(1

Wiesner finds that if a plant is subjected to periods of one minute's illumination alternating with 2' darkness the same heliotropic effect is produced as by constant illumination. He explains this by supposing that the Nachwirkung lasts during the 2' darkness, so that it is immaterial whether the 2' are spent in light or darkness.

What I want to get at is:— what, according to this view, ought to be the normal action of light when there are no dark periods.

Wiesner must make one of two assumptions. He might say (I) that after 1' illumination the plant is incapable of being further affected by light until 2' have elapsed; or that (ii) during the period of Nachwirkung it can still be affected.

If ii is true then the effect of alternate light & dark periods cannot be equal to continuous light unless the sensitiveness to light is heightened by the periods of darkness. This we imagined to be the case and we drew an analogy between the phenomena & those of contract on in the case of our senses.

If on the other hand (i) is true, then it seems

(2

that light the effect mode of action of light on a plant is normally to produce one of alternate periods of activity receptivity and absence of receptivity. These would correspond to exhaustion & repose; but is it not the existence of exhaustion and repose which gives rise to the sense of contrast?

If this is so, then whichever assumption (I or II) we make we shall establish as analogy with the phenomena of contrast.

[2v]

FD

Light

contrast