The prayer plant, scientifically known as Maranta leuconeura, folds its leaves upward every evening in a predictable, daily rhythm. This characteristic behavior is why the plant earned its common name, as the leaves appear raised in a position similar to hands clasped in prayer. This leaf movement is a natural, routine action for the plant, demonstrating how it actively responds to its environment over a 24-hour cycle.
Nyctinasty: The Plant’s Sleep Cycle
The phenomenon of the prayer plant’s daily leaf movement is called nyctinasty, which translates roughly to “sleep movement.” Nyctinasty is a nastic movement, meaning the direction of the movement is independent of the stimulus, which is the shift from light to darkness. The plant’s internal biological clock, known as the circadian rhythm, controls the timing of this movement.
The change in light intensity triggers a signal that governs the timing of the leaf folding. As dusk approaches, the plant raises its leaves from their horizontal, daytime position to a vertical one. This adjustment is an expression of photoperiodism, the plant’s response to the duration of light and dark periods. This programmed response continues even if the plant is kept in constant darkness, demonstrating its ingrained 24-hour cycle.
The Mechanics of Leaf Movement
The physical mechanism allowing the leaves to move centers on a specialized, cushion-like joint called the pulvinus. This motor organ is located at the base of the leaf petiole, the stalk attaching the leaf blade to the stem. The pulvinus contains thin-walled motor cells on opposing sides that expand and contract to drive the leaf’s position.
The movement is achieved through rapid, localized changes in turgor pressure, the internal water pressure within the plant cells. When the plant folds its leaves, it initiates the controlled movement of ions, particularly potassium (K+) and chloride (Cl-), into or out of the motor cells. This movement creates an osmotic gradient, causing water to quickly follow the ions by osmosis, which leads to a change in cell volume.
To raise the leaf, motor cells on one side of the pulvinus rapidly lose water and shrink, while cells on the opposite side take on water and swell. This differential swelling and shrinking acts like a hinge, forcing the leaf to move from a flat to a vertical orientation. The process is reversible and is repeated daily.
Biological Significance of Closing Leaves
The evolutionary reason for nyctinasty is not fully settled, but several scientific hypotheses suggest leaf closure provides the plant with a survival advantage.
Minimizing Pathogens and Water Loss
One theory proposes that folding the leaves upward reduces the surface area exposed to the cool, damp night air. This minimizes the accumulation of dew, which can foster the growth of fungal pathogens. The upward positioning may also help with water conservation by reducing the rate of water loss through evaporation during the night.
Defense Mechanism
Some botanists suggest the movement is a defense mechanism. The vertical orientation may make the leaves less appealing or accessible to nocturnal herbivores. Alternatively, it could make insects already feeding on the plant more visible to their predators.
Temperature Regulation
Another possibility relates to temperature regulation. The leaves folding together may help insulate the plant’s delicate surfaces against temperature fluctuations. Nyctinasty likely persists because it offers multiple benefits that contribute to the plant’s overall health and survival in its native tropical environment.