Transcript Chapter 20

Chapter 16
Reproduction in Plants
Objectives
Students should be able to:
• Define asexual reproduction as the
process resulting in the production of
genetically identical offspring from one
parent
• Define sexual reproduction as the
process involving the fusion of nuclei to
form a zygote and the production of
genetically dissimilar offspring
• Identify and draw, using a hand
lens if necessary, the sepals,
petals, stamens and carpels of
one, locally available, named,
insect-pollinated, dicotyledonous
flower, and examine the pollen
grains under a microscope
• State the functions of the sepals,
petals, anthers and carpels
• Use a hand lens to identify and describe
the anthers and stigmas of one, locally
available, named, wind-pollinated
flower, and examine the pollen grains
under a microscope
• Outline the process of pollination and
distinguish between self-pollination and
cross-pollination
• Compare, using fresh specimens, an
insect pollinated and a wind-pollinated
flower
• Describe the growth of the pollen
tube and its entry into the ovule
followed by fertilisation
(production of endosperm and
details of development are NOT
required)
16.1 What is Reproduction?
• No organism lives forever
• To ensure continuity of species,
organisms must produce new
organisms like themselves
 Production of new organisms is
called reproduction
Two types of reproduction
1. Asexual reproduction
2. Sexual reproduction
Asexual Reproduction
• Asexual Reproduction is process
resulting in the production of
genetically identical offspring from
one parent, without the fusion of
gametes
• Gametes are reproductive cells
containing half the chromosomes
as the normal body cell
Asexual Reproduction
Identical offspring produced
– Mitosis occurs.
– A cell divides to give 2 identical
daughter cells
– Each daughter cells has the same no.
of chromosomes as parent cell.
– These daughter cells have identical
genes as the parent cell. They are
also called clones.
Parent with 2n
chromosomes
2n
Cell divides
2n
Two daughter
cells formed,
each with 2n
chromosomes
(mitosis)
2n
• No. of chromosomes in cells varies with
the species
Sexual Reproduction
• Is the process involving the fusion
of two gametes to form a zygote,
resulting in the production of
genetically dissimilar offspring.
• Gametes are produced in a process
known as meiosis
• In meiosis,
– A cell divides to form 4 daughter cells
called gametes
– Each gamete has half the number of
chromosomes as the paent cell.
– When gametes fuse during
fertilisation, the normal number of
chromosomes is restored.
Parent cell
with 2n
chromosomes
2n
nucleus
Cell divides
(meiosis)
n
n
n
n
Four gametes formed, each with
n chromosomes
How are gametes involved in sexual
reproduction?
• The process of fusion of gametes is
called fertilisation
• The nuclei of the two gametes (male
and female) fuse to form a zygote.
• Zygote contains hereditary information
from two different gametes, hence
offspring produced is genetically
different from both parents
16.2 Sexual reproduction in
Flowering Plants
• New plants produced from seeds
• Seeds are found in fruits
• Each fruit is produced by a flower
• Hence, flowers bear the
reproductive organs of flowering
plants
General Parts of a Flower
• Pedicel
– Is a flower stalk
• Receptacle
– Enlarged end of the flower stalk on
which the other parts of the flower
are borne
• Sepals**
– Modified leaves
– Function : enclose and protect the other
parts of the flower in the bud stage
– Sepals together forms calyx
• Petals**
– Modified leaves and forms the most
obvious (conspicuous) part of the flower
– insect-pollinated flowers, petals are
• Brightly coloured to attract insects for
pollination
• Provide Landing platform for insects
– Petals together form the corolla
carpel
Parts of a Flower
stigma
carpel
style
anther
ovary
ovule
stamen
filament
Sepal
• Sepals are modified leaves which enclose and protect the other parts of
the flower in the bud stage.
• All the sepals together make up the calyx.
• The sepals usually form the outermost layer of floral leaves.
• However, some flowers, for example, Hibiscus, have another layer of
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floral
leaves
outside
the
sepals,
whichPte.make
up the epicalyx of18 the
flower.19
Parts of a Flower
stigma
carpel
style
anther
ovary
ovule
stamen
filament
Sepal
Receptacle
The receptacle is the
enlarged end of the
flower stalk which bears
the other parts of the
flower.
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Parts of a Flower
stigma
carpel
style
anther
stamen
ovary
ovule
filament
Sepal
Receptacle
Pedicel
The pedicel is the flower stalk. Some
flowers have no pedicels and are
attached to the plant directly at the
base. They are called sessile flowers.
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Parts of a Flower
stigma
carpel
style
anther
stamen
ovary
ovule
Sepal
Receptacle
filament
Petal
Petals are modified leaves forming the most
obvious (conspicuous) part of the flower. All the
petals together make up the corolla. In insectpollinated
Pedicelflowers, petals:
• are brightly coloured to attract insects for
pollination; and
• provide a platform for insects to land.
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Parts of a Flower
stigma
Petal
carpel
style
ovary
anther
ovule
stamen
filament
Sepal
Receptacle
Pedicel
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• Stamens** (Androecium)
– Male part of the flower
– Consists of a filament and an anther
– All stamens together make up the
androecium
• Filament
– Stalk that holds the anther
– Anther contains a vascular bundle (xylem
and phloem)
• Anther**
– Consists of two lobes
– Each lobe contains two pollen sacs
– Pollen sacs contain pollen grains
Parts of a Stamen
Side view
of a stamen
Stamen
• The male part of the flower.
• It consists of an anther and a
filament.
• All the stamens together make
up the androecium (plural:
androecia).
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Parts of a Stamen
vascular bundle
Side view
of a stamen
Stamen with
upper half of
anther cut off
Stamen
Filament
The filament is the stalk that holds the anther in a suitable position to
disperse the pollen. The anther also contains a vascular bundle. The
vascular bundle contains:
• phloem which translocates sucrose and amino acids to the developing
anther. Sucrose provides the energy for cell division. Amino acids are
used for synthesis of proteins and new protoplasm.
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• xylem which transports water and mineral salts to the anther.
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Parts of a Stamen
pollen sacs
vascular bundle
pollen grains
anther lobes
Side view
of a stamen
Stamen
Filament
Stamen with
upper half of
anther cut off
Anther
The anther consists of two lobes. Each lobe contains
two pollen sacs. In the pollen sacs are the pollen
grains. The anther produces pollen grains. When the
anther matures, it splits open to release the pollen
grains.
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Parts of a Stamen
pollen sacs
vascular bundle
pollen tube
nucleus
pollen grains
anther lobes
Side view
of a stamen
Stamen
Filament
generative
nucleus
Stamen with
upper half of
anther cut off
Anther
Mature pollen grain
Pollen grains
• Pollen grains contain the male gametes,
which are produced by meiosis.
• The nuclei in pollen grains contain the
haploid number of chromosomes.
• Each pollen grain has two nuclei, the
generative nucleus and the pollen tube
nucleus or vegetative nucleus.
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• Carpel** (pistil or gynoecium)
– Female part of the flower
– Consists of:
• an ovary,
• a style above the ovary
• and one or more stigmas
• Stigma**
– Receives pollen grains
– Mature stigma secretes sugary fluid
that stimulate the pollengrains to
germinate
Parts of a Carpel
Carpel
stigma
• The female part of the
flower.
• It consists of:
style
- an ovary;
- a style above the ovary;
and
ovary
Side view
of a carpel
- one or more stigmas.
• A flower may have one or
more carpels.
• All the carpels together
make up the pistil or
gynoecium (plural:
gynoecia).
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Parts of a Carpel
Stigma
• The stigma is a swollen
structure at the end of the
style.
style
Carpel
ovary
• It receives the pollen
grains.
• The mature stigma
secretes a sugary fluid that
stimulates the pollen grains
to germinate.
Side view of
a carpel
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Parts of a Carpel
Style
Stigma
• The style is a stalk that
connects the stigma to the
ovary.
• It holds the stigma in a
suitable position to trap pollen
grains.
Carpel
ovary
Side view
of a carpel
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Parts of a Carpel
stigma
style
ovary wall
definitive
nucleus
ovule
ovum
funicle
L.S. of a
carpel
placenta
Ovary
• The ovary contains one or
more ovules.
• Within each mature ovule is
a female gamete or egg cell
called the ovum (plural: ova)
and the definitive nucleus.
The ovum is produced by
meiosis. It is therefore
haploid.
• The ovules are attached to a
region in the ovary called the
placenta. The ovule is
attached to the placenta by a
stalk called the funicle.
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Pollination
• Pollen grains must be transferred
from the anthers to the stigmas for
the fusion of gametes
• Can be done by insects or wind
• Flowers pollinated by insects
possess characteristics distinct
from wind-pollinated flowers
What is Pollination?
• The transfer of pollen grains from
the anther to the stigma
• If the pollen grains are transferred
to a flower in another plant of the
same kind, it is called crosspollination
Self-pollination
• Self-pollination is the transfer of
pollen grains from the anther to
the stigma of the same flower or of
a different flower of the same
plant
Advantages
1. Only one parent plant is required
2. Offspring inherits its genes from
the parent plant. Hence good
qualities passed down to the
offspring
3. High probability of pollination
4. Less pollen grain and energy
wasted in self-pollination
Disadvantages
1. Less varieties are produced.
Hence species is less adapted to
environmental changes
2. Continued self-pollination may
lead to offspring becoming
weaker, smaller and less
resistant to diseases
Cross-pollination
• Cross-pollination is the transfer of
pollen grains from one plant to the
stigma of a flower in another plant
of the same species
Advantages of crosspollination
1. Offspring may have inherited
good qualities from both parents
2. More varieties produced. This
increases the chance of species
surviving environmental changes
3. More viable seeds are produced
Disadvantages of crosspollination
1. Both parents required
2. Depends on external factors such
as insects or wind for pollination
3. Lower probability of crosspollination
4. More energy and pollen grains
wasted
Self Pollination vs Cross
Pollination
• Self Pollination results in the production
of
– offspring that are weaker and
– less adaptable to changes in he
environment
• Cross Pollination may produce
– Offspring with valuable qualities
– Abundant and more viable seeds
– More varieties of offspring
The Structure and Pollination of
an Insect-Pollinated Flower
Structure of Clitoria flower
• Belongs to the legume (pea) family
External features of Clitoria
The Clitoria flower is pollinated by insects. The flower is
complete, bisexual and bilaterally symmetrical.
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External features of Clitoria
pedicel
calyx
1
• The calyx consists of five green sepals.
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External features of Clitoria
pedicel
epicalyx
calyx
2
• The base of the calyx is enclosed by the epicalyx. The
epicalyx consists of two green leaves.
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External features of Clitoria
pedicel
epicalyx
calyx
corolla
• The corolla consists of five brightly-coloured petals
3
of different shapes and sizes.
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External features of Clitoria
pedicel
epicalyx
calyx
corolla
standard
petal
4
standard petal
• The large, deep blue standard petal has lines on it
that guide insects to where the nectar is located.
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External features of Clitoria
pedicel
epicalyx
calyx
corolla
standard petal
nectar
guide
standard petal
4
• The large, deep blue standard petal has lines on it that guide
insects to where the nectar is located.
• Thus, these lines are called nectar guides.
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External features of Clitoria
pedicel
epicalyx
calyx
wing
petal
wing petals
corolla
nectar guide
standard petal
standard petal
5
• Two deep blue wing petals are enclosed by the
standard petal.
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External features of Clitoria
pedicel
keel
petals
epicalyx
wing petal
calyx
wing petals
corolla
nectar guide
standard petal
standard petal
6
• Two small, yellowish-green keel petals are enclosed
by the wing petals. The keel petals enclose the reproductive
organs.
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Stamens and Pistil of Clitoria
Androecium
• The androecium consists of ten stamens with long filaments.
• The filaments of nine of the stamens are fused along most of their
length to form a hollow tube or ‘trough’. This trough encloses the
pistil.
• Nectar, secreted by the flower, collects at the bottom of the stamen
trough. Thus, the nectar can be reached only by an insect with a long
tubular mouth or proboscis, for example, a bee or a butterfly.
stamen trough
Stigma and stamens
of Clitoria
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stigma
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Stamens and Pistil of Clitoria
Androecium
• The androecium consists of ten stamens with long filaments.
• The filaments of nine of the stamens are fused along most of their
length to form a hollow tube or ‘trough’. This trough encloses the
pistil.
• Nectar, secreted by the flower, collects at the bottom of the stamen
trough. Thus, the nectar can be reached only by an insect with a long
tubular mouth or proboscis, for example, a bee or a butterfly.
stamen trough
Stigma and stamens
of Clitoria
The tenth stamen is free.
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stigma
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Stamens and Pistil of Clitoria
Pistil
The pistil consists of a single carpel.
Section of the
carpel of Clitoria
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