Fig. 132.—Simple characteristic curve of phototropic reaction. Excitation increases slowly in the first part and rapidly in the second; it is uniform in the third, and undergoes decline in the fourth part (Erythrina indica).
Experiment 134.—I give below the characteristic curve of excitation (Fig. 132) of the pulvinus of Erythrina indica, traced by the plant itself, and exactly reproduced by photomechanical process. A parallel beam of light from a Nernst lamp was thrown on the upper leaf of the pulvinus, and the increasing positive curvature was recorded on a smoked glass plate which was moved at an uniform rate. The successive dots are at intervals of 20 seconds; the horizontal distances between successive dots are equal, and represent equal increments of stimulus; the vertical distances between successive dots represent the corresponding increments of excitation. The gradient at any point of the curve—increment of excitation divided by increment of stimulus—gives the susceptibility for excitation at that point. The following table will show how the susceptibility for excitation undergoes variation through the entire range of stimulus. The average susceptibility for each point has been calculated from the data furnished by the curve.
TABLE XXX.—SHOWING THE VARIATION OF SUSCEPTIBILITY FOR EXCITATION AT DIFFERENT POINTS OF THE TROPIC CURVE.
| Successive points in the curve. | Susceptibility for excitation. | Successive points in the curve. | Susceptibility for excitation. |
| 1 | 0 | 14 | 6·6 |
| 2 | 0·187 | 15 | 4·4 |
| 3 | 0·44 | 16 | 2·5 |
| 4 | 0·625 | 17 | 1·87 |
| 5 | 0·875 | 18 | 1·5 |
| 6 | 1·25 | 19 | 1·12 |
| 7 | 1·87 | 20 | 0·937 |
| 8 | 3·12 | 21 | 0·75 |
| 9 | 5·0 | 22 | 0·562 |
| 10 | 6·25 | 23 | 0·375 |
| 11 | 8·75 | 24 | 0·25 |
| 12 | 8·87 | 25 | 0·187 |
| 13 | 8·12 | 26 | 0·062 |
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 retardation 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 assimilatory 'building up', A change, is associated 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.