Transcript Chapter 30 Plant Diversity II

Chapter 30
Plant Diversity II: The
Evolution of Seed Plants
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
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• Overview: Feeding the World
• Seeds changed the course of plant evolution
– Enabling their bearers to become the dominant
producers in most terrestrial ecosystems
Figure 30.1
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• Concept 30.1: The reduced gametophytes of
seed plants are protected in ovules and pollen
grains
• In addition to seeds, the following are common
to all seed plants
– Reduced gametophytes
– Heterospory
– Ovules
– Pollen
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Advantages of Reduced Gametophytes
• The gametophytes of seed plants
– Develop within the walls of spores retained
within tissues of the parent sporophyte
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• Gametophyte/sporophyte relationships
Sporophyte
(2n)
Sporophyte
(2n)
Gametophyte
(n)
(a) Sporophyte dependent
on gametophyte
(mosses and other
bryophytes).
Gametophyte
(n)
(b) Large sporophyte and
small, independent
gametophyte (ferns
and other seedless
vascular plants).
Microscopic female
gametophytes (n) in
ovulate cones
(dependent)
Microscopic male
gametophytes (n)
inside these parts
of flowers
(dependent)
Microscopic male
gametophytes (n)
in pollen cones
(dependent)
Figure 30.2a–c
Sporophyte (2n)
(independent)
(c) Reduced gametophyte dependent on sporophyte
(seed plants: gymnosperms and angiosperms).
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Microscopic
female
gametophytes (n)
inside these parts
of flowers
(dependent)
Sporophyte (2n),
the flowering plant
(independent)
Heterospory: The Rule Among Seed Plants
• Seed plants evolved from plants that had
megasporangia
– Which produce megaspores that give rise to
female gametophytes
• Seed plants evolved from plants that had
microsporangia
– Which produce microspores that give rise to
male gametophytes
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Ovules and Production of Eggs
• An ovule consists of
– A megasporangium, megaspore, and
protective integuments
Integument
Spore wall
Megasporangium
(2n)
Megaspore (n)
Figure 30.3a
(a) Unfertilized ovule. In this sectional
view through the ovule of a pine (a
gymnosperm), a fleshy
megasporangium is surrounded by a
protective layer of tissue called an
integument. (Angiosperms have two
integuments.)
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Pollen and Production of Sperm
• Microspores develop into pollen grains
– Which contain the male gametophytes of
plants
• Pollination
– Is the transfer of pollen to the part of a seed
plant containing the ovules
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• If a pollen grain germinates
– It gives rise to a pollen tube that discharges two
sperm into the female gametophyte within the
ovule
Female
gametophyte (n)
Egg nucleus (n)
Spore wall
Male gametophyte
(within germinating
pollen grain) (n)
Discharged
sperm nucleus (n)
Micropyle
Figure 30.3b
Pollen grain (n)
(b) Fertilized ovule. A megaspore develops into a
multicellular female gametophyte. The micropyle,
the only opening through the integument, allows
entry of a pollen grain. The pollen grain contains a
male gametophyte, which develops a pollen tube
that discharges sperm.
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• Pollen, which can be dispersed by air or
animals
– Eliminated the water requirement for
fertilization
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The Evolutionary Advantage of Seeds
• A seed
– Develops from the whole ovule
– Is a sporophyte embryo, along with its food
supply, packaged in a protective coat
Seed coat
(derived from
Integument)
Food supply
(female
gametophyte
tissue) (n)
Embryo (2n)
(new sporophyte)
Figure 30.3c
(c) Gymnosperm seed. Fertilization initiates
the transformation of the ovule into a seed,
which consists of a sporophyte embryo, a
food supply, and a protective seed coat
derived from the integument.
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• Concept 30.2: Gymnosperms bear “naked”
seeds, typically on cones
• Among the gymnosperms are many wellknown conifers
– Or cone-bearing trees, including pine, fir, and
redwood
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• The gymnosperms include four plant phyla
– Cycadophyta
– Gingkophyta
– Gnetophyta
– Coniferophyta
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• Exploring Gymnosperm Diversity
PHYLUM CYCADOPHYTA
PHYLUM GINKGOPHYTA
Cycas revoluta
PHYLUM GNETOPHYTA
Gnetum
Welwitschia
Ovulate cones
Ephedra
Figure 30.4
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• Exploring Gymnosperm Diversity
PHYLUM CYCADOPHYTA
Douglas fir
Common juniper
Wollemia pine
Pacific
yew
Bristlecone pine
Figure 30.4
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Sequoia
Gymnosperm Evolution
• Fossil evidence reveals that by the late Devonian
– Some plants, called progymnosperms, had begun
to acquire some adaptations that characterize
seed plants
Figure 30.5
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• Gymnosperms appear early in the fossil record
– And dominated the Mesozoic terrestrial
ecosystems
• Living seed plants
– Can be divided into two groups: gymnosperms
and angiosperms
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A Closer Look at the Life Cycle of a Pine
• Key features of the gymnosperm life cycle
include
– Dominance of the sporophyte generation, the
pine tree
– The development of seeds from fertilized
ovules
– The role of pollen in transferring sperm to
ovules
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• The life cycle of a pine
2 An ovulate cone scale has two
ovules, each containing a megasporangium. Only one ovule is shown.
Key
1 In most
conifer species,
each tree has
both ovulate
and pollen
cones.
Haploid (n)
Diploid (2n)
Ovule
Megasporocyte (2n)
Ovulate
cone
Pollen
cone
Integument
Longitudinal
section of
ovulate cone
Microsporocytes
(2n)
Mature
sporophyte
(2n)
MEIOSIS
Longitudinal
section of
Sporophyll
pollen cone Microsporangium
Seedling
Micropyle
Megasporangium
Germinating
pollen grain
Pollen
grains (n)
MEIOSIS
(containing male
gametophytes)
Surviving
megaspore (n)
3 A pollen cone contains many microsporangia
held in sporophylls. Each microsporangium
Germinating
contains microsporocytes (microspore mother
pollen grain
cells). These undergo meiosis, giving rise to
Archegonium
haploid microspores that develop into
Egg (n)
Integument
pollen grains.
Female
Seeds on surface
gametophyte
4 A pollen grain
enters through
the micropyle
and germinates,
forming a pollen
tube that slowly
digests
through the
megasporangium.
5 While the
pollen tube
develops, the
megasporocyte
(megaspore
mother cell)
undergoes meiosis,
producing four
haploid cells. One
survives as a
megaspore.
of ovulate scale
Germinating
pollen grain (n)
8 Fertilization usually occurs more
than a year after pollination. All eggs
may be fertilized, but usually only one
zygote develops into an embryo. The
ovule becomes a seed, consisting of an
embryo, food supply, and seed coat.
Figure 30.6
Embryo
(new sporophyte)
(2n)
Food reserves
(gametophyte
tissue) (n)
Seed coat
(derived from
parent
sporophyte) (2n)
Discharged
sperm nucleus (n)
Pollen
tube
FERTILIZATION
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Egg nucleus (n)
6 The female gametophyte
develops within the megaspore
and contains two or three
archegonia, each with an egg.
7 By the time the eggs are mature,
two sperm cells have developed in the
pollen tube, which extends to the
female gametophyte. Fertilization occurs
when sperm and egg nuclei unite.
• Concept 30.3: The reproductive adaptations of
angiosperms include flowers and fruits
• Angiosperms
– Are commonly known as flowering plants
– Are seed plants that produce the reproductive
structures called flowers and fruits
– Are the most widespread and diverse of all
plants
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Characteristics of Angiosperms
• The key adaptations in the evolution of
angiosperms
– Are flowers and fruits
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Flowers
• The flower
– Is an angiosperm structure specialized for
sexual reproduction
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• A flower is a specialized shoot with modified
leaves
– Sepals, which enclose the flower
– Petals, which are brightly colored and attract
pollinators
– Stamens, which produce pollen
– Carpels, which produce ovules
Carpel
Stigma
Stamen
Anther
Style
Ovary
Filament
Petal
Sepal
Receptacle
Figure 30.7
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Ovule
Fruits
• Fruits
– Typically consist of a mature ovary
(a) Tomato, a fleshy fruit with
soft outer and inner layers
of pericarp
(b) Ruby grapefruit, a fleshy fruit
with a hard outer layer and
soft inner layer of pericarp
(c) Nectarine, a fleshy
fruit with a soft outer
layer and hard inner
layer (pit) of pericarp
Figure 30.8a–e
(d) Milkweed, a dry fruit that
splits open at maturity
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(e) Walnut, a dry fruit that
remains closed at maturity
• Can be carried by wind, water, or animals to new
locations, enhancing seed dispersal
(a) Wings enable maple fruits
to be easily carried by the wind.
(b) Seeds within berries and other
edible fruits are often dispersed
in animal feces.
Figure 30.9a–c
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(c) The barbs of cockleburs
facilitate seed dispersal by
allowing the fruits to
“hitchhike” on animals.
The Angiosperm Life Cycle
• In the angiosperm life cycle
– Double fertilization occurs when a pollen tube
discharges two sperm into the female
gametophyte within an ovule
– One sperm fertilizes the egg, while the other
combines with two nuclei in the center cell of
the female gametophyte and initiates
development of food-storing endosperm
• The endosperm
– Nourishes the developing embryo
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• The life cycle of an angiosperm
Key
Haploid (n)
Diploid (2n)
1 Anthers contain microsporangia.
Each microsporangium contains microsporocytes (microspore mother cells) that
divide by meiosis, producing microspores.
Microsporangium
Anther
Microsporocytes (2n)
Mature flower on
sporophyte plant
(2n)
2 Microspores form
pollen grains (containing
male gametophytes). The
generative cell will divide
to form two sperm. The
tube cell will produce the
pollen tube.
MEIOSIS
Microspore (n)
Ovule with
megasporangium (2n)
7 When a seed
germinates, the
embryo develops
into a mature
sporophyte.
Generative cell
Tube cell
Male gametophyte
(in pollen grain)
Ovary
3 In the megasporangium
of each ovule, the
megasporocyte divides by
meiosis and produces four
megaspores. The surviving
megaspore in each ovule
forms a female gametophyte
Seed (embryo sac).
Embryo (2n)
Endosperm
(food
Supply) (3n)
6 The zygote
develops into an
embryo that is
packaged along
with food into a
seed. (The fruit
tissues surrounding the seed are
not shown).
Pollen
grains
MEIOSIS
Germinating
Seed
Stigma
Megasporangium
(n)
Pollen
tube
Sperm
Surviving
megaspore
(n)
Seed coat (2n)
Pollen
tube
Style
Female gametophyte
(embryo sac)
Antipodal cells
Polar nuclei
Synergids
Egg (n)
Pollen
tube
Zygote (2n)
Nucleus of
developing
endosperm
(3n)
Egg
Nucleus (n)
Sperm
(n)
4 After pollination, eventually
two sperm nuclei
are discharged in
each ovule.
FERTILIZATION
Figure 30.10
5 Double fertilization occurs. One sperm
fertilizes the egg, forming a zygote. The
other sperm combines with the two polar
nuclei to form the nucleus of the endosperm,
which is triploid in this example.
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Discharged
sperm nuclei (n)
Angiosperm Evolution
• Clarifying the origin and diversification of
angiosperms
– Poses fascinating challenges to evolutionary
biologists
• Angiosperms originated at least 140 million
years ago
– And during the late Mesozoic, the major
branches of the clade diverged from their
common ancestor
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Fossil Angiosperms
• Primitive fossils of 125-million-year-old
angiosperms
– Display both derived and primitive traits
Carpel
Stamen
5 cm
(a) Archaefructus sinensis, a 125-million-yearold fossil.
(b) Artist’s reconstruction of
Archaefructus sinensis
Figure 30.11a, b
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An “Evo-Devo” Hypothesis of Flower Origins
• In hypothesizing how pollen-producing and
ovule-producing structures were combined into
a single flower
– Scientist Michael Frohlich proposed that the
ancestor of angiosperms had separate pollenproducing and ovule-producing structures
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Angiosperm Diversity
• The two main groups of angiosperms
– Are monocots and eudicots
• Basal angiosperms
– Are less derived and include the flowering
plants belonging to the oldest lineages
• Magnoliids
– Share some traits with basal angiosperms but
are more closely related to monocots and
eudicots
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• Exploring Angiosperm Diversity
BASAL ANGIOSPERMS
Amborella trichopoda
Star anise (Illicium
floridanum)
Water lily (Nymphaea
“Rene Gerard”)
MAGNOLIIDS
Southern magnolia (Magnolia
grandiflora)
Figure 30.12
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Eudicots
Monocots
Magnoliids
Star anise
and relatives
Water lilies
Amborella
HYPOTHETICAL TREE OF FLOWERING PLANTS
• Exploring Angiosperm Diversity
EUDICOTS
MONOCOTS
Monocot
Characteristics
Orchid
(Lemboglossum
fossii)
Eudicot
Characteristics
California
poppy
(Eschscholzia
californica)
Embryos
One cotyledon
Two cotyledons
Leaf
venation
Veins usually
netlike
Veins usually
parallel
Pygmy date palm
(Phoenix roebelenii)
Pyrenean oak
(Quercus
pyrenaica)
Stems
Lily (Lilium
“Enchantment”)
Vascular tissue
usually arranged
in ring
Vascular tissue
scattered
Roots
Barley (Hordeum vulgare),
a grass
Root system
Usually fibrous
(no main root)
Dog rose (Rosa canina), a wild rose
Taproot (main root)
usually present
Pollen
Pollen grain with
one opening
Pea (Lathyrus
nervosus,
Lord Anson’s
blue pea), a
legume
Pollen grain with
three openings
Flowers
Anther
Stigma
Filament
Figure 30.12
Ovary
Floral organs
usually in
multiples of three
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Floral organs usually
in multiples of
four or five
Zucchini
(Cucurbita
Pepo), female
(left) and
male flowers
Evolutionary Links Between Angiosperms and Animals
• Pollination of flowers by animals and transport
of seeds by animals
– Are two important relationships in terrestrial
ecosystems
(a) A flower pollinated by
honeybees. This honeybee is
harvesting pollen and nectar (a
sugary solution secreted by
flower glands) from a Scottish
broom flower. The flower has a
tripping mechanism that arches
the stamens over the bee
and dusts it with pollen, some of
which will rub off onto the stigma
of the next flower the bee visits.
(b) A flower pollinated by hummingbirds.
The long, thin beak and tongue of this
rufous hummingbird enable the animal to
probe flowers that secrete nectar deep
within floral tubes. Before the hummer
leaves, anthers will dust its beak and
head feathers with pollen. Many flowers
that are pollinated by birds are red or
pink, colors to which bird eyes are
especially sensitive.
Figure 30.13a–c
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(c) A flower pollinated by nocturnal animals. Some
angiosperms, such as this cactus, depend mainly on
nocturnal pollinators, including bats. Common
adaptations of such plants include large, light-colored,
highly fragrant flowers that nighttime pollinators can
locate.
• Concept 30.4: Human welfare depends greatly
on seed plants
• No group is more important to human survival
than seed plants
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Products from Seed Plants
• Humans depend on seed plants for
– Food
– Wood
– Many medicines
Table 30.1
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Threats to Plant Diversity
• Destruction of habitat
– Is causing extinction of many plant species
and the animal species they support
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