Introduction:
Lignified or thickened cell walls are a characteristic feature of the xylem tissue in plants. The xylem tissue is responsible for the conduction of water and minerals from the roots to the other parts of the plant. These thickened cell walls provide strength and support to the xylem cells.

Explanation:
1. Structure of the xylem tissue:
The xylem tissue is composed of several types of cells, including vessel elements, tracheids, fibers, and parenchyma cells. The vessel elements and tracheids are responsible for the conduction of water, while the fibers provide mechanical support. The parenchyma cells, on the other hand, have a storage function.

2. Lignification of cell walls:
In the xylem tissue, the cell walls of vessel elements and tracheids undergo lignification. Lignin is a complex polymer that is deposited in the cell walls, making them thick and rigid. Lignin is a hydrophobic substance, which helps in preventing the inward movement of water through the cell walls. This helps in maintaining the structural integrity of the xylem tissue and prevents collapsing of the cells under the pressure of water.

3. Function of lignified cell walls:
The lignified cell walls provide several important functions in the xylem tissue:
- Strength and support: The lignified cell walls provide mechanical strength and support to the xylem cells, allowing them to withstand the tension and pressure generated during water conduction.
- Water transport: The thickened cell walls of vessel elements and tracheids create a continuous pathway for the upward movement of water through the plant. The water molecules move through the pits and perforations present in the cell walls, ensuring efficient water transport.
- Protection against pathogens: The lignified cell walls act as a physical barrier, protecting the xylem tissue from the invasion of pathogens and other harmful microorganisms.
- Longevity: The lignified cell walls are highly durable and resistant to decay, allowing the xylem tissue to function effectively for a long period.

Conclusion:
Lignified or thickened cell walls are a characteristic feature of the xylem tissue in plants. These thick cell walls provide mechanical strength, enable efficient water transport, protect against pathogens, and ensure the longevity of the xylem tissue. Understanding the structure and function of lignified cell walls is essential for comprehending the role of the xylem tissue in plant physiology.