Fig. 3.2-8. Transverse section of root of Clusia (a tree from tropical Central America with no common name in English). The secretory product of this duct was preserved by the fixation process and has taken up a blue stain (secretory products are often dissolved during processing, so ducts typically appear empty on microscope slides). This duct is lined by one layer of secretory parenchyma cells that appear to be completely empty. Many of the surrounding cells have deposits of tannins that have stained as red particles or as solid red masses. In some, the tannins occur only along the wall, so at first glance the tannin might appear to be a red-stained secondary wall, but a secondary wall would not be as rough or irregular as these tannin deposits. The cell at xxx has a bit of front (or back) wall present, showing the primary pit fields as small whitish dots and splotches.