Stomata in action

Whether a stoma is open or closed depends on how much water is plumping up guard cells. The round structures inside the cells are chloroplasts. Guard cells are the only type of plant epidermal cell with these organelles.

A) This stoma is open. When guard cells swell with water, they bend so  a gap (the stoma) opens up between them. The gap allows the plant to exchange gases with air.
B) This stoma is closed. When the guard cells lose water, they collapse against each other so the gap between them closes. A closed stoma limits water loss. It also limits gas exchange.