Final answer:
Plants use the phytochrome system to detect the length of day and night cycles. Fuchsia, a short-day plant, changes color instead of dropping its flowers after seven days to indicate the approaching autumn.
Step-by-step explanation:
Plants use a system called phytochrome to detect the length of day and night cycles. This system allows plants to determine the time of the year and adjust their physiology accordingly. Short-day plants, like Fuchsia, require nights longer than a critical length to flower, typically eight or fewer hours. Instead of dropping their flowers after seven days, Fuchsia plants change color based on the lengthening nights, indicating the approach of autumn.