Transcript Angiosperm_Reproduction - REMC 8 / Kent ISD Moodle VLE

Angiosperm
Reproduction
Flowers
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The angiosperms are nicknamed the "flowering plants"
because nearly all of them produce flowers.
The "flower" is a reproductive structure meant to
propagate the species, not to adorn the hair or clothing of
a human being.
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The "flower" participates in the complex reproductive cycle
of plants known as "Alternation of Generations".
In this cycle, the haploid (n) and diploid (2n) generations
take turns producing each other.
The plant is the diploid sporophyte.
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The flower contains the haploid gametophyte.
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The Plant - A Diploid Sporophyte
When you think of a plant, you are visualizing an adult sporophyte.
► An oak tree, a mulberry bush, a blade of grass, and a thistle are all
sporophytes.
► Because they developed from a seed, these sporophytes MUST be
diploid.
► Thus, a familiar green or woody plant is a diploid sporophyte (2n).
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The diagram above makes clear the idea that the sporophyte
produces spores.
► These spores are NOT airborne spores (like the fungi). Nor are
they gametes (because they do not fuse with other spores).
They are single haploid cells that divide (by mitosis) into a
multicellular structure that produces gametes.
► The male spore is called the microspore, and the female spore is
named the megaspore.
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The Flower Contains The Haploid
Gametophyte
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When you think of a flower, you are visualizing the part of
the plant that houses the gametophyte.
A tulip, a rose, a petunia, and a pea flower all house
gametophytes.
Because the gametophyte develops from a haploid spore
by mitosis, it must be haploid.
► Thus, the flower contains a plant structure called a haploid
gametophyte (n).
► The diagram on page 1 makes clear the idea that the
gametophyte produces gametes.
► These gametes are the familiar gametes of sexual
reproduction - eggs (ova) and sperm.
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Neither the ova or sperms are independently motile…there are no cilia
or flagella involved in their transport.
Gametophytes (n) form gametes by mitosis!
The male gametophyte is named the pollen grain.
The female gametophyte is named the megagametophyte and is
contained within the embryo sac.
When male and female gametes unite, the fertilized egg is diploid and
undergoes repeated mitosis to grow into the adult plant (diploid
sporophyte).
Flowers
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Flowers are highly modified stems (shoots) in angiosperms.
Only angiosperms produce flowers. Flowers are not a part
of the reproductive cycle of any other organism group.
A typical flower is composed of four whorls of modified
leaves.
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The internodes between these whorls are very small.
Flowers are determinate…they stop growing when they reach maturity.
The four whorls are, from bottom to top:
1. Sepals
2. Petals
3. Stamens
4. Pistils (or carpels)
Sepals are the "flower leaves". They are typically green
(but they may be other colors). Sepals are protective they enclose the flower as it develops and at night, when
the flower closes.
► Petals are the most colorful part of the flower. They are
colorful to attract animal and insect pollinators.
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Stamens are the "boy parts". They consist of a stalk called
the filament and a capsule at the end of the stalk called
the anther. It is within the anther that pollen develops.
Pistils are the "girl parts". Pistils (carpels) have a base
called the ovary and a slender neck called the style. At the
top of the carpel is a sticky structure called the stigma.
The stickiness serves as an adherent for pollen.
The Male Sporangium
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The flower (2n) contains both male and female sporangia
(spore-producing organs).
The male sporangium is called a microsporangium (2n) and
is found within the anther. An anther typically has four
microsporangia.
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Cells in the microsporangium called microsporocytes (2n) undergo
meiosis to form 4 microspores. Each microspore is haploid (n).
Each microspore undergoes a single mitotic division to become two
cells, a tube cell and, within it, a generative cell.
The male's 2 non-flagellated sperm cells (n) arise from the generative
cell (n) by mitosis. (Their movement through the pollen tube is
passive.)
The tube cell produces the pollen tube, a tube for sperm delivery to
the egg.
These two cells become encased within a protective,
resistant coating and become a pollen grain.
This pollen grain IS the male gametophyte generation of
the angiosperm.
The Female Sporangium
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The female sporangium (the megasporangium (2n)) is
found within the ovule, a small compartment within the
ovary.
Each megasporangium (2n) contains a cell called a
megasporocyte.
The 2n megasporocyte undergoes meiosis to produce 4
haploid megaspores.
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Three of these megaspores degenerate.
The single megaspore that survives undergoes 3
successive mitotic divisions, producing 8 haploid cells.
This 8 nucleate stage IS the female gametophyte
generation of an angiosperm.
Pollination
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Pollination occurs when the pollen grains (transferred by
wind, animal, gravity, or physical contact) land upon the
stigma.
Pollination leads, ultimately, to fertilization.
Self-pollination occurs when flowers self-fertilize.
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Cross pollination - pollen is transferred to the stigma of a
different plant.
Upon landing, a pollen grain absorbs water and
germinates.
A pollen tube emerges from the pollen grain and grows
downward through the style to the base of the ovary.
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Meanwhile, the germinative cell divides by mitosis and
forms two sperm cells.
The two sperm cells rely on the growth of a pollen tube to
deliver them to the egg cell. They are NOT flagellated
(remember that the generative cell is found on the inside
of the tube cell).
The tip of the pollen tube enters the ovary from the bottom
through a hole called the micropyle.
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The diagram above shows the female gametophyte, the embryo sac
containing 8 haploid cells.
The 3 cells on the end opposite the micropyle are named antipodal
cells.
The 2 cells in the middle of the embryo sac are the polar nuclei.
The large cell by the micropyle is the ovum. The other two are
synergids.
One sperm fertilizes the egg to form the zygote.
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The cell that is fertilized is usually (but not always) the cell
closest to the micropyle, the ovum
One sperm cell (n) fertilizes the ovum (n) (producing a 2n
zygote).
The other sperm cell (n) combines with the two polar
nuclei (n) to form the 3n endosperm. The endosperm
develops into the food-storing tissue of the seed.
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This union of two sperm cells with different cell types
within the embryo sac is called the DOUBLE
FERTILIZATION. It is unique to angiosperms.