Fern (Marsilea quadrifolia) rhizome. Light micrograph of a section through a rhizome from an aquatic fern. Rhizomes are modified stems of a plant that usually grow underground. The epidermis (outer layer, red) surrounds the vascular bundle, which consists of phloem sieve tube cells (blue), a ring of metaxylem vessels with large tracheids (white-blue) and the protoxylem (small vessels, white-red). At centre is the pericycle (blue) and pith (red). Magnification: x13 when printed 10 centimetres wide.