The calybium (: calybia) and the cupule make up the

accessory fruit An accessory fruit is a fruit that contains tissue derived from plant parts other than the Ovary (botany), ovary. In other words, the flesh of the fruit develops not from the floral ovary, but from some adjacent tissue exterior to the carpel (f ...

flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (). The term angiosperm is derived from the Ancient Greek, Greek words (; 'container, vessel') and (; 'seed'), meaning that the seeds are enclosed with ...

s in the family

Fagaceae The Fagaceae (; ) are a family of flowering plants that includes beeches, chestnuts and oaks, and comprises eight genera with around 1,000 or more species. Fagaceae in temperate regions are mostly deciduous, whereas in the tropics, many species ...

. 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 (angiosperms) that is formed from the ovary after flowering. Fruits are the means by which angiosperms disseminate their seeds. Edible fruits in particular have long propaga ...

during its growth and maturation. In some genera (e.g. '' Lithocarpus,

Quercus An oak is a hardwood tree or shrub in the genus ''Quercus'' of the beech family. They have spirally arranged leaves, often with lobed edges, and a nut called an acorn, borne within a cup. The genus is widely distributed in the Northern Hemisp ...

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

Fagus Beech (genus ''Fagus'') is a genus of deciduous trees in the family Fagaceae, native to subtropical (accessory forest element) and temperate (as dominant element of Mesophyte, mesophytic forests) Eurasia and North America. There are 14 accepted ...

''), 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-sized rodents. The squirrel family includes tree squirrels, ground squirrels (including chipmunks and prairie dogs, among others), and flying squirrel ...

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 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 the initial stage of development for 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 sp ...

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, ...

. In the related family Betulaceae, notably in the genera ''

Carpinus Hornbeams are hardwood trees in the plant genus ''Carpinus'' in the family Betulaceae. Its species occur across much of the temperate regions of the Northern Hemisphere. Common names The common English name ''hornbeam'' derives from the hard ...

'' and '' Corylus'', the cupule is replaced by an

involucre In botany, a bract is a modified or specialized leaf, associated with a reproductive structure such as a flower, inflorescence axis or cone scale. Bracts are usually different from foliage leaves in size, color, shape or texture. They also look ...

, 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 as palaeontology or palæontology, is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure geo ...

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