Light micrograph of a transverse section through a three year old lime tree (Tilia sp.) stem, showing the first and second year growth rings. At bottom right is the pith, which forms the central core of the young stem. The remainder of the frame is filled with xylem, a tissue composed of vertical, tube-like cells (seen here in cross-section) which conduct the sap upwards from the tree roots. The xylem cells which grow in the spring (pink) are much larger than the late season cells (green), since more sap is required by the developing leaves & shoots. This difference in size gives rise to the light and dark shading of the annual growth rings. Magnification: x50 at 35mm size.