Any of various cup-shaped structures, especially: 1. The structure that partially or completely encloses the fruits of trees in the Fagaceae, such as the cup encircling the bottom of an acorn and the spiny husk around sweet chestnuts (Castanea saliva). It is formed from fused extensions of the pedicel and is believed by some to provide a link between angiosperms and seed ferns.
2. The structure formed from fused glandular bracts that enclosed the seed in certain of the extinct seed ferns, e.g. Lyginopteris.
3. The hollow spherical structure that contained the ovules in members of the extinct gymnosperm order Caytoniales. Such cupules were borne at the tips of short pinnae on the female reproductive axis and had a pore at the base to enable pollen to enter.