Transpirational pull results from the evaporation of water from the surfaces of
cells in the
leaves. This evaporation causes the surface of the water to recess into the
pores of the
cell wall. By
capillary action, the water forms concave
menisci inside the pores. 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.
Transpiration in leaves creates tension (differential pressure) in the
mesophyll cells. Because of this tension, water is literally being pulled up from the roots into the leaves, helped by
cohesion (the pull between individual water molecules, due to
hydrogen bonds) and
adhesion (the stickiness between water molecules and the
hydrophilic cell walls of plants).