DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
Ash stem. Light micrograph of a transverse section through the woody stem of an ash (Fraxinus excelsior) tree showing four growth rings (concentric circles). The outer layer is composed of cork (blackish-brown), underneath which is the cortex (red-purple), with primary phloem fibres (light red bundles) in the inner cortex. Under this is a layer of phloem sieve tubes with simple sieve plates (dark blue and deep red). Under the phloem is a ring of cambium (deep blue) and under this is a thick layer of wood (xylem, red). The solid pith (centre) is composed of parenchyma cells. Magnification: x14 when printed 10 centimetres wide.
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