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The
Flower
&
Sexual
Reproduction
The Flower: Site of Sexual Reproduction
The flower is the site where sexual reproduction occurs in angiosperms
and it leads to the formation of seeds.
Complete Sexual Cycle
1) The Production of reproductive cells after meiosis.
2) Pollination.
3) Fertilization
Artichoke
4) Seed & Fruit Development.
5) Seed & Fruit Dissemination.
6) Seed Germination.
Note: Angiosperms are the
flowering plants. They belong
to the division Magnoliophyta
(Anthophtya). Remember that
divisions are right below
kingdoms in the taxonomic
hierarchy.
Complete Flower Parts & Their Function
Flowers are made of 4
whorls of modified leaves.
These whorls are stamens,
carpels, sepals and petals.
Complete “Flower
Assignment” before
continuing to the next
slide.
Pollination
The Male Flower Organs Are The Androecium.
The Generative Cell
divides to form sperm
cells.
mitosis
After landing on a stigma (female part),
the dry pollen grain absorbs water from
the stigma, becomes hydrated and
then germinates and grows a pollen
tube. Then sperm cells go down the
tube.
The Pollen Grain is the male gametophyte
(Note: The gametophyte is the gamete
producing plant). It is a 2-celled, haploid
structure that produces sperm cells for
fertilization.
The Female Flower Organs Are The Gynoecium.
Simple & Compound Pistils Compared. Pea has a simple pistil, showing an ovary composed
of a single carpel. Tulip has a compound pistil showing an ovary with 3 fused carpels.
Pea
The Gynoecium can be
a single carpel (simple
pistil) where the ovary
resides. The ovary can
be many carpels.
Compound Pistil – A
group of fused carpels.
The Pea has a single carpel (simple
pistil) with multi ovules present.
Tulip
Ovary – A hollow
structure, having several
chambers (locules).
Locule – Chamber; a cavity of the ovary in which
ovules occur.
Ovule – A rudimentary seed before fertilization.
Stigmas
Locules
Carpels
Pea
1
1
1
Tulip
3
3
3
The tissue within
the ovary to which
an ovule is
attached is called
the Placenta.
This figure indicates 3
types of placentation: a)
parietal, b) axile, & c)
central.
The Embryo Sac is the Female Gametophyte Plant.
Embryo Sac – The female gametophyte of angiosperms contained in
the ovule of a flowering plant. Generally, a 7-celled structure with all
cells being haploid or n .
7 CELLS
3 Antipodal nuclei (Each haploid).
These will form 3 antipodal cells.
1 Endosperm mother cell with 2 haploid nuclei.
This is the central cell.
1 Egg Cell
2 Synergids
(Haploid
or n)
Embryo Sac
Development
in Cotton
3 Megaspores
Disintegrate
(Haploid or n)
Double Fertilization Produces an Embryo & the Endosperm
Within the embryo sac (female), the synergids emit signals, so the pollen tube
(male) will travel to the micropyle of the ovule. In the pollen tube, 2 sperm cells
are traveling.
1) After reaching the embryo sac, one sperm fuses with the egg.
Sperm (n) + egg (n) will form a diploid zygote (2n). Later, the zygote
will form the embryo.
2) The other sperm fuses with the central cell.
Sperm (n) + polar nuclei (polar bodies (2n) of the central cell will
form a triploid (3n) primary endosperm nucleus. Later, the
primary endosperm nucleus will become the endosperm
(the nutritive tissue formed within the embryo sac of seed
plants; it often is consumed as the seed matures, but
remains in the seeds of many monocots).
The double fusion of egg with sperm and polar nuclei with sperm is called
Double Fertilization.
Now the conditions are set for development of the seed and fruit. After
fertilization, the ovule becomes the seed and the ripened ovary becomes the fruit.
In monocots, food reserves remain in the endosperm. In dicots, the food reserves
of the endosperm are transported to the cotyledons and consequently, the mature
dicot seed lacks endosperm.
Apical Meristems: Sites of Flower Development
As we discussed in previous
chapters, the shoot apex or SAM
produces stems and leaves.
At some time during the growing
season, a signal (for example, day
length or temperature) will trigger a
change in the metabolism of certain
shoot apices, thereby starting its
transformation into a floral apex.
This apex now becomes the floral
apical meristem (FAM).
At the FAM, Bracts are the first to
develop.
Bract – a modified leaf, from the axil of
which gives rise to a flower or an
inflorescence.
Inflorescence – a flower cluster.
Floral organs are modified leaves.
Therefore, the flower itself is
really a shortened and modified
branch.
Flowers Vary In Their Architecture
A Complete Flower has all four sets of floral leaves or parts
(Sepals, Petals, Stamens and Carpels).
An Incomplete Flower lacks at least 1 of the floral leaves or
parts.
Perfect Flowers have both male and female parts present in one
flower. These are bisexual flowers. Therefore, a perfect
flower is a complete flower if it also has sepals and petals.
An Imperfect Flower is missing one gender. These are
unisexual flowers. They have either male or female parts.
The types of imperfect flowers are:
Staminate Flowers – possess male parts.
Pistilliate Flowers – possess female parts.
Is a pistillate flower, a complete or incomplete flower?
Must a complete flower be a perfect flower?
Can an incomplete flower be a perfect flower?
Staminate and
pistillate flowers can
occur on the same
plant. Examples are
Corn, Walnut, Oak.
If a plant produces male and
female gametes in the same
flower on one plant or on
different flowers on the same
plant, the plant is monoecious.
Monoecious – A plant that
produces male and female
gametes on the same individual.
Walnut
Dioecious – unisexual, having the male and female elements in different
individuals. Staminate and pistillate flowers on separate individual plants.
So an individual could be classified as male or female. Example:
Asparagus, willow, Kentucky coffee tree, ginkgo.
Symmetry of Flowers
Regular Symmetry – Divide in
different Places and still have 2
equal parts A regular flower has a
corolla made up of similarly
shaped petals equally spaced and
radiating form the enter of the
flower; star shaped flower;
actinomorphic flower.
Irregular Symmetry – Divide
only in one place and still
have 2 equal parts. An
irregular flower has parts so
that only one line can divide
the flower into 2 equal halves
or mirror images;
zygomorhic flower.
Ovary Position
Superior Ovary – The gynoecium is
situated on the receptacle above the
Tulip with
point of origin of the perianth &
Superior
androecium.
Ovary
Inferior Ovary – Gynoecium is below the
apparent points of attachment of the
perianth & androecium. The lower
portions of the 3 outer whorls - calyx,
corolla and androecium are fused in a
Daffodil with
tube called a hypanthium.
Inferior Ovary
Almond (Inferior Ovary),
the hypanthium is not
fused to the ovary.
Often Flowers Occur in Clusters or Inflorescences
Inflorescence – a
flower cluster or
flowering branch.
Raceme –
Simple,
pedicels
(short
branches)
present.
Panicle –
Branched
raceme.
Catkin – Spike with only pistillate or staminate
flowers (See previous slide on Walnut).
Umbel – short floral axis &
flowers arise in an umbrella-like
fashion from the same level.
Spike –
Like a
raceme
but no
pedicel.
Head – Flowers lack
pedicel & are crowed
together (Asteraceace
Family; this includes the
sunflowers or
composites).
Cyme – One
flower on main
axis & other
lateral flowers
present.
Reproductive Strategies:
Self-Pollination & Cross-Pollination
Self Pollination (Selfing) – This is when pollen is transferred from the anther to
stigma on the same flower or pollen from one flower to another flower on
the same plant. Selfing is sexual reproduction because there is a joining of
egg and sperm but the resulting embryo has only one parent. Selfing can
be prevented if a plant is dioecious. Why?
Selfing can also be prevented if the stigma inhibits pollen tube growth or if
on the same plant, the anther releases pollen some time before the stigma
on the same plant matures (Receptive to Pollen).
Selfing can be beneficial if plants are scattered or if pollinators are rare.
Selfing can save energy by putting very low resources into pollen
production. In some selfers, the flowers never open.  These flowers
remain small and budlike, and the pollen falls from the anther sacs onto the
stigma while the flower parts are still close together.
Some seeds are produced with no fertilization at all. This is parthenogenesis.
Cross Pollination (Outcrossing) – This is when pollen is transferred from the
anther of one individual plant to the stigma of another individual plant of the
same species. Cross pollination occurs between genetically distinct
individuals. Cross pollination results in genetic recombination, leading to
genetically diverse offspring. This is important for adaptation.
Pollination is Affected
By Vectors
Vectors transfer pollen from anther
to stigma.
A Pollination Syndrome is when there are unique flower and
pollen traits that adapt a flower for pollination by a particular
vector. In other words, flowers are adapted to be pollinated
by certain vectors. Vectors can be nonliving (abiotic) or biotic
(living).
Biotic Vectors – Insects (Beetles, Flies, Bees and Relatives, Butterflies, Moths),
Birds and Bats. Biotic Vectors carry and transfer pollen and obtain rewards
themselves. Such rewards can be nectar, pollen (can be eaten), waxes and
oils. Flowers and biotic vectors are adapted (or fit) to each other and are
mutualistic (both benefit) in nature.
- A Nectary is a nectar-secreting gland that is found in flowers.
Nectar occurs in tubes or pools. Nectar may be available only for a
precise period of time. Nectar is made mostly sugar.
- In order to find the nectar, biotic vectors follow Nectar Guides on
the plant. Nectar guides are various markings that direct the
pollinator to the flower’s sexual organs and source of nectar. In
addition, some flowers emit strong odors while others may rely on
colors to attract biotic vectors.
- When considering honeybees, they need 18% sugar concentration otherwise
the act of finding nectar would be an energy loss.
- Some bee species take up oil from flowers (2,000 plant species produce oil
instead of nectar) instead of nectar. The energy content of oil – twice that of
sugar – makes it a suitable food for bee larvae.
Some flowers are built with no landing
platform for pollinators. Biotic vectors,
such as hummingbirds must hover.
- Certain flowers open at night for bat
pollination.
In addition, some female
orchids mimic female bees,
which attract male bees.
Abiotic Vectors – Wind or Water. Pollen is carried by nonliving
items. Examples of plants pollinated by abiotic vectors: Grasses,
Many Trees.
Plants with abiotic vectors usually have no nectar. Flowers on these plants
are often small, colorless and odorless. Petals are often lacking and reduced
to small scales.
These plants release much
more pollen than (biotic
vector) flowers. One single
rye or corn plant can release
20 million pollen grains and
one dock plant can release
400 million pollen grains.
Plants must saturate the
surroundings and hope pollen
lands on the stigma of other
plants. Stigmatic surfaces are
enlarged and elaborate, often
extending outside the flower
to receive the pollen. Note:
Wind and insect pollination
are not mutually exclusive
(See Figure 13.20) in your text.
Wind-pollinated Flower. The
architecture of jojoba flowers
(Simondsia chinensis) creates
vortices of air currents that
bring pollen grains to the
stigmatic surfaces.
BIO 141 Botany with Laboratory
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