The membranous remnants of a partial veil which leaves a ring on the stem of a mushroom.
The ring of the sun not covered by the moon in an annular solar eclipse.
Structure in a fern that consists of differentially thick-walled cells on a sporangium that bend and distort as a result of drying.
A ring- or donut-shaped area or structure.
The region in a plane between two concentric circles of different radii.
The space contained between the centre well bore and any external tubing. Sometimes used for separated gas flow.
Any topological space homeomorphic to the region in a plane between two concentric circles of different radius.
The sterile parts of a flower; collectively, the sepals and petals (or tepals).
The sterile, tubelike tissue that surrounds the female reproductive structure in a leafy liverwort.