CHAPTER 2: SEXUAL REPRODUCTION IN FLOWERING PLANTS

Contents:
1. Pre-fertilization: Structures and Events
2. Double Fertilization
3. Post-Fertilization: Structures and Events
4. Apomixis and Polyembryony

❖ Flower is a modified, vegetative shoot with highly condensed nodes and internodes and
specially meant for reproduction.
▪ Flowers are the organs of sexual reproduction in angiosperms.
▪ Flowers are the characteristic feature of angiosperms.
Parts of a Flower: A typical flower consist of four parts (floral whorls) namely
❖ 1. Calyx - the individual unit of calyx are sepals. They may be free of fused.
2. Corolla - the individual unit of corolla are called petals.
3. Androecium - It is the male reproductive part of a flower. The individual units of
androecium are called stamens.
4. Gynoecium - It is the female reproductive part of a flower. The individual units of
gynoecium are called Pistil.
5. The stalk of the flower is called pedicel.

L.S. of a typical flower

❖ Sexual reproduction in angiosperms can be studied under three steps namely
1. Pre-fertilization structure and events.
2. Fertilization
3. Post-fertilization-structure and events
1. Pre-fertilization structure and events:
▪ Hormonal and structural changes in plants leads to development of flower,
▪ Androecium is the male reproductive part of a flower. It consists of a whorl of
stamens.
 ▪ Gynoecium is the female reproductive organ of a flower.

Stamen, Microsporangium and Pollen grain:

❖ Stamen (Microsporophyll):
▪ Stamen is the individual unit of androecium.
▪ The stamen consists of two parts called a filament and anther.
▪ Filament is the long and slender stalk of stamen.
▪ Anther is terminal bilobed fertile structure of a stamen.
▪ The filament is connected to the anther by a sterile structure called connective.
▪ A typical angiosperm anther is bilobed or dithecous.
▪ Each anther has four microsporangia. Hence the anthers in angiosperms are called
Tetrasporangiate.
▪ Microsporangium develops into pollen sac and the pollen sac contains pollen grains.
 T.S. OF YOUNG ANTHER

❖ Microsporogenesis:
▪ The process of formation of microspores from pollen mother cell (PMC) through meiosis
is called microsporogenesis.
▪ The sporogenous tissue of microsporangium differentiated into microspore mother cell
(MMC) or pollen mother cell (PMC).
▪ Each MMC undergoes meiosis and gives rise to four haploid microspores.
▪ A group of four microspores produced from MMC by meiosis is called microspore
tetrad.
▪ On dehydration microspore tetrad dissociated to form four pollen grains.
▪ The pollen grains are released by the dehiscence of anther wall.

Pollen grains:
▪ Pollen grain represents the male gametophyte.
▪ Generally the pollen grains are spherical in shape and are covered by two layered wall.
▪ The outer layer of pollen wall is called exine. It is hard and made up of sporopollenin.
Sporopollenin is a highly resistant organic material. It can withstand high temperature
and strong acids and alkali. No enzyme can degrades sporopollenin.
▪ The exine has apertures called germ pore where sporopollenin is absent. It is through these
germ pores pollen tube comes out during germination.
▪ The inner layer is called intine. It is a thin, continuous layer and made up of cellulose &
pectin.
▪ Mature pollen grain consists of two cells called vegetative cell and a generative cell.
▪ The vegetative cell is larger cell with large irregular nucleus and dense cytoplasm.
▪ The generative cell is a small lens shaped cell that floats in the cytoplasm of vegetative
cell.
 ▪ In majority of angiosperms (60%) the pollen grains shed at this 2-celled stage. In some
others the generative cell divides mitotically to form two male gametes before shedding.
(3- celled stage).

Uses:
1. Pollen grains are rich in nutrients.
2. The pollen tablets and pollen syrup are used as food supplements.
3. Pollen consumptions may increase performance of athletes and race horses.

Harmful effects:
▪ Pollen grain of many plants causes severe allergies and bronchial afflictions.
▪ They may cause chronic respiratory disorders like asthma, bronchitis etc.
▪ Pollen grain of Parthenium (carrot grass) causes pollen allergy (High fever).

❖ Viability of pollen grains:

▪ The viability of pollen grain depends on temperature and humidity.
▪ In wheat and rice the pollen grain lose viability within 30 min. of their release.
▪ In Rosaceae, Leguminoseae and Solanaceae they remain viable for months.
▪ Pollen grain can be preserved for years in liquid nitrogen (–196oC). The stored pollens
can be used as pollen banks in crop breeding programmes

❖ THE PISTIL, MEGASPORANGIUM (OVULE) AND EMBRYO SAC

▪ The Gynoecium (Pistil) is the female reproductive part of a flower.
▪ The gynoecium may be
1. Monocarpellary - Gynoecium with a single carpel
1.2. Multi Carpellary - Gynoecium with more than one carpel.
3. Syncarpous - Gynoecium with fused carpel.
4. Apocarpous - Gynoecium with free carpels.
The pistil has three parts namely stigma, style and ovary.
1) Stigma - The terminal region of pistil. It serves as a landing platform for pollen
grains.
2) Style - The elongated slender part below the stigma.
3) Ovary - The basal bulged part of the pistil. The ovary encloses locules (ovarian
cavity).
▪ Megasporangia or ovules are produced from the placenta inside the ovary.
▪ The number of ovule in the ovary may be one or many.
MEGASPORANGIUM (OVULE) / Structure of mature Anatropous ovule:
▪ Ovule is an integumented megasporangium.
▪ Anatropous ovule is an ovule where the micropyle and funiculus lie side by side.
▪ The ovule is attached to the placenta by a stalk called funicle (Funiculus).
▪ The point of attachment of the body of ovule to the funicle is called hilum.
▪ Each ovule has one or two protective coverings called integuments.
▪ Integument covers the ovule completely except at an opening called micropyle.
▪ Opposite of the micropylar end is the chalaza.
▪ The unilateral outgrowth of funicle is called raphe.
▪ The ovule encloses female gametophyte or embryo sac.

Mature embryo sac in angiosperms consists of

2.3.Egg apparatus - consists of two synergids and one egg. It is found towards the
micropyle. Synergids have filiform apparatus.
3.4.Central cell - has diploid secondary nucleus.
4.5.Three antipodals - haploid cells found towards chalazal end.
Megasporogenesis:
▪ The process of formation of haploid megaspores from diploid megaspore mother cell
(MMC) is called Megasporogenesis.
▪ The ovule has a central mass of diploid tissue called nucellus.
▪ A nucellar cell towards micropylar end is differentiated into megaspore mother cell
(MMC).
▪ The MMC is larger diploid cell with dense cytoplasm and a prominent nucleus.
▪ The MMC undergo meiosis to form four haploid megaspores.

Megaspore Mother Cell (M C) Dyad Tetrad of Megaspores

Female Gametophyte / Embryo sac:
▪ The chalazal megaspore is functional and the other three megaspores degenerate.
▪ The functional megaspore developed into female gametophyte or embryo sac.
▪ Development of embryo sac from a single megaspore is called monosporic development.
▪ The nucleus of functional megaspore divides by mitosis to form two nuclei which
migrates to the opposite pole. It is the 2-nucleated embryo sac.
▪ Two successive free nuclear mitotic divisions lead to 4-nucleate and 8-nucleate stages.
▪ One nucleus from each pole (polar nucleus) migrates towards the center where they fuse
to form diploid secondary nucleus.
▪ The larger cell containing secondary nucleus is called central cell.
▪ Three cells at the micropylar end are organized into egg apparatus.
▪ The egg apparatus consists of two synergids and one egg cell. Synergids have filiform
apparatus. This helps to direct the pollen tube into the synergid.
▪ The three cells at the chalazal end are organized into haploid antipodal cells.
▪ The mature embryo sac in angiosperms is 8- nucleated and 7-celled.

2, 4 AND 8 NUCLEATE EMBRYO SAC MATURE EMBRYO SAC ❖

 POLLINATION:
The transfer of pollen grains from anther to the stigma of a pistil is called pollination.
Kinds of pollination: Pollination is of three types namely
1. Autogamy
2. Geitonogamy
3. Xenogamy
1. Autogamy
▪ Pollination within the same flower is called autogamy.
▪ It involves the transfer of pollen grains from anther to the stigma of the same flower.
▪ In open flowers with exposed anthers and stigma autogamy is rare. In these flowers
a) Shedding of pollens and receptivity of stigma should occur simultaneously.
b) Anthers and stigma should lie close to each other.
Contrivances for autogamy:
▪ Chasmogamous flowers - flowers with exposed anther & stigma are called
chasmogamous flowers. Pollination in flowers with exposed anthers and stigma is
called chasmogamy.
▪ Cleistogamous flowers - flowers which do not open at all are called cleistogamous
flowers.
The pollination in unopened flower buds is called cleistogamy.
-Cleistogamous flower is autogamous and produce assured seed set even in the absence
of pollinator
Ex: Viola, Oxalis, Commelina (produce both chasmogamous and cleistogamous flowers)
2. Geitonogamy:
▪ Transfer of pollen grains from anther of one flower to the stigma of another flower on the
same plant is called geitonogamy.
It is the pollination between the flowers borne on the same plant.
▪ It is functionally cross pollination (involves pollinators) but genetically autogamy
(pollen grains from the same plant).
3. Xenogamy:
▪ Transfer of pollen grains from anther to the stigma of different plant is called xenogamy.
▪ It is commonly called as cross-pollination
▪ It brings genetically different types of pollen grains to the stigma
 *Agents of pollination: Plants use both abiotic and biotic agents for
pollination ▪ Abiotic agent - wind and water (used by few plants).
▪ Biotic agents - animals (used by majority of the plants).
✓ Pollination by wind (Anemophily):
▪ Pollination by wind is called anemophily.
▪ The wind pollinating flowers are called anemophilous flowers.
Characters of wind pollinated flowers:
▪ The flowers produce large number of pollens.
▪ Pollen grains are light and non-sticky. Hence they can be easily carried through wind.
▪ Flowers have well exposed stamens. So that the pollens are easily dispersed into wind.
▪ They have large feathery stigma. It is to trap air-borne pollen grains.
▪ Flowers are small, inconspicuous, colourless and nectarless.
▪ Many small flowers are aggregated into inflorescences (Ex: corn cob).

*Pollination by water (Hydrophily):

▪ Pollination by water is called hydrophily.
▪ Flowers showing pollination by water are called hydrophilous flowers.
Ex: Vallisneria, Hydrilla and Zostera (sea grass).
▪ Aquatic plants like water hyacinth and water lily show pollination by insects or wind
because the flowers emerge above the level of water.
✓ Pollination in Vallisneria and Zostera:
Vallisneria is a submerged, dioecious fresh water plant. It shows( Epihydrophily).
▪ The female flowers reach the surface of water by the long stalk.
▪ The male flowers or pollen grains are released on to the surface of water.
▪ Male flowers pollen grain are carried passively by water current & some of them reach
female flowers.
▪ The anther burst open to release pollen grains. The pollen grains are attached to the large
sticky stigma and bring about pollination.
▪ The pollen grains are protected from wetting by a mucilaginous covering.
Zostera is a marine sea grass. It shows (Hypohydrophily.)
▪ The female flowers remain submerged in water.
▪ The pollen grains are released inside the water.
▪ The pollen grains are long, ribbon like and are carried passively inside the water.
▪ Some of the pollen grains reach the stigma and brings about pollination.
*Pollination by biotic agent:
▪ Majority of flowering plants use different types of animals as pollinating agents.
▪ Bees, butterflies, wasps, ants, moths, birds (sunbirds and humming birds) and bats are the
biotic agents of pollination.
▪ Among the insects Bees are the dominant biotic pollinating agents.
▪ Insect pollinating flowers are very large, colorful, fragrant and rich in nectar.
▪ Small flowers are present in cluster to make them conspicuous.
▪ Flower pollinated by flies and beetles secrete foul odours.
▪ Nectar and pollen grains are the usual floral rewards for insects.
▪ In some species floral rewards are in providing safe places to lay eggs. (Amorphophallus).
▪ A species of moth & Yucca plant cannot complete their life cycle without each other. The
moth deposits its eggs in the locule of the ovary. The flower in turn gets pollinated by
the moth.
▪ Pollen robbers: Many insects may consume pollen or nectar without bringing about
pollination. Such floral visitors are referred as pollen / nectar robbers.
*Out breeding Devices:
▪ Majority of the flowering plants produce bisexual flower and undergo autogamy.
▪ Continuous autogamy or self-pollination results in inbreeding depression.
▪ Hence flowering plants have developed out breeding devices.
▪ The devices to avoid self pollination and to encourage cross-pollination are called
out breeding devices.
▪ They are
1. Pollen release and stigma receptivity is not synchronized (uniform)
2. Anther and stigma are placed at different positions.
3. Self incompatibility (genetic mechanism to prevent self pollination).
4. Production of unisexual flowers.

*POLLEN-PISTIL INTERACTION:
▪ All the events from pollen deposition on the stigma until pollen tube enters the ovule are
together called pollen-pistil interaction.
▪ There is no guarantee for the transfer of right type of pollen grain to the right type of
stigma during pollination.
▪ The pistil has the ability to recognize the compatible or incompatible pollens.
▪ If it is right type, the stigma allows pollens to germinate.
▪ If it is wrong type the stigma rejects pollen by preventing germination.
▪ Recognition of pollen by pistil is influenced by the chemicals like Boron, Inositol and
sucrose level.
Compatible pollination is followed by the germination of pollen grain to produce pollen
tube through the germ pore.
▪ The content of the pollen grain move into the pollen tube.
▪ Pollen tube grows through the tissues of the stigma and style and reaches the ovary.
▪ If the pollen grain is in 2- celled stage the generative cell divides and forms two male
gametes inside the pollen tube.
▪ If the pollen grain is in 3- cell stage the pollen tube carry two male gametes from the
beginning.
▪ Pollen tube enters into the ovule through micropyle and then into the embryo sac through
synergids guided by filiform apparatus.
ARTIFICIAL HYBRIDISATION
▪ It is an important approach of crop improvement programme.
▪ Artificial hybridization is achieved by emasculation and bagging.
*Emasculation:
▪ Removal of anthers from a bisexual flower bud before dehiscence of anther is called
emasculation. It is done by using a pair of forceps.
▪ It is to prevent self pollination.
▪ In unisexual female flower there is no need of emasculation
*Bagging:
▪ It is the process of covering emasculated flower by polythene bag / butter paper.
▪ It is to prevent contamination of stigma of emasculated flower with unwanted pollen.
▪ Small pores should be made in the polythene bag for aeration.
*Crossing (Hand pollination):
▪ It is the transfer of desired pollen grains onto the stigma of emasculated flower.
▪ After crossing the flower is
*re-bagged and allowed for seeds and fruit setting.
 Double fertilization: Entry of pollen tube into one of the synergid and
Fertilization in angiosperms is called double fertilization because it involves two types of
fusion namely syngamy and triple fusion
▪ The pollen tube enters into one of the synergids and releases both the male gametes into
it.
▪ Double fertilization in angiosperms involves 2 processes namely syngamy & triple
fusion.
▪ Syngamy - It is the fusion of one of the male gamete with the egg to form a diploid zygote
It is the first fertilization.
▪ Triple fusion - It is the fusion of second male gamete with the diploid secondary nucleus
to form a triploid nucleus called Primary Endosperm Nucleus (PEN). It involves the fusion of
three haploid nuclei (1 male gamete and two haploid polar nuclei).
(Endosperm development precedes embryo development )

.
FERTILIZED EMBRYO
SAC
* POST- FERTILIZATION: STRUCTURE AND EVENTS
▪ The events of endosperm and embryo development, maturation of ovule into seed and
ovary into fruit are collectively termed as post-fertilization events.
Endosperm:
▪ Endosperm is a nutritive tissue which gives nutrition to the developing embryo.
▪ The development of endosperm takes place before the development of embryo.
▪ The PEN divides repeatedly and form endosperm.
▪ The cells of endosperm are filled with reserve food materials and used for nutrition of
embryo.
▪ The endosperm in angiosperms is triploid and post-fertilized product where as in
gymnosperms it is haploid and pre-fertilized product.
▪ The PEN undergoes successive nuclear divisions and forms a free nuclear condition. This
stage of endosperm development is called free-nuclear endosperm. Ex. The coconut
water
▪ Subsequently cell wall formation takes place and the endosperm becomes cellular.Ex.
white kernel in coconut
.The endosperm is utilized completely by the developing embryo before the maturation of
the seed or it may persist in the mature seed and used up during seed germination.
Embryo:
▪ After fertilization the zygote develops into embryo.
▪ The zygote is formed at the micropylar end of the embryo sac.
▪ Development of zygote starts only after the formation of some amount of
endosperm. ▪ The early stages of embryo development from zygote are called
embryogeny
▪ The early stages of embryo development are similar in both monocots and dicots.
▪ The zygote develops into pro-embryo. Then the pro-embryo develops into
1) Globular stage 2) Heart shaped 3) Matured embryo
 Dicot embryo:
▪ A typical dicotyledonous embryo consists of an embryonal axis and two cotyledons.
▪ Embryonal axis above the cotyledon is called epicotyls.
▪ Terminal part of the epicotyl is the plumule or stem tip (gives rise to the shoot).
▪ Embryonal axis below the cotyledon is called hypocotyl.
▪ The terminal part of the hypocotyl is called the radicle or root tip (gives rise to roots).
▪ The root tip is covered by root cap.

Monocot embryo:
▪ The monocot embryo consists of only one cotyledon
▪ The single cotyledon in grass family is called
scutellum.
▪ Scutellum is situated towards one side of the embryonal axis.
▪ The radicle and root cap are enclosed by a sheath called coleorhiza.
▪ The portion of the embryonal axis above the level of scutellum is called epicotyl. ▪ Epicotyl has
plumule which is covered by a membrane called coleoptile.
 SEED
▪ The seed is the final product of sexual reproduction. (fertilization)
▪ Seed is the fertilized ovule produced inside the fruit.
▪ Seed consists of seed coat, cotyledon and an embryonal axis.
▪ The seed coat has two layers called outer testa and inner tegmen.

Types of seeds: The mature seeds are of two types namely

1. Non-albuminous seeds: The mature seed without endosperm are called non-
albuminous seeds. They are also called exalbuminous seeds.
Ex: Ground nut, Pea. Beans

Albuminous seeds: The mature seed in which some part of endosperm is retained are called
albuminous seeds. They are also called endospermic seeds. Ex: Coconut, Castor, Maize,
Wheat, Barley, Sunflower.

*Perisperm
▪ The residual, persistent nucellus in the mature seed is called perisperm.
Ex: Black peeper, Beet.
*changes during the development of seed
▪ Integuments of ovules develop into protective seed coats.
▪ Micropyle remains as a small pore in the seed coat. It helps in the entry of oxygen and
water into the seed during germination.
▪ At maturity water content of the seed is reduced and seeds become relatively dry.
▪ General metabolic activity of the embryo slows down.
▪ The embryo enters into a state of inactivity called dormancy.
▪ If favorable conditions are available (adequate moisture, oxygen, suitable temperature)
the seed germinates.
 *FRUIT
▪ The transformation of ovules into seeds and ovary into fruit occurs simultaneously
. ▪ The wall of the ovary develops into fruit wall. The fruit wall is called pericarp.
▪ Fruits are of two types namely based on pericarp
▪ Fleshy fruits - the pericarp remains fleshy at maturity.
Ex: Guava, Orange, Mango etc.
▪ Dry fruits - the pericarp becomes dry at maturity.
Ex: Groundnut, Mustard etc
▪ The ovary develops into fruit and other floral parts degenerates and fall off.
▪ The fruits may be true fruit or false fruit
a) True fruits: The fruit that develop only from ovary are called true fruits. Ex:
Mango, Guava Orange etc.
b) False fruits: The fruit developing from the ovary and also other parts of a
flower are called false fruits.
Ex: Apple, Cashew, Strawberry (Thalamus contributes to the formation fruit in
addition to ovary).

*Parthenocarpic fruits
▪ The fruits developed without fertilization are called parthenocarpic fruits (seedless
fruits).
▪ Development of fruits without fertilization is called parthenocarpy.
▪ Parthenocarpy can be induced by the use of growth hormones like auxins and gibberellins.
Ex: Banana, Grapes

Advantages of seeds in angiosperms:

▪ The food reserves of seeds provide nutrition to the young seedlings until they are capable
of preparing their own food.
▪ Seeds have better adaptive strategies for dispersal to new habitats.
▪ Hard seed coat gives protection to the young embryo.
▪ Seeds generate new genetic combinations leading to variations.
▪ Seed is the basis of our agriculture.
▪ Dehydration and dormancy of mature seeds helps in storage. This helps to use the seeds
as food throughout the year and also to raise crop in the next season.

Viability of seeds:
▪ Seeds of some species lose viability within few months.
▪ In majority of the species the seeds remain viable for many years.
▪ Oldest seed is that of a lupine, Lupinus arcticus excavated from Arctic Tundra. The seed
germinated and flowered after an estimated record of 10,000 years of dormancy.
▪ A recent record of 2000 years old viable seed is of the date palm, Phoenix dactylifera.
(Discovered during the archeological excavation at King Herod’s palace near the Dead Sea)
APOMIXIS:
▪ The development of seeds without fertilization is called apomixis.
▪ It is a type of asexual reproduction which mimics the sexual reproduction.
▪ It is very common in asteraceae and grasses.
▪ Generally diploid egg is formed without meiosis & develops into seed without
fertilization.
▪ Hybrid seeds increases the productivity but the production of hybrid seeds is costly.
▪ The hybrid seeds are made into apomicts. There is no segregation of characters in the
apomicts progeny. Hence the farmers use the apomictic hybrid seeds new crops year
after year and do not buy hybrid seeds every year.

POLYEMBRYONY:
▪ Occurrence of more than one embryo in a seed is called polyembryony.
Ex: Citrus,