DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
Foxglove root. Light micrograph of a section through the root of a foxglove (Digitalis purpurea) plant, showing secondary thickening. From outer root to centre: layers of cork cells (red); cork cambium (blue); phelloderm and cortex of parenchyma (blue); pericycle (blue ring); ring of outer (primary) and inner (secondary) phloem cells (blue), with sieve tubes and plates (blue); layer of cambium outside the xylem (blue); ring of xylem (red). In the centre of the root there is no pith, but xylem primary metaxylem and protoxylem. The primary cambium has produced secondary xylem, which has filled in-between the xylem arms to form a ring with very large xylem vessels, wood parenchyma and tracheid cells. Magnification: x37 when printed 10 centimetres wide.
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