What is a tylosoid and where is it found?

A tylosoid is an intrusion of a parenchyma cell into the cavity of a vessel or tracheid in xylem tissue. The statement should actually read "tylosis" (not "tylosoid"). Tyloses (plural) are bladder-like outgrowths from adjacent parenchyma cells that protrude through pit cavities into the lumens of vessels or tracheids. They form when parenchyma cells adjacent to vessels grow through pit pairs and expand into the vessel cavity. Tyloses serve several functions: they can block water movement through vessels, preventing excessive water loss from cut or wounded areas; they help compartmentalize infections, preventing pathogen spread through the vascular system; and they are responsible for the durability of heartwood by blocking vessels. Tyloses are common in the heartwood of many trees and contribute to the characteristic properties of various commercial timbers.