False-colour scanning electron micrograph of a horizontal section through oak heartwood, the wood (secondary xylem) of the tree Quercus robur. The image shows the cut end of a tracheary vessel which has become blocked by papery bladder-like ingrowths known as tyloses (yellow). 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. Large vessels like this occur in early (spring) wood; late-wood vessels are much smaller. The tissue surrounding the vessel is wood parenchyma, an unspecialised matrix tissue. Oak is classified as a hardwood. Magnification: x100 at 6x4.5cm size.