The calybium and the cupule make up the

accessory fruit An accessory fruit is a fruit in which some of the flesh is derived not from the floral ovary but from some adjacent tissue exterior to the carpel.Esau, K. 1977. ''Anatomy of seed plants''. John Wiley and Sons, New York. Accessory fruits are u ...

flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants t ...

s 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 ever ...

. These two parts derive from different flower components. The cupule holds and protects the

fruit In botany, a fruit is the seed-bearing structure in flowering plants that is formed from the ovary after flowering. Fruits are the means by which flowering plants (also known as angiosperms) disseminate their seeds. Edible fruits in partic ...

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 hav ...

Quercus An oak is a tree or shrub in the genus ''Quercus'' (; Latin "oak tree") of the beech family, Fagaceae. There are approximately 500 extant species of oaks. The common name "oak" also appears in the names of species in related genera, notably ...

''), 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 nuts they produce. The unrelate ...

Fagus Beech (''Fagus'') is a genus of deciduous trees in the family Fagaceae, native to temperate Europe, Asia, and North America. Recent classifications recognize 10 to 13 species in two distinct subgenera, ''Engleriana'' and ''Fagus''. The ''Engle ...

''), 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. ...

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 In the flowering plants, an ovary is a part of the female reproductive organ of the flower or gynoecium. Specifically, it is the part of the pistil which holds the ovule(s) and is located above or below or at the point of connection with the ...

, 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, fe ...

. In the related family

Betulaceae Betulaceae, the birch family, includes six genera of deciduous nut-bearing trees and shrubs, including the birches, alders, hazels, hornbeams, hazel-hornbeam, and hop-hornbeams numbering a total of 167 species. They are mostly natives of the ...

, notably in the genera ''

Carpinus Hornbeams are hardwood trees in the flowering plant genus ''Carpinus'' in the birch family Betulaceae. The 30–40 species occur across much of the temperate regions of the Northern Hemisphere. Origin of names The common English name ''hor ...

'' and ''

Corylus The hazel (''Corylus'') is a genus of deciduous trees and large shrubs native to the temperate Northern Hemisphere. The genus is usually placed in the birch family Betulaceae,Germplasmgobills Information Network''Corylus''Rushforth, K. (1999). ...

'', the cupule is replaced by an

involucre In botany, a bract is a modified or specialized leaf, especially one associated with a reproductive structure such as a flower, inflorescence axis or cone scale. Bracts are usually different from foliage leaves. They may be smaller, larger, or o ...

, 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