1. Plant Diversity II: The Evolution of Seed Plants
Chapter 30
Plant Diversity II: The Evolution of Seed Plants

2. 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

3. In addition to seeds, the following are common to all seed plants
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

4. Advantages of Reduced Gametophytes
The gametophytes of seed plants
Develop within the walls of spores retained within tissues of the parent sporophyte

5. Gametophyte/sporophyte relationships
Sporophyte dependent
on gametophyte
(mosses and other
bryophytes).
(a)
Large sporophyte and
small, independent
gametophyte (ferns
and other seedless
vascular plants).
(b)
Microscopic female
gametophytes (n) in
ovulate cones
(dependent)
Microscopic female
gametophytes (n)
inside these parts
of flowers
(dependent)
Microscopic male
gametophytes (n)
inside these parts
of flowers
(dependent)
Microscopic male
gametophytes (n)
in pollen cones
(dependent)
Sporophyte (2n),
the flowering plant
(independent)
Sporophyte (2n)
(independent)
Reduced gametophyte dependent on sporophyte
(seed plants: gymnosperms and angiosperms).
(c)
Figure 30.2a–c

6. 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

7. Ovules and Production of Eggs
An ovule consists of
A megasporangium, megaspore, and protective integuments
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.)
Integument
Spore wall
Megasporangium
(2n)
Megaspore (n)

8. 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

9. 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)
Pollen grain (n)
Micropyle
(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.
Figure 30.3b

10. Pollen, which can be dispersed by air or animals
Eliminated the water requirement for fertilization

11. 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
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.
(c)
Seed coat
(derived from
Integument)
Food supply
(female
gametophyte
tissue) (n)
Embryo (2n)
(new sporophyte)
Figure 30.3c

12. Concept 30.2: Gymnosperms bear “naked” seeds, typically on cones
Among the gymnosperms are many well-known conifers
Or cone-bearing trees, including pine, fir, and redwood

13. The gymnosperms include four plant phyla
Cycadophyta
Gingkophyta
Gnetophyta
Coniferophyta

14. Exploring Gymnosperm Diversity
Gnetum
Ephedra
Ovulate cones
Welwitschia
PHYLUM GNETOPHYTA
PHYLUM CYCADOPHYTA
PHYLUM GINKGOPHYTA
Cycas revoluta
Figure 30.4

15. Exploring Gymnosperm Diversity
Douglas fir
Pacific
yew
Common juniper
Wollemia pine
Bristlecone pine
Sequoia
PHYLUM CYCADOPHYTA
Figure 30.4

16. 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

17. 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

18. 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

19. The life cycle of a pine Figure 30.6 Microsporocytes pollen cone
An ovulate cone scale has two
ovules, each containing a mega-
sporangium. Only one ovule is shown. 2
Figure 30.6
Ovule
Megasporocyte (2n)
Integument
Longitudinal
section of
ovulate cone
Ovulate
cone
Pollen
Mature
sporophyte
(2n)
pollen cone
Microsporocytes
grains (n)
(containing male
gametophytes)
MEIOSIS
Micropyle
Germinating
pollen grain
Megasporangium
Sporophyll
Microsporangium
Surviving
megaspore (n)
Archegonium
Egg (n)
Female
gametophyte
pollen grain (n)
Discharged
sperm nucleus (n)
tube
Egg nucleus (n)
FERTILIZATION
Seed coat
(derived from
parent
sporophyte) (2n)
Food reserves
(gametophyte
tissue) (n)
Embryo
(new sporophyte)
Seeds on surface
of ovulate scale
Seedling
Key
Diploid (2n)
Haploid (n)
In most
conifer species,
each tree has
both ovulate
and pollen
cones. 1
A pollen grain
enters through
the micropyle
and germinates,
forming a pollen
tube that slowly
digests
through the
megasporangium. 4
While the
pollen tube
develops, the
megasporocyte
(megaspore
mother cell)
undergoes meiosis,
producing four
haploid cells. One
survives as a
megaspore. 5
A pollen cone contains many microsporangia
held in sporophylls. Each microsporangium
contains microsporocytes (microspore mother
cells). These undergo meiosis, giving rise to
haploid microspores that develop into
pollen grains. 3
The female gametophyte
develops within the megaspore
and contains two or three
archegonia, each with an egg. 6
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. 8
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. 7

20. Concept 30.3: The reproductive adaptations of angiosperms include flowers and fruits
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

21. Characteristics of Angiosperms
The key adaptations in the evolution of angiosperms
Are flowers and fruits

22. Flowers
The flower
Is an angiosperm structure specialized for sexual reproduction

23. 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
Anther
Filament
Stigma
Style
Ovary
Carpel
Petal
Receptacle
Ovule
Sepal
Stamen
Figure 30.7

24. Fruits Fruits Typically consist of a mature ovary Figure 30.8a–e
(b) Ruby grapefruit, a fleshy fruit with a hard outer layer and soft inner layer of pericarp
(a) Tomato, a fleshy fruit with soft outer and inner layers of pericarp
(c) Nectarine, a fleshy fruit with a soft outer layer and hard inner layer (pit) of pericarp
(e) Walnut, a dry fruit that remains closed at maturity
(d) Milkweed, a dry fruit that splits open at maturity

25. Can be carried by wind, water, or animals to new locations, enhancing seed dispersal
Wings enable maple fruits
to be easily carried by the wind.
(a)
Seeds within berries and other
edible fruits are often dispersed
in animal feces.
(b)
The barbs of cockleburs
facilitate seed dispersal by
allowing the fruits to
“hitchhike” on animals.
(c)
Figure 30.9a–c

26. 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

27. The life cycle of an angiosperm
Key
Mature flower on
sporophyte plant
(2n)
Ovule with
megasporangium (2n)
Female gametophyte
(embryo sac)
Nucleus of
developing
endosperm
(3n)
Discharged
sperm nuclei (n)
Pollen
tube
Male gametophyte
(in pollen grain)
Sperm
Surviving
megaspore
(n)
Microspore (n)
Generative cell
Tube cell
Stigma
Ovary
MEIOSIS
Megasporangium
grains
Egg
Nucleus (n)
Zygote (2n)
Antipodal cells
Polar nuclei
Synergids
Egg (n)
Embryo (2n)
Endosperm
(food
Supply) (3n)
Seed coat (2n)
Seed
FERTILIZATION
Haploid (n)
Diploid (2n)
Anther
Style
Microsporangium
Microsporocytes (2n)
Germinating
Anthers contain microsporangia.
Each microsporangium contains micro-
sporocytes (microspore mother cells) that
divide by meiosis, producing microspores. 1
Microspores form
pollen grains (containing
male gametophytes). The
generative cell will divide
to form two sperm. The
tube cell will produce the
pollen tube. 2
When a seed
germinates, the
embryo develops
into a mature
sporophyte. 7
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
(embryo sac). 3
The zygote
develops into an
embryo that is
packaged along
with food into a
seed. (The fruit
tissues surround-
ing the seed are
not shown). 6
After pollina-
tion, eventually
two sperm nuclei
are discharged in
each ovule. 4
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. 5
Figure 30.10

28. Clarifying the origin and diversification of angiosperms
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

29. Fossil Angiosperms
Primitive fossils of 125-million-year-old angiosperms
Display both derived and primitive traits
Carpel
Stamen
Archaefructus sinensis, a 125-million-year-
old fossil.
(a)
Artist’s reconstruction of
Archaefructus sinensis
(b)
5 cm
Figure 30.11a, b

30. 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 pollen-producing and ovule-producing structures

31. The two main groups of angiosperms
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

32. HYPOTHETICAL TREE OF FLOWERING PLANTS
Exploring Angiosperm Diversity
Amborella trichopoda
Water lily (Nymphaea
“Rene Gerard”)
Star anise (Illicium
floridanum)
BASAL ANGIOSPERMS
HYPOTHETICAL TREE OF FLOWERING PLANTS
MAGNOLIIDS
Amborella
Water lilies
Star anise
and relatives
Magnoliids
Monocots
Eudicots
Southern magnolia (Magnolia
grandiflora)
Figure 30.12

33. Dog rose (Rosa canina), a wild rose
Exploring Angiosperm Diversity
Orchid
(Lemboglossum
fossii)
Monocot
Characteristics
Embryos
Leaf
venation
Stems
Roots
Pollen
Flowers
Pollen grain with
one opening
Root system
Usually fibrous
(no main root)
Vascular tissue
scattered
Veins usually
parallel
One cotyledon
Two cotyledons
netlike
usually arranged
in ring
Taproot (main root)
usually present
three openings
Zucchini
(Cucurbita
Pepo), female
(left) and male flowers
Pea (Lathyrus nervosus,
Lord Anson’s blue pea), a legume
Dog rose (Rosa canina), a wild rose
Pygmy date palm
(Phoenix roebelenii)
Lily (Lilium
“Enchant-
ment”)
Barley (Hordeum vulgare),
a grass
Anther
Stigma
California
poppy
(Eschscholzia
californica)
Pyrenean oak
(Quercus
pyrenaica)
Floral organs
usually in
multiples of three
Floral organs usually
in multiples of
four or five
Filament
Ovary
Eudicot
MONOCOTS
EUDICOTS
Figure 30.12

34. 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.
(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.
(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

35. Concept 30.4: Human welfare depends greatly on seed plants
No group is more important to human survival than seed plants

36. Products from Seed Plants
Humans depend on seed plants for
Food
Wood
Many medicines
Table 30.1

37. Threats to Plant Diversity
Destruction of habitat
Is causing extinction of many plant species and the animal species they support