The sensitive plant, Mimosa pudica, is famous for its immediate reaction to physical contact, causing its compound leaves to snap shut quickly when disturbed. This ability to move rapidly sets it apart from most other plant species. The plant also exhibits a slower, more deliberate movement: it closes its delicate leaflets every evening as darkness approaches. This nightly closing and subsequent morning reopening is a biological process governed by the plant’s internal clock.
The Daily Cycle of Leaf Folding
The rhythmic folding of the leaves at dusk and their unfolding at dawn is a light-regulated behavior known as nyctinasty, or “sleep movement.” This is a predictable response to the daily cycle of light and temperature, rather than a reaction to a sudden event. The movement is controlled by the plant’s internal, approximately 24-hour cycle, known as the circadian rhythm. This internal clock allows the plant to anticipate environmental changes.
The nyctinastic movement involves a gradual repositioning of the leaflets over a period of minutes to hours. This slow closing at night is regulated by internal signals tied to the light-dark cycle, preparing the plant for the nighttime hours.
The Physiological Mechanism of Movement
Both the slow nyctinastic closing and the rapid folding are powered by specialized structures called pulvini. These swollen, joint-like motor organs are located at the base of the main leaf stalk and at the base of each leaflet. The movement is a hydraulic action driven by changes in water pressure within the pulvinus cells.
Specifically, the cells on one side of the pulvinus, known as extensor cells, lose turgor, or internal water pressure. Meanwhile, the cells on the opposing side, the flexor cells, maintain their pressure. This loss of turgor is achieved by the swift movement of ions out of the extensor cells. Specialized channels in the cell membranes open, allowing large amounts of ions, primarily potassium and chloride, to exit the cells. Water then rushes out of the cells to follow the highly concentrated ions via osmosis.
As the extensor cells lose water, they become flaccid and collapse, causing the pulvinus to bend downward. This loss of rigidity forces the leaflets to fold upward against the stem. This hydraulic pressure change translates directly into visible movement.
Evolutionary Hypotheses for Nighttime Closing
The purpose of this daily nyctinastic movement is a subject of scientific investigation, with several hypotheses proposed for the benefit it provides.
Defense Against Herbivores
One major theory suggests that folding the leaves at night serves as a defense against nocturnal herbivores, such as grazing insects. By collapsing its large, feather-like foliage, the plant drastically reduces its apparent surface area. This makes it less conspicuous and potentially less palatable to nighttime feeders.
Moisture and Temperature Regulation
Another hypothesis relates to the regulation of moisture and temperature. Keeping the leaves folded reduces the total surface area exposed to the cool, moist night air. This minimizes the risk of water condensation, or dew, forming on the leaves, which could otherwise promote the growth and spread of fungal pathogens. Reducing the amount of water lost through transpiration during the cooler hours may also play a role in water conservation.
Protection from Weather
The nightly folding may also be a way to avoid continuous damage from weather events such as strong winds or heavy rain. By adopting a compact, downward-facing posture, the plant limits mechanical stress on its delicate leaflets. This movement is likely a combination of these factors, providing a suite of protective and regulatory benefits that aid in the plant’s survival.
Rapid Movement Triggered by Physical Contact
Mimosa pudica is classified as seismonasty for its rapid reaction to touch. When the plant is mechanically stimulated, it generates an electrical signal, similar to an animal’s nerve impulse, called an action potential.
This electrical signal travels quickly throughout the plant, propagating the message of disturbance to the pulvini. This rapid, long-distance signal causes the leaflets to snap shut in a fraction of a second. This quick reaction is thought to be a primary defense mechanism, designed to startle or dislodge potential insect pests and grazing animals. By folding its leaves, the plant briefly appears wilted and unappealing, creating an immediate, temporary deterrent that protects its foliage.