Annulus

Annulus [ Botany ]	Dictionary of botanic terminology - index of names
Synonyms: aaa Adjective: aaa Adverbbs: aaa

In geometry:

An annulus is the portion of a plane bounded by two concentric circles in the plane.

In anatomical terminology:

An annulus is a tissue structure forming a ring or arranged in a circle.

In Asclepiadaceae anathomy:

An annolus or corona is the raised inner rim around the centre of a flowers corolla.

The annulus is a projecting central circle found in the flowers of some genera of the Asclepiadaceae family (e.g. Orbea, Huernia, Orbeanthus and Duvalia)
The asclepiads flowers has a pedicle ending with five sepals and a five segmented corolla (Usually it is 5-lobed, but sometimes we can find flowers with more or fewer free lobes) this segment are basally fused to form the annulus a raised shiny circular structure around the base of the gynostegium, it might have some prominent hairs.

Left: floral structure of Huernia

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