The xylem is the principal water-conducting tissue of vascular plants. It consists of tracheary elements, tracheids and wood vessels and of additional xylem fibres. All of them are elongated cells with secondary cell walls that lack protoplasts at maturity.
Tracheids are the chief water-conducting elements plants. Tracheids are elongated cells, closed at both ends. Tracheids look often square in cross-section, the lignified secondary wall is relatively thin. The walls are opened by numerous pits .
Transverse section of vascular bundle of sunflower. This bundle contains tracheary elements of many different sizes , but they all are large enough that they are probably vessel elements rather than tracheids