Ascent of sap is done by xylem tissue.

Explanation

Because it is necessary to supply water and dissolved salts absorbed by roots till leaves. For this definite type of vascular or conducting tissue is necessary. Xylem tissue fulfills all necessary properties for ascent of sap like uptake of water and salts is possible on the basis of cohesive force, Adhesive Force, capillarity and transpiration due to absorbent force theory.