Plant Response as a Means of Physiological Investigation

We have already studied in detail the autonomous responses of the leaflets of Desmodium ; but there are also other leaves which execute similar pulsatory movements, and we have now to investigate the effect of the stimulus of light on such multiply-responding organs, either in initiating these movements or in modifying them. We have also another instance of such multiple pulsations, in the swimming movements of ciliated organisms, of which the causes are at present regarded as very obscure. The treatment of these I include within the present chapter, because I hope to show that they really belong to a class of phenomena essentially similar to the autonomous movements of Desmodium, and that from this point of view all their seeming peculiarities may be very satisfactorily explained.

In the first place, I shall take up the question of multiplyresponding leaves or leaflets. Concerning the effect of light on these, there is a certain amount of discrepancy of observation. Strasburger, for example, referring to such movements in the lateral leaflets of Desmodium gyrans, says they are in no way disturbed by variation in the intensity of light. According to PfefTer, similarly, the autonomous movements of the leaflets of Trifolium pratense are not affected by illumination. But Strasburger again states that the leaflets of this plant, on exposure to light, cease their oscillations.

Investigations on the influence of light on the lateral leaflet of Desmodium gyrans. — (a) In sub-tonic condition.— A clearer insight into this subject will, however, be obtained when I describe my experiments on Desmodium gyrans. I have shown that when this plant is in a sub-tonic condition it behaves like an ordinarily responding plant, such as Biophytum ; a single moderate stimulus then evokes a single response, and strong stimulus multiple responses. Thus a leaflet of Desmodium in a state of standstill has multiple response initiated by the incident stimulus of light. The following record

FlO, 275. Initiation of Multiple Response in Lateral Leaflet of Desmodium originally at Standstill Light applied at x and continued till the end of the sixth response, as shown by the thick line. The responses show a staircase increase with increase of absorbed energy. Pulsations persist for a short time even on the cessation of stimulus. (fig. 275) shows this clearly. The leaflet was in a quiescent condition, but during the application of light it exhibited multiple responses, which, owing to the increasing absorption of energy, showed a staircase enhancement of amplitude. On the cessation of light the energy absorbed maintained the pulsation for some time.

(b) In normal tonic condition. — When the specimen, however, is in a favourable tonic condition, its pulsations being apparently autonomous, the application of excessive stimulus of strong light often brings about fatigue, and under these circumstances I have obtained two different types of response, according to the particular condition of the tissue. In the first of these, progressive fatigue, under the continued action of light, is shown by the gradual diminution of amplitude of pulsation. If the strong stimulus of light, which causes this fatigue, be applied for a short time only, the pulsation whose amplitude was diminished, again on the cessation of stimulus recovers its first amplitude in a staircase manner ; but if the light be long continued, fatigue becomes so great that the pulsatory movements are brought to a stop, at least for a considerable period. We have seen again that an organ sometimes exhibits fatigue somewhat differently from this, that is to say in a periodic manner. In accordance with this fact the leaflets of Desmodium are sometimes seen to display periodic fatigue — that is to say, the pulses wax and wane in groups, an example of which is seen in fig. 276.

Thus we see that in a leaflet at standstill, light, by supplying energy, initiates autonomous movement ; while in an actively pulsating leaflet, strong illumination may bring about such fatigue as to cause cessation of movement At still other times, again, owing to the favourable condition of the tissue, fatigue is slight, and there is no arrest of activity. These facts are sufficient to explain the discrepancy already mentioned, in the observations made by Strasburger and by Pfeffer, on the action of the leaflets of Trifolium pratense under light.

Changes induced in existing anisotropy of Desmodium leaflets. — A very interesting consideration arises at this point as to the differential fatigue caused in the anisotropic organ by strong stimulus. We have seen that the lower half of such an organ is the more excitable, hence we can see the probability of fatigue being relatively greater there. Now, it is the naturally greater excitatory contraction of the lower half of the organ which causes the downstroke of the leaflet

to be the quicker ; but if stimulus produce greater fatigue of the lower half, we can see that its movement will be made somewhat slower, and the upstroke will then become the relatively quicker of the two. This anticipation finds remarkable verification in the photographic record given in fig. 276. We see there, in the first or normal record, from the fineness and steepness of the upward line, representing the downstroke, that this is the quicker movement of the two ; but after the application of strong stimulus of light, at the point marked with an upward

Fig. 276. Photographic Record of Autonomous Pulsations in Lateral Leaflet of Desmodiitm gyrans under Action of Sunlight, showing Periodic Reversals Light applied continuously from arrow onwards. In the first two responses, representing the normal, the downstroke of the leaflet, represented by the up curve, is relatively the quicker. After the application of light, this relation is gradually reversed, till in the fourth and fifth pulses after application it is the upstroke, represented by the down curve, which is pronouncedly the quicker. This reversal is in its turn reversed at the eighth pulsation.

arrow, we observe a gradual change, by which the existing normal difference between up and down strokes is first abolished and then reversed. In the next complete response, after the application, we see that the two strokes have become equally rapid. In the next, the downstroke has become distinctly the slower, and this goes on progressively till we see in the fifth of this series a very remarkable degree of difference between the two, the upstroke being now much the quicker. These reversals are found to be recurrent, further on in the record. Such alternate changes of excitability on the two sides we have noticed even in the case of radial organs ; for

we have seen that such an organ, when acted on unilaterally by strong light, often exhibits to-and-fro oscillations, due to alternate fatigue of the two sides. We thus find, starting with a Desmodium leaflet in a state of standstill, that moderate intensity of light initiates normal pulsatory movements, in which the downstroke is quicker than the upstroke ; but, in a pulsating leaflet, under intense or long-continued light, these beats are reversed, that is to say the upstroke becomes the quicker. Under the action of continuous light, again, these reversals themselves may become periodic or recurrent.

Reversals under intense stimulation seen in all forms of response.— We have already seen that autonomous pulsation is simply ordinary response repeated, owing to excess of energy ; and that the Desmodium leaflet is an ordinary anisotropic organ, in which response may be initiated by any form of stimulation — thermal, photic, chemical, or electrical. We have just seen, further, in the case of photic stimulus, that the character of the response may be reversed by the intensity of the stimulation. Thus the normal response, in which the downstroke is more energetic than the upstroke, may be exchanged for a type of response in which the upstroke is quicker than the down. This law of reversal of response with varying intensity of stimulation has already been shown to" be illustrated in different types of response. For example, in the case of chemical stimulation, we found that the leaf of Mimosa when subjected to the action of a dilute solution of sodium chloride gave an erectile response, whereas when the solution was stronger it responded by depression (pp. 551, 552). In the response of growth, again, very dilute and very strong solutions of a given reagent were shown to produce opposite effects. Thus, while dilute solution of sugar accelerated, a very strong solution retarded, the rate of growth (p. 488).

In the case, then, of an organ which is capable of multiple response, we find that any form of stimulus is competent to initiate such response, and that, with regard to certain characteristics, such as the relative quickness of the up or down movement, it may be exhibited in opposite ways, according to the intensity of stimulus. The swimming movements of ciliated organisms.— It is the automatic character of ciliary movements which has hitherto rendered them a subject of such great perplexity, for these movements are independent of any nervous system for their initiation or control. The impulses that lead to ciliary motion arise in the cilia themselves. There is, again, some difficulty in understanding the mechanics of these movements.

Their automatism may, however, be explained by those considerations which are now familiar to us in similar instances— that is to say, initiation of movement by external stimulus, and its maintenance by excess of latent energy. There is, again, the closest resemblance, from a mechanical point of view, between the movements of the cilia and the movements of Desmodium leaflets. In both cases alike, one of the component strokes is quicker than the other. Again, though in both cases we meet with instances in which the up and down movements take place in the same plane, yet there are also others in which more complicated paths, whether circular or elliptical, are described. The fact that one movement is quicker than the other, and that the strokes are lateral, shows that we have in the cilium "an anisotropic organ, essentially similar to the pulsating pulvini of the lateral leaflets of Desmodium. The only difference between the two cases lies in the fact that we have in the pulvinus a multicellular, and in the cilium a unicellular, organ.

Let us suppose a detached petiole of Desmodium, bearing the lateral leaflets, to be thrown into water contained in a glass trough ; let us further suppose these leaflets to be in a sub-tonic condition, that is to say in a state of standstill. If this vegetable organism be now stimulated by sunlight of moderate intensity, striking it horizontally from one side of the vessel, it will be found that rhythmic excitation is initiated, and that the stroke backwards is much quicker and

more energetic than that forwards. The consequence of this will be, in the vegetable organism as in the case of a man swimming, its forward propulsion. This result depends upon the fact that the lower half of the anisotropic organ is in this case the more excitable. Reversal of the relative activities of the two halves of the dorsi-ventral organ was, however, seen to occur in the case of the leaflets of Desmodium when the intensity of stimulation was very great. Such a reversal, under excessive stimulation, would give rise then to a swimming movement in the opposite direction. We also saw such reversals under continuous stimulation of light becoming periodic (fig. 276). The corresponding swimming response would thus consist of a movement to and fro.

Supposing, however, that the excitability of the upper half of the motile organ had been the greater, it is clear that the normal excitatory response would have taken the form of a backward or negative swimming movement. We thus see the possibility of normal responsive swimming movements of two different types, according to the particular half of the anisotropic motile organ which is the more excitable. Taking, again, that type of swimming in which the response is positive or forward, a stronger intensity of stimulation may give rise to a reversal, or negative movement. And from what has already been said, it will be seen that similar responsive movements may also be expected under forms of stimulation other than light.

Similarity between swimming responses and the ordinary heliotropic responses of radial organs. — Though at first sight it would appear as if there were no connection between the simple responsive curvatures of radial organs and the apparently complicated responsive movements of swimming, yet on a closer analysis we shall find that there is little essential difference between the two; for we have seen that growth itself, or growth-curvature, is simply a phenomenon of multiple responsive movements, which, owing to the rapidity of the individual responses, appears continuous. Hence, when, under moderate stimula-

tion, the organ moves towards the light, or exhibits a positive response, this means that the resultant of its multiple movements is towards the stimulus, like the resultant movement of the ciliated organism towards light. Similarly, under strong photic stimulation, the negative heliotropic movement of the organ corresponds to the swimming of the ciliated organism away from light. In the intermediate case, again, where the stimulated organ shows no resultant curvature, but oscillates about a mean position, we have an instance which is paralleled, in the case of the ciliated organism, by alternate swimming backwards and forwards.

Again, just as in the heliotropic response of a radial organ, the minor pulsations by which it is brought about are too rapid and minute to be easily detected, and we can perceive only the resultant movement of the organ as a whole, so in the case of the ciliated organism the individual beats cannot easily be perceived, and we infer their presence from the resultant motion of the organism as a whole. Phototactic movements. — From the fundamental demonstrations which I have already given of the characteristics of multiple response, and its modification by relative variations of contractility, as between the upper and lower halves of the responding organ, it will be found that the multifarious responsive movements of ciliated organisms, under various forms of stimulus of differing intensities, will have been elucidated.

(a) Two natural types of responsive movements.- -The two opposite types of movement, positive and negative, which we have now theoretically anticipated, are found to be completely illustrated in the case of swarm-spores under the action of light. Thus, for example, in the case of Botrydium granulatum, they respond by a positive swimming movement, or motion towards the light. Again, while certain varieties of Ulothrix exhibit the positive effect, by movement towards light, there is another variety which gives the negative response, by swimming away from it. This .opposition of effects is obviously due, as we have anticipated, to a

natural difference of relative excitabilities, as between the upper and lower halves of the anisotropic swimming organ (p. 695). (b) Responsive movements positive, negative, or intermediate, according to intensity of stimulation. — Turning now to the question of different movements in the same organism as modified by the varying intensity of illumination, examples are furnished by the observations of Stahl and Strasburger. These investigators find that the swarmspores, generally speaking, when the intensity of light is moderate, move towards it, and when stronger, away.

In the lateral leaflets of Desmodinm, again, under continuous illumination, we have observed recurrent reversals of the direction of the more rapid of the two strokes which constitute each individual pulsation. Even this phenomenon finds a curiously exact parallel in the movements of certain swarm-spores of Ulothrix under the continuous action of light, as noticed by Strasburger. These he finds first to retire from the light, then to remain stationary, and again to return towards the light, only then to begin the whole process over again, thus moving to and fro for some time like a pendulum.

Directive action of light. — The movement of the ciliated organism, then, whether towards or away from it, is parallel to the direction of incident light. But a difficult question arises here as to how the organ perceives this direction as it were, and by what mechanism it determines its own course accordingly. At first sight, there appears no reason why rhythmic beats caused by stimulus should not produce propulsive movement in any direction. In this connection, turning to the case of Ulothrix, we find the organism provided with symmetrical pairs of cilia. We also know that the excitatory effect of stimulus of light depends upon its angle of incidence. If, then, light strike the two cilia of a given pair asymmetrically, they will undergo unequal excitation, causing them to execute a turning movement. Thus a stable condition can only be arrived at when excitation is

equal in the two cilia, and this can clearly happen only when the organism has so orientated itself that its axis is parallel to the direction of the incident rays ; and it is evident that in this position the equal beats of both cilia must result either in progressive or in retrogressive motion, parallel to the direction of the incident rays. Thermotaxis. — Thermal stimulation also causes responsive movements towards or away from the source of stimulation ; and these reactions, again, are found to change their signs, with varying intensities of stimulus. Thus, Paramcecia swim towards the warmer side of a vessel which is unequally heated, provided the temperature of the warmer side does not exceed 240 C. The response is in this case, then, seen to be positive ; but when the temperature of the heated side is higher than 2 8° C. the Paramcecia are found to swim away, thus exhibiting a negative response.

Galvanotaxis. — Similar multiple response finds expression in certain Infusoria, in swimming movements of positive and negative character. Thus Verworn finds, for example, that under the excitation of an electrical current, Polytoma swims away from the kathode, and Pleuronema towards it. In these galvanotactic movements, also, we meet with the same recurrent reversals with which we are already familiar in the case of the leaflets of Desmodium. In working with Paramceciumy Arthur W. Greely l found that after being subjected for about half an hour to a moderate current, the organisms which had previously gathered round the kathode reversed their action, and moved towards the anode, only to dart back immediately, again, towards the kathode.

I have demonstrated in Chapter XXXVI. the opposite responsive changes which occur under the action of acids and alkalis respectively. This explains the appropriate modification of galvanotactic response in Paramecium, according as the organism has been reared in an acid or in 1 Science has lost a very promising worker in the early death of this investigator. His experiments on Paramcecium are very valuable and suggestive. an alkaline culture. Thus Greely has found that while alkali-reared Paramcecia invariably give the initial response by swimming towards the kathode, the reverse is generally the case with those which have been subjected to acid — that is to say, the latter as a rule begin by swimming towards the anode.

Chemotactic movements. — Similar effects are, again, observed under chemical stimulation. The opposite reactions of acids and alkalis which have already been described are the occasion of opposite chemotactic responses. Thus Jennings found Paramcecia moving towards, or showing positive reaction to, acids, whereas to alkalis they exhibit negative response, or movement away. Chemotactic reaction, again, is modified by the strength of the solution, that is to say by the intensity of stimulation. The opposite effects of strong and feeble doses (p. 488) are here illustrated by the fact that in many organisms positive response is observed under the action of a weak solution, and negative under a strong. Connected with the subject of chemotaxis is the interesting phenomenon of the response of the antherozoids of ferns and of Selaginella to malic acid, as discovered by Pfeffer. In the response of growth we found that dilute solutions of sugar gave rise to one kind of response, that is to say to acceleration of growth, and that with very strong solutions, say about 10 per cent., the opposite occurred — that is to say, retardation (p. 482). Now, it is found by Pfeffer that whereas the dilute solution of a malate exerted an attractive influence on the antherozoids, o per cent, solution had a repellent effect.

A rhythmic vegetable organ, in a state of standstill, has its autonomous movement renewed by a supply of energy rom incident light. Too strong an intensity of light may, by causing fatigue, arrest such movement. Or the greater fatigue of the more excitable half of the organ may cause a reversal of the relative rapidities of the up and down beats. In Desmodium, under the continuous stimulation of strong light, these reversals are often recurrent. The dovvnstroke, which is at first quicker, becomes less quick than the upstroke, and this may be again and again reversed.

In a ciliated organism the swimming movements are explicable by similar unequal up and down strokes of the anisotropic cilia. When the downstroke is quicker, the organism propels itself forward. When the upstroke is quicker, there is a movement backwards. Such swimming movements, due to multiple response, are initiated by stimuli of various forms. There are two natural types of these responses : positive movement, or swimming towards, and negative, or away from, stimulus. These are determined by the relative excitabilities of the upper and lower halves of the cilium.