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A stoma (pl. stomata) is a microscopic pore on the surface (epidermis) of land plants. It is surrounded by a pair of specialized epidermal cells called guard cells, which act as a turgor-driven valve that open and close the pores in response to given environmental conditions. The presence of countless numbers of stomata is critical for plant function. Typically, the plant epidermis is tightly sealed by wax-coated, interlocking epidermal pavement cells, which protect the plant body from the dry atmosphere and UV-rays. At the same time plants must be able to breathe, or exchange carbon dioxide and oxygen, for photosynthesis and respiration. Stomata act as a gateway for efficient gas exchange and water movement from the roots through the vasculature to the atmosphere (Fig. 1). Transpiration via stomata supplies water and minerals to the entire plant system (Raven 2002)(Fig. 1). When a plant encounters adverse environmental conditions, such as drought, a plant hormone called abscisic acid triggers stomata to shut tightly in order to prevent plants from dehydration and wilting.

Stomatal function is important beyond the level of plant physiology and function, and its significance reaches from evolutionary history to atmospheric and environmental sciences. For instance, the acquisition of stomata is considered one of the key developmental innovations that allowed plants to conquer the terrestrial environment, an event that occurred around 400 million years ago (Edwards et al. 1998; Raven 2002). Stomata impact our present global environment as well. Hetherington and Woodward (2003) estimate that 3 x 1018 grams of carbon pass through stomata and fixed into carbohydrate every year, and in addition the total water content of the atmosphere is estimated to be recycled through stomata every half year (Hetherington and Woodward 2003). Importantly, stomata impact atmospheric environment, and in turn, atmospheric environment drives changes in stomatal patterning. It is generally accepted, both from the studies of fossil records and extant plants, that high carbon dioxide condition decreases numbers/density of stomata (Hetherington and Woodward 2003). The mechanisms that allow plants to adjust their stomatal density have just begun to unravel.

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