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apparatus (rods or mycelia), reproductive patterns (fragmentation, formation of vegetative
conidia or sexual sporangiospores) and their tolerance to acid pH levels.
Actinobacteria are usually set apart because of their unusual metabolic capabilities,
e.g., the production of pigments and antibiotics and the assimilation of acetylene. Species
of the genus Frankia form symbiotic, nitrogen-fixing associations with Casuarinales
and a few other higher plants.
2.1.4
FUNGI
Fungi are eukaryotes with a filamentous vegetative structure called a mycelium. Most
are Eumycota; they have chitinous membranes and, in the most common soil fungi, their
reproductive organs do not possess flagellae. Four main groups commonly occur in soils,
viz., Zygomycota, Ascomycota, Basidiomycota and Deuteromycota ( Fungi imperfecti ).
They differ in the structure of their mycelium and reproductive organs. Zygomycota have
a non-segmented mycelium of which only the growing extremes are living. Ascomycota
and Basidiomycota have segmented mycelia with pores in the transverse septa which
allow successive segments to communicate and live cytoplasm to be translocated
towards the growing ends. Sexual reproduction leads to the formation of spores
produced by several different kinds of organs: sporangia, ascini or basidia (Figure III.2).
Another important group in soil is the Deuteromycota (= fungi imperfecti ), which
comprises species with no known sexual stages.
Finally, yeasts (Ascomycota) are unicellular fungi that are normally poorly represented
in the soil system, although they may occasionally play important roles.
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