CBSE Class 10 Answered

Opening and closing of flowers is a growth movement, without regard for the direction of the stimulus. Many flowers have petals which are open during the day, but which close up at night. These flowers are reacting to light or temperature changes. Other flowers, however, remain open around the clock, while still others have unusual opening and closing habits. But these habits are all related to light or temperature changes.

Heat makes the inner surfaces of a flower's petals grow. So when the temperature goes down, the outer surfaces grow faster than the inner ones, thus making the flower close.

The crocus and morning glory, for example, open as the temperature increases during the day and close as the day gets cooler in the late afternoon.