Final answer:
Plants being able to open and close during day and night is a result of photoperiodism, a biological response to the length of day or night. Plants use a light-sensitive chemical called phytochrome to sense the differences in day length and detect seasonal changes. Short-day and long-day plants flower based on the length of darkness.
Step-by-step explanation:
Plants being able to open and close during day and night is a result of a biological response known as photoperiodism. Photoperiodism is the reaction of plants to the length of day or night, specifically the length of light and dark periods. Many plants sense the length of night as a signal to flower or adjust their physiological processes accordingly.
Plants use a light-sensitive chemical called phytochrome to sense the differences in day length. By sensing the Pr/Pfr ratio at dawn, a plant can determine the length of the day/night cycle and detect seasonal changes. Short-day (long-night) plants flower in late summer and early fall, when nights exceed a critical length, while long-day (short-night) plants flower during the spring when darkness is less than a critical length.