in botany, a generally accepted explanation of the rise of sap in plants by means of intermolecular attractions. Calculation and experiment indicate that the forces of cohesion between water molecules and the forces of adhesion between water molecules and the walls of the vessel cells are sufficient to confer on thin columns of water a tensile strength of at least 30 atmospheres (440 pounds per square inch). This is high enough to permit a thin column of water to be lifted to the top of any tree without breaking the column. The cohesion of water explains only maintenance of the sap column; the explanation for the upward movement of the water is accounted for by a mechanism, called transpiration pull, that involves the evaporation of water from leaves. Thus, the explanation for the upward movement of sap in trees is also called the transpiration–cohesion hypothesis. It accounts for the observed rise of sap and agrees with observed tensions (pressures below zero, negative pressures) in plant stems and gradients of tension between upper and lower parts of the same plant. The stability of water columns in wind-tossed trees is more difficult to understand. It is possible only because the water is enclosed in millions of small compartments (tracheids and vessels).