Researches on Irritability of Plants

pulvinus is the indicating motile organ. The parallelism of the two cases will be clearly understood by reference to fig. 108. With both ascending and descending currents of feeble intensity I have obtained with Mimosa excitatory effects at make exactly corresponding to the effects in a nerve-and- muscle preparation. In order to prove that the excitation is really due to make of the proximal kathode in the case of descending, and of distal kathode in that of ascending,

Fic. 108.—Excitation by descending or ascending currents in nerve- muscle and petiole-pulvinus specimens. Motile organs are to the left. Nerve-muscle (upper figure) ; petiole-pulvinus (lower figure). currents, I took time-records of these, as seen in fig. 10g. The proximal electrode was placed at a distance of 2°5 mm. and the distal at a distance of 15 mm. from the pulvinus. The distance between the two electrodes was therefore 12°5mm. The time-record was taken in the usual manner, the vibrating recorder employed having a frequency of 20 vibrations per second. The applied E.M.F. was 6 volts. The two records were obtained successively, the upper being the one due to descending, and the lower to the ascending, current. An inspection of the records in fig. 10g at once shows that the response with descending current, the kathode being proximal, took place the earlier of the two.

records should be equal to the time taken by excitation to travel the intervening distance of 12°5 mm. between the electrodes. This time-interval should be equal to *%?; where V is the velocity of transmission in the given specimen- The velocity can easily be deduced from the two records themselves. From the upper record we see that response took place after 8 spaces, each representing one-twentieth of a second. The true time taken by stimulus to traverse the distance of 2°5. mm. is therefore 34;—L; where L is the

Fic. 109.—Records of responses to ascending and descending make currents in Mimosa. In the upper record the current was descending, kathode being proximal to pulvinus. In the lower record the current was ascending, kathode being distal. Frequency of vibrating recorder 20 D.V. latent period of the pulvinus, the average value of which we found to be ‘1 second ; hence V = *? = 8:3 mm. per second. From the second record we find the true time taken by excitation to traverse the distance of 15 mm. to be 1'8—L = 17 second. V = “ = 88 mm. per second. The mean velocity obtained from the two records is therefore 8°6 mm. Hence the difference of time in the two cases of excitation by ascending and descending current should be equal to the time taken by the excitation to travel 12°5 mm. the distance between the two electrodes. This difference should there- fore be “3 = 1°4 second. That this is really the case can easily be tested by the simple process of counting the

time-difference in the number of dots in the two records. On counting we find the difference to be 28 spaces, each representing one-twentieth of a second. That is to say, the time difference is actually 1°4 second, precisely what was inferred from theoretical considerations. In the case of animal tissues, single induction-shocks of moderate intensity are known, as regards polar action, to be Fic. 110.—KRecords showing time-difference between re- sponses due to ascending and descending induction- shocks. Distance between two electrodes 5 mm. ; upper record with kathode nearer the _ pulvinus. Vibrating recorder 20 D.V.

effective at the commencement and not at the termination of the current. Hence excitation here, as in the case of constant current, takes place at the kathodic region. In the case of plants also I find the same to hold good. The exciting electrodes from an induction-coil were placed on two points on the petiole, separated from each other by 5 mm. Two records were taken with single induction- shocks of intensity 2, now in descending and afterwards in ascending directions. In the first or upper of these records (fig. 110) the proximal electrode was the kathode ; in the lower, the kathode was distal. It will be seen from these two experiments that if it be the kathode which excited

then the arrival of excitation at the pulvinus will be quicker when the kathode is near and later when the kathode is distant. It is clear from the records that this is what obtains ; excitation is earlier when the kathode is proximal and later when the kathode is distal. We find moreover that the difference of time between the moments of arrival of excitation in the two cases is represented by six spaces, each of ‘05 second. Hence the delay in the second case is equal to °3 second, due to transmission through the additional distance of 5mm. The velocity of transmission is therefore 16 mm. per second, which we have seen is the average velocity of transmission through the petiole of Mimosa.

After obtaining these characteristic effects with Mzmosa under feeble electric currents, we have still to find out whether such effects are peculiar to this plant or whether they are universally present. To answer this question we have to experiment with every species of plant that happens to be provided with a motile indicator. After this has been done we have still to determine the effects of increasing intensity of current brought about by the application of increasing E.M.F.

For the application of a graduated increase of E.M.F. up to 12 volts or thereabouts, a Potential Slide is very convenient. This consists of two platinoid wires stretched side by side. Two spring-contacts E and E’ are carried by a slide between the two wires. When a battery of six storage-cells is applied at the terminals of these wires, at A and B, the difference of electric potential at the two points is 12 volts. When the slide is at the extreme end, to the right, the difference of potential between E and £’ will be zero, whereas in being moved to the left the difference of potential or E.M.F. between E and £’ will continuously increase till at the extreme left it is 12 volts, E being, say, positive, and E’ negative. The electrical current enters the plant

by E, which is therefore the anode. It leaves by E’, which is therefore kathode. The advantage of the slide is that the E.M.F. applied may be gradually increased from zero to maximum, or decreased from maximum to zero. In certain experiments, where we wish to apply a given E.M.F. without causing excitation, this is essential. For we have seen that a sufficiently graduated increase or decrease of current causes no excitation. If excitation be desired, however, it may be induced by a sudden variation of current brought about by suddenly moving the slide either backwards or

CTT TT TTT TTT TTT ATA ETAT TAT ATT AAT TA TTT TAAL AAA AAI) eS forwards, or by suddenly completing or interrupting the main circuit (fig. III). It is important also to know the E.m.F. that has been applied and the intensity of the current flowing through the plant in a given experiment. For the former, a scale fixed on the apparatus may be previously calibrated so as to give the values of the E.M.F. at the different positions of the sliding-contacts, it being understood that the same number of storage-cells are always applied at the terminals. Or we may place a voltmeter to measure the applied E.M.F. between the two sliding-contacts.

For the measurement of the currents that flow through the plant, a micro-amperemeter is used. Each division of this instrument indicates one-millionth of an ampere. The currents which usually have to be measured vary from one to a hundred micro-amperes. A single instrument provided with appropriate shunts enables us to measure all these intensities. Another point which should be borne in mind is the possi- bility of induction-currents being generated in the measuring instruments themselves, when the main current is suddenly turned on and off. In order to guard against any disturbance that might be caused in this way, it is necessary at the moment of experimenting on the plant to put the voltmeter and the micro-ammeter out of action for the time. On the completion of each experiment the E.M.F. and the current employed are measured by means of the voltmeter and the micro-ammeter.

In experiments where still higher voltage may be required this potential slide cannot be used, as the wires become unduly heated. We may in such a case modify the potential slide by replacing the platinoid wire with two parallel troughs containing copper-sulphate solution. In place of the spring-contacts two copper plates are here employed, dipping into the solution. These, forming the moving contacts E and E’, are attached to the slide. With this form of electrolytic potential slide a graduated E.M.F. up to 100 volts may be easily obtained. The terminals A and B are connected with a battery of the required number of storage-cells from the installation in the laboratory.

A more convenient arrangement is a portable set of storage-cells, with a Potential Keyboard, by means of which any voltage up to 100 volts may be readily obtained by steps of 2 volts at a time. Small storage-cells of flat form may be obtained 6 cm. broad, 2°5 cm. in depth, and 6 cm. in height. Fifty of these cells, giving a total E.M.F. of 100 volts, may be packed in a small box 25 by 30 cm., having a height of 7.5 cm. Ten of these cells are arranged in units, and forty in four groups of ten each.

series, the positive end of the first being connected with the first projecting button. The negative end of each cell I, 2, 3, and so on, is led to its own button. A key K carries a radial arm, which during a rotation from left to right will make successive connections with the projecting buttons, and thus cause the E.M.F. between E and E’ to rise by increments of 2 volts at a time, the maximum E.M.F. thus obtained by contact with the tenth button being 20 volts (fig. 112). A second key, kK’, not shown in the diagram, may in a similar

manner make contact with other buttons, which are con- nected with terminals of cells in groups of ten. Thus the number of cells which may be included between the two electrodes may be varied from 0 to 50, and the derived E.M.F. may thus be raised from o to 100 volts by steps of 2 volts at a time. Having thus found suitable means of applying currents of increasing intensity, we shall in the course of subsequent chapters study in detail the effects of feeble, moderate, and strong currents on various sensitive plants.

In Mimosa, under feeble intensity of current, excitation takes place only at the make of the kathode. In this the polar reaction in plant is the same as in animal tissues. The effects of feeble ascending and descending currents in the petiole of Mimosa are parallel to those in nerve-and- muscle preparations. In both cases excitation takes place earlier when the kathode is nearer the responding organ. As in animal, so also in Mimosa, single induction-shocks of moderate intensity are, as regards polar action, effective at the commencement and not termination of the current. In taking records with single ascending and descending induction-shocks, response is found to take place earlier when the kathode is proximal.

Polar effects of feeble and moderate currents on (1) leaflets of Mzmosa, (2) leaflets of Biophytum, (3) leaflets of Neptunia, (4) leaflets of Averrhoa cavambola, (5) leaflets of Averrhoa bilimbi, and (6) primary leaf of Mimosa—Excitation with feeble current only at kathode-make— Excitation at kathode-make and anode-break, under moderate current—Tabular statement of results. In the previous chapter I described some typical experi- ments showing the effects of feeble electrical currents on the pulvinus of Mimosa. In order to demonstrate that such effects are universal, it will be necessary to extend this investigation to every kind of motile organ possessing requisitive sensitiveness. I here give a list of the specimens employed.

First, the leaflets of Mimosa form very sensitive speci- mens, though there are certain experimental difficulties to be overcome. These leaflets are borne on four sub-petioles that radiate from the end of the main petiole. While the excitatory effect is shown in the primary pulvinus by causing the fall of the leaf, the same effect is seen in the pulvinules by the folding of the leaflets upwards. Second, the leaflets of Neptunia are moderately sensitive. This plant flourishes in tanks, but can with care be grown in pots. The excitatory effect is exhibited by the folding upwards of the leaflets.

Third, the leaflets of Biophytum sensitivum are very sensitive and serve the purpose of the investigations in an admirable manner. These leaflets show excitation by Fourth, the leaflets of Averrhoa carambola are only moderately sensitive. They close downwards on excitation. Fifth, the leaflets of Averrhoa bilimbi possess more or less the same kind of excitability as A. carambola. Under excitation these leaflets fold downwards. Sixth, and lastly, we have the primary pulvinus of Mimosa whose polar reactions have already been briefly referred to. In this and a subsequent chapter I shall give a more exhaustive series of experiments in connection with the response of the leaf of this plant.

These practically exhaust the list of plant-organs possessed of sensitiveness sufficient for our purpose. It will be my object: in this chapter to give a detailed description of the reactions of these various specimens. We shall find that the character of the reaction will depend on the intensity of the electrical current. With a feeble current a characteristic effect is induced which will be designated’as that of Type I. As the acting E.M.F. is gradually increased, the effect is transformed into that of Type II. The characteristic effects of feeble and moderate currents on various sensitive plant-organs will now be dealt with in some detail.

In these experiments, I shall generally speaking employ the Bi-polar method, which possesses many advantages over the Mono-polar method already described. In the Bi-polar method the electrical contacts are made with two points, at or near two different motile-organs. When the commutator is turned to the right, the right contact is suddenly made kathode and the left anode. We are now able to note the excitatory effects, if any, at kathode-make, Km, to the right, and at anode-make, A’m, to the left. The dash affixed in this latter case will always be used to distin- guish the contact on the left side. The circuit is now broken, and we observe the effects of kathode-break, Kb, and anode- break, A’b. The commutator is next tilted to the left, making the right anode, and enabling us to note the effects of K’m and Am. The circuit is then broken, producing the

results K’b and Ab. It will be noticed that in the second half of this cycle of the operation we subject the experiment to the further test of corroboration by reversal. The make-effect may be observed simultaneously at both anode and kathode at the beginning of the experiment. The break-effect also may be observed immediately after- wards, provided there has been no excitation during make. But if such excitation has occurred, the current may be maintained till the leaf or leaflet affected has re-erected itself, with accompanying recovery of excitability. The time required for this process is from Io to I5 minutes in the leaf of Mimosa and about 3 minutes in the case of the leaflet of Biophytum. It is the sudden variation of the current, and not its continuance, that causes conspicuous excitation. The current may now be interrupted and the break-effect observed.

If it is desired to study the effect of break, apart from a previous over-continued maintenance of current, con- ceivably inducive of fatigue, we may proceed as follows: By means of the potentiometer-slide already described the current is initiated and brought to a maximum, so gradually as to cause no excitation. The break can now be effected suddenly, in order to note any excitatory effect that may be characteristic. It will be seen that by adopting this method the necessity for maintaining the current, while waiting for the recovery of excitability, is avoided. In order to avoid unnecessary repetition I may state here that I have employed both methods in studying the break- effect, and that as a matter of fact, in ordinary circum- stances and within limits, there is but little modification of normal results induced by the previous maintenance of current, the intensity of which at its maximum is of the order of millionths of an ampere.

The distinctive effects of Types I. and II. forming the subject of the present chapter, are brought about the former by relatively feeble and the latter by moderate currents. With a given specimen, increasing the E.M.F, from zero, a minimal value of current is reached at which the effect characteristic of Type I. begins to be evident. On continuing to increase the E.M.F., the same characteristic effect is still for a time obtained, until a critical value of the current is reached above which the effect observed is transformed into that which is characteristic of Type II. There is thus a given range within which we obtain the characteristic effect of a particular type.

The minimally effective current for the induction of any given type of excitatory effect is modified by the condition of the specimen, being relatively low when the plant is highly excitable. Thus age, season, and temperature are all modifying factors. Different species of plants, again, exhibit different susceptibilities to excitation by a constant current. Thus the leaflets of Mimosa pudica and Biophytum sensitivum are highly susceptible, whereas the leaflets of Averrhoa are much less so.

It must also be borne in mind that under the same E.M.F. the intensity of the exciting current will depend on the electrical resistance interposed between the two contacts by the intervening tissue. This resistance, it is obvious, will vary with the distance between the two contacts and the character of the specimen. Thus the resistance offered by a specimen of Mimosa, where one contact is at or near the pulvinus and the other on the stem, 7 cm. below, will be of the order of 0°5 million ohm ; whereas that interposed by the same length of a thin petiole of Biophytum will be something like 15 million ohms. Hence it will be seen that with different plants the value of the E.M.F. applied does not by itself give a correct idea of the intensity of the exciting current. In a series of experiments with the same plant, where the resistance is constant, an increasing E.M.F. does actually connote an increasing current. But in different specimens it should not be assumed that the higher E.M.F. necessarily means a higher value of the exciting current. With a high E.M,F. we may have a feeble current and vice

versa, on account of the very different resistances offered by different specimens. To give some idea of the order of magnitude of the current found efficient in causing excitation, I shall, in typical cases, specify its value as read by an interposed micro- ammeter. The unit of current found convenient for these investigations is, as I have stated elsewhere, one-millionth part of an ampere, designated as a micro-ampere or sym- bolised as 10-* ampere.

Having given a general description of the method employed, we will now study the effects of feeble and moderate currents in inducing excitation. I shall begin by de- scribing the responsive effects seen in the leaflets of Mimosa. The bi-polar method has the advantage, as already stated, that the effects at anode and kathode are simultaneously displayed ; moreover, the selection of the leaflets instead of the primary pulvinus as indicators of the excitatory effects enables us not only to observe the occurrence of local excitation but also to watch its propagation, as shown in a very striking manner by their serial closure.

Effect of feeble current. The sensibility of leaflets of this plant is considerable, being even greater than that of the leaf. The conducting power of the sub-petiole is of the same order as that of the main petiole. Selecting a young leaf of Mimosa, I made appropriate connections with two neigh- bouring sub-petioles, the length interposed between the two contacts being 4 cm. I shall distinguish the right of these as R and the left as L. In making repeated experiments on the secondary petioles of Mimosa bearing the leaflets, much trouble is occasioned by the travelling of the excitation to the primary pulvinus, in consequence of which the leaf is apt to fall and thereby interrupt the electric circuit. This is obviated by means of a special electrode-holder. A lateral rod, carrying a piece