DENDROLOGY

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: -Lignin helps in the transportation of water in stems. Hence, it is present in xylem cells, that’s why they are heavily lignified and make up the wood of woody plants.

Detailed explanation-3: -Xylem vessels are cells that develop a specifically ornamented secondary cell wall to ensure their vascular function, conferring both structural strength and impermeability.