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ascus

(pl. asci)

Terms discussed: ascocarp (pl. ascocarps), ascomycete (pl. ascomycetes), ascomycetous, bitunicate, dehiscence, fissitunicate, inoperculate, operculate, operculum (pl. operculi), prototunicate, unitunicate


Topics:
asci
dehiscence
operculate, operculum, inoperculate
bitunicate, fissitunicate, prototunicate, unitunicate

See Also:
hymenium


       

asci



Image of Ascomycota from Eugen Gramberg (1913) Pilze unserer Heimat
Ascomycota
Some fungi produce their sexual spores in long, sausage-shaped sacs called asci. These fungus are called ascomycetes and are placed in the phylum Ascomycota. There are usually eight spores to an ascus. This is achieved by the ascus generating four sex cells by the normal process of meiosis, and then each of those four cells splitting. There are some asci, though, that produce many more spores than that (thousands, in a few cases), and I'm not sure how that works. Also, in some cases (see the next picture) the spores themselves are septate, containing more than one cell; in that case, I believe that all the cells in one spore are clones of one another.

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dehiscence



Image of Antennaria from Christian Gottfried Daniel Nees von Esenbeck & A. C. F. Henry (1837) Das System der Pilze: part one
Antennaria
When an ascus releases its spores, it dehisces, popping all the spores out at the same time. Dehiscence is sometimes used as a term for other sorts of "breaking open and releasing spores", like the rupture of puffballs.

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Photo of Sarcoscypha occidentalis by John Denk
Sarcoscypha occidentalis
In the larger Discomycetes such as the morels and cup fungi, not only does a single ascus expel all its spores at once, but generally all (or most) of the asci on a fruiting body dehisce at the same time, creating a smoky cloud of spores hanging in the air over the mushroom. A sudden, slight change in light, moisture, or air pressure often sets them off - - blowing a slight puff of air on the fruiting body often does the trick.

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operculate, operculum, inoperculate


Some asci have a "lid" that can be seen under the microscope in both an opened and unopened state; this type of ascus is called operculate and the ascus lid is called an operculum. In the operculate ascomycetes, the spores are blown out through the opening. In the inoperculate ascomycetes, the tip of the ascus usually has a small pore which is stuffed with loose material similar to that of the ascus wall. The spores are discharged through this pore. See below for some other types of dehiscence.

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bitunicate, fissitunicate, prototunicate, unitunicate


These terms refer to the walls of the ascus: an ascus with a clearly differentiated inner and outer wall is called bitunicate, one with only one wall is called unitunicate. These two terms are also applicable to other walled structures such as spores. A fissitunicate ascus is a term distinguished by some people from bitunicate to refer to an ascus where the inner wall pops completely out of the outer wall during dehiscence. A prototunicate ascus is a thin-walled ascus that releases its spores by deliquescing, rather than by rupture.

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