A tylosis is an outgrowth from vertical or ray parenchyma cells through bordered pits into the lumen of a xylem vessel element. This study reports additional information on the ontogeny of tyloses, formation of the developing tylosis wall, and chemical composition of the tylosis wall. The development and structure of tyloses in several species of oak (Quercus) were studied with the transmitting and scanning electron microscopes. The tylosis wall was layered with a complement of wall layers found in xylem elements. Cellulose and lignin were constituents of the tylosis wall
