MARTIN SHIELDS / SCIENCE PHOTO LIBRARY MARTIN SHIELDS / SCIENCE PHOTO LIBRARY
Common bean (Phaseolus vulgaris) seedling at 12:00 noon. The leaves are oriented in the horizontal plane to maximize photosynthesis. In the next photo in the sequence (10:00 p.m.) the leaves have moved to a sleeping vertical position (even though lights are on). These movements are governed by an internal circadian clock (circadian rhythm) that stimulates variations in turgor pressure between day and nighttime.
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