Stoma

Stomata (singular stoma) are gaps, mostly on the underside of leaves for gas exchange. Carbon dioxide enters the plant through these openings for photosynthesis and waste oxygen exits through these same openings. Some environmental oxygen also enters through stomata for respiration (the breakdown of stored sugar for energy). Also excess water is released into the atmosphere through these pores in a process called transpiration. The opening and closing of stomata are controlled by sodium-potassium pumps.

Stomata can be seen under a microscope by taking nail varnish impressions of the leaf.