noun, plural: tracheids
The xylem is the vascular tissue responsible for the upward conduction of water and nutrients from the roots. The xylem tissue moves water and nutrients to various parts of the plant such as shoots and leaves. Its major components include xylem parenchyma, xylem fibers, tracheids, and xylem vessels.
Tracheids are one of the two types of tracheary elements of vascular plants. (The other being the vessel elements). A tracheid cell loses its protoplast at maturity. Thus, at maturity, it becomes one of the non-living components of the xylem. Tracheid cells are tube-like with tapering ends. The average length is 5 to 6 mm. The cells have primary and secondary cell walls. The secondary cell wall is lignified and forms in between the primary cell wall and the plasma membrane in various thickening patterns. The most common patterns are annular, spiral, scalariform, reticulate, and pitted. When viewed from the top, the cells appear angular and polygonal. Tracheids have pits, which are concave depressions on the cell wall. In general, each of the pits has a complimentary pit on the neighboring tracheid cell. These pits that are complimentary are called pit pairs. Through them, water can move across from one tracheid cell to another.