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Thecalybium (pl.: calybia) and thecupule make up theaccessory fruit offlowering plants in the familyFagaceae. These two parts derive from different flower components.[citation needed]
The cupule holds and protects thefruit during its growth and maturation. In some genera (e.g.Lithocarpus,Quercus), it only partly encloses the single nut, while in others (e.g.Castanea,Fagus), it fully encloses the two or more nuts, and splits open at maturity into four valves to release the nuts. It is derived from the vegetative part of the flower (its attachment to the rest of the plant). It is covered by numerous scales. In some (e.g.Castanea), the scales are developed into sharp spines, giving the nut protection fromsquirrels and other seed predators, while in others (e.g. mostQuercus), they are not. InLithocarpus, the cupule is very hard and bone-like in texture.
The calybium is the fruit proper. It develops from aninferior ovary, meaning it is initially encased in the future cupule. Technically the calybium is anut, as its ovary wall becomes dry with theembryo loosely enclosed inside, and remains closed untilgermination. In the related familyBetulaceae, notably in the generaCarpinus andCorylus, the cupule is replaced by aninvolucre, which differs in being more leafy in appearance, but performs a similar role in protecting the developing nuts.