involucre

1. A whorl of bracts around or beneath a condensed inflorescence, such as a capitulum or umbel. It resembles and performs the function of the calyx of a single simple flower.
2.   The sheath surrounding a group of archegonia or antheridia in certain liverworts, such as Sphaerocarpus and its allies.
3.   A tubular upgrowth of the thallus surrounding the base of the sporophyte in members of the Anthocerotae (hornworts).