Many plants exhibit a rhythmic behavior on about a 24‐hour cycle, such as the flowers that open in late afternoon every day. This regular repetition of growth or activity on approximately a 24‐hour cycle is called a circadian rhythm. All sorts of metabolic processes are circadian, such as cell divisions in root tips, and protein or hormone synthesis. Sleep movements of leaves are well‐known circadian rhythms, as are the opening and closing of night‐blooming or day‐blooming flowers.
Circadian rhythms are endogenous, meaning they are controlled by an internal timing mechanism called the biological clock of the plant. Although circadian rhythms are not triggered by an external stimulus, the environment does set and keep the biological clock in harmony with external changes such as darkness and light. The resetting of the biological clock is called entrainment. Entrainment, for example, keeps the circadian periodicity of flowering in sequence with light and dark periods even as the day lengths change seasonally. The biological clock keeps the plant responding appropriately for each season by measuring the changing day lengths. The mechanism by which it does this involves the pigment phytochrome.