The daily cycle of light and darkness dictates a surprising behavior in many flowering plants, which appear to “go to sleep” by folding their petals. This phenomenon, where blossoms open with the morning sun and tightly close as dusk approaches, is a precise biological movement. It is a programmed response to changes in the surrounding environment, primarily light intensity and temperature shifts. This active, regulated process ensures the plant’s survival and reproductive success.
The Biological Mechanism of Nyctinasty
The physical movement of petals folding inward is a non-directional response to a stimulus, a category of plant motion known as nastic movements. When tied to the alternation of day and night, this action is termed nyctinasty, or sleep movement. The mechanism relies on changes in cell size within the petal tissue, often driven by shifts in internal water pressure (turgor).
In some plants, specialized cells at the base of the petals or leaves, sometimes part of a motor organ called a pulvinus, undergo rapid, reversible changes. These cells actively pump ions, causing water to follow due to osmosis, which alters the internal turgor. A gain in turgor on one side of a petal base and a loss on the other side creates an asymmetrical force, causing the petal to bend and the flower to close.
Other flowers sensitive to temperature utilize a mechanism involving differential cell growth. The cells on the inner and outer surfaces of the petals grow at different rates in response to temperature changes. When the temperature drops at night, the cells on the outer side of the petal base may elongate faster than the inner cells, forcing the petal to curve inward and seal the flower.
Common Examples of Night-Closing Flowers
Among the most recognizable flowers that demonstrate this nighttime folding are those that bloom early in the spring, like the Crocus. The cup-shaped blossoms are highly sensitive to both light and temperature, often closing not only at night but also on heavily overcast days. When closed, the three outer sepals and three inner petals overlap tightly, forming a slender, upright spire.
Tulips also exhibit a strong thermonastic response, where the primary trigger for closure is a decrease in ambient temperature. As the air cools in the evening, the petals curve inward, creating a characteristic closed goblet shape. This movement is so sensitive that a temperature change of just a few degrees Celsius can initiate the opening or closing process.
Poppies are another common example, with their delicate, often brightly colored petals closing to form a crumpled, pointed bud at night. The closure is thought to be a mechanism to protect the large amount of exposed pollen from the moisture of evening dew. Keeping the structure dry preserves reproductive viability.
The large, flat flowers of the Water Lily (genus Nymphaea) show a clear circadian rhythm, opening in the morning and closing in the late afternoon or evening. These aquatic plants perform a folding action where the petals and sepals curl inward, often creating a watertight seal. This closure protects the reproductive parts from moisture and may also help to regulate the flower’s internal temperature.
Evolutionary Advantages of Nocturnal Closure
The act of closing petals at night offers several survival benefits that have been favored through natural selection. One primary advantage is the protection of the delicate reproductive structures, the stamens and pistils, from adverse weather conditions. Closing the flower shelters pollen and ovules from the damaging effects of cold temperatures, heavy dew, or rain.
Moisture can severely impair the function of pollen, making it less viable and less likely to be successfully transferred by pollinators the following day. By closing up, the plant effectively creates a small, dry microclimate around its reproductive center, preserving the pollen’s quality.
Nocturnal closure also represents an efficient conservation of the plant’s resources. Keeping the flower tightly shut minimizes the loss of heat and water vapor through transpiration during the cooler night hours. Furthermore, it conserves valuable energy and nectar by signaling to potential nighttime feeders that the flower is not currently receptive. This strategy focuses the plant’s resources on attracting specific daytime pollinators, maximizing the chances of successful reproduction.