The calybium and the cupule make up the accessory fruit of flowering plants in the family

Fagaceae The Fagaceae are a family of flowering plants that includes beeches, chestnuts and oaks, and comprises eight genera with about 927 species. Fagaceae in temperate regions are mostly deciduous, whereas in the tropics, many species occur as evergre ...

. These two parts derive from different flower components. The cupule holds and protects the fruit during its growth and maturation. In some genera (e.g. ''

Lithocarpus ''Lithocarpus'' is a genus in the beech family, Fagaceae. Trees in this genus are commonly known as the stone oaks and differ from ''Quercus'' primarily because they produce insect-pollinated flowers on erect spikes and the female flowers have ...

, Quercus''), it only partly encloses the single nut, while in others (e.g. ''

Castanea The chestnuts are the deciduous trees and shrubs in the genus ''Castanea'', in the beech family Fagaceae. They are native to temperate regions of the Northern Hemisphere. The name also refers to the edible nut (fruit), nuts they produce. Th ...

, 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 from

squirrel Squirrels are members of the family Sciuridae, a family that includes small or medium-size rodents. The squirrel family includes tree squirrels, ground squirrels (including chipmunks and prairie dogs, among others), and flying squirrels. Squ ...

s and other seed predators, while in others (e.g. most ''Quercus''), they are not. In ''Lithocarpus'', the cupule is very hard and bone-like in texture. The calybium (plural: calybia) is the fruit proper. 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 An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

loosely enclosed inside, and remains closed until

germination Germination is the process by which an organism grows from a seed or spore. The term is applied to the sprouting of a seedling from a seed of an angiosperm or gymnosperm, the growth of a sporeling from a spore, such as the spores of fungi, fer ...

. 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.

References

* (2003): A Miocene rodent nut cache in coastal dunes of the Lower Rhine Embayment, Germany. ''

Palaeontology Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...

'' 46(6): 1133-1149. {{doi, 10.1046/j.0031-0239.2003.00337.x Fagaceae Fruit morphology