DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
Lime tree stem. Light micrograph of a transverse section through the outer portion of a three-year-old woody stem of a lime (Tilia cordata) tree. The upper cortex (far right) is composed of a layer of collenchyma (red-brown). The lower cortex has secondary bands of triangular phloem composed of phloem fibres (black-white) and sieve tubes (red-brown). Under the phloem is a ring of cambium (blackish circle) and a thick ring of wood (xylem, red) composed of xylem vessels (large circular holes) and tracheid cells. Magnification: x103 when printed 10 centimetres wide.
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