annulus

noun

(L. annulus: finger ring) (pl. annuli) A ragged ring of tissue that remains on the stalk of a mushroom or toadstool; also called a velum, it is formed from the ruptured membrane that originally covered the lower surface of the cap. Syn. velum. The region of the wall of a fern sporangium that is specialized for spore dispersal; it consists of cells that are thickened except on their outer walls; on drying out, the cells contract and the sporangium ruptures, releasing the spores; the annulus springs back into position when the residual water in the cells vaporizes and any remaining spores are dispersed. In stegocarpous mosses a zone of variously differentiated cells between the capsule urn and operculum, facilitating opening of the capsule. Cf. valve.