The vibrant orange-spotted Tiger Lily (Lilium lancifolium or Lilium tigrinum) is a common sight in summer gardens. This perennial species, native to Asia, belongs to the larger lily family. The sight of flowers opening with the sun and closing at dusk is a widespread natural phenomenon, often called “sleep movements.” This observable change in posture has led many gardeners to wonder whether the Tiger Lily also closes after sunset.
Tiger Lilies and Their Nighttime Behavior
The short answer to whether the Tiger Lily closes at night is no. Unlike many garden flowers that exhibit dramatic folding, the Tiger Lily is considered non-nyctinastic, meaning it does not perform significant daily closing movements. The flower’s structure provides a physical explanation for this lack of movement. Tiger Lilies possess six prominent, thick structures known as tepals, which are the combined petals and sepals common to the lily family.
These tepals are characteristically reflexed, meaning they curve strongly backward toward the stem, giving the flower its distinctive open, downward-facing shape. This robust, curved structure makes complete or partial closure mechanically impractical. The flowers are long-lasting and remain fully open for several days, maintaining their shape regardless of the time of day. Any minor change in petal angle is imperceptible to the casual observer compared to species that visibly shut tight.
The Science of Nyctinasty
The opening and closing movements observed in many flowers are classified as nyctinasty, a plant behavior regulated by the day-night cycle. This movement is not driven by growth but by rapid, reversible changes in water pressure within specialized cells. The mechanism centers around motor organs called pulvini, which are thickened areas located at the base of petals or leaves.
The pulvinus contains flexor and extensor motor cells that work in opposition. Movement is generated when water and the ions that regulate it rapidly shift between these cell types, creating a turgor pressure difference. When extensor cells swell with water and flexor cells lose it, the resulting asymmetry causes the organ to bend or move. This process is governed by the plant’s internal biological clock, known as a circadian rhythm, which is influenced by light cues in a process called photonasty.
Evolutionary Reasons for Flower Closure
The ability to close flowers at night evolved because it offers several advantages to certain plant species. One primary theory suggests that closure protects the delicate pollen and ovules from becoming saturated with dew or moisture overnight. Dry pollen is lighter and more easily transferred by daytime pollinators, improving reproductive success.
Energy conservation is another benefit, as the plant reduces the resources needed to maintain an open, showy structure when primary pollinators are inactive. Flower closure also helps protect reproductive parts from cold temperatures or nocturnal herbivores. Tiger Lilies are primarily pollinated by diurnal insects like butterflies, bees, or hummingbirds, meaning they do not need to synchronize with night-active species. Their robust, unscented, and open structure suggests a successful evolutionary path that did not require the nightly closing mechanism.