Cohesion tension theory is a theory of ascent of sap. It was originally proposed by Dixon and Joly in 1894 and Askenasy (1895), then it was greatly supported by Renner (1911, 1915), Curtis and Clark (1951), Bonner and Galston (1952) and Gramer and Kozlowski (1960).
This theory however describes the movement of water from roots to the leaves of a plant. Because of osmosis water from soil reach the xylem of roots of a plant. Water molecules are bonded to each other by hydrogen bonding, hence water form a string of molecules during its movement toward xylem. The water molecules stick together and get pulled up by the force called tension. This force is exerted because of the evaporation at the surface of the leaf.
Image describing cohesion adhesion tension theory
The theory is based on the following features:
Xylem vessels are tubular structures extending from roots to the top of the plants. Cells are placed one above the other, with their end walls perforated forming a continuous tube. These are supported by xylem tracheids which are characterised by having pores in their walls .one end of xylem tube is connected with the root hairs via pericycle, endodermis and cortex and another end is connected with the sub stomatal cavity in the leaves via mesophyll cells. This tube is filled with water.
The water is filled inside the xylem capillaries and due to cohesion and adhesion properties of water, it forms a continuous water column. The water column cannot be broken or pulled away from the xylem walls because of cohesion and adhesion of water.
An important factor which can discontinue the water column is the introduction of air bubbles in the xylem. Copeland (1902) believed that air bubbles enter into the xylem which breaks the tensile strength of water column, but Scholander et al. (1957) have shown that the air does not block the entire conducting system. Even if air bubbles were introduced, the individual water columns were unbroken and continuous with each other both in the vertical and lateral directions through the pits present in the cell wall.
Conclusion: It is clear from the above discussion that water column in the xylem are continuous .They extend from sub stomatal cavities in leaves to the roots. These unbroken water columns are just like steel ropes which are extended from leaves to the roots.