AP BIOLOGY

Detailed explanation-1: -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. Companion cells-These cells are found inside the phloem of angiosperms.

Detailed explanation-2: -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-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: -The secondary cell walls of xylem are composed of polysaccharide fibres embedded in a lignin matrix, providing mechanical support for the plant.

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.