Plant Diversity II: The Evolution of Seed Plants

1. Chapter 30

Plant Diversity II: The
Evolution of Seed Plants
PowerPoint® Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

2. Overview: Transforming the World

• Seeds changed the course of plant evolution,
enabling their bearers to become the dominant
producers in most terrestrial ecosystems.
• A seed consists of an embryo and nutrients
surrounded by a protective coat.
• The gametophytes of seed plants develop
within the walls of spores that are retained
within tissues of the parent sporophyte.
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3.

What human reproductive organ is functionally similar to this seed?

4. Seeds and pollen grains are key adaptations for life on land

• In addition to seeds, the following are common
to all seed plants:
– Reduced gametophytes
– Heterospory
– Ovules
– Pollen
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5.

Gametophyte / sporophyte relationships in different plant groups
PLANT GROUP
Gametophyte
Sporophyte
Mosses and other
Ferns and other seedless
nonvascular plants
vascular plants
Dominant
Reduced, dependent on
gametophyte for nutrition
Reduced, independent
(photosynthetic and
free-living)
Dominant
Seed plants (gymnosperms and angiosperms)
Reduced (usually microscopic), dependent on surrounding
sporophyte tissue for nutrition
Dominant
Gymnosperm
Sporophyte
(2n)
Microscopic female
gametophytes (n) inside
ovulate cone
Sporophyte
(2n)
Gametophyte
(n)
Angiosperm
Microscopic
female
gametophytes
(n) inside
these parts
of flowers
Example
Microscopic male
gametophytes (n)
inside pollen
cone
Sporophyte (2n)
Gametophyte
(n)
Microscopic
male
gametophytes
(n) inside
these parts
of flowers
Sporophyte (2n)

6. Heterospory: The Rule Among Seed Plants

• The ancestors of seed plants were likely
homosporous, while seed plants are
heterosporous.
• Megasporangia produce megaspores that give
rise to female gametophytes.
• Microsporangia produce microspores that give
rise to male gametophytes.
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7. Ovules and Production of Eggs

• An ovule consists of a megasporangium,
megaspore, and one or more protective
integuments.
• A fertilized ovule becomes a seed.
• Gymnosperm megaspores have one
integument.
• Angiosperm megaspores usually have two
integuments.
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8.

From ovule to seed in a gymnosperm
Integument
Spore wall
Immature
female cone
Megasporangium
(2n)
Megaspore (n)
(a) Unfertilized ovule

9. Pollen and Production of Sperm

• Microspores develop into pollen grains, which
contain the male gametophytes.
• Pollination is the transfer of pollen from the
male to the female part containing the ovules.
• Pollen eliminates the need for a film of water
and can be dispersed great distances by air or
animals.
• If a pollen grain germinates, it gives rise to a
pollen tube that discharges two sperm into the
female gametophyte within the ovule.
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10.

From ovule to seed in a gymnosperm
Female
gametophyte (n)
Spore wall
Egg nucleus (n)
Male gametophyte
(within a germinated
pollen grain) (n)
Micropyle
(b) Fertilized ovule
Discharged
sperm nucleus (n)
Pollen grain (n)

11. The Evolutionary Advantage of Seeds

• A seed develops from the whole ovule.
• A seed is a sporophyte embryo, along with its
food supply, packaged in a protective coat.
• Seeds provide some evolutionary advantages
over spores:
– They may remain dormant for days to years,
until conditions are favorable for germination.
– They may be transported long distances by
wind or animals.
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12.

From ovule to seed in a gymnosperm
Seed coat
(derived from
integument)
Food supply
(female
gametophyte
tissue) (n)
Embryo (2n)
(new sporophyte)
(c) Gymnosperm seed

13.

From ovule to seed in a gymnosperm
Integument
Female
gametophyte (n)
Seed coat
(derived from
integument)
Spore wall
Egg nucleus (n)
Immature
female cone
Male gametophyte
(within a germinated
pollen grain) (n)
Megasporangium
(2n)
Discharged
sperm nucleus (n)
Micropyle
Megaspore (n)
(a) Unfertilized
Food supply
(female
gametophyte
tissue) (n)
ovule
(b) Fertilized
ovule
Pollen grain (n)
Embryo (2n)
(new sporophyte)
(c) Gymnosperm
seed

14. Gymnosperms bear “naked” seeds, typically on cones

• The gymnosperms have “naked” seeds not
enclosed by ovaries and exposed on modified
leaves - cones. There are four phyla:
– Cycadophyta (cycads)
– Gingkophyta (one living species: Ginkgo biloba)
– Gnetophyta (three genera: Gnetum, Ephedra,
Welwitschia)
– Coniferophyta (conifers, such as pine, fir, and
redwood).
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15.

• Seed plants can be divided into two clades:
gymnosperms and angiosperms.
• Gymnosperms appear early in the fossil record
and dominated the Mesozoic terrestrial
ecosystems.
• Gymnosperms were better suited than
nonvascular plants to drier conditions.
• Today, cone-bearing gymnosperms called
conifers dominate in the northern latitudes.
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16. Phylum Ginkgophyta

• This phylum consists of a single living species,
Ginkgo biloba.
• It has a high tolerance to air pollution and is a
popular ornamental tree.
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17.

Gymnosperm
Ginkgo biloba
Pollen-producing tree with fleshy seeds

18.

Gymnosperm
Ovulate cones
Welwitschia

19. Phylum Coniferophyta

• This phylum is by far the largest of the
gymnosperm phyla.
• Most conifers are evergreens and can carry out
photosynthesis year round.
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20.

Gymnosperms: Conifers perform year round photosynthesis
Douglas fir

21.

Gymnosperms:
Conifers
Sequoia - One of
the Largest and
Oldest Living
Organisms
Giant Sequoia: 2,500 tons / 1,800 - 2,700 years old

22. The Life Cycle of a Pine: A Closer Look

• Three key features of the gymnosperm life
cycle are:
– Dominance of the sporophyte generation.
– The transfer of sperm to ovules by pollen.
– Development of seeds from fertilized ovules.
• The life cycle of a pine provides an example.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

23.

Life Cycle
of a Pine
Key
Haploid (n)
Diploid (2n)
Ovule
Ovulate
cone
Pollen
cone
Megasporocyte (2n)
Integument
Microsporocytes
(2n)
Megasporangium
Pollen (2n)
Pollen grain
grains (n) MEIOSIS
MEIOSIS
Mature
sporophyte
(2n)
Microsporangia
Microsporangium (2n)
Seedling
Archegonium
Female
gametophyte
Seeds
Food
reserves
(n)
Seed coat
(2n)
Embryo
(2n)
Sperm
nucleus (n)
Pollen
tube
FERTILIZATION
Egg nucleus (n)
Surviving
megaspore (n)

24. The reproductive adaptations of angiosperms include flowers and fruits

• Angiosperms are seed plants with reproductive
structures called flowers and fruits.
• They are the most widespread and diverse of
all plants.
• All angiosperms are classified in a single
phylum: Anthophyta.
• The name comes from the Greek anthos,
flower.
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25. Flowers - Specialized for Sexual Reproduction

• The flower is an angiosperm structure specialized for
sexual reproduction. It is a specialized shoot with up
to four types of modified leaves:
– Sepals - enclose the flower
– Petals - brightly colored and attract pollinators
– Stamens - produce pollen on their terminal anthers
– Carpels - consist of an ovary containing ovules at
the base and a style holding up a stigma, where
pollen is received.
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26.

Structure of an Idealized Flower
Stigma
Stamen
Anther
Carpel
Style
Filament
Ovary
Petal
Sepal
Ovule

27. Fruits

• A fruit typically consists of a mature ovary but
can also include other flower parts.
• Fruits protect seeds and aid in seed dispersal.
• Mature fruits can be either fleshy or dry.
• Various fruit adaptations help disperse seeds
by wind, water, or animals to new locations.
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28.

Fruits
Tomato
Ruby grapefruit
Nectarine
Hazelnut
Milkweed

29.

Fruit Adaptations
for Seed Dispersal
Wings
Seeds within berries
Barbs

30. The Angiosperm Life Cycle

• The flower of the sporophyte is composed of
both male and female structures.
• Male gametophytes are contained within pollen
grains produced by the microsporangia of
anthers.
• The female gametophyte = embryo sac,
develops within an ovule contained within an
ovary at the base of a stigma.
• Most flowers have mechanisms to ensure
cross-pollination between flowers from
different plants of the same species.
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31.

• A pollen grain that has landed on a stigma
germinates and the pollen tube of the male
gametophyte grows down to the ovary.
• Sperm enter the ovule through a pore opening
called the micropyle.
• Double fertilization occurs when the pollen
tube discharges two sperm into the female
gametophyte within an ovule.
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32.

Double Fertilization: Produces
Zygote 2n and endosperm (food) 3n
• One sperm fertilizes the egg forming a zygote.
• The other sperm combines with two nuclei and
initiates development of food-storing
endosperm.
• The endosperm nourishes the developing
embryo.
• Within a seed, the embryo consists of a root
and two seed leaves called cotyledons.
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33.

Life Cycle of an Angiosperm
Key
Haploid (n)
Diploid (2n)
Mature flower on
sporophyte plant
(2n)
Microsporangium
Microsporocytes (2n)
Anther
MEIOSIS
Ovule (2n) Microspore
(n)
Ovary
Germinating
seed
MEIOSIS
Megasporangium
(2n)
Embryo (2n)
Endosperm (3n)
Seed coat (2n) Seed
Nucleus of
developing
endosperm
(3n)
Male gametophyte
(in pollen grain)
Pollen
(n)
grains
Stigma
Pollen
tube
Megaspore
(n)
Antipodal cells
Female gametophyte Central cell
(embryo sac)
Synergids
Egg (n)
Generative cell
Tube cell
Sperm
Style
Pollen
tube
Sperm
(n)
FERTILIZATION
Zygote (2n)
Egg
nucleus (n)
Discharged sperm nuclei (n)

34. Angiosperm Phylogeny

• The ancestors of angiosperms and
gymnosperms diverged about 305 million years
ago.
• Angiosperms may be closely related to
Bennettitales, extinct seed plants with
flowerlike structures.
• Amborella and water lilies are likely descended
from two of the most ancient angiosperm
lineages.
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35.

Angiosperm evolutionary history
Living
gymnosperms
Microsporangia
(contain
microspores)
Bennettitales
Amborella
Water lilies
Most recent common ancestor
of all living angiosperms
Star anise and
relatives
Monocots
Magnoliids
Eudicots
Ovules
(a) A possible ancestor of the
angiosperms?
300
250
200
150
100
Millions of years ago
(b) Angiosperm phylogeny
50
0

36. Angiosperm Diversity

The two main groups of angiosperms are:
monocots - one cotyledon
eudicots (“true” dicots) - two cotyledons.
• More than one-quarter of angiosperm species
are monocots.
• More than two-thirds of angiosperm species
are eudicots.
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37.

Angiosperms:
Monocots
and
Eudicots
Monocot
Characteristics
Eudicot
Characteristics
Embryos
One cotyledon
Two cotyledons
Leaf
venation
Veins usually
parallel
Veins usually
netlike
Stems
Vascular tissue
usually arranged
in ring
Vascular tissue
scattered
Roots
Taproot (main root)
usually present
Root system
usually fibrous
(no main root)
Pollen
Pollen grain with
one opening
Pollen grain with
three openings
Flowers
Floral organs
usually in
multiples of three
Floral organs usually
in multiples of
four or five

38. Evolutionary Links Between Angiosperms and Animals

• Pollination of flowers and transport of seeds by
animals are two important relationships in
terrestrial ecosystems.
• Clades with bilaterally symmetrical flowers
have more species than those with radially
symmetrical flowers.
• This is likely because bilateral symmetry
affects the movement of pollinators and
reduces gene flow in diverging populations.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

39.

EXPERIMENT
Can Flower Shape Influence Speciation Rate?
Time since divergence
from common ancestor
“Bilateral” clade
“Radial” clade
Common
ancestor
Compare
numbers
of species
Mean difference
in number of species
RESULTS
3,000
2,000
1,000
0
Bilateral
symmetry (N = 15)
Radial
symmetry (N = 4)

40. Human welfare depends greatly on seed plants

• No group of plants is more important to human
survival than seed plants.
• Plants are key sources of food, fuel, wood
products, and medicine.
• Our reliance on seed plants makes
preservation of plant diversity critical.
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41. Products from Seed Plants

• Most of our food comes from angiosperms. Six
crops (wheat, rice, maize, potatoes, cassava,
and sweet potatoes) yield 80% of the calories
consumed by humans.
• Modern crops are products of relatively recent
genetic change resulting from artificial
selection.
• Many seed plants provide wood.
• Secondary compounds of seed plants are used
in medicines.
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42.

43. Threats to Plant Diversity

• Destruction of habitat is causing extinction of
many plant species.
• Loss of plant habitat is often accompanied by
loss of the animal species that plants support.
• At the current rate of habitat loss, 50% of
Earth’s species will become extinct within the
next 100–200 years.
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44.

Summary
Five Derived Traits of Seed Plants
Reduced
gametophytes
Heterospory
Microscopic male and
female gametophytes
(n) are nourished and
protected by the
sporophyte (2n)
Male
gametophyte
Female
gametophyte
Microspore (gives rise to
a male gametophyte)
Megaspore (gives rise to
a female gametophyte)
Ovules
Integument (2n)
Ovule
(gymnosperm)
Megaspore (2n)
Megasporangium (2n)
Pollen
Pollen grains make water
unnecessary for fertilization
Seeds
Seeds: survive
better than
unprotected
spores, can be
transported
long distances
Integument
Food supply
Embryo

45.

Plant Evolutionary Relationships: Clades
Charophyte green algae
Mosses
Ferns
Gymnosperms
Angiosperms

46. You should now be able to:

1. Explain why pollen grains were an important
adaptation for successful reproduction on
land.
2. List the four phyla of gymnosperms.
3. Describe the life history of a pine; indicate
which structures are part of the gametophyte
generation and which are part of the
sporophyte generation.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

47. You should now be able to:

4. Identify and describe the function of the following
floral structures: sepals, petals, stamens, carpels,
filament, anther, stigma, style, ovary, and ovule.
5. Explain how fruits may be adapted to disperse
seeds.
6. Diagram the generalized life cycle of an angiosperm;
indicate which structures are part of the gametophyte
generation and which are part of the sporophyte
generation.
7. Describe the current threat to plant diversity caused
by human population growth.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings