Researches on Irritability of Plants

We have seen that with strong current there is excitation at both kathode and anode; on the break of the current, excitation was now found to take place only at the anode. Thus we have the excitatory effect of Type II].—namely Km Am Ab—established independently by an experiment on the leaflets of Mimosa. Employing the minimal current essential to the mani- festation of Type III. we find the excitation at anode- make somewhat localised, whereas that at kathode-make is transmitted to a considerable distance. But on increasing the current the power of excitatory transmission of anode- make is greatly enhanced.

when the current had been increased to 12 micro-amperes, excitation took place at both kathode-make and kathode- break and also at anode-make and anode-break. Here we have the characteristic effects of Type IV.—namely, Km Kb Am Ab. The following table gives a synopsis of these results :— Intensity of current. Excitation induced. | Characteristic type. 2°5 Km Ab Type II. 5 | Km Am Ab Type III. 2 Km Kb Am Ab Type IV. These conclusions were uniformly supported by the results of no fewer than thirty different sets of experiments on the leaflets of Mimosa. The only variations between different experiments lay in the fact that with less sensitive specimens a relatively higher current was required. Thus in a leaf which was somewhat older, and therefore less sensitive, the intensity of the current necessary for Type I. was I micro-ampere ; for Type II. 3 micro-amperes ; for Type III. 7-4 micro-amperes ; and for Type IV., 15 micro- amperes.

With leaflets of Biophytum I obtained results which were practically the same as those given by leaflets of Mimosa. Thus with a given experimental specimen, the characteristic effect, Km of Type I. was exhibited under a current-intensity of *5 micro-ampere. When the current was increased to 2 micro-amperes the response was transformed to Km Ab, that of Type II. At 3°5 micro-amperes the characteristic effects of Type I1I—Km Am Ab—made their appearance.

And finally, with a current of Io micro-amperes the typical response of Type IV. was obtained—namely, Km Kb Am Ab. These results are shown in the following tabular state- Ment =— Intensity of current. Excitation induced. Characteristic type. These results were confirmed by fifty sets of experiments on different specimens of Biophytum. These leaflets are, as already explained, relatively insensitive and therefore require a higher E.M.F. with higher current. With these specimens also I obtained, as before, the four types of effects in their usual sequence. Thus ina given experiment a current of 4 micro-amperes gave Km,

Intensity of current. Excitation induced. | Characteristic type. 4 Km Type I. II Km Ab Type II. 20 Km Am Ab Type III. 30 Km Kb Am Ab Type IV. the effect indicative of Type I.; when the current was now raised to II micro-amperes excitation at Km Ab, constituting Type II., was obtained. With a current of 20 micro- amperes the result was Km Am Ab, or Type III. And finally, with 30 micro-amperes the characteristic reaction of Type IV.—namely, Km Kb Am Ab—was observed.

These results were confirmed with ten different speci- mens. I have thus described about a hundred experiments with different species of sensitive plants which conclusively demonstrate the existence of Types III. and IV. of polar * reaction. There remains an alternative hypothesis in regarding the effects seen in Types III. and IV., as in some way due to the production of secondary poles. The exact physical conditions under which the formation of the secondary pole is possible so as to cause complications in the excitatory phenomena are clearly shown in the experiments of Engel- mann and Biedermann on the ureter of rabbit. In an insulated specimen, under a moderate current, it was found that excitation took place only at the kathode at make and at anode at break. But when the specimen was laid on a good conducting support, such as salt clay, then the polar reactions were found to be exactly reversed—that is to say, the anode excited at make and the kathode at break. This opposition of effects under differing circum- stances is explained by the fact that in the second of these cases we have a conducting-sheet which gives rise to diffusion of the current and a rich development of secondary poles. Thus, opposite to the kathode, there are produced numerous secondary anodes, and conversely, opposite to the anode, there are numerous secondary kathodes. It is these secondary kathodes which are effective in causing excitation in the neighbourhood of the anode at make. In the neigh- bourhood of the primary kathode, on the other hand, excita- tion is prevented by the depressing influence of the secon- dary anodic points. The presence of secondary poles thus induces an apparent reversal of the normal effects, which is simultaneous at the two electrodes. The simultaneity

of the reversal is therefore a presumptive evidence for the occurrence of secondary poles. It may be asked next whether the effects seen in Mimosa as Types III. and IV., opposed as these are to Pfluger’s Laws, could be explained away by any of the physical conditions of the experiment. In answer to this it may be pointed out that in the experiments described, which were carried out according to the bi-polar method, the electrical connections were made directly on the two contractile pulvini themselves, the thread forming a complete loop round each organ, which was thus equally and throughout its circumference anode or kathode, as the case might be. The organ in this case, moreover, was isolated and free, like the insulated ureter. Hence there was an absence of all those conditions which might favour the formation of secondary poles. Had there been any such possibility we should have expected to see the reversal of normal effects, more or less, from the beginning, and the reversal should have occurred at the two bi-polar contacts simultaneously.

Instead of this, we found on the contrary that in the first two stages, with feeble and moderate currents, the reactions of the organ were absolutely normal. It was only after this, with the same specimen and with identical connections, that by merely increasing the current we obtained the effect characteristic of the third type— namely simultaneous excitation of kathode and anode at make, and excitation of anode at break. Had there been any induction of secondary pole, the result would have been excitation at anode at make and at kathode at break.

From these considerations, in addition to others already given, it appears that the normal effect of strong currents on Mimosa and other sensitive plants is to cause excitation at both kathode and anode at make, and anode only at break. These results, and those of Type IV. which immediately succeed, are definite and different from either of the two types that had previously occurred. These characteristic excitations, moreover, cannot very well be explained on the assumption of hypothetical secondary poles. The view that these effects are physiological will become strengthened when in the course of the next chapter we shall observe the modification of polar effects under physiological changes.

As moderate increase of intensity of current transforms the polar reaction from Type I. to Type II., so also further increase in the intensity of current gives rise to reactions indicative of other types. Pfluger’s Law is not a complete statement of the polar action of currents. Under strong intensities of current, two additional types of reaction—Type III. and Type IV.— make their appearance. These are included in the following supplementary LAw of POLAR EFFEcT of STRONG

Under the action of strong current, excitation takes place at the make of kathode and make and break of anode. Under still stronger currents, excitation takes place at the make and break of both kathode and anode. These four characteristic types of reactions are serially observed in all sensitive plants under increasing intensities of current. Modification of polar reaction under tissue-changes—Effect of age— After-effect of moderate stimulation—Modified polar effect : excitation, at kathode-make and anode-make; excitation at kathode-make, kathode-break, and anode-make—General review of polar reactions.

In the course of my investigations into the effects of currents on the pulvini of different leaves of Mimosa, I found that during the beginning of the summer season the normal effects which have been described occurred uniformly, in their proper sequence. But later in the year, at the end of the rainy season, when the plants had begun to seed, I was puzzled by the appearance of certain new and unexpected types of response with which I had not hitherto been familiar. Thus in the case of certain leaves, with a moder- ate current the excitatory reaction took place only at make of kathode and make of anode. It will be remem- bered that in Amphistigma also this was the characteristic response observed by Verworn. With certain other leaves, again, under fairly strong current, the polar excitations took place at the make of kathode, at the break of kathode, and at the make of anode. This again was like the responsive reaction in Actinosphaerium observed by Kihne. I found a very large number of leaves which gave these specific reactions, which precluded the idea of their being accidental. These were most frequently exhibited late in the season and also in winter.

I made many experiments with a view to solve these anomalies, and found that the normal polar reactions of plants are modifiable, to a greater or less extent, by various physiological factors. Among the most important of these may be mentioned the influence of age and of season. Experiments on the effect of age in the modification of response will first be described. It is impossible to dissociate from the consideration of the age of a given leafits past history as regards the stimulus ofsunlight. If we examine a plant we find that the youngest leaf is quite green, not having as yet been long exposed to the action of light. The next below it will be older, and owing to the longer action of the sunlight, reddish-brown in colour.. The leaves lower down will be older again and still more russet in tint. In this way the sequence of the leaves in point of age, from above downwards, corresponds to the other sequence of duration of exposure to stimulus of light.

It will be remembered that we may broadly classify the condition of a tissue under three different phases : first, the pre-optimum or sub-tonic state; second, the optimum ; and third, the post-optimum condition, in which we see an approach towards the condition of fatigue. Moderate stimulation, as we have seen, will carry a tissue out of the pre-optimum towards the optimum condition, with con- sequent enhancement of excitability. Excessive or too- prolonged stimulation, on the other hand, will carry it to the post-optimum condition with the characteristic depression of its excitability.

From this point of view alone, then, we might expect that the uppermost or youngest leaf of Mimosa would be in the pre-optimum and therefore less sensitive condition ; that the sensitiveness of the leaves should attain a maximum as we descend lower in the plant ; and that after this has been reached, continuing to descend, the excitability of the different leaves will be progressively decreased. Represent- ing these gradations by means of a curve, there would be at first an ascent, then a climax, and after this asharp turn and

descent. Independent of or concomitant with this will be the changes of excitability, more or less obscure, which are brought about by increasing age. Before subjecting this inference to the test, I will describe an experiment which shows that the after-effect of moderate stimulation on a sub-tonic tissue is to raise its susceptibility to polar excitation. For this purpose I used a specimen which appeared to be in a pre-optimum condition, and took its records under

Fic. 127..-Transformation of Type II. to Type III. as after-effect of previous stimulation. First two responses taken before, and last three after, tetanisation. polar excitation by an identical current, before and after the application of tetanising shocks of moderate intensity and duration. Under an E.M.F. of 16 volts, the records obtained show excitation (fig. 127) at kathode-make and anode-break, characteristic of Type II. The record was then stopped and tetanising electric shocks of moderate intensity applied for 2 minutes. After a period of rest of 15 minutes, the record of polar excitation T was taken once more, the applied

E.M.F. being the same as before—namely, 16 volts. It will be remembered that before tetanisation 16 volts, though effective for Ab excitation, had been ineffective for Am excitation. After tetanisation, however, the Am excitation became effective. The after-effect of moderate stimulation had thus been to transform Km Ab, the polar reaction of Type II., to Km Am Ab of Type III. As already mentioned, from the point of view of stimulus the youngest green leaf at the top of the plant, say Lr, may be regarded as being in a sub-tonic condition. A leaf lower down, L2, must be taken as having been previously

subjected to moderate stimulation. From the experiment just described, then, we should expect that one identical current would evoke response of a higher type from L2 than from Lt. In order to test this inference I took a plant and made bi-polar connections with the second and fourth leaves from the top, the former of these being very young. Besides these, a second pair of bi-polar connections was also made, between the fifth and seventh leaves in descent, and these will be referred to as L3, which was old, and L4 which was very old. The results obtained from the second pair will be dealt with separately.

with Lr and Lz. The mode of procedure was to apply a current which was gradually increased till successive types of responsive reactions were observed in one or other of the two leaves :— It is apparent that while a maximum current of 8 micro- amperes induced responses of Type I. in the very young leaf L1, a very much feebler current, of 5°6 micro-amperes was sufficient in the case of L2 to induce Type II. A similar relative exaltation of effect in slightly older specimens occurs in subsequent types also. Thus while I2°7 micro-amperes gives rise in Li to Type II., half that intensity is enough to induce Type III. in Lz. And lastly, while 20 micro-amperes gives Type III. in Li, the same current gives Type IV. in Lz.

Having thus verified our inference with regard to the relative excitabilities of two young leaves, we may expect contrasted results with older leaves, where the excitability will be on the wane. Table V. shows the comparative effects of increasing currents on leaves of the same plant, L3 and L4, which are old and very old respectively. For greater facility of comparison the effects on Li and Lz are repeated. micro-amperes | micro-amperes | micro-amperes | micro-amperes

From this experiment it will be seen, (1) that the general excitability, having reached a maximum in L2, has under- gone a progressive decline with age; (2) that while the excitatory efficiency of Km has undergone but slight decline, that of Abis very marked. This is seen in the fact that while Ab excitation was exhibited by the youngest leaf Lr under 12°7 micro-amperes, and by L2 under 5°6 micro-amperes, no such excitation was exhibited by L3 and L4 under a current as strong as Ig micro-amperes. The decline in excitatory efficiency of Am, on the other hand, was but slight ; in Lr anode-make excitation was induced by 20 micro-amperes, in L2 by 6:3 micro-amperes, in L3 by 20 micro-amperes, and in L4 by 23 micro-amperes. It will be noticed that in the two leaves L3 and L4 there was no excitation of type Km Ab.

We will now consider the possibility of the appearance of new types of polar reaction by the progressive diminution of Ab excitation. A reference to Table V. will show that by Fic. 128.—Abrupt transition from Type I. to Type III., with the vanishing of Type II. its greater loss of excitatory efficiency Ab has been reduced to the level of the excitatory reaction of Am. Normally speaking, the excitation induced by anode-break Ab takes place earlier than that caused by anode-make Am. And this is the reason why Km Ab, or Type II., precedes Km Am Ab,

or Type III. But now when Am is equal to Ab the latter cannot precede, and the two occur at the same minimal intensity of current. Hence the sequence of excitation with increasing current is I., Km; II., absent; and III., Km Am Ab. A series of records are shown in fig. 128, in which Type I.,Km., passes abruptly to Type III.,Km Am Ab, as the acting E.M.F. is slightly increased from 5 to 6 volts. It may be stated here that late in the season, and with somewhat old leaves, I was frequently puzzled by this vanishing of Type II.; but the quantitative explanation which has just been given will adequately account for this.

We have seen that under certain conditions the Ab excitation declines at a greater rate than that of Am, and have considered the resulting effect when the _two become equal. In certain circumstances it is quite conceivable that this relative decline of susceptibility to excitation by anode-break might proceed further. It may then happen that Am becomes more effective than Ab. The sequences of excitatory effects would then be I., Km; II’., Km Am; III., Km Am Ab. It will here - be noticed that to avoid adding to the number of types, I have designated Km Am-—which takes the place of Km Ab—as IV’. or transitional II. In the accompanying table these theoretical modifications of type, due to the decline in excitatory efficiency of Ab, will be seen displayed in a convenient form. Under normal conditions the excitatory

efficiency of Ab is greater than Am; under modification it may become (a) equal to, or (b) less efficient than, Am. The existence of modification (a) with the vanishing of Km Ab has already been demonstrated. We next come to the theoretical possibility of a modified type Km Am. As already stated, in certain circumstances, especially late Fic. 129—Polar reactions, Km, Km Am, Km Am Ab, under gradually increasing current. in the season and in older specimens, I have not infrequently met with this type. A series of records are reproduced in fig. 129, in three cycles of effects of increasing current. In these the acting E.M.F. was increased from 6 to 8 volts and then to 10 volts. It will be seen that the stage Km Ab has apparently vanished, its place being taken by Km Am. An account of results obtained with four different specimens, under gradually increasing E.M.F., is given in the following table, the reaction under a particular type being indicated by an inclined cross.

vanishing of Type Km Ab and a substitution of the transi- tional type Km Am. It may be mentioned here that in some cases this characteristic excitation, at make only of both kathode and anode, was found to persist through an extended range of current-intensities. The resemblance of this type to the response of Amphistigma is obvious. If the excitatory efficiency of Ab were to fall still further, it might be reduced even lower than the excitatory effective- ness of kathode-break. In sucha case, with a given current, excitation would be less for Ab than for Am or Kb. Hence the anode-break effect would vanish and the excitatory formula for this type under a certain intensity of current would be Km Kb Am, and this we may call III., Tvansitional. It is interesting to find that this particular responsive modification is not seldom obtained, especially with leaves the petioles of which are very thin.

The following table shows such results obtained from four different specimens :— A record which exhibits the Km Kb Am excitation is given in fig. 130. current on the subject. In the first place there are many who hold the universality of Pfluger’s Law. Against their views is brought the fact that in the unfibrillated protoplasm of Protozoa the reactions observed are more or less opposed to those of Pfluger’s generalisation. Next are those who hold with Verworn that, in view of the anomalous reactions of unfibrillated protoplasm in Protozoa, there could not possibly be any law of polar reactions of universal applicability.

There is, however, a third view still possible. I have shown that the reactions of fibrillated and unfibrillated protoplasm are not necessarily opposed, since the plant, within certain limiting values of current, shows reactions identical with those which are normal in animal tissues. If then we find that the generally recognised reactions of stage I.—namely, response at kathode-make only—can be transformed into those of stage II.—response at kathode- make and anode-break—by merely increasing the intensity of current, then there can be no inherent impossibility in further transformations into Types III. and IV. under still further increase of the current-intensity.

The possibility of such transformations has now I think been demonstrated by the experiments described in this and previous chapters on numerous species of sensitive plants. It has also been shown, further, that these polar reactions were subject to variation under physiological modification of the tissues. And finally, in plants whose reactions were already established as identical with those of animal tissues, it has been shown that—under the separate or joint action of an increasing current and physiological modification— responses could be obtained which were similar to the so-called anomalous responses characteristic of Protozoa.