What is transpiration pull? How is it caused?
Transpiration pull or the suction force is the force which aids in drawing the water upward from roots to leaves. In leaves, some amount of water is used for photosynthesis and excess water is released into atmosphere through openings called as stomata.
Water molecules stick to one another through cohesion forming a column in the xylem. The negative pressure created by transpiration pull exerts a force on the water particles causing their upward movement in xylem. This theory is based on the upward movement of water from the root to aerial parts of the plant body which is called as ascent of sap.