TABLE x.x.x.--SHOWING THE VARIATION OF SUSCEPTIBILITY FOR EXCITATION AT DIFFERENT POINTS OF THE TROPIC CURVE.

+--------------------------------------+

Successive points

Susceptibility

in the curve.

for excitation.

+-------------------+------------------+

1 ... ...

0

2 ... ...

0187

3 ... ...

044

4 ... ...

0625

5 ... ...

0875

6 ... ...

125

7 ... ...

187

8 ... ...

312

9 ... ...

50

10 ... ...

625

11 ... ...

875

12 ... ...

887

13 ... ...

812

14 ... ...

66

15 ... ...

44

16 ... ...

25

17 ... ...

187

18 ... ...

15

19 ... ...

112

20 ... ...

0937

21 ... ...

075

22 ... ...

0562

23 ... ...

0375

24 ... ...

025

25 ... ...

0187

26 ... ...

0062

+--------------------------------------+

The induced excitation is seen to be increased very gradually from the zero point of susceptibility, known as the latent period at which no excitation takes place. In the second part of the excitation curve, the rate of increase is vary rapid; the maximum rate is nearly reached at point 11 of the curve and remains fairly constant for a time. This is the median range where equal increment of stimulus induces equal increment of excitation. The susceptibility for excitation then falls rapidly, and increase of stimulus induces no further increase of tropic curvature. The maximum tropic curvature was, in the present case, reached in the course of nine minutes. The attainment of this maximum depends on the excitability of the tissue, and the intensity of incident stimulus. The characteristics that have been described are not confined to the phototropic curve but exhibited by tropic curves in general.

Similar characteristics have been found in the curve for electric stimulus (Fig. 130a), and will also be met with in the curve for geotropic stimulus (Fig. 161).

I may here refer incidentally to the three types of responses exhibited by an organ to successive stimuli of uniform intensity; these appear to correspond to the three different regions of tropic curve; in the first stage, the plant exhibits a tendency to exhibit a 'staircase' increase of response; in the intermediate stage, the response is uniform; and in the last stage, the responses show a 'fatigue' decline.

For purpose of simplicity, I first selected the simple type of phototropic curve, where the specimen employed was in a favourable tonic condition, and the stimulus was, from the beginning, above the minimal.

Transverse conduction, which induces neutralisation or reversal into negative, was moreover absent in the specimen. I shall now take up the more complex cases: (1) where the condition of the specimen is slightly sub-tonic, (2) where the stimulus is gradually increased from the _sub-minimal_, and (3) where the specimen possesses the power of transverse conduction.

EFFECT OF SUB-MINIMAL STIMULUS.

It is unfortunate that the terms in general use for description of effective stimulus should be so very indefinite. A stimulus which is just sufficient to evoke excitatory _contraction_ is termed the minimal, stimulus below the threshold being tacitly regarded as ineffective. The employment of sensitive recorders has, however, enabled me to discover the important fact that stimulus below the minimal, though ineffective in inducing excitatory _contraction_, is not below the threshold of perception. The plant not merely perceives such stimulus, but also responds to it in a definite way, by _expansion_ instead of _contraction_. I shall designate the stimulus below the minimal, as the _sub-minimal_. There is a critical point, which demarcates the sub-minimal stimulus with its expansive reaction from the minimal with its responsive contraction.

The _critical_ stimulus varies in different species of plants. Thus the same intensity of light which induces a r.e.t.a.r.dation of growth in one species of plants will enhance the rate of growth in another. Again, the critical point will vary with the _tonic_ level of the same organ; in an optimum condition of the tissue, a relatively feeble stimulus will be sufficient to evoke excitatory contraction; the critical point is therefore low for tissues in tonic condition which may be described as _above par_. In a sub-tonic condition, on the other hand, strong and long continued stimulation will be necessary to induce the excitatory reaction. The critical point is therefore high, for tissues in a condition _below par_. Stimulus below the critical point will here induce the opposite physiological reaction, _i.e._, expansion. The physico-chemical reactions underlying these opposite physiological responses have, for convenience, been distinguished as the "A" and "D"

change (pp. 143, 223). The a.s.similatory 'building up', A change, is a.s.sociated with an increase of potential energy of the system; the dissimilatory 'break down', D change, on the other hand, is attended by a run-down of energy.

Stimulus was shown (p. 225) to give rise to _both_ these reactions, though the A effect is, generally speaking, masked by the predominant D effect. The "A" change is quicker in initiation, while the "D" effect developes later; again the "A" effect under moderate stimulation may persist longer. Thus owing to the difference in their time-relations the A effect is capable of being unmasked at the onset of stimulus or on its sudden cessation. For the detection of the relatively feeble expansive A effect, a special recorder is required which combines lightness with high power of magnification. The earlier expansive reaction and acceleration of rate of growth, followed by normal r.e.t.a.r.dation, are often found in the response of growing organs. The corresponding effect of unilateral stimulation, even when direct, is a transient expansion at the proximal side, inducing a convexity of that side and movement away from stimulus (negative curvature); this is followed by contraction and concavity with normal positive curvature. The interval between the A and D effects is increased with increasing sub-tonicity of the specimen. But it nearly vanishes when the excitability of the specimen is high, and the two opposite reactions succeed each other too quickly for the preliminary A reaction to become evident. It is probable that in such a case the conflict between the two opposite reactions prolongs the latent period. But in other instances a preliminary expansive response is found to herald the more p.r.o.nounced contractile response. Example of this is seen in figure 129 given in page 344.

The A effect was detected in the records referred to above by its earlier appearance. Its longer persistence, after moderate stimulation, is also to be found on the cessation of moderate stimulation. This was seen in the _acceleration_ of growth which was the after-effect of stimulation (Figs. 104, 115). The presence of two conflicting physiological reactions is also made evident on sudden cessation of long continued stimulation. This particular phenomenon of "overshooting" will be more fully dealt with in a subsequent chapter.

Owing to the difference in the time relations of the two opposing activities, A and D, a phase difference often arises in their respective maxima. It is probably on this account that rhythmic tissues originally at standstill, exhibit under continued stimulation a periodic up and down-movement, which persists even on the cessation of the stimulus. The persistence of after-oscillation depends, moreover, on the intensity and duration of previous stimulation.[18]

[18] "Plant Response"--p. 293, etc.

The facts given above cannot be explained by the prevalent theory that stimulus acts merely as a releasing agent, to set free energy which had been previously stored up by the organism, like the pull of a trigger causing explosion of a charged cartridge. It is true that in a highly excitable tissue, the external work performed and the run down of energy are disproportionately greater than the energy of stimulus that induces it. But in a sub-tonic tissue, stimulus induces an effect which is precisely the opposite; instead of a depletion, there is an enhancement of potential energy of the system. Thus the responding leaf instead of undergoing a fall becomes erected; growing organs similarly exhibit a 'building up' and an acceleration of rate of growth, in contrast with the usual 'break down' and depression of the rate. It is obvious that these new facts relating to the action of stimulus necessitate a theory more comprehensive and satisfactory than the one which has been in vogue.

THE COMPLETE PHOTOTROPIC CURVE.

I have explained the characteristics of the simple phototropic curve in which the tropic curvature, on account of the favourable tonic condition and strong intensity of incident light, was positive from the beginning, and in which the curvature reached a maximum beyond which there was no subsequent reversal. If the intensity of the stimulus be feeble or moderate, the quant.i.ty of light incident on the responding organ at the beginning may fall below the critical value, and thus act as a sub-minimal stimulus. This induces as we have seen (p. 344) a negative tropic curvature; continued action of stimulus, however, converts the preliminary negative into the usual positive. The preliminary negative curvature may be detected by the use of a moderately sensitive recorder with a magnification of about 30 times. It is comparatively easy to obtain the preliminary negative response in specimens which are in a slightly sub-tonic condition.

Semi-conducting tissues exhibit under continued stimulation, a neutralisation and reversal into negative (p. 331). Since this reversal into negative usually takes place under prolonged exposure to exceedingly strong light, it is difficult to obtain in a single curve all the different phases of transformation. I have, however, been fortunate in obtaining a complete phototropic curve which exhibits in a single specimen all the characteristic changes from a preliminary negative to positive and subsequent reversal to negative. I shall describe two such typical curves obtained with the terminal leaflet of _Desmodium gyrans_ and the growing seedling of _Zea Mays_.

_Complete phototropic curve of a pulvinated organ: Experiment 135._--A continuous record was taken of the action of light of a 50 c.p.

incandescent lamp, applied on the upper half of the pulvinus of the terminal leaflet of _Desmodium gyrans_. This gave rise: (1) to a negative curvature (due to sub-minimal stimulus) which lasted for 3 minutes. The curve then proceeded upwards, at first slowly, then rapidly; it then rounded off, and reached a maximum positive value in the course of 18 minutes; after this the curve underwent a reversal, and complete neutralisation occurred after a further period of 24 minutes (Fig. 133). Beyond this the induced curvature is negative.

[Ill.u.s.tration: FIG. 133.--Complete phototropic curve given by pulvinated Eq. organ. Positive curvature above, and negative curvature below the horizontal zero line. Preliminary negative phase of response due to sub-minimal stimulus. The positive increases, attains a maximum, and undergoes a reversal. Successive dots at intervals of 30 seconds.

Abscissa represents duration of exposure and quant.i.ty of incident light.

(Terminal leaflet of _Desmodium gyrans_.)]

_Complete phototropic curve of growing organs: Experiment 136._--I obtained very similar effects by subjecting the seedling of _Zea Mays_ to unilateral light from an arc lamp for two hours. The characteristic of this curve is similar to that given by the terminal leaflet of _Desmodium gyrans_. At the first stage, the sub-minimal stimulation is seen to induce a negative curvature, transformed into positive after an interval of 10 minutes. The maximum positive curvature is reached after 50 minutes, and neutralisation completed in a further period of 43 minutes (Fig. 134). After this the response became transformed into negative.

[Ill.u.s.tration: FIG. 134.--Complete phototropic curve of a growing organ (_Zea Mays_).]

In a complete phototropic curve we may thus distinguish 4 distinct stages:--

(1) The stage of sub-minimal stimulation.

(2) The stage of increasing positive curvature culminating in a maximum.

(3) The stage of neutralisation.

(4) The stage of complete reversal into negative.

The curve thus crosses the zero line of the abscissa twice; the first crossing takes places _upwards_ at the critical point of stimulation which demarcates the sub-minimal from the minimal. The second crossing downwards occurs beyond the point of complete neutralisation.

In a tissue in which transverse conductivity is absent, and the stimulus applied from the beginning is above the minimal, the simple tropic curve is confined to the second stage (see Fig. 132).

WEBER'S LAW.

If we neglect the preliminary negative portion under sub-minimal stimulus, the curve of excitation under increasing photic stimulation obeys what is known as Weber's law. This is equally true of modes of stimulation other than that of light as is seen in figure 130 of the contractile effect of continued electric stimulus on growth; the excitatory effect is also seen to reach a limit.

Weber's law is applicable for a limited range of stimulation. For the quant.i.tative relation fails in the region of sub-minimal stimulus, where the physiological reaction is _qualitatively_ different, namely expansion instead of contraction. This holds good even in the case of animal tissues, for here also my recent experiments show that two opposite reactions--expansion and contraction--take place under stimulus, and that a very feeble stimulus tends to induce expansion instead of contraction. The responsive reaction of a kitten under gentle caressing strokes must be _qualitatively_ different from that of a blow.

The psychological effects under the two treatments evidently differ qualitatively rather than quant.i.tatively.[19]

[19] "It has been argued by James that the feeling does not cause, but is caused by the bodily expression.... Munsterberg concludes that the feeling of agreeableness is the mental accompaniment and outcome of reflexly produced movements of extension, and disagreeableness of the movement of flexion."

Schafer--Text Book of Physiology, Vol. II, p. 975 (1900).

SUMMARY.

The excitation curve exhibits a slow ascent in the first part; in the second part the gradient is steep, indicating rapid rise in excitation; in the third part it is uniform; and in the last part the curve rounds off and the rate of ascent becomes very small.

The susceptibility for excitation is feeble at the beginning; it increases very rapidly with increasing stimulus; finally it undergoes a fall, increase of stimulus inducing no further enhancement of excitation.

In a complete phototropic curve the first part is negative; this is due to the physiological expansion induced by sub-minimal stimulus. The curve then crosses the abscissa upwards, and the positive curvature reaches a maximum. This is followed by neutralisation and reversal into negative; the curve crosses the zero line and proceeds in the negative direction.

Weber's law is not applicable for the entire range of stimulation. The quant.i.tative relation fails in the region of sub-minimal stimulus, where the physiological reaction is _qualitatively_ different.

x.x.xIII.--THE TRANSMITTED EFFECT OF PHOTIC STIMULATION

_By_

SIR J. C. BOSE,

_a.s.sisted by_

JYOTIPRAKASH SIRCAR, M.B.

Plant organs exhibit, as we have already seen, a heliotropic curvature under direct stimulation. Still more interesting is the transmitted effect of light giving rise to a curvature. Thus if the tip of the seedling of wheat be exposed to light, the excitation is transmitted lower down into the region which acts as the responding organ. Growth is very active in this particular zone, and the change of growth, induced by the transmitted effect of stimulus, brings about a curvature by which the tip of the seedling bends towards light. The seedling thus appears to be differentiated into three physiological zones subserving three different functions. The tip is the perceptive zone, the intervening distance between the tip and the growing region is the zone of conduction, and the growing region is the responsive zone. These differentiations are shown in a striking manner by certain Paniceae, _Setaria_ for example. In this seedling the tapering sheathing leaf or cotyledon is about 5 mm. in length, and it is the upper part of the cotyledon that is most sensitive to light. Below the sheathing leaf is a narrow length which will be distinguished as the hypocotyl, and where growth is very active. The apex of the leaf perceives the stimulus, and the effect is transmitted to the hypocotyl, which responds by becoming curved so that the seedling bends towards light.