Question

what is the ascent of sap

Answer 1

The ascent of sap in the xylem tissue of plants is the upward movement of water and minerals from the root to the crown. ... The conducting cells in xylem are typically non-living and include, in various groups of plants, vessels members and tracheids

Answer 2

The word ascent means 'to rise up'.

Ascent of sap is nothing but a mechanism in plants which helps in the transport of the water present in the soil through suction by the root cells and the xylem cells.
The process is governed by the absorption of water by different kinds of cells in a tree from the roots to the leaves. Leaf cells generally don't participate in this process. But, they are responsible for the loss of water through stomatal openings.