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37Reproduction in
Flowering Plants
Chapter 37 Key Concepts
37.1 Most Angiosperms Reproduce
Sexually
37.2 Hormones and Signaling Determine
the Transition from the Vegetative to
the Reproductive State
37.3 Angiosperms Can Reproduce
Asexually
Investigating Life: What Signals Flowering?
By determining that poinsettias are short-
day plants, and developing a short,
compact variety, Albert and Paul Ecke
helped make them the most popular
potted plants in the United States.
How did an understanding of angiosperm
reproduction allow floriculturists to develop
a commercially successful poinsettia?
Key Concept 37.1 Focus Your Learning
• The embryo sac is the female
gametophyte and the pollen grain is the
male gametophyte.
• A pollen grain germinates to form a
pollen tube, which grows through the
style to the embryo sac.
Key Concept 37.1 Focus Your Learning
• Angiosperms prevent self-fertilization
by physically separating male and
female gametophytes and by genetic
self-incompatibility.
• The embryo and endosperm develop
within the seed, enclosed within a fruit
derived from the ovary wall.
37.1 Most Angiosperms Reproduce Sexually
Most angiosperms reproduce sexually—
producing the genetic diversity that is the
raw material for evolution.
It involves mitosis, meiosis, and
alternation of haploid and diploid
generations.
37.1 Most Angiosperms Reproduce Sexually
Differences in sexual reproduction
between angiosperms and vertebrate
animals:
• Meiosis in plants produces spores, then
mitosis produces gametes.
• In animals, meiosis produces gametes
directly.
37.1 Most Angiosperms Reproduce Sexually
In most plants, multicellular diploid and
haploid life stages alternate; in animals,
there is no multicellular haploid stage.
Cells that will form gametes are
determined in the adult plant, usually in
response to environmental conditions; in
animals, the germline cells are
determined before birth.
In-Text Art, Ch. 37, p. 787 (1)
37.1 Most Angiosperms Reproduce Sexually
A complete flower has four concentric
groups of organs arising from modified
leaves:
In-Text Art, Ch. 37, p. 787 (2)
37.1 Most Angiosperms Reproduce Sexually
Carpels are female sex organs; contain
developing female gametophytes.
Stamens are male sex organs; contain
developing male gametophytes.
Most angiosperms are “perfect”—flowers
have both stamens and carpels.
Imperfect flowers have only stamens or
only carpels.
Figure 37.1 Perfect and Imperfect Flowers (Part 1)
37.1 Most Angiosperms Reproduce Sexually
Plants that bear both male and female
flowers on an individual plant:
monoecious (“one house”).
Plants that bear either male-only or
female-only flowers on an individual
plant: dioecious.
Figure 37.1 Perfect and Imperfect Flowers (Part 2)
Figure 37.1 Perfect and Imperfect Flowers (Part 3)
37.1 Most Angiosperms Reproduce Sexually
The haploid gametophytes develop from
haploid spores in the flower:
• Megagametophytes (female) are called
embryo sacs; develop in the ovules.
• Microgametophytes (male) are called
pollen grains; develop in anthers on
the stamens.
Figure 37.2 Sexual Reproduction in Angiosperms
37.1 Most Angiosperms Reproduce Sexually
A megasporocyte undergoes meiosis to
produce 4 haploid megaspores; 3
undergo apoptosis.
The surviving megaspore undergoes 3
mitotic divisions with no cytokinesis to
produce 8 haploid nuclei. Cell wall
formation leads to a gametophyte
(embryo sac) with 7 cells and 8 nuclei.
37.1 Most Angiosperms Reproduce Sexually
At one end of the gametophyte are 3
cells—the egg and 2 synergids.
Synergids attract the pollen tube.
Antipodal cells at the opposite end usually
degenerate.
The central cell has 2 polar nuclei.
37.1 Most Angiosperms Reproduce Sexually
Microsporocytes undergo meiosis to
produce 4 haploid microspores.
Each develops a cell wall and divides
mitotically to form 2 haploid cells in each
pollen grain:
• Tube cell forms the pollen tube that
delivers sperm to the embryo sac.
• After pollination, the generative cell
divides by mitosis to form 2 sperm cells.
37.1 Most Angiosperms Reproduce Sexually
Pollination: Transfer of pollen from
anther to stigma.
Germination of the pollen grain involves
uptake of water from the stigma and
growth of the pollen tube through the
style to reach the ovule.
Downward growth is guided by a chemical
signal released by the synergids.
Figure 37.3 Pollen Tubes Begin to Grow
37.1 Most Angiosperms Reproduce Sexually
In some plants, such as peas, self-
pollination occurs before the flower
opens, resulting in self-fertilization. This
leads to homozygosity, which reduces
genetic diversity.
Most plants have mechanisms to prevent
self-fertilization:
37.1 Most Angiosperms Reproduce Sexually
1. Physical separation of male and female
gametophytes:
In dioecious species, pollination occurs
only when one plant pollinates another.
In monoecious species, separation of
male and female flowers can reduce
self-fertilization. In some species the
two flower types bloom at different
times.
37.1 Most Angiosperms Reproduce Sexually
2. Genetic self-incompatibility:
Some plants are self-incompatible;
they reject pollen from their own
flowers.
The plant must determine whether the
pollen is genetically similar or not.
The S locus genes encode proteins in
the pollen and style that interact during
the recognition process.
37.1 Most Angiosperms Reproduce Sexually
The S locus has many alleles.
When pollen carries an allele that
matches an allele of the recipient pistil,
the pollen is rejected.
The rejected pollen either fails to
germinate or is prevented from growing
through the style.
Figure 37.4 Self-Incompatibility (Part 1)
Figure 37.4 Self-Incompatibility (Part 2)
37.1 Most Angiosperms Reproduce Sexually
Double fertilization:
• One synergid degenerates when the
pollen tube arrives and the 2 sperm
cells are released into its remains.
• One sperm cell fuses with the egg cell,
forming a diploid zygote.
• The other sperm cell fuses with the two
polar nuclei in the central cell, forming
a triploid (3n) cell.
Figure 37.5 Double Fertilization (Part 1)
Figure 37.5 Double Fertilization (Part 2)
37.1 Most Angiosperms Reproduce Sexually
The zygote nucleus begins mitotic division
to form the new sporophyte embryo.
The triploid nucleus undergoes mitosis to
form the endosperm. It will later be
digested by the developing embryo for
energy and building blocks.
37.1 Most Angiosperms Reproduce Sexually
Fertilization initiates growth and
development of the embryo, endosperm,
integuments, and carpel.
Integuments are tissues surrounding the
ovule that develop into the seed coat.
The carpel becomes the wall of the fruit
that surrounds the seed.
37.1 Most Angiosperms Reproduce Sexually
As seeds develop, they lose water and
become dormant.
The ovary and seeds develop into a fruit.
Fruits function to
• Protect the seed from damage and
infection
• Aid in seed dispersal
37.1 Most Angiosperms Reproduce Sexually
Some fruits consist only of ovary and
seeds, some include other flower parts.
Some species produce fleshy edible
fruits; some fruits are dry and inedible.
Figure 37.6 Angiosperm Fruits (Part 1)
Figure 37.6 Angiosperm Fruits (Part 2)
Figure 37.6 Angiosperm Fruits (Part 3)
Figure 37.6 Angiosperm Fruits (Part 4)
37.1 Most Angiosperms Reproduce Sexually
Seeds of some species fall to the ground
near the parent plant.
If a plant has successfully reproduced, its
location is likely to be favorable for the
next generation.
But the offspring may then be competing
with the parent, and there is no
guarantee that conditions will remain
favorable in that spot.
37.1 Most Angiosperms Reproduce Sexually
Wider dispersal of seeds helps spread
genetic diversity and increases the
probability that some seeds will find
suitable conditions for growth.
Many fruits help seeds disperse over long
distances.
37.1 Most Angiosperms Reproduce Sexually
Some fruits have “wings” (e.g., maple) or
feathery structures (e.g., thistle) for wind
dispersal.
In-Text Art, Ch. 37, p. 791 (1)
37.1 Most Angiosperms Reproduce Sexually
Some fruits hitch rides on animals as
burs.
In-Text Art, Ch. 37, p. 791 (2)
37.1 Most Angiosperms Reproduce Sexually
Water disperses fruits such as coconuts;
they can float thousands of miles
between islands.
Seeds may travel through an animal’s
digestive tract and be deposited at some
distance from the parent plant.
37.1 Most Angiosperms Reproduce Sexually
Seed development is under control of the
hormone abscisic acid (ABA).
During early seed development ABA
levels are low, and increase as the seed
matures.
This stimulates the endosperm to
synthesize seed storage proteins and
proteins that prevent cell death as the
seeds dry.
37.1 Most Angiosperms Reproduce Sexually
ABA prevents developing seeds from
germinating prematurely on the parent
plant (vivipary).
• These seedlings are unlikely to survive
and cannot establish in the soil.
• In seed crops such as wheat, the grain
is damaged if it has started to sprout.
• ABA plays a key role in maintaining
seed dormancy.
Key Concept 37.1 Learning Outcomes
• Compare the processes of male and
female gamete formation.
• Describe the mechanisms that guide the
growth of a pollen tube.
• Describe and compare two methods for
preventing self-fertilization in
angiosperms.
Key Concept 37.1 Learning Outcomes
• Analyze the relationship between the
diversity of fruits and their ability to
disperse seeds.
• Relate fruit development to seed
development.
Key Concept 37.2 Focus Your Learning
• Although in terms of flowering, many
plants are classified as either short-day
plants (SDPs) or long-day plants (LDPs),
night length is actually the cue that
controls flowering.
• Receptors for the photoperiodic signal for
flowering are located in the leaf, and a
signal travels to the apical meristem.
Key Concept 37.2 Focus Your Learning
• The protein florigen converts a vegetative
meristem into a reproductive meristem.
Several genes are involved in the
regulation and transport of florigen.
• Temperature or gibberellin can induce
flowering in some plants.
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
Flowering is a major event in a plant’s life.
When a plant is old enough, it can
respond to internal or external signals
(such as light or temperature) to start
reproduction.
Or flowering occurs as part of a
predetermined developmental program.
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
Plants fall into three categories in terms of
maturation and flowering:
• Annuals complete life cycle in one
year; little or no secondary growth.
After flowering, most of their energy is
used to develop seeds and fruits and
the plant dies.
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
• Biennials take two years to complete
the life cycle.
They produce vegetative growth the
first year and store carbohydrates in
underground roots (carrots) or stems
(celery).
In the second year, stored
carbohydrates are used to produce
flowers and seeds.
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
• Perennials live three or more years.
They typically flower every year and
keep growing for another season.
In some species the reproductive
cycle repeats each year. Others grow
vegetatively for many years, flower
once, and die.
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
During vegetative growth, apical
meristems continuously produce leaves,
stems, and axillary buds (indeterminate
growth).
If an apical meristem becomes an
inflorescence meristem it produces
bracts and new meristems in the angle
between bract and stem.
Figure 37.7 Flowering and the Apical Meristem (Part 1)
Figure 37.7 Flowering and the Apical Meristem (Part 2)
Figure 37.7 Flowering and the Apical Meristem (Part 3)
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
The new meristems may be inflorescence
meristems, or floral meristems, which
give rise to a flower.
An inflorescence is an orderly cluster of
flowers.
Floral meristems produce 4 whorls of
organs—sepals, petals, stamens, and
carpels, with very short internodes
(determinate growth).
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
Genes for flowering have been studied in
Arabidopsis.
The expression of 2 meristem identity
genes starts a cascade of gene
expression.
The genes encode transcription factors
LEAFY and APETALA1, which are
necessary and sufficient for flowering.
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
Meristem identity gene products trigger
expression of floral organ identity
genes.
They encode transcription factors that
determine whether cells in the floral
meristem will be sepals, petals,
stamens, or carpels.
Figure 19.11 Organ Identity Genes in Arabidopsis Flowers (Part 1)
Figure 19.11 Organ Identity Genes in Arabidopsis Flowers (Part 2)
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
Floral organ identity genes are activated
in response to external cues (day length,
temperature) or internal cues
(hormones).
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
Photoperiod (day length)
• Studies began with two observations:
A tobacco variety grew to 5 meters, but
did not flower before frost killed it.
Soybean farmers planted seeds at
intervals to stagger the harvest, but the
plants all flowered at the same time.
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
• Greenhouse experiments measured
the day length required for different
plant species to flower.
Maryland Mammoth tobacco flowered
when day length became shorter than
14 hours, as it does in December.
Other plants (e.g., soybeans and
henbane) flowered only when days
were long.
Figure 37.8 Day Length and Flowering
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
Short-day plants (SDPs) flower when
the day is shorter than a critical
maximum.
• Coffee, morning glory,
chrysanthemums, poinsettias,
Maryland Mammoth tobacco
They flower in late summer or fall.
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
Long-day plants (LDPs) flower when the
day is longer than a critical maximum.
• Spinach, lettuce, clover
They flower in midsummer.
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
Photoperiodic plants actually measure
length of night, not day.
• Experiments with cocklebur, (SDP):
Day length was varied in one group,
night length in another. The critical
night length was 9 hours.
Figure 37.9 Night Length and Flowering
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
• Other experiments showed that
interruption of the dark period by light,
even briefly, nullified the effect of a long
night.
• Effects of red light interruptions could
be reversed with far-red light, indicating
that a phytochrome was the receptor.
Investigating Life: The Flowering Signal
Hypothesis: Red light participates in the
photoperiodic timing mechanism.
Method:
Grow plants under short-day conditions,
but interrupt the night with light of
different wavelengths.
Investigating Life: The Flowering Signal, Experiment
Investigating Life: The Flowering Signal
Conclusion:
When plants are exposed to red (R) and
far-red (FR) light in alternation, the final
treatment determines the effect.
Phytochrome is the photoreceptor.
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
Expression of the gene CONSTANS (CO)
follows a circadian rhythm.
Experiments with Arabidopsis, an LDP,
show that flowering is determined by
interactions between photoreceptors and
the CO protein.
Peak CO expression is late in the day—in
the afternoon on long days, but after
dark on short days.
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
On long days, the active forms of
phytochrome and blue-light receptors
activate pathways that stabilize the CO
protein, which promotes flowering.
This process does not occur on short
days.
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
Early experiments indicated that receptors
for photoperiod occur in the leaf.
• “Masking” experiments: Either buds or
leaves were covered to determine
which organ receives the light stimulus.
A diffusible chemical must travel from the
leaf to the bud meristem.
Figure 37.10A The Flowering Signal Moves from Leaf to Bud (Part 1)
Figure 37.10A The Flowering Signal Moves from Leaf to Bud (Part 2)
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
Other evidence:
• If a photoperiodically induced leaf is
immediately removed from a plant, the
plant does not flower.
• If cocklebur plants are grafted together
and only one plant is exposed to
inductive long nights, all the plants
flower.
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
• If an induced leaf from one species is
grafted onto a non-induced plant of a
different species, the recipient plant
flowers.
The diffusible signal was named florigen,
but the nature of the signal has only
recently been explained.
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
Three genes are involved in flowering:
1. FT (FLOWERING LOCUS T) codes for
florigen, which is small and can travel
through plasmodesmata.
FT is synthesized in phloem companion
cells of the leaf, diffuses into adjacent
sieve elements, and flows to the apical
meristem.
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
2. CO (CONSTANS) codes for a
transcription factor that activates
synthesis of FT. CO is also expressed
in phloem companion cells in the leaf.
3. FD (FLOWERING LOCUS D) codes
for a protein that binds to FT in the
apical meristem. The complex
activates promoters for meristem
identity genes.
Figure 37.11 Florigen and Its Molecular Biology (Part 1)
Figure 37.11 Florigen and Its Molecular Biology (Part 2)
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
The FT gene is involved in photoperiod
signaling in many species:
• Transgenic plants that express
Arabidopsis FT gene at high levels
flower regardless of day length.
• Transgenic Arabidopsis plants that
express high levels of FT homologs
from other species flower regardless of
day length.
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
In Arabidopsis (an LDP):
• At night, Pfr is gradually converted
back to Pr, which stimulates breakdown
of CO protein.
• In the morning and during the day, CO
protein levels go down.
• By the end of the day, Pr levels go
down, allowing CO to accumulate.
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
The key to flowering is a high level of CO,
which is related to a low level of Pr.
In long days, there is not a lot of dark time
for all the Pfr to be converted to Pr, and a
long day causes more conversion of Pr
to Pfr.
Low levels of Pr result in high levels of CO
and the transcription of genes for
flowering.
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
In some plants, flowering is signaled by
cold temperatures: vernalization.
• Example: Winter wheat is planted in the
fall, grows into a seedling, overwinters,
and flowers the next spring.
If it is not exposed to cold in its first year,
it will not flower normally the next year.
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
In strains of Arabidopsis that require
vernalization, FLC (FLOWERING
LOCUS C) encodes a transcription
factor that inhibits expression of FT and
FD in the florigen pathway.
Cold temperature inhibits synthesis of
FLC protein, allowing FT and FD to be
expressed.
Figure 37.12 Vernalization
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
Epigenetics plays a role in vernalization:
• Before vernalization, chromatin at the
promoter of the FLC gene is relaxed;
and DNA can be transcribed.
• During vernalization, chromatin
remodeling results in more compact
chromatin and reduced expression of
FLC.
Figure 37.13 Chromatin Remodeling during Vernalization (Part 1)
Figure 37.13 Chromatin Remodeling during Vernalization (Part 2)
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
Gibberellins are also involved in flowering.
Application of gibberellins to Arabidopsis
buds results in activation of the meristem
identity gene LEAFY, which in turn
promotes the transition to flowering.
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
Some plant species flower on cue from an
“internal clock.”
In some tobacco strains, flowering is
initiated in the terminal bud when the
stem has grown four phytomers in
length.
The position of the bud determines
transition to flowering.
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
A substance may form a gradient along
the length of the plant.
• The root may produce a flowering
inhibitor, so the plant must reach a
certain height before concentration of
the inhibitor is low enough for flowering.
• The inhibitor is unknown, but it may act
by decreasing the amount of FLC.
37.2 Hormones and Signaling Determine the Transition from the
Vegetative to the Reproductive State
A positional gradient that acts on FLC is
consistent with other mechanisms that
converge on LEAFY and APETALA1.
In-Text Art, Ch. 37, p. 800 (1)
Key Concept 38.2 Learning Outcomes
• Give evidence showing that night
length, rather than day length, is the
cue that triggers flowering.
• Describe evidence that a diffusible
chemical travels from the leaf to the
bud meristem to initiate flowering.
Key Concept 38.2 Learning Outcomes
• Describe the three genes involved in
florigen production and action,
including where they are active, their
functions, and their interactions.
• List factors other than photoperiodism
and genetic triggering of florigen that
can initiate flowering, and give
evidence for each.
Key Concept 38.3 Focus Your Learning
• Sexual and asexual reproduction
provide separate and distinct
advantages to plants.
• Asexual, or vegetative, reproduction in
plants occurs by changes in vegetative
(nonreproductive) organs.
• In apomixis, flowers produce clones;
the technique has the potential to
produce self-reproducing hybrids.
37.3 Angiosperms Can Reproduce Asexually
Asexual reproduction produces offspring
genetically identical to the parent.
If a plant is well adapted to its
environment, asexual reproduction can
preserve and spread that successful
genotype.
37.3 Angiosperms Can Reproduce Asexually
Stems, leaves, and roots are the
vegetative organs of a plant.
Asexual reproduction often occurs by
modification of vegetative organs; also
called vegetative reproduction.
Strawberries produce runners, or
stolons—horizontal stems that form
roots at intervals and can develop into
new plants.
37.3 Angiosperms Can Reproduce Asexually
Shoot tips that sag to the ground and
develop roots—new plant grows from the
branch tip (e.g., blackberry).
Potatoes form enlarged underground
stems called tubers that can produce
new plants from the “eyes.”
Rhizomes are horizontal underground
stems that give rise to new shoots (e.g.,
bamboo).
Figure 37.14 Vegetative Organs Modified for Reproduction (Part 1)
37.3 Angiosperms Can Reproduce Asexually
Bulbs and corms are short, vertical,
underground stems.
Bulbs have fleshy, modified leaves for
food storage—a large, underground bud.
These can give rise to new plants (e.g.,
lilies, onions, garlic).
Corms are mostly stem tissue and lack
modified leaves (e.g., crocuses, gladioli).
Figure 37.14 Vegetative Organs Modified for Reproduction (Part 2)
37.3 Angiosperms Can Reproduce Asexually
Leaves can be the source of new
plantlets, as in Kalanchoe.
Suckers are shoots produced by roots.
Many grasses and trees, such as
aspens, form interconnected stands of
genetically identical individuals.
Figure 37.14 Vegetative Organs Modified for Reproduction (Part 3)
37.3 Angiosperms Can Reproduce Asexually
Plants that commonly reproduce
asexually often live in unstable
environments.
Plants with stolons and rhizomes are
often pioneers on sand dunes, (e.g.,
beach grasses). Rapid reproduction
allows them to survive shifting sands,
and their roots help stabilize the dune.
37.3 Angiosperms Can Reproduce Asexually
Vegetative reproduction is efficient in an
unchanging environment, but can have
disadvantages if conditions change.
English elm (Ulmus procera) was
introduced by the ancient Romans as a
clone. When Dutch elm disease struck,
the clonal population lacked genetic
diversity and was wiped out.
37.3 Angiosperms Can Reproduce Asexually
Vegetative reproduction is used
extensively in agriculture.
Stem cuttings inserted into soil will often
grow into a new plant, especially if
treated with auxin.
37.3 Angiosperms Can Reproduce Asexually
Grafting is the process of attaching a bud
or piece of stem from one plant onto the
root or stem of another plant.
The root-bearing plant is the stock; the
part grafted onto it is the scion.
The vascular cambia of each must grow
together so that water and minerals can
be transported to the scion. Usually
closely related species are used.
Figure 37.15 Grafting
37.3 Angiosperms Can Reproduce Asexually
Meristem culture: Pieces of shoot apical
meristem are cultured to generate
plantlets, which can then be planted in
the field.
• Good when uniformity is desired, as in
forestry, or to produce virus-free plants,
as with strawberries and potatoes.
37.3 Angiosperms Can Reproduce Asexually
Apomixis: Asexual production of seeds.
• The megasporocyte fails to undergo
meiosis, resulting in a diploid egg cell,
which then forms an embryo and seed.
• Or, diploid cells from the integument
around the embryo sac form a diploid
embryo sac, and the sac forms an
embryo and seed.
37.3 Angiosperms Can Reproduce Asexually
Apomixis produces clones, and occurs
naturally in some crop plants such as
citrus and Kentucky bluegrass.
Some important crops, such as corn, are
grown as hybrids and cannot be selfed
to get more seeds.
If a hybrid had a gene for apomixis, its
offspring would be genetically identical
to itself.
37.3 Angiosperms Can Reproduce Asexually
An intensive search is on for genes for
apomixis that could be introduced into
desirable crops and allow them to be
propagated indefinitely.
Such a gene has been identified in corn,
but the yield of the variety that contains it
is low.
Key Concept 37.3 Learning Outcomes
• Compare and contrast sexual and
asexual reproduction in plants,
including both the end result and the
advantages or disadvantages.
• List types of locations where
vegetatively reproducing plants might
occur, and give reasons for their
occurrence in these locations.
• Describe how apomixis occurs and how
this might be useful in agriculture.
Investigating Life: What Signals Flowering?
Poinsettias are short-day plants. To
control photoperiod, the plants are
grown in greenhouses, and photoperiod
is carefully regulated.
How did an understanding of angiosperm
reproduction allow floriculturists to develop
a commercially successful poinsettia?
Investigating Life: What Signals Flowering?
A shorter variety with more branching was
discovered, which was propagated
asexually by grafting to native plants.
New varieties have been generated by
conventional sexual reproduction.