Tyloses are balloon like structures produced by the xylem and ray parenchyma seen adjacent to the xylem vessels.
After a period of activity the vessels are blocked by these structures and vessels become non functional. The balloon like structures enter the lumen of the vessels through the pit cavities of it. Later the walls become lignified and wood become rigid and durable. Tyloses are characteristic of some angiospermic woods.
Tylose may be single or may be divided into a multiple structure. Normally they are present in the heart wood.