Silver birch twig. Coloured scanning electron micrograph (SEM) of a section through a silver birch (Betula pendula) twig. At the centre of the twig (top centre left) is the pith. Radiating out from the centre are concentric growth rings (red). Each ring represents one years growth. The majority of the stem consists of xylem tissue, which is made up of larger vessels (holes) and supporting tissue. The xylem transports water and mineral nutrients from the roots throughout the plant. The vessels have thick lignin walls that also provide structural support for the twig. Surrounding the xylem is a ring of cambium (bright green) a layer of actively dividing cells.