## Abstract

1. Excised leaves of Bryophyllum fedtschenkoi show an endogenous rhythm in their rate of carbon dioxide output when they are kept in darkness and at a constant temperature. The effects of various constant ambient temperatures and of changes in temperature on the phase and period of this rhythm have been investigated. 2. Both the phase and period of a rhythm initiated by transferring leaves from light to darkness are determined by the constant ambient temperature between 16 and 31 degrees C. The time between the onset of darkness and the first peak of the rhythm (the transient, T$_{l}$) determines the phase. This time is 16$\cdot $8 $\pm $ 0$\cdot $3 h at 31 degrees C and 24$\cdot $7 $\pm $ 0$\cdot $3 h at 16 degrees C. The period reaches a steady value after the first peak of the rhythm and is temperature compensated to a greater extent than the transient. It is 20$\cdot $8 $\pm $ 0$\cdot $4 h at 31 degrees C and 23$\cdot $9 $\pm $ 0$\cdot $2 h at 16 degrees C. The ratio of (period at 16 degrees C)/(period at 26 degrees C) is 1$\cdot $06 whereas, that of (transient at 16 degrees C)/(transient at 26 degrees C) is 1$\cdot $06 3. The rhythm is inhibited in leaves held at 36 degrees C but begins again when the temperature is reduced to 26 or 16 degrees C. The first peaks occur 18 to 19 h and 24 to 25 h respectively after the change to the lower temperature. These times are similar to those between the end of an inhibitory light treatment and the first peak of the rhythm in leaves held at 26 and 16 degrees C. 4. When leaves kept in darkness at either 26 or 16 degrees C are exposed to 36 degrees C for several hours it is found that: (a) the phase of the rhythm is reset by a 6 h treatment given between, but not at the crests of, the peaks; (b) an approximately equal phase shift is induced by raising the temperature from 16 to 36 degrees C for 10 min or 6 h between the peaks providing the treatments terminate at the same time. The amount of shift is determined by the time at which the treatments end and not by their duration. This is because the first post-treatment peak always occurs 24 to 25 h after the end of the stimulus. Increasing the temperature from 26 to 36 degrees C for 3 or 6 h between the peaks also induces an approximately equal phase shift. The magnitude of the shift is again determined by the time at which the treatment ends and not by its duration. In these cases the first post-treatment peaks occur 18 to 19 h after the end of the treatment. Treatments of 15 min and 1 h duration induce smaller phase shifts which are approximately proportional to the duration of the treatment. 5. When leaves maintained at 26 degrees C and in darkness are chilled (0 to 2 degrees C) for several hours it is found that: (a) a 4 h treatment shifts the phase of the rhythm when it is given at the crest of a peak but not between the peaks; (b) a 4, 8 or 12 h stimulus given at the crest of a peak induces a phase delay which is approximately equal to the duration of the treatment. In contrast, while a 4 h stimulus given between the peaks has no detectable effect on the phase, extending the treatment to 8 or 12 h results in a shift which is less than, but a function of, the duration of the treatment. 6. Pulse-type stimuli within the range where the steady-state period is only slightly dependent upon temperature can induce phase shifts. The phase is shifted by increasing the temperature from 16 to 26 degrees C for 6 h between the peaks but is unaffected by similar treatment at the crest of a peak. Lowering the temperature from 26 to 16 degrees C for 6 h has little or no effect on the phase when the treatment is given at either position in the cycle. 7. Step-type changes in temperature from 26 to 16 degrees C and vice versa induce the expected changes in the period of the rhythm. However, the phase is shifted when the temperature is increased between the peaks but not at the crest of a peak. In contrast, it is delayed by reducing the temperature at both points in the cycle. 8. 36 degrees C treatments of a few hours' duration shift the phase of the rhythm when given between, but not at the crests of, the peaks. In contrast, low temperature (0 to 2 degrees C) stimuli induce a phase shift when given at the crests of, but not between, the peaks. Light and a temperature of 36 degrees C have closely similar effects on the basic oscillating system of the cells. Evidence indicating that a temperature compensating mechanism forms an integral part of this oscillating system is discussed. Mechanisms are tentatively suggested for the induction of phase shifts in the oscillations of the dark carbon dioxide fixation system by light and temperature stimuli.