DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
Sunflower stem. Light micrograph of a section through a young stem from a sunflower (Helianthus annuus), showing one of the many vascular bundles. The outer stem is covered with a thin epidermis (dark blue). Under the epidermis is a layer of flexible collenchyma (dark blue) for support. The cortex and pith is composed of parenchyma tissue (light blue). The vascular bundles have an outer layer of sclerenchyma tissue (crimson). Next is the phloem (dark blue) with phloem tubes, parenchyma and companion cells. Then the xylem (red) and at the end of the xylem are patches of fibres (red). In-between the phloem and xylem is the cambium (light blue). Magnification: x37, when printed 10 centimetres wide.
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