The stomata are the openings of the epidermis through which the process of transpiration occurs. The atmospheric air is having low humidity so lower water potential. The inter cellular space in the mesophyll cells are accumulated with water vapour so water potential is high. Since the water potential in the atmosphere is lower than the water potential of the mesophyll cell, the water vapour diffuses out through stomata. This process is called as transpiration. This reduces the water potential of the mesophyll cells which then draws water from the adjacent tissues and at last from the conducting tissues of the leaf. Since the conducting tissue leaves are in contact with stem and at last with the root. Thus result of transpiration creates a tension in the through the mesophyll cells, xylem column this tension is known as transpiration pull. The transpiration pull exerted on the continuous water column in the xylem lifts up the water to the tip of the tall tree through the xylem vessels.
