Xylem vessels are made up of hollow cells designed to carry water and minerals from the roots of a plant to the trunk, with altered cell walls to allow for the passage of one vessel to another. They also provide structural support to vascular plants.
Xylem vessels are made up of cells known either as tracheids or vessel members. All vascular plants have tracheids, which are less specialized but have a special cell wall known as a pit membrane. This structure prevents the passage of damaging air bubbles from one xylem vessel to another. These are typically the only cells found in the xylem of gymnosperms, such as pine trees, and seedless vascular plants, such as ferns. Vessel members are more specialized cells with areas that lack any cell wall or membrane, known as perforations. These allow for easy passage of water between vessels, but also allow easier passage of air bubbles that can cause fractures and disruptions to the xylem. Vessel members are the principal components of xylem in flowering plants.
Xylem cells grow within the lengthening tips of roots and shoots. In woody plants, they are the bulk of the plant tissue and grow in rings as the plant expands. The inner rings die as the plant grows, remaining in place to provide structural support.