Plant Response as a Means of Physiological Investigation

I shall now describe the line of investigation by which it is possible to demonstrate the fact that the two halves of an anisotropic organ possess different excitabilities with regard to geotropic stimulus ; but before entering upon this, it is necessary to obtain a clear idea of the direction of response, in relation to direction of the force which causes stimulation of gravity. In the case of an apogeotropic stem, laid horizontally, we have vertical lines of force striking the organ from above, and the curvature induced makes it turn upwards to meet, as it were, these lines of force. We have here an example which is analogous to the response of the organ to the rays of light, in which the responsive movement consists in a similar curvature upwards to meet them.

The fact that the upper and lower halves of an anisotropic organ are unequally excited by light has just been demonstrated by means of torsional response, where it was shown that lateral excitation induced a twisting movement by which the less excitable was made to face the incident rays. By a similar torsional response of the anisotropic organ we are able not only to demonstrate the unequal geotropic excitability of the upper and lower halves, but also, by noting which of the two is made to face the lines of gravitational force, to determine which it is that is the less excitable to this particular form of stimulus.

Torsional response to lateral geotropic stimulation.— For this experiment I took a leaf of Erythrina indica, whose pulvinus I find to be very sensitive to geotropic action. Under normal conditions the leaflets of this plant place themselves horizontally. I now took one of these, say the terminal, and adjusted the plant so that its lateral edge was vertical. The dorsal and ventral halves of its pulvinus were thus equally subjected to geotropic action. This experiment, it is to be noted, was carried out in a dark room, where the only stimulus acting was geotropic. It will be seen, from the first part of the left-hand record in fig. 271, that under geotropic stimulus a torsional response was here obtained, which was exactly similar to that given under the action of light in fig. 270. The angular movement was in this case at the rate of about "11° per minute, and it was the dorsal surface of the organ that was eventually turned upwards — that is to say, so as to face the lines of force of gravity. This experiment, then, conclusively demonstrates the fact that the two halves of the dorsi-ventral organ are unequally sensitive to geotropic stimulus, and that it is the upper which is the less sensitive.

Modification of torsional geotropic response by artificial variation of differential excitability. — The fact that the upper half is, under normal conditions, the less excitable to geotropic stimulus is capable of further and striking demonstration, by the tests of reversal, and acceleration of torsional response. I have already explained, as will be remembered, that if the existing difference of excitability were reversed, the direction of torsional response would also be reversed ; and that if, on the contrary, the difference were by any means increased, the rate of torsional movement would be correspondingly accelerated. In the first part of fig. 271 is seen the natural torsional response of the terminal leaflet of Erythrina indica. The excitability of the lower half of the pulvinus was now abolished, by the local application of chloroform, at the point marked with an upward arrow, and the torsional response to geotropic stimulation was thus found to be reversed, from plus '11° to minus '05° per minute. Hence it was now the lower half of the pul-

vinus — artificially rendered the less excitable of the two — that was found turning upwards, to face the rays of incident force of gravity. The converse of this experiment would consist in still further reducing the excitability of the upper half of the pulvinus, in which case the normal differential excitability of the two halves would be increased, and the geotropic response enhanced. The right-hand record in fig. 271 shows that this is what actually occurs. The first part of the record displays the normal torsional response, and that which follows, after the local application of chloroform on the upper surface — indicated by the arrow from above — exhibits, by its increased steepness, the enhanced character of this response.

We have thus seen, by means of torsional response, that the differential character of the excitability of the two halves of an anisotropic organ under the stimulus of light is in every way paralleled by their differential excitability to the stimulus of gravity. The geotropic response of the organ, as a whole, is neither simple positive nor negative, but may be more fittingly described as differential. And as we saw with regard to photic stimulus that there was no need for the assumption of any specific dia-heliotropic sensibility in plagiotropic stems or in leaves — their movements being fully accounted for by the mechanical considerations arising out of their differential excitability to

Fig. 271. Records showing Torsional Response to Geotropic Stimulus and Induced Modifications in Terminal Leaflet of Erythrina indica In the figure to the left is seen the normal torsion under lateral geotropic action, and its reversal when the differential excitability of the organ is reversed by the application of chloroform to the lower half (f ). In the figure to the right is seen the enhancement of the torsional response when the natural differential excitability is increased by the application of chloroform to the upper half of the organ (|).

stimulus in general— so similarly with regard to geotropic stimulus, there is no necessity for the assumption of any specific dia-geotropic sensibility. Autonomous torsion. — We have now seen how torsional response is induced in an anisotropic organ. We have next to deal with that interesting class of phenomena which consists of the natural torsional movements of growing organs. It is to be remembered that, in referring to these torsional movements, we mean the torsional growth-movements of certain stems themselves about their own axes. The term positive torsion in such cases means a movement which appears, when looked at from above, to take place in the same direction as the movement of the hands of a watch, the term negative being used in the contrary sense.

Such torsional movements are found to occur in the stems of climbing plants. In some of these they are of a positive, and in others of a negative character, while in still others again they alternate, the natural torsion being, say, positive at one period, and negative at another. Now, we have seen in the case of leaves that, owing to different rates of growth in the antagonistic upper and lower halves of the petiole, autonomous epinastic or hyponastic movements occur in a rectilinear direction. When the growth of the upper half is predominant, the movement of the organ is downwards, and vice versa. More complex, however, is that case in which the line of maximum growth is a spiral revolving about an axis, thus bringing about a growth-movement which causes a torsion of the organ round that axis. In the case of epinasty and hyponasty, it was said that the rectilinear movement induced was due, not to a total absence of growth on either side, but to the relatively greater growth of one of the antagonistic halves. Similarly, in these torsional movements we have to remember the existence of antagonistic elements in the stem, and it is the predominant growth of one of these, the right-handed or lefthanded, that brings about the resultant positive or negative torsion.

order to take a record of these torsional movements, a stem is taken and held securely fixed at one end — say the lower — the other being left free. On this free end, the mirror, by the reflected light from which the record is to be made, is attached ; and a long thread from above is tied to the tip of the specimen, in order to prevent its falling to one side by its own weight. The limpness of this supporting thread allows the torsional movements of the specimen to occur without hindrance.

In taking records of the natural torsional response of plants at various temperatures, I have obtained results which are in a general way similar to the records made of longitudinal growth-response (p. 446) — that is to say, the response is enhanced up to the optimum, which is at or near 35° C. Above this, response is diminished. These facts will be seen from the following table, which gives the absolute rate of the angular movements of the torsional response at different temperatures.

The specimens employed in these and the following experiments were climbing stems of Porana paniculata, a plant belonging to the Campa?iulacece> which normally exhibits a strong negative torsion. Table giving the Absolute Rate of Angular Movements of Torsional Response at various Temperatures An interesting effect was observed, however, in this case of torsional response, when the temperature was raised above 43° C, which was apparently different from what occurred in ordinary longitudinal growth-response at the same temperature. It was found in the latter case that at 44° C. or thereabouts growth underwent an apparent arrest ;

but this was shown not to be due to any actual arrest brought about by heat-rigor, for the rhythmic activity induced at this high temperature was even greater than before (p. 432). Now, in the case of torsional response, I find that though the rate of torsion undergoes a continuous decrease up to 420 C. or 430 C, yet beyond this an unexpected increase is induced. Thus in the experiment just described, the torsional movement at 450 C. was at least temporarily enhanced to "8° per minute. This phenomenon may be due to abnormal relaxation at these high temperatures. Or there is another possible explanation. It has been said that the resultant torsion is due to the differential growthactivities of two antagonistic elements. Now, it may be that at 440 C. or thereabouts the growth of the less excitable of these antagonistic elements may undergo the same kind of arrest as we have seen to occur in the longitudinal growth of a radial organ. The sudden increase observed at and above 440 C. in the rate of natural torsion may then be due to the withdrawal of the resistance previously offered by the growthactivity of the antagonistic element.

Effect of increased suctional activity. — Another interesting point, in connection with the occurrence of autonomous torsion, lies in the fact that its rate is enhanced by any means which tends to increase internal energy. One example of this has just been seen in the effect induced by rise of temperature. Another is found in the application of warm water to the base of the specimen, a process which has already been shown, in the case of radial organs, to cause, by means of the consequent increase of suctional activity, a sudden enhancement of the rate of autonomous growth (p. 430). I find, similarly, that the application of warm water to its base enhances the torsional response of a torsioning plant

Effect on natural torsion of unilateral application of jht— We have seen that when an anisotropic organ is Lterally excited by external stimulus, a response torsion ikes place, by which the less excitable side is made to face the stimulus. Now, in a naturally torsioning organ, such an j induced torsional movement must obviously be opposite in direction to the natural movement caused by internal energy. \ This will be found to be illustrated in the modification induced in autonomous torsion by the unilateral application of light, as shown in fig. 272, where the first part of the curve shows the normal negative torsion of the plant. At the point marked with the upward arrow, light from a thirty-two candle-power electric lamp was allowed to strike the stem from one side. It will be seen that the excitatory effect of this external stimulus is first exhibited in the retardation of

Fig. 272. Retardation and Reversal of Normal Torsional Movement by the unilateral Action of Light in Porana panicvlaia Light applied at f retards, and in the course of two minutes reverses, the normal movement of negative torsion, making it positive. On cessation of light at { * \ there is recovery of the original negative torsion. the normal rate of torsion, culminating in its abolition, succeeded by actual reversal of direction. The normal negative was thus converted into positive torsion. On the cessation of the external stimulus the normal torsion was gradually resumed. The effect described is best observed in vigorous specimens in which the natural torsional movement is fairly strong.

In studying the action of light on autonomous longitudinal growth, we found that when a series of responses to the action of light were recorded, the first effect of incident ight, if the specimen were in a sub-tonic condition, would be, by increasing the internal energy, to give rise to an enhanced rate of growth; but when the normal tonic condition had been attained by absorption of light, the subsequent responses would be by a movement opposite to that of growth-elongation— that is to say, a contraction or retardation of growth. In experimenting on the effect or light on the autonomous torsional movements of the stems of certain specimens of Ipomcea, I have met with results exactly parallel — that is to say, the natural rate of torsional movement, in this case negative, was transitorily enhanced by the first incidence of light, but declined and underwent reversal after continued action of stimulus. In the second and subsequent responses this preliminary enhancement was found to have disappeared, the effect of light now being a retardation and subsequent reversal of the natural torsional movement.

Effect of electric current— I have often observed a very interesting effect, as induced by a constant current flowing along the length of the organ. During the continuance of the current the normal torsional response is first decreased and then reversed. On the cessation of current there is recovery and restoration of the normal direction of torsion. If the current maintained be strong or long continued, the induced reversal may become more or less persistent. Such an induced reversal of torsion, moreover, is independent of the direction of current.

Effect of gravity. — I shall next describe a series of effects which I have only been able to obtain with any degree of certainty under favourable conditions. Where natural torsion is feeble the retarding effect of gravity which I am about to describe is such as to arrest torsion, and it is difficult to decide whether such arrest is accidental or induced. When the specimen, on the other hand, is too vigorous, the retardation is not easily observed, probably because it tends to be masked by the natural movement. In practice, therefore, the best specimens are those characterised

by moderate vigour and uniformity of natural torsion during a considerable length of time. In studying the effect of geotropic stimulus on radial organs, we have seen that its effective intensity was greater when the specimen was inclined at an angle of 1350 to the vertical than when at 900 (p. 501). It occurred to me, then, that the effect of geotropic stimulus might possibly be detected, by means of variations induced in natural torsional response at different angles of inclination. Thus, we might first take a record of the normal rate of torsion, occurring when the tip of the organ was in its natural position upwards. We might next take a record of torsion, the stem being held horizontal ; and we might finally take a record with the tip held vertically downwards.1 The effect of geotropic stimulus might then be seen in the retardation induced in the normal torsion. I give here a summary of results obtained from two different specimens of Porana paniculate and from a specimen of Ipomcea.

In the first case, the record taken in the normal upright position gave a rate of movement of negative torsion through minus twenty-three divisions of the scale per minute. The record of torsion in the horizontal position was found to give a very much diminished rate, being now only minus seven divisions per minute ; and finally, with the tip held downwards, the torsional movement of the specimen was found to have undergone an absolute reversal to positive — that is to say, the tip now moved with the hands of a watch at a rate of plus five divisions of the scale per minute.

In the second case, the normal negative torsion in the erect position was at the rate of minus twenty divisions per 1 Here we must bear in mind, with regard to the effective angle of inclination, a certain difference as between radial and torsioning organs. In the former, the line of growth may be taken as vertical, and coincident with the axis, whereas in the latter it is spiral, or inclined at some undetermined angle to the axis of the organ. Hence any inclination of the growth-line of a radial organ is measured by the angle between the organ and the vertical. In the case of the torsioning organ, however, when it is held vertically downwards, its growth-line makes an angle — not of i8o° to the vertical, but — of i8o° less by the degree of its inclination to the axis,

minute. This was reduced in the horizontal position to minus fourteen divisions per minute, and in the vertically downward position it was found to have undergone reversal to plus nine divisions per minute. In the next series of experiments I took records from specimens held alternately up and down and up again. This was done in order to eliminate the effect of any chance variation. The specimen employed was Ipomcea. The torsional response in the first up position was at the rate of minus sixteen divisions per minute. When held in the inverted position, the rate was found reduced to minus ten divisions per minute, and when once more placed in its normal vertical position, the Ipomcea exhibited an increased rate of movement — that is to say, the torsion now took place at the rate of minus twenty divisions of the scale per minute. All these results tend to show that the action of stimulus of gravity is to retard the autonomous torsion, this effect being at a maximum when the specimen is in a position at 1800 from the vertically upright.

The twining of stems. — The twining movements of certain stems are the result of various contributing factors, the relative values of which may differ in different cases. One such factor, suggested by Von Mohl and denied by others, may lie in the irritability of the stem to the contact of its support. Such response to unilateral pressure was found to occur in various organs (p. 497), and probably plays an important part, in some cases, in the phenomenon of twining. Some connection would also seem to exist, in many instances, between autonomous torsion and twining. In the first place, most of the twining plants also exhibit autonomous torsion. Again, just as we have various types of torsioning organs— some characterised by positive, others negative, and others again by positive alternating with negative, or negative with positive, torsions — so in twining stems also, we see some which exhibit positive-directioned twining, others negative, and others again alternately one and the other. A plant, moreover, which has twined, shows, if inverted, a tendency

towards reversal or untwining. This is analogous to the effect of inversion on autonomous torsion, in which, as we have seen, the original torsion tends to be reversed. The effect of gravity is known to be a very important factor in the induction of twining. It is believed that we have here a third type of geotropic action, which is neither positive nor negative, but lateral in its character. Under lateral geotropism, a curvature is induced in the twining stem in a horizontal plane. Since we have found, however, that even such diverse effects as positive and negative geotropic actions are not to be regarded as due to different specific sensibilities, it would be interesting to inquire whether lateral geotropism also is not simply a case of ordinary apogeotropism in combination with some other tendency, say that of autonomous torsion.

At any rate, that this erectile or apogeotropic action is always an element in the process, is shown by the fact that the coils in the older portion of the stem become drawn out, showing that the ascending movement predominates in that region. The whole subject is, however, extremely complicated, owing to the presence of many factors and their different relative intensities. And each of these may again be subject to modification, under the action of external stimulus, as was seen to be the case with autonomous torsion under the influence of light, gravity, and the variation of internal energy.

The observed effect of torsion, by which the upper surface of the leaf is turned to face the light, is not due to any specific dia-heliotropic sensibility. Similar torsional response is induced by the lateral application of any other form of stimulus, such as thermal or chemical. Such torsional response is also obtained from a compound strip made of two unequally contracting inorganic materials, such as ebonite and stretched india-rubber, when stimulus is applied to one of the lateral flanks.

The general law of induced torsional response is that the less excitable side of the organ is made to face the incident stimulus. If the excitability of the lower half of the pulvinus be artificially abolished by the local application of chloroform, the torsional responsive movement is reversed. The leaf now executes a movement by which its lower surface is made to face the light. On artificially increasing the natural difference, as between the two halves, by altogether abolishing the excitability of the upper, the normal torsional response is accelerated.

That the upper and lower halves of a dorsi-ventral organ are unequally sensitive to geotropic stimulus is shown by the fact that they give torsional response, under which the less excitable half is carried so as to face the incident lines of gravitational force. An artificial increase of this natural difference of excitability has the effect of accelerating the rate of torsional response. The artificial reversal, on the other hand, of these relative excitabilities has the effect of reversing the direction of the natural responsive movement.

From these considerations it is seen that none of the responsive movements caused in dorsi-ventral organs by light or gravitation are due to any specific dia-heliotropic or dia-geotropic sensibility, and that they are in reality due to the differential excitability of the two halves of the organ. The autonomous torsional movement of growth increases in rate up to an optimum temperature, after which it begins to decrease. There may be a second acceleration after the attainment of the first minimum.