Transpiration is the process in which water is lost in the form of vapor.
Mechanism of closing and opening of stomata is as follows:
Stomatal transpiration primarily involves minute pores. Stomata consist of two guard cells encircled by accessory cells. The opening and closing of stomata are regulated by the turgidity of guard cells. When guard cells become turgid through endosmosis, their lateral thin and elastic walls stretch, causing the stoma to open during the day. Conversely, at night, when guard cells become flaccid due to exosmosis, the walls relax, leading to stoma closure. The changes in osmotic potential of guard cells drive the processes of endosmosis and exosmosis.