Download Chapter 38 - Angiosperm Reproduction and Biotechnology

Figure 38.1
CAMPBELL
BIOLOGY
Figure 38.1a
Flowers of Deceit
TENTH
EDITION
Reece • Urry • Cain • Wasserman • Minorsky • Jackson
!  Insects help angiosperms to reproduce sexually
with physically distant members of their own
species
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!  For example, male long-horned bees mistake
Ophrys flowers for females and attempt to mate
with them
Angiosperm Reproduction
and Biotechnology
!  The flower is pollinated in the process
!  Unusually, the flower does not produce nectar and
the male receives no benefit
Lecture Presentation by
Nicole Tunbridge and
Kathleen Fitzpatrick
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Concept 38.1: Flowers, double fertilization, and
fruits are key features of the angiosperm life
cycle
!  Many angiosperms lure insects with nectar; both
plant and pollinator benefit
!  Plant life cycles are characterized by the
alternation between sporophyte (spore-producing)
and gametophyte (gamete-producing) generations
!  Mutualistic symbioses are common between
plants and other species
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!  Angiosperms can reproduce sexually and
asexually
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Flower Structure and Function
!  In angiosperms, the sporophyte is the plant that
we see; they are larger, more conspicuous and
longer-lived than gametophytes
!  Flowers are the reproductive shoots of the
angiosperm sporophyte; they attach to a part of
the stem called the receptacle
!  The angiosperm life cycle is characterized by
“three Fs”: f lowers, double fertilization, and f ruits
!  Flowers consist of four floral organs: carpels,
stamens, petals, and sepals
!  Stamens and carpels are reproductive organs;
sepals and petals are sterile
!  Angiosperms are the most important group of
plants in terrestrial ecosystems and in agriculture
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Figure 38.2
Stigma
Stamen
Anther
Video: Flower Blooming (Time Lapse)
Carpel
Style
Filament
!  A carpel has a long style with a stigma on which
pollen may land
Ovary
!  Complete flowers contain all four floral organs
!  At the base of the style is an ovary containing one
or more ovules
!  Incomplete flowers lack one or more floral
organs, for example stamens or carpels
!  Clusters of flowers are called inflorescences
!  A single carpel or group of fused carpels is called
a pistil
Petal
!  A stamen consists of a filament topped by an
anther with pollen sacs that produce pollen
Sepal
Ovule
Receptacle
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Figure 38.3a
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Figure 38.3aa
Figure 38.3ab
Bilateral symmetry
!  Much of floral diversity represents adaptation to
specific pollinators
!  Four general trends can be seen in the evolution
of flowers
!  Bilateral symmetry
Musk mallow
(radial symmetry)
!  Reduction in the number of floral parts
Musk mallow
(radial symmetry)
!  Fusion of floral parts
“Bramley” orchid
(bilateral symmetry)
!  Location of ovaries inside receptacles
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“Bramley” orchid
(bilateral symmetry)
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Figure 38.3b
Figure 38.3ba
Reduction in number of floral parts
Figure 38.3bb
Figure 38.3c
Fusion of floral parts
Bloodroot
Bloodroot
Star of Bethlehem
Drooping trillium
Hedge bindweed
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Drooping trillium
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Figure 38.3ca
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Figure 38.3cb
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Figure 38.3d
Figure 38.3da
Ovaries located inside receptacles
Ovary
Hedge bindweed
Star of Bethlehem
Ovary
Stone plant
(longitudinal
section)
Stone plant
(longitudinal
section)
Ovary
Japanese quince
(longitudinal section)
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Figure 38.3db
Figure 38.4-1
Carpel
The Angiosperm Life Cycle: An Overview
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Mature flower on
sporophyte plant
(2n)
!  The angiosperm life cycle includes
Anther
MEIOSIS
Carpel
Anther
Generative cell
Tube cell
Tube nucleus
MEIOSIS
Ovary
Ovule with
MEIOSIS
megasporangium
(2n)
Pollen
grains
!  Pollination
!  Double fertilization
Female
gametophyte
(embryo sac)
!  Seed development
Ovary
Japanese quince
(longitudinal section)
Key
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Figure 38.4-3
Carpel
Anther
MEIOSIS
Carpel
Ovary
Ovule with
MEIOSIS
megasporangium
(2n)
Female
gametophyte
(embryo sac)
Antipodal cells
Polar nuclei
in central cell
Synergids
Egg (n)
Male
gametophyte
(in pollen
grain) (n)
Generative cell
Tube cell
Tube nucleus
Megasporangium
(2n)
Surviving
megaspore (n)
Integuments
Micropyle
Stigma
Pollen tube
Sperm
Tube nucleus
Female
gametophyte
(embryo sac)
Nucleus of
developing
endosperm
(3n)
Egg
nucleus (n)
FERTILIZATION
Key
Haploid (n)
Diploid (2n)
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MEIOSIS
Embryo (2n)
Endosperm (3n)
Seed coat (2n)
Style
Figure 38.4b
Seed
Antipodal cells
Polar nuclei
in central cell
Synergids
Egg (n)
Microspore (n)
Male
gametophyte
(in pollen
grain) (n)
Generative cell
Tube cell
Tube nucleus
Megasporangium
(2n)
Surviving
megaspore (n)
Integuments
Micropyle
Carpel
Anther
Pollen
grains
Microsporangium (pollen sac)
Microsporocytes (2n)
MEIOSIS
Stigma
Pollen tube
Sperm
Tube nucleus
Style
Key
Haploid (n)
Diploid (2n)
Zygote (2n)
Egg
nucleus (n)
Microspore (n)
Male
gametophyte
(in pollen
grain) (n)
Generative cell
Tube cell
Tube nucleus
Ovary
Ovule with
MEIOSIS
megasporangium
(2n)
Key
Haploid (n)
Diploid (2n)
FERTILIZATION
Key
Discharged sperm nuclei (n)
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Haploid (n)
Diploid (2n)
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Microsporocytes (2n)
Ovary
Ovule with
MEIOSIS
megasporangium
(2n)
Germinating
seed
Pollen
grains
Pollen
grains
Megasporangium
(2n)
Surviving
megaspore (n)
Integuments
Micropyle
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Figure 38.4a
Microsporangium (pollen sac)
Anther
Mature flower on
sporophyte plant
(2n)
Microspore (n)
Generative cell
Tube cell
Tube nucleus
Haploid (n)
Diploid (2n)
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Figure 38.4-4
Microsporangium (pollen sac)
Microsporocytes (2n)
Mature flower on
sporophyte plant
(2n)
Antipodal cells
Polar nuclei
in central cell
Synergids
Egg (n)
Microspore (n)
Male
gametophyte
(in pollen
grain) (n)
Key
Haploid (n)
Diploid (2n)
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Microsporangium (pollen sac)
Microsporocytes (2n)
Mature flower on
sporophyte plant
(2n)
Microspore (n)
Male
gametophyte
(in pollen
grain) (n)
!  Gametophyte development
Figure 38.4-2
Microsporangium (pollen sac)
Microsporocytes (2n)
Discharged sperm nuclei (n)
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Megasporangium
(2n)
Surviving
megaspore (n)
Integuments
Micropyle
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Figure 38.4c
Figure 38.4d
Pollen
grains
Antipodal cells
Female
gametophyte
(embryo sac)
Megasporangium
(2n)
Surviving
megaspore (n)
Integuments
Micropyle
Polar nuclei
in central cell
Synergids
Egg (n)
Female
gametophyte
(embryo sac)
Tube
nucleus
Style
Video: Flowering Plant Life Cycle (Time Lapse)
Embryo (2n)
Endosperm (3n) Seed
Seed coat (2n)
Stigma
Pollen
tube
Sperm
Nucleus of
developing
endosperm
(3n)
Animation: Plant Fertilization
Antipodal cells
Polar nuclei
in central cell
Synergids
Egg (n)
Style
Zygote (2n)
Egg
nucleus (n)
FERTILIZATION
Key
Key
Haploid (n)
Diploid (2n)
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Haploid (n)
Diploid (2n)
Discharged sperm
nuclei (n)
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Gametophyte Development
!  Angiosperm gametophytes are microscopic and
their development is obscured by protective
tissues
Development of Female Gametophytes
(Embryo Sacs)
!  The megaspore divides without cytokinesis,
producing one large cell with eight nuclei
Development of Male Gametophytes in
Pollen Grains
!  The embryo sac, or female gametophyte,
develops within the ovule
!  This cell is partitioned into a multicellular female
gametophyte, the embryo sac
!  Pollen develops from microspores within
the microsporangia, or pollen sacs, of anthers
!  Within an ovule, two integuments surround a
megasporangium
!  Each microspore undergoes mitosis to produce
two cells: the generative cell and the tube cell
!  One cell in the megasporangium undergoes
meiosis, producing four megaspores, only one of
which survives
!  A pollen grain consists of the two-celled male
gametophyte and the spore wall
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Pollination
Double Fertilization
Seed Development
Methods of Pollination
!  In angiosperms, pollination is the transfer of
pollen from an anther to a stigma
!  Fertilization, the fusion of gametes, occurs after
the two sperm reach the female gametophyte
!  After double fertilization, each ovule develops into
a seed
!  The transfer of pollen from anthers to stigma can
be accomplished by wind, water, or animals
!  After landing on a receptive stigma, a pollen grain
produces a pollen tube that grows down into the
ovary and discharges two sperm cells near the
embryo sac
!  One sperm fertilizes the egg, and the other
combines with the two polar nuclei, giving rise to
the triploid food-storing endosperm (3n)
!  The ovary develops into a fruit enclosing the seed
!  Wind-pollinated species (e.g., grasses and many
trees) release large amounts of pollen
!  This double fertilization ensures that endosperm
only develops in ovules containing fertilized eggs
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!  When a seed germinates, the embryo develops
into a new sporophyte
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Figure 38.5a
Figure 38.5aa
Abiotic pollination by wind
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Figure 38.5ab
Figure 38.5ac
Pollination by bees
Common dandelion Common dandelion
under normal light under ultraviolet
light
Hazel carpellate
flower (carpels
only)
Hazel carpellate
flower (carpels
only)
Hazel staminate flowers
(stamens only) releasing
clouds of pollen
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Common dandelion
under normal light
Hazel staminate flowers
(stamens only) releasing
clouds of pollen
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Figure 38.5ad
Figure 38.5b
Figure 38.5ba
Pollination
by bats
Pollination by moths
and butterflies
Figure 38.5bb
Pollination
by flies
Anther
Moth
Anther
Moth
Stigma
Long-nosed bat feeding
on cactus flower at night
Moth on yucca flower
Blowfly on carrion
flower
Pollination by birds
Common dandelion
under ultraviolet
light
Stigma
Long-nosed bat feeding
on cactus flower at night
Moth on yucca flower
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Hummingbird drinking nectar of columbine flower
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Figure 38.5bc
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Video: Bat Pollinating Agave Plant
Video: Bee Pollinating
Figure 38.5bd
Blowfly on carrion
flower
Hummingbird drinking nectar of columbine flower
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From Seed to Flowering Plant: A Closer Look
Endosperm Development
Figure 38.6
!  Coevolution is the joint evolution of interacting
species in response to selection imposed by each
other
!  The development of a seed into a flowering plant
includes several stages
!  Endosperm development usually precedes embryo
development
!  Endosperm development
!  Many flowering plants have coevolved with
specific pollinators
!  In most monocots and many eudicots, endosperm
stores nutrients that can be used by the seedling
!  Embryo development
!  The shapes and sizes of flowers often correspond
to the pollen transporting parts of their animal
pollinators
!  Seed germination
!  Seedling development
!  For example, Darwin correctly predicted a moth with
a 28-cm-long tongue based on the morphology of a
particular flower
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!  In other eudicots, the food reserves of the
endosperm are exported to the cotyledons
!  Seed dormancy
!  Flowering
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Animation: Seed Development
Structure of the Mature Seed
Figure 38.7
Embryo Development
Ovule
Endosperm
nucleus
Zygote
!  The first mitotic division of the zygote splits the
fertilized egg into a basal cell and a terminal cell
!  The embryo and its food supply are enclosed by a
hard, protective seed coat
Integuments
!  The basal cell produces a multicellular suspensor,
which anchors the embryo to the parent plant
!  The seed enters a state of dormancy
Zygote
Terminal cell
Basal cell
!  The terminal cell gives rise to most of the embryo
!  A mature seed is only about 5–15% water
Proembryo
!  The cotyledons form and the embryo elongates
Suspensor
Basal cell
Cotyledons
Shoot apex
Root apex
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Suspensor
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Seed coat
Endosperm
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Figure 38.8
Figure 38.8a
Seed coat
Figure 38.8b
Epicotyl
Hypocotyl
Radicle
!  In some eudicots, such as the common garden
bean, the embryo consists of the embryonic axis
attached to two fleshy cotyledons (seed leaves)
Cotyledons
Seed coat
Endosperm
Cotyledons
Epicotyl
Hypocotyl
(a) Common garden bean, a eudicot with thick cotyledons
!  Below the cotyledons the embryonic axis is called
the hypocotyl and terminates in the radicle
(embryonic root); above the cotyledons it is called
the epicotyl
Endosperm
Cotyledons
Epicotyl
Hypocotyl
Radicle
Hypocotyl
Cotyledons
Radicle
Radicle
(b) Castor bean, a eudicot with thin cotyledons
Scutellum
(cotyledon)
!  The plumule comprises the epicotyl, young leaves,
and shoot apical meristem
Coleoptile
Coleorhiza
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Epicotyl
Seed coat
Seed coat
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Pericarp fused
with seed coat
Endosperm
Epicotyl
Hypocotyl
Radicle
(a) Common garden bean, a eudicot with thick cotyledons
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(c) Maize, a monocot
(b) Castor bean, a eudicot with thin cotyledons
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Figure 38.8c
Seed Dormancy: An Adaptation for Tough
Times
Scutellum
(cotyledon)
Coleoptile
Coleorhiza
!  The seeds of some eudicots, such as castor
beans, have thin cotyledons
Pericarp fused
with seed coat
Endosperm
Epicotyl
Hypocotyl
Radicle
!  A monocot embryo has one cotyledon
!  Grasses, such as maize and wheat, have a special
cotyledon called a scutellum
!  Two sheathes enclose the embryo of a grass
seed: a coleoptile covering the young shoot and a
coleorhiza covering the young root
(c) Maize, a monocot
!  Seed dormancy increases the chances that
germination will occur at a time and place most
advantageous to the seedling
!  The breaking of seed dormancy often requires
environmental cues, such as temperature or
lighting changes
!  Most seeds remain viable after a year or two of
dormancy, but some last only days and others can
remain viable for centuries
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Figure 38.9
Figure 38.9a
Foliage leaves
Cotyledon
Seed Germination and Seedling Development
Hypocotyl
!  Germination depends on imbibition, the uptake of
water due to low water potential of the dry seed
!  In many eudicots, a hook forms in the hypocotyl,
and growth pushes the hook above ground
!  The radicle (embryonic root) emerges first; the
developing root system anchors the plant
!  Light causes the hook to straighten and pull the
cotyledons and shoot tip up
Epicotyl
Foliage leaves
Cotyledon
Cotyledon
Cotyledon
Hypocotyl
Hypocotyl
Hypocotyl
Epicotyl
Cotyledon
Radicle
Hypocotyl
Seed coat
(a) Common garden bean
!  Next, the shoot tip breaks through the soil surface
Cotyledon
Hypocotyl
Foliage leaves
Coleoptile
Radicle
Coleoptile
Seed coat
(a) Common garden bean
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(b) Maize
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Radicle
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Flowering
Fruit Structure and Function
Figure 38.9b
!  In maize and other grasses, which are monocots,
the coleoptile pushes up through the soil creating
a tunnel for the shoot tip to grow through
Foliage leaves
Coleoptile
(b) Maize
Coleoptile
!  A fruit is the mature ovary of a flower
!  It protects the enclosed seeds and aids in seed
dispersal by wind or animals
!  Flowering is triggered by a combination of
environmental cues and internal signals
!  In some fruits, such as soybean pods, the ovary
wall dries out at maturity, whereas in other fruits,
such as grapes, it remains fleshy
Radicle
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!  The flowers of a given plant species are
synchronized to appear at a specific time of the
year to promote outbreeding
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Figure 38.10
Figure 38.11
Figure 38.11a
Carpels
!  Fruits are classified based on their developmental
origin
!  Simple fruits develop from a single or several
fused carpels
Carpels
Stamen
Petal
Style
Ovary
Stamen
Stigma
Ovule
Pea flower
!  Aggregate fruits result from a single flower with
multiple separate carpels
Raspberry flower
Stigma
Seed
Ovary
Pineapple
inflorescence
Each segment
develops
from the
carpel
of one
flower
Stamen
Pea fruit
(a) Simple fruit
Stamen
Sepal
Ovule
Carpel
(fruitlet)
!  Multiple fruits develop from a group of flowers
called an inflorescence
Flowers
Pineapple fruit
(c) Multiple fruit
Pea flower
Apple flower
Remains of
stamens and styles
Sepals
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Raspberry flower
Carpel
(fruitlet)
Stigma
Ovary
Receptacle
Apple fruit
(d) Accessory fruit
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Ovule
Seed
Seed
Raspberry fruit
(b) Aggregate fruit
Stigma
Ovary
(in receptacle)
Stamen
Pea fruit
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Stamen
Ovary
Stamen
Stigma
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(a) Simple fruit
Raspberry fruit
(b) Aggregate fruit
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Figure 38.11b
Flowers
Petal
Stigma Style
!  Fruit dispersal mechanisms include
!  Water
Ovary
(in receptacle)
!  Wind
Apple flower
Remains of
stamens and styles
Sepals
Seed
Pineapple fruit
(c) Multiple fruit
!  An accessory fruit contains other floral parts in
addition to ovaries
Stamen
Sepal
Ovule
Pineapple
inflorescence
Each segment
develops
from the
carpel
of one
flower
Animation: Fruit Development
!  Animals
Receptacle
Apple fruit
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(d) Accessory fruit
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Figure 38.12a
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Figure 38.12aa
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Figure 38.12ab
Figure 38.12ac
Dispersal by water
Dispersal by wind
Giant seed of
the tropical Asian
climbing gourd
Alsomitra macrocarpa
Coconut seed
embryo, endosperm,
and endocarp inside
buoyant husk
Dandelion fruit
Dandelion fruit
Dandelion “seeds” (actually one-seeded fruits)
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Winged fruit of a maple
Giant seed of
the tropical Asian
climbing gourd
Alsomitra macrocarpa
Coconut seed embryo,
endosperm, and endocarp
inside buoyant husk
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Dandelion “seeds” (actually one-seeded fruits)
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Tumbleweed
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Figure 38.12ad
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Figure 38.12ae
Figure 38.12b
Figure 38.12ba
Dispersal by animals
Fruit of puncture vine
(Tribulus terrestris)
Winged fruit of a maple
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Squirrel hoarding seeds or fruits
underground
Fruit of puncture vine
(Tribulus terrestris)
Tumbleweed
Ant carrying seed with
attached “food body”
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Seeds dispersed in black bear feces
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Figure 38.12bb
Figure 38.12bc
Figure 38.12bd
Concept 38.2: Flowering plants reproduce
sexually, asexually, or both
!  Many angiosperm species reproduce both
asexually and sexually
!  Sexual reproduction results in offspring that are
genetically different from their parents
!  Asexual reproduction results in a clone of
genetically identical organisms
Squirrel hoarding seeds or fruits
underground
Ant carrying seed with
attached “food body”
Seeds dispersed in black bear feces
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Figure 38.13
Mechanisms of Asexual Reproduction
Advantages and Disadvantages of Asexual and
Sexual Reproduction
!  Fragmentation, separation of a parent plant into
parts that develop into whole plants, is a very
common type of asexual reproduction
!  Apomixis is the asexual production of seeds from
a diploid cell
!  In some species, a parent plant’s root system
gives rise to adventitious shoots that become
separate shoot systems
!  Asexual reproduction is also called vegetative
reproduction because progeny arise from mature
vegetative fragments
!  All genetic material is passed to the progeny
!  Asexual reproduction can be beneficial to a
successful plant in a stable environment
!  However, a clone of plants is vulnerable to local
extinction if there is an environmental change
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Figure 38.14
Figure 38.14a
Mechanisms That Prevent Self-Fertilization
!  Sexual reproduction generates genetic variation
that makes evolutionary adaptation possible
!  Many angiosperms have mechanisms that make it
difficult or impossible for a flower to self-fertilize
!  However, only a fraction of seedlings survive
!  Dioecious species have staminate and carpellate
flowers on separate plants
!  Some flowers can self-fertilize to ensure that every
ovule will develop into a seed
(a) Staminate flowers (left) and carpellate flowers (right)
of a dioecious species
!  However, many species have evolved
mechanisms to prevent selfing
Stamens Styles
Styles
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Figure 38.14b
Thrum flower
Staminate flower
Stamens
Pin flower
(b) Thrum and pin flowers
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Figure 38.14c
!  Others have stamens and carpels that mature at
different times or are arranged to prevent selfing
!  The most common is self-incompatibility, a
plant’s ability to reject its own pollen
!  Researchers are unraveling the molecular
mechanisms involved in self-incompatibility
Stamens Styles
Styles
Stamens
!  Some plants reject pollen that has an S-gene
matching an allele in the stigma cells
Thrum flower
Carpellate flower
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Pin flower
!  Recognition of self pollen triggers a signal
transduction pathway leading to a block in growth
of a pollen tube
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Totipotency, Vegetative Reproduction, and
Tissue Culture
Vegetative Propagation and Grafting
Test-Tube Cloning and Related Techniques
!  Totipotent cells, those that can divide and
asexually generate a clone of the original
organism, are common in plants
!  Vegetative reproduction that is facilitated or
induced by humans is called vegetative
propagation
!  Humans have devised methods for asexual
propagation of angiosperms
!  Many kinds of plants are asexually reproduced
from plant fragments called cuttings
!  Most methods are based on the ability of plants to
form adventitious roots or shoots
!  A callus is a mass of dividing, undifferentiated
totipotent cells that forms where a stem is cut and
produces adventitious roots
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!  A twig or bud can be grafted onto a plant of a
closely related species or variety
!  Plant biologists have adopted in vitro methods to
create and clone novel plant varieties
!  The stock provides the root system
!  A callus of undifferentiated totipotent cells can
sprout shoots and roots in response to plant
hormones
!  The scion is grafted onto the stock
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Figure 38.15
Figure 38.16
Concept 38.3: People modify crops by breeding
and genetic engineering
!  Some pathogenic viruses can be eliminated by
excising virus-free apical meristems for tissue
culture
!  People have intervened in the reproduction and
genetic makeup of plants for thousands of years
!  Plant tissue culture also facilitates the production
of genetically modified (GM) plants
(a)
(b)
(c)
!  Hybridization is common in nature and has been
used by breeders to introduce new genes
!  Maize, a product of artificial selection, is a staple in
many developing countries
Developing root
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Figure 38.16a
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Plant Breeding
Plant Biotechnology and Genetic Engineering
Figure 38.16b
!  Mutations can arise spontaneously or can be
induced by breeders
!  Plant biotechnology has two meanings
!  In a general sense, it refers to innovations in the
use of plants to make useful products
!  Plants with beneficial mutations are used in
breeding experiments
!  In a specific sense, it refers to use of GM organisms
in agriculture and industry
!  Desirable traits can be introduced from different
species or genera
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!  Transgenic organisms are those that have been
engineered to express a gene from another
species
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Figure 38.17
Figure 38.18
Reducing World Hunger and Malnutrition
!  Genetically modified plants may increase the
quality and quantity of food worldwide
!  Nutritional quality of plants is being improved
!  For example, “Golden Rice” is a transgenic variety
being developed to address vitamin A deficiencies
among the world’s poor
!  Some transgenic crops have been developed to
produce the Bt toxin, which is toxic to insect pests
!  Other crops are able to tolerate herbicides or resist
specific diseases
Non-Bt maize
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!  For example, transgenic cassava have increased
levels of iron and beta-carotene and reduced
cyanide-producing chemicals
Bt maize
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Reducing Fossil Fuel Dependency
The Debate over Plant Biotechnology
!  Biofuels are fuels derived from living biomass,
the total mass of organic matter in a group of
organisms
Issues of Human Health
!  Some biologists are concerned about risks of
releasing GM organisms (GMOs) into the
environment
!  One concern is that genetic engineering may
transfer allergens from a gene source to a plant
used for food
!  Biofuels can be produced by rapidly growing crops
such as switchgrass and poplar
!  Widespread adoption of Bt cotton in India has led
to a 41% decrease in insecticide use and an 80%
reduction in acute poisoning cases
!  Some GMOs have health benefits
!  For example, maize that produces the Bt toxin has
90% less of a cancer-causing toxin than non-Bt
corn
!  Biofuels would reduce the net emission of CO2, a
greenhouse gas
!  Bt maize has less insect damage and lower
infection by Fusarium fungus that produces the
cancer-causing toxin
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Possible Effects on Nontarget Organisms
Addressing the Problem of Transgene Escape
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Figure 38.UN01a
!  Many ecologists are concerned that the growing of
GM crops might have unforeseen effects on
nontarget organisms
!  Perhaps the most serious concern is the possibility
of introduced genes escaping into related weeds
through crop-to-weed hybridization
!  Efforts are underway to prevent this by introducing
!  Male sterility
!  Apomixis
!  This could result in “superweeds” that would be
resistant to many herbicides
!  Transgenes into chloroplast DNA (not transferred
by pollen)
!  Strict self-pollination
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Figure 38.UN01aa
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Figure 38.UN01ab
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Figure 38.UN01b
Figure 38.UN02
Tube
nucleus
One sperm will
fuse with the
egg, forming a
zygote (2n).
One sperm cell will fuse with the
2 polar nuclei, forming an endosperm
nucleus (3n).
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Figure 38.UN03
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