A pulvinus is a joint
A joint is the location at which two or more bones make contact. They are constructed to allow movement and provide mechanical support, and are classified structurally and functionally.-Classification:...

-like thickening at the base of a plant leaf
A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....

 or leaflet that facilitates growth-independent (nyctinastic
Nyctinasty is the circadian rhythmic nastic movement of higher plants in response to the onset of darkness. Examples are the closing of the petals of a flower at dusk and the sleep movements of the leaves of many legumes....

 and thigmonastic
Thigmonasty or seismonasty is the nastic response of a plant or fungus to touch or vibration. It differs from thigmotropism in that it is independent of the direction of the stimulus. For example, tendrils from a climbing plant are thigmotropic because they twine around any support they touch. ...

) movement. It consists of a core of vascular tissue
Vascular tissue
Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. There are also two meristems associated with vascular tissue:...

 within a flexible, bulky cylinder of thin-walled parenchyma
Parenchyma is a term used to describe a bulk of a substance. It is used in different ways in animals and in plants.The term is New Latin, f. Greek παρέγχυμα - parenkhuma, "visceral flesh", f. παρεγχεῖν - parenkhein, "to pour in" f. para-, "beside" + en-, "in" + khein, "to pour"...

 cells. A pulvinus is also sometimes called a geniculum.

Pulvinar movement is caused by changes in turgor pressure leading to a contraction or expansion of the parenchyma tissue. The response is initiated when sucrose is unloaded from the phloem
In vascular plants, phloem is the living tissue that carries organic nutrients , in particular, glucose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word "bark"...

 into the apoplast
Within a plant, the apoplast is the free diffusional space outside the plasma membrane. It is interrupted by the Casparian strip in roots, air spaces between plant cells and the cuticula of the plant....

. The increased sugar concentration in the apoplast decreases the water potential
Water potential
Water potential is the potential energy of water per unit volume relative to pure water in reference conditions. Water potential quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, or matrix effects such as surface tension...

 and triggers the efflux of potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...

 ions from the surrounding cells. This is followed by an efflux of water resulting in a sudden change of turgor pressure in the cells of the pulvinus. The process is similar to the mechanism of stoma
In botany, a stoma is a pore, found in the leaf and stem epidermis that is used forgas exchange. The pore is bordered by a pair of specialized parenchyma cells known as guard cells that are responsible for regulating the size of the opening...

tal closure.

Common examples for pulvinar movements include the night closure movement of legume leaves and the touch response of the sensitive plant (Mimosa pudica
Mimosa pudica
Mimosa pudica , is a creeping annual or perennial herb often grown for its curiosity value: the compound leaves fold inward and droop when touched or shaken, re-opening minutes later...

). Nyctinastic movements (sleep movements) are controlled by the circadian clock and light signal transduction through phytochrome
Phytochrome is a photoreceptor, a pigment that plants use to detect light. It is sensitive to light in the red and far-red region of the visible spectrum. Many flowering plants use it to regulate the time of flowering based on the length of day and night and to set circadian rhythms...

. Thigmonastic movements (touch response) appear to be regulated through electrical and chemical signal transduction spreading the stimulus throughout the plant.
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