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
– A F
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
Flower – A Fascinating
Organ of Angiosperms
Pre-fertilisation : Structures
SEXUAL REPRODUCTION IN
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?
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
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
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
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
(a) Transverse section of a mature anther; (b) Enlarged view of one microsporangium
showing wall layers; (c) A dehisced anther
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
Figure 2.4 Scanning electron micrographs of a few pollen grains
When once they are shed, pollen grains have to land on the stigma
before they lose viability if they have to bring about fertilisation. How long
do you think the pollen grains retain viability? The period for which pollen
grains remain viable is highly variable and to some extent depends on the
prevailing temperature and humidity. In some cereals such as rice and
wheat, pollen grains lose viability within 30 minutes of their release, and
in some members of Rosaceae, Leguminoseae and Solanaceae, they
maintain viability for months. You may have heard of storing semen/
sperms of many animals including humans for artificial insemination. It
is possible to store pollen grains of a large number of species for years in
liquid nitrogen (-196
C). Such stored pollen can be used as pollen banks,
similar to seed banks, in crop breeding programmes.
2.2.2 The Pistil, Megasporangium (ovule) and Embryo sac
The gynoecium represents the female reproductive part of the flower. The
gynoecium may consist of a single pistil (monocarpellary) or may have
more than one pistil (multicarpellary). When there are more than one,
the pistils may be fused together (syncarpous) (Figure 2.7b) or may be
free (apocarpous) (Figure 2.7c). Each pistil has three parts (Figure 2.7a),
the stigma, style and ovary. The stigma serves as a landing platform
for pollen grains. The style is the elongated slender part beneath the
stigma. The basal bulged part of the pistil is the ovary. Inside the ovary
is the ovarian cavity
(locule). The placenta is located inside the ovarian
cavity. Recall the definition and types of placentation that you studied in
Figure 2.6 Pollen products
Pollen grains are rich in nutrients. It has become a fashion in recent
years to use pollen tablets as food supplements. In western countries, a
large number of pollen products in the form of tablets and syrups are
available in the market. Pollen consumption has been claimed to increase
the performance of athletes and race horses (Figure 2.6).
SEXUAL REPRODUCTION IN FLOWERING PLANTS
Class XI. Arising from the placenta are the megasporangia, commonly
called ovules. The number of ovules in an ovary may be one (wheat,
paddy, mango) to many (papaya, water melon, orchids).
The Megasporangium (Ovule) : Let us familiarise ourselves with the
structure of a typical angiosperm ovule (Figure 2.7d). The ovule is a small
structure attached to the placenta by means of a stalk called funicle.
The body of the ovule fuses with funicle in the region called hilum. Thus,
hilum represents the junction between ovule and funicle. Each ovule has
one or two protective envelopes called integuments. Integuments encircle
the ovule except at the tip where a small opening called the micropyle is
organised. Opposite the micropylar end, is the chalaza, representing the
basal part of the ovule.
Enclosed within the integuments is a mass of cells called the nucellus.
Cells of the nucellus have abundant reserve food materials. Located in the
nucellus is the embryo sac or female gametophyte. An ovule generally has
a single embryo sac formed from a megaspore through reduction division.
Megasporogenesis : The process of formation of megaspores from the
megaspore mother cell is called megasporogenesis. Ovules generally
differentiate a single megaspore mother cell (MMC) in the micropylar region
Figure 2.7 (a) A dissected flower of Hibiscus showing pistil (other floral parts have been removed);
(b) Multicarpellary, syncarpous pistil of Papaver ; (c) A multicarpellary, apocarpous
gynoecium of Michelia; (d) A diagrammatic view of a typical anatropous ovule
(a) Parts of the ovule showing a large megaspore mother cell, a dyad and a tetrad of
megaspores; (b) 1,2, 4, and 8-nucleate stages of embryo sac and a mature embryo sac;
(c) A diagrammatic representation of the mature embryo sac.
of the nucellus. It is a large cell containing dense cytoplasm and a
prominent nucleus. The MMC undergoes meiotic division. What is the
importance of the MMC undergoing meiosis? Meiosis results in the
production of four megaspores (Figure 2.8a).
Female gametophyte : In a majority of flowering plants, one of the
megaspores is functional while the other three degenerate. Only the
functional megaspore develops into the female gametophyte (embryo
sac). This method of embryo sac formation from a single megaspore is termed
monosporic development. What will be the ploidy of the cells of the nucellus,
MMC, the functional megaspore and female gametophyte?
SEXUAL REPRODUCTION IN FLOWERING PLANTS
Let us study formation of the embryo sac in a little more detail.
(Figure 2.8b). The nucleus of the functional megaspore divides mitotically
to form two nuclei which move to the opposite poles, forming the
2-nucleate embryo sac. Two more sequential mitotic nuclear divisions
result in the formation of the 4-nucleate and later the 8-nucleate stages
of the embryo sac. It is of interest to note that these mitotic divisions are
strictly free nuclear, that is, nuclear divisions are not followed immediately
by cell wall formation. After the 8-nucleate stage, cell walls are laid down
leading to the organisation of the typical female gametophyte
or embryo sac. Observe the distribution of cells inside the embryo sac
(Figure 2.8b, c). Six of the eight nuclei are surrounded by cell walls and
organised into cells; the remaining two nuclei, called polar nuclei are
situated below the egg apparatus in the large central cell.
There is a characteristic distribution of the cells within the embryo
sac. Three cells are grouped together at the micropylar end and constitute
the egg apparatus. The egg apparatus, in turn, consists of two synergids
and one egg cell. The synergids have special cellular thickenings at the
micropylar tip called filiform apparatus, which play an important role in
guiding the pollen tubes into the synergid. Three cells are at the chalazal
end and are called the antipodals. The large central cell, as mentioned
earlier, has two polar nuclei. Thus, a typical angiosperm embryo sac, at
maturity, though 8-nucleate is 7-celled.
In the preceding sections you have learnt that the male and female gametes
in flowering plants are produced in the pollen grain and embryo sac,
respectively. As both types of gametes are non-motile, they have to be
brought together for fertilisation to occur. How is this achieved?
Pollination is the mechanism to achieve this objective. Transfer
of pollen grains (shed from the anther) to the stigma of a pistil is
termed pollination. Flowering plants have evolved an amazing array
of adaptations to achieve pollination. They make use of external
agents to achieve pollination. Can you list the possible external
Kinds of Pollination : Depending on the source of pollen, pollination
can be divided into three types.
Autogamy : In this type, pollination is achieved within the same
flower. Transfer of pollen grains from the anther to the stigma of the
same flower (Figure 2.9a). In a normal flower which opens and
exposes the anthers and the stigma, complete autogamy is rather
rare. Autogamy in such flowers requires synchrony in pollen release
and stigma receptivity and also, the anthers and the stigma should
lie close to each other so that self-pollination
can occur. Some plants such as Viola
(common pansy), Oxalis, and Commelina
produce two types of flowers –
chasmogamous flowers which are similar to
flowers of other species with exposed anthers
and stigma, and cleistogamous flowers which
do not open at all (Figure 2.9c). In such flowers,
the anthers and stigma lie close to each other.
When anthers dehisce in the flower buds,
pollen grains come in contact with the stigma
to effect pollination. Thus, cleistogamous
flowers are invariably autogamous as there is
no chance of cross-pollen landing on the
stigma. Cleistogamous flowers produce
assured seed-set even in the absence of
pollinators. Do you think that cleistogamy is
advantageous or disadvantageous to the
Geitonogamy – Transfer of pollen grains from
the anther to the stigma of another flower of
the same plant. Although geitonogamy is
functionally cross-pollination involving a
pollinating agent, genetically it is similar to
autogamy since the pollen grains come from
the same plant.
Xenogamy – Transfer of pollen grains from
anther to the stigma of a different plant (Figure
2.9b). This is the only type of pollination which
during pollination brings genetically different
types of pollen grains to the stigma.
Agents of Pollination : Plants use two abiotic (wind
and water) and one biotic (animals) agents to achieve
pollination. Majority of plants use biotic agents for
pollination. Only a small proportion of plants use
abiotic agents. Pollen grains coming in contact with
the stigma is a chance factor in both wind and water
pollination. To compensate for this uncertainties and
associated loss of pollen grains, the flowers produce
enormous amount of pollen when compared to the
number of ovules available for pollination.
Figure 2.9 (a) Self-pollinated flowers;
(b) Cross pollinated flowers;
(c) Cleistogamous flowers