hypanthium

The hypanthium is a floral structure that appears as a cup- or tube-shaped expansion at the base of a flower. It forms from the fusion or connation of the basal portions of sepals, petals, and stamens with the receptacle, and it often surrounds or partly encloses the ovary. The presence and size of the hypanthium vary among species and can influence the overall shape of the flower.

In perigynous flowers, the hypanthium encircles the ovary without completely enclosing it, while in epigynous flowers the ovary sits within or below the floral cup and is often considered inferior. The hypanthium can be a prominent feature or relatively inconspicuous, and its tissues may contribute to the developing fruit in some taxa.

A key functional aspect is that the hypanthium can contribute to fruit tissue, especially in certain cultivated plants. For example, in apples and pears, the hypanthium (along with surrounding receptacle tissue) expands to form a substantial portion of the edible fruit flesh. In other groups, the hypanthium remains small and has little fruit-related impact.

The hypanthium is of interest in plant morphology and systematics because its presence, absence, and degree of development can aid in identifying genera and understanding floral evolution. It is distinct from structures such as nectaries or the primary ovary itself, though it often interacts with these parts in flower and fruit development.