Collected Physical Papers/Electric Response in Ordinary Plants Under Mechanical Stimulation
XXI
ELECTRIC RESPONSE IN ORDINARY PLANTS UNDER MECHANICAL STIMULATION
Discovery of the similarity of response in inorganic substances and in animal tissues led me to the investigation of responsive phenomena in the intermediate region of life of plants.
The action of stimulus on living substances is usually detected by two different methods. In the case of motile organs, stimulus causes a change of form. Mechanical response may thus be obtained in a contractile tissue such as a muscle. But in others, nerve for example, stimulus causes no visible change; the excitation of the tissue may, however, be detected by characteristic electromotive changes. The advantage of the electric mode of detection of response is its universal applicability. In cases where mechanical response is available, as in the muscle, it is found that records of mechanical and electrical responses are very similar to each other.
The intensity of electrical response in animal tissue is modified by the physiological activity of the tissue. When this activity is in any way depressed, the intensity of electrical response is also correspondingly diminished. When the tissue is killed, the electrical response disappears altogether.
Burdon Sanderson, Munck, and others found electric response to occur only in sensitive plants. I wished to find out whether the responsive electric variation was confined merely to organs of plants which exhibit such remarkable mechanical movements, or whether it was a universal phenomenon characteristic of all plants and of all their different organs. My attempt was moreover directed in determining throughout the whole range of response phenomena a parallelism between the animal and the vegetable. That is to say, I desired to know, with regard to plants, what was the relation between the stimulus and response; what were the effects of superposition of stimuli; whether fatigue was present, and in what manner it affected the response; what were the effects of extremes of temperature; whether chemical reagents exercised any influence on the plant-response, as anæsthetics and poisonous drugs affected the responses of nerve and muscle.
Fig. 84. Response Recorder.
Experimental Arrangements
The galvanometer used is a sensitive dead-beat D'Arsonval. A current 1/109 ampere gave a deflection of 1 mm. at a distance of 1 metre.
The Response Recorder.—In these response-curves the ordinate represents the intensity of E. M. variation, and the abscissa the time. The curves are obtained directly, by tracing the excursion of the galvanometer spot of light on a revolving drum. The drum, on which is wrapped the paper for receiving the record, is driven by clockwork (fig. 84). 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 an inclined mirror. A stylographic pen attached to a carrier rests on the writing surface. The carrier slides over a rod parallel to the drum. On stimulation, the resulting excursion of the spot of light is followed by moving the carrier which holds the pen; after , the cessation of stimulus the excitatory effect slowly disappears, and the galvanometer spot returns gradually to its original position. The response and recovery are thus directly traced on the recording surface. We can calibrate the value of the deflection by applying a known electromotive force 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; this speed can, however, be increased to suit any particular experiment. Very accurate records can thus be obtained in a very simple manner. A large number of records might be taken by this means in a comparatively short time.
Photographic Recorder.—The records may also 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 galvanometer spot of light. The records given in this paper are accurate reproductions of those obtained by one of these two methods.
Graduation of the Intensity of Stimulus
The important conditions for securing quantitative results are (1) the maintenance of uniform intensity of stimulation in certain experiments and (2) the graduated increase of intensity in others.
Stimulus of torsional vibration.—I find that torsional vibration affords a very effective method of stimulation. The plant-stalk may be fixed at one end, the other end being held in a tube provided with clamping jaws, A rapid torsional to-and-fro vibration can 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 means of a graduated circle. The plant chamber and the vibrational stimulator is shown in fig. 85.
Intensity of Stimulus dependent on Amplitude of Vibration
A block is produced by clamping the plant in the middle at C; increased stimulation is found to occur in the tissue by increased amplitude of vibration.
The Physiological Character of Response
l now proceed to demonstrate that the response given by the plant is physiological and that it affords an accurate index of the vital activity of the plant. For it will be found that whatever tends to exalt or depress this activity also tends to increase or diminish the intensity of electric response.
The test applied by physiologists, in order to discriminate as to the physiological nature of the response consists in observing the effects of anæsthetics, poisons, and exceedingly high temperatures, all of which are known to depress or destroy the activity of life.
Effect of Anæsthetics and Poisons.—I took 30 leaf-stalks of horse-chestnut, and divided them into 3 batches, of 10 each. One batch was kept in water, to serve as control experiment, another in chloroform water, and the third in a 5 per cent. solution of mercuric chloride.
Average response of stalks kept in water = 23 divisions.
Average response of stalks kept in chloroform = 1 division.
The response of stalks kept in poisonous mercuric chloride was completely abolished.
Effect of High Temperature.—A leaf-stalk is taken, and using the block method strong responses are obtained at both ends A and B. The stalk is now immersed for a short time in water at 60° C. After this treatment the responses are found to be completely abolished. As all the external conditions were the same in the two series of experiment, the only difference being that in one the stalk was alive, and in the other killed, we have here a 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 that half in hot water. The stalk is replaced in the apparatus; repetition of the experiment shows that whereas the A half gives strong response, the end B gives none.
Uniform Responses
Uniform responses may be obtained with certain plants in a vigorous condition, and when sufficient period of rest is allowed for complete recovery. The record given below shows the remarkable uniformity of response given by radish. The response of plant is by the induced galvanometric negativity of the excited tissue just as in the tissue of the animal.
Fig.86. Uniform responses (Radish).
Fatigue
The responses however exhibit fatigue by shortening the intervening period of rest. The first four responses in fig. 87 show that there is a complete recovery of the tissue one minute after the application of the stimulus.
Fig. 87. Appearance of fatigue in plant under shortened period of rest.
The molecular condition is exactly the same at the end of the recovery as at the beginning of stimulation; the second and succeeding response-curves therefore are exactly similar to the first, provided sufficient interval in each case has been allowed for complete recovery.
The rhythm was next changed and stimuli of the same intensity as before applied at intervals of half a minute, instead of one. It will he noticed that these responses in the second part appear much feebler than those in the first set, in spite of the equality of stimulus. The original rhythm of one minute was now restored and the succeeding curves at once show restoration of the original amplitude of response.
Increased Response under increasing Stimulus
I will now show that the electric response is not merely a qualitative phenomenon, but that increased intensity of stimulus always gives rise to a definite increase in the amplitude of response.
In order to obtain the simplest type of effects, not complicated by secondary phenomena, it is necessary to choose specimens which exhibit little fatigue. Having obtained such a specimen I took records of responses for increasing stimuli caused by increasing amplitudes of vibration.
In the record given (fig. 88) the amplitude of vibration was increased from 2°.5 to 12°.5 by steps of 2°.5
Fig. 83. Increasing responses to increasing vibrational stimuli; the vertical line to the right represents .1 volt.
(cauliflower-stalk). It will be noticed the remarkably definite manner in which the response increases with the stimulus. The rise is at first rapid, but with high intensities of stimulus there is a tendency for the response to approach a limit.
Superposition of Stimuli
Additive effect.—There is apparently little or no response when the stimulus is feeble. But even a subminimal stimulus, though singly ineffective, becomes effective by the summation of several. This is shown in fig. 89, where the first record 𝑎 is the response to a feeble stimulus, and the second b is the response to the same stimulus repeated in quick succession, thirty times.
Fig. 89. Additive effect of singly ineffective stimuli in plant.
Effect of Steam
The plant was mounted in a chamber into which steam could be introduced. I had chosen a specimen which gave regular responses. On the introduction of
Fig. 90. Effect of steam in abolishing response with death of plant. The two records to the left give responses at 20° C., after which steam was introduced.
steam, with consequent rise of temperature, there was a transitory augmentation of excitability. But this quickly disappeared, and in five minutes the response disappeared with the death of the plant (fig. 90).
Effects of Anaesthetics and Poisons
The most important test by which vital phenomena are differentiated is the influence of narcotics and poisons on response. I have already shown how plants which previously gave strong response, did not, after application of an anæsthetic or poison, give any response at all. In those cases it was the last stage only that could be observed. But it appeared important to be able to trace the growing effect of anæsthetisation or poisoning throughout the process.
Effect of Chloroform.—The mode of experiment was (1) to obtain a series of normal responses to uniform stimuli, applied at regular intervals of time, say one minute, the record being taken the while on a photographic plate. (2) Without interrupting this procedure, the anæsthetic agent, chloroform vapour, was
Fig. 91. Effect of chloroform.
blown into the closed chamber containing the plant. It will be seen how rapidly chloroform produced depression of response which afterwards culminated with death and abolition of response (fig. 91).
Exactly similar effects were obtained with chloral, also with formalin. These were applied in the form of solution on the tissue at the two leading contacts.
It has been shown that the electric response is a faithful index of physiological action and that such a response is given by all plants and by their different organs. It has also been shown that in the matters of response by induced galvanometric negativity, of modification of response by high and low temperatures, they are strictly correspondent to similar phenomena in muscle and nerve. Judged by the final criterion of the effect produced by anæsthetics and poisons, these electric responses in plants fulfil with animal tissues the test of vital phenomenon.
The electro-physiological investigation on plants will undoubtedly throw much light on the response phenomena in the animal. 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 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. The essential factors which modify response can, therefore, be better determined under the simpler conditions which obtain in plant life.