Question

Explain all the reasons that lignified cell walls are important in vascular plants. List the tissues that are lignified.

Answer 1

• Lignification involves the deposition of phenolic polymers called lignins on the extracellular polysaccharidic matrix.
• Lignin forms key structural materials in the support tissues of vascular plants.
• Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity and do not rot easily.
• Lignin fills the spaces in the cell wall between cellulose, hemicellulose, and pectincomponents, especially in vascular and support tissues: xylem tracheids, vessel elements and sclereid cells.
• It is covalently linked to hemicellulose and therefore cross-links different plant polysaccharides, conferring mechanical strength to the cell wall
• It is particularly abundant in compression wood but scarce in tension wood, which are types of reaction wood.
• Lignin plays a crucial part in conducting water in plant stems.
• This allows the xylem to withstand pressure changes as water moves through the plant.
• The polysaccharide components of plant cell walls are highly hydrophilic and thus permeable to water, whereas lignin is more hydrophobic.
• The crosslinking of polysaccharides by lignin is an obstacle for water absorption to the cell wall.
• Thus, lignin makes it possible for the plant's vascular tissue to conduct water efficiently.

Lignified tissues

1. Mature sclerenchyma cells are usually dead cells that have heavily thickened secondary walls containing lignin. The cells are rigid and nonstretchable  
2. Xylem is a tissue system consisting of specialized cells with lignified secondary cell walls responsible for water movement through the plant.