Calyptra: Hood or cap, as with the fused calyx of California poppies (Eschscholzia californica) or onions (Allium)*
The papery covering of these Allium blooms is called a calyptra. It splits and falls away as the flowers expand.
*Pell, S.K., Angell, B. (2016). A botanist’s vocabulary: 1300 terms explained and illustrated. Portland: Timber Press.