Some flowers fold their petals inward as dusk approaches, creating a closed, bud-like structure before reopening with the morning light. This rhythmic movement, often called a “sleep movement,” is a highly regulated response to environmental changes. This daily cycle serves several important biological purposes that help ensure the flower’s survival and reproductive success. The mechanism behind this closing and opening is a complex interplay of internal timing and external stimuli, demonstrating the plant’s ability to respond to the twenty-four-hour cycle of day and night.
The Mechanism of Nyctinasty
The rhythmic closing and opening of petals is a form of nastic movement known as nyctinasty, which is a response independent of the stimulus’s direction. This movement is primarily driven by rapid, reversible changes in the internal water pressure, known as turgor pressure, within specialized motor cells. In some flowers, these motor cells are located in a swollen, joint-like structure called the pulvinus, found at the base of the petal or leaf stalk.
The physical movement occurs because of an unequal change in turgor pressure between the cells on the upper and lower sides of the pulvinus. When the flower closes, water and ions, such as potassium, move out of the cells on one side, causing them to shrink, while the opposing side maintains its turgidity. This differential shrinking and swelling creates a bending motion that forces the petals to fold inward.
The plant perceives the shift from day to night through internal timing mechanisms, or circadian rhythms, and light-sensitive proteins called photoreceptors. Phytochromes detect changes in the ratio of red to far-red light, which signals the approach of sunset and triggers the movement. In many flowers, the movement is achieved by differential growth, where the inner petal cells grow faster in the morning to open the flower, and the outer petal cells grow faster at night to close it.
Evolutionary Reasons for Flower Closure
Flower closure provides several protective benefits. One significant advantage is the protection of delicate reproductive organs, such as the stamens and pistils, from potential cold damage. Closing the petals helps reduce exposure to the cold night air and prevents frost from harming the flower’s pollen and ovules.
The closed structure also provides a barrier against excess moisture, shielding the pollen and nectar from dew or rain. Wet pollen becomes heavy and sticky, making it less likely to be successfully carried away by pollinators. Keeping the interior dry ensures the reproductive material remains viable for the next day’s activity.
Flower closure also plays a role in optimizing pollination efficiency by managing energy. The tightly shut petals conserve the flower’s resources, such as its fragrant oils, concentrating them for release during the day when target pollinators are active. Closing also excludes non-target nocturnal insects that might consume nectar without effectively transferring pollen, focusing the flower’s energy on daytime visitors like bees and butterflies.
Common Examples of Sleeping Flowers
Many familiar garden plants exhibit this noticeable sleep movement. The Tulip is a classic example, reliably closing each evening and reopening when exposed to sufficient warmth and light in the morning. This closing is driven by differential cell growth, where temperature changes primarily affect the growth rate of the cells on the inner and outer sides of the petals.
Crocuses also perform a similar temperature-dependent closure, sealing their petals as the temperature drops in the late afternoon and evening. This behavior, sometimes called thermonasty, is important for early spring bloomers facing a higher risk of freezing temperatures overnight. The California Poppy is another well-known example, closing its bright orange petals mainly to protect its pollen and nectar from the heavy dew common in its native habitat.
The African Daisy, or Cape Marguerite, folds up its petals every night, a response highly sensitive to light levels. In contrast to flowers that close at night, the Evening Primrose demonstrates reverse nyctinasty, opening its petals rapidly at dusk and staying open throughout the night. This adaptation attracts nocturnal pollinators like moths, which are active only after the sun has set.