CBSE Class 12 Biology

Revision Notes
CHAPTER-02
SEXUAL REPRODUCTION IN FLOWERING PLANTS

Reproduction ensures continuity of species

generation after generations as the older individuals
undergo senescence and die. Flowering plants shows
sexual mode of reproduction and bears complex
reproductive units as male and female reproductive
units along with accessary structures.

Flower is a modified stem which functions as a

reproductive organ and produces ova and/or pollen. A
typical angiospermic flower consists of four whorls
of floral appendages attached on the receptacle:
calyx, corolla, androecium (male reproductive organ
consisting of stamens) and gynoecium (composed of
ovary, style and stigma) .

Pre-fertilisation: Structures and Events

• Several structural and hormonal changes lead to
formation and development of the floral primordium.
Inflorescence is formed that bears floral buds and
then flower.

• In flowers, male (androecium) and female (gynoecium)

differentiate and develops in which male and female
gametes are produced.
Stamen, Microsporangium and Pollen Grain :
• Stamen consists of long and slender stalk called
filament and generally bilobed anthers. Each lobe
contains two theca (dithecious).

• The anther is four-sided structure consisting of four

microsporangia, two in each lobes.

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• Microsporangia develop further and become

pollen sacs which contain pollen grains.

• Microsporangium is generally surrounded by four

layered walls- the epidermis, endothecium, middle
layer and tapetum. Innermost layer tapetum
nourishes the developing pollen grains.

• Sporogenous tissues- It is compactly arranged

homogenous cells which are present at centre of
each microsporangium when the anther is young..

Microsporogenesis- The process of the formation and

differentiation of microspores (pollen grains) from
microspore mother cells (MMC) by reductional division
is called microsporogenesis.
• The cells of sporogenous tissues undergo meiotic
division to form microspore tetrad. As the anther
mature and dehydrate, the microspore dissociate and
develops into pollen grains.

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Pollen grain represents the male gametophytes. Pollen

grains are made of 2 layered Wall,

1. Exine :- Made of sporopollenin- most resistant organic

matter known.It can withstand high temperatures and
strong acids and alkali. No enzyme can degrade
sporopollenin

2. Intine :-
-Thin and continuous layer
- Made of cellulose and pectin

3. Germ pores
- apertures on exine where sporopollenin is absent
- forms pollen tube.

4. A plasma membrane surrounds cytoplasm of pollen

grain.

MATURE POLLEN
— A mature pollen consist of 2 cells with nucleus
(Vegetative and Generative)

VEGETATIVE CELL

Bigger
Abundant food reserve
Large irregular nucleus
Responsible for the development of pollen grain

GENERATIVE CELL

Small
Involves in syngamy (fuse with an egg)

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Dense cytoplasm and nucleus

Pollen grains of many species e.g Parthenium

cause severe allergies and bronchial diseases in
some people and leads to chronic respiratory
disorders– asthma, bronchitis, etc.

• Pollen grains are rich in nutrients and are used as

pollen tablets as food supplements.

• Viability of pollen grain varies with species to

species and should land on stigma before this period to
germinate. Pollen grains of large number of species
0
are stored in liquid nitrogen at temperature – 196 ,

called pollen bank.

The Pistil, Megasporangium (Ovule) and Embryo sac

• Gynoecium may consists of single pistil
(monocarpellary) or more than one pistil
(polycarpellary) which may be fused
(syncarpous) or free (apocarpous).

e.g Multicarpellary and syncarpous pistil- Papaver

Multicarpellary and apocarpous pistil- Michelia

• Each pistil has three parts the stigma, style and

ovary. Inside the ovary is ovarian cavity (locule). The
placenta is located inside the ovarian cavity.
Megasporangia (ovules) arise from placenta.

Megasporangium (ovule)
Ovule is a small structure attached to placenta.
Funicle – stalk by which ovule is attached to
placenta
Hilum- junction between ovule and funicle
Integuments- protective envelops
Micropyle- small opening at the tip of ovule into
where pollen tube enters

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Chalaza- basal part of ovule
Nucellus (2n)-mass of cells enclosed in

integuments. Has abundant food reserve.

Megasporogenesis- The process of formation of
megaspore from megaspore mother cell by meiotic
division is known as megasporogenesis. This process
takes place in ovule

Ovule differentiates a single megaspore mother

cell (MMC) in the micropylar region of nucellus. MMC
undergoes meiotic division that results into the
production of four megaspores.

• In most of the flowering plants three megaspores

degenerate. 1megaspore develops into female
gametophyte (embryo sac).

• The nucleus of functional megaspore divides

mitotically to form two nuclei which move to opposite
poles to form 2-nucleate embryo sac. Two more
sequential mitotic division results into 8-nucleate
embryo sac.

• Six of the eight nuclei surrounded by cell wall and

remaining two nuclei (polar nuclei) are situated below
the egg apparatus.

• Three cells are grouped at micropylar end to

constitute egg apparatus and three cells at chalazal
end forms antipodal cells. At maturity ,embryosac is
8-nucleate and 7 celled.

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Pollination – transfer of pollen grains from anther to

stigma.

a) Autogamy
- transfer of pollen grain from anther to
stigma of same flower.

i. Cleistogamous – flower which do not open.

cleistogamous flowers are 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. e.g Viola (common
pansy), Oxalis, and Commelina.

ii. Chasmogamous- exposed anther and stigma.

b) Geitonogamy – transfer of pollen grains from anther
to stigma of different flower of same plant.
Geitonogamy is functionally cross-pollination
involving a pollinating agent, genetically it is similar
to autogamy since the pollen grains come from the
same plant

c) Xenogamy- transfer of pollen grain from anther to

stigma of different plant’s flower of same species.

Agents of pollination includes abiotic (water, wind) and

biotic (insects, butterfly, honey bee etc. large number
of pollen grains are produced by plants using abiotic
mode of pollination as most of pollen grains are
wasted during transfer.

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Adaptations in flowers for
Pollination Ex : Fresh water plants-
Vallisneria, Hydrilla
I. Wind Pollination Sea grass- Zostera

pollen grains :– light, non-

sticky, winged
Main features of wind and
anther :- well exposed
water pollinated plants -
produce pollen grains in
stigma :- large and
feathery large no.
flower :- one ovule, - do not produce nectar
arranged as inflorescence

: ,
Ex corn cob cotton, date
III. Insect Pollination
palm - Flowers : large, colourful,
fragrant, rich in nectar -
II.Water Pollination Pollen grains : sticky
- Bryophytes, - Stigma : sticky
Pteridophytes, Algae
pollen grains : protected Certain rewards to
by mucilaginous covering pollinators:
nectar and (edible) pollen
grains as foods
provide safe place for
laying eggs

Ex : Amorphophallus, Yucca

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Outbreeding Devices- the various mechanisms take
discourage self-pollination and encourage cross
pollination as continued self-pollination leads to
inbreeding depression. It includes

• Pollen release and stigma receptivity not

synchronized.

• Anther and stigma are placed at different position.

• Inhibiting pollen germination in pistil.

• Production of unisexual flowers.

Pollen pistil interaction – the pistil has ability to

recognize the compatible pollen to initiate post
pollination events that leads to fertilisation. Pollen
grain produce pollen tube through germ pores to
facilitate transfer of male gametes to embryo sac.

Artificial Hybridization
Crossing diff varieties of species- hybrid
individual- with desirable characters of the
parent plants
desired pollen grains for pollination- stigma
protected from contamination
Emasculation : removal of anther
Bagging : flower covered- bag made up of
 butter-prevent contamination of stigma from
unwanted pollen

Bagged flower- attains receptivity- mature pollen

grains- dusted on the stigma – rebagged fruits
allowed to develop

Double Fertilisation- after entering the one of the

synergids, each pollen grain releases two

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male gametes. One male gametes fuse with egg
Syngamy) and other male gametes fuse with two polar
(
nuclei (triple fusion) to produce triploid primary
endosperm nucleus (PEN). Since two types of fusion
takes place in an embryo sac the phenomenon is called
double fertilisation. The PEN develops into the
endosperm and zygote develops into embryo.

Post fertilisation events include endosperm and

embryo development, maturation of ovules into
seeds and ovary into fruits.

Endosperm- the primary endosperm cell divides

many time to forms triploid endosperm tissue
having reserve food materials.

Two types of endosperm development :

(i) Free nuclear type (common method)
(ii) Cellular type

(a) Non-albuminous- endosperm completely utilized-

before maturation of seeds. e.g pea, groundnut

(b) Albuminous- a portion of endosperm remain in

mature seeds. e.g wheat, maize, castor

Embryo- Embryo develops at the micropylar end of

the embryo sac where the zygote is located.

Embryogeny – early stages of embryo development

.The zygote gives rise to the proembryo and
subsequently to the globular, heart-shaped and
mature embryo.

Embryo consists of:

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- embryonal axis
- cotyledons
- plumule
- radicle

Monocotyledonous Seed
- Scutellem = Cotyledon
- Coleorrhiza:
undifferentiated sheath
covering radical & root cap

- Coleoptile: sheath
covering plumule

Seed
- Fertilized and mature
ovule develops into seed.
Seed consists of:
- cotyledon(s)
- embryonal axis
- Seed coat- double layered-
formed by integuments

Testa (outer coat)

Tegmen (inner coat)
- Micropyle:- small opening
on seed coat, it facilitates
entry of H2O & O2 into seeds
(for germination)

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- Hilum:- scar on seed coat
- Seed - Albuminous / Non-Albuminous
- Perisperm : remnants of nucellus that is
persistent. Ex: Black pepper - Dormancy:
state of inactivity

• The wall of ovary develops into wall of fruit called

pericarp. In true fruits only ovary contributes in
fruit formation by in false fruit thalamus also
contributes in fruit formation.

Apomixis
- Form of asexual reproduction- mimics sexual
reproduction- seed formed without fertilisation
- Formation of apomictic seeds :
• diploid cell (formed without meiosis)- develop into
embryo without fertilization • cells of nucellus (2n)
surrounding embryo sac- protrude into embryo sac-
develop into embryos. Ex. Citrus and Mango.

Polyembryony
- Occurrence of more than one embryo in a seed
- Often associated with apomixes. Ex: Citrus, groundnut
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