The common observation of a flower folding its petals as darkness falls is real, but whether the plant doing the closing is a true lily depends entirely on its genus. This nightly movement is a precise biological behavior, finely tuned by evolutionary pressures to maximize the plant’s chance of reproductive success. Understanding this phenomenon requires looking past the common name to the specific plant species and the specialized cellular mechanisms that govern its daily cycle.
Identifying the Flowers That Close
Many plants with “lily” in their common name are not members of the genus Lilium, the true lilies. True lilies, such as Asiatic or Oriental lilies, typically remain open for an extended period, often lasting one to two weeks, and do not exhibit a pronounced nightly closing motion. Their floral structure is built for longevity and continuous availability to pollinators.
Plants that do close their flowers at night fall into different groups, including Daylilies and Water Lilies. Daylilies (Hemerocallis) are named because each individual flower opens in the morning and withers by nightfall. While this results in a “closed” appearance at night, it is not a reversible closing mechanism, as the flower is simply dying.
In contrast, Water Lilies (Nymphaea species) display a genuine, reversible closing movement. Diurnal varieties open mid-morning and close late in the afternoon or evening. Some tropical water lilies are even nocturnal, opening at dusk and closing again in the morning, specifically targeting night-flying moths or bats for pollination.
The Science Behind Nyctinasty
The rhythmic opening and closing of petals, known as nyctinasty, is a form of plant movement controlled by an internal biological clock. It is primarily driven by changes in light and temperature. The mechanics rely on specialized motor organs at the base of the petals or leaves, where the cells rapidly gain or lose water pressure.
When the flower needs to open or close, ions, most commonly potassium, are quickly moved into or out of these motor cells. This ion flux causes water to follow by osmosis, leading to a swift volume change in the cells and resulting in the petal movement.
The plant’s internal, roughly 24-hour cycle, or circadian rhythm, regulates the timing of this ion movement. Light-sensing pigments, such as phytochromes, act as a trigger, detecting the ratio of red to far-red light to signal the onset of day or night.
Evolutionary Reasons for Closing
The nightly folding of petals offers several distinct evolutionary advantages. One primary benefit is the physical protection of the flower’s delicate reproductive organs. Closing the petals forms a protective dome, safeguarding pollen from environmental hazards.
This closed structure shelters the pollen from cold nighttime temperatures, which can reduce its viability. It also prevents the pollen from becoming saturated with heavy dew or rain, as wet pollen is less likely to be successfully transferred by insects.
The timing of opening and closing is also a crucial energy and pollination strategy. By closing at night, the flower conserves its scent and nectar resources for the daytime hours when its specific, preferred pollinators, like bees or butterflies, are most active.