Active transport of mineral ions into the xylem by endodermal cells increases the water potential ($\psi$) of the xylem sap, driving the osmotic movement of water from the cortex.

During peak daytime transpiration, the diameter of a tree trunk increases because high tension within the xylem vessels pushes the lignified walls outwards.

At the endodermis, the waterproof Casparian strip blocks the apoplast pathway, forcing water and solutes to cross the selectively permeable cell-surface membranes of endodermal cells into the cytoplasm.

Cohesion is the attractive force between polar water molecules and the hydrophilic cellulose and lignin in the xylem vessel walls, supporting the water column.