Transpiration Pull Theory
This is also called as cohesion-tension theory put forward by Dixon and Joly (1894) and supported by Renner, Curtis and Clark. This theory is based on a number of features.
Cohesion and Adhesion
Mutual attraction between water molecules is called cohesion and this force may have a value as high as 350 atmospheres. The wall of the tracheids and vessels which transport water are made up of lignin and cellulose and have high affinity for water and this is called adhesion. Xylem vessels have perforated end walls and form a tubular structure from roots to the shoot tip. This provides a continuous channel for movement of water which cannot be pulled away from xylem wall due to cohesive and adhesive properties.
The transpiration taking place through leaves causes negative pressure or tension in xylem sap which is transmitted to the root. This is called