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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 Generalite des Phenomenes Moleculaires produits par l'Electricite sur la matiere Inorganique et sur la matiere Vivante.' (Travaux du Congres 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 Reponse Electrique dans les Metaux, les Tissus Animaux et Vegetaux.' (Societe 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 anaesthetics 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 ANAESTHETICS AND POISONS
Effect of anaesthetics, 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—Resume 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 ANAESTHETICS
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).
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.
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]
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.
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. Anaesthetics, like chloroform, gradually diminish, and finally altogether abolish, electrical response.
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, anaesthetic, 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 Schaefer, 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 (AEsculus 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.
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.
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.
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.
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.
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.
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 deg. and 67 deg. 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.
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).
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 anaesthetics 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).
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 anaesthetics, 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 anaesthetics and poisons.—Ordinary anaesthetics, 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 anaesthetised or poisoned.
Experiments on the effect of anaesthetics 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 deg..
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 deg. 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.
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).
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).
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]
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]
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.
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 deg. vibration, and the second set B, with an augmented response, for stimulus intensity of 90 deg. vibration. On reverting in C to stimulus intensity of 45 deg., 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.
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 deg.) 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).
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.
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.
(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.
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).
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.
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.
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.
(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).
(B) 1. The vibrational stimulus was increased from 2.5 deg. to 5 deg. to 7.5 deg. to 10 deg. to 12.5 deg. 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 deg. .044 volt 5 deg. .075 volt 7.5 deg. .090 volt 10 deg. .100 volt 12.5 deg. .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).
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.
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 deg. 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 deg. 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 deg. C.). I then placed it for fifteen minutes in a cooling chamber, temperature -2 deg. 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 deg. 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.
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 deg. 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 deg. 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.
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 deg. 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 deg. 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 deg. to 55 deg. C.
TABLE SHOWING EFFECT OF HIGH TEMPERATURE IN ABOLISHING RESPONSE
Temperature Galvanometric response (100 dns. = .07 V.)
(1) {17 deg. C 70 dns. {53 deg. " 4 "
(2) {17 deg. " 160 " {53 deg. " 1 "
(3) {17 deg. " 100 " {50 deg. " 2 "
(4) {17 deg. " 80 " {55 deg. " 0 "
(5) {17 deg. " 40 " {60 deg. " 0 "
(6) {17 deg. " 60 " {55 deg. " 0 " |
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