Bulk flow of xylem sap.

The movement of fluid in vessels is called bulk flow.

Water vapor exits the leaf via stomata in a process called transpiration; this negative pressure (tension) pulls water in the xylem into the leaf.

Water molecules in the xylem form a continuous column by cohesion and adhesion.

The transpirational pull is transmitted from the leaves to the root, and together with root pressure facilitates uptake of water from the soil.