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Vitamin C crystals. Differential interference contrast light micrograph of crystals of vitamin C (ascorbic acid). The main dietary sources of vitamin C are citrus fruits and green vegetables. Vitamin C is required to maintain healthy connective tissue and heal wounds. As an antioxidant, it may protect against viruses and cancer-causing substances. Magnification: x25 when printed at 10 centimetres wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/KARL GAFF / SCIENCE PHOTO LIBRARY)