Ash wood. Light micrograph of a tangential longitudinal section through wood of the Ash tree, Fraxinus excelsior. Running top to bottom (purple) are thick-walled xylem vessels known as tracheids. Between some of the tracheids are lenticular- shaped medullary rays (red) seen in cross- section. The rays consist of rounded parenchyma cells that form radial lines perpendicular to the tracheids. At left, sieve cells (dotted) are seen. Tracheids function in conducting water up the stem, as well as giving mechanical support to the tree. The ray system is thought to transport water across the wood. Illuminated by phase contrast. Magnification: x200 at 35mm size.