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Marrow stem. Light micrograph of a longitudinal radial section through the stem of a marrow (Cucurbita pepo) plant, showing the various tissues of three vascular bundles. Each vascular bundle contains (from left to right): Outer stem parenchyma cells, three rows of perivascular thick-walled fibres (red-orange), six rows of large celled parenchyma cells (blue), two rows of phloem sieve tubes with sieve plates (dark red), six rows of thin cambium tissue cells (light blue), a large reticulately-thickened vessel (red), five rows of wood parenchyma tissue (deep blue), large parenchyma (blue). Magnification: x4 when printed 10 centimetres wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR KEITH WHEELER/SCIENCE PHOTO LIBRARY)