Researches on Irritability of Plants

human tongue. The further fact that the excitation occurs only at the point of kathode at make and at anode at break shows that the effect transmitted is not physical but physiological. Multiple excitation by constant current.—According to the mechanical theory multiple excitation can only occur under separate hydro-mechanical disturbances caused by multiple blows. Itis, however, found that as in the rhythmic tissue of the animal, so also in that of the plant, multiple excitations are induced by the action of a constant current (p. 249).

Characteristic effect of temperature on polar excitation.— Excitation of animal nerve by induction shock is enhanced by warmth and depressed by cold. The reverse is the case when the stimulation is caused by constant current. The excitatory effect here is depressed by warmth and exalted by cold. These specific effects are found repeated in the conducting petiole of Mimosa. The excitation caused by induction shock is depressed by cold and enhanced by warmth. But as in the animal nerve, so also in the petiole of Mimosa, these effects are reversed in excitation under the polar action of a constant current. The excitation is now enhanced by cold and depressed by warmth. Minimal excitation becomes maximal under cold and ineffective under warmth (p. 247).

The crucial test of the excitatory character of the transmitted impulse is furnished by the action of various physiological blocks which arrest the transmission of excitation. Paralysis of conduction by cold.—The local application of increasing cold on the conducting petiole retards and finally arrests conduction of excitation. As an after-effect of the application of cold the conducting power is paralysed for a considerable length of time. The lost conducting power may, however, be quickly restored by tetanising electric shocks (p. 164).

also be arrested by the action of electrotonic block. This arrest persists during the continuation of the blocking current, the conductivity being restored on its cessation (p. 167). Block of conduction by action of poison.—Finally, the conductivity of a selected portion of a petiole may be abolished by the local application of poison. The abolition of conducting power proceeds slowly under the action of copper sulphate solution and quickly under potassium cyanide (p. 173).

These results prove conclusively that the transmission of excitation in plant is a process fundamentally similar to that which takes place in the animal, being in the one case as in the other a propagation of protoplasmic change. When stimulus is applied directly on the responding organ there is induced an excitatory fall of the leaf, con- comitant with contraction and negative turgidity-variation. This particular reaction is designated as the DIRECT EFFECT oF StimuLus. When, on the other hand, a feeble stimulus is applied at a distance there occurs only a positive or erectile response of leaf with concomitant expansion and positive turgidity-variation. This particular reaction is designated as the INDIRECT EFFECT OF STIMuULUs. If the intervening tissue be highly conducting and the stimulus sufficiently strong, then the excitatory negative effect masks the positive. In such a case the response to indirect application of stimulus is negative—that is to say, the same as caused by direct stimulation. But, if the intervening tissue be semi-conducting, or if the stimulus be feeble or applied at too great a distance, then there is induced the positive or INDIRECT EFFECT (p. 196).

These two opposite reactions are found to take place in various plants under definite conditions and under diverse forms of stimuli. The two opposite effects are demonstrated independently by means of mechanical and electric responses. Two diametrically opposite effects are thus induced by an identical stimulus, depending on direct or indirect appli- cation. The existence of the positive or the indirect effect of stimulus has hitherto been unsuspected. It must be taken into full consideration in unravelling the complexities of reaction in a responding organ.

The effect at the responding region of a strong excitation transmitted through a short distance or through a good conducting channel, is negative, being the same as the effect under direct stimulation. The response is by negative turgidity-variation, contraction, fall of leaf, and electrical change of galvanometric negativity. This is the direct effect of stimulus. The effect of feeble stimulus transmitted through a great distance or through a semi-conducting channel, is positive. The responsive reaction is by positive turgidity- variation, expansion, erection of leaf, and electrical change of galvanometric positivity. This is the indirect effect of stimulus.

In taking records of electric response it is often found that, while a single moderate stimulus gives rise to a single response, a strong stimulus gives rise to a multiple series of responses. Similarly in Biophytum and Averrhoa, while a moderate stimulus gives rise to a single mechanical response, a strong stimulus gives rise to a multiple series of responses. These multiple responses are induced by various modes of strong stimulation, such as induction shock, constant current, strong light, thermal shock, and chemical excitation (p. 285).

of increasing intensity, the amplitudes of the successive responses remain the same. The response is, therefore, on ‘all-or-none principle.’ After the stimulus intensity has reached a certain limit the excess of absorbed energy finds expression in multiple responses. Certain plant tissues have thus the power of holding the excess of stimulus latent, to be given out later in the form of recurrent responses (p. 282). The characteristics of the response of Biophytum are like those of cardiac tissue of the animal. Both are charac- terised by a long refractory period and response on ‘ all-or- none principle.’ In both a single moderate stimulus gives rise to a single response and strong stimulus to a multiple series of responses.

There is no strict line of demarcation between the phenomenon of multiple and of spontaneous response so called. Under very favourable circumstances of absorption of excess of energy from without an ordinary responding plant like Biophytum will become converted to an auto- matically responding plant like Desmodium gyrans. No satisfactory theory has been offered in explanation of the so-called spontaneous movement. It has, however, been shown in this and in my previous work that there is no such thing as an absolutely spontaneous movement, but that every movement is the result of the action of stimulus which has been stored up. That this is the case may be demonstrated in the case of Desmodium by isolating the leaflet from external sources of stimulation. The effect of run down of stored up energy is then seen in the gradual stoppage of the pulsatory movement. In this condition of standstill response occurs under fresh stimulation. If the depletion of energy has not been excessive, then a moderate stimulus gives rise to a multiple series of responses. But,

under greater depletion, a strong stimulus evokes only a single response (p. 316). Desmodium leaflets in a state of standstill give a single response to a single induction shock of moderate intensity. In a typical case the latent period was found to be ‘4 second, the apex time 45 seconds, and the period of relaxation 120 seconds. The response curve exhibits a flattened top. In summer a single pulsation of a vigorous leaflet of Desmodium is accomplished in the course of about I00 seconds. The quicker down movement is completed in 4I seconds, the maximum rate being -9 mm. and average rate ‘44 mm. per second. The period of up movement is slower, being 60 seconds ; maximum rate of up movement is ‘56 mm. per second, the average rate being *3 mm. per second.

The pulsating activity of the detached leaflet of Des- modium can be maintained uniform for several hours by subjecting it to a moderate internal hydrostatic pressure. When the internal hydrostatic pressure is increased, the limit of diastolic or up movement is increased ; the con- tractile movement being opposed, the systolic limit is decreased. Under increasing external load the pulsation is decreased in amplitude and is finally arrested (p. 301).

The rhythmic tissues of the plant exhibit characteristics which are extraordinarily similar to those of the rhythmic tissue of the animal. The cardiac tissue of the animal has a long refractory period; the tissue takes no account of a stimulus which falls within the refractory period. This is also characteristic of the response of the rhythmic tissue of Desmodium (p. 310). The rhythmic tissues, animal and vegetal alike, are incapable of tetanus. By the application of Stannius’ ligature the pulsation of the heart is arrested at diastole. A similar arrest at

diastole is found to take place in the pulsation of Desmo- dium by the application of ligature below the motile organ (p. 303). The pulsating leaflet of Desmodium, like the pulsating heart, is more susceptible to excitation at diastole than at systole. An extra pulsation is induced by an induction shock applied during the diastolic phase (p. 319). Trans- mitted excitation affects the normal pulsations of rhythmic tissues—animal and vegetal—in a similar manner. In certain circumstances the effect is one of inhibition; in other circumstances the effect is one of acceleration (p. 320).

Still more remarkable are the similarities of effect of temperature and of chemical reagents on the rhythmic pulsations in animal and plant. The effect of lowering of temperature on the rhythmic pulsation of Desmodium gvyrans is similar to that on the pulsation of a frog’s heart. Lowering of temperature enhances the amplitude, but reduces the frequency of pulsation of both. The pulsation of Desmodium leaflet is arrested at the minimum temperature of about 17°C. Arrest takes place at systole; gradual warming revives the pulsation, which undergoes a staircase increase with enhancing diastolic expansion (p. 326).

Rise of temperature induces enhanced frequency and diminished amplitude of pulsation. During rise of tempera- ture to about 43° C. there is a tendency of arrest towards diastole. The systolic contraction undergoes continuous diminution during rise of temperature. During the fall of temperature there is a gradual enhancement of systolic contraction (p. 330). The temperature maximum at which arrest of pulsation takes place may be as high as 45° C. Above this temperature there is a tendency to contraction and permanent arrest under heat-rigor.

The effect of drugs on the rhythmic pulsation is modified by the tonic condition of the plant, the strength of the reagent, and the duration of application. Vapour of alcohol and dilute carbonic acid induce a transient enhancement of amplitude with prolongation of period. Stronger application induces an arrest of pulsation. Dilute vapour of ether and carbon disulphide induce a temporary arrest, revival taking place after quick substitu- tion of fresh air. The action of chloroform is more intense than that of ether.

Copper-sulphate solution causes a permanent arrest of pulsation. The poisonous reaction of potassium cyanide is more powerful than that of copper sulphate. A very striking characteristic modification in the rhythmic activity of animal tissue is found in the antagon- istic action of acid and alkali on the pulsation. Application of dilute acid induces in the heart an atonic reaction with arrest of pulsation in the relaxed or diastolic condition. The effect of alkali is the very reverse of this, the arrest taking place in systole. These specific effects are repro- duced in an astonishing manner in the rhythmic pulsation of Desmodium. Dilute solution of lactic acid induces in the pulsating leaflet an arrest at diastolic relaxation. The application of dilute sodium hydrate induces, on the other hand, exactly the opposite effect of arrest at systole (p. 339).

At the beginning of this work we took up the question of the possibility of detecting internal changes in a plant by subjecting it to a questioning shock. It has been shown how the plant can be made to record its answer to an impinging testing stimulus, and how the effects of environ- mental changes may be read in the script made by the plant itself. It has been shown that the variations in the plant’s, physiological activity, under changing external conditions

may be gauged by the waxing or waning of its response. It has been shown also how numerous and varied are the factors that go to make up the complexity of the responses in the plant. It has been shown that stimulus may be modified in its effect, according as it is direct or indirect, according as it is feeble or strong. The modifying influence of the tonic condition of the tissue has also been shown, depending on whether it was normal, sub-tonic, or fatigued. In the numberless permutations and combinations of these varied factors lies the infinite complexity of the responsive phenomena of life.

In surveying the response of living tissues we find that there is hardly any phenomenon of irritability observed in the animal which is not also found in the plant. The various manifestations of irritability in the plant have been shown to be identical with those in the animal. From the standpoint of the theory of evolution this will be found highly significant. It may be confidently predicted that the recognition of this unity of response in animal and plant will in no small degree further the progress of plant physiology. Many difficult problems in animal physiology, moreover, will find their solution in the experimental study of corresponding problems under simpler conditions of vegetable life. The study of the responsive reactions in plants. must, therefore, be regarded as of fundamental im- portance in the elucidation of various phenomena relating to the irritability of living tissues.

‘s PAGE Mimosa 1. Response to indirect thermal stimulus . 3 2 4 PS 2. Response to constant current 5 : : 2 5 Bey 3. Response to condenser discharge . ; 30 4. Excitatory response induced by sudden application of cold 5 : #7) LOO 5. Greater pmeiency, of breaks ene nese : ‘ 53 6. Response due to stimulation of upper half of pulvinus . 45 7. Determination of apex time : : : 37 8. Determination of time relations of response : : Si g. Additive effect of stimulus . : ; F : «54 to. Influence of temperature . : : = ~<60 Ir. Relation between stimulus and meouonee 3 ; : 62 12. Determination of work performed by Mimosa : 57, 13. Effect of load on work performed i 3 i «56 14. Determination of rate of work . : ; : - 58 Neptunia oleracea 15. Response of leaf of Neptunia to electric stimulus . + 43

Biophytum sensitivum 16. Response of leaflet of Biophytum « é - - é 42 17. Response of terminal leaflet of Desmodium to electrical stimulation . j z s 3 . ; 7 45 Ordinary plant 18. Response of ordinary plants to stimulus : : ee. / 23. Staircase response in Mimosa : é : E Sey 24. Modification of tonicity by stimulus . “ : 2 79 25. Alternating response . : : - : 5 a SO 26. Fatigue reversal in Mimosa < : - : Pn 27. Effect of sudden darkness . : c Pea: 28. Abolition of motile response by absorption of water 7° 88 29. Restoration of motile excitability by action of glycerin. 88 30. Stimulating effect of ozone . : : 5 . ESS 31. Effect of carbonic-acid gas . : - : - 5) Gn 32. Effect of alcohol E p 4 3 3 3 - oni 33. Effectofether . é fs : 3 - 4 92 34. Effect of carbon disulphide . : c - = oe 35. Effect of coal gas 2 : : ‘ : 2 . 93 36. Effect of chloroform . 5 ; 93 37- Action of ammonia in abolition of excitability ‘ » oH 38. Abolition of excitability and death of plant under sul- phuretted hydrogen - : : eB 5 ~ OS 39. Toxic effect of nitrogen dioxide . ; ; F 9S 40. Poisonous action of sulphur dioxide . : : 96

41. Death record of Mimosa . : Bf aioy- 42. Abolition of mechanical response after death : « LOS 43. Determination of death point of Desmodium . é ~ rod 44. Death point of bean plant . : “ - 04 45. Determination of death point in ordinary plant 3 ; = Low 46. Lowering of death point under fatigue 3 2 106 47. Translocation of death point under the action of dilute poison . : 4 ; A ; : ; - 106 48. Identity of latent period in successive experiments a LS 49. Determination of latent period in highly excitable Mimosa 116 50. Value of latent period unaffected by abies of

recorder : Bins. 51. Simultaneous ecdnce in fae lelectrenial ace é ; Leal 52. Effect of intensity of stimulus on latent period . « 25 53. Constancy of latent period under maximal stimulus e027 54- Effect of optimum condition on latent period - owes 55. Effect of fatigue ; ‘ 4 ; A : ot Leg 56. Effect of temperature . ; f : ‘ + ago Biophytum sensitivum 57- Determination of latent period of Biophytum - 5 285 Block of conduction by local application of copper sulphate in petiole of Biophytum . :

Effect of mercuric chloride in inducing block of con- duction . - Time record showing block of pondeton 7 CuSO, in petiole of Mimosa 3 Determination of velocity by the direct method . Determination by the differential method . Effect of intensity of stimulus on velocity After-effect of stimulus on conductivity Determination of velocity of transmission of excitation in Biophytum sensitivum . Determination of direction of preferential conductivity -

Dual impulse under thermal stimulation Effect of chemical stimulus Action of induction shock . Diphasic response under constant Been Effect of condenser discharge Mimosa 86. Dual impulse under induction shock . . : Bee 151s 87. Double response under thermal shock . ‘ 186 88. Effect of intensity of stimulus in inducing positive or negative response . . SO, 89. Effect of distance of point of application of stunalis - 189 90. Masking of positive impulse by predominant negative . 194 91. Unmasking of positive by suppression of negative by cold 195

Mimosa 92. Effect of make of kathode on the leaf of Mimosa . “5 A202 93. Time difference between excitation by ascending and descending currents : = 204 94. Time difference between effects of ascending and descend- ing induction shock 4 : 206 95. Effect of moderate current at spake) of kathode and break of anode 2 : A r Zao 96. Effect of feeble current on leaflet as Manon F ‘ BP eri) 97. Effect of moderate current on leaflet of Mimosa . =. f220 Biophytum sensitivum 98. Effect of feeble current F “ aie : Mee 99. Effect of moderate current . 4 c : : ated Neptunia oleracea roo. Excitatory effect of feeble current 4 - i s) 225 tor. Excitatory effect of moderate current . : . + 226 Averrhoa carambola toz. Polar effect of feeble current F A : F «2226 103. Polar effect of moderate current . : é - i 6227)

104. Effect of feeble current at make of kathode . oe” 228 105. Effect of moderate current at make of kathode and break of anode Fi ? - 5 ‘ : «, S220, 106. Effect of strong current on Mimosa . F . « 255 107. Effect of strong current on leaflets of Mimosa . = ee 108. Effect of strong current on Biophytum sensitivum errr ey: 10g. Effect of strong current on Averrhoa carambola . . 263 110. Effect of very strong current on leaf of Mimosa . « 256 111. Effect of very strong current on leaflets of Mimosa . 261 112. Effect of very strong current on leaflets of Biophytum . 262

113. Effect of very strong current on Averrhoa carambola. 263 After-effect of moderate stimulation . : ‘ - Effect of age ; : f : 2 2 : Modified response KmAm - : : - ‘ : Comparison of relative sensitiveness of plant and animal to polar excitation . Depression of excitability to induction shock by the action of cold . Enhancement of soe to pola Boao of hohe by the action of cold : Effect of application of kathode on pulsation of Dees modium . Arrest at systole by the make of fathode, and diastolic expansion by the break of kathode .