Biology Reference
In-Depth Information
Fungi
Fungi are organisms having no chlorophyll,
reproducing by sexual and asexual spores, not
by fission like bacteria, and typically possessing
a mycelium or mass of interwoven threads
(hyphae) containing well-marked nuclei.
According to Hawksworth (1991), there are
about 4,300 valid genera, and many more that
are synonyms, and about 70,000 species living
as parasites or saprophytes on other organisms or
their residues. More than 8,000 species cause
plants disease. Fungi are divided into three king-
doms and eleven phyla.
Kingdom: Fungi
Phylum: Chytridiomycota
Phylum: Zygomycota
Phylum: Ascomycota
Phylum: Basidiomycota
Kingdom: Stramenopila
Phylum: Oomycota
Phylum: Hyphochytriomycota
Phylum: Labyrinthulomycota (slime molds)
Kingdom: Protists
Phylum: Plasmodiophoromycota (endoparasitic
slime molds)
Phylum: Dictyosteliomycota (Dictyostelid slime
molds)
Phylum: Acrasiomycota (Acrasid slime molds)
Phylum: Myxomycota (true slime molds)
Oomycetes, Zygomycetes and
Chytridiomycetes were formerly listed as
subclasses within the class Phycomycetes.
Oomycota, Zygomycota and Chytridiomycota
are now generally accepted as separate phyla
of fungi. The mycelium of these three phyla has
many nuclei which are not marked off by cross-
wells (or nonseptate mycelium) except where
reproductive structures arise, a condition known
as coenocytic. Asexual reproduction is by means
of spores borne in sacs called sporangia. The
Zygomycota have sexual spores called zygo-
spores which are formed by the union of two
similar sex cells or gametes; the Oomycota have
sexual spores called oospores formed from dis-
similar gametes; the Chytriodiomycota have nei-
ther type of sexual spore; the Ascomycota have
septate mycelium and sexual spores in asci; the
Basidiomycota have septate mycelium, fre-
quently with clamp connections, and sexual
spores; the Myxomycota have thalli as a motile
mass of protoplasm (a plasmodium or
myxamoeba - no mycelium) which is
transformed into a mass of small, aseptate
resting spores that on germination form motile
cells with or without flagella. The Myxomycota
include protists with amoeboid thalli and their
status as fungi often has been questioned. The
thalli of the Myxomycota are naked, amoeboid,
plasmotic masses without cell walls and are
termed plasmodia or pseudoplasmodia. They are
also able to move by the formation of pseudopo-
dia and by plasma-streaming. The Plasmodio-
phoromycetes is the only class of the
Myxomycota which includes parasites of vascu-
lar plants. The best known species is
Plasmodiophora brassicae , which causes “club
root” of cabbage.
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