Transcript Propagating Plants Sexually - Oconto Falls Agricultural Education

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Bring to class several varieties of corn or
soybeans. If plants are not available, use a seed
catalog that contains color pictures of plant
varieties. Ask students about the significance of
variety. Ask if they can tell you how varieties
are obtained? Ask how genetic variation in
agricultural crops has impacted our world food
resources? Show students a variety of bean
seeds. What do the seeds have in common?
What is different about them? How does
germination unlock the “secret” within each
seed?
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1 Explain sexual reproduction of plants and
examine its importance in plant survival.
2 Examine the role of meiosis in the development
of gametes.
3 Define pollination and investigate the different
methods of pollination.
4 Describe the fertilization process in flowering
plants and explain how seeds are formed.
5 Describe the process of seed germination and
what conditions are necessary for germination to
occur in a seed.
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anaphase
anemophily
cell
chromosomes
cotyledons
cross-pollination
cytokinesis
diploid
double fertilization
embryo plant
endosperm
entomophily
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fertilization
gametes
germination
haploid
hybrid
hydrophily
incompatibility
meiosis
metaphase
mitosis
pollenizer
pollination
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prophase
scarification
seed
seed coat
self-pollenization
self-pollination
sexual reproduction
stratification
telophase
turgor
zoophily
zygote
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I. Sexual reproduction in plants
involves the union of the male
pollen with the female egg and
results in the formation of a seed.
Sexual reproduction enables a
plant to produce new
combinations of genetic
information that may add to the
vigor of the developing young
plant.
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A. Most plants reproduce
their own kind in nature
through sexual reproduction
that forms some type of
seed. A seed is a living entity
that serves as a bridge
between generations of a
plant.
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B. The recombining of genetic material in a
species gives it the means to adapt to changes
in the environment. Some of the changes may
be beneficial and some may not. As conditions
of the environment change over time, the
beneficial changes in plant genetics will allow
the plant species to survive. As plants continue
to reproduce, they pass genes onto their
offspring, which enables them to survive.
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II. A cell is the basic unit of life and consists of the
cell membrane, cytoplasm, and the nucleus.
A. Chromosomes are found in the nucleus; they
carry genetic units, known as genes, that govern
specific traits. Chromosomes are found in pairs in
all cells except in the reproductive cells. Corn has
20 chromosomes or 10 pairs. Both members of the
chromosome pair contain the same genes in the
exact same location or loci on the chromosome.
Therefore, for any one trait there exists a pair of
genes responsible for its expression. The
following lists the number of chromosomes that
each plant contains: corn—20, barley—14,
alfalfa—32, potato—48, cotton—52, carrot—18,
garden pea—20, lettuce—18, and wheat— 42.
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B. Cells can reproduce by two main methods
in multicellular organisms—mitosis and
meiosis.
1. Mitosis is a type of asexual reproduction in
which two new cells are created from the
original cell. Each new cell has the same
number of chromosomes as the parent cell
and is genetically identical to the parent cell.
Mitosis continues throughout an organism’s
life. The following are the five phases in cell
division, with the first four making up
mitosis:
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a. Prophase: The nuclear membrane begins to break
down, spindle fibers appear at each end of the cell,
and chromosomes become visible as paired bundles.
b. Metaphase: Spindle fibers attach to the
chromosomes as they group together.
c. Anaphase: The chromosomes divide and separate,
moving to opposite ends of the cell.
d. Telophase: New nuclear membranes form around
the groups of chromosomes, the spindle fibers
disappear, and the chromosomes uncoil.
e. Cytokinesis: In this final phase, the organelles
move to the new nuclei, and the cell completes
division by forming a plate between the two new
complete cells.
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2. Meiosis is cell division that
creates four new cells from the
original parent cell resulting in four
sex cells called gametes. Meiosis
occurs in the flower (in
angiosperms) to form the cells from
which the pollen grains and the
embryo sac (which contains the egg)
develop.
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a. The four daughter cells have half a set of chromosomes.
Also, meiosis results in a random assortment of parental
genes. Therefore, each of the four cells is genetically
different from each other and the parent cell.
b. The steps of meiosis include: Interphase, Prophase I,
Metaphase I, Anaphase I, Telophase I, Interphase,
Prophase II, Metaphase II, Anaphase II, Telophase II, and
Cytokinesis.
c. Sexual reproduction takes place when the gametes
(sperm and egg cells), created by meiosis, unite and
fertilization occurs. The offspring produced by sexual
reproduction receives half of its genetic information from
the female parent and the other half from the male parent.
A zygote results from the fusion of gametes. Thus, a new
plant is created by sexual reproduction.
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III. The reproductive process begins
with pollination.
A. Pollination is the transfer of the
male sperm carried in the pollen to the
female part of the flower, the stigma.
Some plants rely on wind and water to
transfer the pollen to the stigma. Other
plants depend on animals to help with
pollination. Birds, insects, bats, and
other animals are attracted to brightly
colored or scented flowers.
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1. Pollination by wind is called anemophily.
2. Pollination by water is referred to as
hydrophily.
3. When vertebrates including birds, bats, and
other animals, pollinate flowers, the process is
called zoophily.
4. The process of insects transferring pollen is
called entomophily.
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B. Pollination can be self-pollination, self-pollenization,
and cross-pollination.
1. When the pollen of a plant pollinates a flower on the
same plant without the influence of a pollinator, it is
called self-pollination. Many plants have the ability to
self pollinate, others do not. Some plants have genetic
mechanisms that prevent their pollen from growing a
pollen tube on a style of the same plant. This condition
is called incompatibility.
2. When a plant is self-pollinated through the efforts of
a pollinator, it is said to be self-pollenization.
3. When the pollen of a plant pollinates the flower on
another plant of the same species, this is called crosspollination. Cross-pollination requires a pollinator
and an external pollenizer. A pollenizer is the plant
that provides the pollen.
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C. Once the pollen lands on the
stigma, it grows a thin pollen tube
down the style to the ovary. The cell
within the grain of pollen divides to
form two sperm nuclei. The sperm
cells or male gametes travel down
the pollen tube to an ovule that
holds the egg or the female gamete.
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IV. Fertilization is the fusion of sperm
with an egg. The egg and the sperm
contain a single set of chromosomes and
are said to be haploid. Fertilization
unites the single set of chromosomes in
the sperm nucleus with the single set of
chromosomes in the egg nucleus. This
enables the fertilized egg, or zygote, to
have a complete pair of chromosomes. A
cell with two sets of chromosomes is
termed diploid.
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A. In flowering plants fertilization is unlike
that of any other living organism. This is
because both sperm nuclei from the pollen
grain are involved in fertilization, and this
fertilization process of flowering plants is
called double fertilization.
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1. The first fertilization occurs when one of the sperm
fuses with the egg. In this process, the sperm carries
genetic material from the male part of the flower. The
egg contains genetic material from the female part of
the flower. The resulting zygote contains genetic
material from both the male and female parts of a
flower. Through cellular division the zygote becomes
the embryo or immature plant.
2. In the second fertilization, the other sperm nucleus
fuses with two polar nuclei in the ovule. The
combination of the three nuclei makes a triploid cell
that multiplies by mitosis. It eventually develops into
the endosperm, which is the food storage tissue in the
seed.
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B. Following fertilization, the ovule develops into a seed,
which consists of a seed coat, an embryo plant, and stored
food.
1. The seed coat is a protective shell surrounding the
embryo and endosperm. It protects the seed from drying
and from physical injury. The seed coat helps in
determining when conditions for germination, or the
beginning of growth, are right.
2. The embryo plant is a little plant that eventually grows
and develops into the mature plant. It remains dormant
within the seed. It has a stem, root, and one or two seed
leaves called cotyledons. Monocot embryos have one seed
leaf and dicot embryos have two seed leaves.
3. The endosperm is the food storage tissue in the seed,
particularly in monocots. Dicots store the bulk of their food
in the two cotyledons. The food storage is necessary for the
young seedling until it is able to manufacture its own food.
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C. The ovary ripens and becomes the fruit.
Fruit serves the purpose of protecting the seeds
from drying and also aids in seed dispersal.
D. Both the male sperm and female egg
contribute genetic information to the new
embryo plant. The union of sperm and egg
results in new combinations of genetic
information. These combinations produce new
traits that add to the vigor of the offspring. The
offspring that results from the new
combination of genes are known as a hybrid.
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V. Germination is the end of the period of
dormancy for the embryo plant that occurs
when favorable conditions are present for
growth and development of the seedling plant.
The seedling plant has a root system, stem, and
leaves to produce the food necessary for the
young actively growing plant.
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A. This process by which an embryo plant inside
the seed changes into a developing seedling is
known as germination. Many important crops are
grown from seed. For example, corn, soybeans,
cotton, and vegetables are all started from seeds.
Some plants produce seeds that germinate
immediately, others produce seeds that remain
dormant, perhaps for years, until conditions are
optimal for growth and survival. Optimal
conditions for germination include favorable
moisture, temperature, oxygen, and sometimes
light.
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1. The first important step in germination is
usually the absorption of water. Water enters
the seed by diffusion. The moisture triggers
several important processes. Water also causes
the seed to expand and creates pressure within
the seed. This pressure, known as turgor,
causes the rupturing of the tough seed coat.
The primary root, or radicle, emerges through
the ruptured seed coat.
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2. Water stimulates the production of plant
hormones that signal the seed to convert
stored food to energy for the embryo plant. In
addition, growth hormones are behind the
growth of the radicle that begins absorbing
water and nutrients for the embryo once
outside the seed.
3. Seeds germinate at a wide range of
temperatures. Some plants require very
warm germination temperatures, and other
plants must have cool temperatures.
However, the optimum temperature range
for most seeds is between 65°F and 80°F.
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4. All seeds need oxygen to germinate.
Oxygen is necessary for cellular respiration.
It is important that the soil not be so wet
that it restricts the availability of oxygen.
Lack of oxygen may result in the death of
the embryo plant within the seed.
5. Seeds of some plants need exposure to
light before they will germinate. Seeds of
other plants do not need light to germinate
or germinate poorly when exposed to light.
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B. Two additional mechanisms affect germination
in certain plant seeds.
1. The first is stratification, which is a required
period of cold temperature. This is especially
important for plants that produce fruit in the fall. If
the seeds of these plants germinated immediately,
they would die from the cold winter temperatures.
These plants survive by producing seeds that
remain dormant and will not germinate until the
warmer spring temperatures.
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2. The second mechanism is scarification, which
causes the breakdown of the seed coat. The tough
protective seed coat prevents diffusion of both
water and oxygen into the seed. The seed coat
must be broken before germination can begin.
Plants have evolved many interesting methods to
accomplish this task. The seed coat can be
damaged or broken by the acid produced in an
animal’s stomach; by soil microorganisms; by
repeated freezing and thawing; by mechanical
stress from the grinding in the gizzard of birds;
and by being stepped upon, chewed etc.
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1. What is sexual reproduction of plants and
why is it important to plant survival?
2. What is the role of meiosis in the
development of gametes?
3. What is pollination, and what are the
different methods of pollination?
4. What is the fertilization process in flowering
plants, and how are seeds formed?
5. What is the process of seed germination, and
what conditions are necessary for germination
to occur in a seed?