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38
Reproduction in
Flowering Plants
In angiosperms, flowers contain sex organs
 Nearly all reproduce sexually, many reproduce asexually
as well
 Sexual reproduction produces new gene combinations
and diverse phenotypes
 Asexual reproduction produces clones of genetically
identical individuals
○ Tubers, stolons, grafts, runners, bulbs, rhizomes
This dune plant is
reproducing asexually
by setting out runners
In agriculture, both modes are important
 Many annual crops are grown from seeds (e.g., grains
[grasses], soybean, etc.)
 Other crops such as strawberries, potatoes, and bananas, are
produced asexually
 Navel oranges are seedless, and must be propagated
asexually. All descend from one mutant tree
Seedless navel
orange
Stawberry plant
with stolen
www.hort.cornell.edu/4hplants/Fruits/Images/
www.biology.iastate.edu/Courses/202L/Ne
w%20Site%20S05/27AngioReprod/reprod
A complete flower has four concentric groups
of organs arising from modified leaves:
 Perfect flowers have both stamens and carpels
 Imperfect flowers have only stamens or only
carpels
Monoecious (“one house”)
 Plants that bear both male and female flowers on
an individual plant
Dioecious
 Plants that bear either male-only or female-only
flowers on an individual plant
Flowers
A complete flower has four groups of organs
(modified leaves):
 Carpels
 Stamens
 Petals
 Sepals
Flowers - Carpels
Carpels are female sex organs
 Pistil – composed of one or
more carpels
○ Style – stalk of pistil, with
stigma at the end
 Ovary – at base of pistil
○ containing ovules, each of
which contain a
megasporangium
○ Female gametophyte
develops in the
megasporangium
Flowers - Stamens
Stamens are male sex organs
 Each has a filament with two-lobed anther, containing
four fused microsporangia.
 Male gametophyte develops in microsporangia.
Flowers - Petals
Petals make up corolla
 Often colored to attract pollinators
 Sepals make up the calyx
 All flower parts attached to stem tip or receptacle
Nipplewort (Lapsana communis)
http://oregonstate.edu/dept/nurseryweeds/weedspeciespage/lapsana
Alternation of generations
 multicellular diploid generation
alternates with a multicellular
haploid generation.
 In angiosperms, 2n sporophyte
generation is larger and more
conspicuous
 Sporophyte produces flowers
○ 1n gametophytes develop
from 1n spores in sporangia
Pink and white lady’s slipper
(Cypripedium reginae)
within flower
http://www.sos.state.mn.us/student/artwork/State-Flower.gif
 The two haploid cells within pollen
grain are:
○ generative cell
(mitosis  2 sperm cells)
○ tube cell
Lilium
pollen grain
 Further development is delayed
until pollen arrives at a stigma
http://botit.botany.wisc.edu/Interface/weblessons/Diversity/
plants/Vascular%20Plants/Magnoliophyta/images
Pollination:
 transfer of pollen
from anther to
stigma
Photo 38.8 Stigma of
Hibiscus sp.; pollen
grains adhere to sticky
stigma. SEM.
Evolution of pollen made it possible for male
gametes to reach female gametophyte
without water
 This selective advantage allowed pollen-
bearing plants to colonize land
Many mechanisms have evolved for pollen
transport
 In many species, wind transports pollen
 The flowers have sticky or featherlike stigmas,
and pollen grains are produced in great
numbers
Most angiosperms rely on animals (insects, birds,
and bats) to transfer pollen
 Flowers are pigmented, shaped, and scented
to attract the pollinating animal
 The animals receive nutrition from the
flowers (nectar and pollen)
 The relationship is mutually beneficial
Preventing Self-fertilization
In some (e.g. peas), self
pollination occurs before
flower opens, resulting in
self-fertilization
  leads to homozygosity,
which reduces genetic
diversity
 Most plants have mechanisms
to prevent self-fertilization
Self-pollinated flower of a peanut plant is produced on a
slender stalk (pedicel) near the base of the plant.
Following pollination and fertilization, the flower stalk
curves downward and pushes into the soil. Below
ground, the ovary develops into a 2-seeded pod.
○ Separation of male and female
flowers on monoecious plants
○ Recognition of self pollen
http://waynesword.palomar.edu/images
Preventing self-fertilization
 Genetic self-incompatibility
○ When plants reject pollen from their
own flowers  promotes
outcrossing and increases
heterozygosity
○ must determine whether pollen is
genetically similar or not
○ S gene is responsible for selfincompatibility
S gene locus encodes proteins in pollen and
style that interact during recognition
process
 This locus has many alleles
 When pollen carries an allele that
matches an allele of the recipient pistil,
the pollen is rejected
Germination of pollen grain involves growth of
pollen tube in stigma
 Pollen tube grows through style to reach
ovule
Mature pollen grains consist of two cells
 The larger tube cell encloses the generative
cell
○ Tube cell nucleus directs growth of
pollen tube through style tissue to
embryo sac
 Generative cell undergoes mitotic division
to produce 2 sperm cells (1n)
○ 1 will fertilize the egg
○ 1 will join with 2 polar nuclei in large central
cell to form endosperm
Double
fertilization
Fertilization initiates growth and development of
the embryo, endosperm, integuments, and carpel
 Integuments are the tissues surrounding
megasporangium  develop into seed coat
 Carpel becomes wall of the fruit that surrounds seed
After double fertilization
 First mitotic division of zygote results in asymmetric division of cytoplasm
 One daughter cell becomes embryo,
 other becomes a supporting structure suspensor
 Asymmetric division establishes polarity and longitudinal axis of new plant
In eudicots, embryo develops into heart stage as
cotyledons begin to form
Capsella (Shepherd'sPurse ) “heart”
embryo (dark purple
structure) with
suspensor
www.texarkanacollege.edu/~mstorey/1407
Further elongation gives rise to torpedo stage
 Tissues begin to differentiate
 Shoot apex forms between cotyledons
 Root apex forms at other end
 Both contain meristematic cells
In later stages of development, seed loses water
and becomes dormant
 Integuments – tissues surrounding
megasporangium develop into seed coat to protect
embryo
Mature corn “seed”. This fruit
type, a "grain“, is
characterized by an ovary wall
fused to the seed coat. Monocot
seeds usually have lots of starch
filled endosperm.
http://www.lima.ohio-state.edu/academics/biology/images
Fruit
 Develops from ovary wall  carpel becomes wall of fruit
surrounding seed
 May consist only of mature ovary and seeds, or include
other structures
 Fruits protect seed from damage by animals and
infection by microbes
An apple, is a pome type
fruit. The fleshy part
one eats is the receptacle;
the mature ovary is
actually just the core.
www.csdl.tamu.edu/FLORA/tfp
http://buytaert.net/cache/images-miscellaneous-2005-dandelion-700x700.jpg
Fruits also facilitate seed dispersal
 Some fruits (dandelion) have a
“parachute” for wind dispersal.
 Some fruits float on water
(coconuts).
 Some cling to animals (burrs).
Fruits of sandbur
or sandspur
http://fortdesoto.com/images/pool/
 Other seeds are eaten by
Fruit-eating iguana
animals and pass
through the digestive
tract before germinating
 Some species produce
fleshy, edible fruits, such
as tomatoes
http://www.richard-seaman.com/Reptiles/BelizeAndGuatemala/IguanaEatingFruit.jpg
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.
Plants fall into three categories in terms of maturation and flowering:
 Annuals complete life cycle in one year and have little or no
secondary growth.
○ After flowering, most of their energy is used to develop seeds
and fruits
 Biennials take two years to complete life cycle.
○ Produce vegetative growth during first year and store
carbohydrates in underground roots (carrots) or stems (onion)
○ In second year, stored carbohydrates are used to produce
flowers and seeds
 Perennials live three or more years.
○ 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 (Agave)
Environmental cues can trigger transition to the
flowering state
Photoperiodism is control of flowering and other
responses by length of day or night
 First studied in 1920s in mutant tobacco plants
 Plants would not flower until day length was shorter
than 14 hours — critical day length
Wild type and gigantea (gi) mutant
Arabidopsis plants. Mutation delays
flowering, allowing for large growth.
http://3e.plantphys.net/images/ch24/wt2405a.jpg
Asexual reproduction can involve vegetative
parts: tubers, stolons, runners
Asexual production of seeds is called
apomixis (dandelions, citrus, blackberries)
 Seeds are genetically identical to parent
 Sometimes requires pollination
Photo 38.26 Dandelion heads, with
asexually produced seeds ready for
dissemination on the wind.
Vegetative reproduction is used extensively in
agriculture
 Stem cuttings inserted into soil will often grow into a
new plant, especially if treated with auxin (“rooting
hormone”)
Row of Dahlia cuttings
http://www.gardenbuddies.com/forum/messages/60521/1099136.jpg
Grafting
 Process of attaching bud or piece




of stem from one plant onto root
or stem (with roots) of another
plant
Root-bearing plant is stock, the
part grafted onto it is scion
Vascular cambia of each must
grow together so water and
minerals can be transported to
scion.
Usually closely related species are
used.
Most fruit marketed in U.S. from
grafted trees
Grafted pink
lemon tree on root
stock
http://www.beyondgutenberg.com/Peek/Garden
/LemonTree/PinkLemonadeGraft.jpg
Researchers are investigating ways to reproduce
plants by tissue culture.
 Many plant cells are totipotent —
undifferentiated cells can give rise to entire
plants
 Culturing tiny bits of apical meristem can
eliminate plant viruses
Cattleya orchid
www.ppm4plant-tc.com/
http://members.tripod.com/~rsauleda/
 Treatment with hormones can cause apical meristems
to produce millions of plants in a year
 Used for strawberries and potatoes to produce virusfree plants
Tissue culture of
commercial
strawberries
http://www.noursefarms.com/images/staticPages/tour/tissueCulture.jpg
38.3 How Do Angiosperms
Reproduce Asexually?
Recombinant DNA also used with tissue
culture to produce plants with disease
resistance, increased nutritive value, etc.
Golden Rice (right) is
bioengineered to produce
and accumulates provitamin A (β-carotene) in
the grain, often in short
supply in developing
countries.
www.goldenrice.org/image/