Fig. 216.—Post-maximum after-effect of light on response of leaf of Cassia. There is an over-shooting on cessation of light at arrow within a circle.
Experiment 225.—In Cassia, as in Mimosa, light acting from above induces at first an erectile movement which reaches a maximum; after this there is a neutralisation and reversal. In the record given in figure 216, light from a small arc lamp acting on the upper half of the pulvinus for 48 minutes gave the maximum positive curvature; this was completely neutralised by further exposure to light for 20 minutes. Light was cut off at neutralisation and there was a sudden fall beyond the equilibrium position, which was more rapid than the movement under light. The after-effect of prolonged exposure is thus an 'over-shooting' beyond the normal position of equilibrium.
RESPONSE OF Mimosa TO DARKNESS AT DIFFERENT PARTS OF THE DAY.
I now tried the effect of darkness on the movement of Mimosa, and was surprised to find that while artificial darkness caused a sudden fall of the leaf in the afternoon, it had no such effect in the forenoon.
Experiment 226.—Successive records were taken of the effect of artificial darkness for two hours, alternating with exposure to light for two hours. The plant was subjected to darkness by placing a piece of black cloth over the glass cover from 12 to 2 p.m., it was exposed to light from 2 to 4 p.m. and darkened once more from 4 to 6 p.m.
The record given in figure 217 shows that the leaf had been moving upwards under the action of light (positive phototropism); darkness commenced at the point marked with a thick dot. The after-effect on the stoppage of light is seen to be in the same direction as under light; this persisted for ten minutes followed by recovery which was complete by 2 p.m., as seen in the horizontal character of the curve. On restoration of light (at the point marked with the second thick dot) the leaf moved upwards till the positive phototropic movement attained a maximum in the course of an hour and twenty minutes, after which neutralisation set in, and by 4 p.m. the positive phototropic effect had become partially neutralised. Artificial darkness at the third thick dot caused a rapid down-movement which overshot the position of equilibrium. The difference of after-effect in the forenoon and in the afternoon lies in the fact that in the first case it was the pre-maximum after-effect; but in the second case the after-effect was post-maximum. I have already shown in the previous chapter that the pre-maximum after-effect of light is a short-lived movement in the same direction as under light, while post-maximum after-effect was a rapid over-shooting downwards beyond the equilibrium position. These characteristics are also found in the after-effects of light in Mimosa.
Fig. 217.—Effect of periodic alternation of light L, and of darkness D, on the response of Mimosa leaf. The first darkness causes the pre-maximal after-effect of slight erection followed by recovery. The subsequent application of light from 2 to 4 p.m. caused erectile movement followed by partial neutralisation by 4 p.m. Stoppage of light at the third thick dot caused a sudden fall of leaf below the position of equilibrium.
The responses of Mimosa on the cessation of light described above took place in the course of experiments which lasted for more than six hours. Objection may be raised that during this long period the temperature variation must have produced certain effects on the response. In order to meet this difficulty, I carried out the following experiments which were completed in a relatively short time. I have already explained how the period of experiment could be shortened by suitable increase of the intensity of light. The experiment was commenced inside a room at noon and completed by 2 p.m.; the temperature variation during this period was less than 1°C.