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Chapter 18
The Plant Life Cycle
Lectures by
Gregory Ahearn
University of North Florida
Copyright © 2009 Pearson Education, Inc..
18.1 What Is The Life Cycle Of Plants?
 Asexual reproduction requires only one
parent and produces clones.
• Offspring plants are produced by mitotic
division of the parent plant’s cells, so the
offspring is genetically identical to the parent.
• Asexual reproduction is more rapid and
efficient than is sexual reproduction, and can
allow the plant to spread quickly when
circumstances permit.
• However, most plants reproduce sexually at
least some of the time.
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18.1 What Is The Life Cycle Of Plants?
 Two types of plant bodies alternate in the
sexual life cycle.
• As plant reproduction proceeds, the two plant
body types alternate; this kind of life cycle is
known as alternation of generations.
• In alternation of generations, each body type
produces reproductive cells that give rise to
the other body types.
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18.1 What Is The Life Cycle Of Plants?
 Two types of plant bodies alternate in the
sexual life cycle (continued).
• The alternating body types are called the
sporophyte and the gametophyte.
• The sporophyte is composed of diploid cells
that produce spores (cells that develop into
adult organisms).
• The gametophyte has haploid cells and
makes gametes.
• All plants have both sporophyte and
gametophyte phases.
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18.1 What Is The Life Cycle Of Plants?
 In ferns, the sporophyte and gametophyte are
separate, independent plants; consider the life
cycle of a fern.
• To reproduce, the fern sporophyte bears reproductive
cells that undergo meiosis to produce haploid spores.
• The spores travel on the wind, land on the soil, and
begin to grow by mitosis into a haploid plant—the fern
gametophyte.
• The gametophyte produces both sperm and eggs.
• The sperm and egg unite to form a new diploid
sporophyte.
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clusters of
spore cases
mature
sporophyte (2n)
Meiotic cell
division in the
sporophyte
produces
haploid spores
spore case
A sporophyte
emerges from the
gametophyte, undergoes
mitotic cell division,
develops, and grows
young
sporophyte (2n)
egg (n)
spores (n)
A spore germinates,
undergoes mitotic cell
division, develops, and
grows into a gametophyte
young
gametophyte (n)
mature
gametophyte
zygote (2n)
Sperm and eggs form by
mitotic cell division and fuse,
producing a diploid zygote
within the gametophyte
fusion to
form zygote
haploid (n)
sperm (n)
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mature
gametophyte
(n)
diploid (2n)
Fig. 18-1
18.1 What Is The Life Cycle Of Plants?
 Alternation of generations is less obvious in
flowering plants.
• In flowering plants, gametophytes are
microscopic and do not live independently of
the sporophyte.
• The gametophytes develop within flowers that
are part of the sporophyte body.
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18.1 What Is The Life Cycle Of Plants?
 Alternation of generations is less obvious in
flowering plants (continued).
• The gametophytes can be female (eggs) or
male (sperm).
• The male and female gametophytes grow
from two types of spores that are formed by
meiosis within the flowers of the sporophyte.
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18.1 What Is The Life Cycle Of Plants?
 Alternation of generations is less obvious in
flowering plants (continued).
• One type of spore, the megaspore, arises
from a precursor cell called the megaspore
mother cell, which undergoes mitosis to form
the female gametophyte called an embryo
sac.
• The embryo sac remains permanently within
the flower and includes an egg cell.
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18.1 What Is The Life Cycle Of Plants?
 Alternation of generations is less obvious in
flowering plants (continued).
• The other type of spore, the microspore,
arises from a precursor cell called the
microspore mother cell and develops into the
male gametophyte—a tough, watertight,
pollen grain.
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18.1 What Is The Life Cycle Of Plants?
 Alternation of generations is less obvious in
flowering plants (continued).
• The pollen grain is carried by the wind to the
flower’s female gametophyte; the sperm is
liberated from the pollen grain and fertilizes
the female gametophyte.
• The resulting zygote becomes an embryonic
plant enclosed within a seed that will develop
into a new sporophyte.
Copyright © 2009 Pearson Education Inc.
Mother cells
develop in the
ovary and anthers
The seed germinates
and the embryo develops
into a sporophyte (2n) by
anther
mitotic cell division
ovule
megaspore mother cell (2n)
nuclei
ovary
Meiotic cell
division produces
haploid spores
microspore
mother cell (2n)
embryo
microspore (n)
seed
gametophyte (n)
(pollen)
A zygote (2n) produces an
embryo by mitotic cell division
megaspore (n)
sperm
cells (n)
Mitotic cell division
forms male and female
gametophytes
pollen
tube
Mitotic cell division
forms sperm and an egg
A pollen tube
burrows to the
embryo sac
Fertilization
occurs within the
female gametophyte,
producing a zygote
polar
nuclei
gametophyte (n)
(embryo sac)
egg cell (n)
haploid (n)
diploid (2n)
Fig. 18-2
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18.2 What Is The Structure Of Flowers?
 Flowers evolved from leaves.
• Flower parts are actually highly modified
leaves, shaped by natural selection into
structures that foster the transfer of pollen
from one plant to another.
• Complete flowers—such as petunias, roses,
and lilies—consist of a central axis on which
four sets of modified leaves are attached:
sepals, petals, stamens, and carpels.
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18.2 What Is The Structure Of Flowers?
 Flowers evolved from leaves (continued).
• The sepals are located at the base of the
flower and protect the flower bud.
• The petals are above the sepals and are
brightly colored.
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18.2 What Is The Structure Of Flowers?
 A complete flower
petal
anther
filament
sepal
stamen
petal
stigma
sepal
style
ovary
carpel
stigma
anthers
style
(a) A dicot flower in cross-section
(b) An amaryllis (monocot) flower
Fig. 18-3
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18.2 What Is The Structure Of Flowers?
 Flowers incorporate male and female
reproductive structures.
• The male reproductive structures, the
stamens, are attached just above the petals.
• They consist of a long and slender filament
that supports an anther, the structure that
produces pollen.
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18.2 What Is The Structure Of Flowers?
 Flowers incorporate male and female
reproductive structures (continued).
• The female reproductive structures, the
carpels, occupy the uppermost position in the
flower.
• They contains a sticky stigma for catching
pollen, mounted on an elongated style; the
style connects the stigma with the ovary.
• Inside the ovary are one or more ovules in
which the female gametophytes develop
• When mature, the ovules become seeds and
the ovary will develop into a fruit.
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18.2 What Is The Structure Of Flowers?
 Flowers incorporate male and female
reproductive structures (continued).
• Incomplete flowers lack one or more of the
four floral parts.
• For example, in plants that have separate
male and female flowers, the male flowers
lack carpels and the female flowers lack
stamens.
• Incomplete flowers may also lack sepals or
petals.
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18.2 What Is The Structure Of Flowers?
 Some plants have separate male and
female flowers.
Fig. 18-4
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18.3 What Are the Gametophytes of
Flowering Plants?
 The pollen grain is the male gametophyte.
• Each anther produces thousands of haploid
microspores, each of which divides once, by
mitosis, to produce a pollen grain.
• As the pollen grain matures, it produces two
haploid sperm cells.
• A tough, protective surface coat develops
around the pollen grain, which is released
from the anther to the wind currents.
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18.3 What Are the Gametophytes of
Flowering Plants?
 Pollen grains
Fig. 18-5
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18.3 What Are the Gametophytes of
Flowering Plants?
 The embryo sac is the female gametophyte.
• Within the ovules in the ovary, large haploid
megaspores undergo mitotic divisions.
• Three nuclear divisions produce eight haploid
nuclei, but plasma membranes divide the
cytoplasm into seven, not eight, cells.
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18.3 What Are the Gametophytes of
Flowering Plants?
 The embryo sac is the female gametophyte
(continued).
• There are three small cells at each end, each
containing one nucleus, and one remaining
large cell in the middle containing two nuclei,
called polar nuclei.
• This seven-celled organism, called the embryo
sac, is the haploid female gametophyte.
• The egg is the central small cell at the bottom
of the embryo sac, located near an opening in
the ovule.
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18.4 How Does Pollination Lead To
Fertilization?
 Sexual reproduction in plants requires
fertilization—the fusion of sperm and egg—
but fertilization can occur only after
pollination, when a pollen grain lands on a
stigma.
 Pollinators may be attracted by feeding
opportunities.
• Flowers that are pollinated by animals are
often colorful, conspicuous, and fragrant.
• Showy flowers typically contain nectar, which
attracts insects and hummingbirds.
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18.4 How Does Pollination Lead To
Fertilization?
 Most wind-pollinated
flowers, such as
those of grasses
and oaks, are
inconspicuous and
unscented.
Fig. 18-6
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18.4 How Does Pollination Lead To
Fertilization?
 Some flowers have adaptations that help
ensure pollination by insects.
• Many flowers are pollinated by moths or
butterflies and have nectar-containing tubes to
accommodate the long tongues of these
insects.
• Flowers that smell like dung or rotting flesh
attract beetles.
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18.4 How Does Pollination Lead To
Fertilization?
 Bees are attracted to certain flowers by both
their smell and the color of their flowers
(white, blue, yellow, or orange).
“bee vision”
human vision
farred
red
orange
yellow
green
blue
violet
nearUV
human
bee
700
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600
500
wavelength
400
Fig. 18-7
18.4 How Does Pollination Lead To
Fertilization?
 Flowers that smell
like dung or rotting
flesh attract beetles.
Fig. 18-8
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18.4 How Does Pollination Lead To
Fertilization?
 Birds can be pollinators, too.
• Hummingbirds are among the few important
vertebrate pollinators.
• Flower shapes match the long bills of the birds
and have no resting place for insect
pollinators.
• Most hummingbirds pollinate flowers that are
red or orange.
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18.4 How Does Pollination Lead To
Fertilization?
 Pollination may be followed by fertilization.
• When a pollen grain lands on the stigma of a
flower, a remarkable chain of events begins.
• The pollen grain elongates to form a tube that
grows down the style toward an ovule in the
ovary.
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18.4 How Does Pollination Lead To
Fertilization?
 Pollination may be followed by fertilization
(continued).
• Inside the embryo sac, one sperm fertilizes
the egg, forming a diploid zygote.
• A second sperm enters the large central cell
and fuses with both polar nuclei, forming a
triploid nucleus.
• This process is called double fertilization.
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mature pollen grain
sperm cells (n)
tube cell nucleus
coat
pollen
tube
sperm
(n)
Pollination occurs
when pollen grains
land on a stigma
stigma
A pollen tube grows
down through the style to
the ovary; two sperm
travel within it
style
“Double fertilization”
occurs within the ovule
tube cell
nucleus
polar
nuclei
ovule
One sperm
fuses with
the two
nuclei in the
central cell
ovary
egg (n)
egg
pollen tube
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One sperm
fuses with
the egg
Fig. 18-9
18.5 How Do Seeds And Fruits Develop?
 After double fertilization, the diploid zygote
and the cell with a triploid nucleus continue
to develop, forming an embryo and food
reserve, respectively.
 Certain parts of the flower develop into
seeds, and other parts develop into fruits.
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18.5 How Do Seeds And Fruits Develop?
 The seed develops from the ovule and
embryo sac.
• The embryo sac and surrounding cells of the
ovule develop into a seed.
• First, the zygote develops into an embryo, and
then the triploid cell undergoes repeated
mitotic divisions to form the endosperm, a
food storage tissue.
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18.5 How Do Seeds And Fruits Develop?
 The embryo consists of three parts:
• The shoot
• The root
• One or two cotyledons, or seed leaves
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18.5 How Do Seeds And Fruits Develop?
 Seed development
central cell (3n)
(becomes
endosperm)
endosperm
endosperm (3n)
root
seed
coat
embryo
(2n)
zygote (2n)
(becomes
embryo)
sperm
embryo
(becomes
seed coat)
shoot
cotyledons
pollen tube
(a) Ovule
(b) Developing seed
(c) Mature seed
Fig. 18-10
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18.5 How Do Seeds And Fruits Develop?
PLAY
Animation—Reproduction in Flowering Plants
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18.5 How Do Seeds And Fruits Develop?
 The fruit develops from the ovary wall and
helps disperse seeds.
• The seed is surrounded by the ovary, which
develops into a fruit; the function of most fruits
is to disperse the seeds.
• Each type of fruit disperses seeds in a
different way.
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18.5 How Do Seeds And Fruits Develop?
 Development of fruit and seed from flower
parts
ripening
sepal
filament
style
ovary
wall
“flesh” of
pepper
ovary
pepper
fruit
seed
anther
petal
stigma
ovule
pepper flower
embryo
pepper fruits
Fig. 18-11
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18.5 How Do Seeds And Fruits Develop?
 Lightweight fruits can be dispersed by wind.
Fig. 18-12
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18.5 How Do Seeds And Fruits Develop?
 Floating fruits can be dispersed by water.
Fig. 18-13
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18.5 How Do Seeds And Fruits Develop?
 Clingy or tasty fruits can be dispersed by
animals.
Fig. 18-14
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18.6 How Do Seeds Germinate And Grow?
 The stage at which a seed or spore begins
to grow and develop is known as
germination.
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18.6 How Do Seeds Germinate And Grow?
 Many seeds do not germinate immediately,
but do so after a period of dormancy during
which they will not germinate; dormancy
avoids two problems.
• First, if the seed were to germinate while still
enclosed in a fruit and hanging from a tree, it
might exhaust its food reserves before it ever
touched the ground.
• Second, environmental conditions that are
suitable for seedling growth may not coincide
with seed maturation.
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18.6 How Do Seeds Germinate And Grow?
 The root emerges first.
• During germination, the embryo breaks
dormancy and emerges from the seed; it
absorbs water and bursts the seed coat.
• The root is the first structure to emerge from
the seed coat, and much of the water it
absorbs is transferred to the shoot.
• The shoot cells lengthen and push up through
the soil.
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18.6 How Do Seeds Germinate And Grow?
 The shoot tip must be protected.
• The shoot must push through the soil without
scraping away the apical meristem, since its
tip is not protected by a root cap.
• In monocots, a tough sheath called a
coleoptile encloses the shoot tip like a glove
and pushes aside the soil particles as it grows;
once in air, the coleoptile tip degenerates.
• Dicots do not have coleoptiles, but instead
have shoots that form a hook that encases the
apical meristem.
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18.6 How Do Seeds Germinate And Grow?
 Seed germination
Corn (monocot)
seed coat
leaves
coleoptile
coleoptile
endosperm
leaves
root
cotyledon
root tip
primary
root
branch
roots
Fig. 18-15
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18.6 How Do Seeds Germinate And Grow?
 Cotyledons nourish the sprouting seed.
• Food stored in the seed provides the energy
for sprouting.
• In dicots, the cotyledons absorb the
endosperm while the seed is developing and
are full of food by the time the seed
germinates.
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18.6 How Do Seeds Germinate And Grow?
 Cotyledons nourish the sprouting seed
(continued).
• In other dicots, the cotyledons stay below the
ground, shriveling up as the embryo absorbs
their stored food.
• Monocots retain most of their food reserve in
the endosperm until germination, when it is
digested and absorbed by the cotyledon as
the embryo grows; the cotyledon remains
below ground.
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18.6 How Do Seeds Germinate And Grow?
 Development is regulated throughout the life
cycle.
• Once in the air, the apical meristem of the
shoot and of the root begin to divide, giving
rise to the structure of the mature plant.
• The regulation of growth and maturation is
controlled by plant hormones.
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18.7 What Are Plant Hormones, And How
Do They Act?
 Plant hormones are chemicals produced by
cells in one part of the plant body and
transported to other parts of the plant where
they exert specific effects.
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18.7 What Are Plant Hormones, And How
Do They Act?
 Five major classes of plant hormones have
been identified.
• Auxins: influence elongation of cells
• Gibberellins: promote elongation of cells in
stems
• Cytokinins: promote cell division
• Ethylene: causes fruit to ripen
• Abscisic acid: helps the plant withstand
unfavorable conditions
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18.7 What Are Plant Hormones, And How
Do They Act?
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18.8 How Do Hormones Regulate The Plant
Life Cycle?
 An illustration of how hormones regulate growth
and development of a plant at each stage of its life
cycle
• Abscisic acid maintains seed dormancy, slows down
the metabolism of the embryo within the seed,
preventing its growth through autumn until the
following spring.
• Gibberellin stimulates germination; germination is
induced by the manufacture of gibberellin as abscisic
acid is turned off.
• Auxin controls the orientation of the sprouting
seedling, and controls the responses of both roots
and shoots to light and gravity.
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18.8 How Do Hormones Regulate The Plant
Life Cycle?
 Auxin stimulates shoot elongation away
from gravity and toward light.
• Auxin is first synthesized in the shoot tip and
then moves down the stem to stimulate cell
elongation.
• If the stem is not exactly vertical, auxin
accumulates on the stem’s lower side.
• This induces these cells to elongate rapidly,
forcing the stem to bend upward.
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18.8 How Do Hormones Regulate The Plant
Life Cycle?
 Auxin stimulates shoot elongation away
from gravity and toward light (continued).
• Once the shoot is vertical, auxin is evenly
distributed around the stem and it grows
straight up.
• This is an example of gravitropism, directional
growth with respect to gravity.
• Auxin also influences a growing shoot’s
phototropism, or growth toward light.
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18.8 How Do Hormones Regulate The Plant
Life Cycle?
 Auxin may control the direction of root
growth.
• Auxin is transported from the shoot to the root,
and if the root is not vertical, the root cap
causes the auxin to accumulate on the lower
side of the root, where the auxin inhibits cell
elongation.
• Therefore, cell elongation in the lower side of
the root is inhibited and the root bends
downward; once the root is vertical, the auxin
becomes evenly distributed and the root
grows straight down.
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The shoot tip
produces auxin
(blue dots)
Auxin accumulates
on the lower side,
stimulating cell
elongation and bending
the shoot upward
shoot
tip
germinating seed
Auxin enters the root,
and the root cap cells direct
auxin to the lower side
root tip
root
cap
Root cell elongation
is inhibited by auxin,
so the root bends
downward
Fig. 18-17
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18.8 How Do Hormones Regulate The Plant
Life Cycle?
PLAY
Animation—Plant Hormones
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18.8 How Do Hormones Regulate The Plant
Life Cycle?
 Stem and root branching is influenced by
auxin and cytokinin.
• In stems, auxin by itself inhibits lateral bud
sprouting; however, a combination of auxin
and cytokinin in particular proportions
promotes sprouting.
• In many plants, the interaction between auxin
and cytokinin produces an orderly progression
of bud sprouting from the bottom to the top of
the shoot.
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18.8 How Do Hormones Regulate The Plant
Life Cycle?
 Stem and root branching is influenced by
auxin and cytokinin (continued).
• Branching in roots is stimulated by auxin and
is inhibited by cytokinin.
• Branching is inhibited at the root tip and is
stimulated at root locations closer to the
above-ground shoot tip.
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18.8 How Do Hormones Regulate The Plant
Life Cycle?
 Role of auxin and
cytokinin in lateral
bud sprouting
auxin
shoot tip
high
Lateral buds are inhibited
by high auxin levels
Lateral buds develop into
branches (optimal ratio
of auxin to cytokinin)
Branch roots develop (optimal
ratio of cytokinin to auxin)
high
cytokinin
Branch roots are inhibited
by high cytokinin levels
root tip
Fig. 18-18
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18.8 How Do Hormones Regulate The Plant
Life Cycle?
 Hormones coordinate the development of
seeds and fruit.
• Flowering is followed by pollination,
fertilization, and the development of seeds
and fruit.
• When a flower is pollinated, the pollen
releases auxin or gibberellin, which stimulates
the ovary to being developing into a fruit.
• If fertilization follows, the developing seeds
release more auxin or gibberellin into the
ovary tissues, and the ovary grows larger,
storing starches and forming the mature fruit.
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18.8 How Do Hormones Regulate The Plant
Life Cycle?
 When the seeds mature, the fruit ripens and
changes color and sugar content so that it
will be attractive to
animals that can
disperse its seeds.
 Ripening is
stimulated
by ethylene.
Fig. 18-19
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18.8 How Do Hormones Regulate The Plant
Life Cycle?
 Senescence and dormancy prepare the
plant for winter.
• As winter approaches, fruits and leaves drop
from the plant after undergoing a rapid aging
called senescence.
• During this process, cytokinin production in
the roots slows down, and fruits and leaves
produce less auxin.
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18.8 How Do Hormones Regulate The Plant
Life Cycle?
 Senescence and dormancy prepare the
plant for winter (continued).
• A layer of thin-walled cells called an
abscission layer forms at the base of the leaf
stalk.
• Ethylene, released by aging leaves and
ripening fruit, stimulates the abscission layer
to produce an enzyme that digests the cell
wall holding the leaf or fruit to the stem.
• Metabolism slows, and the plant enters its
long winter sleep.
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18.8 How Do Hormones Regulate The Plant
Life Cycle?
 The abscission layer
bud
abscission layer
leaf
petiole
Fig. 18-20
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