What are the four main forces which contribute to the ascent of sap (upward movement of water and minerals) ? Name them and explain in short.

The four main forces which contribute to the ascent of sap are:

1. Root pressure — Root pressure builds up sufficient force to push the sap in the xylem vessels up to a certain height and may be enough for herbaceous plants.
2. Capillary force — Capillarity (narrow diameter) of xylem vessels causes the water from a lower level to rise to fill up the vacuum created by the loss of water due to transpiration from the leaves. Narrower the diameter of a tube, greater will be the height of water rising in it exerting a force called capillary force.
3. Transpiration pull — As the water is lost from the leaf surface by transpiration, more water molecules are pulled up due to the tendency of water molecules to remain joined (cohesion), and thus to produce a continuous column of water through the stem.
4. Adhesion — It causes the water to stick to the surface of cells thus drawing more water molecules from below when the leaf cells lose water during transpiration. This pulling force (or suction force) provided by the leaves is important for tall trees.