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Anatomical description of a mustard (Brassica napus) plant showing its characteristic four petal flower head, stamen, seed pods, leaf arrangements and seeds. Image from Franz Eugen Koehler archive, Koehlers Medicinal Plants, Germany 1887. Image reproduced from Wikimedia Commons Repository, https://en.wikipedia.org/wiki/Rapeseed (accessed 19 August 2019), copyright lapsed.
