coleoptile
Concise Oxford English Dictionary © 2008 Oxford University Press:
coleoptile /ˌkɒlɪˈɒptʌɪl/
▶noun Botany a sheath protecting a young shoot tip in a grass or cereal.
– origin C19: from Gk koleon ‘sheath’ + ptilon ‘feather’.
'coleoptile' also found in these Oxford entries:

