Fig. 192.—Diurnal movement of the leaflet of Cassia alata. Closure movement commenced at 5 p.m. and completed by 9 p.m. Leaflets began to open at 5 a.m.
DIURNAL MOVEMENT OF THE LEAFLET OF Cassia alata.
Experiment 206.—I next obtained the diurnal record of the leaflet, from 4 p.m. till 1 p.m. next day. The leaflets remain open from 1 p.m. to 4 p.m. and the record of this period is therefore omitted. In the diurnal record (Fig. 192) the first thick dot was made at 4 p.m. and successive thick dots are at intervals of an hour, the thinner dots being at intervals of 15 minutes. It will be seen that a rapid movement of closure was initiated at 5 p.m. when the light is undergoing a rapid diminution. The movement of closure is completed at about 9 p.m. The leaflets remain closed till 5 a.m. next morning, after which they begin to open; this opening may commence even an hour earlier. It should be borne in mind in this connection, that since light and rise of temperature are antagonistic in their reactions, the effects of light and fall of temperature would be concordant; and the opening in the early hours may possibly be hastened by the low temperature in the morning. The leaflets open to their utmost by 9 a.m., and they remain open till the afternoon. The plant is so extremely sensitive to light that any slight fluctuation is followed by responsive movement of the leaflet. Thus the transitory passage of a cloud is marked in the record by a short-lived closure movement.
Fig. 193.—The day and night positions of the petiole and terminal leaflet of Desmodium gyrans.
DIURNAL MOVEMENT OF THE TERMINAL LEAFLET OF Desmodium gyrans.
Both the petiole, and the terminal leaflet of this plant exhibit very marked nyctitropic movement. The petiole is raised and becomes almost erect in the evening, while the pulvinus of the terminal leaflet exhibits a sharp curvature downwards (Fig. 193).
Experiment 207.—The petiole was held fixed, and the terminal leaflet attached to the recording lever. I have already explained that light falling on the pulvinus from above, induces an up-movement of the leaflet, which is thus erected under light of moderate intensity. If the light be strong, the transversely conducted excitation induces a partial neutralisation; very intense light may even cause a reversal into down-movement. Under natural conditions, day-light acting from above induces an up-movement; darkness, on the other hand, induces a rapid movement of fall. The leaflets sometimes exhibit autonomous pulsations; but the diurnal movement is very strong and the daily curve appears as a single large pulse on which smaller autonomous pulsations may become superposed.
