Although many flowers close up at night, not all of them do. Those that open their petals during the day and then close them at night are reacting to light or temperature changes. Other flowers, such as tudaylilies, remain open for twenty-four hours. Other flowers have unusual opening and closing habits. Crocuses, poppies, and morning glories, for example, open as the temperature increases during the day and close as the day gets cooler in the late afternoon. The flower called the four-o'clock closes in the morning and opens again late in the afternoon, right around four o'clock. Moonflower, night-blooming jasmine, evening primrose, angel's trumpet, night phlox, and night-blooming cereus open only at dusk or at night. Some plants also react to touch and close up their leaves and “play dead” if a hand or twig brushes against them. For example, if you touch a mimosa plant, it will fold its leaves and the stalk will droop.
Flowers close their petals at night because they are protecting the pollen and other reproductive parts of the plant that are inside from the cold and rain. Also, many flowers are pollinated by insects and birds that are active during the day, so there is no reason to be open at night. However, some types of flowerssuch as some types of tropical fruit plants and varieties of cactus flowers and related plantsare pollinated by bats at night, and these flowers will be open at night and closed during the daytime.
