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Floral Organ Development in Lavatera trimestris of the Malvaceae (from Duchartre, 1845).
Darwin read this paper at some point between 1853 and 1857. In illustrations 1 to 4, the epicalyx, calyx, and stamens can be seen as extremely young primordia. (Image courtesy of the Library of the Arnold Aboretum of Harvard University.)
