Researches on Irritability of Plants

But the most serious difficulty of all is that introduced by the edge of the attracting electro-magnet. It is known that the magnetic intensity of a pole is strongest at its edges. Should the writing-index by chance be placed exactly symmetrically, as regards the right and left edges of the pole, then the two lateral pulls, being equal, will neutralise each other, and the index will vibrate to and fro perpendi- cularly to the recording-surface. But should it be placed, however slightly, nearer to one edge than to the other, then one of the two pulls will be in excess, and the index will be drawn to one side, thus producing a disturbance in the record not due to the excitatory pull of the leaf. Even if, at the beginning, the index had been placed in a

strictly symmetrical position, the movement of the writer caused by the excitation of the leaf would draw it into an asymmetrical position, resulting in a one-sided pull which would seriously interfere with the reliability of the record. It is therefore absolutely necessary so to arrange matters that the electro-magnet shall be without laterality. This condition I was able to fulfil by making the pole of the electro-magnet in the form of either a cylinder or a ring. The axis, from which is suspended the writing-index, is accurately supported, perpendicular to the plane of the circular section of the magnetic pole at its centre. Every- thing was thus made symmetrical, and as there was no laterality there could be no tendency whatsoever for the index to execute its to-and-fro vibrations in any other direction than that which was perpendicular to the plane of the terminal pole of the magnet. As this plane may be adjusted parallel to the glass recording-surface, the tapping movement of the writing-index can be made to take place perpendicularly to the recording-surface.

Next, in order to overcome the difficulty of the irregular timing of those electrical impulses which are to maintain the recording-index or writer in a state of periodic vibra- tion, I devised the Resonant Recorder. If we know the natural frequency of vibration of the recording-index, and if by means of some mechanism we can send periodic currents of exactly the same frequency through the electro- magnet, then the intermittent magnetic pulls will exactly synchronise with the natural swings of the writing-index. Owing to this perfect tuning the index will now resonate, breaking out into a persistent and regular vibration of con- siderable amplitude. In practice, all that is necessary in order to secure this is to take a long steel reed, which in the course of its regular vibration will periodically interrupt the electro-magnet circuit of the vibrator coil. The reed

itself is maintained in a state of persistent vibration by the usual electro-magnetic arrangement. I shall for the sake of convenience refer to this reed-interrupter as the Coercer ; and the writing-index simply as the vibrating-recorder or Vibrator. The reed was at first purposely selected of too great a length, so that the natural frequency of the coercer should be slower than that of the vibrator. The free end of the coercing reed carries a platinum wire which, dipping into a cup of mercury, completes the electric circuit. The other end is clamped, and by shifting the clamping-point the vibrating length of the reed is gradually and continuously shortened. This has the effect of gradually raising the vibration-frequency of the interrupting reed. A time soon comes when the frequency of the coercer is exactly the same as that of the vibrator. The latter, which has been hitherto more or less inert, now suddenly breaks out, as foreseen, into very regular and sustained vibrations of large amplitude. For some purposes it is important that the vibration-frequency of the recording-index should have a definite value. The various frequencies most suitable for these researches were I0, 20, 50, 100, and 200 vibrations per second. The exciting reed was previously calibrated by means of frequency-meters, or standard tuning-forks, to give these values. Then, with great expenditure of time and patience, different vibrating-recorders having various standardised frequencies were constructed. For this we have to select fine-steel wires of differing lengths and thick- nesses. The final tuning is accomplished by careful filing or hammering. Filing the tip of the vibrator raises the frequency ; and hammering of the wire, near the point of suspension, lowers it.

A general flattening, along the whole length, tends to maintain the vibration in a definite plane, otherwise the free tip is apt to execute an elliptical vibration. The tuning of the vibrator with the coercer is not very difficult when the vibration-frequency is low, say 10 per second. But when the frequency is high, say 100 or 200 times in a second, an exact tuning is essential. The slightest variation of the length of the coercer will either bring about full resonance or make it entirely ineffective. When the tuning is nearly but not quite perfect, then we have the phenomenon of beats. In this case, in the successive dots of the record, there will be periodic blanks. For the purpose of exact adjustment of length of the coercer I employ a micrometer-screw, by means of which the most delicate adjustment of length may be carried out.

For periodic interruption the coercing and vibrating coils may be put in series, but I find it is much easier to obtain a persistent vibration when the coercer coil is placed in a multiple arc with the vibrator coil. An electro-motive force of 4 volts should be sufficient for the purpose of maintain- ing a steady vibration of both the coercer and the vibrator. Having thus secured the requisite perfection of the resonating-writer, it is necessary to describe the complete apparatus by which to obtain records of responses in Mzmosa and other sensitive plants. For this purpose we require a slide-carrier to hold the recording-plate, and this is to be dropped at a definite speed, without jar ; also the clockwork by which it is to be actuated. Besides these is needed some special means by which the recording-point may be brought to the proper distance from the recording-surface. It is necessary, again, that the response-movement of the writer should be absolutely parallel to the writing-surface, and that its tip or contact-point should be capable of delicate adjust- ment as regards distance. It should be possible, moreover, to bring this writing-point to any position on the recording- surface that may be required. I will now proceed to relate the devices by means of which all these conditions have been met. Some of these will be seen in fig. 3, which illustrates only the upper part of the Resonant Recorder.

A gunmetal upright, the upper part of which is of trian- gular section, stands on a large disc of the same metal, which is screwed to a larger wooden base-board. The slide-carrier, holding the glass recording-plate, moves up and down the top part of the upright. It is essential to have this slide so accurately fitted that the plate-carrier may be able to drop Fic. 3.—Upper part of Resonant Recorder. (From a Photograph.) Thread from clock, not shown, passes over pulley P, letting down recording-plate; s’, screw for adjusting distance of writing-point from plate; s, screw for vertical adjustment; 1, tangent-screw for exact adjustment of plane of movement of recorder, parallel to writing-surface ; axis of writer supported perpendicularly at centre of circular end of magnet; c, coercer; M, micrometer-screw for adjustment of length of coercer; v, vibrating recorder; Gc, smoked- glass plate.

smoothly and uniformly, without any jerking whatsoever. I have sometimes attained the same end by mounting the plate-carrier on wheels and letting it slide down vertical rails. a clock or a phonograph motor, according to what may be the requirement of the speed. The quarter-plate size (Iz x 8 cm.) is convenient for record, as it is not too large for book illustration. The suspending thread passing over pulleys is wound round the winding-wheel of the clock. This wheel is provided with click and ratchet, which allow it to be wound without interfering with the axis of the clock. Thus winding of the wheel in the left-handed direction pulls up the recording-slide. The running-down of the clock then allows the slide to fall at a uniform rate. The various speeds found necessary for different records were such that the entire length of the plate, 1r cm., travelled past the recording-point in .5 second, 6 seconds, 15 minutes, I hour, or 3 hours. The first two of these rates were obtained from a phonograph motor employing two different-sized wheels. The last three were obtained by attaching three different- sized wheels to the clock-axis which carries the minute-hand. In these slow rates the movement of the plate is quite uniform from the beginning, but when, as in the first two cases, this has to be dropped at a relatively high speed, a short time will elapse, equivalent at most to the first fourth of the plate, before it becomes quite uniform. Should uniformity of such movement be specially desired for the record, it must be commenced after passing this first fourth. But on account of the chronographic signals which accompany the record, this uniformity is not absolutely essential, for they give us the data from which the time-relations of the curve may be derived.

I may here refer to a few practical points with regard to the preparation of the glass for record and its subse- quent fixing. In order to produce an even layer of smoke on the recording-plate, it is moved over the gas-flame from a bat’s-wing burner ; and this deposit of smoke will be improved if the gas has been previously passed through a jar containing a small quantity of benzine. After the record is taken, it is fixed by pouring carefully over it a dilute solution of canada balsam in xylol. It is

afterwards easy to reproduce this record by contact-print on a photographic paper. It is sometimes necessary to have the recording-point brought two or more times to the same place, in order that the successive records may be rendered the more strictly comparable. This is accomplished by a rack and pinion to adjust the height of the platform carrying the plant. When the platform is lowered, the petiole, which is attached to one arm of the recording-lever, pulls it down, and the recording-point is moved to the left. When the platform is raised, then by the action of the counterpoise attached to the other arm of the lever the index is moved in the opposite direction. In this way the recording-point can be brought to any position that is desired. The same end is secured through adjustments of a micrometer by which the carrier of the writing-index is raised or lowered.

Another adjustment that is necessary is the bringing of the recording-point near to the writing-surface without actual contact ; so that, when the index is set in a state of resonance, it may trace a dotted line. The necessary adjustment is brought about by means of a micrometer-screw at the top of the instrument, by which the lever can be made to approach or recede from the writing-surface. When the speed of the plate is slow, the successive dots may be so close together as to appear like a continuous line.

More troublesome is the adjustment necessary to render the plane of movement of the index exactly parallel to the writing-surface. If this be omitted, the writing-point in one part of the record, say to the right, will be too far away to strike the surface, whereas in another part, say to the left, it will press against the plate and lose its freedom. This difficulty I have been able to overcome by mounting the vertical rod, carrying the writer, inside another tube. An attached tangent-screw, T, then causes a very slow rotation

of the vertical rod, either in the right-hand or the left- hand direction. By this means it is possible to bring the Fic. 4.—General view of the whole apparatus, and the electrical connec- tions by means of which excitatory shock of a definite duration may be given to the plant; duration of shock determined by metronome which completes electrical circuit. (From a Photograph.) plane of movement of the writing-index exactly parallel to that of the writing-surface. The complete apparatus for

obtaining response of Mimosa is shown in the accompanying illustration (fig. 4). Having thus given an account, in some detail, of the practical working of the Resonant Recorder, it will now be well to show a pair of curves which demonstrate, in a marked manner, the advantage of intermittent over continuous contact in the making of these records (fig. 5). These represent two successive experiments on the same leaf, under identical stimulation of an electrical shock. The recording-plate was here moving at a mode- rately high speed. The lower record was taken with continuous con- tact, and the upper with the same recorder but in astate of vibra- tion, giving intermittent contact. The vibration- frequency was Io times per second. Stimulus was applied at the point marked by the vertical line. A comparison of Ee ewo records will Fic. 5.—Advantage of intermittent over show that owing to the Seoaguacns meee in obtaining re- relative loss of freedom, cords. due to friction, in the continuous contact, the latent period, or the interval between stimulus and initiation of response, is prolonged and the amplitude of the response itself reduced. In the case of the intermittent contact, on the other hand, we see that besides the freedom from this particular error we have the further advantage that the record itself contains its own time-marks, the successive dots being at intervals of one-tenth of a second.

for the excitation of the plant tissue, perfect methods of stimulation, the intensity of which can either be maintained constant or varied in a perfectly known manner. We have moreover to render the successive stimulations, and consequent scripts of the plant, a perfectly automatic process; so that the experimenter may be comparatively relieved of personal participation in the securing of the records. This will have the incomparable advantage of having no element of personal error in the results so obtained. The question of the effects of the various forms of stimulus will be dealt with in the next chapter.

In the response-records of plants, errors are introduced on account of friction of the writing-point against the recording-surface. These errors are eliminated by the method of intermittent instead of continuous contact for the record. By employ- ing the principle of resonance, the writer is made to vibrate to and fro at a known and definite rate. The record consists of series of dots giving definite time-intervals. The record is thus its own chronogram.

Different methods of stimulating the plant : mechanical, chemical, thermal, and electrical—Difficulties of securing quantitative stimuli—Direct and indirect stimulation—Ideal modes of stimulation—Electro- thermic stimulation—Stimulation by constant current—Stimulation by condenser-discharge—Non-polarisable electrodes—Direct, extra- electrodal, and intra-electrodal stimulation—Stimulation by induction shock—Effects of make- and break-shock—Excitation by tetanising shock.

give rise to excitation, and it is a very remarkable fact that the same stimuli exercise a similar excitatory influence on the pulvinus of Mimosa. Classifying these stimuli, we find that they are :— 1. Mechanical.—A blow will excite animal muscle and cause mechanical response. A similar effect is induced by a mechanical blow in the pulvinus of Mimosa. A prick or cut also will cause contraction in either. 2. Chemical. — Various chemical agents are found to induce excitation in both animal and vegetal contractile tissues. Thus dilute hydrochloric acid or ammonia causes excitation of both muscle and pulvinus.

3. Thermal.—The application of a hot wire will induce responsive contraction in both cases. 4. Electrical—_The muscle may be excited by an induc- tion-shock. The pulvinus of Mzmosa is also excited by such shocks. Other modes of electrical stimulation, such as that of condenser-discharge and that of the applica- tion of a constant electrical current, are found effective in causing excitation of animal tissues. It will be seen in the course of the present chapter that plant tissues also may be excited by similar methods.

In all these cases excitation may be either direct or indirect. In the case of muscle, with its attached nerve, we may cause excitation directly by applying the various forms of stimulus on the muscle itself, or indirectly by applying them on the nerve. In the latter case excitation is transmitted by the conducting-tract—the nerve—and reaching the muscle after a brief and definite interval, induces there the usual contraction. Taking the case of Mimosa, we may similarly have either direct or indirect excitation. Excitation is direct when it is applied, say, on the contractile pulvinus itself. It is indirect when it is applied on the petiole, at a distance from the pulvinus. Certain tissues in the petiole conduct this excitation, which, reaching the pulvinus after a definite interval, induces a responsive contraction.

It is usually maintained that in the case of Mimosa there is no true conduction of excitation, but that this con- tention is not justified will be fully demonstrated in a sub- sequent chapter. We have, then, in correspondence to the nerve and muscle preparations of the animal, plant-speci- mens, consisting of petiole and pulvinus. Indirect excitation for specific experiments is effected in the animal through the nerve, and in the plant through conducting-strands embedded in a tissue, as in the petiole.

Although we thus have various forms of stimulus at our disposal for inducing individual and isolated responsive contractions in Mimosa, yet we are confronted with very great difficulties when we wish to obtain a series of uniform excitations for quantitative investigation. It is obvious that chemical forms of stimulus would be impossible for successive excitations. The objection to a mechanical blow, as the stimulus to be employed, lies in its liability to cause a mechanical jar and thus to disturb the record.

The ideal form of stimulation would be one the inten- sity of which might be maintained uniform in successive experiments, or varied in a definite and known manner. Another great obstacle to be overcome in practice is the avoidance of injury which is caused by the stimulus itself. The application of stimulus above a critical intensity induces a depression or abolition of excitability of the tissue. As the result of long investigation for the purpose of securing various forms of quantitative stimulus, I find that one mode of thermal and three modes of electrical stimulation may be rendered practicable for our purpose. These four different methods will be described in some detail below.

It is evident that touching the specimen with a hot wire, though effective, is not a form of stimulus that is capable of quantitative application or of repetition. It is apt, moreover, unless very great precautions are taken, to injure the of heat. A loop of fine platinum-wire is made Fis. 6.—Electro-thermic stimulator for eeeeea cound th oS uniform stimulation; metronome em- P . € peti ployed in place of key k, for closing ole which is to be ex- circuit for definite length of time. cited, and is connected with an electrical circuit by means of fine flexible silver-wire (fig. 6). The circuit can be completed by a metronome inter- rupter, the current from the battery flowing for a definite length of time during, say, a single or definite number of beats of the metronome. This produces a sudden thermal shock, enough to cause excitation. Successive uniform stimuli can

thus be applied. By means of a variable resistance included in the circuit, the intensity of the stimulus can be increased or diminished. Care must, however, be taken that the heat produced in the platinum loop shall not be such as to scorch or otherwise injure the tissue. I find that injury from scorching may be avoided by adding a drop of water at the point of contact and after- wards removing excess of water by blotting-paper. This thin film of water protects the tissue from a burn. It is, again, not absolutely necessary to place the platinum wire in contact with the plant. Excitation will take place if the heating-wire is in close proximity. How practicable this form of stimulus may be rendered will be observed from the record (fig..7) of two suc- cessive excitations by this method, which are seen to Fic. 7.—Response records of Mimosa be uniform. For Corea

under indirect thermal stimulation. electrical investigations it ment is admirably fulfilled by the thermal mode of stimu- lation. Another mode of stimulation—namely, that of thermal radiation—can also be employed, though not so conveniently asthe former. A certain area may be rendered radiant by the passage of an electrical current. A Nernst electrical lamp can be conveniently utilised for the purpose. This, when rendered incandescent, gives out radiation of constant intensity. This radiation consists not only of light rays but also of a large proportion of obscure heat-rays. The excitatory value of the latter is more efficient than the luminous rays. The radiant surface of the Nernst lamp is suitably placed in front of a concave metal mirror, by

means of which the rays can be focused upon any point that is desired. I have found that Mzmosa and other sensitive plants show certain very remarkable excitatory effects under the action of a constant current. The characteristic feature of these is that excitation is not induced during the passage of the current but only at its initiation or cessation. The excita- tory effect in this case is further conditioned by the point Fic. 8.—Responses to stimulation by constant electric current.

of entry, or anode, and that of exit, or kathode. The specific characteristics of this mode of stimulation will be found fully described in the chapter on the Polar Effects of Currents in Excitation. It need only be mentioned here that, in the matter of all these peculiar effects, the plant tissue behaves in a manner exactly similar to the animal tissue. A series of records obtained from Mimosa by the stimulus of a constant current are shown in fig. 8.

Another practical method of stimulation is that of condenser discharge. The condenser consists essentially of two conducting-plates—which may be two sheets of tin-foil—separated by a sheet of non-conducting material, such as mica or paraffined paper. The capacity of the condenser is increased by enlarging the effective area of the plates. The diagram in fig. 9 illustrates this mode of excitation. By increasing the number of cells, the charging E.M.F. may be increased until a suitable value is obtained which is efficient for excitation. This will depend on the excitability of the plant-specimen. About 2 volts charging ‘5 microfarad will in general be found sufficient. K is a special spring-key by which the condenser may be charged

Fic. 9.—Direct stimulation by condenser discharge; c, condenser, k, key. (a) intra-electrodal and (b) indirect extra-electrodal mode of stimulation. or discharged. The plant to be excited is included in the electrical circuit. When kK is pressed down, the condenser is charged, the instantaneous charging current passing in one direction. The upper arrow in the diagram shows the direction of this charging current. When the key is released, it springs back and discharges the condenser. The instantaneous discharge current now flows in a reverse direction (fig. 9).