NCERT Textbook - Sexual Reproduction in Flowering Plants NEET Notes | EduRev

Biology Class 12

Created by: Sushil Kumar

NEET : NCERT Textbook - Sexual Reproduction in Flowering Plants NEET Notes | EduRev

 Page 1


Are we not lucky that plants reproduce sexually? The
myriads of flowers that we enjoy gazing at, the scents and
the perfumes that we swoon over, the rich colours that
attract us, are all there as an aid to sexual reproduction.
Flowers do not exist only for us to be used for our own
selfishness. All flowering plants show sexual reproduction.
A look at the diversity of structures of the inflorescences,
flowers and floral parts, shows an amazing range of
adaptations to ensure formation of the end products of
sexual reproduction, the fruits and seeds.  In this chapter,
let us understand the morphology, structure and the
processes of sexual reproduction in flowering plants
(angiosperms).
2.1 FLOWER – A FASCINATING ORGAN OF
ANGIOSPERMS
Human beings have had an intimate relationship with
flowers since time immemorial.  Flowers are objects of
aesthetic, ornamental, social, religious and cultural value
– they have always been used as symbols for conveying
important human feelings such as love, affection,
happiness, grief, mourning, etc.  List at least five flowers
of ornamental value that are commonly cultivated at
2.1 Flower – A Fascinating
Organ of Angiosperms
2.2 Pre-fertilisation : Structures
and Events
2.3 Double Fertilisation
2.4 Post-fertilisation: Structures
and Events
2.5 Apomixis and
Polyembryony
CHAPTER 2
SEXUAL REPRODUCTION IN
FLOWERING PLANTS
Page 2


Are we not lucky that plants reproduce sexually? The
myriads of flowers that we enjoy gazing at, the scents and
the perfumes that we swoon over, the rich colours that
attract us, are all there as an aid to sexual reproduction.
Flowers do not exist only for us to be used for our own
selfishness. All flowering plants show sexual reproduction.
A look at the diversity of structures of the inflorescences,
flowers and floral parts, shows an amazing range of
adaptations to ensure formation of the end products of
sexual reproduction, the fruits and seeds.  In this chapter,
let us understand the morphology, structure and the
processes of sexual reproduction in flowering plants
(angiosperms).
2.1 FLOWER – A FASCINATING ORGAN OF
ANGIOSPERMS
Human beings have had an intimate relationship with
flowers since time immemorial.  Flowers are objects of
aesthetic, ornamental, social, religious and cultural value
– they have always been used as symbols for conveying
important human feelings such as love, affection,
happiness, grief, mourning, etc.  List at least five flowers
of ornamental value that are commonly cultivated at
2.1 Flower – A Fascinating
Organ of Angiosperms
2.2 Pre-fertilisation : Structures
and Events
2.3 Double Fertilisation
2.4 Post-fertilisation: Structures
and Events
2.5 Apomixis and
Polyembryony
CHAPTER 2
SEXUAL REPRODUCTION IN
FLOWERING PLANTS
20
BIOLOGY
homes and in gardens. Find out the names of five more flowers that are
used in social and cultural celebrations in your family.  Have you heard
of floriculture – what does it refer to?
To a biologist, flowers are morphological and embryological marvels
and the sites of sexual reproduction.  In class XI, you have read the various
parts of a flower.  Figure 2.1 will help you recall the parts of a typical
flower. Can you name the two parts in a flower in which the two most
important units of sexual reproduction develop?
2.2 PRE-FERTILISATION: STRUCTURES AND EVENTS
Much before the actual flower is seen on a plant, the decision that the plant
is going to flower has taken place. Several hormonal and structural changes
are initiated which lead to the differentiation and further development of
the floral primordium. Inflorescences are formed which bear the floral buds
and then the flowers. In the flower the male and female reproductive
structures, the androecium and the gynoecium differentiate and develop.
You would recollect that the androecium consists of a whorl of stamens
representing the male reproductive organ and the gynoecium represents
the female reproductive organ.
Figure 2.1 A diagrammatic representation of L.S. of a flower
Page 3


Are we not lucky that plants reproduce sexually? The
myriads of flowers that we enjoy gazing at, the scents and
the perfumes that we swoon over, the rich colours that
attract us, are all there as an aid to sexual reproduction.
Flowers do not exist only for us to be used for our own
selfishness. All flowering plants show sexual reproduction.
A look at the diversity of structures of the inflorescences,
flowers and floral parts, shows an amazing range of
adaptations to ensure formation of the end products of
sexual reproduction, the fruits and seeds.  In this chapter,
let us understand the morphology, structure and the
processes of sexual reproduction in flowering plants
(angiosperms).
2.1 FLOWER – A FASCINATING ORGAN OF
ANGIOSPERMS
Human beings have had an intimate relationship with
flowers since time immemorial.  Flowers are objects of
aesthetic, ornamental, social, religious and cultural value
– they have always been used as symbols for conveying
important human feelings such as love, affection,
happiness, grief, mourning, etc.  List at least five flowers
of ornamental value that are commonly cultivated at
2.1 Flower – A Fascinating
Organ of Angiosperms
2.2 Pre-fertilisation : Structures
and Events
2.3 Double Fertilisation
2.4 Post-fertilisation: Structures
and Events
2.5 Apomixis and
Polyembryony
CHAPTER 2
SEXUAL REPRODUCTION IN
FLOWERING PLANTS
20
BIOLOGY
homes and in gardens. Find out the names of five more flowers that are
used in social and cultural celebrations in your family.  Have you heard
of floriculture – what does it refer to?
To a biologist, flowers are morphological and embryological marvels
and the sites of sexual reproduction.  In class XI, you have read the various
parts of a flower.  Figure 2.1 will help you recall the parts of a typical
flower. Can you name the two parts in a flower in which the two most
important units of sexual reproduction develop?
2.2 PRE-FERTILISATION: STRUCTURES AND EVENTS
Much before the actual flower is seen on a plant, the decision that the plant
is going to flower has taken place. Several hormonal and structural changes
are initiated which lead to the differentiation and further development of
the floral primordium. Inflorescences are formed which bear the floral buds
and then the flowers. In the flower the male and female reproductive
structures, the androecium and the gynoecium differentiate and develop.
You would recollect that the androecium consists of a whorl of stamens
representing the male reproductive organ and the gynoecium represents
the female reproductive organ.
Figure 2.1 A diagrammatic representation of L.S. of a flower
21
HUMAN REPRODUCTION
SEXUAL REPRODUCTION IN FLOWERING PLANTS
2.2.1 Stamen, Microsporangium and Pollen Grain
Figure 2.2a shows the two parts of a typical stamen – the long and slender
stalk called the filament, and the terminal generally bilobed structure
called the anther. The proximal end of the filament
is attached to the thalamus or the petal of the
flower. The number and length of stamens are
variable in flowers of different species.  If you were
to collect a stamen each from ten flowers (each from
different species) and arrange them on a slide, you
would be able to appreciate the large variation in
size seen in nature.  Careful observation of each
stamen under a dissecting microscope and making
neat diagrams would elucidate the range in shape
and attachment of anthers in different flowers.
A typical angiosperm anther is bilobed with
each lobe having two theca, i.e., they are dithecous
(Figure 2.2). Often a longitudinal groove runs
lengthwise separating the theca. Let us understand
the various types of tissues and their organisation
in the transverse section of an anther (Figure 2.3 a).
The bilobed nature of an anther is very distinct in
the transverse section of the anther. The anther is
a four-sided (tetragonal) structure consisting of
four microsporangia located at the corners, two
in each lobe.
The microsporangia develop further and
become pollen sacs.  They extend longitudinally
all through the length of an anther and are packed
with pollen grains.
Structure of microsporangium: In a transverse
section, a typical microsporangium appears near
circular in outline.  It is generally surrounded by four wall layers
(Figure 2.3 b)– the epidermis, endothecium, middle layers and the tapetum.
The outer three wall layers perform the function of protection and help in
dehiscence of anther to release the pollen.  The innermost wall layer is the
tapetum.  It nourishes the developing pollen grains. Cells of the tapetum
possess dense cytoplasm and generally have more than one nucleus.  Can
you think of how tapetal cells could become bi-nucleate?
When the anther is young, a group of compactly arranged homogenous
cells called the sporogenous tissue occupies the centre of each
microsporangium.
Microsporogenesis : As the anther develops, the cells of the sporogenous
tissue undergo meiotic divisions to form microspore tetrads.  What would
be the ploidy of the cells of the tetrad?
Figure 2.2 (a)A typical stamen;
(b)three–dimensional cut section
 of an anther
(b)
(a)
Page 4


Are we not lucky that plants reproduce sexually? The
myriads of flowers that we enjoy gazing at, the scents and
the perfumes that we swoon over, the rich colours that
attract us, are all there as an aid to sexual reproduction.
Flowers do not exist only for us to be used for our own
selfishness. All flowering plants show sexual reproduction.
A look at the diversity of structures of the inflorescences,
flowers and floral parts, shows an amazing range of
adaptations to ensure formation of the end products of
sexual reproduction, the fruits and seeds.  In this chapter,
let us understand the morphology, structure and the
processes of sexual reproduction in flowering plants
(angiosperms).
2.1 FLOWER – A FASCINATING ORGAN OF
ANGIOSPERMS
Human beings have had an intimate relationship with
flowers since time immemorial.  Flowers are objects of
aesthetic, ornamental, social, religious and cultural value
– they have always been used as symbols for conveying
important human feelings such as love, affection,
happiness, grief, mourning, etc.  List at least five flowers
of ornamental value that are commonly cultivated at
2.1 Flower – A Fascinating
Organ of Angiosperms
2.2 Pre-fertilisation : Structures
and Events
2.3 Double Fertilisation
2.4 Post-fertilisation: Structures
and Events
2.5 Apomixis and
Polyembryony
CHAPTER 2
SEXUAL REPRODUCTION IN
FLOWERING PLANTS
20
BIOLOGY
homes and in gardens. Find out the names of five more flowers that are
used in social and cultural celebrations in your family.  Have you heard
of floriculture – what does it refer to?
To a biologist, flowers are morphological and embryological marvels
and the sites of sexual reproduction.  In class XI, you have read the various
parts of a flower.  Figure 2.1 will help you recall the parts of a typical
flower. Can you name the two parts in a flower in which the two most
important units of sexual reproduction develop?
2.2 PRE-FERTILISATION: STRUCTURES AND EVENTS
Much before the actual flower is seen on a plant, the decision that the plant
is going to flower has taken place. Several hormonal and structural changes
are initiated which lead to the differentiation and further development of
the floral primordium. Inflorescences are formed which bear the floral buds
and then the flowers. In the flower the male and female reproductive
structures, the androecium and the gynoecium differentiate and develop.
You would recollect that the androecium consists of a whorl of stamens
representing the male reproductive organ and the gynoecium represents
the female reproductive organ.
Figure 2.1 A diagrammatic representation of L.S. of a flower
21
HUMAN REPRODUCTION
SEXUAL REPRODUCTION IN FLOWERING PLANTS
2.2.1 Stamen, Microsporangium and Pollen Grain
Figure 2.2a shows the two parts of a typical stamen – the long and slender
stalk called the filament, and the terminal generally bilobed structure
called the anther. The proximal end of the filament
is attached to the thalamus or the petal of the
flower. The number and length of stamens are
variable in flowers of different species.  If you were
to collect a stamen each from ten flowers (each from
different species) and arrange them on a slide, you
would be able to appreciate the large variation in
size seen in nature.  Careful observation of each
stamen under a dissecting microscope and making
neat diagrams would elucidate the range in shape
and attachment of anthers in different flowers.
A typical angiosperm anther is bilobed with
each lobe having two theca, i.e., they are dithecous
(Figure 2.2). Often a longitudinal groove runs
lengthwise separating the theca. Let us understand
the various types of tissues and their organisation
in the transverse section of an anther (Figure 2.3 a).
The bilobed nature of an anther is very distinct in
the transverse section of the anther. The anther is
a four-sided (tetragonal) structure consisting of
four microsporangia located at the corners, two
in each lobe.
The microsporangia develop further and
become pollen sacs.  They extend longitudinally
all through the length of an anther and are packed
with pollen grains.
Structure of microsporangium: In a transverse
section, a typical microsporangium appears near
circular in outline.  It is generally surrounded by four wall layers
(Figure 2.3 b)– the epidermis, endothecium, middle layers and the tapetum.
The outer three wall layers perform the function of protection and help in
dehiscence of anther to release the pollen.  The innermost wall layer is the
tapetum.  It nourishes the developing pollen grains. Cells of the tapetum
possess dense cytoplasm and generally have more than one nucleus.  Can
you think of how tapetal cells could become bi-nucleate?
When the anther is young, a group of compactly arranged homogenous
cells called the sporogenous tissue occupies the centre of each
microsporangium.
Microsporogenesis : As the anther develops, the cells of the sporogenous
tissue undergo meiotic divisions to form microspore tetrads.  What would
be the ploidy of the cells of the tetrad?
Figure 2.2 (a)A typical stamen;
(b)three–dimensional cut section
 of an anther
(b)
(a)
22
BIOLOGY
As each cell of the sporogenous tissue is capable of giving rise to a
microspore tetrad. Each one is a potential pollen or microspore mother
cell (PMC). The process of formation of microspores from a pollen mother
cell through meiosis is called microsporogenesis. The microspores, as
they are formed, are arranged in a cluster of four cells–the microspore
tetrad (Figure 2.3 a).  As the anthers mature and dehydrate, the microspores
dissociate from each other and develop into pollen grains (Figure 2.3 b).
Inside each microsporangium several thousands of microspores or pollen
grains are formed that are released with the dehiscence of anther
(Figure 2.3 c).
Pollen grain: The pollen grains represent the male gametophytes.  If you
touch the opened anthers of Hibiscus or any other flower you would find
deposition of yellowish powdery pollen grains on your fingers. Sprinkle
these grains on a drop of water taken on a glass slide and observe under
(c)
(a) (b)
Figure 2.3 (a) Transverse section of a mature anther; (b) Enlarged view of one microsporangium
showing wall layers; (c) A dehisced anther
Page 5


Are we not lucky that plants reproduce sexually? The
myriads of flowers that we enjoy gazing at, the scents and
the perfumes that we swoon over, the rich colours that
attract us, are all there as an aid to sexual reproduction.
Flowers do not exist only for us to be used for our own
selfishness. All flowering plants show sexual reproduction.
A look at the diversity of structures of the inflorescences,
flowers and floral parts, shows an amazing range of
adaptations to ensure formation of the end products of
sexual reproduction, the fruits and seeds.  In this chapter,
let us understand the morphology, structure and the
processes of sexual reproduction in flowering plants
(angiosperms).
2.1 FLOWER – A FASCINATING ORGAN OF
ANGIOSPERMS
Human beings have had an intimate relationship with
flowers since time immemorial.  Flowers are objects of
aesthetic, ornamental, social, religious and cultural value
– they have always been used as symbols for conveying
important human feelings such as love, affection,
happiness, grief, mourning, etc.  List at least five flowers
of ornamental value that are commonly cultivated at
2.1 Flower – A Fascinating
Organ of Angiosperms
2.2 Pre-fertilisation : Structures
and Events
2.3 Double Fertilisation
2.4 Post-fertilisation: Structures
and Events
2.5 Apomixis and
Polyembryony
CHAPTER 2
SEXUAL REPRODUCTION IN
FLOWERING PLANTS
20
BIOLOGY
homes and in gardens. Find out the names of five more flowers that are
used in social and cultural celebrations in your family.  Have you heard
of floriculture – what does it refer to?
To a biologist, flowers are morphological and embryological marvels
and the sites of sexual reproduction.  In class XI, you have read the various
parts of a flower.  Figure 2.1 will help you recall the parts of a typical
flower. Can you name the two parts in a flower in which the two most
important units of sexual reproduction develop?
2.2 PRE-FERTILISATION: STRUCTURES AND EVENTS
Much before the actual flower is seen on a plant, the decision that the plant
is going to flower has taken place. Several hormonal and structural changes
are initiated which lead to the differentiation and further development of
the floral primordium. Inflorescences are formed which bear the floral buds
and then the flowers. In the flower the male and female reproductive
structures, the androecium and the gynoecium differentiate and develop.
You would recollect that the androecium consists of a whorl of stamens
representing the male reproductive organ and the gynoecium represents
the female reproductive organ.
Figure 2.1 A diagrammatic representation of L.S. of a flower
21
HUMAN REPRODUCTION
SEXUAL REPRODUCTION IN FLOWERING PLANTS
2.2.1 Stamen, Microsporangium and Pollen Grain
Figure 2.2a shows the two parts of a typical stamen – the long and slender
stalk called the filament, and the terminal generally bilobed structure
called the anther. The proximal end of the filament
is attached to the thalamus or the petal of the
flower. The number and length of stamens are
variable in flowers of different species.  If you were
to collect a stamen each from ten flowers (each from
different species) and arrange them on a slide, you
would be able to appreciate the large variation in
size seen in nature.  Careful observation of each
stamen under a dissecting microscope and making
neat diagrams would elucidate the range in shape
and attachment of anthers in different flowers.
A typical angiosperm anther is bilobed with
each lobe having two theca, i.e., they are dithecous
(Figure 2.2). Often a longitudinal groove runs
lengthwise separating the theca. Let us understand
the various types of tissues and their organisation
in the transverse section of an anther (Figure 2.3 a).
The bilobed nature of an anther is very distinct in
the transverse section of the anther. The anther is
a four-sided (tetragonal) structure consisting of
four microsporangia located at the corners, two
in each lobe.
The microsporangia develop further and
become pollen sacs.  They extend longitudinally
all through the length of an anther and are packed
with pollen grains.
Structure of microsporangium: In a transverse
section, a typical microsporangium appears near
circular in outline.  It is generally surrounded by four wall layers
(Figure 2.3 b)– the epidermis, endothecium, middle layers and the tapetum.
The outer three wall layers perform the function of protection and help in
dehiscence of anther to release the pollen.  The innermost wall layer is the
tapetum.  It nourishes the developing pollen grains. Cells of the tapetum
possess dense cytoplasm and generally have more than one nucleus.  Can
you think of how tapetal cells could become bi-nucleate?
When the anther is young, a group of compactly arranged homogenous
cells called the sporogenous tissue occupies the centre of each
microsporangium.
Microsporogenesis : As the anther develops, the cells of the sporogenous
tissue undergo meiotic divisions to form microspore tetrads.  What would
be the ploidy of the cells of the tetrad?
Figure 2.2 (a)A typical stamen;
(b)three–dimensional cut section
 of an anther
(b)
(a)
22
BIOLOGY
As each cell of the sporogenous tissue is capable of giving rise to a
microspore tetrad. Each one is a potential pollen or microspore mother
cell (PMC). The process of formation of microspores from a pollen mother
cell through meiosis is called microsporogenesis. The microspores, as
they are formed, are arranged in a cluster of four cells–the microspore
tetrad (Figure 2.3 a).  As the anthers mature and dehydrate, the microspores
dissociate from each other and develop into pollen grains (Figure 2.3 b).
Inside each microsporangium several thousands of microspores or pollen
grains are formed that are released with the dehiscence of anther
(Figure 2.3 c).
Pollen grain: The pollen grains represent the male gametophytes.  If you
touch the opened anthers of Hibiscus or any other flower you would find
deposition of yellowish powdery pollen grains on your fingers. Sprinkle
these grains on a drop of water taken on a glass slide and observe under
(c)
(a) (b)
Figure 2.3 (a) Transverse section of a mature anther; (b) Enlarged view of one microsporangium
showing wall layers; (c) A dehisced anther
23
HUMAN REPRODUCTION
SEXUAL REPRODUCTION IN FLOWERING PLANTS
a microscope.  You will really be amazed at the variety of architecture –
sizes, shapes, colours, designs – seen on the pollen grains
from different species (Figure 2.4).
Pollen grains are generally spherical measuring about
25-50 micrometers in diameter. It has a prominent two-layered
wall.  The hard outer layer called the exine is made up of
sporopollenin which is one of the most resistant organic material
known. It can withstand high temperatures and strong acids
and alkali. No enzyme that degrades sporopollenin is so far
known. Pollen grain exine has prominent apertures called germ
pores where sporopollenin is absent.  Pollen grains are well-
preserved as fossils because of the presence of sporopollenin.
The exine exhibits a fascinating array of patterns and designs.
Why do you think the exine should be hard?  What is the
function of germ pore?  The inner wall of the pollen grain is
called the intine.  It is a thin and continuous layer made up of
cellulose and pectin.  The cytoplasm of pollen grain is
surrounded by a plasma membrane.  When the pollen grain is
mature it contains two cells, the vegetative cell and generative
cell (Figure 2.5b).  The vegetative cell is bigger, has abundant
food reserve and a large irregularly shaped nucleus. The
generative cell is small and floats in the cytoplasm of the
vegetative cell.  It is spindle shaped with dense cytoplasm and
a nucleus.  In over 60 per cent of angiosperms, pollen grains
are shed at this 2-celled stage. In the remaining species, the
generative cell divides mitotically to give rise to the two male
gametes before pollen grains are shed (3-celled stage).
Pollen grains of many species cause severe allergies and bronchial
afflictions in some people often leading to chronic respiratory
disorders – asthma, bronchitis, etc. It may be mentioned that Parthenium
or carrot grass that came into India as a contaminant with imported wheat,
has become ubiquitous in occurrence and causes pollen allergy.
Figure 2.5 (a) Enlarged view of
a pollen grain tetrad; (b) stages
of a microspore maturing into a
pollen grain
Figure 2.4 Scanning electron micrographs of a few pollen grains
(a)
(b)
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