False-colour scanning electron micrograph of a vertical section through oak heartwood, the wood (secondary xylem) of the tree Quercus robur. The vertical tube-like structure (left) is a tracheary vessel which has become blocked by bladder-like ingrowths 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. Large vessels like this occur in early (spring) wood; late-wood vessels are much smaller. The rest of the tissue seen in the image is wood parenchyma, an unspecialised matrix tissue. Oak is classified as a hardwood. Magnification: x300 at 35mm size.