stoma

noun

(Gr. stoma, stomatos: mouth) (pl. stomas, stomata). A pore, large numbers of which are present in the epidermis of leaves (especially on the undersurface) and young shoots. The stomata function in gas exchange between the plant and the atmosphere. Each stoma is bordered by two semicircular guard cells (specialized epidermal cells), whose movements (due to changes in water content) control the size of the aperture. The term stoma is also used to mean both the pore and its associated guard cells. (The Funariaceae have single guard cell with an opening in the center). Also stomata, stomate.