xylem, the principal water-conducting tissue in vascular plants. In woody stem tissues, the secondary xylem forms the wood. The cambium differentiates xylem tissue on its inside. A single ring of xylem is produced each year with the first-formed vessels (in spring) being larger than those of late wood. This is how the annual rings which help to assess the age of a plant are formed (see oak). In the grapevine, the vessels are large and porous so that its wood is very water conductive, a feature of vines generally. In autumn, however, vessels may become blocked by structures which plug the tubes, called tyloses, formed by the ‘ballooning’ of adjacent cell material into the vessel through pits in the walls; some vessels remain functional for up to seven years, but most become blocked by tyloses in their second or third year. The secondary xylem of quercus forms the wood from which oak barrels are made. (See also phloem.)
