This theory was proposed by Dixon and Jolly (1894). It is most important and widely accepted physical theory. This theory is based on the following features,
(1) Cohesion and adhesion:
The water molecules remain attracted by a force called cohesion. The force by which water molecules remain adherel to the lateral wall of trachea and trancheids of xylem is called adhesion. The cohesive and adhesive properties of water form an unbroken continuous water column in xylem. Such water column is continuous from roots to the leaves.
(2) Transpiration pull or tension:
Transpiration pull is a force created due to transpiration. OR a negative force or tension or it is pull created as a result of transpiration which is transmitted down to the root. As transpiration occurs, mesophyll cells of leaf loose water in the form of water vapour. Due to the loss of water vapours, the cytoplasm of mesophyll cells becomes concentrated and its osmotic concentration ( OC ) osmotic pressure ( OP ) and DPD are increased so they absorb water from the adjacent cells.
This process continues (evaporation absorption of water from the adjacent cells evaporation) upto the xylem vessels of leaf, then to the xylem vessels of petiole, stem and root. In other words, this tension is transmitted all the way down to the unbroken column of water through the stem to the absorbing parts or the root. So entire water column is pulled up.
Merits of transpiration pull theory:
(1) The force created by transpiration pull and cohesion is known to be capable of lifting the water column even to a height of 2000 m.
(2) The cohesive force of water is up to 350 atmospheres. The transpiration force created at the region of leaf is only 20 -50 atmospheres. It falls on the continuous water column but column does not break because the cohesive force of 350 atm can overcome this.
