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annulus

annulus
1. One of a series of concentric rings or bands of varying width and opacity which are formed in the scales of bony fish. Winter rings are often narrower and denser than summer rings. The number of annuli is indicative of the age of a fish.

2. In the fruit bodies of certain agarics, a remnant of the partial veil that adheres as a ring of tissue around the stipe.

3. A ring of cells around the sporangium of some ferns. The walls of these cells become progressively thicker around the circumference, so inducing tension as they dehydrate, eventually causing the rupture of the sporangium (the point of breaking is called the stomium). The lid of the sporangium then curls back, releasing the tension induced by the annulus in a spring-like motion serving to aid spore dispersal.

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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.

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annulus

annulus
1. In the fruit bodies of certain agarics, a remnant of the partial veil that adheres as a ring of tissue around the stipe.

2. A ring of cells around the sporangium of some ferns. The walls of these cells become progressively thicker around the circumference so inducing tension as they dehydrate, eventually causing the rupture of the sporangium. (The point of breaking is called the stomium.) The lid of the sporangium then curls back, releasing the tension induced by the annulus in a spring-like motion serving to aid spore dispersal.

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annulus

annulus One of a series of concentric rings or bands of varying width and opacity which are formed in the scales of bony fish. Winter rings are often narrower and denser than summer rings. The number of annuli is indicative of the age of a fish.

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annulus

an·nu·lus / ˈanyələs/ • n. (pl. -li / -ˌlī/ ) technical a ring-shaped object, structure, or region.

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annulus

annulus (planetary) A ring-like structure used, for example, to describe a discrete ring of ejecta around a martian rampart crater.

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annulus

annulus (an-yoo-lŭs) n. (in anatomy) a circular opening or ring-shaped structure.
annular adj.

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