Xylem's structure serves a duel purpose; support and transport. Cells that will become xylem add more material to the primary cell walls that most plant cells lay down. The thicker walls can be either disconnected rings or extensive secondary cell walls that cover the cell almost completely. Secondary thickenings are made up of cellulose and lignin. Lignin is a tough organic compound that makes wood strong and dense. When the cell walls of xylem conducting cells are complete they die. The contents within the cell disintegrate and leave a strong hollow cylinder filled with water. Water can travel in kind of a straight line due to the fact that these cells are stacked on top of each other. Tracheids are conducting cells of primitive vascular plants and they're long and extremely thin cells. Important in the transport is rays. Rays run from the center of the tree laterally supplying materials to the living tissues in the vascular cambium and phloem.
Flowering plants have a quite efficient xylem. The last act in life of some conducting cells is to digest parts of their end walls, forming holes, not just pits (where little or no extra material has been added to the primary cell wall). These holes allow water to flow rapidly from one cell to the next. Hollow xylem tubes are called vessels while the cells of the vessels are called vessel elements. More support for after the cell's contents have died are fibers, which are long and thin cells.
Changes in the Xylem of Woody Plants
Xylem in woody plants changes a lot during their lives. There is new wood formed that is called growth rings. Annual rings are generally formed in temperate climates where there are definite seasons. When the xylem gets old it can get plugged...