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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 ?
LEAF DESCRIPTION: NEEDLE-LIKE, 1/2" to 3/4" long, 2 broad white bands underneath, each having 5 to 6 stomatic lines, no stomatic lines above, 2-sided.
LEAF DESCRIPTION: NEEDLE-LIKE, 3/8" long, 4-sided, 1 to 4 stomatic lines on all surfaces, obtuse tip, densely arranged, stiff.
LEAF DESCRIPTION: NEEDLE-LIKE, 1" long, 4-sided, 4 to 5 stomatic lines on all sides, acuminate tip.
LEAF DESCRIPTION: NEEDLE-LIKE, 1" long, 4-sided, acuminate tip, 4 or 5 stomatic lines on all sides, radially arranged and dense.
LEAF DESCRIPTION: NEEDLE-LIKE, 1/2" to 3/4" long, 4-sided, 1 to 2 stomatic lines on upper surfaces, 3 to 4 lines on lower surfaces, acute tip, needles more dense on upper side of branches.
LEAF DESCRIPTION: NEEDLE-LIKE, 2 1/2" to 3 1/2" long, rigid, sharply acuminate, stomatic lines on both surfaces, appearing "brush-like" at branch tips, arranged in FASCICLES (BUNDLES) OF 2.
LEAF DESCRIPTION: NEEDLE-LIKE, 3" to 6" long, stiff, 12 to 14 stomatic lines, straight or slightly incurved, margins finely serrated.
LEAF DESCRIPTION: NEEDLE-LIKE, 3 1/2" to 4 1/2" long, obtuse tip, margins finely serrated, 3-sided, stomatic lines on the two ventral surfaces.
LEAF DESCRIPTION: NEEDLE-LIKE, 5" to 8", long, acuminate tip, finely serrated margins, stomatic lines on all surfaces.
LEAF DESCRIPTION: NEEDLE-LIKE, 3/4" to 1 1/4" long, 2 stomatic bands beneath having 5 or 6 gray lines, acute or obtuse tip.
LEAF DESCRIPTION: NEEDLE-LIKE, 1/2" long, margins finely serrated, rounded tip, tapering from base to tip, furrowed above, 2 white stomatic bands underneath.