Fern (Pteridium aquilium) rhizome. Polarised 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 (white-blue) of the inner cortex, the endodermis (red) and pericycle (pink). At centre is the metaxylem with long thick tracheid vessels (white-yellow) and protoxylem (small vessels, white). The xylem is surrounded by the phloem sieve tube cells (yellow). Magnification: x13 when printed 10 centimetres wide.