CHAPTER 2
SEXUAL REPRODUCTION IN
FLOWERING PLANTS
2.1 Flower – A Fascinating
Organ of Angiosperms
2.2 Pre-fertilisation : Structures Are we not lucky that plants reproduce sexually? The
and Events myriads of flowers that we enjoy gazing at, the scents and
the perfumes that we swoon over, the rich colours that
2.3 Double Fertilisation attract us, are all there as an aid to sexual reproduction.
2.4 Post-fertilisation: Structures Flowers do not exist only for us to be used for our own
and Events selfishness. All flowering plants show sexual reproduction.
A look at the diversity of structures of the inflorescences,
2.5 Apomixis and
flowers and floral parts, shows an amazing range of
Polyembryony
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
2019-2020
, BIOLOGY
Figure 2.1 A diagrammatic representation of L.S. of a flower
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
20 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.
2019-2020
, SEXUAL
HUMANREPRODUCTION IN FLOWERING PLANTS
REPRODUCTION
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 b). 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 (b)
corners, two in each lobe.
The microsporangia develop further and (a)
become pollen sacs. They extend longitudinally
all through the length of an anther and are packed Figure 2.2 (a) A typical stamen;
with pollen grains. (b) three–dimensional cut section
Structure of microsporangium: In a transverse of an anther
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 21
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?
2019-2020
SEXUAL REPRODUCTION IN
FLOWERING PLANTS
2.1 Flower – A Fascinating
Organ of Angiosperms
2.2 Pre-fertilisation : Structures Are we not lucky that plants reproduce sexually? The
and Events myriads of flowers that we enjoy gazing at, the scents and
the perfumes that we swoon over, the rich colours that
2.3 Double Fertilisation attract us, are all there as an aid to sexual reproduction.
2.4 Post-fertilisation: Structures Flowers do not exist only for us to be used for our own
and Events selfishness. All flowering plants show sexual reproduction.
A look at the diversity of structures of the inflorescences,
2.5 Apomixis and
flowers and floral parts, shows an amazing range of
Polyembryony
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
2019-2020
, BIOLOGY
Figure 2.1 A diagrammatic representation of L.S. of a flower
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
20 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.
2019-2020
, SEXUAL
HUMANREPRODUCTION IN FLOWERING PLANTS
REPRODUCTION
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 b). 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 (b)
corners, two in each lobe.
The microsporangia develop further and (a)
become pollen sacs. They extend longitudinally
all through the length of an anther and are packed Figure 2.2 (a) A typical stamen;
with pollen grains. (b) three–dimensional cut section
Structure of microsporangium: In a transverse of an anther
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 21
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?
2019-2020
