Biology Reference
In-Depth Information
Monoblepharidales
Chytridiomycota
Saprophytes in water, most of which grow on
submerged twigs and fruit;
The thalli are usually vesicular, occasionally fil-
amentous, and are transformed to sporangia,
gametangia or resting spores. The
Chytridiomycetes are the only members of the
kingdom Fungi that produce motile cells. Motile
cells may function as zoospores, or as gametes,
are radially symmetrical, with a single, posteri-
orly directed whip-lash type flagellum. The
Chytridiomycetes are the only class in this
phylum.
thallus of much-
branched delicate hyphae.
Plasmodiophorales
The placement of this order has always been
uncertain. Some put it with the Myxomycetes,
the slime molds, others between the Myxomy-
cetes and the true fungi. Some have considered
it a family in the Chytridiales. This single class is
placed in the kingdom, Protists, phylum,
Plasmodiophoromycota which contains a single
class, Plasmodiophoromycetes. There is also
only one family, Plasmodiophoraceae, in this
order. Parasitic, assimilative phase a
multinucleate thallus within host cells, chiefly
of vascular plants, often causing hypertrophy;
germinating in place by amoeboid, occasionally
uniciliate, zoospores.
Plasmodiophoraceae
Chytridiales
This order is defined on the basis of zoospore
ultrastructure. Most members are water-or soil-
inhabiting fungi; many of the former are parasitic
on algae and water molds, many of the latter on
vascular plants. A few parasitize animal eggs and
protozoa while others are saprobic on the
decaying remains of dead plants. Simple fungi
which have almost no mycelium, the thallus at
maturity acting as a single sporangium, or divid-
ing to become a sorus of sporangia; zoospores
posteriorly uniflagellate.
The only family in this
order
but with
two
important
genera:
Plasmodiophora
,
causing
club
root,
and
Spongospora
, causing potato scab.
Oomycota
The thalli may be vesicular, often irregular, but
are usually filamentous. Sporangia on germina-
tion release biflagellate zoospores. One flagellum
is the whip-lash type and the other the tinsel type.
Motile sex cells are absent. Sessile gametangial
cells conjugate and form an oogonium containing
one or several egg cells (see
Fig. 1
).
The Oomycota are related to autotrophic algae
with similar characters. The Oomycetes are the
only class in this phylum; however, the small
classes Hyphochytriomycetes and Labrinthu-
lomycetes may also be included.
Spizellomycetales
Members of this order are diverse and include
plant and fungal parasites and free-living
saprobes inhabiting both soil and water. There is
great morphological variation in the group, and
examples of both endogenous and exogenous
development. Most species are monocentric.
Blastocladiales
Saprophytes in water or soil; genera are charac-
terized by thick-walled, resistant sporangia, usu-
ally with pitted walls. Another feature is the
prominent membrane-bound nuclear cap present
in zoospores and planogametes.
Hyphochytriales
Zoospores anteriorly uniflagellate, usually
formed outside the sporangium. The order
