Ascent of sap:
The water absorbed by root hair also contains dissolved mineral salts in it and it is called sap. The process of upward transportation of sap against the force of gravity from root to leaves through xylem vessels is called the Ascent of sap.
Theories of Ascent of sap:
There are two major theories by which the ascent of sap takes place are given below:
1. Root Pressure Theory:
Stephen and Hales (1727) showed that the turgor pressure of all the root hairs and cell-to-cell osmosis through the cells of the root cortex result in the pericycle. The cumulative hydrostatic pressure or root pressure thus created in the cells of the pericycle force the accumulated water and the dissolved minerals through the dead cell wall of the xylem vessels.
This result in the upward movement or ascent of sap through the xylem vessels. Since maximum root pressure is utilized in crossing the wall of the dead xylem vessel. It can only force the absorbed water up to a height of 3-4 meters suitable for lower plants, but not for big trees.
2. Cohesive-Adhesive Force and Transpiration Pull Theory:
Dixon and Jolly (1894) propounded this theory. According to them, the water entering the xylem vessel encounters two different forces. These forces operate mainly because of the thin diameter of the xylem vessel like that of a capillary tube. These forces include the cohesive force between the water molecules and the adhesive force between the water molecules and the xylem wall.
These two forces create a continuous water column within the xylem vessel from the root to the leaves. The loss of water from the leaves due to transpiration creates a vacuum in the leaves, which creates a suction pressure on the water in the xylem vessel and lifts the water column. This force is known as Transpiration Pull.
As the water column is raised in the xylem vessel, new water molecules are added from the lower end to the column due to root pressure. Thus the continuity of the process is maintained. This theory is considered to be the most legitimate in explaining the ascent of sap.