The daily opening and closing of flower petals is a common biological process driven by the plant’s internal clock and external environmental cues. This rhythmic movement, known as nyctinasty, allows plants to maximize successful reproduction while protecting themselves from harsh conditions. Many familiar flowers exhibit this behavior, aligning their daily activity with the sun’s cycle. This movement ensures pollen is available for daytime pollinators or is shielded from nocturnal dew and cold.
The Biological Mechanism of Movement
Flower movements are a form of nastic response, meaning the direction of movement is not dependent on the direction of the stimulus. The petals move because of rapid, controlled changes in cell size within the flower structure. This is often achieved through a change in turgor pressure, the internal water pressure that pushes against a plant cell’s wall, in specialized motor cells at the base of the petals.
When a flower opens, cells on the outer side of the petal bases rapidly gain water, swell, and push the petals outward. Conversely, when the flower closes, cells on the inner side may swell, causing the petals to fold inward. This turgor-driven movement is fully reversible and can be repeated. In some cases, the movement is caused by differential growth, where cells on one side of the petal grow faster than those on the other, a mechanism that can be irreversible.
Flowers That Respond to Sunlight
A specific type of nyctinasty, called photonasty, is governed by changes in light intensity. These flowers open as light levels rise in the morning and close as the sun sets or if clouds block the sunlight. This ensures the flower is only open when its primary pollinators, such as bees and butterflies, are most active, preventing the waste of pollen and nectar.
The common Dandelion and Oxalis (wood sorrel) are classic examples of this light-driven response. Their petals unfurl as soon as they receive sufficient bright light and snap shut when the light diminishes. This closing action also serves a protective function, shielding the flower’s reproductive parts from cold night air and dew, which can damage the pollen.
Gazania, often called the Treasure Flower, displays strong photonastic behavior, requiring full sun to fully open its daisy-like blooms. If Gazania is planted in a shaded spot, or if the day is heavily overcast, its flowers will remain closed. For these flowers, light intensity is the direct trigger that dictates when they are visible and accessible to foraging insects.
Flowers That Respond to Temperature
While light is the main cue for many flowers, others are sensitive to changes in ambient temperature, a response known as thermonasty. Primary examples are Tulips and Crocuses, which can open and close repeatedly based on temperature fluctuations. The opening in a Tulip is caused by cells on the inner surface of the petals growing faster when the temperature rises above a certain threshold.
When the temperature drops, cells on the outer surface begin to grow faster, causing the petals to curve inward and close the flower. This differential growth mechanism results in permanent, unequal cell elongation, which is distinct from the reversible water pressure changes seen in other plants. This temperature-sensitive movement is believed to help warm the flower’s reproductive parts, attracting early-season pollinators.
Other flowers, like the Evening Primrose, operate on a strong, internal circadian rhythm that dictates their timing regardless of environmental shifts. These flowers open rapidly at dusk and remain open all night, often coinciding with the activity of nocturnal pollinators like hawk moths. While temperature and light are often intertwined, the predictable opening of the Evening Primrose demonstrates a biological clock is the overriding factor for its daily rhythm.