Sexual reproduction in flowering plants

Sexual Reproduction in Flowering
Plants
Shubhadeep Bhattacharjee/ Eureka Classes

Hit Points
•Flower
•Male reproductive structures
•Formation of male gametes
•Female reproductive structures
•Formation of female gametes
•Pollination & its different types
•Fertilization
•Formation of zygote
•embryogenesis
• Formation of fruit & seeds

Flower
• Sexual reproductive parts of
angiosperms
• Develops post vegetative phase
Formation of flower
• Hormone induced structural changes
• Apical region of the shoots, and internodes become condensed- formation
of the floral primordia
• Leaves at the terminal end are modified into floral whorls- Calyx
• Inflorescences are formed within the floral primordia.

Parts of flower
• Thalamus
• Floral whorls
Calyx- Sepals
Corolla- Petals
Androecium- Stamens
Gynoecium- Carpels/ Pistils

Function of floral members
• Calyx
i. protect flowers in bud conditions
ii. When green- photosynthetic in nature
iii. When colored- flag apparatus for pollination
• Corolla
i. Color/ scent to attract pollinators
ii. Base contains nectar glands
iii. Helps in protection

• Androecium
i. production of microspores
• Gynoecium
i. production of megaspores,
fruits and seeds

The male reproductive unit: Stamen
• Microsporophyll
• Consists of two parts: Filament and anther

• Types of stamen arrangements
i. Circular
ii. Whorled
iii. Didymous
iv. Didynamous
v. Tetradynamous

• Filament
long and slender stalk
proximally attached to thalamus/ petals/ tepals
distally bears the anther
• Anther
fertile part of the stamen
separated into lobes
based on the no. of lobes: monothecous/ dithecous

• Anatomy of the anther
anther lobes separated anteriorly by deep groove
anther lobes separated posteriorly by sterile connective tissue
each anther lobe bifurcated into one/ two theca
the theca is a hollow cavity
four pollen sacs- four corners of the anther
Microsporangium

• Microsporangium/ pollen sacs
covered by well defined common epidermis of the anther
• Development
develop hypodermally
archesporial cells-- parietal cells -- sporogenous cells

------------------------------------------------------------
-------

• Components of the microsporangial wall
derived from the parietal cells
outer layer: endothecium
1-3 middle layers
1layer tapetum

• Endothecium
develop fibrous thickenings
consists primarily of dead cells
Stomium
• Middle layers
degenerate to provide nourishment to the
MMCs
• Tapetum
bi-nucleated
nourishment of the MMCs
secretion of enzymes and hormones
Ubisch granules
pollen kitt
compatibility proteins

• Sporogenous cells
fill the whole inner cavity of the theca
increase in no. as anther grows

• Microsporogenesis
 sporogenous cells matures into
Microspore Mother Cells (MMCs)
 Callose formation
 separated MMCs undergo
haploid division
 formation of the
Tetrads of Microspores

• Pollen grains/ Male gametophyte
Oval/ ellipsoidal/ triangular/ lobed/ crescent shaped
10-200 uM
SPORODERM- outer covering of a pollen grain
OUTER EXINE
INNER INTINE

• Intine
-Pectocellulosic in nature
-Enzymatic protein deposits
• Exine
- Highly resistance fatty
substance- sporopollenin
- Proteins for compatibility
reactions
- outer “extexine”/ sexine
- Inner “ëndexine”/ nexine

- Extexine is further divided into three layers
- inner continuous foot layer
- middle baculate layer
- outer tectum
- Pollenkitt
- Germ pores/ germinal furrows

• Development of the male gametophyte
• Precocius development
• Vacuolation
• nucleus grows in size
• Shifts near the cell wall
• Protoplast divides into
Smaller generative cell
Larger vegetative cell/ Tube cell
Pre-Pollination Development Post-Pollination Development
Visit slide#64

https://www.youtube.com/watch?v=tYj-QYDM6Vw

The Female Reproductive Unit: Pistil/ Carpel
• Gynoecium
• Free unit of gynoecium: Pistil
• Pistil has three parts-
Stigma
Style
Ovary

• Stigma
terminal receptive part of the pistil
landing platform for the pollen grains
• Style
hollow tube
connects the stigma with the ovary
• Ovary
basal bulged part
contains the ovules

Ovarian cavity/ Locules
Placenta
From the placenta megasporangia
arises- Ovules
No. of ovules 1: Wheat/ Mango/ Paddy
No. of ovules >1: Papaya/ Orchids

• Segregation of flowering plants based on the Carpels
Type Specification Example
Monocarpellary One carpel Mango
Bicarpellary Two carpels Sunflower
Tricarpellary Three Carpels Lillies
Multicarpellary More than three carpels Rose
Apocarpous Free carpels Michelia
Syncarpous Fused carpels Hibiscus

• Structure of the ovule
Ovarian chambers- locules
Integumented megasporangium
Attachments:
Placenta
Funiculus
Hilum
Raphe

Body:
Nucellus
Integuments
Micropyle
Chalaza
Female gametophyte or the embryo sac is located in the micropylar
end of the nucellus.

• Types of ovules (based on orientation of the body of the ovule)

• Ovule Development
Ovule develops as primordium over placenta
Ovular primordium  Nucellus
Integuments develop from the nucellar base- Chalazar end
Integuments cover the whole of nucellus except the micropyle
Hypodermal region of the nucellus Micropyler end  Primary
Archesporial cell
Archesporial cell outer parietal cell & Inner Sporogenous cell
Sporogenous cell Megaspore Mother cell/ Megasporocyte
Megasporocyte 4 haploid megaspores (Megasporogenesis)

• Development of the embryo sac
- 4 megaspores obtained post-meiosis of the Megaspore Mother
Cell
- Only the Chalazal megaspore remains viable
- Functional megaspore enlarges
- Nucleus undergoes three mitotic divisions 8 nuclei generated
- Megaspore cytoplasm becomes vacuolated
(…. Continued)

(….Continued)
-8 Nuclei Segregated into three groups
I. 3 celled Micropylar end  Egg Apparatus
II. 2 celled  Central  Polar nuclei
III. 3 Celled  Chalazal end  Antipodal cells

• Anatomy of the embryo sac
- 8 nuclei  7 cells  3 micropylar+ 1 central + 3 chalazal
3 Micropylar cells
- Egg Apparatus
- Arranged in a triangular fashion
- One of the cells is larger Egg/ Oosphere
- Peculiar wall thickenings Filliform Apparatus towards the
embryo sac membrane
- Central Vacuole- Micropylar end
- Nucleus- Chalazal end

- Remaining two cells  Synergids/ help cells
- Filliform apparatus in the micropylar region
- Lateral Hook
- Chalazal vacuole
- Central nucleus
- The egg/ oosphere – Single female gamete of the
embryo sac
- Synergids help in nourishment & guiding the
in pollen tube to the egg post fertilization

3 Chalazal Cells Antipodal cells
- Vegetative cell of the embryo sac
- Degenerates soon after formation
- Sources nutrition from the surrounding nucellar cells
Central Cell
- 2 polar nuclei
- Highly vacuolated
- 2 polar nuclei may fuse together Secondary fusion nucleus
** All the cells of the embryo sac are haploid except for the central cell which becomes
diploid post fusion of the two polar nuclei

Pollination
• Transfer of pollen grains from the anther to the stigma
Pollination
Self
pollination
Cross
pollination

• Self pollination
- Transfer of pollen from the anther of one flower to the stigma
of the same flower or a genetically similar flower.
Autogamy
- It is the type of pollination where a flower is pollinated by its
own pollen grains

• Autogamy is of three types:
1. Homogamy.
- chasmogamous flowers
- Anthers and stigma
brought together by growth,
bending or folding.
E.g., Catharantus, Mirabilis, Potato

2. Cleistogamy
- Cleistogamous/ Chasmogamous flowers
- Flowers remain closed during pollination
E.g., Commelina, Balsam, Oxalis and Viola

3. Bud pollination
- Anthers and stigmas ripen before opening of the floral buds
- Self pollination happens as a rule
e.g., Pea, Wheat and rice

Geitonogamy
- Pollen from one flower are transferred to the stigma of another
flower belonging to the same plant or some other genetically
similar plant

ADVANTAGES OF SELF POLLINATION???
DISADVANTAGES OF SELF
POLLINATION???

• Cross Pollination/ Xenogamy
- Transfer of pollen from the anther of one flower to the stigma
of a genetically different flower
- Requirement of an external agency
- External agency can be biotic: Insects/ birds/ snails
External agency can be abiotic: wind/ water
- CP is named after the agency that assists it
** Allogamy refers to pollinations where two separate flowers are involved (either
genetically similar or different). As a result both Geitonogamy and Xenogamy fall
under this category.

1. Pollination by wind (Anemophily)
- Pollen grains are light in weight, non-sticky, dry and winged
- Well-exposed stamens for easy dispersal of the pollen grains in
the wind
- The stigma is sticky, large and have appendages to tap pollen
- Numerous flowers (florets) are packed together to form
infloroscence
e.g., Maize, Grass

2. Pollination by water (hydrophily)
- Commonly observed in algae, bryophytes, pteridophytes and
some angiosperms
- Vallisneria and Hydrilla- submerged freshwater plants
- Zostera- marine angiosperm
- in Vallisneria, female flower stalk is coiled to reach the water surface
to catch pollen carried by the water currents
- Pollen grains are long ribbon like structures to be carried by water
current
- Pollen grains are protected from wetting because by mucilaginous
covering

3. Pollination by animals (Zoophily)
- Pollination through the agency of animals
- Most common zoophilic agents: Insects
- Birds/ Bats/ Snails/ Humans
- Primates and other arboreal rodents
- Flowers have adaptations to facilitate pollinations
by animals
E.g., Asteraceae and Lamiaceae

Adaptations in flowering plants to facilitate cross-pollination
i. Turn-pipe/ Lever-mechanism
E.g., Salvia
-

ii. Mutual dependency
E.g., Fig and wasp Blastophaga

iii. Pseudocopulation/ sexual deceit
- Ophrys orchid and Colpa moth

Outbreeding devices to ensure cross-pollination
Out-breeding device Modification Example
Dicliny • Unisexual flowers.
• Homogamy not possible.
• The plants can however be both
mono- or dioecious.
Mulberry (dioecious)
Papaya (dioecious)
Maize (monoecious)
Dichogamy • Anther and Stigma mature at
different times.
• Happens in bisexual flowers.
• Protoandry: Anther matures
earlier
• Protogyny: Stigmas mature
earlier.
Sunflower/ Salvia (Protoandry)
Mirabilis/ Gloriosa/ Plantago
(Protogyny)

Outbreeding Device Modification Example
Pre-potency Pollen grains of other flower
germinates more rapidly than self
pollens.
Apple
Grape
Self-sterility Pollen grains of a flower do not
germinate on the stigma of the
same flower.
Tobacco
Potato
Crucifers
Heterostyly Styles and stamen are variantly
tall/short.
Jasmine
Primrose
Lythrum
Herkogamy Mechanical device to prevent self
pollination
Pansy
Calotropis

Pollen-Pistil Interaction
Pollen grains of a no. of plants settle over a stigma.
But only the right pollen belonging to the same species would germinate
whereas, the others fail to do so.
Compatibility and incompatibility of the pollen-pistil is determined by set of
special proteins called as the compatibility complex.
Compatible proteins are able to absorb water and nutrients from the surface of
the stigma.
They germinate and produce pollen tubes.
Pollen tubes grow into the style.
The growth and path through the style is also determined by another set of
specialized chemotropic response.

Post-pollination development of the male
gametophyte
Vegetative cell/ Tube cell
• On the stigma the pollen grains absorb water and nutrients through
its germ pores.
• The vegetative cell enlarges.
• It comes out from one of the germ pore/ germinal furrow and is
called as the Pollen tube.
• The pollen tube is covered by the intine.
• It secretes pectinases and other hydrolytic enzymes to create a
passage for it in the style.
• The tube nucleus descends to the tip of the pollen tube.

Generative cell
• The generative cell passes down the pollen tube.
• It divides into two male gametes.
• The male gametes are lenticular/ spherical in outline.
• The male gametes have a very large nucleus, surrounded by a very
thin layer of cytoplasm.
• The tube nucleus degenerates completely.

Structural anatomy of the pollen tube
• Dense cytoplasm towards the tip of the pollen tube.
• Two male gametes.
• One degenerated tube nucleus.
• Behind the tip the cytoplasm becomes highly vacuolated.
• Plugs of callose- separates the older parts.

Fertilization
• Fusion of male and female gametes is called as fertilization.
• In angiosperms, the male gametes are brought to the egg containing
female gametophyte- Siphonogamy
• Only the pollen tube passes down the stigma, the pollen grain
remains atop the stigma.
• Pollen tubes travel intercellularly secreting pectinases and other
hydrolytic enzymes
• Boron-inositol sugar complex- Pollen tube guiding system.

• Once the pollen tube has reached the ovary, its growth is directed by
the obturator.
• The pollen tube enters the ovule, either through
micropyle (Porogamy, e.g., Lily)
Chalaza (Chalazogamy, e.g., Casuarina)
Integuments (Mesogamy, e.g., Cucur-bita)

• Synergids secrete chemicals which guides the pollen tube towards the
micropylar end of the embryo sac.
• Pollen tube pierces one of the synergids.
1 FERTILIZATION
• One male gamete fuses with the egg/ oosphere- Generative
fertilization/ Syngamy
• Diploid Zygote/ oospore (2n)
• Vacuole+ Plasmodesmal degeneration
• Zygote Embryo

2 FERTILIZATION
• The nucleus of the second male gamete (n) fuses with the Fusion
nucleus (2n)
• Triploid primary endosperm nucleus (3n)
• Central Cell Primary Endosperm Cell (PEC)
• Vegetative fertilization Triple fusion

Post-Fertilization events
• External sex apparatus of the flower deteriorates
• Endosperm formation
• Embryo formation
• Fruit formation
• Seed formation

1. Endosperm formation
Depending upon the mode of its formation, angiosperm
endosperm is of three types:
1.1. Nuclear Endosperm
1.2. Cellular Endosperm
1.3. Helobial Endosperm

1.1. Nuclear Endosperm

1.2. Cellular Endosperm

1.3. Helobial Endosperm

2. Embryogeny/ Embryo formation
Sum total of changes that occur during
the development of a mature embryo
from the zygote.
2.1. Two celled Pro-Embryo
The zygote elongates and undergoes transverse division forming a large
basal cell and a small apical or terminal cell.
The basal cell is called Suspensor cell - Micropylar end
The smaller terminal cell is called Embryo cell  Antipodal end

2.2. Formation of Suspensor and embryo octant
• The suspensor divides transversely forming the
6-10 celled filamentous suspensor.
• The terminal cell of the suspensor towards the
micropylar end is called as HAUSTORIUM.
• The terminal cell of the suspensor towards the
chalazal end is called HYPOPHYSIS.

• The embryo cell undergoes two vertical
divisions and one transverse division to
form eight cells- Embryo octant
• These eight cells are arranged in two layers:
Epibasal- Terminal end Gives rise to
cotyledons and plumule
Hypobasal- Near the suspensor Gives
rise to hypocotyl

2.3. Embryonic Dermatogen- Globular stage
• The eight embryonic cells divide periclinally
to produce an outer layer of protoderm or
dermatogen.
• The inner cells differentiate to form the
procambium and ground meristems.

2.4. Heart shaped stage
• Development of the Plumule from the epibasal cells
• Development of the Cotyledons from the epibasal cells.
• Development of the Radicle from the hypophysis.
• Two cotyledons differentiate from the sides with the
plumule in the center.

2.5. The mature dicot embryo
• The cotyledons elongate
• The plumule remain undifferentiated

Post Embryogenic modifications
• The ovule enlarges
• Integuments harden
• Embryo undergoes rest
• Ovule gets transformed into the
seed

3. Seed formation
• Seed: Fertilized & mature ovule
• A typical seed consists of
1-2 seed coats
1-2 cotyledons
Embryonal axis
• The integuments of the ovule develop into seed coat
• The outer integument develop into the outer seed coat- Testa
• Inner integument develop into the inner seed coat- Tegmen.

• Seed coat has a small opening in the micropylar end to allow the
passage to water and oxygen during seed germination
• Hilum persists as a scar on the seed coat.

• Endospermic/ albuminous seeds:
In some seeds, the endosperm persists in the seed as a food
storage tissue. Such seeds are called endospermic seeds.
Eg., Castor, Maize, wheat, barley, sunflower.

• Nonendospermic/ exalbuminous seeds:
In such seeds the endosperm is completely eaten up by the
growing embryo. The food for development is stored in the cotyledons.
Such seeds are characterized by huge cotyledons.
E.g., Pea, Gram, Bean, Ground-nut.

• Perisperm:
In some seeds, the nucellus persists. The remaining of the
nucellus which persists in the seed is called Perisperm.

The state of dormancy:
* Embryo fully mature
* Growth inhibitors
* Abscission of funiculus
* Hardening of the integuments
* The moisture content of the seed reaches 10- 15%

4. Fruit formation
• The fruit wall is derived from the wall of the developing ovary and is
called the Pericarp
• In some fruits, along with the ovary the thalamus or other floral parts
also develop into the fruit. These are called as false fruits.
E.g., Apple, Strawberry and cashew
• Fruits in which only the ovary matures into the fruit are called as true
fruits.
E.g., Mango

Development of fruit
Role of Pollination:
1. Pollination is essential for fertilization and hence development of
the seeds and fruits.
2. It prevents abscission or dropping of the ovary
3. Pollen grain contains small amounts of growth hormones. Auxin
allows growth of the ovary along with the pollen tube.
4. Pollen tube stimulate auxin synthesis
5. As soon as fertilization is over, the fertilized ovules start producing
various types of growth hormones

Role of the developing seeds:
1. Developing seeds can produce all three types of growth promoting
hormones- Auxin/ Gibberellins/ Cytokinins
2. These hormones help in overall development of the fruit.

Parthenocarpy
• Development of seedless fruits.
Vegetative parthenocarpy:
Seedless fruit can develop without the stimulus of pollination.
E.g., Pear and Fig.
Simulative parthenocarpy:
Stimulus of pollination is required without the actual process of
fertilization or seed formation.
E.g., Grapes

Apomixis and Polyembryony
• Apomixis:
Is a mode of asexual reproduction that mimics sexual reproduction
but, produces seeds without fertilization.
E.g., Asteraceae and grasses.

• Polyembryony:
*Formation of multiple embryos.
*One embryo develops from the destined
egg cell + sperm cell fertilization
*The other embryo is developed from
diploid tissues such as integuments or
the nucellus.

Sexual reproduction in flowering plants

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