Calybium and cupule

: calybia) and the cupule make up the accessory fruit of flowering plants in the family Fagaceae.

In some (e.g. Castanea), the scales are developed into sharp spines, giving the nut protection from squirrels and other seed predators, while in others (e.g. most Quercus), they are not.

It develops from an inferior ovary, meaning it is initially encased in the future cupule.

Technically the calybium is a nut, as its ovary wall becomes dry with the embryo loosely enclosed inside, and remains closed until germination.

In the related family Betulaceae, notably in the genera Carpinus and Corylus, the cupule is replaced by an involucre, which differs in being more leafy in appearance, but performs a similar role in protecting the developing nuts.