A part of the plant
The small,microscopic openings on the underside of a leaf is called Stomata(plural,stoma).
Stomata are shut at night to conserve water. When there is no light, plants cannot fix carbon dioxide. Stomata are usually open to allow entrance of carbon dioxide. A side effect is that water is lost from the leaves.There is an exception to this, however. CAM plants such as cacti and succulents open the stomata at night, store carbon dioxide as acids, and close them during the day to prevent dessication from harsh desert conditions.