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Ch. 30: Plant Diversity II:
Seed Plants
1
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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
 Seeds can disperse over long distances by wind
or other means
 In addition to seeds, the following are common to
all seed plants:
Reduced gametophytes
Ovules
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Heterospory
Pollen
Figure 30.2
PLANT GROUP COMPARISONS
Mosses and other
nonvascular plants
Gametophyte Dominant
Sporophyte
Ferns and
other seedless
vascular plants
Reduced,
Independent
(photosynthetic
and free-living)
Reduced, dependent
on gametophyte for Dominant
nutrition
Seed plants (gymnosperms and angiosperms)
Reduced (usually microscopic), dependent on
surrounding sporophyte tissue for nutrition
Dominant
Gymnosperm
Sporophyte
(2n)
Sporophyte
(2n)
Microscopic female
gametophytes (n)
inside ovulate cone
Gametophyte
(n)
Example
Gametophyte
(n)
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Microscopic
male
gametophytes
(n) inside pollen
cone
Sporophyte (2n)
Angiosperm
Microscopic female
gametophytes (n)
inside these parts
of flowers
Microscopic
male
gametophytes
(n) inside
these parts
of flowers
Sporophyte (2n)
Heterospory: The Rule Among Seed Plants
 Seed plants are heterosporous
 The dependent gametophytes of seed plants are
microscopically small and develop from spores that
are retained within tissues of the parent sporophyte
 Within the parent sporophyte are the Megasporangia
which produce megaspores that give rise to female
gametophytes
 Microsporangia produce microspores that give rise
to male gametophytes
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Pollen and Production of Sperm
 Microspores develop into pollen grains, which contain the
male gametophytes
 Pollination is the transfer of pollen
to the part of a seed plant 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 sperm into the female gametophyte within
the ovule
 A seed develops from the whole ovule
 A seed is a sporophyte embryo, along with its food supply
(in the endosperm), packaged in a protective seed coat ,
the integuments
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An ovule consists of a megasporangium, megaspore, and one
or more protective integuments
Immature
ovulate cone
Integument (2n)
Megaspore (n)
Female
gametophyte (n)
Spore wall
Egg nucleus
(n)
Megasporangium
(2n)
Micropyle
Pollen
grain (n)
(a) Unfertilized ovule
Seed coat
Spore
wall
Discharged
sperm nucleus
(n)
Male
Pollen tube
gametophyte (n)
(b) Fertilized ovule
Food
supply
(n)
Embryo (2n)
(c) Gymnosperm seed
Gymnosperm megasporangia have one integument
Angiosperm megasporangia usually have two integuments
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Concept 30.2: Gymnosperms bear “naked”
seeds, typically on cones
• Gymnosperms were better suited than
nonvascular plants to drier conditions.
• Cone-bearing gymnosperms called conifers
dominate in the northern latitudes
• Gymnosperms means “naked seeds”
 The seeds are exposed on sporophylls that
(typically) form cones.
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The Life Cycle of a Pine
 Three key features of the gymnosperm life cycle
are
 Miniaturization of their gametophytes
 Development of seeds from fertilized ovules
 The transfer of sperm to ovules by pollen
 The life cycle of a pine provides an example
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• The pine tree is the sporophyte and produces sporangia
in male and female cones. Most have both types of cones
• Small cones contain hundreds of microsporangia held on
small sporophylls. Each cone produces microspores that
develop into pollen grains containing the male
gametophyte.
• The familiar larger cones contain ovules that undergo
meiosis, which produce haploid megaspores that develop
into female gametophytes (retained in the sporangia
cone)
• It takes nearly three years from cone production to mature
seed
• During pollination, windblown pollen falls on the ovulate
cone and grows into the ovule through the micropyle-fertilization follows
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Figure 30.4
Ovule
Megasporocyte (2n)
Ovulate cone
Integument
Pollen cone
Microsporocytes
Pollen
(2n)
grains (n)
Mature
sporophyte
(2n)
Megasporangium
(2n)
MEIOSIS Germinating MEIOSIS
pollen grain
Microsporangia
Microsporangium (2n)
Seedling
Surviving
megaspore (n)
Archegonium
Seeds
Female
gametophyte
Food
reserves Seed
coat (2n)
Sperm
nucleus (n)
Pollen
tube
Embryo
(new sporophyte)
(2n)
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Key
FERTILIZATION
Egg
nucleus (n)
Haploid (n)
Diploid (2n)
Generations and Seed Production:
The gymnosperm embryo, the new sporophyte, is
surrounded by the female gametophyte which nourishes
the embryo. Therefore,
The ovule develops into a seed, which consists of an
embryo (new sporophyte), its food supply (derived from
gametophyte tissue), and a seed coat derived from the
integuments of the parent tree (parent sporophyte)-- 3
generations in 1!
It takes three years from the appearance of young cones
on a tree to the formation of mature seeds.
The scales of ovulate cone separate and the seeds are
typically dispersed by the wind.
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 The gymnosperms consist of four phyla
 Cycadophyta (cycads)
 Gingkophyta (one living species: Ginkgo biloba)
 Gnetophyta (three genera: Gnetum, Ephedra,
Welwitschia)
 Coniferophyta (conifers, such as pine, fir, and
redwood)
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Phylum Cycadophyta
 Individuals have large cones and palmlike leaves
 Unlike most seed plants, cycads have flagellated sperm
 These thrived during the Mesozoic, but most of the few
surviving species are endangered
Cycas revoluta
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Figure 30.7b
Phylum Ginkgophyta
This phylum consists of a single
living species, Ginkgo biloba
Like the cycads, this group also
has flagellated sperm
It has a high tolerance to air
pollution and is a popular
ornamental tree
Ginkgo biloba
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right picture from nature2health.blogspot.com
Phylum Gnetophyta
 This phylum comprises three genera: Gnetum,
Ephedra, and Welwitschia
 Species vary in appearance, and some are tropical
whereas others live in deserts
Ovulate
cones
Welwitschia
Gnetum
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Ephedra
Phylum Coniferophyta
• This phylum is by far the largest of the gymnosperm
phyla
• Most conifers are evergreens and can carry out
photosynthesis year round
• Named for the cone, which is a cluster of scalelike
sporophylls
• Needle shaped leaves are adapted to dry conditions as
well as more easily shed snow
• Waxy cuticles and stomates located in pits help limit
water loss
• Examples include fir, spruce, hemlock, yew, sequoia,
cedar, pine, cypress, tamarack
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Douglas fir
Sequoia
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European larch
Figure 30.7e
Phylum
Coniferophyta
Common juniper
Wollemi pine
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Bristlecone pine
Concept 30.3: The reproductive adaptations of
angiosperms include flowers and fruits
 Angiosperms are the most widespread and diverse of all
plants. Angiosperms now dominate more terrestrial
ecosystems though conifers still dominate in some
regions including the northern latitudes.
 All angiosperms are classified in a single phylum,
Anthophyta, from the Greek anthos for flower
 Angiosperms are seed plants with two key adaptations:
 Reproductive structures called Flowers
 and Fruits, the mature ovaries where seeds are
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Flowers
 The flower is an angiosperm structure specialized for sexual
reproduction
 Many species are pollinated by insects or animals, while
some species are wind-pollinated
 A flower is a specialized shoot with up to four types of
modified leaves called floral organs:
 Sepals, which enclose the flower
 Petals, which are brightly colored and attract pollinators
 Stamens, which produce pollen-- A stamen consists of a
stalk called a filament, with a sac called an anther where
the pollen is produced
 Carpels, which produce ovules--A carpel consists of an
ovary at the base and a style leading up to a stigma,
where pollen is received
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Figure 30.8
Stigma
Stamen
Anther
Carpel
Style
Filament
Ovary
Petal
Sepal
Ovule
Receptacle
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 Flowers that have all four organs are called complete
flowers
 Those lacking one or more organs are called
incomplete flowers
Symmetry
 Flowers may have radial symmetry or bilateral
symmetry
 For flowers with radial symmetry, any imaginary line
through the central axis divides the flower into two
equal parts
 In bilateral symmetry, a flower can only be divided into
two equal parts by a single imaginary line
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Figure 30.9
Location of Stamens
and Carpels
Symmetry
Common
holly
flowers
with
stamens
Sepal
Radial
symmetry
(daffodil)
Fused petals
Stamens
Carpel
Bilateral
symmetry
(orchid)
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Common
holly
flowers
with
carpels
Nonfunctional
stamen
Location of Stamens and Carpels
 Most species have flowers with both functional stamens
and carpels, but in some species they occur on separate
flowers= monoecious
 Flowers with stamens may be on the same plant as
those with carpels, or they may occur on different plants
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Fruits
• A fruit is formed when the ovary wall thickens and
matures
• A fruit typically consists of a mature ovary but can also
include other flower parts
• Fruits protect seeds and aid in their dispersal
• Mature fruits can be either fleshy or dry
• Various fruit adaptations help disperse seeds
• Examples include burrs, milkweed fuzz, maple
helicopters, nuts, pits, and seeds in all different types
of fruits
• Seeds can be carried by wind, water, or animals to new
locations
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Figure 30.10
Tomato: a berry
Apple: a pome
Ruby grapefruit: a hesperidium
Nectarine: a drupe
Pumpkin: a pepo
Hazelnut: a nut
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Milkweed: a follicle
Figure 30.11
▼
 Seeds provide some
evolutionary advantages
over spores
▼
 They may remain
dormant for days to
years, until conditions
are favorable for
germination
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Wings
▼
▼
 Seeds have a supply
of stored food
 They may be
transported long
distances by wind or
animals
Mechanisms that
disperse seeds by
explosive action
Barbs
Seeds within
berries and
other edible
fruits
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, or embryo sac, develops within
an ovule contained within an ovary at the base of a stigma
 Most flowers have mechanisms to ensure crosspollination between flowers from different plants of the
same species
 A pollen grain that has landed on a stigma germinates and
the pollen tube of the male gametophyte grows down to the
ovary
 The ovule is entered by a pore called the micropyle
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 Double fertilization occurs when the 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 central cell of the female
gametophyte and initiates development of food-storing
endosperm
 The triploid endosperm nourishes the developing
embryo
 Within a seed, the embryo consists of a root and two
seed leaves called cotyledons
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Figure 30.12
Carpel
Anther
Mature flower on
sporophyte plant
(2n)
Germinating
seed
Nucleus of
developing
endosperm
(3n)
Microsporangium
Microsporocytes (2n)
MEIOSIS
Microspore
(n)
Generative cell
Tube cell
Ovule with
megasporangium
Tube nucleus
(2n)
Male gametophyte
(in pollen grain)
(n)
Ovary
Pollen
MEIOSIS
grains
Stigma
Pollen tube
Megasporangium
Surviving
Sperm
(2n)
megaspore
Embryo (2n)
Integuments
(n)
Tube
Endosperm (3n)
nucleus
Seed
Micropyle
Seed coat (2n)
Antipodal cells
Polar nuclei
Style
Female
in central cell
gametophyte
(embryo sac)
Synergids
Egg (n)
Zygote (2n)
Egg
nucleus (n)
FERTILIZATION
Key
Discharged sperm nuclei (n)
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Haploid (n)
Diploid (2n)
Angiosperm Diversity
 Angiosperms comprise more than 250,000 living
species
 Previously, angiosperms were divided into two main
groups
 Monocots (one cotyledon)
 Dicots (two dicots)
 DNA studies suggest that monocots from a clade while
dicots are paraphyletic. The clade eudicot (“true”
dicots) includes most dicots
 The rest of the former dicots form several small lineages
and include water lilies, magnolias, and laurels
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Figure 30.17c
Monocots
Orchid
(Lemboglossum rossii)
Pygmy date palm (Phoenix roebelenii)
Monocots
More than one-quarter of
angiosperm species are
monocots
The largest groups are the
orchids, grasses, and palms
Barley (Hordeum vulgare), a grass
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Figure 30.17d
Eudicots
Snow pea (Pisum
Dog rose (Rosa canina), a wild rose
sativum), a legume
Pyrenean oak
(Quercus pyrenaica)
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Eudicots
More than two-thirds of
angiosperm species are
eudicots
Eudicots include the large
legume family and the
economically important rose
family
Figure 30.16
Embryos
Leaf venation
One
cotyledon
Veins usually
parallel
Stems
Roots
Pollen
Flowers
Monocot
Characteristics
Root system
Vascular tissue usually fibrous Pollen grain
scattered
with one
(no main root)
opening
Floral organs
usually in
multiples
of three
Eudicot
Characteristics
Vascular tissue
Two
Veins usually usually arranged
cotyledons
netlike
in ring
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Taproot
Pollen grain
(main root)
with three
usually present
openings
Floral organs
usually in
multiples of
four or five
Evolutionary Links with Animals
 Animals influence the evolution of plants and vice
versa
 For example, animal herbivory selects for plant
defenses
 For example, interactions between pollinators and
flowering plants select for mutually beneficial
adaptations
 Bilateral symmetry affects the movement of pollinators
and reduces gene flow in diverging populations
 Plants with bilateral symmetry may have increased
rates of speciation
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Figure 30.15
Stamens
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Concept 30.4: Human welfare depends on seed
plants
 Seed plants are key sources of food, fuel, wood products,
and medicine
 Our reliance on seed plants makes preservation of plant
diversity critical
 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
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Table 30.1
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Threats to Plant Diversity
• Destruction of habitat is causing extinction of many plant
species
• In the tropics 55,000 km2 are cleared each year
• At this rate, the remaining tropical forests will be
eliminated in 200 years
• Loss of forests reduces the absorption of atmospheric
CO2 that occurs during photosynthesis
• 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
• The tropical rain forests may contain undiscovered
medicinal compounds
39
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Figure 30.18
A satellite image
from 2000 shows
clear-cut areas in
Brazil surrounded
by dense tropical
forest.
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By 2009, much
more of this same5 km
tropical forest had
been cut down.
Figure 30.UN05
Five Derived Traits of Seed Plants
Reduced
gametophytes
Heterospory
Microscopic male and
female gametophytes
(n) are nourished and
protected by the
sporophyte (2n)
Male
gametophyte
Female
gametophyte
Microspore (gives rise to
a male gametophyte)
Megaspore (gives rise to
a female gametophyte)
Ovules
Integument (2n)
Ovule
(gymnosperm)
Megaspore (n)
Megasporangium (2n)
Pollen
Pollen grains make water
unnecessary for fertilization
Seeds
Seeds: survive
better than
unprotected
spores, can be
transported
long distances
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Seed coat
Food supply
Embryo