An ascent of sap is the upward passage of water from a plant's roots to the tip.
The rise of sap is based on the mechanism, i.e. water creates a continuous column in the tracheary elements from the root hairs to the tip of the plant.
The water molecules are held together by the force of attraction known as cohesion.
Because water has a great tensile strength, the water column in xylem containers does not break.
The transpirational pull the water column from root to leaves.