Hypanthium

A hypanthium (plural hypanthia) is a floral structure consisting of the base of the sepals, petals, and stamens fused together. These are present in several plant families such as Rosaceae, Grossulariaceae, and Fabaceae. In some cases, it can be so deep and narrow that the flower can appear to have an inferior ovary.

In the Eucalyptus, the hypanthia are sometimes called gum nuts.

Geometrics
The various shapes of hypanthium are very useful in identifying species. A diversity of geometric forms are associated with the hypanthium including obconic forms found on the Toyon, and saucer-shapes found in some Mitellastra.