Transpirational pull
Transpirational pull is the main phenomenon driving the flow of sap in the xylem tissues of large plants.Transpirational pull results ultimately from the evaporation of water from the surfaces of cellss in the interior of the leaves. This evaporation causes the surface of the water to pull back into the pores of the cell wall. Inside the pores, the water forms a concave meniscus. The high surface tension of water pulls the concavity outwards, generating enough force to lift water as high as a hundred meters from ground level to a tree's highest branches.
