Fern (Osmunda regalis) petiole. Light micrograph of a section through a petiole (leaf stalk) from a royal fern, showing the epidermis (outer layer, red), chloroplasts (blue), endodermal cells (purple) and phloem sieve tubes (green). In the middle is the xylem, consisting of the metaxylem (large thick-celled, brown) and protoxylem (small thin-celled, brown). Magnification: x13 when printed 10 centimetres wide.