DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
Common daisy root. Light micrograph of a section through the root of a common daisy (Bellis perennis) plant, showing secondary thickening. From outside inwards: layers of cork cells (red); cork cambium (green); phelloderm and cortex of parenchyma (green); pericycle (green ring); ring of outer (primary) and inner (secondary) phloem cells (dark green, small) with sieve tubes and plates (light green); thick layer of cambium cells outside of the xylem (light green); ring of xylem (red). At centre there is no pith but the xylem, primary metaxylem and protoxylem make up of four star-shaped arms of the four medullary rays (green). The primary cambium has produced secondary xylem, in-between the xylem arms to form a ring with very large xylem vessels, wood.
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