Water rises through xylem vessels because of two qualities:
Cohesion: Water molecules are weakly attracted to each other via hydrogen bonds
Adhesion: Water molecules form hydrogen bonds with the xylem cell wall
These properties create a suction effect (or transpiration pull) in the xylem
The xylem has a specialised structure to facilitate transpiration:
The inner lining is composed of dead cells that have fused to create a continuous tube
These cells lack a cell membrane, allowing water to enter the xylem freely
The outer layer is perforated (contains pores), allowing water to move out of the xylem into the leaves
The outer cell wall contains annular lignin rings which strengthens the xylem against the tension created by the transpiration stream