Final answer:
Transpiration is the loss of water from the plant through evaporation at the leaf surface. It drives water movement in the xylem and is controlled by factors such as vapor pressure deficit and relative humidity. Water is pulled up from the roots by negative pressure created during transpiration.
Step-by-step explanation:
Transpiration is the loss of water from the plant through evaporation at the leaf surface. It is the main driver of water movement in the xylem. Transpiration is caused by the evaporation of water at the leaf-atmosphere interface; it creates negative pressure (tension) equivalent to -2 MPa at the leaf surface. This value varies greatly depending on the vapor pressure deficit, which can be negligible at high relative humidity (RH) and substantial at low RH. Water from the roots is pulled up by this tension. At night, when stomata shut and transpiration stops, the water is held in the stem and leaf by the adhesion of water to the cell walls of the xylem vessels and tracheids, and the cohesion of water molecules to each other. This is called the cohesion-tension theory of sap ascent.