Explain the secondary cell wall.

Secondary cell wall characteristics:

• Location: Deposited internal to the primary cell wall (between primary wall and plasma membrane)
• Timing: Forms after cell has stopped growing
• Composition:
  • High cellulose content (organized in parallel microfibrils)
  • Lignin (in woody tissues) – provides rigidity and waterproofing
  • Hemicellulose, pectin (in lesser amounts)
• Structure: Multiple layers (often three: S1, S2, S3) with different microfibril orientations
• Function:
  • Provides mechanical strength
  • Supports water transport (xylem vessels)
  • Protection against pathogens

Distribution: Particularly prominent in tracheids, vessels, sclerenchyma fibers, and sclereids.