Stomates (or stomas) are openings on the underside of a leaf, which allow gases to pass in and out of the leaf. The pictures show these stomates as narrow gaps. Two guard cells control the size of the opening. The guard cells have chloroplasts, which are visible as dark dots in the cell. During the day the stomates open to allow carbon dioxide gas to enter the leaf. During night the stomates close to minimize the loss of water vapor from the leaf.