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{{Short description, A ring of specialized cells on the sporangium
An annulus in botany is an arc or a ring of specialized cells on the
In leptosporangiate ferns, the annulus located on the outer rim of the
In
The Fern Sporangium: A Unique Catapult.
''Science'' 335 (6074): 1322. Plant anatomy
sporangium
A sporangium (; from Late Latin, ) is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. Virtually all plants, fungi, and many other lineages form sporangia at some point in their life cy ...
. These cells are arranged in a single row, and are associated with the release or dispersal of spores.
Ferns
In leptosporangiate ferns, the annulus located on the outer rim of the sporangium
A sporangium (; from Late Latin, ) is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. Virtually all plants, fungi, and many other lineages form sporangia at some point in their life cy ...
and serves in spore dispersal
In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, f ...
. It consists typically of a ring or belt of dead water-filled cells with differentially thickened cell walls that stretches about two-thirds around each sporangium in leptosporangiate ferns
The Polypodiidae, commonly called leptosporangiate ferns, formerly Leptosporangiatae, are one of four subclasses of ferns, and the largest of these, being the largest group of living ferns, including some 11,000 species worldwide. The group has ...
. The thinner walls on the outside allow water to evaporate quickly under dry conditions. This dehiscence causes the cells to shrink and a contraction and straightening of the annulus ring, eventually rupturing the sporangial wall by ripping apart thin-walled lip cells on the opposite side of the sporangium. As more water evaporates, air bubbles form in the cells causing the contracted annulus to snap forward again, thus dislodging and launching the spore
In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, f ...
s away from the plant. The type and position of the annulus is variable (e.g. patch, apical, oblique, or vertical) and can be used to distinguish major groups of leptosporangiate ferns.
Mosses
In moss
Mosses are small, non-vascular flowerless plants in the taxonomic division Bryophyta (, ) '' sensu stricto''. Bryophyta (''sensu lato'', Schimp. 1879) may also refer to the parent group bryophytes, which comprise liverworts, mosses, and hor ...
es, an annulus is a complete ring of cells around the tip of the sporangium, which dissolve to allow the cap to fall off and the spores to be released.
Footnotes
* Noblin et al. (2012The Fern Sporangium: A Unique Catapult.
''Science'' 335 (6074): 1322. Plant anatomy