[…] it has been transformed, except in a few taxa where lobules are always explanate, into a galeate water-sac, which is formed wholly from the lobule, with the external surface from the adaxial leaf surface.
1992, Rudolf M[athias] Schuster, The Hepaticae and Anthocerotae of North America: East of the Hundredth Meridian, volume V, New York, N.Y.: Columbia University Press, page 4