A relatively primitive tracheary cell in the xylem of many plants that has a lignified secondary cell wall and usually lacks a living protoplast at maturity. Tracheids may be distinguished from * vessels as they lack perforation plates and are generally smaller in diameter and greater in length. The only perforation in tracheids are the pits in the secondary cell wall, which are most concentrated on the end walls. In gymnosperms (except Gnetales) and some primitive angiosperms that lack vessels, the tracheids are the only cells specialized to transport water up the plant. Compare fibre , fibre-tracheid. |
