Cross-sections of cambial zone and differentiating xylem of
trees. A. Cross-sections viewed under an ordinary light microscope (24 March) and pairs of ordinary (left) and polarizing (right) light microscope images for the same field (27 April, 22 June, 24 August and 7 October). B. Number of cells in cambial zone and differentiating xylem. Cells in differentiating xylem were categorized into expanding cells, thickening cells and lignifying cells in accordance with ordinary and polarizing light microscope observations.