When a plant organ is subjected to the continued action of unilateral stimulus of light, it exhibits increasing tropic curvature, which in certain cases reaches a limit; in other instances a reversal takes place, seen in neutralisation, or in the conversion of the positive into negative curvature. I shall in this chapter enter into a detailed study of the phototropic curve, and determine its characteristics.
As the vague terminology at present in use has been the source of much confusion, it is necessary here to define clearly the various terms which will be employed in this investigation. It is first of all necessary to distinguish between cause and effect, between external stimulus and the excitation induced by it. As regards stimulus itself I have shown elsewhere[16] that its effective intensity becomes summated by repetition. This was demonstrated by the two following typical experiments carried out with the pulvinus of Mimosa.
(1) The intensity of a single electric shock of intensity of 0·5 unit was found to be ineffective in inducing excitation; but it became effective on being repeated four times in rapid succession.
(2) The same specimen was next subjected to a feebler stimulus of intensity of 0·1 unit, and it required a repetition of 20 times for the stimulus to become effective.
The total stimulus in the first case was 0·5 × 4 = 2, and this was found to be the same as 0·1 × 20 = 2 in the second case. Thus the intensity of stimulus is increased by repetition; in the limiting case where the interval between successive stimulus is zero, the stimulus becomes continuous. Bearing in mind the additive effects of stimulus we see that its effective intensity increases with the duration of application. This important conclusion found independent support from the results of Experiment 133 given in the last chapter.
We shall now take up the general question of the characteristics of the phototropic curve, which gives the relation between increasing stimulus and the resulting excitation. As regards stimulus we found that its effectiveness increases with the duration of application. The induced excitation in growing organs may be measured by concomitant retardation of growth caused by stimulus. In the excitation curves which will be presently given, the abscissae represent increasing stimulus and ordinates the resulting excitation. This excitation curve may be obtained by making the plant record on a moving plate its retardation of growth by means of the High Magnification Crescograph. I reproduce below two records of the effects of continuous photic and electric stimulation. The ordinate of the 'excitation curve' (Fig. 130) exhibits increasing incipient contraction (retardation of growth) culminating in an arrest of growth; the abscissa represents increasing stimulus consequent on increased duration of application. The record shows that the incipient contraction is slight at the first stage; it increases rapidly in the second stage; finally, it declines and reaches a limit. The excitatory reaction is thus not constant throughout the entire curve of excitation, but undergoes very definite and characteristic changes. We shall find similar characteristics in the phototropic curves under unilateral stimulus which will be given presently. The explanation of the similarity is found in the fact that the tropic curvature is also due to incipient contraction or retardation of the rate of growth, which remains confined to the directly stimulated proximal side of the organ.
Fig. 130.—Effects of continuous (a) electric, and (b) photic stimulation on rate of growth. Abscissa represents duration of application of stimulus. Note induced retardation, and arrest of growth.
For facility of explanation of what follows, I shall have to use a new and necessary term, susceptibility, to indicate the relation of cause and effect, of stimulus and resulting excitation. Susceptibility is thus = Excitation⁄Stimulus. Different organs of plants exhibit unequal susceptibilities; some undergo excitation under feeble stimulus, while others require more intense stimulus to induce excitation. But even in an identical organ the susceptibility undergoes, as we have seen, a characteristic variation, being feeble at the beginning of the excitation curve, considerable in the middle, and becoming feeble once more towards the end of the curve. The most difficult problem that faces us is an explanation of this characteristic difference in different parts of the tropic curve.