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Lecture Outline
Introduction
Plants and Fungi—A Beneficial Partnership
A. The lives of modern plants and fungi are intertwined.
1. We depend on plants and, indirectly, fungi for much of our food.
2. Plants are often harmed by fungi.
3. On the other hand, mycorrhizal fungi aid nearly all plants in the wild.
B. Mycorrhizae are rootlike structures made of both fungi and plants. The fungi help plants obtain
nutrients and water, and they protect plant roots from parasites, in exchange for food the plants
make by photosynthesis.
Preview: Mycorrhizae are also discussed in Module 32.11.
1. Modern agricultural practices, such as killing the parasitic fungi with fungicides, may
disrupt mycorrhizal fungi, forcing the need for fertilizer.
2. Some farmers try to reduce the effects of the addition of fungicides and alternate fungicides
by adding mycorrhizal fungus.
C. Plants and fungi evolved together as life moved onto land. This observation is supported by the
presence of mycorrhizae in the earliest plant fossils.
I. Plant Evolution and Diversity
Module 17.1 Plants evolved from green algae.
A. Plants may have evolved from green algae about 500 mya. The ancestor is not known, but
analysis of nucleic acid sequences, cell structure, and biochemistry indicate that a group of
green algae, the charophyceans (Figure 17.1A, a simple charophycean), and plants shared a
common ancestor (Figure 17.1B, a complex charophycean).
B. The oldest plant fossils indicate that the correct adaptations necessary for life on land appeared
about 475 mya. Early plants thrived on moist shorelines where space was essentially limitless,
sunlight was unlimited, and CO2 was abundant. Initially there were few, if any, herbivorous
animals and plant pathogens.
Module 17.2 Plants have adaptations for life on land.
A. Review: In the two-kingdom system, Linnaeus classified algae as plants. In the five-kingdom
system, algae are protists (Module 16.18). Plants (like trees, grass, and flowers) are in the
kingdom Plantae in either system.
B. The definition of plants as multicellular, eukaryotic photosynthesizers also describes
multicellular algae (Modules 16.22 and 16.24).
C. Multicellular algae, such as sea lettuce, are the most complex algae and are adapted for life in
water, while plants are adapted for life on land (Figure 17.2A). For the multicellular algae, the
resources, including water, carbon dioxide, and minerals, are in direct contact with their tissues,
and waste products can be washed away. Water supports and suspends the body of the alga.
Holdfasts anchor the alga, and all other parts can be photosynthetic. Water provides a means of
dispersal for gametes and offspring.
D. Life on land imposes problems. Water and nutrients are concentrated in the ground, while
carbon dioxide and light are most abundant above the ground. Air provides no support against
the force of gravity and will dry out reproductive cells.
E. Adaptations to a terrestrial environment in plants include the following:
1. Obtaining resources from two locations: Plants have three organs (roots, stems, and leaves)
that are involved with resource acquisition. Most water and nutrients are obtained from the
soil via the roots (in association with mycorrhizae). Roots also provide a means to anchor
the plant to the ground. Stems support the leaves that obtain CO2 and light for
photosynthesis. Growth of both the roots and shoots (stems) occurs at the apical meristems.
Vascular tissues are used to distribute water and nutrients throughout the plant (Figure
17.2B). Xylem, which is made of dead cells, transports water and minerals up from the
roots. Phloem, which is made of living cells, transports sugars.
2. Supporting the plant body: Plants contain a chemical called lignin, which thickens and
strengthens the cell walls of vascular tissues that are involved in supporting the plant
structure.
3. Maintaining moisture: In an effort to reduce moisture loss, a waxy coating called a cuticle
covers the leaves and stems. Gases cannot easily pass through the cuticle, so small pores
called stomata (singular, stoma) are used for the exchange of CO2 and dioxygen. Two cells
regulate the closing and opening of the stomata, open during the day and closed at night to
reduce water loss.
4. Reproducing on land: Desiccation and dispersal are two key elements that must be
overcome by land plants. Gametangia (singular, gametangium) are protective structures of
cells that contain the eggs and prevent drying out. The sperm either swim to the egg through
a thin film of water or are brought to the egg in the form of pollen. Once fertilized, the
developing embryo remains attached to the parent plant for nourishment. Thus plants are
classified as embryophytes. All plants alternate between two life cycles, haploid and
diploid. Some plants produce seeds and rely on the wind or animals for dispersal. Some
plants produce structures called sporangia (singular, sporangium), which contain haploid
cells called a spores. A spore can produce a new plant without being fertilized.
F. A functional definition of plants is a multicellular, photosynthetic eukaryote with multiple
adaptations for land including apical meristems, dependent embryos, multicellular gametangia,
alternating haploid and diploid generations, and spores produced in sporangia.
Module 17.3 Plant diversity provides clues to the evolutionary history of the plant kingdom.
A. Plant phylogeny shows branching of several plant lineages, reflecting major evolutionary steps.
These are previewed here, and details are added in the modules that follow (Figure 17.3A).
B. The first lineage of plants arose, most likely from ancestral charophyceans, approximately 475
mya. This lineage gave rise to modern bryophytes, a group of plants that includes liverworts,
hornworts, and mosses. The moss lineage most likely gave rise to vascular plants.
C. Like other plants, bryophytes have a cuticle and embryos that develop within gametangia.
However, many bryophytes lack vascular tissue for water conduction and internal support. Like
their algal ancestors, bryophytes have flagellated sperm and depend on water for reproduction.
The bryophytes are grouped together as nonvascular plants but are not one clade (Figure
17.3B).
D. Vascular plants originated approximately 420 mya from the moss lineage. The development of
lignin allowed the plants to stand upright. There are two clades that are informally grouped as
seedless vascular plants and include the lycophytes (club mosses, Figure 17.3C) and the
widespread pterophytes (ferns, Figure 17.3C).
E. Ferns are a modern example of a seedless vascular plant and use flagellated sperm to fertilize
the egg and a haploid spore for dispersal. The leaves (fronds) develop from stems that grow
along the ground (Figure 17.3C). Ferns are found in shady temperate forests and are most
diverse in the tropics.
F. Seed plants evolved around 360 mya and today make up more than 90% of all living plant
species. There are two adaptations responsible for the success of this group.
1. Seeds that provide the embryonic plant with a protective coating and a food supply. Seeds
promote dispersal to diverse habitats without drying out.
2. These plants also produce pollen, vehicles that transfer nonflagellated sperm to the female
parts of plants.
G. The earliest seed plants to appear were the gymnosperms (naked seed). The name of these
plants is based on their seeds not being contained within a specialized chamber. The largest
living group of gymnosperms is the conifers (cone bearing trees) such as pine, spruce, and fir.
Other gymnosperms include the cycads and ginkgo tree.
H. Flowering plants (angiosperms) evolved from the seed plant line approximately 140 mya.
Flowers are complex reproductive structures that develop a seed in a protective chamber. The
majority of modern plant species (257,000) are angiosperms.
I.
Summary of land plant adaptations:
1. Dependent embryos
2. Vascular tissue with lignin
3. Production of seeds
4. Reproductive structures called flowers
Preview: Angiosperm structure is discussed in detail in Chapter 31.
II. Alternation of Generations and Plant Life Cycles
Module 17.4 Haploid and diploid generations alternate in plant life cycles.
A. Review: Ulva life cycle (Figure 16.24C); human life cycle (Figure 8.13).
B. Plant life cycles alternate between multicellular, diploid (2n), and haploid (n) adults, unlike
animals that are diploid after fertilization and as adults produce single-celled haploid gametes
(Figure 17.4). The alternating life cycle of plants is referred to as an alternation of generations.
C. A gametophyte is the haploid generation of plants and produces gametes (sperm and eggs) by
mitosis. Fertilization of the egg by sperm results in a diploid plant.
D. A sporophyte is the diploid generation of plants and produces spores by meiosis.
E. Some algae have an alternating life cycle (ulva); however, the closest ancestor to plants (the
charophyceans) does not. It is, therefore, thought that plants developed this trait independently.
NOTE: Compare the mosses (dominant gametophytes, semiparasitic sporophytes, flagellated
sperm) to the conifers or flowering plants (dominant sporophytes, semiparasitic, reduced
gametophytes, pollen).
Module 17.5 Mosses have a dominant gametophyte.
A. The green growth we see consists mostly of gametophytes (Figure 17.3B).
NOTE: Adult gametophytes consist of nonvascular stems and leaves and threadlike holdfasts.
For water and minerals, they depend on rain or on water flowing along the stem from the
ground surface.
B. The moss life cycle is a five-step process (Figure 17.5):
1. Gametophytes produce sperm and eggs in male and female gametangia on separate plants.
Flagellated sperm require a film of water in which to swim to female gametangia where
fertilization takes place.
2. The zygote remains in the female gametangium.
3. The zygote remains attached and, dividing by mitosis, develops into a sporophyte.
4. Meiosis occurs in the sporangium and produces haploid spores at the tip of the sporophyte
stalk. Spores are released from the sporangium.
5. The spores develop into haploid gametophyte plants by mitosis.
NOTE: Adult sporophytes lose their chlorophyll and become more dependent on the female
gametophytes. The sporangia are adapted to dispersing spores by means of movable teeth
around the sporangium opening, on wet/dry cycles.
Module 17.6 Ferns, like most plants, have a dominant sporophyte.
NOTE: Ferns have underground stems with small roots, and all parts are vascularized. The
fronds are the principal organs of photosynthesis.
A. The fern fronds we see are sporophytes. The five-step life cycle of a fern is similar to a moss
(Figure 17.6):
1. Gametophytes produce flagellated sperm and eggs (like mosses). The sperm need a thin
film of water to reach the egg.
2. Like mosses, fern zygotes remain in the female gametangium.
3. The zygote develops into an independent sporophyte.
4. The sporangia in clusters on the frond’s underside (yellow dots) produce haploid spores by
meiosis.
5. Spores are dispersed by wind and grow into small, heart-shaped gametophytes by mitosis.
NOTE: Fern sporangia have a unique mechanism to release their spores. As the sporangia
dry out, thick-walled cells along the “backs” of the sporangia pull the thin-walled “front”
cells into tension. In a fraction of a second, the front cells rupture, scattering the spores into
the air.
B. Today, about 95% of all plants (including seed plants) have a dominant sporophyte generation.
C. Seeded plants (as sporophytes) house the gametophyte and all reproductive stages (spores, eggs,
sperm, zygote, and embryo).
Module 17.7 Seedless plants dominated vast “coal forests.”
A. Two clades of seedless vascular plants once where the dominant plant on Earth. The mosses
and ferns, collectively called the lycophytes, are illustrated in the painting in Figure 17.7.
B. During the Carboniferous period (360 to 290 mya) vast forests grew in tropical, swampy areas
over what are now Eurasia and North America.
C. The forests included dominant lycophytes (giant woody trees) and horsetails (both seedless
plants more primitive than ferns), and tree ferns (not seen in the figure).
D. The compressed remains of these forests survive as deposits of coal and other fossil fuels. Coal
was pivotal to the industrial revolution and is still used today to generate electricity. The vast
Carboniferous forest removed large quantities of CO2, causing global cooling. Now we are
adding the CO2 back to the atmosphere by burning fossil fuels, causing global warming.
E. At the end of the Carboniferous period, the climate turned cooler and drier, providing the
conditions for further plant evolution. The evolution of pollen was key to the adaptation of seed
plants to dry land.
F. The most successful of the early seed plants were the gymnosperms. They grew alongside
seedless plants in Carboniferous swamps. Gymnosperms survived the drying of these swamps
and became the dominant type of plant. Gymnosperms (particularly the cycads) became the
main food for herbivorous dinosaurs starting in the Mesozoic era.
G. Conifers (cone-bearing plants) are the most successful of several groups of gymnosperms
because they are adapted to dry-cool climates. Conifers have needle-like leaves with a small
surface area and a thick cuticle, which help them resist drying. Most conifers do not shed their
leaves and, thus, are able to photosynthesize year round.
Module 17.8 A pine tree is a sporophyte with tiny gametophytes in its cones.
A. Cones are a significant adaptation to land, protecting all reproductive structures: sporangia,
microscopic gametophytes, and zygotes. The seven steps of the pine life cycle are presented in
Figure 17.8.
1. Female cones are larger. Scales bear a pair of ovules (sporangia with a covering, or
integument). Within the ovule, one of the four products of meiosis develops into a tiny,
multicellular, “female” gametophyte.
2. Male cones are smaller, and scales bear many sporangia that make spores by meiosis.
Spores develop into tiny male gametophytes (pollen grains), each consisting of a few cells.
Pollen grains house the cells that will develop into sperm.
NOTE: The particular pollen grains in Figure 17.8 are typical of the pine family: The
central cell and cells within it compose the gametophyte. The two lateral “cells” are
outgrowths of the cell wall that function as floats.
3. Pollen is dispersed by wind. Pollination occurs when a pollen grain lands near an ovule and
grows into it.
4. Meiosis occurs in the ovule where a haploid spore cell begins to develop into the female
gametophyte. Months later, a few of the female gametophyte’s cells inside the ovule
function as eggs.
5. The pollen tube delivers a male gametophyte’s nucleus (functioning as a nonflagellated
sperm) to fertilize an egg. Fertilization occurs a year or more after pollination.
6. The zygote develops into the embryonic sporophyte, and the remaining ovule develops into
the other parts of the seed (the integument becomes the seed coat, and the female
gametophyte becomes a food supply).
7. Seed dispersal is by wind or animal. Under favorable conditions, the seed germinates and
the embryo grows into a tree.
B. All the reproductive stages are held in the cone, and the development of the ovule is the key
adaptation. An ovule is a protective device for all the female steps and is the site of pollination,
fertilization, and embryonic development. The ovule eventually becomes a seed.
Module 17.9 The flower is the centerpiece of angiosperm reproduction.
Preview: The life cycle of the flowering plant (Modules 31.9–31.15).
A. Flowers expose an angiosperm’s sexual parts and are the sites for pollination and fertilization
(Figure 17.9A).
B. Flowers are short stems with the following modified leaves: sepals, to protect flower buds;
petals, to attract animal pollinators; stamens, the male parts (holding the anthers, in which
pollen develops); carpels, the female parts (consisting of sticky stigma to trap pollen; and the
ovary, which bears ovules and later develops into fruit) (Figure 17.9B).
Module 17.10 The angiosperm plant is a sporophyte with gametophytes in its flowers (Figure 17.10).
A. The life cycle of the angiosperm is very similar to that of the gymnosperm (review Figure 17.8).
The main plant is the sporophyte, and the gametophytes live on the sporophyte. Unlike
gymnosperms, which have cones, angiosperms have flowers and package the seeds in fruit.
B. Flowers protect all microscopic reproductive structures: sporangia, male and female
gametophytes, and zygotes. The seven steps of the angiosperm life cycle are presented in Figure
17.10.
1. Meiosis in the anthers leads to haploid spores that undergo mitosis and develop into the
male gametophytes (pollen grains).
2. Meiosis in the ovules leads to haploid spores that undergo mitosis and develop into the
female gametophytes, each of which produces an egg inside the ovules.
3. Pollen is dispersed by either wind or insects. Pollination occurs when a pollen grain lands
on the stigma. The pollen grows into the ovary and delivers one nucleus (again functioning
as a nonflagellated sperm) to the egg nucleus in the ovule.
4. Fertilization usually occurs after pollination to form a zygote.
5. Following fertilization, the zygote develops into the embryonic sporophyte in the seed.
6. The ovary develops into the fruit.
7. Under favorable conditions, the seed germinates and the embryo develops into a mature
sporophyte.
C. Preview: Further details about the flowering plant life cycle, including the unique adaptation of
double fertilization, are covered in Modules 31.10–31.14.
Module 17.11 The structure of a fruit reflects its function in seed dispersal.
A. Fruit is the developed (ripened) ovary of a flower and aids in seed dispersal.
B. Fruits ripen quickly, within one growing season.
C. Some fruits are adapted for wind dispersal of seeds (Figure 17.11A).
D. Others are adapted to hitch a ride on animals (Figure 17.11B).
E. Fleshy, edible fruits are attractive to animals as food. Seeds of these fruits usually pass
unharmed through the animal’s digestive tract and are deposited, with fertilizer, far from the
parent plant (Figure 17.11C).
Module 17.12 Connection: Agriculture is based almost entirely on angiosperms.
A. Gymnosperms supply almost all our lumber and paper, but food comes from angiosperms. Dry
fruits such as corn, wheat, and rice are consumed by humans and livestock worldwide. Fleshy
fruits such as tomatoes, squash, cucumbers, cherries, apples, strawberries, and oranges are also
grown around the world. Humans also eat specialized stems and roots such as sweet potatoes,
cassava, and potatoes.
B. Angiosperms are also used for fiber, medication, perfumes, and as a source of fine hardwoods
such as oak, cherry, and walnut. Several popular drinks come from angiosperms (tea and coffee)
and never forget the cacao tree, which provides cocoa and chocolate.
C. Agriculture began when humans decided to cultivate grains in an effort to control food supplies.
Selective breeding produced better plants, and today, genetic engineers are developing new and
improved plants.
Module 17.13 Interactions with animals have profoundly influenced angiosperm evolution.
A. The mutual evolution of two species such as flowers and bees is referred to as coevolution (see
Chapter 36).
B. Most flowering plants depend on insects, birds, or mammals for pollination and seed dispersal.
C. Most land animals depend on flowering plants for food.
D. Pollen and nectar provide food for pollinators. Bees are one of the most highly coevolved
groups of pollinators (Figure 17.13A).
E. Flowers that are pollinated by hummingbirds are usually pink or red (Figure 17.13B). The
location of the nectar for some flowers is deep within the floral tube. A hummingbird can
obtain the nectar with its long tongue and, in the process, collect and distribute pollen from
flower to flower.
F. Flowers that are pollinated at night by bats or moths are usually large and light-colored (Figure
17.13C).
G. These mutual dependencies have been favored by natural selection and threatened by human
interaction as well.
Module 17.14 Connection: Plant diversity is a nonrenewable resource.
A. Plant biodiversity is being reduced at an unprecedented rate. The majority of plant genetic
diversity is found in the rain forests of the world.
B. The loss of plants is harmful in a variety of ways:
1. Twenty-five percent of all prescription medicine comes from plants, and fewer than 5,000
of the more than 295,000 known plant species have been researched as possible medicines.
Table 17.14 lists just a few of the plants used as medicines.
2. Animals are perishing along with the plants.
3. Plant loss reduces air and water quality.
C. Approximately 50 million acres of forest are cleared every year. In an effort to reduce plant
diversity loss, scientists have joined with the UN and an organization called the All Species
Foundation in an effort to catalog all species in the next 25 years.
D. Efforts are also under way by the UN to conserve most plant species and learn how to manage
forests in a sustainable and environmentally sound manner.
III. Fungi
Module 17.15 Fungi absorb food after digesting it outside their bodies.
A. The Kingdom Fungi includes heterotrophic eukaryotes with body structures and reproduction
unlike any other organism. Like animals, fungi digest their food, but do it externally by
excreting powerful enzymes to obtain the resulting small molecules as nutrients by absorption.
B. The close association between plants and fungi illustrates their codependence. However, fungi
are more closely related to animals than plants. Fungi cannot produce their own food by
photosynthesis and plants cannot decompose and return valuable nutrients to the soil (Figure
17.15A). Review the introduction to Chapter 17.
C. Fungi are ubiquitous in ecosystems and are essential decomposers. Not all fungi are beneficial,
parasitizing plants and animals for nutrition.
D. Fungi are composed of hyphae (filamentous cell-like units, singular, hypha) that branch into a
complex feeding network of cells called mycelium (plural, mycelia, Figures 17.15B and C).
E. The hyphae of the mycelium extend rapidly into the food source, developing a huge surface
area from which digestive enzymes are secreted and through which the digested food is
absorbed (Figure 17.15C).
F. Some fungi produce mushrooms, external reproductive bodies composed of above-ground
tightly packed hyphae attached to underground mycelium (Figure 17.15A).
G. The hyphae cell walls are composed of chitin (nitrogen-containing polysaccharide) which is
identical to the chitin in insects. The cells are attached end to end and have large pores that
allow free movement of cellular components. A large mycelium can add as much as a kilometer
of hyphae each day in an effort to obtain food. One fungus in Oregon covers 2,200 acres and is
estimated to be 2,600 years old.
Module 17.16 Fungi produce spores in both asexual and sexual life cycles.
A. The fungal life cycle is complex but most can reproduce asexually or sexually (Figure 17.16).
Fungi that reproduce sexually fuse two haploid hyphae of different mating types after sexual
signaling molecules have been released. The fusion leads to the heterokaryotic stage, which is a
cell with two distinct haploid nuclei.
B. Hours to years may pass before the nuclei fuse to form the short-lived diploid phase. Once the
diploid zygote is formed, the cell undergoes meiosis and haploid spores are formed and
dispersed. Spores can travel great distances by wind and water and will germinate if conditions
are favorable (the presence of moisture and food).
C. Molds are fast growing, asexually producing fungi that produce spores at the tips of specialized
hyphae. Molds are easily found on old bread or neglected fruit in the refrigerator.
D. Yeasts are single-celled fungi found in liquids or moist habitats including animal tissue. Yeasts
reproduce asexually by cell division or by budding.
Module 17.17 Fungi can be classified into five groups.
A. Over 100,000 fungi species have been described and it is estimated by biologists that there are
1.5 million species of fungi. Classification systems for fungi are often based on the sexual
reproduction structures. Those fungi that cannot reproduce sexually are informally classified as
imperfect fungi.
B. Most fungi lack flagella, which were once used as a criterion to classify fungi. The
classification systems used now rely on molecular analysis. Scientists estimate that the common
ancestor of animals and fungi dates back to 1.5 bya. The oldest known fungal fossil, however, is
only 460 million years old.
C. Figure 17.17A shows one hypothesis for the phylogenetic tree of fungi. Most biologists
recognize five fungal phyla. The five phyla and their characteristics are presented below:
1. Chytrids are the only fungi with flagella and are thought to be the oldest fungi. They are
found almost ubiquitously (lakes, ponds, and soil), and are decomposers of and parasitic to
plants, protists, and animals.
2. Zygote fungi (zygomycetes) have a characteristic resistant zygosporangium with spores
formed by meiosis (Figure 17.17B). This group includes the fast-growing molds that spoil
food and can also be animal parasites.
3. Arbuscular mycorrhizal fungi (glomeromycetes) were once classified with the zygomycetes
but genetic analysis indicates that they are truly a separate clade. The tips of the hyphae that
invade the roots form miniature tree-like structures called arbuscles (Figure 17.17C). Most
plants (90%) have a symbiotic relationship with this fungal group. The fungi deliver
phosphates and other minerals to the plants and receive nutrients in return (symbiotic
relationship).
4. Sac fungi (ascomycetes) are named for their sac-like structures called asci that produce
spores sexually. Ascomycetes are found in marine, freshwater, and land environments.
They range in size and structure from unicellular to complex structures such as a morel
(Figure 17.17D). This group includes devastating plant pathogens and symbionts (green
algae or cyanobacteria with fungi from lichen, Module 17.20).
5. Club fungi (basidiomycetes) include the familiar mushroom, puffball, and shelf fungi
(Figure 17.17E). They are named for the spore-producing structure called a basidium. This
fungal group is an excellent decomposer of woody plants (lignin). This group also has two
destructive plant parasites (rust and smut).
Module 17.18 Fungal groups differ in their life cycles and reproductive structures.
A. The life cycle of the fungus zygomycetes is illustrated in Figure 17.18A. When food is
plentiful, the hyphae can asexually produce haploid spores in sporangia at the tips of upright
hyphae. However, when food is scarce, sexual reproduction begins.
1. Mycelia of opposite mating types join to make cells from two parents.
2. The zygosporangium develops into a thick-walled structure.
3. The thick-walled structure can survive under harsh conditions for months. Once conditions
improve, the parental nuclei fuse, forming a zygote.
4. Haploid spores are produced from the zygote by meiosis.
B. The process of reproduction for ascomycetes is similar to zygomycetes. When conditions are
favorable, they reproduce asexually. However, if conditions are severe, they reproduce
sexually.
C. The life cycle of mushroom (a basidiomycete) is presented in Figure 17.18B and involves five
steps.
1. A heterokaryotic mycelium forms by fusion of two different mating types.
2. A mushroom develops and grows.
3. Specialized cells from the gills on the underside of the mushroom contain the diploid nuclei
from nuclei fusion.
4. Haploid spores are formed by meiosis and are then released. A single mushroom can release
a billion spores.
5. Provided the conditions are correct, germination takes place and a haploid mycelium grows.
D. Much of the success of fungi is due to the high asexual reproductive capacity and the ability to
maintain genetic diversity through sexual reproduction.
Module 17.19 Connection: Parasitic fungi harm plants and animals.
A. Of the 100,000 species of known fungi, about 30% are parasites. Parasitic fungi are the most
serious plant pests. Particularly dangerous are nonnative parasites, such as the fungus that
causes Dutch elm disease (Figure 17.19A).
B. Approximately 80% of all plant diseases are caused by fungi. Between 10% and 50% of fruit
harvested around the world is destroyed by fungi. Some parasitic fungi, such as corn smut, can
attack developing seeds (Figure 17.19B).
C. Fungi cause a few diseases in humans. The term used for a fungal infection is mycosis. Flour
made from grain infested with ergots can cause gangrene, nervous system disorders, and even
death (Figure 17.19C). LSD (lysergic acid diethylamide) comes from one of the toxins (lysergic
acid) that has been isolated from ergots.
D. Some systemic fungal infections can be fatal, particularly in people weakened by other diseases.
Less serious are the fungal infections of the outer layers of the skin known as ringworm and
athlete’s foot. Fortunately, fungicidal ointments can control most of these fungal parasites. One
type of fungus is so deadly (Coccidioides) that it is considered a potential biological weapon.
E. Yeast infections of the vagina are usually caused by some chemical, microbial, or immunologic
change in the body and are classified as opportunistic infections. Other opportunistic fungal
infections are on the rise due to the emergence of AIDS, which suppresses the immune system.
Module 17.20 Lichens consist of fungi living mutualistically with photosynthetic organisms.
A. Lichens are associations of millions of green algae or cyanobacteria held in a tangled network
of fungal hyphae (Figure 17.18B).
B. The fungus receives food from the photosynthesis of its partner. The alga or cyanobacterium
receives housing, water, and the minerals trapped by the hyphal network.
C. All lichen fungi and most lichen algae and cyanobacteria cannot grow independently. The
relationship is so complete that they are classified together as a single species.
D. Lichens are able to survive in habitats where neither partner (nor any other multicellular
organism) could grow alone (Figure 17.18A).
E. Lichens play important ecological roles in soil formation on rock surfaces and as food for
animals.
F. Some lichens are thousands of years old.
NOTE: Cyanobacterium-containing lichens fix nitrogen and play important roles in the nitrogen
cycle (Module 37.18).
Module 17.21 Fungi also form mutualistic relationships with animals.
A. Some fungal species have developed mutualistic relationships with animals. Ruminants have
fungi in their gut to assist in the digestion of cellulose and other plant material.
B. Ants called leaf cutters have a close relationship with fungi, so much so that both depend on the
other for their existence (a mutualistic association). This relationship has been evolving for
more than 50 million years (Figure 17.21).
Module 17.22 Connection: Fungi have enormous ecological benefits and practical uses.
A. Plants and fungi have evolved together since they first inhabited the land. The close relationship
between plants and fungi is obvious when one looks to the role played by fungi. The
decomposition of matter and recycling of nutrients is essential to the survival of all plants.
B. Humans use fungi in several practical ways, such as the decomposition of pollutants and
carcinogens. Fungi are also used as a source of food (e.g., mushrooms, cheese, truffles, bread,
beer, and wine) (Figure 17.22A).
C. Fungi are a source of antibiotics. It was discovered that the fungus Penicillium could inhibit the
growth of the bacterium Staphylococcus aureus. Subsequently, the antibiotic penicillin was
developed (Figure 17.22B).
D. Fungi have played an important role in eukaryotic molecular biology, as sources of human
proteins, and for industrial applications such as in the paper industry.