Water lilies, aquatic plants belonging to the genus Nymphaea, are known for their striking blooms that float on the water’s surface. The majority of common varieties close their flowers at night. This daily opening and closing is a form of plant movement known as nyctinasty, a response to environmental cues tied to the 24-hour cycle. This biological rhythm serves important functions for the plant’s survival and reproduction.
The Diurnal Closing Pattern
The most commonly observed water lilies are day-blooming (diurnal) species, which follow a predictable schedule dictated by the sun. These flowers typically begin to open shortly after sunrise, unfurling their petals to reveal reproductive structures during the brightest part of the day. Closure usually occurs in the late afternoon or early evening, often around sunset, as light intensity fades.
This rhythmic pattern was historically part of Linné’s floral clock. Not all water lilies follow this exact pattern; some tropical species are night-blooming, opening only after dusk to attract nocturnal pollinators. However, the vast majority of water lilies observed in ponds and gardens exhibit this daily closing before dark.
The Mechanism of Movement
The physical movement of the petals is a nastic movement, a plant response independent of the stimulus direction. This floral rhythm is controlled internally by the plant’s circadian clock, synchronized by external factors like light and temperature. The movement is achieved through reversible changes within specialized cells located along the petals, not through muscle-like contraction.
The core mechanism involves the rapid expansion and shrinkage of these flexible cells, particularly in the intermediate segment of the petal. When the flower opens, these cells expand, and when it closes, they shrink, driven by changes in turgor pressure. This cellular action is regulated by the plant hormone auxin, which coordinates the floral movement rhythm. Auxin promotes the cell expansion necessary for unfurling and helps trigger the closing process.
Ecological Role of Flower Closing
The closing pattern serves distinct evolutionary and ecological purposes, centered on protection and pollination efficiency. Closing the petals at night acts as a physical barrier, shielding the delicate reproductive organs from environmental hazards. This action protects the pollen and female structures from cold temperatures, heavy dew, or rain, which could damage them or wash away the pollen.
The closed flower also protects the inner parts from nocturnal herbivores and pests that might feed on the pollen or petals. The closing functions as a specific pollination strategy to ensure successful transfer of genetic material. Day-blooming flowers limit access to only active diurnal pollinators, maximizing the chance of a successful visit.
In some species, such as the fragrant water lily (Nymphaea odorata), the closing is part of a complex two-day cycle. On the first day, the flower is female and traps visiting insects in a pool of fluid, ensuring foreign pollen deposition. It then closes overnight, holding the insect captive, and on the second day, the flower converts to the male stage, dusting the released insect with its own pollen to carry to the next flower.