Fern (Pteridium aquilium) rhizome. Light micrograph of a section through a rhizome from a bracken plant. Rhizomes are modified stems of a plant that usually grow underground. Shown here are the parenchyma cells (pink) of the inner cortex, the endodermis (red) and pericycle (pink). At top centre is the metaxylem with long thick tracheid vessels (yellow-orange) and protoxylem (small vessels, red). The xylem is surrounded by the phloem sieve tube cells (red). Magnification: x13 when printed 10 centimetres wide.