DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
Sunflower root. Light micrograph of a transverse section through the root of a sunflower (Helianthus annuus) plant, showing primary tissue. From outer root to centre: single layer of cork cells (brown); cortex of parenchyma (blue); remains of the pericycle (blue). In the centre of the root, there is no pith, but the xylem, primary metaxylem and protoxylem make up the star-shaped arms (red). Next is a ring of outer primary phloem between the xylem (blue, small cells), with sieve tubes and plates (blue); layer of cambium (red). Magnification: x37 when printed 10 centimetres wide.
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