Biology – Structure of transport tissues | e-Consult
Structure of transport tissues (1 questions)
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Xylem vessel elements are elongated, living cells that form continuous tubes throughout the plant. Their primary function is the long-distance transport of water and mineral ions from the roots to the rest of the plant. The key structural features that facilitate this function are:
- Cell Wall: The cell wall is composed primarily of lignin, a complex polymer that provides significant structural support and rigidity. This is crucial for maintaining the vessel's integrity under negative pressure (tension) created by transpiration.
- Perforation Plates: At the ends of xylem vessel elements, the cell walls are perforated, forming open ends called perforation plates. This allows water to flow freely between vessels, creating a continuous conduit.
- Lack of Cytoplasm at Maturity: Mature xylem vessel elements are dead at maturity and lack cytoplasm, nuclei, and vacuoles. This reduces the internal volume of the vessel, minimizing resistance to water flow. The cell walls become highly lignified, further reducing resistance.
- Pits: Pits are thin areas in the secondary cell walls of xylem vessels. They allow water to move laterally between adjacent vessels, preventing blockage and ensuring continuous water flow. There are two types: perforate pits (with a complete break in the cell wall) and non-perforate pits (with only a thin break).
- Tracheids: While xylem vessel elements are present in all vascular plants, tracheids are also involved in water transport, particularly in gymnosperms. Tracheids are narrower and have tapering ends, which create more resistance to water flow than vessel elements. They also have pits, but these are typically wider and more open than those in vessel elements. Tracheids are essential for water transport in smaller plants and are found in all vascular plants.
In summary, the lignified cell walls, perforation plates, lack of internal contents, and presence of pits all contribute to the efficient and continuous transport of water through the xylem.