Xylem

Xylem is the tissue that gives a vascular plant its plumbing for water and dissolved minerals, running from root to leaf; it was named by Nageli in 1858 from the Greek xylos, wood, and its older synonym, hadrome, comes from Haberlandt (1914). Xylem laid down by the procambium of the apical meristem is primary xylem, while xylem added later by the lateral vascular cambium is secondary xylem; within primary xylem, the earliest-formed elements (smaller, in developing organs) are protoxylem, and the elements formed afterward (larger, in mature organs) are metaxylem. The spatial relationship between protoxylem and metaxylem gets its own vocabulary: exarch, where protoxylem sits toward the periphery and metaxylem toward the centre (the normal condition in roots); endarch, the reverse - protoxylem central, metaxylem peripheral (the normal condition in stems); centrarch, where a single vascular strand has protoxylem in the very centre surrounded by metaxylem (e.g. Selaginella); and mesarch, the same central-protoxylem arrangement but with several vascular strands developing (e.g. Ophioglossum). Structurally, xylem is a complex tissue built of four cell types working together: tracheids (dead, elongated, tapering, imperforate cells with bordered pits, the sole water-conducting cells of gymnosperms and pteridophytes), vessels/trachea (dead, wider, perforated tubes formed end-to-end from vessel elements, the chief water conductors of angiosperms), xylem parenchyma (the only living cells in xylem, storing food and assisting conduct …