Scanning electron micrograph of a vertical section through the wood (secondary xylem) of the Scots pine, Pinus sylvestris, showing the vertical, tube-like structures known as tracheids. In softwoods, like pine, these carry out both sap- conducting and mechanical functions; the large early (spring) wood tracheids seen here are primarily concerned with conduction, whereas the much smaller late ones act primarily as structural elements. The thickened walls of the tracheids are marked by lines of tiny holes, known as pits. These allow the exchange of substances with adjacent cells. Magnification: x750 at 8x10 inch size.