annulus

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annulus

[′an·yə·ləs]
(anatomy)
Any ringlike anatomical part.
(botany)
An elastic ring of cells between the operculum and the mouth of the capsule in mosses.
A line of cells, partly or entirely surrounding the sporangium in ferns, which constricts, thus causing rupture of the sporangium to release spores.
A whorl resembling a calyx at the base of the strobilus in certain horsetails.
(mathematics)
The ringlike figure that lies between two concentric circles.
(mycology)
A ring of tissue representing the remnant of the veil around the stipe of some agarics.