The opening and closing of stomata are controlled by the guard cells. When water flows into the guard cells, they swell up and the curved surface causes the stomata to open. When the guard cells lose water, they shrink and become flaccid and straight thus closing the stomata
Guard cells regulate the passage of water vapor, oxygen, and carbon dioxide into and out of the leaf. When the guard cells are very turgid, they pull apart, opening the stoma. When the guard cells lose water and become flaccid, they collapse, closing the stoma.
