Flower buds ofPharbitis nilcut from plants growing in the field opened rapidly when kept in darkness for 8 hr followed by continuous light at 20–25°C, but those kept in darkness for 4 hr opened promptly oniy when the temperature during the following light period was kept at 23°C or lower. Buds exposed to continuous light at 25°C did not open, but those exposed to continuous light at 23°C opened slowly. At a lower temperature, the buds opened rapidly even in continuous light. When the buds were placed in darkness at 25°C at 13:30, 17:30 and 21:30 (artificial light from 17:30 to 21:30), they opened about 10 hr after the onset of darkness regardless of the time of the onset of darkness, but when the buds were kept at 20°C in light from 13:30, 17:30 and 21:30, they opened at 3:30–5:30 regardless of the time of transfer to the lower temperature. The biological clock which controls the time of flower-opening is suggested to be easily reset by a light-off signal, but not by a shift from a normal to lower temperature (20°C). At the lower temperature, the time of flower-opening probably is determined by the time of the latest preceding light-off (or light-
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