AbstractA theoretical model for photoperiodic time measurement in higher plants is presented. It is based on the concept that the diurnal change in responsiveness to the phytochrome pigment system is dependent upon a circadian rhythm in membrane functioning and configuration, which is reflected in a parallel rhythm in membrane binding capacity for phytochrome Pfr. Phytochrome in turn, being an integrated part of the membrane clock, is acting as a “Zeitgeber” by mediating the external light stimuli in phase‐controlling of the membrane oscillation. Phytochrome and the circadian clock is thus integrated into one structural and functional unity.Applied to the field of photoperiodic control of flowering the model is shown to account for most of the conflicting experimental results in both short‐day and long‐d