The New Student's Reference Work/Annulus
An'nulus. A structure developed in the wall of the spore-case (sporangium) of the ordinary ferns to assist in discharging the spores. See Filicales.
See also Annulus on Wikipedia, and the disclaimer.
An'nulus. A structure developed in the wall of the spore-case (sporangium) of the ordinary ferns to assist in discharging the spores. See Filicales.