Fungi are any member of a diverse group of organisms
that—unlike plants and animals—obtain food by absorbing nutrients from an
external source. The fossil record suggests that fungi were present 550 million
years ago and may have evolved even earlier. Today thousands of different types
of fungi grow on and absorb food from substances such as soil, wood, decaying
organic matter, or living plants and other organisms. They range from tiny,
single-celled organisms invisible to the naked eye to some of the largest living
multi-cellular organisms. In Michigan for example, the underground portion of
an individual Armillaria mushroom, a type of fungus, extends more than 12
hectares (30 acres). Other fungi are among the longest-lived organisms on
Earth—some lichens, a living partnership of a fungus and an alga, are thought
to be more than 4,500 years old.
A large and widely distributed group of organisms, fungi
perform activities essential to the functioning of all natural ecosystems. They
are among the foremost decomposers of organic matter, breaking down plant and
animal remains and wastes into their chemical components. As such, fungi play a
critical role in the recycling of minerals and carbon. Fungi’s value to
humankind is inestimable. Certain types of fungi, including several types of
mold, have proven extremely valuable in the synthesis of antibiotics and
hormones used in medicine and of enzymes used in certain manufacturing
processes. Some fungi, such as mushrooms and truffles, are considered tasty
delicacies that enhance a wide variety of recipes. Not all fungi are
beneficial—some damage agricultural crops, cause disease in animals and humans,
and form poisonous toxins in food.
Common fungi include mushrooms, puffballs, truffles, yeasts,
and most mildews, as well as various plant and animal pathogens (disease
agents), such as plant rusts and smuts. Some experts estimate that there are
1.5 million fungus species, of which approximately 100,000 have been
identified. The unique characteristics of fungi led scientists to classify
these important organisms into a separate kingdom, Kingdom Fungi (also known as
Mycetae). Certain fungus-like organisms, such as downy mildews, water molds
(also known as oomycetes), and slime molds, once classified as fungi, are now
placed in the Kingdom Protista.
Fungi lack chlorophyll, the green pigment that enables
plants to make their own food. Consequently, fungi cannot synthesize their own
food the way plants do. In order to feed, fungi release digestive enzymes that
break down food outside their bodies. The fungus then absorbs the dissolved
food through its cell walls.
Depending as they do on outside sources for food, fungi have
developed various living arrangements that enhance their opportunities for food
absorption. Some fungi live as parasites, feeding on living plants, animals,
and even other fungi. Certain fungus parasites injure plants and animals,
causing millions of dollars of damage to farm animals, crops, and trees each
year. For example, the fungus Ophiostoma ulmi, which causes Dutch elm disease,
has killed more than half of the elm trees in North America since it was
accidentally introduced there in the 1930s.
Fungi that obtain their food by breaking down dead organisms
or substances that contain organic compounds, such as starch and cellulose, are
called saprobes or saprophytes. While they are invaluable decomposers of
organic material, saprobes can also cause food spoilage and destroy wood
products. During the American Revolution (1775-1783), more British ships were
destroyed by wood-digesting saprobes than by enemy attack. Some saprobes even
grow in aviation fuels, where they breakdown the fuels, destroying their
usefulness.
Some fungi also form highly specialized relationships with
other organisms (see Symbiosis). For example, the roots of most plants develop
a mutually beneficial association with fungi to form mycorrhizae. Mycorrhizae
greatly increase the nutrient-absorbing capacity of the plant root—the fungus
absorbs minerals from the soil and exchanges them for organic nutrients
synthesized by the plant. Fungi also form mutualistic associations with various
animals. For example, leaf-cutting ants cut pieces of leaves and bring them
into their underground nests, where they feed them to certain fungi. These
fungi primarily live in ant nests, and the ants eat nothing but the fungi. Some
termites and wood-boring beetles use fungi to break down the cellulose in wood,
making the wood easier for the insects to digest. Plant parasites such as rusts
invade plant cells via specialized structures called haustoria that absorb
nutrients from the cell. (Encarta Encyclopedia)
