Polarised light micrograph of a vertical section through the wood (secondary xylem) of a pine tree, Pinus sp. , showing the structure around a knot (blue oval, right). Knots are formed where side branches leave the main trunk of a tree; the blue oval is effectively a cross section through the side branch. The rest of the image shows sap- conducting tracheids (pink, elongated cells) and medullary rays (short chains of smaller bluish cells). The tracheids have thick walls studded with small pores (known as pits) which allow the sap to flow from one cell to another. Magnification: x60 at 6x7 cm size.