Transpiration Pull Hypothesis

(Transport of Water and Minerals by the Xylem)


1 A negative pressure ( tension ) is created on the surface of the film of water coating mesophyll cells when evaporation occurs from its surface. (This occurs during transpiration).

2 This tension pulls on the interconnected chain of water molecules within the xylem which extends from the leaf through the stem to the tips of the root.

3 The water molecules in the chain are held together by hydrogen bonds which exist between neighbouring water molecules. ( cohesion)

4 The chain of molecules is prevented from being pulled down because each water molecule in the chain is attracted to the walls of the xylem by hydropyllic attraction between water and the cellulose in the cell walls. (Adhesion)

5 Hence the water column which is held together by cohesion and prevented from lowering by adhesion is pulled up by the tension generated from above by transpiration.

6 Solutes which are dissolved in the water also are pulled up within the xylem.