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Biological clocks of about 24 hours' duration are called circadian clocks.
Many body functions follow circadian rhythms controlled by light intensity.
Changes in day length can control seasonal responses (such as flowering,
mating, or migration) in both plants and animals. Circadian clocks: Many organisms maintain internal clocks that control rhythms of approximately 24 hours (circadian rhythms). In vertebrates, the pineal body (epiphysis) appears to control these rhythms in response to daily fluctuations in light intensity. Body temperature, hormone levels, neurotransmitter levels, and immune responses all follow circadian rhythms. In constantly dim light, these rhythms continue for days, though they slowly drift from exactly 24-hours duration. Bright light resets circadian clocks to conform to natural daylight rhythms. Rapid resetting of the clock by more than an hour produces "jet lag," increasing the need for rest and depressing the immune system. Seasonal responses (photoperiodism) in plants: Plants need to flower or their seeds need to sprout at particular times of the year. Pollen production must occur at the same time in all members of a population if pollination is to occur. The timing of these events often depends on increasing or decreasing day length and can be artificially manipulated by gradually changing rhythms of light and darkness. "Long day" (or "short night") plants flower in response to dark periods below some critical duration. "Short day" (or "long night") plants require dark periods above a certain duration before flowering; brief flashes of brightness will interrupt these dark periods and inhibit flowering. Seasonal responses (photoperiodism) in animals: Mating seasons, including seasonal changes in hormone levels and secondary sex characteristics, are controlled in many species by changing day lengths. Artificial increases in the duration of daytime light intensities induce springtime responses; artificial decreases in daylight produce fall-like responses. Seasonal migrations in migratory species are also controlled this way, as is fat deposition in species that need to store up fat for migration or for overwintering. Artificial shortening or lengthening of dark periods can induce migration in seasonally inappropriate directions, accumulation of winter fat, and similar responses. |
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