The most widely accepted theory for the ascent of sap in the tree is the continuous movement of water from the stomata of the leaves to the atmosphere. Transpirational pull and cohesion theory of Dixon and Joly is the most widely accepted theory for the ascent of sap in trees. OR
In plants, water rises in the stem through xylem elements.
Water rises due to the phenomenon of cohesion and transpiration pull theory given by Dixon and Joly. It is the widely accepted theory for the ascent of water or plant sap. Cohesion is the property of molecules of the same substance to remain together. During transpiration when there is a scarcity of water in the aerial region then transpiration pull occurs. Due to cohesion between water molecules and transpiration pull water rises upwards in the stem.
So the correct answer is 'Transpirational pull and cohesion theory of Dixon and Joly'.

Transpiration pull and cohesion tension theory:

The most widely accepted theory of ascent of sap is the transpiration pull and cohesion tension theory. This theory explains how water is transported from the roots to the leaves of a plant against gravity.

Transpiration pull:
Transpiration is the process of water loss from the leaves of a plant in the form of vapor. When water is lost through the stomata in the leaves, it creates a negative pressure or tension in the xylem vessels. This negative pressure is known as the transpiration pull.

Cohesion-tension:
The cohesion-tension theory explains how water molecules are pulled up through the xylem vessels due to the cohesive forces between the water molecules and the adhesive forces between water molecules and the walls of the xylem vessels. The process is as follows:

1. Transpiration at the leaves creates a negative pressure in the xylem vessels.
2. This negative pressure pulls the water molecules upwards.
3. The cohesive forces between water molecules help in maintaining a continuous column of water in the xylem vessels.
4. The adhesive forces between water molecules and the walls of the xylem vessels prevent the water from separating from the vessel walls.

Root pressure:
Root pressure is another force that aids in the ascent of sap. It is the pressure exerted by the roots of a plant that helps push water up through the xylem vessels. This pressure is created by active pumping of mineral ions into the xylem by the roots.

Capillary force theory:
The capillary force theory suggests that water is pulled upwards through the xylem vessels due to capillary action. Capillary action is the ability of water to rise in narrow tubes against gravity. While capillary action does play a role in the movement of water in some plants, it is not the primary mechanism responsible for the ascent of sap.

Atmospheric pressure theory:
The atmospheric pressure theory suggests that the movement of sap is due to changes in atmospheric pressure. However, this theory is not widely accepted and does not fully explain the movement of water in plants.

In conclusion, the transpiration pull and cohesion tension theory is the most widely accepted explanation for the ascent of sap in plants. It takes into account the negative pressure created by transpiration, the cohesive and adhesive forces between water molecules, and the active pumping of ions by the roots.