Burdock stem. Polarised light micrograph of a cross-section through the stem of a burdock plant (Arctium lappa). The outer layers below the epidermis are the collenchyma (red patches) and cortex (deep purple). Below these are the vascular bundles. The phloem (red-brown) separates an outer cap of fibres (yellow) and the xylem (yellow with brown vessels). Most of the stem consists of the large central pith (light blue), which consists of parenchyma cells filled with starch grains. Magnification: x8 when printed at 10 centimetres across.