Question

Why is lignin present in the xylem?

Answer 1

Xylem is the water and mineral conducting transportation tissue in plants.Lignin is a normal metabolite of plant metabolism but may be present due to disease or abnormality but it is very crucial for plants. Xylem contains cells like parenchyma , fibres, tracheids and vessels.

Complete answer:
Xylem vessels transport Water and minerals upwards through the plant stem. It consists of dead and hollow cells.
Its walls are lignified which allows it to survive pressure changes as water moves. It also prevents infections.
Lignin, which is a complex organic polymer that provides structural support to the tissues of vascular plants. Lignins are present in cell walls of wood and bark as they provide rigidity. It is hydrophobic, that is, it is water insoluble but lipophilic ( lipid soluble) and does not allow water to pass through. So, lignin makes it easier for vascular tissues to conduct water.

Additional information:
Suberin, cutin and lignins are some other substances present in higher plants. They are macromolecules which form a protective layer. Suberin, which is a polyester biopolymer, is lipid soluble and made of long fatty acid chains and glycerol.
Suberin is a water insoluble and lipid soluble macromolecule found in plant cell walls, it provides properties like insulation or protection. Cells containing suberin form the periderm, the tissue that covers secondary stems.

Note:
Lignin provides rigidity and prevents bacterial infections which is very important for plant cells.Another substance which is also known as suberin also serves as the same function as provided by lignin. Lignin has protective functions against the environment, so it is present in cells like bark etc.