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1. Botany an epidermal pore, present in large numbers in plant leaves, that controls the passage of gases into and out of a plant
2. Zoology anatomy a mouth or mouthlike part
3. Surgery an artificial opening made in a tubular organ, esp the colon or ileum



a slitlike opening in the epidermis of aboveground organs of plants. The stoma is bordered by two guard cells, which are usually bean-shaped. The walls of the guard cells that face the stoma are thick, while the opposite walls are thin. The stoma leads to a large intercellular space, the substomatal cavity. The stoma is often surrounded by two or more cells that differ in shape from ordinary epidermal cells.

Stomata are found in the epidermis of all aboveground parts of the plant containing chlorophyll but are especially numerous in leaf epidermis (100–300 per sq mm). They regulate the exchange of gas and water vapor between the atmosphere and the cells of the plant by increasing and decreasing in width. Stomatal movement is effected by changes in the turgor of the guard cells. When turgor is increased, the thin parts of their walls stretch and are drawn away from the stoma. The walls that face the stoma are distended in the same direction, and the stoma opens. When the turgor of the guard cells decreases, the stoma closes. Change in the turgor of the guard cells occurs as a result of reversible conversion of starch, which is osmotically inactive, into osmotically active sugars. However, according to some data, potassium ions play an important, possibly leading, role in regulating the turgor of the guard cells. Scientists are currently investigating these data with the intention of formulating a new hypothesis of the mechanism of stomatal movement.

At night, the stomata of most plants are closed, and gas exchange and transpiration are minimal. During the day, when the weather is fair, the stomata are open. Carbon dioxide gas readily enters the internal tissues of the plant, and oxygen formed as a result of photosynthesis is released with water vapor into the atmosphere.



A small opening or pore in a surface.
One of the minute openings in the epidermis of higher plants which are regulated by guard cells and through which gases and water vapor are exchanged between internal spaces and the external atmosphere.
References in periodicals archive ?
It is known and reported by several authors in the literature, that water stress provides strong impacts to plants' metabolism, the reduction in photosynthetic capacity and stomatal conductance being considered limiting and triggering several other harmful processes (FERNER et al.
Reduction of stomatal conductance in plants under water stress (125% ETc), in the absence of treatment with initial defoliation, results from a regulating behavior of the water balance, according to Mollard et al.
Unlike photosynthesis response to soil moisture stress, stomatal conductance ([g.
Stomatal densities and plant morphometry: We tagged all plants within each plot, and for each plant, we measured basal stem circumference with a standard measuring tape, taking measurements just above the first pair of aerial roots.
Drought-inhibition of photosynthesis in C3 plants: stomatal and non-stomatal limitations revisited.
Besides, from the time of deficit irrigation began to be applied, investigations have shown a decrease in the stomatal conductance in order to control water loss via transpiration and to avoid leaf turgor loss (stress avoidance mechanism) (Connor, 2005).
2012c, d) investigated the regulation of stomatal conductance by exposing plants to various concentrations of MG in the model plant Arabidopsis, the results showed that MG can act as a signalling molecule by inducing stomatal closure without reducing the viability of guard cells at low concentration ([less than or equal to] 1 mM), but high MG concentrations ([greater than or equal to] 1 mM) were cytotoxic.