AP BIOLOGY

Detailed explanation-1: -Both lignin and cellulose are found in the rigid cell walls of the xylem cells (those that conduct water) in the primitive plant Selaginella.

Detailed explanation-2: -Xylem cells The xylem is specialised to transport water up the stem of a plant and into the leaves. Xylem vessels are made up of a series of connected dead xylem cells. The end walls of the dead cells are broken to allow water to move through. A substance called lignin strengthens the cell walls of xylem cells.

Detailed explanation-3: -The walls of xylem cells are lignified (strengthened with a substance called lignin ). This allows the xylem to withstand pressure changes as water moves through the plant.

Detailed explanation-4: -In woody plants, secondary xylem constitutes the major part of a mature stem or root and is formed as the plant expands in girth and builds a ring of new xylem around the original primary xylem tissues.