The largest division of the fungus kingdom, with more than 32,000
described species, is the Ascomycota,
or sac fungi. Many wood-rotters, saprobes, powdery mildews,
parasites, and mycorrhizae belong in this division, as do the
morels and truffles. They produce sexual spores inside sac-like
cells called asci
(singular: ascus).
Each ascus usually contains eight spores. The asci
are usually located in a fruit body called an ascoma.
These fruitbodies vary in size from microscopic in many moulds,
to the 20 cm (8 inch) tall fruit bodies of morels. There are
three types:
A cleistothecium
is completely enclosed. Thousands of spores are produced inside
asci
within this fruitbody and released when the outer wall breaks
down. Usually less than 1 mm across, this type is characteristic
of the powdery mildews.
A perithecium
is flask-shaped, usually less than 1mm tall, and with a narrow
opening. It also produces spores inside. Perithecia are often
grouped together in a matrix known as a stroma.
Spores are either shot or squeezed out of the narrow opening.
Fungi of Pyrenomycetes have perithecia. Examples are Xylaria
(Dead Man's Fingers), Claviceps and Ophiostoma.
An apothecium
is open and usually shaped like a cup, saucer, or urn (although
the fruit bodies of morels are also apothecia). When ripe, apothecia
release their spores explosively. In addition to the morels,
Peziza and Helvella species also have apothecia.
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