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Haematoxylin crystals. Polarized light micrograph of a section through crystals of haematoxylin. Haematoxylin is a compound extracted from the logwood tree that binds to and forms salts with acidic, or basophilic, compounds containing negative charges (such as DNA and RNA) and stains them dark blue of violet. Together with eosin, its makes up haematoxylin and eosin stain, one of the most commonly used stains in histology. Magnification: x200 when printed at 10 centimetres tall. (KEYSTONE/SCIENCE PHOTO LIBRARY/MAREK MIS/SCIENCE PHOTO LIBRARY)