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: -Xylem is a tissue consisting of dead, hollowed-out cells that form a system of pipes. 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: -In some cells, notably the water-conducting tracheids and vessel elements of xylem tissues in vascular plants, walls are strengthened by secondary walls with complex patterns of thickening and by impregnation with the polyphenolic polymer lignin.

Detailed explanation-4: -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-5: -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.