24–1 Reproduction With
Cones and Flowers
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24–1 Reproduction With
Cones and Flowers
Alternation of Generations
Alternation of Generations
In seed plants, gametophytes are found within
tissues of the sporophyte plant.
In gymnosperms, they are found inside cones.
In angiosperms, they are found inside flowers.
Cones and flowers represent two different methods
of reproduction.
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24–1 Reproduction With
Cones and Flowers
Life Cycle of Gymnosperms
Life Cycle of Gymnosperms
Reproduction in gymnosperms takes place in cones,
which are produced by a mature sporophyte plant.
Gymnosperms produce two types of cones: pollen
cones and seed cones.
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24–1 Reproduction With
Cones and Flowers
Life Cycle of Gymnosperms
Pollen Cones and Seed Cones
Pollen cones produce the male gametophytes, also
called pollen grains.
One of the haploid nuclei in the pollen grain will divide
later to produce two sperm nuclei.
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Life Cycle of Gymnosperms
Seed cones produce female gametophytes and are
generally larger than pollen cones.
Female gametophytes develop in two ovules located
near the base of each scale.
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24–1 Reproduction With
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Life Cycle of Gymnosperms
Within the ovules, meiosis produces haploid cells that
grow and divide to produce female gametophytes.
Each gametophyte may contain hundreds or
thousands of cells.
When mature, each gametophyte contains a few
large egg cells, ready for fertilization. egg cells.
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24–1 Reproduction With
Cones and Flowers
Life Cycle of Gymnosperms
Pollination
The gymnosperm life cycle typically takes two years
to complete.
The cycle begins as male cones release pollen grains.
Pollen grains are carried by the wind and reach
female cones.
There, some pollen grains are caught in a sticky
secretion on one of the scales of the female seed
cone.
This sticky material, known as a pollination drop,
ensures that pollen grains stay on the female cone.
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24–1 Reproduction With
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Life Cycle of Gymnosperms
If pollen grains land on and enter an ovule, pollination
occurs.
When the pollen grain lands near an ovule, the grain
splits open and begins to grow a pollen tube, which
contains two haploid sperm nuclei, and releases
sperm near an egg.
Egg cells
Discharged
sperm nucleus
Pollen tube
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Life Cycle of Gymnosperms
One sperm nuclei disintegrates when they reach the
female gametophyte. The other fertilizes the egg.
If sperm from another pollen tube reaches the female
gametophyte, more than one egg may be fertilized,
but just one embryo develops.
Fertilization produces a diploid zygote which develops
into a new sporophyte plant.
Zygote (2N)
(new sporophyte)
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24–1 Reproduction With
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Life Cycle of Gymnosperms
This zygote grows into an embryo and is encased
within what will develop into a seed.
The seed consists of three generations of the life
cycle: outer seed coat is part of the old sporophyte,
haploid cells surrounding the embryo are female
gametophyte, and embryo is new sporophyte.
Gametophyte tissue
Embryo (2N)
Seed
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Seed coat
(old sporophyte)
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Life Cycle of Gymnosperms
Male Cones
Pollen
cone
Mature sporophyte
Pollen grain (N) (male
gametophytes)
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24–1 Reproduction With
Cones and Flowers
Life Cycle of Gymnosperms
Cone scale
Female Cones
Diploid cell (2N)
Ovule
Seed
cone
Ovules
Four haploid
cells (N)
Female
gametophyte
(N)
Mature sporophyte
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24–1 Reproduction With
Cones and Flowers
Structure of Flowers
Structure of Flowers
Flowers are reproductive organs that are composed
of four kinds of specialized leaves:
•Sepals
•Petals
•Stamens
•Carpels
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24–1 Reproduction With
Cones and Flowers
Structure of Flowers
Sepals enclose the bud before it opens and protect
the flower while it is developing.
In many plants, sepals are green and closely resemble
ordinary leaves.
Sepal
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24–1 Reproduction With
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Structure of Flowers
Petals are often brightly colored and are found just
inside the sepals.
Petals attract insects & other pollinators to the flower.
Because they do not produce reproductive cells,
sepals and petals are sometimes called sterile
leaves.
Petal
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24–1 Reproduction With
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Structure of Flowers
The male parts of a flower consist of an anther and
a filament, which together make up the stamen.
Anther
Stamen
Filament
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24–1 Reproduction With
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Structure of Flowers
An anther is an oval sac where meiosis takes place,
producing pollen grains.
Anther
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24–1 Reproduction With
Cones and Flowers
Structure of Flowers
The filament is a long, thin stalk that supports an
anther.
Filament
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Structure of Flowers
The innermost floral parts are carpels, also called
pistils, which produce the female gametophytes.
Some flowers have several carpels fused together to
form a single compound carpel.
Stigma
Style
Carpel
Ovary
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24–1 Reproduction With
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Structure of Flowers
Each carpel has a broad base forming an ovary.
The ovary contains one or more ovules where female
gametophytes are produced.
Ovary
Ovule
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Structure of Flowers
The narrow stalk of the carpel is the style.
Style
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24–1 Reproduction With
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Structure of Flowers
At the top of the style is the stigma—a sticky portion
where pollen grains frequently land.
Stigma
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24–1 Reproduction With
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Structure of Flowers
Parts of a Typical Flower
Stamen
Stigma
Anther
Filament
Style
Carpel
Ovary
Ovary
Petal
Sepal
Ovule
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Structure of Flowers
A typical flower produces both male and female
gametophytes.
In some plants, male and female gametophytes are
produced in separate flowers on the same individual.
Corn has separate male and female flowers on the
same plant.
The tassel is a flower that produces male
gametophytes, and the silk is the style of a flower that
produces the female gametophyte.
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24–1 Reproduction With
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Life Cycle of Angiosperms
Life Cycle of
Angiosperms
Reproduction in
angiosperms takes
place within the
flower.
Following pollination
and fertilization, the
seeds develop
inside protective
structures.
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24–1 Reproduction With
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Life Cycle of Angiosperms
Think of the beginning of the angiosperm life cycle as
when the mature sporophyte produces flowers.
Each flower contains anthers and an ovary.
Inside the anthers, each cell undergoes meiosis and
produces haploid spore cells.
Each spore cell becomes a single pollen grain.
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24–1 Reproduction With
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The nucleus of each
pollen grain undergoes
one mitotic division to
produce two haploid
nuclei.
Life Cycle of Angiosperms
Pollen grains (N)
(male gametophyte)
Stigma
The pollen grain usually
stops growing until it is
released from the
anther and deposited on
a stigma.
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24–1 Reproduction With
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Life Cycle of Angiosperms
In the ovule, a single diploid cell undergoes meiosis
to produce four haploid cells.
Haploid cell
(N)
Ovule
Ovary (2N)
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Life Cycle of Angiosperms
Embryo sac (N)
(female gametophyte)
Only one of the four cells
undergoes mitosis to
produce eight nuclei.
The eight nuclei and the
surrounding membrane are
called the embryo sac.
Sperm
Pollen tube
Egg cell
Polar nuclei
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24–1 Reproduction With
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Life Cycle of Angiosperms
Embryo sac (N)
(female gametophyte)
The embryo sac is the
female gametophyte.
One of the eight nuclei is
the egg nucleus—the
female gamete.
Sperm
Pollen tube
Egg cell
Polar nuclei
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24–1 Reproduction With
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Life Cycle of Angiosperms
When fertilization takes place, this cell becomes the
zygote that grows into a new sporophyte plant.
Endosperm
(3N)
Zygote (2N)
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24–1 Reproduction With
Cones and Flowers
Pollination
Pollination
Most gymnosperms and some angiosperms are wind
pollinated, whereas most angiosperms are pollinated
by animals.
Botanists suggest that insect pollination is the factor
largely responsible for the displacement of
gymnosperms by angiosperms during the past 100
million years.
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24–1 Reproduction With
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Pollination
Wind pollination
• is less efficient than animal pollination
• relies on weather and sheet numbers
Animal pollination
• plants have bright colors and sweet nectar to
attract animals
• Animals have body shapes that enable them to
reach nectar deep within certain flowers
• benefits both the plants and the animals that
pollinate them
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Fertilization in
Angiosperms
Fertilization in Angiosperms
Pollen grains (N)
(male gametophyte)
Pollen tubes
If a pollen grain lands
on the stigma of a
flower of the same
species, it grows a
pollen tube.
Ovule
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24–1 Reproduction With
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The generative
nucleus within the
pollen grain divides
and forms two sperm
nuclei.
Fertilization in Angiosperms
Pollen grains (N)
(male gametophyte)
Pollen tubes
The pollen tube
grows into the style,
reaches the ovary,
and enters the ovule.
Ovule
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24–1 Reproduction With
Cones and Flowers
Life Cycle of Angiosperms
One of the sperm nuclei
fuses with the egg
nucleus to produce a
diploid zygote.
The zygote will grow
into the new plant
embryo.
Zygote (2N)
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24–1 Reproduction With
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Fertilization in Angiosperms
The other sperm
nucleus fuses with two
polar nuclei in the
embryo sac to form a
triploid (3N) cell.
This cell will grow into a
food-rich tissue known
as endosperm, which
nourishes the seedling
as it grows.
Endosperm
(3N)
Zygote (2N)
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24–1 Reproduction With
Cones and Flowers
Fertilization in Angiosperms
Because two fertilization events take place between
the male and female gametophytes, this process is
known as double fertilization.
This may be one of the reasons why the angiosperms
have been so successful.
In gymnosperms, the food reserve built up in seeds is
produced before fertilization takes place. If an ovule
is not fertilized, those resources are wasted.
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