1. (in botany) A pore, large numbers of which are present in the epidermis of leaves (especially on the undersurface) and young shoots. Stomata function in the exchange of carbon dioxide and oxygen between the plant and the atmosphere and are the principal route of evaporative water loss. Each stoma is bordered by two semicircular guard cells, whose movements (due to changes in water content) control the size of the aperture and hence the movement of gases and water vapour. The density of stomata is determined both by genetics and by the environmental conditions to which the plant is exposed during its development; some plants may have a stomatal density of up to 20 000 per square centimetre. A decline in stomatal density in some species over the past 100 years is thought to be associated with the marked increase in atmospheric levels of carbon dioxide during that time. The term stoma is also used to mean both the pore and its associated guard cells. See stomatal conductance.

2. (in anatomy) Any small opening or pore, especially an artificial opening connecting an internal cavity or organ with the body surface.