Calatheas, a popular group of tropical houseplants, move their leaves in a regular, cyclical movement tied directly to the 24-hour light and dark cycle. Sometimes referred to as ‘prayer plants,’ they spread their leaves widely during the day. As evening approaches, the leaves lift and fold upward, flattening out again by morning.
The Mechanism of Leaf Movement
The physical ability of the Calathea leaf to lift and lower rests on a specialized structure called the pulvinus, a swollen, hinge-like joint located at the base of the leaf blade where it meets the petiole. This joint acts as the plant’s motor, driving the movement without any muscle tissue.
The process relies on turgor pressure, the force exerted by water inside the plant cell. When night approaches, motor cells on one side of the pulvinus rapidly expel water and ions like potassium (\(\text{K}^+\)). This loss causes those cells to shrink, while cells on the opposite side remain full and turgid. This pressure difference physically bends the pulvinus, pulling the leaf into its vertical, closed position.
Why Calatheas Move Their Leaves
The scientific term for this light-driven movement is nyctinasty, which is governed by the plant’s internal biological clock, or circadian rhythm. The movement relates to both light collection and protection. During daylight hours, the leaves lie flat and horizontal to maximize the surface area exposed to indirect light for photosynthesis.
Folding the leaves upward helps minimize the exposed surface area to cooler nighttime air, conserving heat and reducing water loss through transpiration. The vertical orientation also allows dew or rainwater to run off the leaf surface more effectively. This runoff can deter the growth of fungal spores and prevent pests from settling. The movement is an adaptation that helps the Calathea survive the changing conditions in its native tropical understory.
Observing Movement and Plant Health Checks
A healthy Calathea displays nyctinastic movement, typically beginning its upward fold at dusk and returning to its open, horizontal state around sunrise. Observing this daily motion is a way to check on a plant’s overall well-being. Since the movement requires significant energy and proper hydration, a plant that is not moving is often signaling a problem.
If the leaves remain flat and horizontal at night, the plant may be receiving insufficient light during the day, which disrupts the light-sensing mechanism that triggers the movement. A lack of movement can also be an early indication of severe dehydration, as the plant cannot maintain the turgor pressure needed to manipulate the pulvinus. Ensuring the plant is in a location with consistent, bright indirect light and maintaining consistently moist soil will support the healthy opening and closing of the leaves.