| Fig. 218. | Fig. 219. | Fig. 220. |
Fig. 218.—Pre-maximum after-effect of light in Mimosa.
Fig. 219.—After-effect at maximum.
Fig. 220.—Post-maximum after-effect exhibiting an 'over-shooting' below position of equilibrium.
In the above records light was applied at arrow, and stopped at the second arrow enclosed in a circle.
After-effect at pre-maximum: Experiment 227.—Light from an 100 c.p. incandescent lamp was focussed on the upper half of the pulvinus of Mimosa for 8 minutes, after which the light was turned off. The after-effect was a persistence of previous movement followed by recovery (Fig. 218).
After-effect at maximum: Experiment 228.—Continued action of light for 18 minutes induced maximum positive curvature as seen in the upper part of the curve becoming horizontal. On the stoppage of light, there was a recovery to the original position of equilibrium (Fig. 219).
After-effect at post-maximum: Experiment 229.—A fresh specimen of plant was taken for this experiment; it exhibited maximum positive curvature after an exposure of 20 minutes; continuation of light for a further period of 17 minutes produced complete neutralisation. Stoppage of light at this point, gave rise to a rapid down-movement (Fig. 220) below the equilibrium position.
The experiments that have been described show that the rapid fall of the leaf of Mimosa in the afternoon is due to 'over-shooting' which is the after-effect of prolonged action of light.
We are now in a position to give a full explanation of the different phases of diurnal movement of the leaf of Mimosa. The fall of the leaf commences from its highest position at thermal-dawn at 6 a.m. in the morning and continued till the thermal-noon at 2 p.m. This is the thermo-geotropic reaction due to rise of temperature. In the forenoon the phototropic action is positive, and the fall of the leaf, due to rise of temperature, is brought about in opposition to the action of light. The temperature begins to fall after 2 p.m. and the leaf begins to erect itself, and in the absence of any disturbing factor would have continued its up-movement till next morning. But light undergoes a rapid diminution after 5 p.m. and the after-effect of light is an 'over-shooting' in a downward direction. This fall continues till about 9 p.m., after which the leaf erects itself under thermo-geotropic action of falling temperature, the maximum erection being attained at the thermal-dawn at about 6 a.m.