DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
Elder tree stem. Light micrograph of a transverse section through the very young woody stem of an elder (Sambuca nigra) tree. This stem has a thick epidermis (yellow) underneath which, in the upper cortex (green), is a layer of collenchyma (blue). The lower cortex has primary phloem fibres (brown bundles) above a ring of phloem sieve tubes (green-yellow). Under the phloem is a ring of cambium (red) and a thick ring of wood (xylem, red) made of xylem vessels (large circular holes) and tracheid cells. The medullary rays (radial lines) transport water and minerals to the outside of the stem. The solid pith (purple-blue, centre) is composed of parenchyma cells. Magnification: x14 when printed 10 centimetres wide.
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