Transpiration is the name given to the process of evaporation of water molecules from the stomatal pores of the leaves. When the water gets evaporated from the leaves, it creates negative pressure (tension) equivalent to –2 MPa at the leaf surface. The water from the roots move upward in the xylem due to this suction ( or tension). During the day, transpiration is the major driving force for upward movement of water in the xylem.
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