The xylem is a specialised structure that functions to facilitate the movement of water throughout the plant

• It is a tube composed of dead cells that are hollow (no protoplasm) to allow for the unimpeded movement of water
  

• Because the cells are dead, the movement of water is an entirely passive process and occurs in one direction only (from roots to shoots)
  

• The cell wall contains numerous pores (called pits), which enables water to be transferred between cells
  

• Walls have thickened cellulose and are reinforced by lignin, so as to provide strength as water is transported under tension
  

Xylems can be composed of tracheids (all vascular plants) and vessel elements (certain vascular plants only)

• Tracheids are tapered cells that exchange water solely via pits, leading to a slower rate of water transfer

• In vessel elements, the end walls have become fused to form a continuous tube, resulting in a faster rate of water transfer

Xylem Structure