A tissue that transports water and dissolved mineral nutrients in vascular plants. In flowering plants it consists of hollow vessels that are formed from cells (vessel elements) joined end to end. The end walls of the vessel elements are perforated to allow the passage of water. In nonflowering vascular plants, such as conifers and ferns, the constituent cells of the xylem are called tracheids. In young plants and at the shoot and root tips of older plants primary xylem is formed by the apical meristems (see protoxylem; metaxylem). In plants showing secondary growth this xylem is replaced in most of the plant by secondary xylem, formed by the vascular cambium. The walls of the xylem cells are thickened with lignin, the extent of this thickening being greatest in secondary xylem. Xylem contributes greatly to the mechanical strength of the plant: wood is mostly made up of secondary xylem. See also fibre. Compare phloem.