Typically gated openings such as associated with plant leaves.
It is through stomata that both enter and leave leaves, but also through occurs (movement of out of plants). In particularly, molecular oxygen exits leaves as produced via the light reaction of photosynthesis and carbon dioxide enters as required by the dark reaction.
Given the need for plants to have waterproofed covering of their tissues (), due to the nature of air unless is at 100%, there correspondingly is a need to have passages through this covering to nevertheless allow for . These passages, as found in most plants, are stomata.