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 violet); cortex of parenchyma (violet); ring and remains of the pericycle (dark violet). At the centre of the root there is no pith, but xylem primary metaxylem (centre) and protoxylem, made up of star-shaped arms (grey); next is the primary phloem, in-between the xylem, with small cells (dark-violet) and sieve tubes. Magnification: x13 when printed 10 centimetres wide.
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