As regards negative phototropic response of the root of Sinapis, it will be shown (p. 376) to be brought about by algebraical summation of the effects of direct and indirect photic stimulus.
SUMMARY.
The normal positive phototropic curvature is modified by transverse conduction of true excitation to the distal side of the organ.
The extent of neutralisation or reversal due to internal conduction of excitation from the proximal to the distal side of the organ depends: (a) on the intensity of the incident stimulus, (b) on the conductivity of the organ in a transverse direction, (c) on the thickness of the intervening tissue, and (d) on the relative excitability of the distal as compared to the proximal side.
The dia-phototropic position is not one of indifference, but of balanced antagonistic reactions of two opposite sides of the organ.
The supposition that direct sunlight is phototropically ineffective is unfounded. The response is fully vigorous, but the first positive curvature may in certain cases be neutralised by the transmission of excitation to the distal side.
Under light of strong intensity and long duration, the transmitted excitation to the distal side neutralises, and finally reverses the positive into negative curvature.
The positive-phototropic, the dia-phototropic, and the negative phototropic curvatures are not due to three distinct irritabilities but are brought about by a fundamental excitatory reaction remaining localised or increasingly transmitted to the distal side.
[12] Jost—Ibid—p. 462.
[13] Jost—Ibid—p. 464.