1

CHAPTER

SEXUAL
REPRODUCTION
IN FLOWERING
PLANTS
Topics
1.1 Flower and Pre-Fertilisation
1.1.1 Flower
1.1.2 Pre-Fertilisation: Structures and Events
1.1.3 Pollination
1.1.4 Artificial Hybridisation
1.1.5 Double Fertilisation
1.2 Post-Fertilisation: Structures and
Events
1.2.1 Development of an Endosperm
1.2.2 Development of an Embryo
1.2.3 Development of a See
1.2.3 Formation of a Fruit
1.2.4 Some Special Mechanisms of Reproduc-
tion
4|Sexual Reproduction In Flowering Plants
All living organisms reproduce through asexual Gynoecium
and sexual reproduction. Sexual reproduction is The gynoecium, also known as the female reproductive
predominantly observed in higher organisms, such as organ, is composed of one or multiple carpels. Each
flowering plants. This process involves the fusion of carpel comprises three essential parts: the stigma,
male and female haploid (n) gametes, resulting in the which is the receptive part, the style, a long tube located
formation of a diploid (2n) zygote. beneath the stigma, and the ovary, the basal part of the
The zygote then develops into an embryo, eventually style responsible for bearing ovules. This structure
forming a diploid plant body. In flowering plants, plays a crucial role in the generation of megaspores, as
these steps occur within a specialized reproductive well as the development of fruits and seeds.
organ, the flower. In this chapter, we will explore all
1.1.2 Pre-Fertilisation: Structures and
aspects of sexual reproduction in flowering plants.
Events
1.1 Flower And Pre-Fertilisation Pre-fertilisation events refer to the series of
1.1.1 Flower occurrences that take place before fertilisation. These
events involve various hormonal and structural
The flower is the primary reproductive organ in transformations that contribute to the maturation and
angiosperms. It is a modified shoot where the specialization of floral organs or flowers.
internodes are highly condensed, and the leaves are
transformed into floral parts. Within the blossom, the androecium and gynoecium
undergo differentiation and growth to produce the
A typical flower consists of four distinct parts or whorls: male and female gametophytes, correspondingly. The
the calyx, corolla, androecium, and gynoecium. various components and processes associated with
These parts are attached to a base known as the pre-fertilisation are elaborated upon below.
receptacle or thalamus. The calyx and corolla are sterile Stamen, Microsporangium and Pollen
and considered non-essential or accessory whorls, Grain
while the androecium and gynoecium are fertile and
considered essential or reproductive whorls. The Here is a brief description of the rewrite process:
various parts of a flower are briefly discussed below. Stamen:
A stamen is the male reproductive organ of an
Calyx angiosperm, comprising two components:
The outermost layer of the flower is known as the
i. Filament: a lengthy, slender stalk originating from
calyx, consisting of individual units called sepals.
the thalamus or petal of the flower (epipetalous
Sepals are leaf-like structures that are typically green
condition) at its proximal end.
in colour and serve to safeguard the flower during its
early stages of development. ii. Anther: the end structure usually bilobed. The
quantity and size of stamens differ among various
Corolla
flower species.
The second whorl of the flower, located inside the
Structure of an Anther
calyx, consists of the petals. Often brightly coloured
In angiosperms, a typical anther is bilobed, with
and fragrant, these individual units of the corolla serve
two theca in each lobe (dithecous), separated by a
to attract pollinators and enhance the overall beauty of
longitudinal groove.
the flower, aiding in the pollination process.
Androecium
The male reproductive structure, known as the stamen,
is a vital component of the plant’s reproductive system.
Comprising of an anther, filament, and connective, the
stamen plays a crucial role in producing microspores,
specifically pollen grains, which contain male gametes
enclosed within the anther lobes.
Stigma

Anther Style

Filament Ovule
Petal Ovary

Sepal Receptacle

Stem (a) A typical stamen; (b) Three dimensional cut

Longitudinal Section (LS) of a flower section of an Anther
 Sexual Reproduction In Flowering Plants|5
When viewed in cross-section, the anther appears PMCs then undergo meiotic division to give rise to
as a tetragonal structure, with four microsporangia four haploid microspores, forming a microspore
positioned at the corners (tetrasporangiate), two in tetrad.
each lobe. These microsporangia eventually mature The anthers undergo maturation and dehydration,
into pollen sacs that are filled with pollen grains. causing the microspores to separate from each other
Structure of a Microsporangium and the microspore tetrad, eventually growing into
In a standard microsporangium, there are four layers of pollen grains. Thousands of microspores or pollen
walls enclosing it: the epidermis, endothecium, middle grains are produced within each microsporangium.
layers, and tapetum. The epidermis, endothecium, and Upon the rupturing of the anther wall during anther
middle layers serve a protective role and aid in anther dehiscence, these pollen grains are released into the
dehiscence for the pollen grain’s release. environment.
The tapetum, the innermost layer, supports the growth
of microspores or pollen grains. Tapetal cells are rich
in cytoplasm and typically contain multiple nuclei.

Pollen grains

A matured dehisced Anther

Structure of Male Gametophyte (Pollen Grain)
It is a single-celled, haploid spore, containing a
single nucleus and produced abundantly through
meiosis within the microspore mother cell. Their
characteristics, including size, shape, colour, and
pattern, differ across different species.
Pollen grains are typically spherical in shape, with a
diameter ranging from 25 to 50 micrometers. Encased
within a two-layered wall known as the sporoderm,
they possess:
i. An outer hard layer called the exine, crafted
from sporopollenin - an exceptionally sturdy
organic material capable of withstanding extreme
conditions like high temperatures and strong acids
or alkalis. It resists degradation by enzymes,
ensuring the preservation of pollen as fossils.
Notably, pollen grains feature a distinctive distal
aperture known as the germ pore for germination,
devoid of sporopollenin.
ii. An inner layer referred to as the intine, which is
a delicate, uniform structure primarily comprising
(a) Transverse section of a young Anther; (b) cellulose and pectin.
Enlarged view of one microsporangium showing
wall layers
In the initial stages of development, the
microsporangium is filled with a cluster of closely
packed homogenous cells known as the sporogenous
tissue, located at the core of the anther.
Microsporogenesis Scanning electron micrographs of a few pollen
Microsporogenesis refers to the process where grains
microspores are formed from a pollen mother cell
through meiosis. The diploid cells in the sporogenous During mitotic division, pollen grains divide to
tissue undergo growth and transformation into produce two distinct cells.
microspores or Pollen Mother Cells (PMC). These
6|Sexual Reproduction In Flowering Plants
i. The first one is the Vegetative Cell, larger in size in future plant breeding programs.
with ample reserve food and an irregularly shaped Pistil, Megasporangium and Embryo Sac
large nucleus.
Here are the main points, summarized briefly.
ii. The second one is the Generative Cell, smaller in
size and spindle-shaped with dense cytoplasm and Pistil
nucleus. This cell resides within the cytoplasm of The gynoecium serves as the female reproductive
the vegetative cell. organ of the flower. It can be composed of a single
pistil or carpel (monocarpellary), two carpels
(bicarpellary), three carpels (tricarpellary), or multiple
carpels (multicarpellary).

Carpels
Stigma

Style Stigma

(a) Enlarged view of a pollen grain tetrad; (b) Syncarpous

Stages of a microspore maturing into a pollen grain ovary

In more than 60% of flowering plants, pollen grains

are typically released when they are at a 2-celled Ovary
stage, consisting of vegetative and generative cells.
However, in the remaining species, the generative Thalamus
cell undergoes mitotic division to produce two
a b c
male gametes before the shedding of pollen grains,
resulting in a 3-celled stage with one vegetative cell (a) A dissected flower of Hibiscus showing pistil
and two male gametes. Subsequently, the vegetative (other floral parts have been removed);
cell continues to grow, eventually forming the pollen
tube in these species. (b) Multicarpellary, syncarpous pistil of Papaver;
(c) A multicarpellary, apocarpous gynecium of
Pollen Allergy michelia
Allergies and bronchial infections can be triggered by
pollen grains from various plant species, potentially The pistil can exhibit syncarpy, where multiple pistils
leading to conditions like asthma, bronchitis, and are fused together as seen in papaver and solanum, or
more. For instance, plants like Parthenium or carrot it can be apocarpous where carpels remain separate, as
grass, which were introduced to India as contaminants seen in Michelia rose.
in imported wheat, have now spread widely and are A typical pistil comprises a stigma, an elongated
causing pollen allergies. style, and a swollen ovary. Within the ovary lies the
Pollen Products ovarian cavity (locule) where the placenta is situated.
Pollen grains are a powerhouse of nutrients. In Megasporangia (ovules) are produced from the
Western countries, a wide array of pollen products, placenta within the ovary.
such as tablets and syrups, are commonly utilized to Structure of Megasporangium (Ovule)
enhance the performance of athletes and racehorses. The ovule, a tiny structure connected to the placenta
Pollen Viability via a stalk known as the funicle, transforms into a
The ability of pollen to germinate is known as pollen seed post-fertilisation. The junction point between the
viability, which is influenced by temperature and ovule and funicle, termed as hilum, eventually forms
humidity. The duration of pollen viability varies a scar on the seed.
among species; for example, it lasts about 30 minutes Each ovule is shielded by one or two protective layers
in wheat and rice, while in some members of the called integuments, enveloping the nucellus and
Solanaceae, Rosaceae, and Leguminosae families, it leaving a small opening known as the micropyle.
can last for several months. After pollen grains are
released, they must land on the stigma before they The chalaza, situated at the base of the ovule opposite
lose viability to achieve fertilization. to the micropyle, is surrounded by integuments,
housing a mass of cells referred to as the nucellus.
Pollen grains can be preserved for years in liquid These nucellus cells are abundant in reserve nutrients.
nitrogen at -196°C in pollen banks, enabling their use
 Sexual Reproduction In Flowering Plants|7
Typically, an ovule contains a singular embryo sac sac.
or female gametophyte within the nucellus, evolving iv. Subsequent divisions generate the 4-nucleate and
from a megaspore via reductional division. 8-nucleate stages of the sac.
v. The central cell forms by polar nuclei fusion.
vi. The sac matures into eight nuclei and seven cells,
including the egg apparatus and the antipodals.
vii. These stages in megagametogenesis mark
the formation and maturation of the female
gametophyte in angiosperms.

A diagrammatic view of a typical anatropous ovule

Megasporogenesis
Megasporogenesis is the process through which
megaspores are formed from the Megaspore Mother
Cell or MMC in angiosperms.
Taking place within the nucellus of the ovule, this 1.1.3 Pollination
process begins early on, prior to complete integument
enclosure. The MMC, a sizable cell with dense Male and female gametes developed in the
cytoplasm and a noticeable nucleus, undergoes gametophytes must unite through fusion, necessitating
enlargement and meiotic division. the movement of gametes from one location to another.

Initially, it splits transversely into a pair of cells Pollination refers to the process of transferring pollen
known as the megaspore dyad, which subsequently grains from the anther of a flower to the compatible
undergo further transverse division, resulting in the stigma of the same or a different flower. In order to
formation of a linear array of four haploid cells called achieve successful pollination, flowering plants have
the megaspore tetrad. developed diverse adaptations. There are three main
types of pollination based on the origin of the pollen
Among these, only one megaspore positioned at the grains.
chalazal end remains viable, while the other three
undergo degeneration. Autogamy (Self-Pollination)
Autogamy is a unique form of pollination that occurs
within the same flower. It involves the transfer of
pollen from the anthers to the stigma of the same
flower. Plants like wheat, rice, pea, and others have
evolved specific adaptations to facilitate autogamy.
i. One adaptation is cleistogamous flowers, which
Parts of the ovule showing a large megaspore remain closed with their anthers and stigma in
mother cell, a dyad and a tetrad of megaspore. close proximity, ensuring self-pollination and
seed production even in the absence of external
This type of embryo sac progression is termed pollinators. Examples of plants with cleistogamous
monosporic development, as seen in examples like flowers include Viola, Mirabilis, Commelina, and
Polygonum. Oxalis.
Megagametogenesis ii. Another adaptation for autogamy is homogamy,
Female gametophyte development, known as where the anthers and stigma of a flower mature
megagametogenesis, involves several stages in simultaneously, promoting self-pollination.
angiosperms: This phenomenon is observed in plants like
i. The functional haploid megaspore is the initial Catharanthus and Oxalis.
cell.
ii. It enlarges to become the embryo sac.
iii. The nucleus divides to form a 2-nucleate embryo
8|Sexual Reproduction In Flowering Plants
Do you know? Xenogamy (Cross-Pollination)
Chasmogamous Flowers Cross-pollination is a vital process in plant
Chasmogamous flowers are those with exposed reproduction that entails the movement of pollen
reproductive organs. The anthers and stigmas of these grains from the anther of one plant to the stigma of
flowers come together through processes such as growth, another plant. This mechanism plays a crucial role in
bending, or folding. This structure allows pollen from introducing genetically diverse pollen grains to the
another flower to land on the stigma. Examples include stigma during pollination. Examples of plants that
Catharanthus and Mirabilis. rely on cross-pollination include papaya, maize, and
many others.

Geitonogamy
Autogamous cross-pollination is a unique
reproductive process observed in certain plant
species like Cucurbita. This phenomenon involves
the transfer of pollen grains from the anther of one
flower to the stigma of another flower on the same
Cross-pollination
plant, albeit on separate branches. Although it mimics
cross-pollination in terms of requiring a pollinating Adaptations for Cross-Pollination or Out-
agent, genetically, it resembles autogamy due to breeding Devices
the self-origin of the pollen grains. This process is
Continued self-pollination increases the risk of
commonly found in plants that exhibit monoecious
inbreeding depression. Therefore, flowering plants
characteristics, providing an interesting example
have evolved various mechanisms to discourage self-
of how nature’s intricate mechanisms facilitate
pollination and promote cross-pollination, including:
reproduction within a single plant.
i. Dichogamy: In some bisexual flowers, the
anther and stigma do not mature simultaneously.
Dichogamy is divided into two types:
• Protandry: The anthers mature before the
stigma, as seen in sunflower, sweet pea, Salvia,
Jasminum, and others.
• Protogyny: The stigma matures before the
anthers, as seen in Magnolia, custard apple,
peepal, rose, and others.

Self-pollination
 Sexual Reproduction In Flowering Plants|9
ii. Heterostyly: In some species, the anther and Adaptations for Wind Pollination
stigma are positioned differently, preventing pollen Anemophily, also known as wind pollination, is the
from contacting the stigma of the same flower. predominant abiotic method of pollination in plants.
Examples include Oxalis, Primula, and Linum. Wind-pollinated flowers exhibit specific adaptations:
Both dichogamy and heterostyly prevent autogamy. i. They are small, colorless, inconspicuous, and lack
iii. Self-incompatibility: This genetic mechanism nectar.
prevents pollen from fertilizing the ovules of the ii. These flowers typically feature a single ovule per
same flower by inhibiting pollen germination or ovary and are densely clustered in inflorescences,
pollen tube growth within the pistil. Examples resembling tassels found in corn cobs.
include tobacco and potato. iii. Stamen are prominently exposed to facilitate easy
iv. Dicliny or Unisexuality: This condition involves pollen dispersal.
the presence of only unisexual flowers, which iv. The pollen grains are small, light, dry, dusty, non-
prevents autogamy but not geitonogamy. Examples adhesive, and occasionally possess wings.
include castor and maize.
v. Stigmas are large, hairy, feathery, or branched to
v. Herkogamy: In this mechanism, structural capture airborne pollen.
abnormalities in the sex organs of flowers, such as
in orchids, act as barriers to self-pollination. vi. Common examples of wind-pollinated flowers
include grasses, sugarcane, bamboo, coconut, and
vi. Dioecy: In species like papaya, both autogamy maize.
and geitonogamy are prevented because male
and female flowers are found on separate plants,
meaning each plant is either male or female.
Agents of Cross-Pollination
Cross-pollination in angiosperms requires external
agents for the transfer of pollen from the anther to the
stigma since pollen cannot do so independently. These
agents can be categorized into two main groups,
playing a crucial role in the pollination process.
1. Abiotic Agents:
• Wind (Anemophily): Pollen is carried by the
wind to other flowers. Common in grasses and
trees like pines and oaks.
• Water (Hydrophily): Pollen is transported
through water in aquatic plants such as
Vallisneria and some seaweeds.
2. Biotic Agents:
• Insects (Entomophily): Insects like bees,
butterflies, moths, and beetles transfer pollen
while visiting flowers for nectar. Examples A wind-pollinated plant (Maize)
include sunflowers, roses, and orchids. Adaptations for Wafer Pollination
• Birds (Ornithophily): Birds, particularly Water pollination, known as hydrophily, is a rare
nectar-feeding species like hummingbirds, phenomenon in flowering plants, primarily seen
pollinate flowers such as hibiscus and certain in about 30 genera, mainly monocotyledons. The
cacti. characteristics of water-pollinated flowers are:
• Bats (Chiropterophily): Bats pollinate i. These flowers include small, colourless,
nocturnal flowers like those of the baobab tree inconspicuous, odourless, and nectarless blooms,
and agave. which are typical in lower plant groups like algae,
bryophytes, and pteridophytes.
• Animals (Zoophily): Various animals, including
monkeys, rodents, and humans, can transfer ii. These flowers have long and sticky stigmas, such
pollen between flowers. as those found in Vallisneria, Hydrilla, and Zostera
(sea-grasses).
To enhance the likelihood of pollen grains encountering
the stigma and minimize pollen loss, plants have iii. Pollen grains in water-pollinated species are
implemented diverse strategies. Among these are: often protected from moisture by a mucilaginous
covering.
10|Sexual Reproduction In Flowering Plants
iv. For instance, in Vallisneria, female flowers surface viii. Plants like Salvia exhibit specialized adaptations to
with a long stalk while male flowers release pollen promote cross-pollination by insect visitors.
on the water’s surface, which is then carried by
water currents.

Insect pollination
Pollen-Pistil Interaction
Male & female plant Vallisneria The process starting from the deposition of pollen on
the stigma to the penetration of the pollen tube into
v. Sea grasses have female flowers submerged in
the ovule is known as pollen-pistil interaction. This
water, with pollen grains released into the water,
intricate process encompasses the recognition of
where they passively travel to the stigma for
pollen followed by either the inhibition or promotion
pollination.
of its growth. This crucial phenomenon plays a pivotal
Do you know? role in the fertilization of angiosperms, influencing the
Not all aquatic plants use water for pollination. In fact, in compatibility or incompatibility between pollen and
a majority of aquatic plants, the flowers emerge above the pistil. The steps involved in this process include
surface of the water and are pollinated by insects or wind,
Recognition of Compatible Pollen
similar to land plants. Examples of such plants include
The pistil is endowed with the remarkable capacity
water hyacinth and water lily.
to distinguish and embrace the suitable pollen,
belonging to the same species, while discarding any
Adaptations for Insect Pollination
unsuitable pollen grains, regardless of their species.
Insect pollination, known as entomophily, involves
This compatibility is achieved through an ongoing
specific adaptations in flowers to attract and facilitate
exchange facilitated by the chemical constituents of
the pollination process. Key adaptations are:
the pollen engaging with those of the pistil. Once
i. These include large, colourful, and fragrant flowers the correct pollen is identified, the pistil instigates
with abundant nectar to entice insects. subsequent post-pollination processes that culminate
ii. Clustering multiple flowers into inflorescences in fertilisation.
enhances visibility, especially for smaller blooms. Growth of a Pollen Tube
iii. Nectar glands and strong fragrances further attract Pollination facilitates the transfer of pollen grains to
insects, while sticky pollen grains and stigmas aid the stigma, where they absorb water and swell before
in pollen transfer. germinating to form a pollen tube through a germ
pore. The tube enables the transportation of pollen
iv. Some flowers emit foul odours to draw specific grain contents, growing through stigma and style
pollinators like flies and beetles. tissues with the help of hydrolytic enzymes. As the
v. Providing rewards such as nectar and pollen grains tube navigates through, it releases certain enzymes to
ensures continued visits from pollinators. break down surrounding tissues.
vi. Unique interactions, such as that between Yucca The pollen tube advances intercellularly and
plants and moths, showcase intricate mutual chemotropically, following a chemical gradient. In
dependencies for reproduction. plants with 2-celled pollen shedding, the generative
cell divides to form two male gametes within the
vii. Some insects may exploit floral resources without
stigma as the pollen tube develops.
aiding in pollination, dubbed as pollen/nectar
robbers.
 Sexual Reproduction In Flowering Plants|11
Subsequently, it makes its way into one of the
synergids via the filiform apparatus located at the
micropylar end. The growth of the pollen tube is
guided by the filiform apparatus through the secretion
of specific chemical substances. Collectively, these
events constitute the pollen-pistil interaction, a crucial
aspect that plant breeders leverage to manipulate
interactions, even in cases of incompatible pollination,
to produce desired hybrids.
Additionally, the pollen tube has the capacity to access
the ovule either through the chalaza or via the funicle,
which are termed as chalazogamy and mesogamy,
respectively.
1.1.4 Artificial Hybridisation
Crossing different species, and sometimes even
genera, is a method used to combine desired traits for
the creation of more valuable commercially superior
plant varieties. Plant breeders have utilized this
technique in their crop enhancement programs. The
process involves two key methods:
i. Emasculation: The removal of the anther from the
bud before it releases pollen is essential, especially
(a) LS of pistil showing path of pollen tube growth; in cases where the female parent produces bisexual
(b) enlarged view of an egg apparatus showing flowers.
entry of pollen tube into a synergid; (c) discharge of ii. Bagging: After emasculation, the flower is
male gametes into a synergid and the movement of covered with a suitable bag made of material like
the sperms, one into the egg and the other into the butter paper to prevent any unwanted pollen from
central cell reaching the stigma. Once the stigma is ready,
Conversely, in plants with 3-celled pollen shedding, collected pollen grains are applied, and the fruit is
the pollen tube carries the two male gametes from the allowed to develop.
outset of its growth. For unisexual female parents, emasculation is
Entry of Pollen Tube into the Ovule unnecessary, and the female bud can be directly
Upon reaching the ovary, the pollen tube typically bagged. Pollen grains are then applied to the stigma
penetrates the ovule through the micropylar end in a for fertilization.
process known as porogamy.

Artificial Hybridisation
12|Sexual Reproduction In Flowering Plants
1.1.5 Double Fertilisation zygote, a process known as syngamy or generative
fertilization. The zygote develops into the embryo.
In angiosperms, double fertilization is a unique
process involving the fusion of two male gametes with The second male gamete fuses with the two polar
two different cells of the same female gametophyte, nuclei or secondary nucleus in the central cell, forming
resulting in the formation of two distinct structures. a triploid Primary Endosperm Nucleus (PEN). This
This process, first demonstrated by Nawaschin in process is called triple fusion because it involves the
1898 in Lilium and Fritillaria, is a key characteristic fusion of three haploid nuclei. Following triple fusion,
of angiosperms. During double fertilization, both the central cell becomes the Primary Endosperm Cell
male gametes play a role in fertilization. (PEC), which develops into the endosperm.
The pollen tube enters the ovule and releases the Since both syngamy and triple fusion occur within the
two male gametes into the cytoplasm of one of the embryo sac, this phenomenon is referred to as double
synergids, which then begins to degenerate. One male fertilization.
gamete fuses with the egg cell to form a diploid (2n)

Double Fertilisation

1.2 Post-Fertilisation: Structures kernel represents the cellular endosperm.

and Events 1.2.2 Development of an Embryo

The main post-fertilization events include the During the initial stage of plant zygote development,
development of the endosperm and embryo, as well known as embryogeny, a well-defined process unfolds
as the maturation of ovules into seeds and the ovary where the zygote transforms into an organized
into fruit. These processes occur shortly after double cluster of cells referred to as the embryo, located at
fertilization. the micropylar tip of the embryo sac. This embryo
possesses the capacity to evolve into a fully-developed
1.2.1 Development of an Endosperm plant. In most cases, zygotes only divide once a
Endosperm development precedes embryo sufficient amount of endosperm is present to ensure
development. The process unfolds through several adequate nourishment for the growing embryo.
key stages: In the context of angiosperms, the zygote typically
i. The primary endosperm cell divides repeatedly undergoes an asymmetric mitotic division, resulting
to form triploid endosperm tissue, rich in reserve in the formation of two cells with distinct destinies.
food materials crucial for nurturing the emerging i. The smaller daughter cell, characterized by
embryo. its dense cytoplasm, is positioned towards the
ii. The primary endosperm tissue goes through chalazal extremity, identified as the terminal,
continuous free nuclear divisions, generating apical, or embryonal cell.
free nuclei and transitioning to a free-nuclear ii. Conversely, the larger daughter cell located at
endosperm state. the micropylar end is known as the basal cell,
iii. Wall formation initiates from the periphery towards which undergoes a transverse division to generate
the centre, transforming the endosperm into a suspensor cells. The zygote progresses into the
cellular structure. Notably, coconut water from proembryo stage and advances into the globular,
tender coconuts comprises free-nuclear endosperm heart-shaped, and ultimately mature form.
with numerous nuclei, while the surrounding white
 Sexual Reproduction In Flowering Plants|13

Despite variations in seed structure, the early stages of embryo development exhibit similarities between
monocots and dicots.

Structure of Dicot and Monocot Embryos Structure of a Seed

In a dicotyledonous embryo, you can find an embryonal A typical seed consists of a seed coat, cotyledon and an
axis along with two cotyledons. The upper part of the embryonal axis. The seed coat is often double layered,
embryonal axis, known as the epicotyl, ends in either formed by the integuments. The outer integument
the plumule or the stem tip. Below the cotyledons lies forms the outer seed coat or testa, which is hard and
the hypocotyl, which leads to the root tip, or radicle. inner integument forms the inner seed coat or tegmen.
The root tip is shielded by a root cap and examples
of dicotyledonous plants include mangoes, apples,
radishes, roses, and more.

A typical dicot emtry, LS of grass embry(monocot)

Monocot embryos have a single cotyledon known
as the scutellum located at one end of the embryonal
axis, with the radical and root cap enclosed in a sheath
called colcorhiza at the opposite end. Structure of some seeds
The upper part of the embryonal axis, above the The embryonic leaves, known as the contedons,
scutellum, is termed the epicotyl, which contains the are typically plump and filled with nutrients. They
shoot apex and a few leaf primordia surrounded by a usually come in a pair and contain abundant reserves
hollow foliar structure called the coleoptile. Examples of food. The micropyle serves as a small opening in
of plants with this embryo structure include grass, the seed coat, allowing water and oxygen to enter
banana, bamboo, and palm. the seed. As the seed matures, its moisture content
1.2.3 Development of a Seed decreases, resulting in relatively dry seeds (with 10-
15% moisture by mass). The hilum denotes the point
In angiosperms, double fertilisation initiates the
where the seed is attached to the stalk.
metamorphosis of an ovule into a seed, commonly
referred to as a fertilised ovule. These developed In some cases, like black pepper and beet seeds, traces
seeds germinate within the protective encasement of of the nucellus may persist, forming what is known as
fruits, ensuring their propagation. a perisperm – a residual and enduring part of the seed.
14|Sexual Reproduction In Flowering Plants
Types of Seeds
Seeds can be categorized into two main types based
Do you know? on the presence or absence of endosperm.
Seed Dormancy
Seed dormancy refers to a period when the metabolic i. Endospermic or Albuminous Seeds: These
activity of the embryo decreases, causing it to enter seeds contain endosperm, such as wheat, maize,
a state of inactivity. Under favourable conditions, sunflower, castor, etc.
the dormant seed resumes growth and germinates to ii. Non-endospermic or Exalbuminous Seeds: These
form new plants. seeds lack endosperm as it is fully utilized during
embryo development, like pea, bean, groundnut,
etc.

Furthermore, seeds can also be classified based on the number of cotyledons they have, leading to the distinction
between monocots and dicots.
The major differences between monocot and dicot seeds include:
Types Monocot Dicot
Seed Structure Monocot seeds typically have a single Dicot seeds usually have two cotyledons.
cotyledon (seed leaf) inside the seed coat.
Cotyledon Appearance The cotyledon in monocots is usually thin Dicot cotyledons are often broader and
and elongated. thicker.
Root Development Monocot seeds produce a single embryonic Dicot seeds typically develop two
root (radicle) from the base of the embryo. embryonic roots (radicles) known as
primary roots.
Vein Pattern in Leaves Monocot leaves usually have parallel veins. Dicot leaves generally have a branched
network of veins.
Floral Parts Monocot flowers typically have floral Dicot flowers usually have floral parts in
parts in multiples of three (e.g., petals in multiples of four or five.
multiples of three).
Stem Vascular Bundles Monocot stems have scattered vascular Dicot stems have vascular bundles
bundles. arranged in a ring.

Advantages of Seeds Seed Dispersal

Advantages of seeds include: Seed dispersal is the process of moving or transferring
i. Foundation of Agriculture: Seeds form the basis seeds from the parent plant to enable their spread
of agriculture, crucial for food production and across a vast geographic region.
plant propagation. Significance of Seed Dispersal
ii. Nutritional Support: Stored food within seeds Seed dispersal plays a crucial role as it prevents
sustains their growth until they can sustain overcrowding near the parent plant, which can lead
themselves independently. to intense competition among seedlings and with the
iii. Long-Term Storage: Seeds can be stored for parent plant itself.
extended periods as a food source. For example, This competition primarily revolves around vital
Lupinus arcticus in the Arctic Tundra has yielded resources such as light, space, water, and nutrients.
viable seeds after dormancy lasting up to 10,000 When there is competition for these essential factors,
years. Similarly, date palms (Phoenix dactylifera) it hampers the overall growth of the plants, negatively
from King Herod’s Palace near the Dead Sea have impacting their development.
produced viable seeds after dormancy of 2000 Therefore, effective seed dispersal is essential to
years. ensure the healthy and sustainable growth of plant
iv. Genetic Recombination: As products of sexual populations by reducing competition and allowing
reproduction, seeds contribute to genetic diversity each plant to access the necessary resources for its
through recombination. growth and survival.
v. Dispersal and Adaptation: Seeds are adapted for 1.2.4 Formation of a Fruit
dispersal, enabling plants to colonize new areas
After fertilization, a fruit emerges from the ovary
and employ adaptive strategies for survival and
through cell division and differentiation. The ripened
reproduction.
 Sexual Reproduction In Flowering Plants|15
ovary transforms into a fruit which can either be iii. Parthenocarpic fruits are those that grow without
fleshy like guava, tomato, and cucumber, or dry and fertilization, like bananas. This phenomenon of
leathery like pea, bean, and mustard. The ovary wall fruit formation without fertilization is known as
evolves into the pericarp, the protective fruit wall parthenocarpy which can be initiated through the
consisting of three layers – epicarp, mesocarp, and use of growth hormones, resulting in seedless
endocarp. Pericarp safeguards the seeds and aids in fruits.
their dispersal.
Fruits come in various seed arrangements, from
solitary seeds like mango to fruits with numerous tiny
seeds seen in orchids, Orobanche, Striga, Ficus, and
other parasitic species.
Significance of Fruit Formation
The significance of fruit formation in plants is
multifaceted and crucial for their reproductive success
and ecological interactions. Here are key points
highlighting its importance:
Two types of fruit
i. Seed Dispersal: Fruits often serve as vehicles for
seed dispersal, aiding plants in colonizing new 1.2.5 Some Special Mechanisms of
habitats and reducing competition with parent Reproduction
plants. Various mechanisms such as wind, water,
While seeds are typically the result of fertilization,
animals, and gravity help disperse seeds from their
some flowering plants have developed unique
parent plant.
strategies to either create seeds or generate multiple
ii. Protection and Nutrition: Fruits provide embryos from a single fertilized egg. The following
protection to seeds from environmental stresses and section explores these distinctive methods:
predators. The fleshy or tough outer layer of fruits
Apomixis
shields seeds during development and dispersal.
Additionally, fruits may contain nutrients that aid i. Asexual Reproduction Resembling Sexual
in the initial growth of the seedling. Reproduction: Apomixis is a form of asexual
reproduction that mimics sexual reproduction by
iii. Attraction of Dispersal Agents: Fruits attract producing seeds without the fusion of gametes. It
animals, birds, and other organisms that consume is observed in certain species of Asteraceae and
them. These agents aid in seed dispersal by grasses.
consuming the fruit and transporting seeds to new
locations through digestion or by carrying them in ii. Production of Viable Seeds: Apomictic seeds are
their fur, feathers, or beaks. viable and develop without the need for pollination
or sexual reproduction. They can originate from
iv. Reproductive Strategy: Fruit formation is a part various sources such as segments of fruit (like
of the reproductive strategy of flowering plants mango stem) or male gametic content of pollen (as
(angiosperms). It ensures the successful transfer seen in Cyperus), as well as other vegetative parts.
of genetic material (seeds) to new environments,
enhancing the species’ survival and adaptation. iii. Formation of Diploid Egg Cells: In some species,
diploid egg cells are formed directly without
v. Human and Ecological Importance: Fruits are undergoing reductional division, leading to the
a valuable food source for humans and wildlife, development of embryos without fertilization.
contributing to biodiversity and ecosystem
stability. They also play roles in cultural practices, iv. Utilization in Hybrid Seed Industry: Apomixis
economies, and the ecology of plant-animal is widely utilized in the hybrid seed industry. By
interactions. converting hybrids into apomicts, the segregation
of traits in hybrid progeny is avoided. This allows
Types of Fruits farmers to save costs since they do not need to
Fruits can be categorized based on their formation. purchase hybrid seeds annually, instead using
These are: apomictic seeds to cultivate new crops each year.
i. True fruits originate from the ovary of a flower Polyembryony
and are not connected to any non-carpellary Polyembryony is when multiple embryos develop
element. Examples include mango and tomato. within a single seed. This phenomenon can be
ii. False fruits develop from the ovary and additional observed in gymnosperms, where it is often triggered
floral parts like thalamus, such as apple, cashewnut, by the division of the growing embryo. Certain species
and strawberry. of plants such as Citrus (orange and lemon), onion,
16|Sexual Reproduction In Flowering Plants
groundnut, and certain mango varieties are known to 13. What are chasmogamous flowers? Can cross-
exhibit polyembryony. pollination occur in cleistogamous flowers? Give
Parthenocarpy reasons for your answer. (NCERT, 2M)

Parthenocarpy, the process of producing seedless 14. Why is geitonogamy also referred to as genetical
fruit, comes in two forms: vegetative and stimulative. autogamy? (All India 2010, 2M)
Vegetative parthenocarpy enables seedless fruits like 15. What is self-incompatibility? Why does self-
pears to grow without the need for pollination. On pollination not lead to seed formation in self-
the other hand, stimulative parthenocarpy, as seen in incompatible species? (NCERT)
grapes, necessitates the pollination stimulus but does
16. Protandry and protogyny effectively prevents
not involve the fertilization process.
autogamy. How? What is the collective term for
both?
Topic Practice 1 17. Write two adaptations undertaken by sea-grasses
1. Give the name of the parts of an angiosperm in order to increase their chances of fertilisation.
in which the development of male and female 18. Pollination takes place via various pollinating
gametophyte takes place.  (NCERT, 1M) agents.
2. An anther with malfunctioning tapetum often 19. Write the characteristic feature of anther, pollen
fails to produce viable male gametophytes. Give and stigma of a wind pollinated flower.
any one reason. (Delhi 2013, 1M)
20. Angiosperms bearing unisexual flowers are said
3. In the TS of a mature anther given below, identify to be either monoecious or dioecious. Explain
‘a’ and ‘b’ and mention their functions. with the help of one example each.
4. Arrange the following terms in a correct  (All India 2016, 1M).
developmental sequence.
21. What is meant by emasculation? When and why
Pollen grain, sporogenous tissue, microspore does a plant breeder employ this technique?
tetrad, pollen mother cell, male gametes.
 (NCERT, 2M)
 (NCERT, 2M)
22. Mention the advantages of emasculation and
5. A pollen grain in angiosperm at the time of bagging in artificial hybridisation in plants
dehiscence from an anther could be 2-celled or bearing unisexual and bisexual flowers.
3-celled. Explain, how are the cells placed within
 (Delhi 2020, 2M)
the pollen grain when shed at a 2-celled stage.
23. In angiosperms, zygote is diploid while primary
 (All India 2017, 2M)
endosperm cell is triploid. Explain.
6. Give an example of a plant which came into India
 (All India 2013, 2M)
as a contaminant and is a cause of pollen allergy.
 (All India 2014, 1M)
Explanations
1. The parts of an angiospermic flower in which the
7. Gynoecium of a flower may be apocarpous or
development of male and female gametophyte
syncarpous. Explain with the help of an example
takes place are pollen sac of anther and nucellus
each. (Delhi 2016, 2M)
of ovule, respectively.
8. Why do we call embryo sac monosporic?
2. An anther with malfunctioning tapetum often
or fails to produce viable male gametophytes due to
What is meant by monosporic development of lack of nutrients.
female gametophyte? (NCERT, 1M) 3. In the given figure ‘a’ is sporogenous tissue and
9. A mature embryo sac in a flowering plant may ‘b’ is tapetum.
possess 7 -cells, but 8 -nuclei. Explain with the (i) Sporogenous tissue has cells which are potential
help of a diagram only. (Delhi 2017, 2M) Pollen Mother Cell (PMC) or microspore mother
10. Name all the haploid cells present in an unfertilised cell and give rise to microspore tetrad.
mature embryo sac of a flowering plant. Write the (ii) Tapetum nourishes the developing
total number of cells in it. (All India 2013, 2M) microspores or pollen grains.
11. Write two differences between male and female 4. The correct developmental sequence is as follows
gametophytes. Sporogenous tissue → Pollen mother cell →
12. How are cleistogamous flowers different from Microspore tetrad → Pollen grain Male gametes
chasmogamous flowers ? Which of the two would 5. In about 60% of angiosperms, pollen grains are
assure seed set even in the absence of pollinators?
 Sexual Reproduction In Flowering Plants|17
shed at the 2 -celled stage. However, in about 40% Cross-pollination cannot occur in cleistogamous
flowering plants, the generative cell may further flowers because they remain in bud throughout
divide mitotically to give rise to two male gametes their life and form seeds via self-pollination.
and pollen grains are shed at this 3-celled stage. 14. Geitonogamy is functionally a cross-pollination,
The placement of cells within the pollen grain involving a pollinating agent. But genetically, it is
when shed at 2-celled stage can be visualised as similar to autogamy since, the pollen grains come
Diagrammatic representation showing placement from the same plant. So, it is also referred to as
of two cells within a pollen grain genetical autogamy.
6. Pollen grains of Parthenium came into India as a 15. Refer to text on page no. 7 .
contaminant and is a cause of pollen allergy.
16. In order to prevent autogamy, certain plants like
7. Gynoecium of a flower is called apocarpous when Salvia, adopt dichogamy wherein the maturation
the carpels are free, e.g. in Ranunculus. Whereas time of both anther and stigma differs. If anther
it is called syncarpous when the carpels are fused, mature earlier than the stigma it is called
e.g. in Petunia. protandry. On the other hand, if the stigma
8. An embryo sac is called monosporic because it becomes receptive earlier the condition is called
develops from one of the four megaspores formed protogyny. Both these conditions effectively
from the division of megaspore mother cell. prevent self-pollination.
9. A typical angiospermic embryo sac is 8-nucleated 17. In order to increase chances of fertilisation, sea-
and 7 -celled. grasses have undertaken the following adaptations
10. An unfertilised embryo sac of angiosperm is (i) Long and sticky stigmas.
composed of 8 -nuclei and 7 cells. Among 8 (ii) Female flowers are submerged in water and
nuclei, 6 are enclosed by cell walls and organised pollen are released inside the water.
into cell, while the remaining 2 nuclei (called
polar nuclei) are situated above the egg apparatus. 18. In wind pollinated flowers,
Out of 6 cells, 3 are grouped at micropylar end (i) Anthers are well exposed for easy dispersal.
and constitute the egg apparatus, made up of 2
(ii) Stigma is large and feathery to trap pollens.
synergids and 1 egg cell. The other 3 are located
at chalazal end and are called antipodals. All the (iii) Pollen grains are small, light, dry, dusty and
cells of the embryo sac are haploid except the non-sticky.
central cell which is first binucleate and then 19. Plant bearing flowers of both sexes, i.e. staminate
becomes diploid due to the fusion of polar nuclei. and pistillate flowers are called monoecious, e.g.
11. Differences between male and female Zea (maize).
gametophytes are as follows When both sexes, i.e. staminate and pistillate
Male gametophyte Female gametophyte flowers, are present on different plants; these
plants are called dioecious, e.g. Carica papaya
It is derived from a It is derived from a (papaya).
microspore, which is megaspore, which
produced inside the is formed inside the 20. Emasculation is the process of removal of stamens
pollen chamber of nucellus of an ovule. from a bisexual flower before they shed pollen in
anther. order to prevent self-pollination and permit cross-
pollination of stigma with desired pollen. During
The microspores are The megaspores do not
breeding, the breeder wants to make sure that the
shed at maturity and shed from the ovule.
desired pollen grains are used for pollination and
transferred the process They from anthers to
the stigma is protected from contamination (from
called pollination. stigma by develop to
an unwanted pollen.
form embryo sac within
the ovule. A plant breeder employs this technique to select
and incorporate specific traits in the progeny.
12. Cleistogamous flowers are closed flowers with
their anthers and stigma lying close to each other, 21. Emasculation is the removal of anthers from
e.g. Oxalis. On the other hand, chasmogamous bud before its dehiscence. In unisexual plants, it
flowers are flowers with exposed sex organs, e.g. is not required while in bisexual plants, it helps
Catharanthus. in preserving pollens and preventing unwanted
pollination. Bagging is advantageous in both
Out of the two, cleistogamous flowers assure seed
type of plants. This protects them from any
set even in the absence of pollinators.
contamination by pollen grains.
13. Chasmogamous flowers are the bisexual, open
22. Zygote is the product of a fusion of a haploid male
flowers with exposed anthers and stigma.
gamete and haploid female gamete, i.e. egg cell.
18|Sexual Reproduction In Flowering Plants
Therefore, it is diploid, 15. Why is apple called a false fruit? Which part(s) of
Male gamete + Egg → Zygote the flower forms the fruit? (NCERT, 1M)

(n)+(n)→(2n) 16. Apomixis is a type of asexual reproduction.

Comment.
Whereas, primary endosperm cell is a product of
fusion of secondary nucleus (2n) and a haploid 17. Give reason why hybrid seeds are to be produced
male gamete. So, it is triploid. year after year. (Delhi 2011, 2M)

Secondary nucleus + Male gamete → Primary Explanations

endosperm cell …………… 1. The post-fertilisation events in angiosperms are
as follows
Topic Practice 2 (i) Development of endosperm

1. List post-fertilisation events in angiosperms. (ii) Development of embryo

 (Delhi 2014, 2M) (iii) Development of seed

2. Why do you think that the zygote is dormant for (iv) Fruit formation
sometime in a fertilised ovule? (NCERT, 2M) 2. There is an adaptation in angiospermic plants
3. Most zygotes in angiosperms divide only to provide assured nutrition to the developing
when certain amount of endosperm in formed. embryo. Most zygotes divide only after certain
Comment. amount of endosperm is formed. Therefore,
zygote remains dormant for sometime in a
4. Endosperm is a mass of nutritive cell. What fertilised ovule.
impact would lack of endosperm create on the
growing embryo? 3. Endosperm is a nutritive tissue that gives
fundamental supplements to the growing
5. Identify in the figure showing a stage of embryo organism.
development in a dicot plant and mention its
function. (All India 2016, 1M) Thus, in this way most zygotes in case of flowering
plants divide only after a certain measure of
6. Which part of the dicot embryo emerges first endosperm is obtained.
above the ground when the seed germinates?
4. Endosperm stores carbohydrates, fats, proteins,
7. What is the position of plumule in monocot etc. In the absence of endosperm, growth of
embryo? embryo would be poor. This would further result
8. Give the name of the common functions that in formation of non-viable seeds.
cotyledons and nucellus perform. 5. A is cotyledon. It is the storehouse of food in
 (NCERT Exemplar, seeds.
9. Given below is a section of maize grain. Identify 6. Hypocotyl is the first part of dicot embryo
and state its function. (All India 2016C, that emerges above the ground during seed
germination.
10. Why do the integuments of an ovule harden and
the water content gets highly reduced as the seed 7. The position of plumule in monocot embryo is
matures? (All India 2011, 2M) lateral.
11. Seeds are fertilised mature ovule which possess 8. The common functions of cotyledons and
stored food. In what category will a castor seed nucellus are
be placed? Why? (i) storage of reserve food material.
12. Differentiate between pericarp and perisperm. (ii) nourishment of embryo and embryo sac,
 (Delhi 2017C, 2M) respectively.
13. Name the parts of pistil which develop into fruits 9. A is endosperm. It provides nutrition to the
and seeds. (NCERT Exemplar, 1M) developing embryo.
14. Differentiate between 10. Integuments of ovule harden to form tough
protective seed coats. Water content gets reduced
(i) Hypocotyl and epicotyl
to keep the seed drier and maintained in dormant
(ii) Coleoptile and coleorhiza stage. However, the micropyle remains as a
(iii) Integument and testa small pore in the seed coat. This facilitates the
entry of oxygen and water into the seed during
(iv) Perisperm and pericarp
germination.
 (NCERT, 2M)
11. A castor seed contains endosperm. Therefore,
 Sexual Reproduction In Flowering Plants|19
it will be categorised as an endospermic or while testa is the outermost protective covering
albuminous seed. of the seed coat.
12. Differences between pericarp and perisperm are (iv) Refer to Q. No. 12.

Pericarp Perisperm 15. Apple is called a false fruit because it develops

from the thalamus (enlarged structure at the base
It is the covering of It is unused nucellus in
of the flower) instead of ovary.
fruits that develops from the seed.
ovary wall. 16. Apomixis is a type of asexual reproduction in
It is a part of the fruit. It is a part of the seed. which viable seeds can be produced without
fertilisation and zygote formation through gamete
It is usually fleshy or dry. It is usually dry. fusion.
It is found in all fruits. It is present in only a few
On the contrary, in sexual reproduction seeds
seeds.
are produced through gametic fusion following
13. The parts of pistil which develop into fruits and fertilisation.
seed are ovary and ovules, respectively.
17. If the seeds are collected from the hybrid variety
14. (i) Refer to text on page no. 13-14. are sown, the plants in the progeny will segregate
(ii) Refer to text on page no. 14. and will not maintain the hybrid characters. So,
hybrid seeds are to be produced year after year.
(iii) Integument is the outer covering of ovules,