These are the small pores in plant stems or leaves that allow carbon dioxide in and oxygen and water vapor out. Each tiny hole is surrounded by a pair of cells called guard cells. These cells control whether a stoma is open or closed. Plants that live in dry places may keep their stomata closed during the day to prevent water loss. Plants in wetter places aren’t so predictable. They can close or open their stomata in response to changing conditions. They may close their stomata at night, for instance, or when the weather is too dry or wet.
In a sentence
When stomata are closed, pressure inside the plant makes water molecules condense and push out of gaps on the edges and tips of leaves — a process called guttation.