Elm stem. Light micrograph (LM) of a cross-section through a woody stem of an elm (Ulmus procera) tree. This shows the thick layer of cork (phellem) on the outside of the stem which insulates the stem, prevents water loss and the ingress of pathogens, and reduces fire damage. Behind this is the thin outer ring of cortex and phloem. This surrounds the three thick rings of woody xylem tissue that indicates it is 2 years old. In the centre there are the thin-walled cells of the pith. Magnification: x60 when printed 10 centimetres wide.