Internal biological clocks in plants that regulate daily cycles of physiological processes, typically following a roughly 24-hour pattern. These rhythms control activities like leaf movement, flower opening, and metabolic processes even in constant environmental conditions.
From Latin 'circa' meaning 'about' and 'dies' meaning 'day,' literally 'about a day.' The term was coined in the 1950s by chronobiologist Franz Halberg, though the phenomenon in plants was observed much earlier by astronomers noting daily leaf movements.
Plants actually have better internal clocks than most humans! Their circadian rhythms are so precise that some flowers open and close at specific times, and 18th-century botanist Carl Linnaeus created a 'flower clock' that could tell time based on which flowers were open.
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