It is necessary here to make special reference to the confusion that arises from want of precision in the use of the term stimulus, used indifferently to denote both the cause and the resulting effect. An external agent, say light, causes certain excitatory change in the tissue, and we refer to the agent which induces it, as the _stimulus_.

Thus in the instance cited above, light is the _stimulus_, and it is the stimulus-effect that is transmitted to a distance. But in physiological literature no distinction is made between the stimulus and its effect, hence arises frequent use of the phrase 'transmission of stimulus'. It is obvious that it is not light but its effect that is transmitted.

Such want of precision in the use of the term stimulus would not have seriously affected the truth about the description of facts, had the transmitted effect been only of one kind. In a nerve-and-muscle preparation, the velocity of transmission of excitation is so great, that it completely masks the positive impulse (a.s.suming the existence of such an impulse). The effect of indirect stimulation is, therefore, the same as that of direct stimulation. Any indefiniteness in the use of the term stimulus for its transmitted effect does not, in animal physiology, seriously militate against the observed facts. But lack of precision in the employment of the term in plant physiology leads to hopeless confusion. For owing to the semi-conducting nature of vegetable tissue, the transmitted effect is not of a definite sign, but may be positive or negative; in the first case, the response is by expansion, in the latter, by contraction. Thus the transmitted effect will be very different in the two cases, according as the intervening tissue is a good or a bad conductor. These facts accentuate the urgent necessity of revision of our existing terminology.

I have shown that the effects of other forms of stimuli are also transmitted from the perceptive to the responding region along the intervening path of conduction. Thus the petiole of _Mimosa_ perceive any form of stimulus applied to it, and the induced excitation is conducted to the distant pulvinus to evoke the familiar responsive fall of the leaf. The pulvinus, moreover, perceives and responds to direct stimulation. In a nerve-and-muscle preparation the responding muscle is alike perceptive and responsive.

But in _Setaria_ we meet with certain characteristics of reaction which are quite inexplicable. Thus if

"the seedling be illuminated on one side, a sharp heliotropic curving takes place at the apex of hypocotyl. The curvature makes itself apparent only if the cotyledon be illuminated from one side whether the hypocotyl be exposed to light or not. If the cotyledon be shaded and the light be permitted to fall on one side of the hypocotyl, no heliotropic curving takes place. Hence we may conclude that it is only the cotyledon that is sensitive to the light stimulus, and it is only the hypocotyl which can carry out the movement. The excitation which the light effects in the cotyledon must be transmitted to the hypocotyl and curvature takes place only from such a transmitted excitation. We have thus in this case a definite organ for the perception of the stimulus of light, viz., the cotyledon, and as Rothert has shown, it is more specially the apex of that organ that is the sensitive part: on the other hand, the motile organ, the hypocotyl, is some distance away from the sensitive organ, and in it the power of perception is entirely absent. From the behaviour of these organs we may draw the further conclusion that perception and heliotropic excitation are two distinct phenomena, which depend on different properties of the protoplasm and which are independent of each other.... We may, therefore, conclude from this experiment that these two types of excitation are fundamentally distinct processes, for it is only after indirect or transmitted and not after direct excitation that a reaction occurs in the case of the seedlings of the Paniceae".[20]

[20] Jost--_Ibid_--p. 468.

The noteworthy deductions on the above facts are:--

(1) That the motile organ in _Setaria_ is totally devoid of perception, since direct action of light induces no effect.

(2) That perception and heliotropic excitation are two distinct phenomena, which depend on different properties of the protoplasm, and which are independent of each other.

Though the conclusions thus arrived at appear to follow from the facts that have been observed, yet it is difficult to accept the inference, that a responding organ should be totally devoid of the power of perception, and that excitation and perception are to be regarded as dependent on different properties of protoplasm. It therefore appeared necessary to re-investigate the subject of the perceptive power of the cotyledon, and the responding characteristics of the hypocotyl.

The criterion employed for test of perception is the movement induced in response to stimulus. The responsive _mechanical_ movement is rendered possible only by the contractility of the organ, and mechanical and anatomical facilities offered by it for unhampered movement. The petiole of _Mimosa_ when locally stimulated does not itself exhibit any movement. The fortunate circ.u.mstance of the presence of a motile pulvinus in the neighbourhood enables us to recognise the perceptive power of the petiole, since it transmits an impulse which causes the fall of the leaf. There is no motile pulvinus in ordinary leaves, and stimulation of the petiole gives rise to no direct or transmitted motile reaction; from this we are apt to draw the inference that the petiole of ordinary leaves are devoid of perception. This conclusion is, however, erroneous, since under stimulus the petiole exhibits the electric response characteristic of excitation. Moreover my electric investigations have shown that every living tissue not only perceives but also responds to stimulation.[21] Hence considerable doubt may be entertained as regards the supposed absence of perception in the hypocotyl of _Setaria_.

[21] "Response in the Living and Non-Living"--p. 17.

I shall in the present paper describe my investigations on the mechanical response of _Setaria_ under direct and indirect stimulation which will be given in the following order:--

(1) The response to unilateral stimulation of the tip of the seedling.

(2) The response of growing hypocotyl to direct stimulation.

(3) Summated effects of direct and indirect stimulation.

EXPERIMENTAL ARRANGEMENTS.

_The Recorder._--The pull exerted by the tropic curvature of the seedling is very feeble; it was therefore necessary to construct a very light and nearly balanced recording lever. A long gla.s.s fibre is supported by lateral pivots on jewel bearings. The seedling is attached to the short arm of the lever by means of a coc.o.o.n thread. The recording plate oscillates to and fro once in a minute; the successive dots give therefore the time relations of the responsive movement. The positive curvature towards light is recorded as an up-curve, the negative curvature being represented by a down-curve.

[Ill.u.s.tration: FIG. 135.--Arrangement for local application of light to the tip and the growing region. O, O', apertures on a metallic screen.

Light is focussed by a lens on the tip, and on the growing region at o, o'. Figure to the right shows front view of the shutter resting on a pivot and worked by string, T.]

_Arrangement for local stimulation by light._--The device of placing tin foil caps on the tip employed by some observers labours under the disadvantage, that it causes mechanical irritation of the sensitive tip.

The appliance seen in figure 135 is free from this objection and offers many advantages. A metallic screen has two holes O and O'; these apertures are illuminated by a parallel beam of light from an arc lamp.

A lens focusses the light from O, on the hypocotyl, and that from O', on the tip of the cotyledon. A rectangular pivoted shutter S, lies between the apertures O and O'. In the intermediate position of the shutter, light acts on both the tip and the growing region. The shutter is tilted up by a pull on the thread T, thus cutting off light from the growing region; release of the thread cuts off light from the tip. Thus by proper manipulation of the shutter, the tip or the growing hypocotyl, or both of them, may be subjected to the stimulus of light. The experiment was carried out in a dark room, special precaution being taken that light was screened off from the plant except at points of localised stimulation.

[Ill.u.s.tration: FIG. 136.--Response of seedling of _Setaria_ to unilateral stimulation of the tip applied at dotted arrow.

Note preliminary negative curvature reversed later into positive.]

EFFECT OF LIGHT AT THE TIP OF THE ORGAN.

_Experiment 137._--If the tip of the seedling of _Setaria_ be illuminated on one side, it is found that a _positive_ curvature (_i.e._, towards light) is induced in the course of an hour or more. But in obtaining record of the seedling by unilateral stimulation of the tip, I found that the immediate response was not towards, but away from light (negative curvature). The latent period was about 30 seconds and the negative movement continued to increase for 25 minutes (Fig. 136).

This result, hitherto unsuspected, is not so anomalous as would appear at first sight. Indirect stimulus, unilaterally applied, has been shown to give rise to two impulses: a quicker positive and a slower excitatory negative. The former induces a convexity on the same side, and a movement away from stimulus (negative curvature); the excitatory negative, on the other hand, is conducted slowly and induces contraction and concavity, and a movement towards the stimulus (positive curvature).

In semi-conducting or non-conducting tissues, the excitatory negative is weakened to extinction during transit, and the positive reaction with negative curvature persists as the initial and final effect.

But in _Setaria_ the excitatory negative impulse is transmitted along the parenchyma which is moderately conducting; the speed of transmission of heliotropic excitation is, according to Pfeffer, one or two mm. in five minutes or about 04 mm. per minute. Thus under the continued action of light, the excitatory impulse will reach the growing region, and by its predominant reaction neutralise and reverse the previous negative curvature.

Inspection of figure 136 shows that this is what actually took place; the intervening distance between the tip of the cotyledon and the growing region in hypocotyl was about 20 mm., and the beginning of reversal from negative to positive curvature occurred 29 minutes after application of light. The velocity of transmission of excitatory impulse under strong light is thus 07 mm. per minute. The positive curvature continued to increase for a very long time and became comparatively large. This is for two reasons: (1) because the sensibility of the tip of the cotyledon is very great, and (2) because the positive curvature induced by longitudinally transmitted excitation is not neutralised by transverse conduction (see below).

[Ill.u.s.tration: FIG. 137.--Effect of application of light to the growing hypocotyl at arrow induced positive phototropic curvature followed by neutralisation. Application of indirect stimulus at dotted arrow on the tip gave rise at first to negative, subsequently to positive curvature.

(Seedling of _Setaria_).]

RESPONSE TO UNILATERAL STIMULUS IN THE GROWING REGION.

_Experiment 138._--The growing region of the hypocotyl of _Setaria_ is supposed to be totally devoid of the power of perception. In order to subject the question to experimental test, I applied unilateral light on the growing region of the same specimen, after it had recovered from the effect of previous stimulation. The response now obtained was vigorous and was _ab-initio_ positive. Direct stimulus has thus induced the normal effect of contraction and concavity of the excited side. The belief that the hypocotyl of _Setaria_ is incapable of perceiving stimulus is thus without any foundation. The further experiment which I shall presently describe will, however, offer an explanation of the prevailing error. On continuing the action of unilateral light, the positive curvature after attaining a maximum in the course of 15 minutes, underwent a diminution and final neutralisation (Fig. 137). On account of this neutralisation the seedling became erect after an exposure of 30 minutes; in contrast with this is the increasing positive curvature under unilateral illumination of the tip (Fig. 136) which continues for several hours. The explanation of this neutralisation under direct stimulation of the growing region is found in the fact that transverse conduction of excitation induces contraction at the distal side of the organ and thus nullifies the positive curvature. The seeming absence of tropic effect under direct stimulation is thus not due to want of perception, but to balanced antagonistic reactions on opposite sides of the organ.

EFFECT OF SIMULTANEOUS STIMULATION OF THE TIP AND THE HYPOCOTYL.

Though stimulation of the hypocotyl results in neutralisation, yet the illumination of one side of the organ including the tip and hypocotyl is found to give rise to positive curvature. This will be understood from the following experiment.

After the neutralisation in the last experiment light was also applied to the tip from the right side at the dotted arrow (Fig. 137). The record shows that this gave rise at first to a negative curvature (away from light); under the continued action of light, however, the negative was subsequently reversed to a positive curvature, towards light.

Inspection of the curve shows another interesting fact. The positive curvature induced by direct stimulation is very much less than that brought out by indirect stimulation. This is due to two reasons: (1) the sensitiveness of the tip of the organ is, as is well known, greater than that of the hypocotyl, (2) the positive curvature under direct stimulation cannot proceed very far, since it is neutralised by transverse conduction of excitation.

It will be seen from the above that the illumination of the tip practically inhibits the neutralisation and thus restores the normal positive curvature. The question now arises as to how this particular inhibition is brought about.

ALGEBRAICAL SUMMATION OF THE EFFECTS OF DIRECT AND INDIRECT STIMULATIONS.

An instance of inhibition, though of a different kind, was noticed in the response of the tendril of _Pa.s.siflora_ (p. 296); the under side of the organ is highly sensitive, while the upper side is almost insensitive. Stimulation of the under side of the tendril induces a marked curvature, but simultaneous stimulation of the diametrically opposite side inhibits the response. This neutralisation could not be due to the antagonistic contraction of the upper side since the irritability of that side is very slight. I have shown that the inhibition results from the two antagonistic reactions, contraction at the proximal side due to direct stimulation and expansion caused by the positive impulse from the indirectly stimulated distal side.

We have in the above an algebraical summation of the effects of direct and indirect stimulations. The longitudinally transmitted effect of indirect stimulus in _Setaria_ may, likewise, be summated with the effect of direct stimulus. The phenomenon of algebraical summation is demonstrated in a very striking and convincing manner in the following experiment, which I have been successful in devising.

_Experiment 139._--I have explained, (Expt. 126) that unilateral application of stimulus of light on the upper half of the responding pulvinus of _Mimosa_ induces an up or positive curvature, followed by a neutralisation and even a reversal into negative, the last two effects being brought about by transverse conduction of excitation to the distal side. When the incident light is of moderate intensity, the transmitted excitation only suffices to induce neutralisation without further reversal into negative; while in this state of balanced neutralisation let us apply indirect stimulus by throwing light on the stem at a point directly opposite to the leaf (Fig. 138).

[Ill.u.s.tration: FIG. 138.--(_a_) Diagrammatic representation of direct application of light (v) on the pulvinus and the indirect application on the stem (-->) (_b_) Record of effect of direct stimulus, positive curvature followed by neutralisation. Superposition of the positive reaction of indirect stimulus induces erectile up-response followed by down movement due to transmitted excitatory impulse (_Mimosa_).]

Two different impulses are thus initiated from the effect of indirect stimulus. In the present case the positive reached the responding pulvinus after 30 seconds and induced an erectile movement of the leaf; the excitatory negative impulse reached the organ 4 minutes later and caused a rapid fall of the leaf. The record (Fig. 138) shows further that the previous action of direct stimulus which brought about neutralisation, does not interfere with the effects of indirect stimulus. The individual effects of direct and indirect stimulus are practically independent of each other; hence their joint effects exhibit algebraical summation.

We are now in a position to have a complete understanding of the characteristic response of Paniceae to transmitted phototropic excitation.

(1) Local stimulation of the tip gives rise to two impulses, positive and negative. The former induces a transient negative movement (away from light); the latter causes a permanent and increasing positive curvature towards light.

(2) Local stimulation of the growing hypocotyl gives rise to positive curvature, subsequently neutralised by the transverse conduction of excitation to the distal side. The absence of tropic effect in the growing region is thus due not to lack of power of perception, but to balanced antagonistic reactions of two opposite sides of the organ.

(3) The effects of direct and indirect stimulations are independent of each other; hence, on simultaneous stimulations of the tip and the growing hypocotyl, the effects of indirect stimulus are algebraically summated with the effect of direct stimulus (neutralisation). The indirect stimulation of the tip on the right side gives rise to two impulses, of which the expansive positive reached the right side of the responding region earlier, inducing convexity and movement away from stimulus (negative curvature). This is diagrammatically shown in Fig.

139. Had the intervening tissue been non-conducting, the slow excitatory negative impulse would have failed to reach the responding region, and the negative curvature induced by the positive impulse would prove to be the initial as well as the final effect. In the case of _Setaria_, however, the excitatory impulse reaches the right side of the organ after the positive impulse; the final effect is therefore an induced concavity and positive curvature (movement towards stimulus).