Layman D. Biology Demystified: A Self-Teaching Guide

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through the air, many of them falling down onto a neighboring area suitable

as a food source. When the spore lands, it sprouts and starts absorbing nutri-

ents. The spore grows out to create a hypha (HIGH-fah), a thin, thread-like

ﬁlament. The word, hypha, comes from the Greek for ‘‘web.’’ As its literal

meaning suggests, each individual hypha branches repeatedly in a web-like

manner as it grows. And two adjacent hyphae (HIGH-fee) of diﬀerent mating

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Fig. 8.1 The fungi: A kingdom of molds, yeasts, and mushrooms.

Fig. 8.2 The mushroom as a body plan for a fungus.

types can fuse together and branch even more extensively. The result is a

mycelium (my-SEE-lee-um) or ‘‘fungus nail,’’ actually a web-like network

consisting of thousands of branching hyphae or ﬁlaments. The mycelium of

a mushroom is mostly located underground. The mycelium ﬁnally pushes up

above the earth, creating the ﬂeshy fruiting body or reproductive structure we

call a mushroom. Quite surprisingly, what we consider a solid mushroom is

actually an extensive web or network of hyphae (ﬁlaments), packed tightly

together! And this umbrella-shaped mushroom is just the above-ground fruit-

ing body (reproductive structure) of a much more broad and extensive myce-

lium hidden below the surface. [Study suggestion: Buy some fresh whole

mushrooms and slice them thinly. Using a strong magnifying glass, look for

a network of hyphae or slender ﬁlaments within each cap.]

The Major Groups of Fungi

More than 100,000 diﬀerent species of fungi have been identiﬁed, yet mycol-

ogists (my-KAHL-uh-jists) – ‘‘those who specialize in the study of fungi’’

(myc) – usually group all of these species into just four diﬀerent phyla. The

names of these phyla are very tongue-twisting! Nevertheless, a helpful hint is

that each phylum name ends with the suﬃx, -mycetes (my-SEE-teez), which

comes from the Greek for ‘‘fungus’’ (mycet). So, look for and recognize

-mycetes, and you know a fungus is somehow involved!

PHYLUM BASIDIOMYCETES (THE CLUB FUNGI)

A good fungal phylum to start with is the Phylum Basidiomycetes (bah-sid-

e-oh-my-SEE-teez). The Phylum Basidiomycetes includes mushrooms,

puﬀballs, and the shelf fungi often seen growing on the barks of trees.

This particular fungal phylum, then, is quite familiar to those of us who

like to walk through woods and ﬁelds (or even stroll through our own

backyard).

The mushrooms get the ﬁrst part of their phylum name from their basidia

(bah-SID-e-uh) – the ‘‘little bases’’ (basidi) growing out from the gills under

their caps (see Figure 8.3). This group is therefore called the Basidiomycetes

(‘‘little base fungi’’) or club fungi because of the tiny, ‘‘club’’-shaped basidia

present along the edges of their gills. The basidia are important because they

produce the mushroom’s spores, which can be blown oﬀ from under the cap

and distributed long distances by the wind.

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PHYLUM ASCOMYCETES (THE SAC FUNGI)

You might also encounter various members of the Phylum Ascomycetes (as-

koh-my-SEE-teez) or ‘‘leather bag’’ (asco-) ‘‘fungi’’ as you walk through a

forest. This group is commonly called the sac fungi, because they produce

spores within their sac-like or bag-like caps. Consider, for instance, the very

popular morel mushroom, often nicknamed spongy-cap due to the wrinkled,

sponge-like appearance of its cap.

Much less dramatic members of Ascomycetes, however, are the unicellular

yeasts. Yeast cells reproduce by smaller cells budding oﬀ and eventually

separating from larger ones. Yeast belonging to the genus Saccharomyces

(sak-ah-roh-MY-seez) are the main ‘‘sugar’’ (sacchar) ‘‘fungi’’ (myc). This

genus of yeasts has been used to help make bread, beer, and wine since

ancient times. The yeast cells are added to raw bread dough or a vat of

grape juice, and they soon begin to consume lots of sugar molecules in the

dough or juice. Initially, these Saccharomyces organisms operate aerobically,

utilizing oxygen. The yeast cells give oﬀ CO

gas as a waste product, and

these gas bubbles are what make bread dough rise! Later, when the bread is

ﬁnally baked, the yeast cells are killed by the high temperature of the oven.

During winemaking, in contrast, the sealed wine vats become anaerobic,

after the active yeast cells consume all available O

. The yeast cells then

switch their metabolism from aerobic to anaerobic, and carry out the process

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Fig. 8.3 The basidia: ‘‘Little bases’’ or ‘‘clubs’’ on mushroom gills.

of alcoholic fermentation (fer-men-TAY-shun). By this process, yeast cells

produce both carbon dioxide and ethyl alcohol (common drinking alcohol)

as waste products. When the fermenting wine reaches an alcohol content of

about 12–16%, the yeast cells become poisoned and die from too much

alcohol!

Some common types of molds are also considered members of the Phylum

Ascomycetes (sac fungi). In general, a mold is a fuzzy coating of fungus

growing on the surface of some food or animal or plant substances, when

they are decaying or left for too long in a moist, warm place. A familiar

example of such a fuzzy mold is the genus Penicillium (pen-ih-SIL-e-um),

named from the Latin for ‘‘brush.’’ The Penicillium fungi are bluish-colored

molds growing on bread, fruits, and cheeses. They have a somewhat brush-

like appearance under the light microscope, and several Penicillium species

produce penicillin, the powerful, bacteria-killing, antibiotic drug.

PHYLUM ZYGOMYCETES (THE ZYGOTE FUNGI)

A third group of fungi are those of the Phylum Zygomycetes (zeye-go-my-

SEE-teez), literally the ‘‘yoked together’’ (zygo-) ‘‘fungi.’’ This phylum of

fungi derives its name from the inclusion of a zygote (ZEYE-goat) within

its life cycle. A zygote consists of two sex cells, called gametes (GAM-eats),

that are literally fused, yoked, or ‘‘married’’ (gamet) together during fertiliza-

tion.

The so-called zygote fungi in this group utilize sexual reproduction – the

fusion of two gametes together to create a zygote. Once formed, the zygote

then divides repeatedly by the process of mitosis (Chapter 5), thereby creating

a new adult organism. The zygotes are enclosed in thick-walled zygospores

(ZEYE-go-spoors), which are shed and sent wafting out through the air to

new locations.

A typical zygomycetic (zeye-go-my-SEE-tick) fungus is the black bread

mold, genus Rhizopus (RYE-zuh-pus) – ‘‘root’’ (rhiz) ‘‘feet’’ (pus).

Interestingly enough, like many fungi, the Rhizopus group can engage in

both sexual reproduction (gametes uniting to form zygotes), as well as asex-

ual (AY-sex-you-al) reproduction that occurs ‘‘without’’ (a-) gametes. For

simplicity, just the asexual reproduction of the black bread mold is displayed

in Figure 8.4.

A spore lands on a piece of white bread, then begins to germinate (JER-

muh-nayt). The landed spore ‘‘sprouts’’ a number of slender, thread-like

hyphae, which soon merge to form a white, extensively branching mycelium,

deep inside the bread slice. Soon, a large number of round-topped sporangia

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(spoh-RAN-jee-ah) or ‘‘seed’’ (spor) ‘‘vessels’’ (angi), appear like tiny black

puﬀballs. The round, black sporangia are attached to the bread surface by

long, narrow stalks. The mold is called Rhizopoda, because the sporangia

and their stalks are ﬁrmly anchored into the bread surface by means of

rhizoids (RYE-zoyds) – blunt, ‘‘root-resembling’’ or ‘‘foot-resembling’’ pro-

jections. When the wind blows, masses of black spores are scattered from the

rounded sporangia, which hold thousands of them. A spore may land on

another piece of bread, and the life cycle of the mold begins anew.

PHYLUM CHYTRIDIOMYCETES (WATER-DWELLING

FUNGI)

A fourth phylum of fungi is the Phylum Chytridiomycetes (KIH-trid-e-oh-my-

SEE-teez) or ‘‘water-dwelling’’ (chytrid) ‘‘fungi.’’ The members of this group

with an extremely tongue-twisting name are more easily called chytrids (KIT-

rids), because they are mainly ‘‘water-dwellers.’’

Recent evidence suggests that the chytrids are the most primitive fungi,

and that they were the ﬁrst type to evolve from protists having ﬂagella.

CHAPTER 8 Fungi: Not Just Mushrooms 131

Fig. 8.4 The asexual life cycle of Rhizopus or black bread mold.

Dwelling in an aqueous (watery) environment, each chytrid has a small,

globe-shaped body. It produces a highly active spore which has a ﬂagellum

attached. In their adult stage, the chytrids make a large mycelium (network)

consisting of an extensive tangle of slender hyphae. These highly branched

hyphae create a large area for the easy absorption of nutrients dissolved in

the surrounding water.

Summary diagram of the fungi

Figure 8.5 provides summary pictures and brief descriptions of each of the

four major phyla of fungi we have been studying.

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Fig. 8.5 A summary of the four major phyla of fungi.

The Lichens: Where Fungi Mix with Algae

Another unusual group closely related to the fungi are the lichens (LIE-kens).

This term derives from ancient Greek and originally meant, ‘‘that which licks

or eats around itself.’’ Lichens are mixtures of fungi with either algae or

bluish-green bacteria, living together in symbiosis (Chapter 7), such that

each type of organism beneﬁts.

Because they contain photosynthetic algae that produce chlorophyll,

lichens may look like greenish moss growing on a rock or tree trunk.

Lichens are not green plants or really any other type of single organism.

Lichens are constructed somewhat like a tough, stale, blueberry pie containing

two diﬀerent types of organisms (Figure 8.6). Like a pie, there is a protective

upper and lower crust of slender, tightly packed hyphae (the fungus partner).

As in the fruiting body (cap and stem) of a mushroom, the middle of the lichen

consists of a mycelium – a loose network of highly branched, thread-like

hyphae. Finally, like thousands of blueberries baked into a pie, small round

algae are inserted into the meshwork of the fungus mycelium.

This relationship is symbiotic (sim-beye-AHT-ik) because the fungal crusts

are tough enough to protect the contained algae from extremes of climate

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2, Web

Fig. 8.6 The lichen as a slice of blueberry pie.

and temperature. The fungus also releases enzymes that partially digest rock

or other hard surfaces, allowing nutrients to be absorbed by the lichen. In

return, the algae produce additional energy for their fungal partners by

carrying out photosynthesis. Thus, lichens make an extremely tough duo

of cooperating organisms that, working together, can survive in some of

the harshest habitats on planet Earth. Brave hikers can see them tightly

clinging to rocks at the tops of frozen mountains, or even thriving in the

empty wasteland of Antarctica!

Parasitic Fungi and Their Victims

Since certain fungi are parasites, it is only to be expected that some will cause

the disease or death of their infected hosts. And those that are potentially

pathogenic are nearly impossible to escape, because fungal spores ﬂoat in the

air, almost everywhere! [Study suggestion: Take the lid oﬀ an unwanted jar of

jelly and leave it open. Soon, you will see the surface completely covered with

a fuzzy substance. To what particular phylum in the Kingdom Fungi, do you

think this fuzzy coating belongs? Its spores may have been produced and

shed many miles away from you!]

FUNGAL PARASITES OF PLANTS

Most of the parasitic fungi attack plants. Dutch elm disease, for instance, is

due to a sac fungus (Phylum Ascomycetes) that was accidentally brought into

the United States by infected logs from Europe during World War I. (The

disease gets its Dutch name from the fact that it was ﬁrst observed in Holland

in 1919.) Spread from tree to tree by bark beetles, Dutch elm disease has

eliminated thousands of native elm trees across the Continental U.S.

Every year, grain farmers lose billions of dollars of crops to fungal infec-

tions. One grain infection with especially important implications for human

health is ergot (ER-gaht), a small black sac fungus in the seed heads of wheat

or rye plants. If any unwary humans or farm animals are so unfortunate as to

eat such infected grain plants, they may come down with ergotism (ER-goh-

tizm). During the Middle Ages in Europe, ergotism was quite common and

was given the alternate name, St. Anthony’s ﬁre. People ate a diet heavy with

rye and other natural grains baked in their bread. Although baking killed the

fungus, it simply released its toxins into the bread. Such poisoning results in

vomiting, cramps, great thirst, and burning in the abdomen. (Hence the old

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1, B-Web

2, B-Web

name, St. Anthony’s ﬁre.) Happily, modern grain processing techniques clean

the ergot fungus from wheat and rye, so that ergotism is rare.

FUNGAL PARASITES OF HUMANS

Records show that when patients go to a hospital and accidentally acquire an

infection, it is more likely to be a fungal infection than one of bacteria or

viruses! And the oﬀending parasite is most likely to be Candida (KAN-dih-

dah) albicans (AL-bih-kans), a ‘‘glowing white’’ (candid) fungus. A close

relative of the yeasts, Candida albicans is a common inhabitant of the

human mouth, skin, intestine, and vagina.

Candidiasis (kan-dih-DIE-ah-sis), or an ‘‘infestation with’’ (-iasis) Candida

fungus, tends to occur whenever the number of these fungi in the human

body becomes very excessive. This may happen when a person is given a

long-term regimen of antibiotics or steroid drugs, which depresses the activity

of the immune (self-defense) system. Whenever too many competing organ-

isms (such as bacteria) are killed oﬀ, or whenever the immune system

becomes too weak, this leaves the door wide open for Candida albicans to

overpopulate part of the body and cause a problem. Scattered white patches

appear in infested areas, such as the female vagina, along with an inﬂamma-

tion. This is a common cause of vaginitis (vaj-in-I-tis) – ‘‘inﬂammation of the

vagina’’ – in women of reproductive age.

Quiz

Refer to the text in this chapter if necessary. A good score is at least 8 correct

answers out of these 10 questions. The answers are found in the back of this

book.

1. Saprophytic fungi:

(a) Absorb small molecules from the cells of living hosts

(b) Are not common enough to be worth studying

(d) Are named from their habit of feeding upon the sap leaking from

injured trees

2. A key diﬀerence between fungi and slime molds:

(a) Some fungi are parasites of living organisms

(b) Most slime molds act as parasites of living hosts

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3, Disorder

(d) There are far more species of slime molds, than of fungi

3. The entire group of unicellular fungi:

(a) Molds

(b) Yeasts

(d) Lichens

4. The fruiting body of a mushroom:

(a) Its extensive underground mycelium

(b) Spores released into the air

(d) The sweet, cool part that always tastes like an apple!

5. The fungal phylum to which morel mushrooms belong:

(a) Zygomycetes

(b) Basidiomycetes

(d) Ascomycetes

6. The basidia are important to a mushroom, since they:

(a) Directly absorb nutrient molecules from decaying organic matter

(b) Produce and hold its spores

(d) Allow the fruiting body to be eﬃciently removed

7. Phylum of fungi involving the union of two gametes:

(a) Ascomycetes

(b) Chytridiomycetes

(d) Basidiomycetes

8. Probably the most primitive group of fungi, and the ﬁrst to evolve from

protists:

(a) The chytrids

(b) Ascomycetes

(d) Bread yeasts

9. Penicillium:

(a) Releases thick-walled zygospores

(b) A bluish-colored genus of mold that produces powerful

antibacterial agents

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