Biology
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Alternation of Generations
Alternation of Generations
All plants have a life cycle in which a diploid sporophyte
generation alternates with a haploid gametophyte
generation.
Male and female gametes of a gametophyte plant join and
form a zygote that begins the next sporophyte generation.
In some plants, the two stages of the life cycle are distinct,
independent plants.
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Cones and Flowers
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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Life Cycle of Gymnosperms
Life Cycle of Gymnosperms
What are the reproductive structures of
gymnosperms?
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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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Life Cycle of Gymnosperms
Pollen Cones and Seed Cones
Pollen cones produce the male gametophytes,
also called pollen grains.
Pollen grain (N)
(male gametophytes)
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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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Life Cycle of Gymnosperms
Within the ovules, meiosis produces haploid cells that grow and
divide to produce female gametophytes.
Each gametophyte contains egg cells.
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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.
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Life Cycle of Gymnosperms
Meiosis
Fertilization
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Life Cycle of Gymnosperms
Male Cones
Pollen
cone
Mature sporophyte
Pollen grain (N) (male
gametophytes)
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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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Life Cycle of Gymnosperms
If pollen grains land on and enter an ovule, pollination
occurs.
A pollen tube grows out of each pollen grain and
releases sperm near an egg.
Egg cells
Discharged
sperm nucleus
Pollen tube
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Life Cycle of Gymnosperms
Fertilization produces a diploid zygote which
develops into a new sporophyte plant.
Zygote (2N)
(new sporophyte)
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Life Cycle of Gymnosperms
This zygote grows into an embryo and is encased
within what will develop into a seed.
Gametophyte tissue
Embryo (2N)
Seed coat
(old sporophyte)
Seed
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Life Cycle of Gymnosperms
The seed is then dispersed by wind.
When conditions are favorable, the seed germinates
and its embryo grows into a seedling.
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Structure of Flowers
What are the reproductive structures of
angiosperms?
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Structure of Flowers
Structure of Flowers
Flowers are reproductive organs that are
composed of four kinds of specialized
leaves: sepals, petals, stamens, and
carpels.
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Structure of Flowers
Sepals enclose the bud before it opens and
protect the flower while it is developing.
Sepal
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Structure of Flowers
Petals are often brightly colored and are found just
inside the sepals.
Petals attract insects and other pollinators to the
flower.
Petal
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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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Structure of Flowers
An anther is an oval sac where meiosis takes place,
producing pollen grains.
Anther
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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.
Stigma
Style
Carpel
Ovary
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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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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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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.
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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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Life Cycle of Angiosperms
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Life Cycle of Angiosperms
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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The pollen grain
usually stops growing
until it is released
from the anther and
deposited on a
stigma.
Life Cycle of Angiosperms
Pollen grains (N)
(male gametophyte)
Stigma
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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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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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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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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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The pollen tube
grows into the style,
reaches the ovary,
and enters the ovule.
Fertilization in Angiosperms
Pollen grains (N)
(male gametophyte)
Pollen tubes
Ovule
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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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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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Fertilization in Angiosperms
Because two fertilization events take place between
the male and female gametophytes, this process is
known as double fertilization.
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Pollination
How does pollination differ between
angiosperms and gymnosperms?
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Pollination
Pollination
Most gymnosperms and some
angiosperms are wind pollinated,
whereas most angiosperms are
pollinated by animals.
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Pollination
Wind pollination
• is less efficient than animal pollination
• relies on weather
Animal pollination
• plants have bright colors and sweet nectar to
attract animals
• benefits both the plants and the animals that
pollinate them
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In a gymnosperm, fertilization occurs inside the
a. ovule.
b. pollen tube.
c. seed cone.
d. pollen cone.
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The pollen-producing structure of angiosperms
is the
a. stigma.
b. carpel.
c. anther.
d. sepal.
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In an angiosperm, a structure that results from
fertilization is the
a. female gametophyte.
b. pollen grain.
c. zygote.
d. ovary.
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Which term applies to the chromosome number
of a seed plant embryo?
a. haploid
b. diploid
c. triploid
d. polyploid
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Pollination in most gymnosperms and some
angiosperms is carried out by
a. water transport.
b. insects.
c. wind.
d. birds and bats.
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