Editing Plant cell (section)

===Xylem===
[[Xylem]] is a complex vascular tissue composed of water-conducting [[tracheid]]s or [[vessel elements]], together with fibres and parenchyma cells. Tracheids<ref name=Tyree>MT Tyree; MH Zimmermann (2003) Xylem structure and the ascent of sap, 2nd edition, Springer-Verlag, New York USA</ref> are elongated cells with lignified secondary thickening of the cell walls, specialised for conduction of water, and first appeared in plants during their transition to land in the [[Silurian]] period more than 425 million years ago (see ''[[Cooksonia (plant)|Cooksonia]]''). The possession of xylem tracheids defines the [[vascular plants]] or Tracheophytes. Tracheids are pointed, elongated xylem cells, the simplest of which have continuous primary cell walls and lignified secondary wall thickenings in the form of rings, hoops, or reticulate networks. More complex tracheids with valve-like perforations called [[pit (botany)|bordered pits]] characterise the gymnosperms. The [[fern]]s and other [[pteridophyte]]s and the [[gymnosperm]]s have only xylem [[tracheid]]s, while the [[angiosperms|flowering plants]] also have [[vessel element|xylem vessel]]s. Vessel elements are hollow xylem cells without end walls that are aligned end-to-end so as to form long continuous tubes. The bryophytes lack true xylem tissue, but their [[sporophyte]]s have a water-conducting tissue known as the hydrome that is composed of elongated cells of simpler construction.