Wiktionary
n. (context botany English) A joint on a plant leaf or petiole that may swell and cause movement of the leaf or leaflet.
Wikipedia
A pulvinus (pl. pulvini) is a joint-like thickening at the base of a plant leaf or leaflet that facilitates growth-independent ( nyctinastic and thigmonastic) movement. Pulvini are common in members of the bean family Fabaceae (Leguminosae) and the prayer plant family Marantaceae.
Pulvini may be present at the base or apex of the petiole or where the leaflets of a compound leaf are inserted into the rachis. They consist of a core of vascular tissue within a flexible, bulky cylinder of thin-walled parenchyma 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 into the apoplast. The increased sugar concentration in the apoplast decreases the water potential and triggers the efflux of potassium 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 stomatal closure.
Common examples for pulvinar movements include the night closure movement of legume leaves and the touch response of the sensitive plant ( Mimosa pudica). Nyctinastic movements (sleep movements) are controlled by the circadian clock and light signal transduction through phytochrome. Thigmonastic movements (touch response) appear to be regulated through electrical and chemical signal transduction spreading the stimulus throughout the plant.
Usage examples of "pulvinus".
From these several reasons and from our having partially traced the development of the pulvinus from an early age, the case seems worth describing in some detail.
The cotyledons are provided with a pulvinus, and its development will hereafter be described.
It is therefore almost certain that the pulvinus itself was not then growing.
In these several species the pulvinus is seated close to the blade of the cotyledon, as is the usual rule with most plants.
The pulvinus is developed imperfectly and to an extremely variable degree, so that apparently it is tending towards abortion.
After a few additional days the illdefined zone of cells becomes distinct, and although it does not extend across the whole width of the petiole, and although the cells are of a green colour from containing chlorophyll, yet they certainly constitute a pulvinus, which as we shall presently see, acts as one.
As the pulvinus is so indistinct at first, the movement probably does not then depend on the expansion of its cells, but on periodically unequal growth in the petiole.
The singular fact of the cotyledons of this plant not sleeping at first is therefore due to the pulvinus not being developed at an early age.
We learn from these two cases of Lotus and Oxalis, that the development of a pulvinus follows from the growth of the cells over a small defined space of the petiole being almost arrested at an early age.
Trifolium, Lotus, and Oxalis some of the species have a welldeveloped pulvinus, and others have none, or one in a rudimentary condition.
As the movements caused by the alternate turgescence of the cells in the two halves of a pulvinus, must be largely determined by the extensibility and subsequent contraction of their walls, we can perhaps understand why a large number of small cells will be more efficient than a small number of large cells occupying the same space.
In these two cases perhaps the pulvinus was accidentally pricked, for on pricking the pulvinus of another cotyledon it rose a little.
At the base of the peduncle there is a mass of small cells, forming a welldeveloped pulvinus, which is exteriorly coloured purple and hairy.
The fact of leaves and cotyledons frequently, or even generally, rising a little in the evening and sinking in the morning, is of interest as giving the foundation from which the specialised sleepmovements of many leaves and cotyledons, not provided with a pulvinus, have been developed.
When the movements of leaves or cotyledons, furnished with a pulvinus and destitute of one, are compared, they are seen to be closely similar, and are apparently effected for the same purpose.