AP BIOLOGY

Detailed explanation-1: -Their collapse during the time they are needed to function is prevented by special thickening in their walls. The inner walls of the xylem vessels are strengthened by the deposits of a substance called Lignin. Lignin is a tough and rigid substance; it prevents the vessel from collapsing.

Detailed explanation-2: -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-3: -Lignin provides mechanical strength and resistance against pathogens, and makes the cell walls impermeable to water, thus enabling the transport of solutes via tracheids in the xylem tissue.

Detailed explanation-4: -Xylem tissue transports water to different parts of the plant. The xylem vessel elements originally are made up of cells that eventually die as a result of lignin buildup in their cell walls, and therefore lost their end walls and cytoplasm.

Detailed explanation-5: -Both lignin and cellulose are found in the rigid cell walls of the xylem cells (those that conduct water) in the primitive plant Selaginella.