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Lime tree stem. Light micrograph of a section through the stem of a lime tree (Tilia europaea). The outer epidermis has been shed and replaced by a layer of cork (dark red). Under the cork is the outer cortex layer of flexible collenchyma tissue (dark blue and red) and a lower cortex of parenchyma (blue). Next is a ring of phloem (blue) with large sieve tubes, companion cells and fibres (pink). The phloem is broken by primary medullary rays (blue, triangular) and thin subsidiary medullary rays (dark blue). Next is a ring of xylem (light pink) with large files of vessels and small woody parenchyma (dark red-brown). There are two yearly growth rings, which have large vessels laid down in the spring and small vessels in the previous autumn. Magnification: (KEYSTONE/SCIENCE PHOTO LIBRARY/DR KEITH WHEELER/SCIENCE PHOTO LIBRARY)