Chapter 17
Plants, Fungi, and the Colonization of Land
Introduction: Plants and Fungi—A Beneficial Partnership
A.)
Plants and fungi colonized land together
B.)
Mycorrhizae, mutually beneficial associations of plant roots
and fungi hyphae, enabled plants to colonize land
– Mycorrhizal fungi absorb water, phosphorus, and other minerals from
soil and make them available to the plant
– The sugars produced by the plant nourish the fungus
PLANT EVOLUTION
DIVERSITY
AND
17.1 Plants have adaptations for life on land
A.) 500 million years ago, the algal ancestors of plants formed a
green carpet on the edge of lakes and coastal salt marshes
B.)
C.)
D.)
Green algae called charophytes are the closest living relatives
of plants
Land plants are a clade, defined by a set of derived characters
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Alternation of haploid and diploid generations
Walled spores produced in sporangia
Male and female gametangia
Multicellular, dependent sporophyte embryos
Life on land offered new opportunities
– Unlimited sunlight
– Abundant CO2
– Initially, few pathogens or herbivores
E.)
Challenges of terrestrial life
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–
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Maintaining moisture within cells
Obtaining resources from soil and air
Supporting body in air
Reproducing and dispersing offspring without water
F.)
In all plants, the zygote develops into an embryo while
attached to and nourished by the parent plant
G.)
Plants are embryophytes, with multicellular, dependent
embryos
17.2 Plant diversity reflects the evolutionary history of the
plant kingdom
A.)
Four key adaptations for life on land distinguish the main
lineages of the plant kingdom
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Dependent embryos (characteristic of all plants)
Lignified vascular tissues
Seeds
Flowers
ALTERNATION
OF GENERATIONS
AND PLANT LIFE CYCLES
17.3 Haploid and diploid generations alternate in plant life
cycles
A.) The haploid gametophyte produces gametes (eggs and
sperm) by mitosis
B.)
Fertilization results in a diploid zygote
C.)
The zygote develops into the diploid sporophyte, which
produces haploid spores by meiosis
D.)
Spores grow into gametophytes
17.4 Mosses have a dominant gametophyte
A.)
Gametophytes make up a bed of moss
– They produce eggs and flagellated sperm in gametangia
– Sperm swim through water to the egg
B.) The zygote develops within the gametangium into a mature
sporophyte, which remains attached to the gametophyte
– Meiosis occurs in sporangia at the tips of the sporophyte stalk
– Haploid spores are released from the sporangium and develop into
gametophytes
17.5 Ferns, like most plants, have a dominant sporophyte
A.)
Fern gametophytes are small and inconspicuous
– They produce flagellated sperm that swim to the egg and fertilize it to
produce a zygote
– The zygote initially develops within the female gametangia but
eventually develops into an independent sporophyte
B.) Sporangia develop on the underside of the leaves of the
sporophyte
– Within the sporangia, cells undergo meiosis to produce haploid spores
– Spores are released and develop into gametophytes
17.6 Seedless plants dominated vast “coal forests”
A.) Seedless plants formed vast ancient forests in low-lying
wetlands during the Carboniferous period (360–299 million years
ago)
– When they died, the plants formed peat deposits that eventually formed
coal
B.) Coal, oil, and natural gas are fossil fuels
– Oil and natural gas formed from marine organisms; coal formed from
seedless plants
– Burning fossil fuels releases CO2, causing climate warming
17.7 A pine tree is a sporophyte with gametophytes in its cones
A.) A pine cone holds all of the tree’s reproductive stages: spores,
eggs, sperm, zygotes, and embryos
– Each scale of the cone contains sporangia that produce spores by
meiosis
– The spores produce gametophytes within the cone
B.)
The male gametophyte is a pollen grain, released from pollen
cones and carried by wind to female cones
C.)
D.)
Female ovulate cones carry two ovules on each stiff scale
– Each ovule contains a sporangium surrounded by the integument
In pollination, a pollen grain lands on a scale in an ovulate
cone and enters an ovule
– The scales then grow together, sealing up the cone
– Within the sealed cone, the gametophytes produce gametes
E.)
Fertilization occurs a year after pollination, when a sperm
moves down a pollen tube to the egg to form a zygote
– The zygote develops into a sporophyte embryo, and the ovule becomes
a seed, with stored food and a protective seed coat
F.)
The seed is a key adaptation for life on land and a major factor
in the success of seed plants
17.8 The flower is the centerpiece of angiosperm reproduction
A.) Flowers contain separate male and female sporangia and
gametophytes
B.)
Flowers usually consist of sepals, petals, stamens (which
produce pollen), and carpels (which produce eggs)
– Sepals enclose the flower before it opens
– Petals attract animal pollinators
C.)
Stamens include a filament and anther, a sac at the top of
each filament that contains male sporangia and releases pollen
D.)
The carpel is the female reproductive structure, including the
ovary
E.)
– The ovary encloses the ovules, which contain sporangia that will
produce a female gametophyte
Ovules develop into seeds; ovaries mature into fruit
17.9 The angiosperm plant is a sporophyte with gametophytes
in its flowers
A,)
The angiosperm life cycle
• Meiosis in the anthers produces haploid spores that form the male
gametophyte (pollen grains)
• Meiosis in the ovule produces a haploid spore that forms a tiny female
gametophyte, including the egg
• A pollen tube from the pollen grain to the ovule carries a sperm that
fertilizes the egg to form a zygote
• Each ovule develops into a seed, consisting of an embryo (a new
sporophyte) with a food supply and a seed coat
• The ovary wall forms a fruit
17.10 The structure of a fruit reflects its function in seed
dispersal
A.)
Fruits, ripened ovaries of flowers, are adaptations that disperse
seeds
– Some rely on wind for seed dispersal
– Some hitch a ride on animals
– Fleshy, edible fruits attract animals
17.11 CONNECTION: Angiosperms sustain us—and add spice
to our diets
A.)
Most human food is provided by the fruits and seeds of
angiosperms
– Corn, rice, wheat, and other grains are dry fruits
– Apples, cherries, tomatoes, and squash are fleshy fruits
– Spices such as nutmeg, cinnamon, cumin, cloves, ginger, and licorice
are also angiosperm fruits
17.12 EVOLUTION CONNECTION: Pollination by animals
has influenced angiosperm evolution
A.)
90% of angiosperms use animals to transfer pollen
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Birds are attracted by colorful flowers, but not scent
Beetles are attracted by fruity odors, but not color
Bats are attracted by large, highly scented flowers
Wind-pollinated flowers produce large amounts of pollen
17.13 CONNECTION: Plant diversity is an irreplaceable
resource
A.) More than 50,000 square miles of forest are cleared every year
– Replanted areas have greatly reduced biological diversity
B.) Loss of forests has greatly reduced diversity of life on Earth
– The loss of plant diversity removes potentially beneficial medicines
– More than 25% of prescription drugs are extracted from plants
FUNGI
17.14 Fungi absorb food after digesting it outside their bodies
A.) Fungi are absorptive heterotrophic eukaryotes that digest their
food externally and absorb the nutrients
B.)
Most fungi consist of a mass of threadlike hyphae making up a
mycelium
– Hyphal cells are separated by cross-walls with pores large enough for
ribosomes, mitochondria, and nuclei to cross
– Some are multinucleate without cross-walls
– Hyphae have a huge surface area to secrete digestive enzymes and
absorb food
C.)
Fungal hyphae are surrounded by a cell wall with chitin
17.15 Fungi produce spores in both asexual and sexual life
cycles
A.)
Many fungal species can reproduce both sexually and asexually
B.)
Fungi produce huge numbers of asexual spores, each of which
can germinate to form a new fungus
C.)
In many fungi, sexual fusion of haploid hyphae leads to a
heterokaryotic stage, in which cells contain two genetically
distinct haploid nuclei
– Hours or centuries may pass before parental nuclei fuse to form a shortlived diploid phase
– Zygotes undergo meiosis inside specialized reproductive structures and
disperse haploid spores
17.16 Fungi are classified into five groups
A.) Sexual reproductive structures are used to classify fungi
– Fungi with no known sexual stage are known as imperfect fungi
B.) Fungi likely evolved from an aquatic, flagellated ancestor shared
with animals
C.) Chytrids, which have flagellated spores, are the earliest lineage
of fungi
D.)
Animals and fungi diverged into separate lineages 1.5 billion
years ago
E.)
The oldest fungal fossils are 460 million years old
F.)
Glomeromycetes
– These fungi form mycorrhizae, in which invasive hyphae branch into
treelike arbuscules within plant roots
– 90% of plants have symbiotic partnerships with glomeromycetes
G.)
Ascomycetes
– Sac fungi form saclike asci, which produce sexual spores
– They range in size from yeasts to elaborate morels and cup fungi
– Some form lichens in association with green algae or cyanobacteria
H.)
Basidiomycetes
– Club fungi are the mushrooms, puffballs, and shelf fungi
– They have club-shaped spore-producing structures called basidia
– These fungi are important forest decomposers
17.17 Fungal groups differ in their life cycles and reproductive
structures
A.)
Hyphae reproduce asexually by producing spores in sporangia
at the tips of upright hyphae
B.)
C.)
When food is depleted, the fungus reproduces sexually
– Mycelia of different mating types join and produce a zygosporangium,
which develops into a thick-walled structure that can tolerate dry, harsh
conditions
– Under favorable conditions, the parental nuclei fuse and the diploid
nucleus undergoes meiosis to form haploid spores
Hyphae of different mating types fuse to form a heterokaryotic
mycelium, which grows to produce a mushroom
– Haploid nuclei fuse to form diploid nuclei in the club-shaped cells called
basidia that line the gills of the mushroom
– Each diploid nucleus undergoes meiosis to form haploid spores
17.18 CONNECTION: Parasitic fungi harm plants and
animals
A.)
B.)
80% of plant diseases are caused by fungi
– Between 10 and 50% of the world’s fruit harvest is lost each year to
fungal attack
– A variety of fungi, including smuts and rusts, infect grain crops
Only 50 species of fungi are parasitic on animals, causing
mycoses
– Human infections include athlete’s foot (caused by ringworm)
– Systemic mycoses are rare but serious fungal infections that spread
through the body from inhaled spores
17.19 Lichens consist of fungi living in close association with
photosynthetic organisms
A.)
Lichens consist of algae or cyanobacteria within a fungal
network
– Many lichen associations are mutualistic
– The fungus receives food from its photosynthetic partner
– The fungal mycelium helps the alga absorb and retain water and
minerals
B.)
Lichens are important pioneers on new land, where they help
to form soil
– Lichens are sensitive to air pollution, because they obtain minerals from
the air
17.20 Some fungi have mutually beneficial relationships with
ants
A.)
Several species of ants and termites cultivate fungal gardens
B.)
The insects feed their fungi with leaves, weeding out
undesirable fungi
– The fungi feed on the leaves
– The ants harvest the swollen hyphal tips
C.)
Farmer insects and fungal “crops” have been evolving together
for over 50 million years
17.21 CONNECTION: Fungi have enormous ecological
benefits and practical uses
A.)
Fungi have many practical uses for humans
– Some fungi can break down toxic pollutants, including pesticides like
DDT and cancer-causing chemicals
– Fungi may be able to clean up oil spills and chemical messes
– We eat many fungi, from mushrooms to cheeses modified by fungi
– Yeasts produce alcohol and cause bread to rise
– Fungi provide antibiotics that are used to treat bacterial disease
– Fungi are playing important new roles in molecular biology and
biotechnology
You should now be able to
§ Describe the key plant adaptations for life on land
§ Describe the alternation of generation life cycle; explain why it
appears that this cycle has evolved independently in algae and
land plants
§ Describe the key events of the moss, fern, and pine life cycles
§ Explain how coal was formed; explain why coal, oil, and natural
gas are called fossil fuels
You should now be able to
§
§
§
§
§
Describe the parts of a flower and explain their functions
Describe the stages of the angiosperm life cycle
Describe angiosperm adaptations that promote seed dispersal
Explain how flowers are adapted to attract pollinators
Compare the life cycles and reproductive structures in the fungal
groups
You should now be able to
§
§
Describe the structure and characteristics of lichens
Describe the positive ecological and practical roles of fungi