A protrusion that resembles a balloon and enters a vessel’s lumen is formed by the expansion of an adjacent xylem parenchyma or medullary ray cell, and then extruded through a pit in the vessel wall. In the past, a singular structure like this was referred to as a “tylosis.” However, current conventions dictate that the suffix “-osis” pertains to a process or condition, so it is now recommended that this structure be called a “tylose.”