Maple stem. Polarised light micrograph of a cross-section through the stem of a field maple (Acer campestre). 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 two thick pink rings of woody xylem tissue that indicates it is 2 years old. In the centre there are the thin-walled cells of the pith (blue).