DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
Black burdock stem. Polarised light micrograph of a transverse section through the stem of a black burdock (Arctium lappa) plant. This stem has two rings of vascular bundles (yellow and red oval structures). Each vascular bundle consists of an outer cap of phloem fibres (yellow), phloem cells (red) and then xylem cells. Phloem cells transport carbohydrates around the plant, while xylem cells transport water. At centre is the pith, which contains starch grains, a store of energy. Magnification: x14 when printed at 10 centimetres wide.
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