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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 ?
In all treatments, in an environment with natural light, the stomata have guard cells with external periclinal walls that present a rather sharp thickening, forming a crest around the external atrium, and underdeveloped substomatic chambers (Figures 3A, E, 4A, C, D).
Stomata in the upper and lower epiderma are large, dense and without correspondence cells.
In adaxial epidermis the leaf epidermal cells were irregular shaped having thick undulate walls, average length of epidermal cells was 155 (120-190) um and the average width was 70 (60-80) um, stomata were present, stomatal type was anisocytic and diacytic.
For the abaxial surface, it was noted that the number of stomata per area was higher in leaves with more developed stages of growth for the susceptible biotype.
The parameters studied were shape and margin of epidermal cells, number of epidermal cells on abaxial and adaxial sides, length and width of leaf epidermis and trichomes, the presence and absence of stomata on each epidermis, type and size of stomata.
In the latter, stomata in abaxial epidermis were shorter in autumn than in spring samples, and epidermal cell number was greater in autumn than in spring in both epidermis (Table 1).
Probably, the role of stomata on the leaf surface in the interaction between forests and the climate has to be reevaluated and established vegetation-climate models need to be revised.
The shape of stomata was elliptical in all species analyzed.
It may be mentioned that the retarded cuticle development, epicuticular waxes deposition and functional stomata apparatus during in vitro culture causes excessive transpiration rates in leaves of plantlets taken out of the culture vessels (POSPISILOVA et al.
Stomata altogether absent in the floral epidermises of Salvia aegyptiaca and Salvia deserti while few diacytic stomata are present in Salvia spinosa.
Finally, the use of stomata in the taxonomy of these groups is also ample (cf.