Elm wood. Coloured scanning electron micrograph (SEM) of a transverse section through wood from an elm tree (Ulmus procera) showing annual growth rings (horizontal bands). A years growth consists of a porous band, which is the slow winter growth, and a more dense band, which is the faster summer growth. The Xylem vessels (holes) are also seen in the wood. The xylem transports water and mineral nutrients from the roots throughout the plant. Its thick lignin walls also provide structural support for the stem.