Xylem with pits. False-colour scanning electron micrograph (SEM) of a radial longitudinal section through wood of an ash tree, Fraxinus excelsior. Here, empty thick-walled xylem tubes known as tracheids are seen, each formed from a single cell. Small round pits (or pores) occur in the walls of the tracheids. Tracheids comprise the non-living element of wood and function in the conduction of water up the stem, as well as giving mechanical support to the tree. The pits allow the sap to flow across from one xylem cell to another. Magnification: x120 at 6x4.5cm size.