Researches on Irritability of Plants

of ebonite, slides up and down on a vertical stand and can be fixed at any height by a screw. The piece of ebonite carries two small plates of brass, each supporting a bored cork through which slides a straight-form non-polarisable electrode. The two pieces of brass can also be adjusted laterally, so as to make the electrical connections with the two sub-petioles at varying distances apart. The main petiole is also supported mechanically and thereby kept

from falling, by a strut attachment underneath, as shown in the diagram, which represents two out of four sub-petioles (fig. 113). Having thus supported the leaf, the next point is to make effective electrical contacts, with middle points on the two neighbouring sub-petioles. In doing this, consider- able difficulty is encountered by reason of the waxy coating on the surface of midrib and leaflets. On this account there is apt to be no proper electrical contact between the moist thread of the electrode and the specimen. This obstacle may be overcome, however, by directing a jet of very dilute solution of ether or alcohol—either of which will dissolve wax—

upon the required point and immediately washing it off with water. A minute speck of kaolin paste is then attached to each of the electrodal spots, with the point of a fine brush. The electrical contact with the moistened thread is now found to be perfect. The E.M.F. is then applied and gradually increased, till the minimally effective intensity is found. If the two sub- petioles be equally excitable, then the same minimal stimulus will excite both to the same extent. If their sensitiveness be slightly different, then the extent of excitatory trans- mission will be different in the two cases. In the present case the excitabilities of the two sub-petioles were practically the same, and the minimally effective E.M.F. was found to be 4 volts. The exciting current was *7 micro-ampere. When the current was suddenly made by turning the Pohl’s commutator to the right, kathode-make, or Km, was effected on R, and anode-make, or A’m, on L. No effect was found to be induced at A’m, but excitation was induced at Km. This excitation not only caused closure of the pair of leaflets at the kathodic point but was irradiated in both directions from this point. Eleven consecutive pairs of leaflets under- went successive closure in the intra-polar tract, and three pairs in the extra-polar—a fact which may be indicated by the symbol 11 « > 3. In this convention it will al- ways be understood that < indicates intra-polar and > extra-polar transmission. Having observed the effects of Km and A’m, the circuit was then broken, thus causing kathode break, Kb, at R, and anode-break, A’b, at L. No excitatory effect was induced.

In order to test these results further, the key was now tilted to the left, making L kathode and R anode. This, it will be noted, is the reverse of the previous arrangement. The excitatory effects were now found to have changed their places, the new kathode K’ at L showing excitation at make, with a similar overflow as before—namely, II < > 7. It is obvious that only by accident could the excitability and conductivity of two different sub-petioles be exactly the

same ; hence the slight difference in the extent of trans- mission in the two cases. As regards the sub-petiole R, the anode-make, Am, showed no excitation. On break of the circuit, again, no excitatory effects were observed at either anode or kathode. This cycle of operations was repeated once more, with precisely similar effects. The results of these experiments will be seen, in proper sequence, in the following tabular statement, which sum- marises the effects caused by currents acting in a given direction—the direct—and its reverse, for the purpose of corroboration. It should be remembered that at any given moment, when a point on the one sub-petiole is made kathode, K, a point on the other is simultaneously made anode, A’. Excitatory contraction or closure is denoted by C, and the direction of the arrow indicates the trans- mission in intra- and extra-polar directions. O denotes absence of excitation.

The results given in the above table may be expressed in a more condensed form as follows :— TABLE II.—LEAFLETS OF Mimosa UNDER FEEBLE CURRENT E.M.F.=4 volts; current = ‘7 micro-ampere In another series the E.M.F. was gradually raised in successive experiments. The specimen in this case was older and less sensitive than the former. The minimally effective E.M.F. was here found to be 8 volts. Successive observations were taken with an E.M.F. of 8 volts, 12 volts, and 14 volts. In all these the excitatory effect only took place at kathode-make. The sole difference was in the increased extent of transmission of excitation with higher voltage. With 8 volts only one pair of leaflets underwent closure ; with 12 volts this was increased to 7 pairs ; with 14 volts to 9 pairs. With still higher voltage the responsive effects became those of Type II., which will be described presently.

I carried out twenty-five sets of experiments with different specimens of Mimosa on the effect of feeble current. The minimally effective E.M.F. was found to vary from 4 to 12 volts, according to the age of the specimen. The young leaflets were excited under a lower E.M.F. than the old. In all these, without a single exception, excitation was found to take place only at the kathode at make. Hence as the result of these direct and reverse experi- ments on the effect of feeble current, undertaken with various specimens of leaflets of Mimosa, we find one invariable result—namely, that kathode excites at make and that no excitation is occasioned by the break of kathode or by the make or break of anode. The excitation formula of the first type is, to follow the parallel nomenclature in the case of animal tissues, KCC, or, as I have preferred for the sake of convenience, the simpler formula Km, which is a short way of saying that excitation took place at the make of kathode. The excitation formula will always be represented by letters in thick type.

Effect of moderate current.—The characteristic effect of the first type is obtained, normally speaking, from an E.M.F. of 4 volts, and continues till the E.M.F. is raised to about 8 volts. In the case of young and excitable specimens we obtain here a transition from the effect of the first to that of the second type. With older specimens, the first type may persist up to I2 volts. Taking an excitable specimen of leaflets of Mimosa, I first applied 4 volts and obtained with it Type I. effect, that is to say, excitation took place only at the make of the kathode. I then raised the E.M.F. to 8 volts. At make, the kathodic point on the right became the seat of excitation, four pairs of leaflets undergoing closure in the extra-polar and six pairs in the intra-polar regions. There was no effect at the anode. At break, however, excitation was initiated at the anode, as evidenced by the serial closure of three pairs of leaflets in the intra-polar and two pairs in the extra-polar regions. This effect was corroborated by a reversal experiment, when the new kathodic point to the left exhibited excitation at make, and the new anodic point to the right exhibited it at break. Thus with current of moderate intensity, excitation took place in the sub-petiole of Mimosa at the make of kathode and break of anode. The polar excitation formula of the leaflets of Mimosa under moderate current may therefore be expressed as KCC, AOC ; or Km Ab.

E.M.F. = 8 volts; current = 1°6 micro-ampere R. Sub-petiole. L. Sub-petiole. Km | Kb | Am | Ab K’m | K’b | fe PUaterty Cc O O Cc Cc O Oo; |- ¢€ | Peel Ny EL rel | | Excitation formula—Km Ab I carried out this experiment on the polar effects of a moderate current on leaflets of Mzmosa with twenty-five different specimens, and obtained invariably the charac- teristic effects of excitation only at the make of kathode and the break of anode. No specimen appears better suited for experiments on polar excitation than the leaflets of Biophytum. There is but little difficulty here in securing good electrical con- nections; the conductivity is moderate, the velocity of transmission being about 3 mm. per second. The suscepti- bility of Biophytum to polar excitation is considerable, an extremely feeble current being sufficient to induce excitation. Its greatest advantage, however, lies in the rapidity of its recovery from excitation, which is practically effected in the course of about 3 minutes. Thus employing the same plant, experiments can be repeated under different conditions without undue waste of time.

Young leaflets are highly excitable and the older speci- mens lessso. Thus we have a wide range of selection accord- ing to the particular characteristics which we wish to bring into prominence. When the plant is highly excitable the bi-polar connections are made, on the middle points of opposite leaves, at a distance of 7 cm. from each other. The electrical resistance is high, being in the case described about 15 million ohms. With older and less sensitive specimens the two con- nections may be made on the same leaf, about 3°5 cm. apart. There will then be about five pairs of leaflets in the intra- polar and about two pairs in each of the extra-polar regions.

When the electrical connections are made with two leaves of fairly equal sensitiveness the excitations induced inthe two willbeequal. But if one leaf be old and the other young,then a given excitatory effect will occur earlier,or under feebler stimulus, in the younger. The minimally effective E.M.F. will thus be lower in this case than in the older. Effect of feeble current.—In order to study the effect of feeble current, I took a pair of leaves of Biophytum whose sensitiveness was approximately equal. The minimally effective E.M.F. was here found to be 4 volts, the current being of the extremely feeble intensity of -5 micro-ampere. The excitatory effect was induced at the right contact, when that point was made kathode, six pairs of leaflets

undergoing closure in the intra- and two pairs in the extra- polar regions. No effect was induced to the left by anode- make A’m. Nor was there any effect induced at break of current, Kb or A’b. On reversal, the left contact showed the excitatory reaction of K’m, two pairs of leaflets under- going closure in the intra- and none in the extra-polar regions. The difference in the extent of transmitted excita- tion as given in Km and K’m is here due to the fact that the left leaf was slightly the older and less sensitive of the two. On break there was no effect induced at either kathode or anode. I carried out in this way more than fifty experiments regarding the excitatory effect of feeble current on the leaflets of Biophytum. The minimally effec- tive E.M.F. in these cases varied from 4 to 8 volts, and the range of E.M.F. through which the effects of Type I. persisted was found to be between a minimum of 4 volts and a maxi- mum of 20 volts or thereabouts. In all these experiments of the first type excitation took place only at kathode-make and not at kathode-break, or anode-make or break. This result was without a single exception. The characteristic excitatory formula for Biophytum, under feeble current, is thus, as in leaflets of Mimosa, Km.

E.M.F. = 4 volts; current = ‘5 micro-ampere R. Contact. i Contact: Km | Kb Am Ab | K’m K’b A’m A’b (G O O | (e O O O Excitation formula—Km Effect of moderate current.—Taking another specimen, I applied an E.M.F. of 22 volts between two points in the same leaf, the intensity of current being 2°I micro-amperes. In carrying out the experiments, I found that the excitation took place at make of kathode and break of anode only. I give a sketch showing these effects of kathode-make and anode-break (fig. 114). The figure at the left shows the depression of leaflets starting from the kathodic point which was on the left. After their recovery the circuit was broken and excitation is seen to have taken place at the anode, on the right, and not at the kathode, on the left. Thus the leaflets of Biophytum, with a moderate current,

Fic. 114.—Illustration to the left shows excitation induced by the make of kathode, that to the right the effect of break of anode. gave the same reaction as the leaflets of Mimosa, the excitatory formula in both being Km Ab. I carried out more than fifty experiments on the effect of moderate currents on the leaflets of Brophytum, and the results without a single exception were as has been described. The tabular statement below shows the results due to a current of moderate intensity on leaflets of Biophytum as given in the typical experiment. Each experiment, it should be remembered, was carried through the usual cycle of direct and reverse currents.

TaBLE V.—EFFECT OF MODERATE CURRENT ON LEAFLETS OF Biophytum the sensibility is much less than that of the leaflets of Mimosa and Biophytum, will next be described. Amongst these the first plant dealt with is Neptunia. This plant is somewhat like Mimosa pudica in appear- ance, except that under natural conditions in Bengal it grows in water, being an inhabitant of the pools amongst the ricefields. In these circumstances it develops curious growths on the stem, which serve as floats. The leaves are much longer than those of Mzmosa, but considerably less sensitive. There are three pairs of sub-petioles, which are articulated to the main petiole. Electrical connections are made with two points at the middle of two sub-petioles which carry numerous leaflets. The sensitiveness of the leaflets of this plant is much less than in Mimosa. The conducting- power is also very moderate, the velocity of transmission of excitation in one of the sub-petioles being 1°5 mm. per second.

Effect of feeble current.—The minimum E.M.F. which will induce excitation is higher in the case of Neftunia than in Mimosa, being 20 volts in the typical instance now to be described. The resulting current was 7 micro-amperes. On going through the usual cycle of direct and reverse experiments, it was found that the kathode alone excited during make. From experiments with five other specimens I obtained the same result without exception. E.M.F. = 20 volts; current = 7 micro-amperes | R. Contact. L. Contact. | | | | Pitas Km | Kb | Am | Ab K’m Kp yl) Am A’b Cc O Oe Ort ene O O O Excitation formula—Km

E.M.F. still further, I found the characteristic effects of Type II. to take place under an E.M.F. of 30 volts, the resulting current being II micro-amperes. Excitation thus takes place, with moderate currents, at the make of kathode and the break of anode. This result was borne out without any exception by a series of experiments on five different specimens. TaBLE VII.—EFFEcT OF MODERATE CURRENT ON LEAFLETS OF Neptunia E.M.F. = 30 volts; current = II micro-amperes R. Contact. L. Contact. | | | Bees | | Km Kb) ame | Ab 5G 1 A’m A’b C ye OE | SG ee | O | Oe Excitation formula—Km Ab

It will be seen that, in this case as in others, with feeble current kathode alone excites at make, and with moderate current excitation takes place at make of kathode and break of anode. These leaflets are as a general rule but slightly sensitive, and require a comparatively high E.M.F. to induce excitation. The bi-polar connections are here made on two points of the same leaf, separated from each other by a distance of 7 cm. The electrical resistance of the petiole is high, being about 5 million ohms when the connections are as described. The velocity of transmission of excitation, also, is relatively feeble, being in a representative case ‘5 mm. per second. Under feeble stimulus the excitation travels through a short distance only ; still feebler excitation remains more or less localised.

Effect of feeble current.—With young and excitable specimens the minimally effective E.M.F. is 10 volts, the resulting current being 2 micro-amperes. Generally speak- ing an E.M.F. of 22 volts and a current of 4 micro-amperes are necessary. On starting the current, a pair of leaflets at the kathode underwent an excitatory fall. No effect was induced at the left anodic contact. At break there was no effect at either. On reversal, the left contact became kathode, resulting in the excitatory fall of a pair of leaflets. This local character of excitation was due to feeble con- ductivity. It is only under much stronger polar excitation that a moderate amount of transmission takes place. At the anode there was on this occasion no effect at make—nor was there any excitation at break of kathode or anode.

This experiment was repeated in the case of ten different specimens, the result being invariably the same. TABLE VIII.—EFFECT oF FEEBLE CURRENT ON LEAFLETS OF Averrvhoa carambola Effect of moderate current.—In order to obtain the characteristic reactions of Type II., the E.M.F. has generally, in the case of Averrhoa carambola, to be raised to 50 volts or of 50 volts and with a current intensity of r1 micro-amperes, excitation took place at kathode at make and at anode at break. The experiment was repeated with ten different specimens, the result being always the same.

The excitatory reaction in this plant is thus charac- teristically the same as in others already described. The excitability of these leaflets is perhaps slightly greater than that of A. carambola. I carried out the usual cycle of observations on the effects of feeble and moderate currents, employing in each case five different specimens. The two bi-polar contacts were made in the case of the same leaf 7 cm. apart. Effect of feeble current.—The excitatory effect of kathode- make characteristic of Type I. was exhibited with an E.M.F. of 10 volts, the current being 6 micro-amperes. The electrical resistance offered by the petiole of this plant was much less than that of A. carambola. All the specimens, without exception, gave the effects characteristic of Type I. under feeble current.

E.M.F. = Io volts; current = 6 micro-amperes pet 4 eee aE: T Se ae R. Contact. | L. Contact. ioe, ae ara | Km | Kb | Am | Ab | K’m K’b | Ae A’b c OF doer ae O € O | O oO Effect of moderate current.—When the E.M.F. was raised to 28 volts, with a resulting current of 16 micro-amperes, the effects induced were characteristic of Type II. That is to say, excitation was shown at the kathode at make and at the anode at break. Repetition of the experiment on other specimens gave similar results.

Avervhoa bilimbi E.M.F. = 28 volts; current = 16 micro-amperes R. Contact. L. Contact. mai | Am.| Ab | Km | Kb | Am | AD Gwin .O O Cc Cc O O ; & I have already described in the previous chapter the polar effects of feeble currents on the pulvinus of Mimosa. In that case the mono-polar method of experiment was employed. The bi-polar method will now be taken up, enabling us to observe the make-effect of anode and kathode in two different organs simultaneously, and, in the same way, the break-effect ; all these to be further corroborated by reversal experiments.

Effect of feeble current.—The bi-polar connections are made either directly with, or in the close vicinity of, two different pulvini, at varying heights on a single main stem of Mimosa. With highly excitable specimens the polar effects characteristic of Type I. may here be obtained with as low an E.M.F. as 2 volts. In another case the applied E.M.F. was 4 volts, and the resulting current 4 micro-amperes. On going through the usual experimental cycle—with direct and reverse currents—it was found that the kathode alone exhibited excitation at make. There was no excitation either at kathode-break or anode-make or break. Similar experiments were carried out on some fifteen different plants of Mimosa, the results without exception being as described. The only variation seen in the case of different individuals

lay in the value of the minimally effective E.M.F., which in less excitable specimens was sometimes as high as 8 volts. E.M.F. = 4 volts; current = 4 micro-amperes R. Leaf L., Leaf Km Kb | estan | Noy | Km | Kb | Anas foe O O lis 2 Cc O O |. ae Effect of moderate current.—Under an increasing E.M.F. the characteristic polar effects seen in the pulvinus of Mimosa are, as in other cases, transformed into those of Fic. 115.—Under 8 volts excitation at kathode-make; under 12 volts excitation at kathode-make and anode-break.

Type II. Thus in a particular case the E.M.F. applied was IO volts, the resulting current being 8 micro-amperes. In carrying the experiment through the usual cycle of direct and reverse currents it was found that the kathode excited at make, and the anode at break. In fig. 115 are shown a series of records given by a different leaf under kathode- make, kathode-break, anode-make, and anode-break. It will be seen that under a feeble current, due to an E.M.F.

of 8 volts, the reactions are those characteristic of Type I. But when the acting current was increased by applying the higher E.M.F. of 12 volts, the responsive characteristics, as was to be expected, changed to those of Type II. Whereas under feeble current the kathode alone had excited at make, now under moderate current excitation occurred at both kathode-make and anode-break. These distinctive effects of the moderate current were obtained with no fewer than fifteen different specimens in spring, and were thus shown to be characteristic of plants in normal condition. Later in the year, however, seasonal changes were seen to induce certain modifications of the polar effects; these will be fully described in another chapter.

It will now be advisable to recapitulate the results described in the present chapter, in order to show how universal these phenomena are. I will first give a tabular statement of the effect of feeble current :— I next give a table showing results obtained with moderate current :— It is here shown that we have examined not one species only but all the species that were available for experiment. More than a hundred experiments have moreover been carried out on different specimens. The result of all these investigations has been to demonstrate clearly the fact that in plants, under a feeble current, it is the kathode alone that excites at make, while under a moderate current excitation takes place by the make of kathode and the break of anode.

Thus, so far from there being any necessary divergence in the excitatory polar reactions of fibrillated and unfibrillated protoplasm, we find on the contrary that they are identical in the plant and in the animal. In other words, the respon- sive characteristics of Type I. and Type II. in plants corre- spond in every respect to those in animal tissues which are classed under Pfluger’s Law as Stage I. and Stage II. This is not by any means to be taken as a complete statement of the law of polar effects. We shall find that if the current be increased still further, certain new types of effects make their appearance which will be described in detail in a subsequent chapter. In the meantime I have to describe certain further characteristics of these polar effects

and the relative sensitiveness of plant and animal to this form of stimulation. These will be dealt with in the follow- ing chapters. The polar effects observed in diverse sensitive plants are in every way similar to those in animal tissues. THE Laws I. With feeble current the kathode excites at make and not at break. The anode excites at neither make nor break. II. With current of moderate intensity the kathode excites at make and not at break. The anode excites at break and not at make.