Sensitive plants exhibit movements, often folding their leaves or petals in response to environmental changes. This behavior, known as nastic movement, is a non-directional response to stimuli. Two types are nyctinasty, or “sleep movements,” triggered by darkness, and thigmonasty, triggered by touch or vibration. These movements are regulated by an internal biological clock, similar to circadian rhythms in animals. Such responses allow plants to adapt and optimize survival.
The Mechanism of Plant Movement
The ability of sensitive plants to move their leaves stems from specialized structures called pulvini. These “motor organs” are located at the base of leaves or leaflets and are composed of thin-walled parenchyma cells that surround a vascular core. Changes in the turgor pressure within these pulvini drive the plant’s movements. Turgor pressure is the internal pressure of water within plant cells, which pushes against the cell walls, providing rigidity.
When a plant needs to move its leaves, water and ions, particularly potassium (K+), are rapidly transported into or out of the pulvini cells. This causes a decrease in turgor pressure, leading to the collapse of these cells, acting like a hinge and causing the leaf to fold or droop. The process is reversible; when turgor pressure is restored, the leaves return to their original position. Some movements, like Mimosa pudica’s, are rapid, occurring within seconds, while others, such as daily sleep movements, are more gradual.
The Purpose Behind Plant Closure
Plants exhibit these closure movements for various adaptive reasons.
Protection from Herbivory
One primary theory suggests protection from herbivory. By folding their leaves at night or upon touch, plants can appear less appealing or accessible to nocturnal feeders, making them harder to consume. This wilting appearance might also trick grazers into thinking the plant is unhealthy or dying.
Water Conservation and Temperature Regulation
Folding leaves reduces the surface area exposed to the cooler night air, potentially minimizing water loss through transpiration, especially when dew is present. Closing leaves can also help protect them from extreme cold or heat fluctuations during the night. Additionally, positioning leaves vertically at night may help shed excess surface water, preventing fungal diseases.
Light Optimization and Pest Avoidance
Leaves position themselves horizontally during the day to maximize sunlight absorption for photosynthesis. At night, a more vertical orientation might prevent photoperiodism disruption by moonlight. These movements can also serve as a pest avoidance strategy. Rapid folding can dislodge insects or or prevent them from settling on the leaves overnight.
Notable Examples of Sensitive Plants
Many plants showcase nastic movements, with some species being well-known for their responses.
Mimosa pudica
The Mimosa pudica, commonly known as the Sensitive Plant or Touch-Me-Not, is perhaps the most famous example. Its fern-like leaves rapidly fold inward and droop when touched, shaken, or exposed to heat, and also perform slower nyctinastic movements by closing at night. This quick response is a form of thigmonasty, thought to deter herbivores.
Prayer Plant (Maranta leuconeura)
The Prayer Plant (Maranta leuconeura) is another popular example, named for its distinctive nyctinastic movements. Its leaves lie flat during the day to capture light and then fold upwards at night, resembling hands clasped in prayer. This daily rhythm is regulated by the plant’s internal clock.
Oxalis Genus
Species within the Oxalis genus, often called shamrock plants, exhibit nyctinastic movements. Their trifoliate leaves spread open during daylight and fold down at night. This movement is influenced by light levels and an internal circadian rhythm, optimizing light absorption during the day and providing night protection.
Bean Family (Fabaceae)
Many plants in the bean family (Fabaceae), including common bean plants and soybeans, display clear sleep movements. Their leaves and leaflets fold up or droop down at sundown and unfold at dawn, a behavior controlled by circadian rhythms.