_Complete phototropic curve of a pulvinated organ: Experiment 135._--A continuous record was taken of the action of light of a 50 c.p. incandescent lamp, applied on the upper half of the pulvinus of the terminal leaflet of _Desmodium gyrans_. This gave rise: (1) to a negative curvature (due to sub-minimal stimulus) which lasted for 3 minutes. The curve then proceeded upwards, at first slowly, then rapidly; it then rounded off, and reached a maximum positive value in the course of 18 minutes; after this the curve underwent a reversal, and complete neutralisation occurred after a further period of 24 minutes (Fig. 133). Beyond this the induced curvature is negative. [Illustration: FIG. 133.--Complete phototropic curve given by pulvinated Eq. organ. Positive curvature above, and negative curvature below the horizontal zero line. Preliminary negative phase of response due to sub-minimal stimulus. The positive increases, attains a maximum, and undergoes a reversal. Successive dots at intervals of 30 seconds. Abscissa represents duration of exposure and quantity of incident light. (Terminal leaflet of _Desmodium gyrans_.)] _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. [Illustration: 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). WEBER'S LAW.