Light micrograph of a section through the needle (leaf) of a pine tree (Pinus sp.) . The centre of the needle is occupied by two vascular bundles (butterfly-shaped region, centre), each one made up of xylem (red and black) and phloem (blue) tissue. These are surrounded by a thick pericycle (large-celled region) and a layer of endodermis (necklace-like ring of large cells). The tissue surrounding the endodermis is known as mesophyll (green and brown speckled layer), which contain the resin ducts (not seen). In the epidermis (surface) are stomata (openings) for gas exchange.