Stomata are tiny pores on leaves that allow gas exchange for photosynthesis. They let carbon dioxide diffuse into the leaf, while oxygen and water vapor can diffuse out. Most stomata are found in the epidermis, particularly on the underside of leaves in many land plants.
Each stoma is controlled by a pair of specialized guard cells. When the guard cells take up water and become turgid, the pore opens; when they lose water, it closes. This adjustable opening helps the plant balance carbon-dioxide intake with water conservation.
Stomata are not the same as trichomes, hydathodes, or lenticels. Trichomes are hairs, hydathodes release liquid water at leaf margins, and lenticels are pores in woody stems and some fruits. Stomata also matter beyond photosynthesis: their opening and closing regulate transpiration, the movement of water vapor from a plant to the atmosphere.