the intensity of one being nearly double that of the other. Having done this, I applied to the same stalk two successive torsional vibrations of 45° and 67° respectively. These successive responses to taps and torsional vibrations are given in fig. 12, and from them it will be seen that these two modes of stimulation may be used indifferently, with equal effect. The vibrational method has the advantage over tapping, that, while with the latter the stimulus is somewhat localised, with vibration the tissue subjected to stimulus is uniformly stimulated throughout its length. [Illustration: FIG. 12.--RESPONSE IN PLANT TO MECHANICAL TAP OR VIBRATION The end B is injured. A tap was given between A and B and this gave the response-curve _a_. A stronger tap gave the response _b_. By means of the handle H, a torsional vibration of 45° was now imparted, this gave the response _c_. Vibration through 67° gave _d_.] #Effectiveness of stimulus dependent on rapidity also.# In order that successive stimuli may be equally effective another point has to be borne in mind. In all cases of stimulation of living tissue it is found that the effectiveness of a stimulus to arouse response depends on the rapidity of the onset of the disturbance. It is thus found that the stimulus of the 'break' induction shock, on a muscle for example, is more effective, by reason of its greater rapidity, than the 'make' shock. So also with the torsional vibrations of plants, I find response depending on the quickness with which the vibration is effected. I give below records of successive stimuli, given by vibrations through the same amplitude, but delivered with increasing rapidity (fig. 13). [Illustration: FIG. 13.--INFLUENCE OF SUDDENNESS ON THE EFFICIENCY OF STIMULUS The curves _a_, _b_, _c_, _d_, are responses to vibrations of the same amplitude, 30°. In _a_ the vibration was very slow; in _b_ it was less slow; it was rapid in _c_, and very rapid in _d_.] Thus if we wish to maintain the effective intensity of stimulus constant we must meet two conditions: (1) The amplitude of vibration must be kept the same. This is done by means of the graduated circle. (2) The vibration period must be kept the same. With a little practice, this requirement is easily fulfilled. The uniformity of stimulation which is thus attained solves the great difficulty of obtaining reliable quantitative values, by whose means alone can rigorous demonstration of the phenomena we are studying become possible. FOOTNOTES: [8] A preliminary account of Electric Response in Plants was given at the end of my paper on 'Electric Response of Inorganic Substances' read before the Royal Society on June 6, 1901; also at the Friday Evening Discourse, Royal Institution, May 10, 1901. A more complete account is given in my paper on 'Electric Response in Ordinary Plants under Mechanical Stimulus' read before the Linnean Society March 20, 1902. I thank the Royal Society and the Linnean Society for permission to reproduce some of my diagrams published in their _Proceedings_.--J. C. B. [9] By this is meant a rapid to-and-fro or complete vibration. In order that successive responses should be uniform it is essential that there should be no resultant twist, i.e. the plant at the end of vibration should be in exactly the same condition as at the beginning. CHAPTER IV ELECTRIC RESPONSE IN PLANTS--BLOCK METHOD Method of block--Advantages of block method--Plant response a physiological phenomenon--Abolition of response by anæsthetics and poisons--Abolition of response when plant is killed by hot water. I shall now proceed to describe another and independent method which I devised for obtaining plant response. It has the advantage of offering us a complementary means of verifying the results found by the method of negative variation. As it is also, in itself, for reasons which will be shown later, a more perfect mode of inquiry, it enables us to investigate problems which would otherwise have been difficult to attempt. When electrolytic contacts are made on the uninjured surfaces of the stalk at A and B, the two points, being practically similar in every way, are iso-electric, and little or no current will flow in the galvanometer. If now the whole stalk be uniformly stimulated, and if both ends A and B be equally excited at the same moment, it is clear that there will still be no responsive current, owing to balancing action at the two ends. This difficulty as regards the obtaining of response was overcome in the method of negative variation, where the excitability of one end was depressed by chemical reagents or injury, or abolished by excessive temperature. On stimulating the stalk there was produced a greater excitation at A than at B, and a current of action was then observed to flow in the stalk from the more excited A to the less excited B (fig. 6). But we can cause this differential action to become evident by another means. For example, if we produce a block, by clamping at C between A and B (fig. 14, _a_), so that the disturbance made at A by tapping or vibration is prevented from reaching B, we shall then have A thrown into a relatively greater excitatory condition than B. It will now be found that a current of action flows in the stalk from A to B, that is to say, from the excited to the less excited. When the B end is stimulated, there will be a reverse current (fig. 14, _b_). [Illustration: FIG. 14.--THE METHOD OF BLOCK (_a_) The plant is clamped at C, between A and B. (_b_) Responses obtained by alternately stimulating the two ends. Stimulation of A produces upward response; of B gives downward response.] We have in this method a great advantage over that of negative variation, for we can always verify any set of results by making corroborative reversal experiments. By the method of injury again, one end is made initially abnormal, i.e. different from the condition which it maintains when intact. Further, inevitable changes will proceed unequally at the injured and uninjured ends, and the conditions of the experiment may thus undergo unknown variations. But by the block method which has just been described, there is no injury, the plant is normal throughout, and any physiological change (which in plants will be exceedingly small during the time of the experiment) will affect it as a whole. [Illustration: FIG. 15.--RESPONSE IN PLANT (FROM THE STIMULATED A TO UNSTIMULATED B) COMPLETELY IMMERSED UNDER WATER The leaf-stalk is clamped securely in the middle with the cork C, inside the tube T, which is filled with water, the plant being completely immersed. Moistened threads in connection with the two non-polarisable electrodes are led to the side tubes _t t'_. One end of the stalk is held in ebonite forceps and vibrated. A current of response is found to flow in the stalk from the excited A to the unexcited B, and outside, through the liquid, from B to A. A portion of this current, flowing through the side tubes _t t'_, produces deflection in the galvanometer.] #Plant response a physiological or vital response.#--I now proceed to a demonstration of the fact that whatever be the mechanism by which they are brought about, these plant responses are physiological in their character. As the investigations described in the next few chapters will show, they furnish an accurate index of physiological activity. For it will be found that, other things being equal, whatever tends to exalt or depress the vitality of the plant tends also to increase or diminish its electric response. These E.M. effects are well marked, and attain considerable value, rising sometimes, as has been said before, to as much as ·1 volt or more. They are proportional to the intensity of stimulus. It need hardly be added that special precautions are taken to avoid shifting of contacts. Variation of contact, however, could not in any case account for repeated transient responses to repeated stimuli, when contact is made on iso-electric surfaces. Nor could it in any way explain the reversible nature of these responses, when A and B are stimulated alternately. These responses are obtained in the plants even when completely immersed in water, as in the experimental arrangement (fig. 15). It will be seen that in this case, where there could be no possibility of shifting of contact, or variation of surface, there is still the usual current of response. I shall describe here a few crucial experiments only, in proof of the physiological character of electric response. The test applied by physiologists, in order to discriminate as to the physiological nature of response, consists in finding out whether the response is diminished or abolished by the action of anæsthetics, poisons, and excessively high temperature, which are known to depress or destroy vitality. I shall therefore apply these same tests to plant responses. #Effect of anæsthetics and poisons.#--Ordinary anæsthetics, like chloroform, and poisons, like mercuric chloride, are known to produce a profound depression or abolish all signs of response in the living tissue. For the purpose of experiment, I took two groups of stalks, with leaves attached, exactly similar to each other in every respect. In order that the leaf-stalks might absorb chloroform I dipped their cut ends in chloroform-water, a certain amount of which they absorbed, the process being helped by the transpiration from the leaves. The second group of stalks was placed simply in water, in order to serve for control experiment. The narcotic action of chloroform, finally culminating in death, soon became visually evident. The leaves began to droop, a peculiar death-discolouration began to spread from the mid rib along the venation of the leaves. Another peculiarity was also observed. The aphides feeding on the leaves died even before the appearance of the discoloured patches, whereas on the leaves of the stalks placed in water these little creatures maintained their accustomed activity, nor did any discolouration occur. In order to study the effect of poison, another set was placed in water containing a small quantity of mercuric chloride. The leaves here underwent the same change of appearance, and the aphides met with the same untimely fate, as in the case of those subjected to the action of chloroform. There was hardly any visible change in the appearance of the stalks themselves, which were to all outer seeming as living as ever, indications of death being apparent only on the leaf surfaces. I give below the results of several sets of experiments, from which it would appear that whereas there was strong normal response in the group of stalks kept in water, there was practically a total abolition of all response in those anæsthetised or poisoned. #Experiments on the effect of anæsthetics and poisons.# A batch of ten leaf-stalks of plane-tree was placed with the cut ends in water, and leaves in air; an equal number was immersed in chloroform-water; a third batch was placed in 5 per cent. solution of mercuric chloride. Similarly a batch of three horse-chestnut leaf-stalks was put in water, another batch in chloroform-water, and a third batch in mercuric chloride solution. I. LEAF-STALK OF PLANE-TREE The stimulus applied was a single vibration of 90°. A. After 24 hours in | B. After 24 hours in | C. After 24 hours in water | chloroform water | mercuric chloride | | [All leaves standing up | [Leaves began to | [Leaves began to droop and fresh--aphides | droop in 1 hour | in 4 hours. Deep alive] | and bent over in | discolouration along | 3 hours--aphides | the veins. Aphides | dead] | dead] | | Electric | Electric | Electric Response | response | response (1) 21 dns. | (1) 1 dn. | (1) 0 dn. (2) 31 " | (2) 1 " | (2) ·25 " (3) 26 " | (3) 2 " | (3) ·25 " (4) 15 " | (4) 0 " | (4) 0 " (5) 17 " | (5) 1 " | (5) ·25 " (6) 23 " | (6) 1·5 " | (6) ·25 " (7) 30 " | (7) 2 " | (7) 0 " (8) 27 " | (8) 1 " | (8) ·25 " (9) 29 " | (9) 1 " | (9) ·25 " (10) 17 " | (10) ·5 " | (10) ·5 " ------------------------+----------------------+----------------------- Mean response 23·6 | Mean 1 | Mean ·15 II. LEAF-STALK OF HORSE-CHESTNUT (1) 15 dns. | (1) ·5 dn. | (1) 0 dn. (2) 17 " | (2) ·5 " | (2) 0 " (3) 10 " | (3) 0 " | (3) 0 " ------------------------+----------------------+----------------------- Mean 14 | Mean ·3 | Mean 0 These results conclusively prove the physiological nature of the response. I shall in a succeeding chapter give a continuous series of response-curves showing how, owing to progressive death from the action of poison, the responses undergo steady diminution till they are completely abolished. #Effect of high temperature.#--It is well known that plants are killed when subjected to high temperatures. I took a stalk, and, using the block method, with torsional vibration as the stimulus, obtained strong responses at both ends A and B. I then immersed the same stalk for a short time in hot water at about 65° C., and again stimulated it as before. But at neither A nor B could any response now be evoked. As all the external conditions were the same in the first and second parts of this experiment, the only difference being that in one the stalk was alive and in the other killed, we have here further and conclusive proof of the physiological character of electric response in plants. The same facts may be demonstrated in a still more striking manner by first obtaining two similar but opposite responses in a fresh stalk, at A and B, and then killing one half, say B, by immersing only that half of the stalk in hot water. The stalk is replaced in the apparatus, and it is now found that whereas the A half gives strong response, the end B gives none. In the experiments on negative variation, it was tacitly assumed that the variation is due to a differential action, stimulus producing a greater excitation at the uninjured than at the injured end. The block method enables us to test the correctness of this assumption. The B end of the stalk is injured or killed by a few drops of strong potash, the other end being uninjured. There is a clamp between A and B. The end A is stimulated and a strong response is obtained. The end B is now stimulated, and there is little or no response. The block is now removed and the plant stimulated throughout its length. Though the stimulus now acts on both ends, yet, owing to the irresponsive condition of B, there is a resultant response, which from its direction is found to be due to the responsive action of A. This would not have been the case if the end B had been uninjured. We have thus experimentally verified the assumption that in the same tissue an uninjured portion will be thrown into a greater excitatory state than an injured, by the action of the same stimulus. CHAPTER V PLANT RESPONSE--ON THE EFFECTS OF SINGLE STIMULUS AND OF SUPERPOSED STIMULI Effect of single stimulus--Superposition of stimuli--Additive effect--Staircase effect--Fatigue--No fatigue when sufficient interval between stimuli--Apparent fatigue when stimulation frequency is increased--Fatigue under continuous stimulation. #Effect of single stimulus.#--In a muscle a single stimulus gives rise to a single twitch which may be recorded either mechanically or electrically. If there is no fatigue, the successive responses to uniform stimuli are exactly similar. Muscle when strongly stimulated often exhibits fatigue, and successive responses therefore become feebler and feebler. In nerves, however, there is practically no fatigue and successive records are alike. Similarly, in plants, we shall find some exhibiting marked fatigue and others very little. #Superposition of stimuli.#--If instead of a single stimulus a succession of stimuli be superposed, it happens that a second shock is received before recovery from the first has taken place. Individual effects will then become more or less fused. When the frequency is sufficiently increased, the intermittent effects are fused, and we find an almost unbroken curve. When for example the muscle attains its maximum contraction (corresponding to the frequency and strength of stimuli) it is thrown into a state of complete _tetanus_, in which it appears to be held rigid. If the rapidity be not sufficient for this, we have the jagged curve of incomplete tetanus. If there is not much fatigue, the upper part of the tetanic curve is approximately horizontal, but in cases where fatigue sets in quickly, the fact is shown by the rapid decline of the curve. With regard to all these points we find strict parallels in plant response. In cases where there is no fatigue, the successive responses are identical (fig. 16). With superposition of stimuli we have fusion of effects, analogous to the tetanus of muscle (fig. 17). And lastly, the influence of fatigue in plants is to produce a modification of response-curve exactly similar to that of muscle (see below). One effect of superposition of stimuli may be mentioned here. [Illustration: FIG. 16.--UNIFORM RESPONSES (RADISH)] [Illustration: FIG. 17.--FUSION OF EFFECT OF RAPIDLY SUCCEEDING STIMULI (_a_) in muscle; (_b_) in carrot.] #Additive effect.#--It is found in animal responses that there is a minimum intensity of stimulus, below which no response can be evoked. But even a sub-minimal stimulus will, though singly ineffective, become effective by the summation of several. In plants, too, we obtain a similar effect, i.e. the summation of single ineffective stimuli produces effective response (fig. 18). [Illustration: FIG. 18.--ADDITIVE EFFECT (_a_) A single stimulus of 3° vibration produced little or no effect, but the same stimulus when rapidly superposed thirty times, produced the large effect (_b_). (Leaf-stalk of turnip.)] #Staircase effect.#--Animal tissues sometimes exhibit what is known as the 'staircase effect,' that is to say, the heights of successive responses are gradually increased, though the stimuli are maintained constant. This is exhibited typically by cardiac muscle, though it is not unknown even in nerve. The cause is obscure, but it seems to depend on the condition of the tissue. It appears as if the molecular sluggishness of tissue were in these cases only gradually removed under stimulation, and the increased effects were due to increased molecular mobility. Whatever be the explanation, I have sometimes observed the same staircase effect in plants (fig. 19). [Illustration: FIG. 19.--'STAIRCASE EFFECT' IN PLANT] #Fatigue.#--It is assumed that in living substances like muscle, fatigue is caused by the break down or dissimilation of tissue by stimulus. And till this waste is repaired by the process of building-up or assimilation, the functional activity of the tissue will remain below par. There may also be an accumulation of the products of dissimilation--'the fatigue stuffs'--and these latter may act as poisons or chemical depressants. In an animal it is supposed that the nutritive blood supply performs the two-fold task of bringing material for assimilation and removing the fatigue products, thus causing the disappearance of fatigue. This explanation, however, is shown to be insufficient by the fact that an excised bloodless muscle recovers from fatigue after a short period of rest. It is obvious that here the fatigue has been removed by means other than that of renewed assimilation and removal of fatigue products by the circulating blood. It may therefore be instructive to study certain phases of fatigue exhibited under simpler conditions in vegetable tissue, where the constructive processes are in abeyance, and there is no active circulation for the removal of fatigue products. It has been said before that the E.M. variation caused by stimulus is the concomitant of a disturbance of the molecules of the responsive tissues from their normal equilibrium, and that the curve of recovery exhibits the restoration of the tissue to equilibrium. #No fatigue when sufficient interval between successive stimuli.#--We may thus gather from a study of the response-curve some indication of the molecular distortion experienced by the excited tissue. Let us first take the case of an experiment whose record is given in fig. 20, _a_. It will be seen from that curve that one minute after the application of stimulus there is a complete recovery of the tissue; the molecular condition is exactly the same at the end of recovery as in the beginning of stimulation. The second and succeeding response-curves therefore are exactly similar to the first, _provided a sufficient interval has been allowed in each case for complete recovery_. There is, in such a case, no diminution in intensity of response, that is to say, no fatigue. We have an exactly parallel case in muscles. _'In muscle with normal circulation and nutrition there is always an interval between each pair of stimuli, in which the height of twitch does not diminish even after protracted excitation, and no fatigue appears.'_[10] [Illustration: FIG. 20.--RECORD SHOWING DIMINUTION OF RESPONSE WHEN SUFFICIENT TIME IS NOT ALLOWED FOR FULL RECOVERY In (_a_) stimuli were applied at intervals of one minute; in (_b_) the intervals were reduced to half a minute; this caused a diminution of response. In (_c_) the original rhythm is restored, and the response is found to be enhanced. (Radish.)] #Apparent fatigue when stimulation frequency increased.#--If the rhythm of stimulation frequency be now changed, and made quicker, certain remarkable modifications will appear in the response-curves. In fig. 20, the first part shows the responses at one minute interval, by which time the individual recovery was complete. The rhythm was now changed to intervals of half a minute, instead of one, while the stimuli were maintained at the same intensity as before. It will be noticed (fig. 20, _b_) that these responses appear much feebler than the first set, in spite of the equality of stimulus. An inspection of the figure may perhaps throw some light on the subject. It will be seen that when greater frequency of stimulation was introduced, the tissue had not yet had time to effect complete recovery from previous strain. The molecular swing towards equilibrium had not yet abated, when the new stimulus, with its opposing impulse, was received. There is thus a diminution of height in the resultant response. The original rhythm of one minute was now restored, and the succeeding curves (fig. 20, _c_) at once show increased response. An analogous instance may be cited in the case of muscle response, where 'the height of twitch diminishes more rapidly in proportion as the excitation interval is shorter.'[11] [Illustration: FIG. 21.--FATIGUE IN CELERY Vibration of 30° at intervals of half a minute.] From what has just been said it would appear that one of the causes of diminution of response, or fatigue, is the residual strain. This is clearly seen in fig. 21, in a record which I obtained with celery-stalk. It will be noticed there that, owing to the imperfect molecular recovery during the time allowed, the succeeding heights of the responses have undergone a continuous diminution. Fig. 22 gives a photographic record of fatigue in the leaf-stalk of cauliflower. [Illustration: FIG. 22.--FATIGUE IN LEAF-STALK OF CAULIFLOWER Stimulus: 30° vibration at intervals of one minute.] It is evident that residual strain, other things being equal, will be greater if the stimuli have been excessive. This is well seen in fig. 23, where the set of first three curves A is for stimulus intensity of 45° vibration, and the second set B, with an augmented response, for stimulus intensity of 90° vibration. On reverting in C to stimulus intensity of 45°, the responses are seen to have undergone a great diminution as compared with the first set A. Here is seen marked fatigue, the result of overstrain from excessive stimulation. [Illustration: FIG. 23.--EFFECT OF OVERSTRAIN IN PRODUCING FATIGUE Successive stimuli applied at intervals of one minute. The intensity of stimulus in C is the same as that of A, but response is feebler owing to previous over-stimulation. Fatigue is to a great extent removed after fifteen minutes' rest, and the responses in D are stronger than those in C. The vertical line between arrows represents ·05 volt. (Turnip leaf-stalk.)] If this fatigue be really due to residual strain effect, then, as strain disappears with time, we may expect the responses to regain their former height after a period of rest. In order to verify this, therefore, I renewed the stimulation (at intensity 45°) after fifteen minutes. It will at once be seen from record D how far the fatigue had been removed. One peculiarity that will be noticed in these curves is that, owing to the presence of comparatively little residual strain, the first response of each set is relatively large. The succeeding responses are approximately equal where the residual strains are similar. The first response of A shows this because it had had long previous rest. The first of B shows it because we are there passing for the first time to increased stimulation. The first of C does _not_ show it, because there is now a strong residual strain. D again shows it because the strain has been removed by fifteen minutes' rest. #Fatigue under continuous stimulation.#--The effect of fatigue is exhibited in marked degree when a tissue is subjected to continuous stimulation. In cases where there is marked fatigue, as for instance in certain muscles, the top of the tetanic curve undergoes rapid decline. A similar effect is obtained also with plants (fig. 24). [Illustration: FIG. 24.--RAPID FATIGUE UNDER CONTINUOUS STIMULATION IN (_a_) MUSCLE; (_b_) IN LEAF-STALK OF CELERY] The effect of rest in producing molecular recovery, and hence in the removal of fatigue, is well illustrated in the following set of photographic records (fig. 25). The first shows the curve obtained with a fresh plant. The effect is seen to be very large. Two minutes were allowed for recovery, and then stimulation was repeated during another two minutes. The response in this case is seen to be decidedly smaller. A third case is somewhat similar to the second. A period of rest of five minutes was now allowed, and the curve obtained subsequently, owing to partial removal of residual strain, is found to exhibit greater response. [Illustration: FIG. 25.--EFFECT OF CONTINUOUS VIBRATION (THROUGH 50°) IN CARROT In the first three records, two minutes' stimulation is followed by two minutes' recovery. The last record was taken after the specimen had a rest of five minutes. The response, owing to removal of fatigue by rest, is stronger.] The results thus arrived at, under the simple conditions of vegetable life, free as they are from all possible complications and uncertainties, may perhaps throw some light on the obscure phenomena of fatigue in animal tissues. FOOTNOTES: [10] Biedermann, _Electro-physiology_, p. 86. [11] Biedermann, _loc. cit._ CHAPTER VI PLANT RESPONSE--ON DIPHASIC VARIATION Diphasic variation--Positive after-effect and positive response--Radial E.M. variation. When a plant is stimulated at any point, a molecular disturbance--the excitatory wave--is propagated outwards from the point of its initiation. #Diphasic variation.#--This wave of molecular disturbance is attended by a wave of electrical disturbance. (Usually speaking, the electrical relation between disturbed and less disturbed is that of copper to zinc.) It takes some time for a disturbance to travel from one point to another, and its intensity may undergo a diminution as it recedes further from its point of origin. Suppose a disturbance originated at C; if two points are taken near each other, as A and B, the disturbance will reach them almost at the same time, and with the same intensity. The electric disturbance will be the same in both. The effect produced at A and B will balance each other and there will be no resultant current. By killing or otherwise reducing the sensibility of B as is done in the method of injury, there is no response at B, and we obtain the unbalanced response, due to disturbance at A; the same effect is obtained by putting a clamp between A and B, so that the disturbance may not reach B. But we may get response even without injury or block. If we have the contacts at A and B, and if we give a tap _nearer_ A than B (fig. 26, _a_), then we have (1) the disturbance reaching A earlier than B. (2) The disturbance reaching A is much stronger than at B. The disturbance at B may be so comparatively feeble as to be negligible. It will thus be seen that we might obtain responses even without injury or block, in cases where the disturbance is enfeebled in reaching a distant point. In such a case on giving a tap near A a responsive current would be produced in one direction, and in the opposite direction when the tap is given near B (fig. 26, _b_). Theoretically, then, we might find a neutral point between A and B, so that, on originating the disturbance there, the waves of disturbance would reach A and B at the same instant and with the same intensity. If, further, the rate of recovery be the same for both points, then the electric disturbances produced at A and B will continue to balance each other, and the galvanometer will show no current. On taking a cylindrical root of radish I have sometimes succeeded in finding a neutral point, which, being disturbed, did not give rise to any resultant current. But disturbing a point to the right or to the left gave rise to opposite currents. It is, however, difficult to obtain an absolutely cylindrical specimen, as it always tapers in one direction. The conductivity towards the tip of the root is not exactly the same as that in the ascending direction. It is therefore difficult to fix an absolutely neutral point, but a point may be found which approaches this very nearly, and on stimulating the stalk near this, a very interesting diphasic variation has been observed. In a specimen of cauliflower-stalk, (1) stimulus was applied very much nearer A than B (the feeble disturbance reaching B was negligible). The resulting response was upward and the recovery took place in about sixty seconds. [Illustration: FIG. 26.--DIPHASIC VARIATION] (2) Stimulus was next applied near B. The resulting response was now downward (fig. 26, _b_). (3) The stimulus was now applied near the approximately neutral point N. In this case, owing to a slight difference in the rates of propagation in the two directions, a very interesting diphasic variation was produced (fig. 26, _c_). From the record it will be seen that the disturbance arrived earlier at A than at B. This produced an upward response. But during the subsidence of the disturbance at A, the wave reached B. The effect of this was to produce a current in the opposite direction. This apparently hastened the recovery of A (from 60 seconds to 12 seconds). The excitation of A now disappeared, and the second phase of response, that due to excitation of B, was fully displayed. #Positive after-effect.#--If we regard the response due to excitation of A as negative, the later effect on B would appear as a subsequent positive variation. In the response of nerve, for example, where contacts are made at two surfaces, injured and uninjured, there is sometimes observed, first a negative variation, and then a positive after-effect. This may sometimes at least be due to the proximal uninjured contact first giving the usual negative variation, and the more distant contact of injury giving rise, later, to the opposite, that is to say, apparently positive, response. There is always a chance of an after-effect due to this cause, unless (1) the injured end be completely killed and rendered quite irresponsive, or (2) there be an effective block between A and B, so that the disturbance due to stimulus can only act on one, and not on the other. I have found cases where, even when there was a perfect block, a positive after-effect occurred. It would thus appear that if molecular distortion from stimulus give rise to a negative variation, then during the process of molecular recovery there may be over-shooting of the equilibrium position, which may be exhibited as a positive variation. #Positive variation.#--The responses given by muscle or nerve are, normally speaking, negative. But that of retina is positive. The sign of response, however, is apt to be reversed if there be any molecular modification of the tissue from changes of external circumstances. Thus it is often found that nerve in a stale condition gives positive, instead of the normal negative variation, and stale retina often gives negative, instead of the usual positive. [Illustration: FIG. 27.--ABNORMAL POSITIVE RESPONSES IN STALE LEAF-STALK OF TURNIP CONVERTED INTO NORMAL NEGATIVE UNDER STRONG STIMULATION[12] The relative intensities of stimuli in the two cases are in the ratio of 1:7.] Curiously enough, I have on many occasions found exactly parallel instances in the response of plants. Plants when fresh, as stated, give negative responses as a rule. But when somewhat faded they sometimes give rise to positive response. Again, just as in the modified nerve the abnormal positive response gives place to the normal negative under strong and long-continued stimulation, so also in the modified plant the abnormal positive response passes into negative (fig. 27) under strong stimulation. I was able in some cases to trace this process of gradual reversal, by continuously increasing the intensity of stimulus. It was then found that as the stimulus was increased, the positive at a certain point underwent a reversal into the normal negative response (fig. 28). [Illustration: FIG. 28. ABNORMAL POSITIVE PASSING INTO NORMAL NEGATIVE IN A STALE SPECIMEN OF LEAF-STALK OF CAULIFLOWER Stimulus was gradually increased from 1 to 10, by means of spring-tapper. When the stimulus intensity was 10, the response became reversed into normal negative. (Parts of 8 and 9 are out of the plate.)] The plant thus gives a reversed response under abnormal conditions of staleness. I have sometimes found similar reversal of response when the plant is subjected to the abnormal conditions of excessively high or low temperature. #Radial E.M. variation.#--We have seen that a current of response flows in the plant from the relatively more to the relatively less excited. A theoretically important experiment is the following: A thick stem of plant stalk was taken and a hole bored so as to make one contact with the interior of the tissue, the other being on the surface. After a while the current of injury was found to disappear. On exciting the stem by taps or torsional vibration, a responsive current was observed which flowed inwards from the more disturbed outer surface to the shielded core inside (fig. 29). Hence it is seen that when a wave of disturbance is propagated along the plant, there is a concomitant wave of radial E.M. variation. The swaying of a tree by the wind would thus appear to give rise to a radial E.M.F. [Illustration: FIG. 29.--RADIAL E.M. VARIATION] FOOTNOTES: [12] For general purposes it is immaterial whether the responses are recorded up or down. For convenience of inspection they are in general recorded _up_. But in cases where it is necessary to discriminate the sign of response, positive response will be recorded up, and negative down. CHAPTER VII PLANT RESPONSE--ON THE RELATION BETWEEN STIMULUS AND RESPONSE Increased response with increasing stimulus--Apparent diminution of response with excessively strong stimulus. As already said, in the living tissue, molecular disturbance induced by stimulus is accompanied by an electric disturbance, which gradually disappears with the return of the disturbed molecules to their position of equilibrium. The greater the molecular distortion produced by the stimulus, the greater is the electric variation produced. The electric response is thus an outward expression of a molecular disturbance produced by an external agency, the stimulus. #Curve of relation between stimulus and response.#--In the curve showing the relation between stimulus and response in nerve and muscle, it is found that the molecular effect as exhibited either by contraction or E.M. variation in muscle, or simply by E.M. variation in nerve, is at first slight. In the second part, there is a rapidly increasing effect with increased stimulus. Finally, a tendency shows itself to approach a limit of response. Thus we find the curve at first slightly convex, then straight and ascending, and lastly, concave to the abscissa (fig. 30). In muscle the limit of response is reached much sooner than in nerve. As will be seen, the range of variation of stimulus in these curves is not very great. When the stimulus is carried beyond moderate limits, the response, owing to fatigue or other causes, may sometimes undergo an actual diminution. [Illustration: FIG. 30.--CURVES SHOWING THE RELATION BETWEEN THE INTENSITY OF STIMULUS AND RESPONSE Abscissæ indicate increasing intensity of stimulus. Ordinates indicate magnitude of response. (Waller.)] I have obtained very interesting results, with reference to the relation between stimulus and response, when experimenting with plants. These results are suggestive of various types of response met with in animal tissues. 1. In order to obtain the simplest type of effects, not complicated by secondary phenomena, one has to choose specimens which exhibit little fatigue. Having procured these, I undertook two series of experiments. In the first (_A_) the stimulus was applied by means of the spring-tapper, and in the second (_B_) by torsional vibration. [Illustration: FIG. 31 Taps of increasing strength 1:2:3:4 producing increased response in leaf stalk of turnip.] (_A_) The first stimulus was given by a fall of the lever through _h_, the second through 2 _h_, and so on. The response-curves clearly show increasing effect with increased stimulus (fig. 31). [Illustration: FIG. 32.--INCREASED RESPONSE WITH INCREASING VIBRATIONAL STIMULI (CAULIFLOWER-STALK) Stimuli applied at intervals of three minutes. Vertical line = ·1 volt.] (_B_) 1. The vibrational stimulus was increased from 2·5° to 5° to 7·5° to 10° to 12·5° in amplitude. It will be observed how the intensity of response tends to approach a limit (fig. 32). TABLE SHOWING THE INCREASED E.M. VARIATION PRODUCED BY INCREASING STIMULUS +----------------------------------------+ | Angle of Vibration | E.M.F | +----------------------------------------+ | 2·5° | ·044 volt | | 5° | ·075 volt | | 7·5° | ·090 volt | | 10° | ·100 volt | | 12·5° | ·106 volt | +----------------------+-----------------+ 2. The next figure shows how little variation is produced with low value of stimulus, but with increasing stimulus the response undergoes a rapid increase, after which it tends to approach a limit (fig. 33, _a_). [Illustration: FIG. 33.--RESPONSES TO INCREASING STIMULI PRODUCED BY INCREASING ANGLE OF VIBRATION (_a_) Record with a specimen of fresh radish. Stimuli applied at intervals of two minutes. The record is taken for one minute. (_b_) Record for stale radish. There is a reversed response for the feeble stimulus of 5° vibration.] 3. As an extreme instance of the case just cited, I have often come across a curious phenomenon. During the gradual increase of the stimulus from a low value there would be apparently no response. But when a critical value was reached a maximum response would suddenly occur, and would not be exceeded when the stimulus was further increased. Here we have a parallel to what is known in animal physiology as the 'all or none' principle. With the cardiac muscle, for example, there is a certain minimal intensity which is effective in producing response, but further increase of stimulus produces no increase in response. 4. From an inspection of the records of responses which are given, it will be seen that the slope of a curve which shows the relation of stimulus to response will at first be slight, the curve will then ascend rapidly, and at high values of stimulus tend to become horizontal. The curve as a whole becomes, first slightly convex to the abscissa, then straight and ascending, and lastly concave. A far more pronounced convexity in the first part is shown in some cases, especially when the specimen is stale. This is due to the fact that under these circumstances response is apt to begin with an actual reversal of sign, the plant under feebler than a certain critical intensity of stimulus giving positive, instead of the normal negative, response (fig. 33, _b_). #Diminution of response with excessively strong stimulus.#--It is found that in animal tissues there is sometimes an actual diminution of response with excessive increase of stimulus. Thus Waller finds, in working with retina, that as the intensity of light stimulus is gradually increased, the response at first increases, and then sometimes undergoes a diminution. This phenomenon is unfortunately complicated by fatigue, itself regarded as obscure. It is therefore difficult to say whether the diminution of response is due to fatigue or to some reversing action of an excessively strong stimulus. From fig. 33, _b_, above, it is seen that there was an actual reversal of response in the lower portion of the curve. It is therefore not improbable that there may be more than one point of reversal. In physical phenomena we are, however, acquainted with numerous instances of reversals. For example, a common effect of magnetisation is to produce an elongation of an iron rod. But Bidwell finds that as the magnetising force is pushed to an extreme, at a certain point elongation ceases and is succeeded, with further increase of magnetising force, by an actual contraction. Again a photographic plate, when exposed continuously to light, gives at first a negative image. Still longer exposure produces a positive. Then again we have a negative. There is thus produced a series of recurrent reversals. In photographic prints of flashes of lightning, two kinds of images are observed, one, the positive--when the lightning discharge is moderately intense--and the other, negative, the so-called 'dark lightning'--due to the reversal action of an intensely strong discharge. In studying the changes of conductivity produced in metallic particles by the stimulus of Hertzian radiation, I have often noticed that whereas feeble radiation produces one effect, strong radiation produces the opposite. Again, under the continuous action of electric radiation, I have frequently found recurrent reversals.[13] #Diminution of response under strong stimulus traced to fatigue.#--But there are instances in plant response where the diminution effect can be definitely traced to fatigue. The records of these cases are extremely suggestive as to the manner in which the diminution is brought about. The accompanying figures (fig. 34) give records of responses to increasing stimulus. They were made with specimens of cauliflower-stalks, one of which (_a_) showed little fatigue, while in the other (_b_) fatigue was present. It will be seen that the curves obtained by joining the apices of the successive single responses are very similar. [Illustration: FIG. 34.--RESPONSES TO INCREASING STIMULUS OBTAINED WITH TWO SPECIMENS OF STALK OF CAULIFLOWER In (_a_) fatigue is absent, in (_b_) it is present.] In one case there is no fatigue, the recovery from each stimulus being complete. Every response in the series therefore starts from a position of perfect equilibrium, and the height of the single responses increases with increasing stimulation. But in the second case, the strain is not completely removed after any single stimulation of the series. That recovery is partial is seen by the gradual shifting of the base line upwards. In the former case the base line is horizontal and represents a condition of complete equilibrium. Now, however, the base line, or line of modified equilibrium, is tilted upwards. Thus even in this case if we measure the heights of successive responses from the line of absolute equilibrium, they will be found to increase with increasing stimulus. Ordinarily, however, we make no allowance for the shifting of the base line, measuring response rather from the place of its previous recovery, or from the point of modified equilibrium. Judged in this way, the responses undergo an apparent diminution. FOOTNOTES: [13] See 'On Electric Touch,' _Proc. Roy. Soc._ Aug. 1900. CHAPTER VIII PLANT RESPONSE--ON THE INFLUENCE OF TEMPERATURE Effect of very low temperature--Influence of high temperature--Determination of death-point--Increased response as after-effect of temperature variation--Death of plant and abolition of response by the action of steam. For every plant there is a range of temperature most favourable to its vital activity. Above this optimum, the vital activity diminishes, till a maximum is reached, when it ceases altogether, and if this point be maintained for a long time the plant is apt to be killed. Similarly, the vital activity is diminished if the temperature be lowered below the optimum, and again, at a minimum point it ceases, while below this minimum the plant may be killed. We may regard these maximum and minimum temperatures as the death-points. Some plants can resist these extremes better than others. Length of exposure, it should however be remembered, is also a determining factor in the question as to whether or not the plant shall survive unfavourable conditions of temperature. Thus we have hardy plants, and plants that are affected by excessive variations of temperature. Within the characteristic power of the species, there may be, again, a certain amount of individual difference. These facts being known, I was anxious to determine whether the undoubted changes induced by temperature in the vital activity of plants would affect electrical response. #Effect of very low temperature.#--As regards the influence of very low temperature, I had opportunities of studying the question on the sudden appearance of frost. In the previous week, when the temperature was about 10° C., I had obtained strong electric response in radishes whose value varied from ·05 to ·1 volt. But two or three days later, as the effect of the frost, I found electric response to have practically disappeared. A few radishes were, however, found somewhat resistant, but the electric response had, even in these cases, fallen from the average value of ·075 V. under normal temperature to ·003 V. after the frost. That is to say, the average sensitiveness had been reduced to about 1/25th. On warming the frost-bitten radish to 20° C. there was an appreciable revival, as shown by increase in response. In specimens where the effect of frost had been very great, i.e. in those which showed little or no electric response, warming did not restore responsiveness. From this it would appear that frost killed some, which could not be subsequently revived, whereas others were only reduced to a condition of torpidity, from which there was revival on warming. I now tried the effect of artificial lowering of temperature on various plants. A plant which is very easily affected by cold is a certain species of Eucharis lily. I first obtained responses with the leaf-stalk of this lily at the ordinary temperature of the room (17° C.). I then placed it for fifteen minutes in a cooling chamber, temperature -2° C., for only ten minutes, after which, on trying to obtain response, it was found to have practically disappeared. I now warmed the plant by immersing it for awhile in water at 20° C., and this produced a revival of the response (fig. 35). If the plant be subjected to low temperature for too long a time, there is then no subsequent revival. [Illustration: FIG. 35.--DIMINUTION OF RESPONSE IN EUCHARIS BY LOWERING OF TEMPERATURE (_a_) Normal response at 17° C. (_b_) The response almost disappears when plant is subjected to -2° C. for fifteen minutes. (_c_) Revival of response on warming to 20° C.] I obtained a similar marked diminution of response with the flower-stalk of Arum lily, on lowering the temperature to zero. My next attempt was to compare the sensibility of different plants to the effect of lowered temperatures. For this purpose I chose three specimens: (1) Eucharis lily; (2) Ivy; and (3) Holly. I took their normal response at 17° C., and found that, generally speaking, they attained a fairly constant value after the third or fourth response. After taking these records of normal response, I placed the specimens in an ice-chamber, temperature 0° C., for twenty-four hours, and afterwards took their records once more at the ordinary temperature of the room. From these it will be seen that while the responsiveness of Eucharis lily, known to be susceptible to the effect of cold, had entirely disappeared, that of the hardier plants, Holly and Ivy, showed very little change (fig. 36). Another very curious effect that I have noticed is that when a plant approaches its death-point by reason of excessively high or low temperature, not only is its general responsiveness diminished almost to zero, but even the slight response occasionally becomes reversed. [Illustration: FIG. 36.--AFTER-EFFECT OF COLD ON IVY, HOLLY, AND EUCHARIS LILY _a._ The normal response; _b._ Response after subjection to freezing temperature for twenty-four hours.] #Influence of high temperature, and determination of death-point.#--I next tried to find out whether a rise of temperature produced a depression of response, and whether the response disappeared at a maximum temperature--the temperature of death-point. For this purpose I took a batch of six radishes and obtained from them responses at gradually increasing temperatures. These specimens were obtained late in the season, and their electric responsiveness was much lower than those obtained earlier. The plant, previously kept for five minutes in water at a definite temperature (say 17° C.), was mounted in the vibration apparatus and responses observed. The plant was then dismounted, and replaced in the water-bath at a higher temperature (say 30° C.) again, for five minutes. A second set of responses was now taken. In this way observations were made with each specimen till the temperature at which response almost or altogether ceased was reached. I give below a table of results obtained with six specimens of radish, from which it would appear that response begins to be abolished in these cases at temperatures varying from 53° to 55° C. TABLE SHOWING EFFECT OF HIGH TEMPERATURE IN ABOLISHING RESPONSE Temperature Galvanometric response (100 dns. = ·07 V.) (1) {17° C 70 dns. {53° " 4 " (2) {17° " 160 " {53° " 1 " (3) {17° " 100 " {50° " 2 " (4) {17° " 80 " {55° " 0 " (5) {17° " 40 " {60° " 0 " (6) {17° " 60 " {55° " 0 " [Illustration: FIG. 37.--THE GLASS CHAMBER CONTAINING THE PLANT Amplitude of vibration which determines the intensity of stimulus is measured by the graduated circle seen to the right. Temperature is regulated by the electric heating coil R. For experiments on action of anæsthetics, vapour of chloroform is blown in through the side tube.] #Electric heating.#--The experiments just described were, however, rather troublesome, inasmuch as, in order to produce each variation of temperature, the specimen had to be taken out of the apparatus, warmed, and remounted. I therefore introduced a modification by which this difficulty was obviated. The specimen was now enclosed in a glass chamber (fig. 37), which also contained a spiral of German-silver wire, through which electric currents could be sent, for the purpose of heating the chamber. By varying the intensity of the current, the temperature could be regulated at will. The specimen chosen for experiment was the leaf-stalk of celery. It was kept at each given temperature for ten minutes, and two records were taken during that time. It was then raised by 10° C., and the same process was repeated. It will be noticed from the record (fig. 38) that in this particular case, as the temperature rose from 20° C. to 30° C., there was a marked diminution of response. At the same time, in this case at least, recovery was quicker. At 20° C., for example, the response was 21 dns., and the recovery was not complete in the course of a minute. At 30° C., however, the response had been reduced to 7·5 divisions, but there was almost complete recovery in twelve seconds. As the temperature was gradually increased, a continuous decrease of response occurred. This diminution of response with increased temperature appears to be universal, but the quickening of recovery may be true of individual cases only. [Illustration: FIG. 38.--EFFECT OF TEMPERATURE ON RESPONSE The response was abolished at the hot-water temperature of 55° C.] TABLE SHOWING DIMINUTION OF RESPONSE WITH INCREASING TEMPERATURE (·01 Volt = 35 divisions) Temperature Response 20° 21 30° 7·5 40° 5·5 50° 4 65° 3 In radishes response disappeared completely at 55° C., but with celery, heated in the manner described, I could not obtain its entire abolition at 60° C. or even higher. A noticeable circumstance, however, was the prolongation of the period of recovery at these high temperatures. I soon understood the reason of this apparent anomaly. The method adopted in the present case was that of dry heating, whereas the previous experiments had been carried on by the use of hot water. It is well known that one can stand a temperature of 100° C. without ill effects in the hot-air chamber of a Turkish bath, while immersion in water at 100° C. would be fatal. In order to find out whether subjection to hot water would kill the celery-stalk, I took it out and placed it for five minutes in water at 55° C. This, as will be seen from the record taken afterwards, effectively killed the plant (fig. 38, w). [Illustration: FIG. 39.--EFFECT OF RISING AND FALLING TEMPERATURE ON THE RESPONSE OF SCOTCH KALE] #Increased sensitiveness as after-effect of temperature variation.#--A very curious effect of temperature variation is the marked increase of sensitiveness which often appears as its after-effect. I noticed this first in a series of observations where records were taken during the rise of temperature and continued while the temperature was falling (fig. 39). The temperature was adjusted by electric heating. It was found that the responses were markedly enhanced during cooling, as compared with responses given at the same temperatures while warming (see table). Temperature variation thus seems to have a stimulating effect on response, by increasing molecular mobility in some way. The second record (fig. 40) shows the variation of response in Eucharis lily (1) during the rise, and (2) during the fall of temperature. Fig. 41 gives a curve of variation of response during the rise and fall of temperature. TABLE SHOWING THE VARIATION OF RESPONSE IN SCOTCH KALE DURING THE RISE AND FALL OF TEMPERATURE Temperature Response Response [Temperature rising] [Temperature falling] 19° C. 47 dns. -- 25° " 24 " -- ^ 30° " ¦ 11 " 23 dns. ¦ 50° " ¦ 8 " 16 " ¦ 70° " v 7 " -- -----> [Illustration: FIG. 40.--RECORDS OF RESPONSES IN EUCHARIS LILY DURING RISE AND FALL OF TEMPERATURE Stimulus constant, applied at intervals of one minute. The temperature of plant-chamber gradually rose on starting current in the heating coil; on breaking current, the temperature fell gradually. Temperature corresponding to each record is given below. Temperature rising: (1) 20°, (2) 20°, (3) 22°, (4) 38°, (5) 53°, (6) 68°, (7) 65°. Temperature falling: (8) 60°, (9) 51°, (10) 45°, (11) 40°, (12) 38°.] #Point of temperature maximum.#--We have seen how, in cases of lowered temperature, response is abolished earlier in plants like Eucharis, which are affected by cold, than in the hardier plants such as Holly and Ivy. Plants again are unequally affected as regards the upper range. In the case of Scotch kale, for instance, response disappears after ten minutes of water temperature of about 55° C., but with Eucharis fairly marked response can still be obtained after such immersion and does not disappear till it has been subjected for ten minutes to hot water, at a temperature of 65° C. or even higher. The reason of this great power of resistance to heat is probably found in the fact that the Eucharis is a tropical plant, and is grown, in this country, in hot-houses where a comparatively high temperature is maintained. [Illustration: FIG. 41.--CURVE SHOWING VARIATION OF RESPONSE IN EUCHARIS WITH THE RISE AND FALL OF TEMPERATURE] #The effect of steam.#--I next wished to obtain a continuous record by which the effects of suddenly increased temperatures, culminating in the death of the plant, might be made evident. For this purpose I mounted the plant in the glass chamber, into which steam could be introduced. I had chosen a specimen which gave regular response. On the introduction of steam, with the consequent sudden increase of temperature, there was a transitory augmentation of excitability. But this quickly disappeared, and in five minutes the plant was effectively killed, as will be seen graphically illustrated in the record (fig. 42). [Illustration: FIG. 42.--EFFECT OF STEAM IN KILLING RESPONSE The two records to the left exhibit normal response at 17° C. Sudden warming by steam produced at first an increase of response, but five minutes exposure to steam killed the plant (carrot) and abolished the response. Vibrational stimulus of 30° applied at intervals of one minute; vertical line = ·1 volt.] It will thus be seen that those modifications of vital activity which are produced in plants by temperature variation can be very accurately gauged by electric response. Indeed it may be said that there is no other method by which the moment of cessation of vitality can be so satisfactorily distinguished. Ordinarily, we are able to judge that a plant has died, only after various indirect effects of death, such as withering, have begun to appear. But in the electric response we have an immediate indication of the arrest of vitality, and we are thereby enabled to determine the death-point, which it is impossible to do by any other means. It may be mentioned here that the explanation suggested by Kunkel, of the response being due to movement of water in the plant, is inadequate. For in that case we should expect a definite stimulation to be under all conditions followed by a definite electric response, whose intensity and sign should remain invariable. But we find, instead, the response to be profoundly modified by any influence which affects the vitality of the plant. For instance, the response is at its maximum at an optimum temperature, a rise of a few degrees producing a profound depression; the response disappears at the maximum and minimum temperatures, and is revived when brought back to the optimum. Anæsthetics and poisons abolish the response. Again, we have the response undergoing an actual reversal when the tissue is stale. All these facts show that mere movement of water could not be the effective cause of plant response. CHAPTER IX PLANT RESPONSE--EFFECT OF ANÆSTHETICS AND POISONS Effect of anæsthetics, a test of vital character of response--Effect of chloroform--Effect of chloral--Effect of formalin--Method in which response is unaffected by variation of resistance--Advantage of block method--Effect of dose. The most important test by which vital phenomena are distinguished is the influence on response of narcotics and poisons. For example, a nerve when narcotised by chloroform exhibits a diminishing response as the action of the anæsthetic proceeds. (See below, fig. 43.) Similarly, various poisons have the effect of permanently abolishing all response. Thus a nerve is killed by strong alkalis and strong acids. I have already shown how plants which previously gave strong response did not, after application of an anæsthetic or poison, give any response at all. In these cases it was the last stage only that could be observed. But it appeared important to be able to trace the growing effect of anæsthetisation or poisoning throughout the process. There were, however, two conditions which it at first appeared difficult to meet. First it was necessary to find a specimen which would normally exhibit no fatigue, and give rise for a long time to a uniform series of response. The immediate changes made in the response, in consequence of the application of chemical reagents, could then be demonstrated in a striking manner. And with a little trouble, specimens can be secured in which perfect regularity of response is found. The record given in fig. 16, obtained with a specimen of radish, shows how possible it is to secure plants in which response is absolutely regular. I subjected this to uniform stimulation at intervals of one minute, during half an hour, without detecting the least variation in the responses. But it is of course easier to find others in which the responses as a whole may be taken as regular, though there may be slight rhythmic fluctuations. And even in these cases the effect of reagents is too marked and sudden to escape notice. [Illustration: FIG. 43.--EFFECT OF CHLOROFORM ON NERVE RESPONSE (WALLER)] For the obtaining of constant and strong response I found the best materials to be carrot and radish, selected individuals from which gave most satisfactory results. The carrots were at their best in August and September, after which their sensitiveness rapidly declined. Later, being obliged to seek for other specimens, I came upon radish, which gave good results in the early part of November; but the setting-in of the frost had a prejudicial effect on its responsiveness. Less perfect than these, but still serviceable, are the leaf-stalks of turnip and cauliflower. In these the successive responses as a whole may be regarded as regular, though a curious alternation is sometimes noticed, which, however, has a regularity of its own. My second misgiving was as to whether the action of reagents would be sufficiently rapid to display itself within the time limit of a photographic record. This would of course depend in turn upon the rapidity with which the tissues of the plant could absorb the reagent and be affected by it. It was a surprise to me to find that, with good specimens, the effect was manifested in the course of so short a time as a minute or so. #Effect of chloroform.#--In studying the effect of chemical reagents in plants, the method is precisely similar to that employed with nerve; that is to say, where vapour of chloroform is used, it is blown into the plant chamber. In cases of liquid reagents, they are applied on the points of contact A and B and their close neighbourhood. The mode of experiment was (1) to obtain a series of normal responses to uniform stimuli, applied at regular intervals of time, say one minute, the record being taken the while on a photographic plate. (2) Without interrupting this procedure, the anæsthetic agent, vapour of chloroform, was blown into the closed chamber containing the plant. It will be seen how rapidly chloroform produces depression of response (fig. 44), and how the effect grows with time. In these experiments with plants, the same curious shifting of the zero line is sometimes noticed as in nerve when subjected similarly to the action of reagents. This is a point of minor importance, the essential point to be noticed being that the responses are rapidly reduced. [Illustration: FIG. 44.--EFFECT OF CHLOROFORM ON RESPONSES OF CARROT Stimuli of 25° vibration at intervals of one minute.] #Effects of chloral and formalin.#--I give below (figs. 45, 46) two sets of records, one for the reagent chloral and the other for formalin. The reagents were applied in the form of a solution on the tissue at the two leading contacts, and the contiguous surface. The rhythmic fluctuation in the normal response shown in fig. 45 is interesting. The abrupt decline, within a minute of the application of chloral, is also extremely well marked. [Illustration: FIG. 45.--ACTION OF CHLORAL HYDRATE ON THE RESPONSES OF LEAF-STALK OF CAULIFLOWER Vibration of 25° at intervals of one minute.] [Illustration: FIG. 46.--ACTION OF FORMALIN (RADISH)] #Response unaffected by variation of resistance.#--In order to bring out clearly the main phenomena, I have postponed till now the consideration of a point of some difficulty. To determine the influence of a reagent in modifying the excitability of the tissue, we rely upon its effect in exalting or depressing the responsive E.M. variation. We read this effect by means of galvanometric deflections. And if the resistance of the circuit remained constant, then an increase of galvanometer deflection would accurately indicate a heightened or depressed E.M. response, due to greater or less excitability of tissue caused by the reagent. But, by the introduction of the chemical reagent, the resistance of the tissue may undergo change, and owing to this cause, modification of response as read by the galvanometer may be produced without any E.M. variation. The observed variation of response may thus be partly owing to some unknown change of resistance, as well as to that of the E.M. variation in response to stimulus. We may however discriminate as to how much of the observed change is due to variation of resistance by comparing the deflections produced in the galvanometer by the action of a definite small E.M.F. before and after the introduction of the reagent. If the deflections be the same in both cases, we know that the resistance has not varied. If there have been any change, the variation of deflection will show the amount, and we can make allowance accordingly. I have however adopted another method, by which all necessity of correction is obviated, and the galvanometric deflections simply give E.M. variations, unaffected by any change in the resistance of the tissue. This is done by interposing a very large and constant resistance in the external circuit and thereby making other resistances negligible. An example will make this point clear. Taking a carrot as the vegetable tissue, I found its resistance plus the resistance of the non-polarisable electrode equal to 20,000 ohms. The introduction of a chemical reagent reduced it to 19,000 ohms. The resistance of the galvanometer is equal to 1,000 ohms. The high external resistance was 1,000,000 ohms. The variation of resistance produced in the circuit would therefore be 1,000 in (1,000,000+19,000+1,000) or one part in 1,020. Therefore the variation of galvanometric deflection due to change of resistance would be less than one part in a thousand (cf. fig. 49). #The advantage of the block method.#--In these investigations I have used the block method, instead of that of negative variation, and I may here draw attention to the advantages which it offers. In the method of negative variation, one contact being injured, the chemical reagents act on injured and uninjured unequally, and it is conceivable that by this unequal action the resting difference of potential may be altered. But the intensity of response in the method of injury depends on this resting difference. It is thus hypothetically possible that on the method of negative variation there might be changes in the responses caused by variation of the resting difference, and not necessarily due to the stimulating or depressing effect of the reagent on the tissue. But by the block method the two contacts are made with uninjured surfaces, and the effect of reagents on both is similar. Thus no advantage is given to one contact over the other. The changes now detected in response are therefore due to no adventitious circumstance, but to the reagent itself. If further verification be desired as to the effect of the reagent, we can obtain it by alternate stimulation of the A and B ends. Both ends will then show the given change. I give below a record of responses given by two ends of leaf-stalk of turnip, stimulated alternately in the manner described. The stalk used was slightly conical, and owing to this difference between the A and B ends the responses given by one end were slightly different from those given by the other, though the stimuli were equal. A few drops of 10 per cent. solution of NaOH was applied to both the ends. It will be seen how quickly this reagent abolished the response of both ends (fig. 47). [Illustration: FIG. 47.--ABOLITION OF RESPONSE AT BOTH A AND B ENDS BY THE ACTION OF NaOH Stimuli of 30° vibration were applied at intervals of one minute to A and B alternately. Response was completely abolished twenty-four minutes after application of NaOH.] #Effect of dose.#--It is sometimes found that while a reagent acts as a poison when given in large quantities, it may act as a stimulant in small doses. Of the two following records fig. 48 shows the slight stimulating effect of very dilute KOH, and fig. 49 exhibits nearly complete abolition of response by the action of the same reagent when given in stronger doses. [Illustration: FIG. 48.--STIMULATING ACTION OF VERY DILUTE KOH] So we see that, judged by the final criterion of the effect produced by anæsthetics and poisons, the plant response fulfils the test of vital phenomenon. In previous chapters we have found that in the matter of response by negative variation, of the presence or absence of fatigue, of the relation between stimulus and response, of modification of response by high and low temperatures, and even in the matter of occasional abnormal variations such as positive response in a modified tissue, they were strictly correspondent to similar phenomena in animal tissues. The remaining test, of the influence of chemical reagents, having now been applied, a complete parallelism may be held to have been established between plant response on the one hand, and that of animal tissue on the other. [Illustration: FIG. 49.--NEARLY COMPLETE ABOLITION OF RESPONSE BY STRONG KOH The two vertical lines are galvanometer deflections due to ·1 volt, before and after the application of reagent. It will be noticed that the total resistance remains unchanged.] CHAPTER X RESPONSE IN METALS Is response found in inorganic substances?--Experiment on tin, block method--Anomalies of existing terminology--Response by method of depression--Response by method of exaltation. We have now seen that the electrical sign of life is not confined to animals, but is also found in plants. And we have seen how electrical response serves as an index to the vital activity of the plant, how with the arrest of this vital activity electrical response is also arrested temporarily, a