False-colour scanning electron micrograph of a vertical section through oak heartwood, the wood (secondary xylem) of Quercus robur. The large, tubular structures are tracheary vessels which have become blocked by bladder-like ingrowths (yellow) known as tyloses. In young sapwood the vessels carry water & nutrients from the roots to the leaves, but after a few years they become blocked & no longer carry sap. The large vessels occur in early (spring) wood; the smaller tubes seen on the right of the photo are late-wood vessels. The compact tissue separating the two types of wood is the tree's ray system. Oak is a hardwood. Magnification: x12 at 6x4.5cm size.