Complete phototropic curve of growing organs: Experiment 136.—I obtained very similar effects by subjecting the seedling of Zea Mays to unilateral light from an arc lamp for two hours. The characteristic of this curve is similar to that given by the terminal leaflet of Desmodium gyrans. At the first stage, the sub-minimal stimulation is seen to induce a negative curvature, transformed into positive after an interval of 10 minutes. The maximum positive curvature is reached after 50 minutes, and neutralisation completed in a further period of 43 minutes (Fig. 134). After this the response became transformed into negative.
Fig. 134.—Complete phototropic curve of a growing organ (Zea Mays).
In a complete phototropic curve we may thus distinguish 4 distinct stages:—
(1) The stage of sub-minimal stimulation.
(2) The stage of increasing positive curvature culminating in a maximum.
(3) The stage of neutralisation.
(4) The stage of complete reversal into negative.
The curve thus crosses the zero line of the abscissa twice; the first crossing takes places upwards at the critical point of stimulation which demarcates the sub-minimal from the minimal. The second crossing downwards occurs beyond the point of complete neutralisation.
In a tissue in which transverse conductivity is absent, and the stimulus applied from the beginning is above the minimal, the simple tropic curve is confined to the second stage (see Fig. 132).