Stomata (singular “stoma” from the Greek term stoma, meaning “mouth”) are specialized pores in the leaves and sometimes in the green portions of the stems, as well as flowers and fruits. Carbon dioxide (CO2) enters the plant through the stomata, while water vapor escapes through the same pores. The guard cells that border the stomata expand and contract to control the passage of water, carbon dioxide (CO2), and oxygen (O2).