DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
Rowan root. Light micrograph of a section through the root of a rowan (Sorbus aucuparia) tree, showing secondary thickening. From outer root to centre: four flattened layers of cork cells (brown-red); a few layers of cortex made up of parenchyma (red) tissue; ring of pericycle (dark-red); ring of outer primary and inner secondary phloem (small cells) sieve tubes (blue); layer of cambium (dark blue ring); secondary xylem (red) made up of tracheids and vertical and horizontal parenchyma. Radiating from the centre of the xylem are files of cells, known as the medullary rays. Magnification: x37 when printed 10 centimetres wide.
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