protrusions of portions of the adjacent parenchyma cells that enter and block xylem vascular cells when those cells become inactive or injured.
tyloses (Wikipedia)
In woody plants, a tylosis (plural: tyloses) is a bladder-like distension of a parenchyma cell into the lumen of adjacent vessels. The term tylosis summarises the physiological process and the resulting occlusion in the xylem of woody plants as response to injury or as protection from decay in heartwood. It is a key process in wall one of the Compartmentalization Of Decay In Trees (CODIT) and other woody plants.
Section of Carpinus betulus wood showing the distinctive dark areas created as a result of tylosis in response to fungal infection