Light micrograph of a radial longitudinal section through the wood (secondary xylem) of the Yew tree, Taxus baccata, showing the vertical, tube- like structures known as tracheids. In softwoods, like yew, these carry out both sap-conducting and mechanical functions. Dark hair-like spirals of secondary thickening are visible on the walls of the tracheids. These serve to strengthen the cell walls. They are laid down while the xylem is still elongating and the spiral pattern allows the cells to stretch longitudinally. The white dots are holes known as pits. They allow sap to flow between adjacent cells. Magnification: x200 at 35mm size.