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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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Seeds
• 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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Seeds
• 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
– Protects gametophytes from environmental
stresses
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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
***Seedless vascular plants have bisexual gametophytes
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Ovules and Production of Eggs
• An ovule consists of
–
A megasporangium, megaspore, & 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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Evolutionary Advantage of Pollen
• 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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Gymnosperms
• Bear “naked” seeds, typically on cones
• Among the gymnosperms are many well-known conifers
– Or cone-bearing trees, including pine, fir, & redwood
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Gymnosperms
• The gymnosperms include four plant phyla
– Cycadophyta
– Gingkophyta
– Gnetophyta
– Coniferophyta
Welwitschia
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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
Seed Plants
• 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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Life Cycle of a Pine
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Angiosperms
• 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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Flowers
• 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
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Flower Anatomy
Stamens
Stamens are composed of an anther and a filament. The
anther contains microsporangia. Microspores and
microgametophytes are produced within the anther.
Ovules
Ovules are structures that will become seeds. They contain
outer protective coverings called integuments and a
megasporangium within the integuments. Within the
megasporangium, megaspores are produced by meiosis. The
megaspores produce megagametophytes, which, in turn,
produce eggs.
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Pistil
All of the female reproductive structures form the pistil.
This includes the stigma, style, and ovary.
Each chamber within a pistil is called a carpel. It evolved
from a leaf that contained sporangia on its edges. Over
evolutionary time, the leaf became curled to enclose the
sporangia as seen in carpels today.
A simple pistil is also called a carpel because it has only one
chamber.
A compound pistil contains several carpels that have become
fused as a result of evolutionary change.
Ovary
The bottom portion of a pistil is the ovary. It contains
ovules. As the reproductive process proceeds, the ovary
enlarges and becomes the fruit and the ovules become
seeds.
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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
Seeds
• 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 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
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.
Products from Seed Plants
• Humans depend on seed plants for
– Food
– Wood
– Many medicines
Table 30.1
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