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

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.

3. What human reproductive organ is functionally similar to this seed?
Figure 30.1 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

5. Gametophyte / sporophyte relationships in different plant groups
Mosses and other nonvascular plants
Ferns and other seedless vascular plants
Seed plants (gymnosperms and angiosperms)
Reduced, independent (photosynthetic and free-living)
Reduced (usually microscopic), dependent on surrounding sporophyte tissue for nutrition
Gametophyte
Dominant
Sporophyte
Reduced, dependent on gametophyte for nutrition
Dominant
Dominant
Gymnosperm
Angiosperm
Sporophyte (2n)
Microscopic female gametophytes (n) inside ovulate cone
Microscopic female gametophytes (n) inside these parts of flowers
Sporophyte (2n)
Gametophyte (n)
Example
Figure 30.2 Gametophyte/sporophyte relationships in different plant groups
Microscopic male gametophytes (n) inside these parts of flowers
Microscopic male gametophytes (n) inside pollen cone
Sporophyte (2n)
Sporophyte (2n)
Gametophyte (n)

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.

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.

8. (a) Unfertilized ovule
From ovule to seed in a gymnosperm
Integument
Spore wall
Immature female cone
Megasporangium (2n)
Figure 30.3a From ovule to seed in a gymnosperm
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.

10. Female gametophyte (n)
From ovule to seed in a gymnosperm
Female gametophyte (n)
Spore wall
Egg nucleus (n)
Male gametophyte (within a germinated pollen grain) (n)
Discharged sperm nucleus (n)
Figure 30.3b From ovule to seed in a gymnosperm
Micropyle
Pollen grain (n)
(b) Fertilized ovule

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.

12. Seed coat (derived from integument)
From ovule to seed in a gymnosperm
Seed coat (derived from integument)
Food supply (female gametophyte tissue) (n)
Figure 30.3c From ovule to seed in a gymnosperm
Embryo (2n) (new sporophyte)
(c) Gymnosperm seed

13. From ovule to seed in a gymnosperm
Seed coat (derived from integument)
Integument
Female gametophyte (n)
Spore wall
Egg nucleus (n)
Immature female cone
Food supply (female gametophyte tissue) (n)
Male gametophyte (within a germinated pollen grain) (n)
Megasporangium (2n)
Discharged sperm nucleus (n)
Embryo (2n) (new sporophyte)
Megaspore (n)
Micropyle
Pollen grain (n)
Figure 30.3 From ovule to seed in a gymnosperm
(a) Unfertilized ovule
(b) Fertilized ovule
(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).

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.

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.

17. Ginkgo biloba Pollen-producing tree with fleshy seeds
Gymnosperm
Figure 30.5 Gymnosperm diversity
Ginkgo biloba Pollen-producing tree with fleshy seeds

18. Gymnosperm
Ovulate cones
Figure 30.5 Gymnosperm diversity
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.

20. Gymnosperms: Conifers perform year round photosynthesis
Figure 30.5 Gymnosperm diversity
Douglas fir

21. Giant Sequoia: 2,500 tons / 1,800 - 2,700 years old
Gymnosperms: Conifers Sequoia - One of the Largest and Oldest Living Organisms
Figure 30.5 Gymnosperm diversity
Giant Sequoia: 2,500 tons / 1, ,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.

23. Life Cycle of a Pine Key Haploid (n) Ovule Diploid (2n) Ovulate cone
Megasporocyte (2n)
Integument
Pollen cone
Microsporocytes (2n)
Mature sporophyte (2n)
Megasporangium (2n)
Pollen grain
Pollen grains (n)
MEIOSIS
MEIOSIS
Microsporangia
Microsporangium (2n)
Surviving megaspore (n)
Seedling
Archegonium
Figure 30.6 The life cycle of a pine
Seeds
Female gametophyte
Food reserves (n)
Sperm nucleus (n)
Seed coat (2n)
Pollen tube
Embryo (2n)
FERTILIZATION
Egg nucleus (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.

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.

26. Stigma Carpel Stamen Anther Style Filament Ovary Petal Sepal Ovule
Structure of an Idealized Flower
Stigma
Carpel
Stamen
Anther
Style
Filament
Ovary
Figure 30.7 The structure of an idealized flower
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.

28. Tomato Ruby grapefruit Nectarine Hazelnut Milkweed Fruits
Figure 30.8 Some variations in fruit structure
Hazelnut
Milkweed

29. Wings Seeds within berries Barbs Fruit Adaptations for Seed Dispersal
Figure 30.9 Fruit adaptations that enhance seed dispersal
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.

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.

32. Zygote 2n and endosperm (food) 3n
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.

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

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
Figure Angiosperm evolutionary history
Ovules
Millions of years ago
(a)
A possible ancestor of the angiosperms?
(b)
Angiosperm phylogeny

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.

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
Root system usually fibrous (no main root)
Taproot (main root) usually present
Figure Angiosperm diversity
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.

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

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.

42. Table 30.1

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.

44. Summary Five Derived Traits of Seed Plants Reduced gametophytes
Microscopic male and female gametophytes (n) are nourished and
protected by the sporophyte (2n)
Male gametophyte
Female gametophyte
Heterospory
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. Charophyte green algae
Plant Evolutionary Relationships: Clades
Charophyte green algae
Mosses
Ferns
Gymnosperms
Angiosperms

46. You should now be able to:
Explain why pollen grains were an important adaptation for successful reproduction on land.
List the four phyla of gymnosperms.
Describe the life history of a pine; indicate which structures are part of the gametophyte generation and which are part of the sporophyte generation.

47. You should now be able to:
Identify and describe the function of the following floral structures: sepals, petals, stamens, carpels, filament, anther, stigma, style, ovary, and ovule.
Explain how fruits may be adapted to disperse seeds.
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.
Describe the current threat to plant diversity caused by human population growth.