Yew wood. Light micrograph of a radial longitudinal section through wood of the Yew tree, Taxus baccata. Here, tracheids (non-living xylem) are seen forming annual growth rings. From right to left, in each ring, the tube-like cells become gradually narrower until the dense line (pink) signifies winter, followed by a rapid change to larger cells of spring. These vertical cells carry out both sap-conducting and mechanical functions. Hair-like spirals of secondary thickening serve to strengthen the cell walls, and allow the cells to stretch lengthwise. White pits in the wall allow sap to flow between adjacent cells. Illuminated by crossed polars. Magnification: x200 at 35mm size.