annulus
annulus
1. (in botany)
a. 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.
b. 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.
2. (in zoology) Any of various ring-shaped structures in animals, such as any of the segments of an earthworm or other annelid.
1. (in botany)
a. 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.
b. 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.
2. (in zoology) Any of various ring-shaped structures in animals, such as any of the segments of an earthworm or other annelid.
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annulus