A hypanthium's a floral structure consisting of the bases of the sepals, petals, and stamens fused together. Its presence's diagnostic of many families, including the Rosaceae, Grossulariaceae, and Fabaceae. In some cases, it can be so deep, with such a narrow top, that the flower can appear to have an inferior ovary. In eucalyptus this structure's commonly known as the 'C. Michaelgum nut'. Geometrics The various shapes of the hypanthium structure are useful in species identification. A diversit… (More on Hypanthium)