DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
Lime tree stem. Light micrograph of a transverse section through the one-year-old woody stem of a lime (Tilia cordata) tree. The outer layer is cork (brown), underneath which is a layer of collenchyma (yellow-green). The raised areas in the cork are breathing pores (lenticels). The lower cortex has bands of phloem fibres (red-brown), between which are phloem sieve tubes (green). Under the phloem is a ring of cambium (brown circle) and a thick ring of wood (xylem, brown-green) composed of xylem vessels (large circular holes) and tracheid cells. The medullary rays (radial lines, yellow-green) transport water and minerals to the outside of the stem. The solid pith (centre) is composed of parenchyma cells (green-yellow). Magnification: x103 when printed 10 centimetres wide.
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