The cohesion theory was proposed by the Dixon and Jolly in 1894. This theory is explained based on the two features;
The cohesive and the adhesive properties of the water molecules.
The transpiration pull exerted on the column of water.
According to this theory the water molecules remain together by cohesive property due to the presence of hydrogen bond. Strong mutual force of attraction will be created between the water molecules when there is large volume of water in column. So they can remain as a continuous water column in the xylem. This force of attraction that binds the molecules together is known as the cohesive force. Along with the cohesive force there is another force of attraction that is between the water molecules and the xylem walls. This force of attraction is known as the adhesive force; it also ensures the continuous water in the xylem.
The water evaporates from the inter cellular space to the outer atmosphere through the stomata and this process is known as the transpiration. The water released to the inter cellular space from the mesophyll cells from there the water is drawn from the xylem water column of the leaf. This continuity creates a tension in the xylem of the leaf and which is then transferred to the xylem of stem and to root. This tension or force is known as the transpiration pull. So this transpiration pull helps in the upward movement of water as continuous water column so that it can reach the transpiring surface of the plant.