Answer:
Stoma (pl. stomata)
In botany, a pore (tiny hole) in the epidermis (outer layer of tissue) of a plant. There are lots of these holes, usually in the lower surface of the leaf. A leaf contains several layers of tissue. The outer layer is the epidermis and is only one cell thick. Stomata occur in the lower epidermis.
Each stoma is surrounded by a pair of guard cells that are crescent-shaped when the stoma is open but can collapse to an oval shape, thus closing off the opening between them. Stomata allow the exchange of carbon dioxide and oxygen (needed for photosynthesis and respiration) between the internal tissues of the plant and the outside atmosphere.
They are also the main route by which water is lost from the plant (water vapour), and they can be closed to conserve water, the movements being controlled by changes in turgidity of the guard cells.
At night the stomata may allow oxygen to diffuse in and carbon dioxide out because only respiration is taking place. However in many plants the stomata close at night, preventing this gas exchange.