Water is carried up a tree through the xylem tissue in a process called transpiration. At the bottom of the tree, the roots absorb the vast majority of water that a tree needs. Above ground, the constant evaporation from the leaves creates a flow of water from roots to shoots. The properties of cohesion and adhesion allow the water to move up a tree regardless of its height: Cohesion allows the individual water molecules to stick together in one continuous stream, while adhesion permits the water molecules to adhere to the cellulose molecules in the walls of xylem cells. When the water reaches a leaf, it evaporates—thus allowing additional water molecules to be drawn up through the tree.