DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
Lime tree stem. Light micrograph of a transverse section through the woody stem of a lime (Tilia cordata) tree. The stem is three-years-old as it has three growth rings (concentric circles). The outer layer is cork (solid red), underneath which is a layer of collenchyma (red). The lower cortex has secondary bands of phloem fibres (black-white), in-between which are phloem sieve tubes (red). Under the phloem is a ring of cambium (blackish-red circle) and a thick ring of wood (xylem, red)composed of xylem vessels (large circular holes) and tracheid cells. The medullary rays (radial lines, solid red) transport water and minerals to the outside of the stem. The solid pith (centre) is composed of parenchyma cells (blue). Magnification: x14 when printed 10 centimetres wide.
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