Plant Response as a Means of Physiological Investigation

An attempt has been made in the case of Vochting's Theory of Recti-petality to account for this recovery, by assuming the action of an unknown regulating power which would tend always to bring the organ back to a straight line ; but, beyond the assumption of an unknown specific power, this theory affords no explanation of the mechanism by which recovery is brought about, and I am able to adduce considerations which obviate the necessity for thus assuming the existence of any such specific agency as that of rectipetality.

In a growing organ which is radial, the tip of the growing region being free, the vertical direction is that in which there is least obstruction to growth, and as long as all the lateral tensions are the same in all directions, there is no reason why the organ in the course of its upward growth should bend permanently on any one side more than on another. We have therefore the normal growth of radial organs in a straight line ; but when stimulus acts unilaterally on the growing region, a sequence of events ensues, which has already been fully explained :

(i) Active contraction is induced during the continuance of stimulus, on the proximal or excited side, with concomitant diminished turgidity, and retardation of growth. (2) The water thus expelled is forced, against tension, into the growing cells of the distal side, raising their turgescence and consequent rate of growth above par. (3) The curvature thus induced is maintained as long as the difference of hydrostatic pressure on the two sides is continued, by the persistent contraction of the proximal, under the action of stimulus.

(4) When the stimulus ceases to act, the active contraction which forced the water against tension to the distal side comes to an end, and there is a rebound of the expelled water to the proximal side. Thus the increased growth of the distal falls, and the decreased growth of the proximal rises, to the normal rate of growth. The unequal tensions on the two sides, which previously maintained the curvature, being now equalised, the organ shows a tendency to straighten itself.

(5) I have also shown that a tissue which has been subjected to stimulus, having absorbed energy and held it latent, exhibits it on the cessation of external stimulus, in the form of a temporary negative after-effect, that is to say an acceleration of growth above the normal. As a result o this fact, the stimulated side of the organ will show an active tendency to neutralise the previous curvature, and return to the straight line. It is thus seen, from facts which I had already established regarding the nature of the after-effect of stimulus, that the recovery of the organ is fully explained, without postulating the existence of any specific power, such as that of rectipetality.

The responsive movement of the plant-organ towards light is due to the excitatory contraction of the side acted upon. The curvature of a growing organ towards light is brought about by the joint action of the induced concavity of the proximal and the convexity of the distal sides. The former is the result of the contraction, negative turgidityvariation, and retardation of growth caused by the stimulus. The latter comes about by the positive turgidity-variation due to forcing-in of expelled water, expansion and acceleration of growth of the distal side.

Such responsive movements take place in organs previously devoid%of circumnutation. The sensitiveness of certain plant-organs to heliotropic stimulus is very great. The terminal leaflet of Desmodium responds under the briefest exposure to feeble candlelight. This necessitates the making of observations on heliotropic effects without the aid of light. The perceptive region for the stimulus of light in the case of the terminal leaflet of Desmodium is the pulvinus.

There is no essential difference between the heliotropic response of a growing and a pulvinated organ. On the cessation of stimulus the recovery of a pulvinated organ is complete ; and this is more or less true also of the recovery of a growing organ from response, if the stimulus have not been excessive. For the explanation of this recovery in a growing organ, it is not necessary to assume the existence of any specific power such as recti-petality. The cessation of the difference of hydrostatic pressure on the two sides — such difference being only maintained during the action of stimulus — together with the accelerated rate of growth on the proximal side, which constitutes the negative after-effect, are quite sufficient to explain the recovery from induced curvature.

Incomplete parallelism between actions of light and of gravitation — Theoretical considerations — Recording microscope — Negative heliotropic curvature induced by stimulation of the tips of root and shoot — Intermediate phases between positive and negative heliotropic response : (a) neutralisation by transverse transmission ; (b) neutralisation by transverse transmission, with multiple response — Localised sensitiveness to light and transmission of excitatory effect — Negative heliotropism of a radial organ — Gradual transition from positive to negative, through intermediate phase of neutrality — Apparent heliotropic insensitiveness of certain tendrils — Negative heliotropism of tendril of Vitis.

We have seen in the chapter on the response due to gravitation, that the responsive curvature of the root is opposite in character to that of the stem, this fact having led to the assumption of specific sensibilities as characteristic of the root-tip. It was there shown, however, that the responsive characteristics of the root were not actually different from those of the shoot, and that the differences in their observed responses were simply a consequence of the fact that in the one case the stimulus of gravity acted indirectly, and in the other directly, upon the responding growing organ. This assumption that the root possessed a definite sensitiveness characteristically different from that of the stem, was apparently supported by certain differences in heliotropic action also, as between shoot and root ; for example, while the hypocotyl of Sinapis bends towards the light, the root is found to bend away from it.

Incomplete analogy between action of light and gravitation.— But I have already explained the fact that the supposed analogy is false ; for while the stimulus of gravity acts, in the case of the root, only on a restricted area of the tip, the stimulus of light is not necessarily restricted in the area of its action. Again, whereas the stimulation caused by statolithic particles is moderate, that caused by light may be of any degree of intensity. The fact that there is no true extended analogy between the action of light and that of gravitation, is seen from the fact that, while gravitation in the case of the root induces a movement opposite to that induced in the stem, in the case of light this is not always so ; for though a few roots turn away from light, in others there is either no resultant movement, or movement towards the light. Again, while the shoot makes a definite curvature with reference to the direction of gravity, in the case of light we shall observe that though under moderate stimulation it turns towards it, yet it will sometimes under stronger stimulation be found to move away. The idea that positive and negative heliotropic curvatures are due to two distinct sensibilities could not be better disproved than by the fact, which will be demonstrated shortly, that the same organ can be made under different conditions of illumination to exhibit the two opposite effects.

Discarding, then, the theory of any specific sensibility, we shall now proceed to show how the movement away from the stimulating light, the so-called negative heliotropic curvature, is brought about. Theoretical considerations. — From the movements already demonstrated (p. 535) as taking place in plant-organs in response to stimulus unilaterally applied, we can see the possibility of such movement becoming negative, or away from stimulus, under three different conditions :

(1) Under longitudinal transmission of the indirect effect of stimulus, when, for example, moderate stimulus is applied to the tip of either shoot or root. (2) Under transverse conduction of the direct excitatory effect of stimulus to the distal side of a radial organ, the proximal side being fatigued by excessive stimulation ; and (3) Under the transverse transmission of excitation to the distal sjde of an anisotropic organ, the distal side being the

more excitable. In this last case, we may obtain a very pronounced negative response in consequence of the relatively greater natural excitability of the distal side. We shall see in the course of this and the following chapters how heliotropic movements other than positive are actually brought about under these different conditions, and in the present chapter we shall study cases which are illustrative of the first two. Plant mounted in cubical glass trough with root in water. Light strikes root, R, unilaterally from the right side. Movement of root observed by microscope, M, the inclined transparent disc of glass, G, giving at the same time the reflected image of the recording pen, P.

Recording microscope. — Since the growing root has to be kept in water, for the purpose of studying the phases of its responsive curvature, the method hitherto employed of obtaining records by the Optic Lever is inapplicable. I therefore devised a different method of observation— that of the Recording Microscope (fig. 244). The method of record will be understood from the figure, where in a cubical glass trough a piece of the stem of Bindweed, with its water root, R, is securely fixed on the surface of the water. Light is made to

strike the root unilaterally, say from the right side. This pencil of light may be so thrown as to act locally on the roottip, or on the growing region, or on both at the same time. The movement of the root towards or away from light is observed through the microscope focussed on the tip. The eye-piece end of the microscope has a disc of glass adjusted at an angle of 45 ° to the vertical. The observer sees the tip of the root directly through the transparent disc, and at the same time the reflected image of the recording point of the pen, lying against the revolving drum below. The two images are at the beginning of the experiment coincident, and the responsive movement of the tip of the root, which takes place afterwards, is easily followed by the observer with the recording pen. Thus we obtain the response-record on the moving surface. This method of the recording microscope can always be used when attachment to the Optic Lever is not possible or not desired.

Negative heliotropic curvature induced by stimulation of the tips of root and shoot. — I have by this method obtained various records of the responses of the root and shoot to the unilateral stimulus of light applied at the tip. Of these I shall give, as a typical example, the record of the root of a seedling of Sinapis nigra, suitably mounted in the cubical trough by means of a cork. The curve seen to the left of fig. 245 represents the negative movement, or movement away from light, of this root, when the tip alone was unilaterally stimulated. This movement was due therefore to the indirect action of the stimulus on the growing responding region. After a period of rest in darkness I next took a record of its movement resulting from the direct unilateral illumination of the growing region. I now obtained a positive responsive curvature, as seen to the right of fig. 245. It will be noticed that this particular movement was relatively smaller than the preceding. We must here remember that the receptivity of an organ is not the same in all its different parts, and the greater negative response induced in this case by the indirect action of stimulus on the

tip is probably due to the higher degree of receptivity possessed by that part of the organ. In taking a third record in a case in which both tip and growing region were simultaneously subjected to unilateral stimulation of light, I found that a resultant responsive movement was induced, which was away from light. That this negative movement, induced by stimulation of the root-tip, is not due to any specific sensitiveness of the root as such, is seen from the fact that on local stimulation of the tip of the shoot, e.g. the flower-bud of Crocus, I obtained a responsive movement away from, whereas unilateral stimulation of the growing region of the peduncle induced a movement towards, light.

Apart from this possible factor, however, of the greater receptivity of the tip, there is another, which tends to make the positive curvature of the growing region of the root relatively ineffective. This region, being acted on unilaterally by light, the proximal excitation often passes by conduction to the distal side, thus neutralising the first positive action. Instances of this will be given in greater detail presently. The negative curvature induced by the action of the tip, depending as it does on the indirect transmission of stimulation, requires as a condition the relative non-conductivity of the intervening tissue to the passage of true excitation. Hence, if the conductivity of such a tissue be not sufficiently feeble, or if the intensity of stimulus be too great, we shall find that the direct effect of stimulus is transmitted to the growing organ, and a positive curvature is induced. This

The curve seen to the left shows the negative response due to stimulation of root-tip. The curve to the right exhibits positive response on stimulation of growing region. explains the positive heliotropic curvature exhibited by many roots. Intermediate phases between positive and negative heliotropic response. — I shall next proceed to demonstrate the induction of negative heliotropic movements in radial shoots, a phenomenon which, for reasons already explained, has no parallel in the case of geotropic action (p. 544). As it has already been said that there is no specific sensibility which determines the positive or negative character of the heliotropic response, it would be interesting to trace out the transitions by which the normal positive is gradually transformed into the negative movement. We have seen that when stimulus is applied unilaterally to a growing region, the positive curvature at first induced is jointly due to the contraction caused by direct stimulation of the proximal and the expansion caused by the indirect stimulation of the distal ; but when the stimulus is strong or long-continued, excitation is transmitted from the proximal to the distal, the contraction of which latter now neutralises the first effect. Hence the normal positive curvature disappears.

(a) Neutralisation by transverse transmission. — The considerations just related explain the curious anomaly that has been observed, by which, while feeble or moderate stimulus of light, or interrupted light, gives rise to well-marked positive heliotropic curvature, the continuous application of stronger light induces a relatively feeble effect. Thus under moderate lighting we often observe strong heliotropic curvature, which disappears under strong sunlight. The curvature induced is, as we have seen, due to the differential action of unilateral stimulus, on the proximal and distal sides ; but when a strong light is used the stimulus becomes internally diffused, and the differential effect on the two sides is reduced in amount or vanishes altogether. Such internal diffusion is due to the fact that, owing to the weak transverse conductivity of the tissue, while a feeble stimulus is not conducted across it, a stronger stimulus is. This consideration, together with the fact that the conductivity of a tissue undergoes seasonal variation,

will be found to offer a satisfactory explanation of various anomalies in heliotropic response. Sinapis, for example, exhibits a strong positive effect in winter, while in hot weather its action is very feeble. It may be supposed that this is due, in some unknown way, to a greater rapidity of growth in warm than in cold seasons. That this, however, cannot be the reason, will be seen from the fact which I have demonstrated, that that contractile response of the plant to external stimulus on which curvature depends is greatest when the rate of growth is at its optimum. The real explanation lies in the fact that the neutralisation, or reversal of normal positive response, caused by transverse conduction, takes place more easily in warmer seasons, the general conducting power being then great. This accounts for the feebler positive response in summer, which culminates in certain instances in an actual reversal into negative (p. 623). . (b) Neutralisation by transverse transmission, with multiple response. — Thus if stimulus be sufficiently strong or long,- continued, the positive curvature will become neutralised, and the organ will return to its original position. I have, however, observed an interesting modification of this neutralisation, in which it is attended by oscillatory movements to and fro about the mean position. We have seen that unilateral stimulus, when its action is long continued, becomes diffused, and thus both sides of the organ become excited. The tissue, moreover, is now possessed of an excess of energy— a condition conducive to the production of multiple response. This fact, together with the periodic and alternate variation of excitability on the two sides, is then found to give rise to oscillatory movements of the kind described.

I give below a record which shows the initiation of these oscillatory movements when the organ had been too long subjected to unilateral stimulus. It will be remembered that the pulvinus of the terminal leaflet of Desmodium executes a positive heliotropic movement, the record of which has been given in fig. 238. In winter, when the conductivity of the tissue is feeble, the leaflet curves towards the light to the maximum extent possible, and remains in that position as long as the light acts. But we have seen that in summer the stimulus is more likely to be internally transmitted to the distal side, the positive effect being thus gradually neutralised. Thus, in the course of an experiment during the summer on the pulvinus of the terminal leaflet of Desmodium, I found, on subjecting it to sunlight from above, that for the first forty minutes the leaflet rose continuously, its tip having moved during that time through a little more than 4 cm. After this there was induced, instead of the continuous movement upwards, a pulsatory movement up

and down (fig. 246). After a series of such movements the leaflet was gradually depressed, the former positive curvature being thus neutralised. The supposed localisation of sensitiveness to light, and the transmission of excitatory effect. I have fully explained the manner in which the effect of stimulus of light applied at a given point is transmitted to the distant growing organ, and the mechanics by which the curvature is induced. In connection with this, a peculiar phenomenon has been observed, which has led to the belief that in seedlings, like that of Avena sativa, the zone for the perception of heliotropic stimulus is confined to the upper region, or tip of the shoot. This conclusion is based on Darwin's observations on the unilateral effect of light on these seedlings. It was found that, generally speaking, when the lower part of the cotyledon was alone exposed to the unilateral light — the upper part being covered with a cap of tinfoil or with an opaque glass tube — there was little curvature induced ; but when such light was allowed to act on the upper part of the seedlings the curvature was con-

Eig. 246. Positive Heliotropic Movement of Terminal Leaflet of Desmodium Converted by Strong and too Longcontinued Stimulus of Light into Oscillatory Movement siderable. From this it was concluded that sensitiveness to light was mainly confined to the upper part of the plant, and that this determined the curvature ; but to this conclusion, that it was the upper rather than the lower part that was sensitive to light, Darwin found and recorded several exceptions, which he regarded as inexplicable. In the case of six seedlings, for instance, of which the upper parts were covered with opaque shields, there was as much curvature induced as in seedlings which were unshielded. In these, therefore, there must have been sensitiveness in the lower parts also, thus negativing the conclusion, drawn from other and more numerous experiments, that it was characteristic of the upper alone.

In order to see if these discrepancies were not capable of explanation, I undertook an investigation into the heliotropic action of light on the seedlings of A vena sativa. The method of screening the upper part of the seedlings from light, which has usually been employed by Darwin and others, labours under the disadvantage that the weight and contact of the tinfoil or the blackened glass tube are not unlikely themselves to set up a certain mechanical irritation, which may have the effect of causing an unknown disturbance in the result. I was therefore desirous of keeping the delicate seedling free from the irritating contact of caps in the course of my own experiments. For this reason I arranged for the localised application of light on upper or lower or both parts of the organ at will, by the method which has already been described of throwing a pencil of light on the required spot in the plant placed in the heliotropic chamber (p. 592). The resultant movement of the organ was now continuously observed and recorded, by means of the Recording Microscope which has been described.

For the sake of clearness I may here forestall matters, by saying that the observed results fall under two types, according to whether the given specimen possesses feeble or high conductivity — that is to say, power of transmitting stimulus to a distance. Taking first the case in which the organ possesses feeble conductivity, I have found that when the extreme tip was stimulated unilaterally, by using a pencil of light from a sixteen candle-power incandescent electric lamp, the result was a negative movement — that is to say, a movement away from light. This is exactly parallel to the response to stimulus of the unopened flower-bud of Crocus and the root-tip of Sinapis, in both of which the indirect effect at the growing region, of stimulus applied on the tip, was seen to be a convexity of the side acted upon, with consequent negative movement of the tip.

I next applied unilateral stimulus to the same specimen a little lower down, and now, owing to the better conductivity of this part of the tissue, the excitation itself was transmitted to the growing region, inducing concavity and positive heliotropic movement. The same effect was found to be produced when stimulus was applied on the growing region itself. It must be remembered that in this case the conducting power of the tissue is not high, hence there is no transmission of stimulus to the distal side, by which, as we have seen, the positive curvature would be neutralised. The long-continued action of light on one side here tends only to increase the positive curvature to a maximum. Thus when one side of the entire seedling is acted upon by light, while the response of the extreme tip tends to induce a slight negative, all the other parts, from immediately below it to the growing region, conspire together to exhibit a much stronger positive heliotropic action. The result is therefore a movement towards the light. Thus we see that in the case of seedlings having feeble conductivity the curvature will be positive, whether it is the lower part only or the entire plant which is exposed to the one-sided action of light. In this fact we find the explanation of those exceptional cases observed by Darwin, in which the seedling was found to bend towards the light, in spite of the upper part being covered.

We shall next take up that type of response in which the tissue of the specimen is rather better conducting. In this case, when the upper part of the organ is locally stimulated, ' the excitation is longitudinally transmitted to the growing region lower down, and induces a concavity there which increases with the duration of the stimulus ; but if the stimulus of light be applied directly on the growing region itself, instead of on the upper part of the specimen, then, by reason of the transverse transmission of excitation to the distal side, we obtain a state of things in which there is no resultant curvature at all. In this case, then, the direct effect of stimulus on the proximal side of the growing region is balanced or neutralised by the transmitted effect on the distal side ; but this condition of balance will be upset if the unilateral stimulus of light, hitherto acting on the growing region alone, be allowed to act simultaneously on the upper part of the specimen also. The longitudinally transmitted stimulus from the upper part being now added to the direct excitation of the proximal side of the growing region, causes an over-balance of responsive effect on that side, resulting in a positive heliotropic curvature. The fact that there is no heliotropic movement, when only the lower part of the seedling is unilaterally acted on by stimulus, is thus not due to any absence in that region of heliotropic sensibility, but to the neutralisation of the proximal effect by the equal excitation of the distal. Such transmission of stimulation along the length of the organ is observed to take place in a specially marked manner in the cotyledon of graminaceous plants. We may account for this by the fact that such organs are parallel-veined — that is to say, the fibro-vascular elements, which we already know as good conductors of excitation, run along their length. This is no doubt the reason of their ready transmission of stimulus to a distance.

Negative heliotropism of a radial organ. — We have seen that when moderate stimulus acts unilaterally on a growing organ a positive curvature is induced, and that, with stronger or long-continued stimulation, this reaches the distal side, producing neutralisation. We shall now proceed tc trace out the continuity of responsive heliotropic effects, from the positive curvature to the negative, through the intermediate phase of neutralisation. We have seen that at a certain definite intensity of illumination the excitations of proximal and distal sides, balancing each other, cause neutralisation. If now the intensity of stimulating light be further increased, it is easy to see that while the stimulation transmitted to the distal side, with the concomitant contraction, is being increased, the excitatory contraction of the proximal will be at the same time decreased, owing to the fatigue brought on by over-stimulation. The result then will be the greater contraction of the distal side, with a consequent negative heliotropic curvature of the organ.