BACTERIA AND OTHER MICROORGANISMS OF THE SOIL - The Soil - page 141

Marasmius oreades is an example of a fungus which causes 'fairy rings' on
lawns or pastures. The fungal hyphae grow out radially from a point where a colony
of the fungus has become established and, eventually, they become manifest in a ring
of deeper coloured grass. As the years go by, this circle of mycelium grows steadily
wider, sometimes at a rate of several inches per year. Several other species of fungi
also develop rings of growth in this way. It is possible that the darker green colour
of the grass is caused by some enrichment of the soil in nitrogen compounds where
fungal growth has occurred. A bare circular area in the soil surface, bordering the
darker green grass can be the result of the grass roots being covered by the fungal my-
celia. Basidiomycetes frequently produce mycelia which can remain viable for long
periods and so, when they have colonized some substrate or other, may still continue
growing towards other substrates some distance away. Fairy ring type basidiomycetes
tend to migrate in pasture soil in this manner ( Figure 48 ).
Some fungal species that belong to the family Zoopagaceae are predators of
nematodes, for example Arthrobotrys oligospora; there are others which feed on
amoebae. Trapping of nematode worms in a ring of fungal hyphae is quite ingenious.
Some animal-trapping fungi possess interwoven hyphae to which the nematodes ad-
here; the fungus then dissolves the nematode cuticle, drives hyphae into the nemat-
ode's body and absorbs the internal organs. In the fungus Dactylaria brochophaga ,
collar-like outgrowths are produced out of three cells of the fungal hypha and develop
into a constricting ring trap. When a nematode passes its head into the opening of the
ring, the cells are stimulated and swell rapidly thus tightly gripping the nematode. The
action is very quick: the nematode is trapped, dies, and its body contents are absorbed
by the fungus.
Yeasts are generally considered as fungi and they occur frequently in soil. They
are oval or spherical unicellular organisms, mostly about 5 to 10 microns in diamet-
er. Usually they reproduce by a budding process; a small outgrowth develops on the
periphery of the cell, it continues to grow until it is large enough to separate from the
mother cell and then becomes a new, independent yeast organism. Many yeasts also
reproduce by a sexual process. Some yeasts grow filamentous hyphae and produce
mycelia, just as do moulds. There are also yeasts, for example Rhodotorula species,
which are capable of fixing atmospheric nitrogen. Although, in general, yeast and
fungi flourish best under aerobic conditions, fermentations by yeasts proceed an-aer-
obically. Bakers' yeast and beer yeasts are different races or varieties of the species
Saccharomyces cerevisiae. Both fungi and yeasts can usually tolerate a fairly wide
range of pH values.