cupule

n. part of the body or of a plant that is cup-shaped

A cupule is a fruit structure found in flowering plants in the family Fagaceae. It holds and protects the nut 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.
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Noun
1. cup-shaped structure of hardened bracts at the base of an acorn
(synonym) acorn cup
(hypernym) cup
(part-holonym) acorn
2. a sucker on the feet of certain flies
(hypernym) sucker

n. acorn, fruit or seed of the oak tree

(n.)
A sucker or acetabulum.
  

(n.)
A cuplet or little cup, as of the acorn; the husk or bur of the filbert, chestnut, etc.