Most plant physiologists now accept the "cohesion-tension theory" as an explanation for the ascent of sap. According to this theory, water moves up the trunk of a tree in narrow, elongated cells near the periphery of the trunk, referred to as the xylem, and does not require the expenditure of metabolic energy. The movement of water only depends upon three important physical-chemical properties of water.
The first important property of water is that it always moves from a region with a more positive water potential, to a region with a more negative potential. Water potential is a measure of the energy available in a solution of water. Thus, water moves out of the leaves and into the air because the water potential of the air is more negative; water moves out of the tree trunk and into the leaves because the water potential of the leaves is more negative; water moves out of the roots and into the trunk because the water potential of the trunk is more negative; and water moves out of the soil and into the roots because the water potential of the roots is more negative.
The second important physical-chemical property of water is that it is a cohesive molecule. In other words, water molecules tend to bind to one another through the formation of hydrogen bonds. The cohesiveness of water molecules gives the thin water columns in a tree trunk a very great tensile strength. This prevents breakage of the water column when great longitudinal stresses are placed upon it as it is pulled out of the leaves and into the air.
The third important property of water is that it adheres very tightly to the walls of xylem cells in the tree's transport pathway. Adhesion of water to these cell walls maintains the full hydration of the pathway for water transport. This prevents breakage of the water column, and allows water transport even when a tree is water-stressed in a dry environment.
Science EncyclopediaScience & Philosophy: Toxicology - Toxicology In Practice to TwinsTree - Tree Taxonomy, History Of Taxonomy, Modern Taxonomy, Cell Layers In A Tree Trunk, Growth Rings - Secondary growth