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 course of plant evolution, enabling their bearers to become dominant producers in most terrestrial ecosystems
Seed consists of embryo and nutrients surrounded by protective coat
Humans starting cultivating wheat, figs, maize, bananas, other wild seed plants ~13,000 years ago

3. In addition to seeds, following are common to all seed plants
Concept 30.1: Seeds and pollen grains are key adaptations for life on land
In addition to seeds, following are common to all seed plants
Reduced gametophytes
Heterospory
Ovules
Pollen
Allows plants to cope with terrestrial conditions (drought, exposure to UV radiation)

4. Advantages of Reduced Gametophytes
Gametophytes of seed plants develop within walls of spores that are retained within tissues of parent sporophyte
Protects delicate female (egg-containing) gametophytes from environmental stresses (desiccation/UV)
Dependent gametophytes obtain nutrients from sporophyte
Free-living gametophytes of seedless plants fend for themselves

5. Fig. 30-2
PLANT GROUP
Mosses and other nonvascular plants
Ferns and other seedless vascular plants
Seed plants (gymnosperms and angiosperms)
Reduced, independent (photosynthetic/free-living)
/visible to naked eye
Reduced (usually microscopic), dependent on surrounding sporophyte tissue for nutrition
Gametophyte
Dominant
Reduced, dependent on gametophyte for nutrition
Sporophyte
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
Ancestors of seed plants were likely homosporous (produce one kind of spore, usually giving rise to bisexual gametophyte), while seed plants are heterosporous
Megasporangia produce megaspores (each has single functioning one) that give rise to female gametophytes
Microsporangia produce microspores (each produces vast numbers of) that give rise to male gametophytes

7. Ovules and Production of Eggs
Ovule consists of
Megasporangium
Megaspore
One/more protective integuments (layer of sporophyte tissue that envelopes/ protects megasporangium)
Gymnosperm megaspores have one integument
Angiosperm megaspores usually have two

8. Pollen and Production of Sperm
Microspores develop into pollen grains, which contain gametophytes enclosed within pollen wall
Pollination: transfer of pollen to part of seed plant containing ovules
Pollen eliminates need for film of water and can be dispersed great distances by air or animals
Flagella lost in most gymnosperms (except some, like ginkgos/cycads)/ all angiosperms
If pollen grain germinates, it gives rise to pollen tube that discharges two sperm into female gametophyte within ovule

9. The Evolutionary Advantage of Seeds
Seed develops from whole ovule and includes sporophyte embryo/food supply packaged in protective coat
Seeds provide some evolutionary advantages over spores
Extra layers of seed coat provide extra protection to embryo over single-celled spores
Seeds have stored food/spores don’t which enables seeds to remain dormant for days to years, until conditions are favorable for germination, and then give them critical support for growth
May be transported long distances by wind or animals

10. Five Derived Traits of Seed Plants
Fig. 30-UN3
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

11. Concept 30.2: Gymnosperms bear “naked” seeds, typically on cones
Gymnosperms have “naked” seeds not enclosed by ovaries
Seeds exposed on modified leaves (sporophylls) that usually form cones (stobili)
Extant gymnosperms include
Cycadophyta (cycads)
Gingkophyta (one living species: Ginkgo biloba)
Gnetophyta (three genera: Gnetum, Ephedra, Welwitschia)
Coniferophyta (conifers, such as pine, fir, and redwood)

12. Gymnosperm Evolution
Fossil evidence reveals that by late Devonian period some plants, called progymnosperms, had begun to acquire some adaptations that characterize seed plants (woody but did not bear seeds)
First seed-bearing plants (gymnosperms) appear early in fossil record ~360 mya, more than million years before first angiosperm fossils
Early seed plants/later lineages all become extinct but morphological/molecular evidence places surviving lineages of seed plants into two monophyletic sister clades, gymnosperms/angiosperms

13. Gymnosperms appear early in fossil record (~305 million years old) during Carboniferous ecosystems still dominated by lycophytes, horsetails, ferns, other seedless vascular plants
Drier climatic conditions in Permian favored spread of gymnosperms which favored key terrestrial adaptations of seeds/pollen, along with thick cuticles, relatively small surface area of needle-shaped leaves
Dominated Mesozoic terrestrial ecosystems
Although angiosperms dominate most terrestrial ecosystems today, cone-bearing gymnosperms (conifers) dominate in northern latitudes

14. Phylum Cycadophyta (Cycads)
Individuals have large cones and palmlike leaves
Thrived during Mesozoic, but relatively few species exist today
sago palm

15. Phylum Ginkgophyta
This phylum consists of single living species, Ginkgo biloba
High tolerance to air pollution/popular ornamental tree, but only pollen-producing trees (fleshy seeds smell rancid as they decay)

16. Phylum Gnetophyta
Phylum comprises three genera (Gnetum, Welwitschia, Ephedra)
Species vary in appearance/some are tropical/others in deserts

17. Phylum Coniferophyta Phylum is by far largest of gymnosperm phyla
Most conifers are evergreens/carry out photosynthesis year round
(1st live one discovered 1994)
(lumber)
(oldest living trees, >4600 years old)
(“berries” are ovule-producing cones of fleshy sporophylls)
(large/old, cousins (redwoods)
grow to >100 m)
(shed needles/cold tolerant)

18. The Life Cycle of a Pine: A Closer Look
Three key features of gymnosperm life cycle are
Dominance of sporophyte generation
Development of seeds from fertilized ovules
Transfer of sperm to ovules by pollen
Life cycle of pine provides example
Pine tree is sporophyte/produces sporangia in male/female cones
Small cones produce microspores (pollen grains), each of which contains male gametophyte
Familiar larger cones contain ovules, which produce megaspores that develop into female gametophytes
It takes nearly three years from cone production to mature seed

19. 4. Pollination occurs when pollen Pollen grain Pollen grains (n)
Fig
Most conifers have both ovulate/
pollen cones on each tree
3. Ovulate cone
(each scale has two ovules
each w/megasporangium)
Key
Haploid (n)
Ovule
Diploid (2n)
Megasporocyte (2n)
Integument
Pollen cone
Microsporocytes (2n)
Mature sporophyte (2n)
Megasporangium (2n)
4. Pollination occurs when pollen
grain reaches ovule, pollen grain
germinates, forms pollen tube
that slowly digests its way through
megasprangium
Pollen grain
Pollen grains (n)
MEIOSIS
MEIOSIS
Microsporangia
2. Microsporangium (2n)
divide by meiosis/produce haploid
microspores which develop pollen
grain (w/male gametophyte)
Surviving megaspore (n)
5. Megasporocyte undergoes
meiosis, producing four haploid
cells/one survives as megaspore
Seedling
Archegonium
Figure 30.6 The life cycle of a pine
Seeds
Female gametophyte Female gametophyte develops
within megaspore, contains two
or three archegonia, each of which
will form egg
7. By time eggs mature, two sperm
cells developed in pollen tube which
extends to female gametophyte
Fertilization occurs when sperm
and egg nuclei unite, 8. usually one
year after pollination.
All eggs may be fertilized, but only
one zygote develops into embryo
Ovule becomes seed w/embryo,
food supply, seed coat
Food reserves (n)
Sperm nucleus (n)
Seed coat (2n)
Pollen tube
Embryo (2n)
FERTILIZATION
Egg nucleus (n)

20. They are most widespread and diverse of all plants
Concept 30.3: The reproductive adaptations of angiosperms include flowers and fruits Characteristics of Angiosperms
Angiosperms
Angiosperms are seed plants with reproductive structures called flowers and fruits (ripened matured ovaries that contain seeds)
They are most widespread and diverse of all plants
All angiosperms are classified in single phylum, Anthophyta
Name comes from Greek anthos, flower

21. Flowers
Flower: angiosperm structure specialized for sexual reproduction
Pollinated by insects/animals (direct contact) or wind (dense populations)
Specialized shoot with up to four whorls (rings) of modified leaves (floral organs)

22. Carpel produce megaspores Stamen Anther where pollen is Style
Fig. 30-7
Stigma
sticky to
receive pollen
Carpel produce megaspores
and their products,
gametophytes
(ovules)
Stamen
produce microspores
that develop into
pollen grains
Anther where pollen is
produced
Style
leads from
stigma to ovary
Filament stalk
Ovary
At base of carpal
Contains one or more ovules
If fertilized, develop into seed
Figure 30.7 The structure of an idealized flower
Petal
Sterile
Brightly colored
Attract pollinators
Sepal
Sterile
Enclose flower
Ovule

23. Fruits
Fruit typically consists of mature ovary (ovary walls thicken) but can also include other flower parts
Protect dormant seeds/aid in their dispersal
Mature fruits can be either fleshy where ovary walls become soft during ripening (oranges, plums, grapes) or dry (beans, nuts, grains)
Various fruit adaptations help disperse seeds
Carried by wind (parachutes/propellers), water, or animals (burrs) to new locations
When eaten, most pass through digestive tract unharmed/deposited with feces

24. The Angiosperm Life Cycle
Flower of sporophyte composed of both male and female structures
Male gametophytes contained within pollen grains produced by microsporangia of anthers
Each has two haploid cells (generative cell that divides, forming two sperm/tube cell that produces pollen tube
Female gametophyte (embryo sac) develops within ovule contained within ovary at base of stigma
Contains only few cells, one is egg

25. Gnetophyta’s double fertilization leads to two embryos only
Most flowers have mechanisms to ensure cross-pollination between flowers from different plants of same species (enhances genetic variability)
Pollen adheres to stigma  pollen tube  micropyle (pore in integuments of ovule)  two sperm cells discharged into embryo sac  double fertilization 
one fertilizes egg forming diploid zygote  sporophyte embryo w/rudimentary root and one or two seed leaves (cotyledon)
other fuses w/two nuclei in large central cell of female gametophyte producing triploid cell (unique to angiosperms)  divides repeatedly  endosperm (tissue rich in starch/other food reserves for embryo)
Double fertilization may prevent flowering plants from squandering nutrients on infertile ovules (synchronizes development of food storage w/development of embryo)
Gnetophyta’s double fertilization leads to two embryos only

26. 1. On anther, each microsporangium contains
Fig
Key
1. On anther, each microsporangium contains
microsporocytes that divide by meiosis,
producing microspores
Anther
Haploid (n)
Diploid (2n)
Microsporangium Microspore develops into
pollen grain. Generative cell
of gametophyte divides, forming
two sperm. Tube cell produces
pollen tube
Mature flower on sporophyte plant (2n)
7. Seed germinated, embryo develops
into mature sporophyte
Microsporocytes (2n)
MEIOSIS
Generative cell
Microspore (n)
Ovule (2n)
Tube cell
Male gametophyte (in pollen grain) (n)
Ovary
Pollen grains
3. In megasporangium of
each ovule, megasporocyte
divides by meiosis,
producing 4 megaspores.
One survives and forms
female gametophyte
4. After pollination, two
sperm cells discharged in
each ovule
Germinating seed
6. Zygote develops into
embryo packaged w/food
into seed (fruit tissues
surround seed)
MEIOSIS
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
Nucleus of developing endosperm (3n)
5. Double fertilization occurs.
One sperm fertilizes egg,
forming zygote.
Other fertilizes central cell,
forming endosperm (food
supply, 3n)
Sperm (n)
FERTILIZATION
Zygote (2n)
Egg nucleus (n)
Discharged sperm nuclei (n)

27. Angiosperm Evolution
Clarifying origin/diversification of angiosperms poses fascinating challenges to evolutionary biologists
Angiosperms originated at least 140 mya
During late Mesozoic, major branches of clade diverged from their common ancestor
By mid-Cretacous (100 mya), adaptive radiation allowed angiosperms began to dominate many terrestrial ecoosystems

28. Fossil Angiosperms
Primitive fossils of 125-million-year-old angiosperms display derived and primitive traits
Fossils, phylogenetic analyses, developmental studies offer insights into origin of flowers
Archaefructus sinensis has anther/seeds but lacks petals/sepals
May belong to earliest-diverging group of angiosperms known
Suggests that ancestors of flowering plants were herbaceous rather than woody
May have originated as aquatic plants (fossils found w/fish fossils/had bulbous structures)
Much debate/need transitional fossil

29. Archaefructus sinensis, a 125-million-year-old fossil
Fig A primitive flowering plant?
Carpel
Stamen
5 cm
(a)
Archaefructus sinensis, a 125-million-year-old fossil
(may represent sister group
to all other angiosperms)
Figure 30.11
(b)
Artist’s reconstruction of Archaefructus sinensis

30. Angiosperm Phylogeny
Which seed plants, including fossil species, are most closely related to angiosperms?
Molecular/morphological evidence suggest that living gymnosperms are monophyletic group whose earliest lineages diverged from ancestors of angiosperms and gymnosperms ~305 mya
Angiosperms may be closely related to Bennettitales, extinct seed plants with flowerlike structures
Also need to work out order in which angiosperm clades diverged from one another
Amborella/water lilies are likely descended from two of most ancient angiosperm lineages

31. Microsporangia (contain microspores)
Fig 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 30.12
Ovules
Millions of years ago
(a)
A possible ancestor of the angiosperms?
Bennettitales, extinct group of
seed plants might be more
closely related to angiosperms
than to gymnosperms
(b)
Angiosperm phylogeny
One current hypothesis of angiosperm evolutionary
Relationships, based on morphological/molecular
evidence
Angiosperms originated at least 140 mya
Dotted line represents uncertain position of
Bennettitales, possible sister group to angiosperms

32. Developmental Patterns in Angiosperms
Egg formation in angiosperm Amborella resembles that of gymnosperms (possible common ancestor)
Scientists are curious how second integument of angiosperms originated (only one in gymnosperms)
Researchers are currently studying expression of flower development genes in gymnosperm and angiosperm species

33. Angiosperm Diversity
Two main groups of angiosperms are monocots (one cotyledon) and eu’dicots (“true” dicots)
Clade eudicot includes some groups formerly assigned to paraphyletic dicot (two cotyledons) group
Basal angiosperms are less derived and include flowering plants belonging to oldest lineages
Comprised of 3 small lineages (Amborella trichopoda, oldest lineage which diverged into  water lilies/star anise
Magnoliids share some traits with basal angiosperms but are more closely related to monocots and eudicots (evolved later)
Include magnolias/laurels/black pepper plants

34. >¼ are monocots >2/3 are eudicots

35. Evolutionary Links Between Angiosperms and Animals
Animals/plants influenced evolution of each other
To keep herbivores from eating roots/leaves/seeds, plants evolved defenses
Pollination of flowers/transport of seeds by animals are two important relationships in terrestrial ecosystems
Clades with bilaterally symmetrical flowers (insect pollinator obtains nectar only when approaching flower form certain direction) have more species than those with radially symmetrical flowers
Likely bilateral symmetry affects movement of pollinators (have to move from flower to flower) and reduces gene flow in diverging populations
Speciation should occur more rapidly (there are more clades w/bilaterally symmetrical flower than radially symmetrical flowers)

36. Mean difference in number of species Bilateral symmetry (N = 15)
Fig
EXPERIMENT
Time since divergence from common ancestor
“Bilateral” clade
Compare numbers of species
Common ancestor
“Radial” clade
RESULTS: flower shape affects rate at which new species
formed
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)

37. Concept 30.4: Human welfare depends greatly on seed plants
No group of plants is more important to human survival than seed plants
Key sources of food, fuel, wood products, and medicine
Our reliance on seed plants makes preservation of plant diversity critical

38. Products from Seed Plants
Most of our food comes from angiosperms
Six crops (wheat, rice, maize, potatoes, cassava, and sweet potatoes) yield 80% calories consumed by humans
Angiosperms feed livestock (takes 5-7 kg of grain to produce 1 kg of grain-fed beef)
Modern crops are products of relatively recent genetic change resulting from artificial selection
Other edible products (tea, coffee, cacao, spices)
Many seed plants provide wood (fuel, paper, construction)
Secondary compounds of seed plants are used in medicines (25% US Rx drugs contain one or more active ingredients extracted or derived from plants)

39. Table 30-1a
Table 30.1

40. Threats to Plant Diversity
Destruction of habitat is causing extinction of many plant species
Greatest problem in tropics, where >½ lives and where population growing fastest
35 million acres of tropical rain forest (size of Iowa) cleared (slash-and-burn) each year/completely gone in years
Loss of plant habitat is often accompanied by loss of animal species that plants support
At current rate of habitat loss, 50% of Earth’s species will become extinct within next 100–200 years
Explored potential uses of tiny fraction of >290,000 known plant species
Our food is based on cultivation of ~24 species of seed plants
<5000 plant species studied for medicinal purposes

41. 3/25/2017 Division Common name Dominant generation Fluid transport
Sperm transport
Dispersal unit
Bryophyta
Mosses
Haploid
Gametophyte
Nonvascu-lar
Flagellated sperm
Spores
Lycophyta
Club mosses
Sporophyte
Vascular
Sphenophyta
Horsetails
Pterophyta
Ferns
Diploid
Coniferophyta
Conifers
Wind-dispersed pollen
Naked Seeds
Anthophyta
Flowering plants
Wind-, or animal-dispersed pollen
Seeds in flowers
3/25/2017

42. Charophyte green algae
Fig. 30-UN4 a. flowers
b. embryos
c. seeds
d. vascular tissues
Charophyte green algae
Mosses
Ferns
Gymnosperms
Angiosperms

43. You should now be able to:
Explain why pollen grains were an important adaptation for successful reproduction on land
List and distinguish among 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
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
Explain the significance of Archaefructus and Amborella
Describe the current threat to plant diversity caused by human population growth