• It has been said that an oak is an acorn’s way of
making more acorns.
• In a Darwinian view of life, the fitness of an
organism is measured only by its ability to replace
itself with healthy, fertile offspring.
• Sexual reproduction is not the sole means by which
flowering plants reproduce.
• Many species can also reproduce asexually, creating
offspring that are genetically identical to themselves.
Introduction
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• Flowers, the reproductive shoots of the angiosperm
sporophyte, are typically composed of four whorls of
highly modified leaves.
2. Flowers are specialized shoots bearing
the reproductive organs of the angiosperm
sporophyte
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• The four kinds of floral organs are the sepals,
petals, stamens, and carpals.
• Their site of attachment to the stem is the receptacle.
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Fig. 38.2
Know these structures and tissues for the
EOC
• Roots
• Stems
• Leaves
• Flowers
• Fruits
• Cones
• Seeds
And these
Stomata
• Guard cells
• Stamen: anther, filament, sperm
• Pistil: style, stigma, ovary, egg
• Sepal, petals
• The stamens and carpels of flowers contain
sporangia, within which the spores and then
gametophytes develop.
• The male gametophytes are sperm-producing structures
called pollen grains, which form within the pollen sacs
of anthers.
• The female gametophytes are egg-producing structures
called embryo sacs, which form within the ovules in
ovaries.
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• The male gametophyte begins its develop within the
sporangia (pollen sacs) of the anther.
• The female gametophyte begins to develop within
the ovules of the ovary
3. Male and female gametophytes develop
within anthers and ovaries, respectively:
Pollination brings them together
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• The development of angiosperm gametophytes involves
meiosis
and mitosis.
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Fig. 38.4
• The male gametophyte begins its development
within the sporangia (pollen sacs) of the anther.
• Within the sporangia are microsporocytes, each of
which will from four haploid microspores through
meiosis.
• Each microspore can eventually give rise to a haploid
male gametophyte.
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• A microspore divides once by mitosis and
produces a generative cell and a tube cell.
• The generative cell will eventually form sperm.
• The tube cell, enclosing the generative cell, produces
the pollen tube, which delivers sperm to the egg.
• This two-celled structure is
encased in a thick, ornate,
distinctive, and resistant wall.
• This is a pollen grain, an
immature male gametophyte.
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Fig. 38.5
• A pollen grain becomes a mature gametophyte
when the generative cell divides by mitosis to form
two sperm cells.
• In most species, this occurs after the pollen grain lands
on the stigma of the carpel and the pollen tube begins to
form.
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• Ovules, each containing a single sporangium, form
within the chambers of the ovary.
• One cell in the sporangium of each ovule, the
megasporocyte, grows and then goes through meiosis,
producing four haploid megaspores.
• In many angiosperms, only one megaspore survives.
• This megaspore divides by mitosis three times,
resulting in one cell with eight haploid nuclei.
• Membranes partition this mass into a multicellular
female gametophyte - the egg sac.
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• At one end of the egg sac, two synergid cells flank
the egg cell, or female gametophyte.
• The synergids function in the attraction and guidance of
the pollen tube.
• At the other end of the egg sac are three antipodal
cells of unknown function.
• The other two nuclei, the polar nuclei, share the
cytoplasm of the large central cell of the embryo
sac.
• The ovule now consists of the embryo sac and the
surrounding integuments (from the sporophyte).
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• Pollination, which brings male and female
gametophytes together, is the first step in the chain
of events that leads to fertilization.
• Some plants, such as grasses and many trees, release
large quantities of pollen on the wind to compensate for
the randomness of this dispersal mechanism.
• At certain times of the year, the air is loaded with
pollen, as anyone plagued by pollen allergies can
attest.
• Most angiosperms interact with insects or other animals
that transfer pollen directly between flowers.
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• After landing on a receptive stigma, the pollen grain
absorbs moisture and germinates, producing a pollen
tube that extends down the style toward the ovary.
• The generative cells divides by mitosis to produce two
sperm, the male gametophyte.
• Directed by a chemical attractant, possibly calcium, the tip
of the pollen tube enters the ovary, probes through the
micropyle (a gap in the integuments of the ovule), and
discharges two sperm within the embryo sac.
5. Double fertilization gives rise to the
zygote and endosperm
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• Both sperm fuse with nuclei in the embryo sac.
• One sperm fertilizes the egg to form the zygote.
• The other sperm combines with the two polar nuclei to
form a triploid nucleus in the central cell.
• This large cell will give rise to the endosperm, a food-
storing tissue of the seed.
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• The union of two sperm cells with different nuclei
of the embryo sac is termed double fertilization.
• Double fertilization is also present in a few
gymnosperms, probably via independent evolution.
• Double fertilization ensures that the endosperm will
develop only in ovules where the egg has been
fertilized.
• This prevents angiosperms from squandering nutrients
in eggs that lack an embryo.
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• After double fertilization, the ovule develops into a
seed, and the ovary develops into a fruit enclosing
the seed(s).
• As the embryo develops, the seed stockpiles proteins, oils,
and starch.
• Initially, these nutrients are stored in the endosperm, but
later in seed development in many species, the storage
function is taken over by the swelling storage leaves
(cotyledons) of the embryo itself.
6. The ovule develops into a seed containing
an embryo and a supply of nutrients
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Wrap your mind around this
• When a plant has babies, they look very different.
• An oak tree, for instance, has pollen grains and
embryo sacs for babies.
• Pollen grains and embryo sacs have oak trees for
babies.
• Confusing?
• Endosperm development usually precedes embryo
development.
• After double fertilization, the triploid nucleus of the
ovule’s central cell divides, forming a multinucleate
“supercell” having a milky consistency.
• It becomes multicellular when cytokinesis partitions the
cytoplasm between nuclei and cell walls form and the
endosperm becomes solid.
• Coconut “milk” is an example of liquid endosperm
and coconut “meat” is an example of solid
endosperm.
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• As the seeds are developing from ovules, the ovary
of the flower is developing into a fruit, which
protects the enclosed seeds and aids in their dispersal
by wind or animals.
• Pollination triggers hormonal changes that cause the ovary
to begin its transformation into a fruit.
• If a flower has not been pollinated, fruit usually does not
develop, and the entire flower withers and falls away.
7. The ovary develops into a fruit adapted
for seed dispersal
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• By selectively breeding plants, humans have
capitalized on the production of edible fruits.
• The apples, oranges, and other fruits in grocery stores
are exaggerated versions of much smaller natural
varieties of fleshy fruits.
• The staple foods for humans are the dry, wind-dispersed
fruits of grasses, which are harvested while still on the
parent plant.
• The cereal grains of wheat, rice, maize, and other
grasses are easily mistaken for seeds, but each is
actually a fruit with a dry pericarp that adheres
tightly to the seed coat of the single seed within.
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