Scanning electron micrograph of a cross section of oak heartwood, Quercus robur. The picture shows (centre), a large xylem vessel, about 3mm in diameter, surrounded by smaller cells of the vascular parenchyma. The large vessel is a tylose; its lumen is blocked by ingrowth from the surrounding cells. The function of xylem vessels in wood is to conduct water upwards from the roots. Tylose formation prevents this movement. In heartwood, as here, it is of little physiological importance to the tree. In sapwood, tyloses may develop as a reaction to pathogens, resulting in wilt disease as soil water fails to reach the canopy leaves. The most well known example of this effect is Dutch Elm disease. Harvested timber that contains tyloses, such as oak, is resistant to the ingress of moisture and of fungal pathogens, and is therefore prized for its durability and resistance to rot