Chapter – 2
Sexual Reproduction
in Flowering Plant
Pre-
Fertilisation Events-
Ø Several hormonal and
structural changes result in the development of a flower
Ø Inflorescences bear the
flower buds, and then the flowers.
Ø Flowers are the
reproductive parts of a plant.
Ø In the flowers, the androecium (male reproductive part) and the gynoecium (female reproductive part) develop.
Androecium
Ø The androecium consists
of whorls of stamen.
Ø The stamen consists of the filament (long and slender stalk) and anther (bilobed
structure).
Ø Filament is attached to
the thalamus or to the petal.
Ø Anther-
· A typical anther is
bilobed and each lobe is dithecous (consists of two theca).
· Theca are separated by
longitudinal groove running lengthwise.
flower
- It is the fascinating repood vetive organ angiosperms.
It consists of -
1) Calyx
2) Corolla
3) Androecium
4) gynoccium
* STAMEN –
Ø Anther - terminal bilobed structure
Ø filament - long
slendero Stalk, Prronimal end of the filament is attached to the petal.
Structure of Anther •
Ø Anther has two lobes (biloted) each lobe consists of the theca . Hence it is dithecous.
Ø Andher is a tetragonal
structure which consists of four microsporangia located two in each lobe. •
Ø Microsporangiom
develops into poller Sacs •
Ø Pollen sacs son
longitudinally and contain pollen grains.
Ø The microsporangia are
located at the corners, two in each theca. They
further develop to form pollen sacs, which contain the pollen grains.
•
Structure of microsporangium
Ø The microsporangium is
surrounded by four wall layers -(epidermis, endothecium, middle layers, and
tapetum).
Ø The outer three layers
are protective and help in dehiscence of anther to release the pollen grains.
The tapetum provides nourishment to the developing pollen grains.
Ø In the young anther,
the sporogenous tissue forms the centre of each
microsporangiu
Microsporogenesis -
· It is the process of
formation of microspore from PMC (Pollen
Mother
Cells)
· . As development occurs in the anther, the
sporogenous tissue undergoes meiosis
to form microspore tetrad.
· Each cell of
sporogenous tissue has capacity to give rise to a tetrad
.hence each cell is a potential pollen or PMC
· As the anther matures,
the microspores get detached from each other and develop
into pollen grains
Pollen
grain
· Represent the male
gamete and are spherical ,having a two – layered wall .
o
Exine
( outer) – hard layer made of sporopollenin, which is exteremly ,acidi and
alkaline conditions, and enzymes
o
Intine
(inner) - thin and continuous layer made
up of cellulose and pectin
· Mature pollen grain contains two cell.
o
vgetative
cell - Large with irregular nucleus, contains food reserves
o Generative cell - Small and floats in the cytoplasm of the vegetative cell
ü . In 60% of the
angiosperms, pollen grains are shed at
2-celled
stage while in others generative cell undergoes mitosis to form two male
gametes (3-celled stage).
ü The viability of pollen
grains after they are shed depends upon temperature and humidity. It ranges
from 30 minutes
to
few months.
Gynaecium and Formation of Female
Gametophyte
ü The gyneecium
represents the female reproductive part of a
flower.
ü It may be
mono-carpellary (one pistil) or multi-carpellary
(many
pistils). In multi-carpellary, the pistils may be fused
in
one (syncarpous) or free (apocarpous).
Each pistil consists of:
o . Stigma - Receives the
pollen grains
o Style -
Elongated, slender part below the stigma
o Ovary - Bulged basal part containing the
placenta,
which
is located inside the ovarian locule (cavity)
o The placenta contains
the megasporangia or ovules.
Megasporangium (Ovule)
ü The ovule is attached
to the placenta by the funicle. The junction of
the ovule and the funicle is called hilum.
ü Each ovule has one or
two protective layers, called integuments, which
cover the rest of the ovule, except for a small opening called micropyle.
ü The chalaza lying on the opposite side of the micropyle
end represents the basal part of the ovule.
ü Nucellus is present within the integuments and contains reserved food. The embryo sac or female gametophyte is located within the nucellus.
Megasporogenesis -
ü The megaspore mother cell (MMC) gets converted into
megaspores by the process of megasporogenesis.
ü The MMC is large and
contains a dense cytoplasm and a prominent nucleus. It undergoes meiosis to
produce four
megaspores.
Female
Gametophyte -
ü In most flowering
plants, only one megaspore is functional while the other three
degenerate. .
ü The single functional megaspore develops into
the female gametophyte. This kind of development is
called monosporic development.
ü The nucleus of the functional megaspore divides
mitotically to form 2 nuclei, which
move towards the opposite ends, forming a 2-nucleate embryo sac. Two more
mitotic divisions ensue, leading to the formation of 4-nucleate and 8-nucleate
embryo sacs.
ü After the 8-nucleate stage, the cell walls are
laid down and the typical female gametophyte (embryo sac) gets
Pollination •
ü It is the process of
transfer of pollen grains from the anther
to the
stigma .
ü Depending on the source
of pollen, pollination can be divided as follows
· Autogamy –
It is the transfer of pollen grains from the anther to the stigma of the same flower. Autogamy requires
the anther and the stigma to lie close. It also requires synchrony in the
pollen release and stigma receptivity.
Plants like Viola, Oxalis,
etc., produce two kinds of flowers-chasmogamous flowers (with exposed anther
and stigma) and cleistogamous flowers (which do not open at all and only
autogamy occurs).
· Geitonogamy –
It is the transfer of
pollens from the anther of one flower to
the stigma of another flower in the same plant. Genetically, it is similar to
autogamy, but it requires pollinating agents.
·
Xenogamy –
It is the transfer of pollen grains from the
anther to the stigma of a different plant. Pollination causes genetically
different types of pollens to be brought to a stigma.
·
Agents of Pollination
ü Plants use air, water
(abiotic agents) and animals (biotic agents) for
pollination.
ü Pollination by wind
o
It
is the most common form of abiotic pollination.
o
Plants
possess well-exposed stamens and large, feathery stigma.
o
Pollens
should be light and non-sticky to be carried
easily
by winds.
o
Wind-pollinated
flowers often have single ovule in the
ovary
and numerous flowers packed in an inflorescence.
o
It
is common in grass.
ü Pollination
by water
o It is
rare in flowering plants, except for some aquatic plants like Vallisneria
and Hydrilla,
o
In most water-pollinated plants, the pollen grains are long and ribbon-like, and are protected from wetting by mucilaginous covering.
o
In
a majority of water plants like water hyacinth and water lily, flowers emerge above the water level and are
pollinated by insects.
ü Pollination by animals
o
.
Majority of flowering plants use butterflies, bees,wasps
etc., for pollination.
o Most of the
insect-pollinated flowers are large, colourful, fragrant, and contain nectar to
attract the animal pollinators. These are called floral rewards.
o
Floral Feward can be in the form of providing safe places
to lay eggs (example: the tallest flower, Amorphophallus)
o
A
symbiotic relationship exists between the plant,
Yucca
and its pollinator moth. The moth is dependent on the plant since the moth
deposits its eggs in the locule of the ovary of the plant, and in return, the
plant is pollinated by the moth.
o
The
pollen grains are sticky and get stuck to the body
of
the pollinator.
Out
Breeding devices
· Repeated self
pollination leads to inbreeding depression
· Plants have developed
methods to prevent self pollination. Autogamy is prevented by following ways:
ü Pollen release and
stigma receptivity not coordinated
ü Different positioning
of the anther and the stigma
ü Production of unisexual
flowers
o
Ways
to prevent both autogamy and geitonogamy:
ü Presence of male and
female flowers on different
plants,
such that each plant is either male or female
(dioecy).
ü
This
mechanism is present in several species of papaya.
Pollen-Pistil Interactions
o
Pollination
does not always ensure the transfer of compatible Pollens.
o
Hence,
the pistil has the ability to recognise the right type of pollen to promote post- pollination events.
o
If the pollen is of the wrong type, the pistil
prevents pollen germination.
o
This interaction is mediated by chemical
components of the pollen and the pistil.
o
Pollen-pistil interaction is a dynamic process
involving
pollen recognition,
followed by promotion or inhibition of the pollen.
o
The
pollen tube reaches the ovary and enters the ovule through the micropyle. Then,
through the filiform apparatus, it reaches synergids.
In this way, the pollen tube grows.
ü Artificial
Hybridisation & Double Fertilisation
o
Artificial hybridisation-
o
it is the method t improve crop yield.
o
In this method, it is essential to ensure that the
right kinds of pollen grains are used , and the stigma is protected from
unwanted pollen grain
o
When the stigma of the bagged flower become receptive
the collected pollen graine are dusted into the stigma ,and then the flower is
rebagged.
o
If the female parent is unisexual , emasculation is
not necessary. In this case, the female bud is directly bagged and when the
stigma turns receptive , suitable pollen grains are dusted into it so as to
allow germination
o
Double Fertilisation
o
When
the pollen grains fall on the stigma, the pollen tube enters one of the synergids and
releases two male gametes.
o
One of the male
gametes moves towards the egg cell and fuses
with it to complete the syngamy to form the zygote.
o
The other male gamete fuses with the two polar
nuclei and forms triploid primary endosperm
nucleus (PEN). This is termed as triple fusion.
o
Since
two kinds of fusion-syngamy and triple fusion-take place, the process is known
as double fertilisation, and is characteristic of flowering plants.
o
After
triple fusion, the central cell becomes the primary
endosperm
cell (PEC).
ü The primary endosperm
nucleus gives rise to the
endosperm,
while the zygote develops into the embryo.
Post-Fertilisation
Events
It includes development of endosperm and
embryo, and maturation of ovules into seeds and ovaries into fruits.
Formation of Endosperm-
ü
The
endosperm develops before the embryo because the
cells
of the endosperm provide nutrition to the developing embryo.
ü
The
primary endosperm nucleus repeatedly divides to give losperm rise to free nuclei.
This stage of development is called free nuclear endosperm.
ü
Cell wall formation occurs next, resulting in
a cellular endospem.
Development
of Embryo -
ü The embryo develops at
the micropylar end of the embryo sac where the zygote is situated.
ü The zygote gives rise first to the pro-embryo, and
then to the globular, heart-shaped, mature embryo.
ü A typical dicot embryo
consists of an embryonal axis and two
cotyledons.
ü The portion of
the embryonal axis above the level of cotyledons is called epicotyl. It
contains the plumule (shoot tip). The portion below the axis is called
hypocotyl. It contains the radicle (root tip). The root tip is covered by the
root cap.
ü only In a monocot embryo, there is one cotyledon. In
ü grass, it is known as
the scutellum, and is situated at one side of the embryonal axis. At its lower
end, the embryonal axis has the radicle
and the root cap enclosed in the coleorrhiza.
ü The epicotyl lies above
the level of the scutellum, and has the shoot apex and leaf primordia enclosed
in hollow structures called coleoptile..
Development of Seeds-
· It is the last stage of
sexual reproduction in angiosperms.
· Seeds are the fertilised ovules that are developed inside a fruit.
· A seed consists of -
o
Seed
coat
o
Cotyledons
o
Embryonal
axis
· Seeds may be albuminous (endosperm present; as in wheat and maize) or non-albuminous
(endosperm absent. since it is consumed by the growing embryo; as in pea and beans).
· Some seeds such as
black pepper and wheat have remnants of nucellus known as perisperm
· The integuments of ovules harden to form the
seed coat, and the micropyle facilitates the entry of oxygen and water into the seed
· As it loses moisture, the seed may enter
dormancy, or if favourable conditions
exist, it germinates.
Development
of Fruits -
· The ovary of a flower
develops into a fruit.
· The walls of the ovary
transform into the walls of the fruit (pericarp).
· Fruits may be fleshy, as in mango and orange,
or can be dry, as in groundnut and mustard.
· In some plants, floral
parts other than the ovary take part in fruit formation, as in
apple and strawberry. In these, the thalamus contributes to fruit formation. Such
fruits are called false fruits (apple pear).
Fruits that develop from the ovary are called true
fruits ( tomato, grapes , banana)
· Some fruits develop
without fertilisation, and are known as parthenocarpic
fruits (example: banana).
Apomixis and Polyembryony
· Some plants produce
seeds without fertilisation. This process
of seed formation
is known as apomixes.
· Apomixis is a form of asexual reproduction
mimicking sexual reproduction.
· In some species,
apomixis occurs as the diploid egg cell is med without meiosis, and develops
into embryo without fertilisation.
· In some varieties of
citrus and mango, the nucellus cells divide and protrude into the embryo sac to
develop into embryos. In such cases, each ovule may contain several embryos and
this condition is called polyembryony.
· Apomixis is important for producing hybrid
varieties of fruits and vegetables, and also for increasing crop yield
multifold.
