What is the cupule that surrounds or supports fruits in many Fagales plants?

In the plant order Fagales, the cupule—often referred to as a hull or husk—is a distinguishing morphological structure that subtends or surrounds the fruit.1

This structure varies in origin and form across different families:

• Fagaceae: In genera such as Quercus (oaks), the cupule is believed to be derived from a highly modified and reduced branch system with associated modified leaves or bracts. It is characterized as an involucre where the bracts are indurated and coherent.12
• Betulaceae: The husks are thought to be composed of several modified bracts. In some genera, these husks may appear leaflike, tubular, or flask-shaped, and they sometimes bear glandular hairs that exude a sticky substance to deter predation.1
• Nothofagus: The four-valved cupule is suggested to be derived from four enlarged and modified stipules located at the base of two bracts beneath the inflorescence.1

The cupule serves to partially or completely enclose the fruit, which may be a nut, samara, or drupe.1