Final answer:
Water is moved upwards in the trunk of a tree primarily through the process of transpiration, which generates a negative pressure in the xylem, owing to the cohesive properties of water molecules.
Step-by-step explanation:
The ultimate source of energy that moves water upward in the trunk of a tree is the process of transpiration, which is the evaporation of water from the plant through the leaves. This process creates a negative pressure (tension) within the xylem, drawing water up from the roots through the plant's vascular system. The cohesion-tension theory explains this ascent, where the cohesive force among water molecules allows them to pull each other along as they evaporate from the mesophyll cells in the leaves.
Transpiration sets up a water potential gradient that causes water to move from areas of higher water potential in the roots to areas of lower water potential at the top of the tree. During the night, when transpiration is reduced, water is retained in the plant's stem and leaves due to the water's adhesion to the cell walls of the xylem vessels and the cohesion among water molecules themselves.