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Land plants have evolved specialised structures known as guard cells to allow gases to enter and leave the leaf. Carbon dioxide cannot pass through the protective waxy layer covering the leaf (cuticle), but it can enter the leaf through a stoma (plural: stomata), flanked by two guard cells. Likewise, oxygen produced during photosynthesis can only pass out of the leaf through the opened stomata.

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stomata |?st?m?t?, st??mät?| plural form of stoma. stoma |?st?m?| noun (pl. stomas or stomata |-m?t?, ?st??mät?| ) Botany any of the minute pores in the epidermis of the leaf or stem of a plant, forming a slit of variable width that allows movement of gases in and out of the intercellular spaces. Also called stomate. • Zoology a small mouthlike opening in some lower animals. • Medicine an artificial opening made into a hollow organ, esp. one on the surface of the body leading to the gut or trachea. DERIVATIVES stomal adjective(Medicine) ORIGIN late 17th cent.: modern Latin, from Greek stoma ‘mouth.’