Ascent of sap

Transpiration (loss of water from aerial parts) creates a pull that draws water up the xylem.
Root pressure pushes water upward through the xylem from below.

The ascent of sap is explained by the cohesion-tension-transpiration pull model.
Transpiration from the leaves creates a pull (tension) on the water column in the xylem.
Cohesion is the mutual attraction between water molecules due to hydrogen bonding; it gives the water column a high tensile strength, i.e. an ability to resist a pulling force.
Adhesion is the attraction of water molecules to the polar surfaces of the walls of the tracheary elements.
Because of cohesion and adhesion the water column remains continuous and does not break or pull away from the walls, even under the strong tension of the transpiration pull.
Transpiration pull is the force that lifts the water; it is the cause of the tension, not the reason why the column does not break.
So rupture and fractionation do not occur because of cohesion and adhesion.