Water and dissolved minerals are carried or pulled upwards towards the leaves through xylem tissue. This is called ascent of sap. This may involve the following:

(A) Cohesion Tension Theory (B) Root Pressure (C) Imbibition

COHESION TENSION THEORY

It is one of the most important theories proposed by Dixon. This theory provides a reasonable explanation of flow of water and minerals upwards from the roots to leaves of plants, in bulk flow or mass flow. This depends on the following:

(i) Cohesion: It is the attraction among water molecules which hold water together, forming a solid chain-like column within the xylem tubes. The water molecules form hydrogen bonds between them.

(ii) Tension: It is provided when this water chain is pulled up in the xylem. Transpiration provides the necessary energy or force. Tension is between the molecules of water by hydrogen bonds. This xylem water tension is strong enough to pull water up to 200 meters (more than 600 feet) in plants.

(iii) Adhesion: It may be added that the water molecules also adhere to the cell walls of xylem cells, so that the column of water in xylem tissue does not break. Composition of cell wall provides necessary adhesion to water molecules that helps water creep up. The cellulose component of cell wall especially has great affinity for water. It can imbibe water.