Example of type A is found in the negative phototropism of the root of Sinapis.
Negative phototropism of root of Sinapis: Experiment 140.—For investigation of the negative phototropism of the root of Sinapis nigra I took record of its movement under unilateral action of light by means of a Recording Microscope, devised for the purpose.[22] When the root-tip alone was stimulated by unilateral light, the root moved away from the source of light. This was due to the longitudinal transmission of positive impulse to the growing region at some distance from the tip. The intervening distance between the tip and the growing region is practically non-conducting, hence the excitatory impulse could not be conducted from the tip. After a period of rest in darkness, I next took record of its movement under direct unilateral illumination of the growing region; the result was at first a positive movement; but this, on account of transverse conduction of excitation under continued stimulation, underwent a neutralisation and slight reversal. In taking a third record, in which both the 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.
Thus in the root of Sinapis, the expansive effect of indirect stimulation of the tip is superposed on that of direct stimulation of the growing region (neutral or slightly negative). The final result is thus a movement away from light or a negative phototropic curvature.
SUMMARY.
The effect induced by stimulus of light is transmitted to a distance, in a manner precisely the same as in other modes of stimulation.
In the Paniceae, the local unilateral stimulation of the tip of the cotyledon induces positive curvature in the growing hypocotyl, at some distance from the tip. This is due to transmitted excitatory effect of indirect stimulation; the earlier positive impulse induces a preliminary negative curvature, which is reversed later by the excitatory negative impulse into positive curvature.
Contrary to generally accepted view the hypocotyl not only perceives but responds to light. The positive curvature induced by direct stimulation is, however, neutralised by transverse conduction of excitation.
The effects of direct and indirect stimulus are independent of each other; the final effect is determined by their algebraical summation.
[20] Jost—Ibid—p. 468.
[21] "Response in the Living and Non-Living"—p. 17.