Transcript SEXUAL REPRODUCTION IN FLOWERING PLANTS

Flower: A Fascinating organ of Angiosperms
Flower has myriads of forms, scents, perfumes, nectar and
colours.
Since times immemorial, flower has been object of admiration,
aesthetic, ornamental, social, religious and cultural value. It has
been as a symbol of conveying human feeling of love, affection,
happiness grief an mounring etc.
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Sexual reproduction is the process of formation and
fusion of haploid gametes resulting in the production
of a diploid zygote that grows to produce a new
individual or offspring. Flower has two types of sex
organs, male stamens and female carpels or pistils.
Meiosis occurs resulting in formation of
microspores or pollen grains in another part of
stamens and megaspores in ovule part of carpel. The
phenomenon is called sporogenesis.
Pre fertilization: Structure and Events.
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A number of hormonal, meristematic and structure
transformations occur prior to initiation of flowering. Shoot
apical meritstem is transformed into reproductive meristum.
It grows to form inflorescence axis over which floral
primordial develop. They grows into floral buds and then
flowers. In the flowers differentiate stamens and carpels(=
pistils).
Stamen, Microsporangium and Pollen grain.
Male reproductive organs of a flower are stamens. They are
collectively called ANDROECIUM. The number, length
and form of stamens is specific for various flowers. Each
stamen has two parts, filament and anther.
FILAMENT: It is sterile, long thread like structure or stalk.
At its proximal end, it is attached to thalamus or petals (in
epipetalous form).
ANTHER: It is knob like, terminal, fertile part of a stamen
which is attached various at the tip of filament.
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A typical angiosperm anther is bilobed with each lobe having two
theca i.e. they are dithecous. Often, a longitudinal groove runs length
wise separating the theca. The bilobed nature of an anther is very
distinct in the transverse section of the anther. The anther is a foursided (tetragonal) structure consisting of four MICROSPORANGIA
located at the corners, two in each lobe.
The micro sporangia develop further and become POLLEN SACS or
POLLEN CHAMBERS. They extend longitudinally all through the
length of an anther and are packed with pollen grains. The whole
anther is covered externally by a common EPIDERMIS.
Structure of micro sporangium
Microsporangium is cylindrical structure that runs on either side of
each anther lobe. It appears circular in transverse section. It consists of
two parts, microsporangial wall and sporogenous tissue.
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MICROSPORANGIAL WALL
It is hypodermal wall having 3-5 layersEpidermis, endothecium, middle layers and the
tapetum. The outer three layers performs the
function of protection and help in dehiscence of
other to release the pollen.
The innermost wall layer is TAPETUM. It
nourishes the developing pollen grains. Cells of
the tapetum possess dense cytoplasm and generally
have more than one nucleus.
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SPOROGENOUS TISSUE
It fills the interior of microsporangium . all the cells are
similar and called sporogenous cells. These cells divide
regularly to form diploid microspore mother cells of the
pollen grain mother cells(MMC or PMC). They are initially
connected by plasmodesmata. Plasmodesmata are broken by
formation of callose layers inner to cell wall. The separated
mother cells round off and undergo meiosis to form four
haploid microspores (tetrad). This phenomenon is known as
MICROSPOROGENESIS. As the anther mature and
dehydrate, the microspores dissociate from each other and
develop into POLLEN GRAINS. Inside each
microsporangium several thousands of microspores/ pollen
grains are formed that are formed that are released with the
dehiscence of anther.
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Pollen Grain: It represents the male reproductive propagule
or young male gametophyte which is formed inside the
anther and is meant for reaching the female reproductive
organ though a pollinating agency.
Pollen grains are generally spherical measuring about 25-50
um in diameter. It has a prominent two-layered wall.
EXINE: It has a hard outer layer made up of sporapollenin
(Zelisch,1932) which is one of the most resistant organic
material known. It can withstand high temperatures and
strong acids and alkali. No enzyme that degrades
sporopollenin is so far known. Pollen grain exine has
prominent apertures called GERM PORES, where
sporopollenin is absent. There are usually 3 germ pores in
dicots and 1 in monocots.
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Pollen grains are well preserved as fossils
because of the presence of sporapollenin
INTINE: It is inner layer of pollen grain wall
which is made up of cellulose and pectin.
Some enzymatic proteins also occur in the
intine (Knox and Heslop- Harrison, 1971).
Intine is normally thin.
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Due to peripheral position of nucleus, the protoplast of
pollen grain divides unequally into two cells – a large tube
or VEGETATIVE CELL and small GENERATIVE CELL.
Tube cell or vegetative cell is rich in vacuoles as well as
reserve food. Slowly its nucleus enlarges and becomes
convoluted.
Generative cell in lenticular to spherical in outline. It has a
thin layer of cytoplasm around a compact nucleus. A layer
of callose develop around the generative cell. It helps in
separation of generative cell from the wall of a pollen grain.
Later on, the callose covering of generative cell dissolves.
In several plants, the pollen grains are shed in 2 celled stage.
In many others, the generative cell divides further into two
male gametes. In such cases pollen grains are shed in 3celled stage.
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Pollen grains of many species cause severe allergies and
bronchial afflictions in some people often leading to chronic
respiratory disorders- asthma, bronchitis etc.
Pollen grains are rich in nutrients. It has become fashionable
to use pollen tablets as food supplements. Their
consumption has been claimed to increase the performance
of athletes and race horses.
POLLEN VIABILITY: The period for which pollen grins
remain functional is called pollen viability. It depends on
the prevailing temperature, humidity and genetic
potentiality.
Eg. 1) Pollen grains of cereals like wheat, rice remain viable
only for 30 minutes.
2) Pollen grains of members of Rosaceae, Fabaceae and
Solanacea remain viable for several months.
Pollen grains can also be cryopreserved in liquid N2
(temperature → - 196 C) and used as POLLEN BANK,
similar to seed bank in crop breeding.
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The Pistil, Megasporangium (ovule) and Embryo sac
Female reproductive organs of the flower are carpel's,
collectively called GYNOECIUM. It may have single carpel
(monocarpellary), two carpel's (bicarpellary), several
carpel's (multicarpellary) .when there are more than one, the
pistils may be fused together (i.e. Syncarpous) or may be
free (apocarpous)
Each pistil has 3 parts.
OVARY: It is basal swollen ovule containing region. Ovary
may have one to many chambers called locules. Ovary
bearing cushions or placentae (Sing-Placenta) occur in the
ovary. Number of ovules in an ovary may be one
(eg. Wheat, paddy, mango, sunflower) to many
(eg. Papaya, watermelon, orchids)
STYLE: A long stalk- like structure.
STIGMA: It is upper broader region which is specialized for
receiving pollen grains.
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Structural of Megasporangium (Ovule): the ovule is
a small structure attached to the placenta by means
of a stalk called funicle. The body of the ovule fuses
with funicle in HILUM.
Thus, hilum represents the junction between ovule
and funicle. Each ovule has one or two protective
envelopes called INTEGUMENTS. It encircle the
ovule except at tip where a small opening called as
MICROPYLE is organized. Opposite the micropylar
end, is the CHALAZA, representing the basal part
of the ovule.
The main body of the ovule consist of parenchy
matous tissue, the NUCELLUS. Located in the
nucleus is the EMBRYOSAC or FEMAL
GAMETOPHYTE.
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MEGASPOROGENESIS: It is the process of
formation of haploid megaspores from diploid
megaspore mother cell or MMC. Ovules generally
differentiate a single megaspore mother cell in the
micropylar region of the nucellus. It is a large cell
contaning dense cytoplasm and a prominent nucleus.
The MMC undergoes meiotic division. Meiosis
results in the production of four megaspores.
Female Gametophyte: In a majority of flowering
plants, one of the megaspore is functional while the
other three degenerate. Only the functional
megaspore develops into the female gametophytes
(embryo Sac). This method of embryo sac formation
from a single megaspore is termed MONOSPORIC
development.
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The nucleus of the functional megaspore divides
mitotically to form two nuclei which move to the
opposite poles, forming the 2- nucleate embryo sac.
Two more sequential mitotic nuclear divisions
results in the formation of the 4- nucleate and later
the 8- nucleate. After this, cell wall gametophyte or
embryo sac.
Six if the eight nuclei are surrounded by cell walls
and organized into cell, the remaining two nuclei,
called polar nuclei are situated below the egg
apparatus in the large CENTRAL CELL.
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In embryo sac, 3 cells are grouped together at the
micropylar and and constitute the EGG
APPARATUS. It consists of two SYNERGIDS and
one EGG cell. The synergids have special cellular
thickenings at the micropylar tip called filiform
apparatus which play an important role in guiding
the pollen tubes into the synergids. Three cells are at
the chalazal end and are called ANTIPODALS. The
large central cells as mentioned earlier, has two
polar nuclei. Thus, a typical angiosperm embryo sac,
at maturity, through 8- nuclear is 7-celled.
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Pollination: It is the process of transfer of
pollen grains from anther and their deposition
aver the stigma of a flower. Depending upon
genetic similarity and dissimilarity between
anther and stigma, pollination is two type, self
pollination and cross- pollination (Xenogamy).
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I] SELF POLLINATION: It is the transfer pf pollen grains
from anthers to the stigma of same or genetically similar
flower. Self pollination is of two types:
(a) AUTOGAMY: (Gk-autos-self; gamos-marriage)
It is self pollination which occurs between anther and
stigma of the same flower. It is of the following types:
Homogamy: The flower are open or chasmogamous. Both
anthers and stigma mature simultaneously. As the anther
dehisce, the pollen fall down due to gravity over the surface
of stigma.
Eg. Convolvulus, Catharanthus, Gargenia, Lilac etc.
Clestogamy: (Gr. Kleisto-closed, gamos-marriage)
The flowers are closed with anthers and stigma lying close
to each other. When the anther dehisce, the pollen grains
come in contact with stigma and pollination takes place.
Eg. Commelina bengalensis, Arachis hypogea (Groundnut),
Balsam, Oxalis, Viola etc.
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b) GEITONOGAMY: (Gk –geiton- neighbour;
gamos-marriage)
It is the transfer of pollen grains from anther of
one flower to stigma of another flower of either the
same (endogamy) or genetically similar plat:
Geitonogamy resembles cross pollination in the
requirement of pollen transfer through a pollinating
agency.
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II] CROSS POLLINATION or XENOGAMY (Gk xenosstrange)
It is the transfer of pollens from anther of one flower to the
stigma of another flower belonging to genetically different
plant.
Cross-pollination can occur only with the help of external
agencies (eg wind, water) and biotic (eg insects, birds, bats,
snails, ants etc). the type of cross- pollination is named after
the agency like.
Anemophily (Gk anemos- wind; philein – to love)
It is a mode of cross pollination performed through the
agency of wind flowers are small and inconspicuous
Flowers are usually colourless, nectarless and odourless.
Anthers are exerted and versatile.
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Pollen grains are small and light. They may
have air sacs or wings. Such pollens can float
in air for long distances.
Stigma are exerted, hairy, feathery or branched
to capture the pollen grains – large number of
pollen grains are produced.
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Pistils commonly posses single ovules.
Eg. Anemophily is common in grasses. Other eg. Are
Amaranthus, Cannabif, Chenopodium, Coconut, Dates,
Mulberrly,Poplar,Willow etc.
(b) HYDROPHYLY: (Gk. –hydro-water; phyilein-to love)
-It is pollination brought about through the agency of water.
-Flowers are small, inconspicuous and light.
-It occurs only in completely submerged plants.
-Perianth is unwettable.
- Flowers are with out nectar and odour.
- Pollen grains are very thin, long, thread –like and lack
exine.
- stigma is long, sticky and unwattable .
Eg. It occurs in about 30 ganera of mostly
monocotyledonous aquatic plants.
Eg. Lemna, Vallisneria, Ceratophyllum, Zostera etc.
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© Entomophily: It is transfer of pollen grains from
anther of one flower to
The stigma of another flower with the help of
insects like moths, bees,
wasps, butterflies, beetles, ants etc.
bees are the most pollinating insects. Insects visit
flowers for containing food (pollen, nectar) or
shelter.
Some insects also use flowers for laying eggs eg.
Amorpholhallus. It has the largest inflorescence and
the tallest flowers with a flower reaching about 1.8
m (6 ft)
Flowers are bright coloured, large and showy
They produce nectar and fragrance.
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Flowers are generally large in size so that they
become conspicuous from a distance.
Majority of flowers have landing platform for
insects.
Pollen grains are covered by a yellow sticky
substance called pollen kitt.
Pollen grains are heavy and stigma is inserted and
sticky.
Eg. Calatropis, Rose, Salvia, Fig, Yucca, Ophrys
etc.
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(d) Ornithophily: (Gk – ornis- bird; philein- to love)
It is cross pollination performed through the agency of
birds pollinating
birds have long beaks, larger than the length of the
corolla tube.
Eg. Sun bird (eg. Nectarninia, Specles) and
humming birds.
flowers are large sizes with bright colours- red, yellow,
orange etc.
Odour is generally absent.
Corolla is often funnel shaped.
Floral parts are strong and leathery.
Nectar is watery with abundant sugar.
Eg. Bombax (red, silk cotton), bignonia, Tecomaria
(honey Suckle), Coral tree,
Graillea (silver oak), Agave (century plant) etc.
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Outbreeding devices:
The following six types of devices promote and ensure
outbreeding through cross pollination:
UNISEXUALITY (Dicliny): Flowers are unisexual or
diclinous. Plants may be monoecious (eg. Maize, castor) or
dioecious (eg. Mulberry, papaya, date palm)
DICHOGAMY: (Gk-dicha- in two, gamos- marriage)
Here in a bisexual flower, anthers and stigmas nature at
different times, so that self pollination is inhibited.
Eg. Clerodendron, Salvia, Sunflower, Rose, Magnolia,
Gloriosa etc.
SELF – STERILITY- Pollen grains of one flower are
incapable of growing over the stigma of same flower due
to the presence of similar sterile genes.
Eg. Crucifers, Potato, Tobacco, Gramineae, Orechidaceae.
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PREPOTENCY: Pollen grains of one flower grow
more rapidly over the stigma of another flower as
compared to the same flower.
Eg. Apple, Grape.
HETEROSTYLY: Flower have different types of
height of styles and stamens.
Eg. Jasmine, Primrose, Lythyrsus, Oxalis.
HERKOGAMY: They are mechanical devices that
prevent self pollination and favour cross pollination
even in homogamous flowers.
Eg. (a) Occurrence of extrose anthers.
 In Gloriosa and Clerodendron, the mature
stigma and anthers occur in different positions.
 In Pansy, anthers are exposed while stigma
occurs inside a flap.
 In Calotrpois pollen grains occur in sacs called
pollinia other eg. Are Salvia, Kalmia,
Aristolochia etc.
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Pollen- Pistil Interaction:
Pollen grains of a no. of plants descend over the stigma of a
flower. All of them do not germinate there. Only the
compatible pollen of the same species are able to germinate.
Germination is connect with compatibility – incompatibility
reaction between proteins presents over the pollen grains
and the stigma. If the reaction is favourable, the pollen
grains pick up water and nutrients from the stigma. It results
in growth of pollen grains to from pollen tubes. Interaction
between pollen tubes and pistil continue till the pollen tubes
reach the ovule.
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In the absence of compatibility- incompatibility
reaction, pollen grains will germinate invariability,
provided water and nutrients are available. Eg.
Vinca, Balsam, Chick Pea, Crotalaria etc.
Therefore understanding the mechanism of
compatibility – incompatibility between the pollen
and the pistil is important. It can help to over come
incompatible reactions by supplying the chemicals
that induce compatibility and allow the pollen to
grow. Plant breeders will then be able to obtain
hybrids between different species.
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Artificial Hybridization:
It is the technique in plant breeding where crosses are made
between different varieties, species and even genera in order
to combine their desirable characters in a single superior
variety. Artificial hybridization is one of the major
approaches in crop improvement. This is carried out by the
two process of EMASCULATION and BAGGING.
Emasculation is the practice of removing anthers in their
bud condition from the bisexual flowers of plants selected
as female parents by means of a pair of fine forceps. It is not
required in case of unisexual flowers.
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Bagging is the covering of both emasculated and
non- emasculated flowers with butter paper/
polythene in their bud condition to prevent
contamination from unwanted pollen. When the
stigmas of emasculated flowers nature, the bags are
removed for a while. The stigmas are dusted with
pollen grains of desired male plants by means of a
brush. The flowers are rebagged till fruits develop.
Flowers are bagged to prevent contamination.
Double fertilization:
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Double fertilization was discovered by Nawschin 1898, in
Fritillaria and Lilium. It was confirmed by in 1899 by
Guignard.
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After entering one of the synergids, the pollen tube release
the two male gametes into the cytoplasm of the synergid.
One of the male gamete move toward the egg cell and fuses
with its nucleus and thus completing the SYNGAMY. This
results in the formation of a diploid cell, the ZYGOTE.
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The other male gamete move towards the two polar nuclei
located in the central cell and fuses with them to produce a
triploid PRIMARY ENDOSPERM NUCLEUS (PEN). As
this involves the fusion of three haploid nuclei it is termed
as TRIPLE FUSION.
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Since two types of fusions, Syngamy and
triple fusion takes place in an embryo sac. The
phenomenon is termed as DOUBLE
FERTILIZATION. The centers cell after the
triple fusion becomes PRIMARY
ENDOSPERM CELL (PEC) and develops
into the endosperm while the zygote develops
into an EMBROYO.
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Post fertilization- Structure and Events
Following double fertilization, events of
endosperm and embryo development, maturation of
ovule(s) into seed(s) and ovary into fruit are
collectively termed POST FERTILIZATION
EVENTS.
Endosperm: Endosperm is highly nutritive tissue
that is formed as a result of triple fusion and
provides nourishment to the developing embryo. In
dicot seeds like beans and peas etc. the endosperm is
completely used up during development and so they
remain without endosperm (non- endospermic). In
seeds of maize and coconuts, the endosperm remain
there in even at maturity stage (endospermic).
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On the basics of its development, it is recognized into three
types:
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NUCLEAR TYPE: In this, the division of primary endosperm
nucleus is not accompanied by the cell wall formation. But later
on, the protoplast collects around each nuclei and then walls are
formed. So the endosperm becomes cellular at the maturity. Eg.
Maize, wheat , rice, and sunflower.
CELLULAR TYPES: In this, the repeated mitotic division of PEN
is flowed by cell wall formation around each nuclei, so the
endosperm becomes cellular from the beginning. Eg. Datura,
Petunia, Magnolia, Impatiens etc.
HELOBIAL TYPE: In this, the first mitotic division of primary
endosperm is followed by incomplete cytokinesis and the
endosperm is formed both by cellular and nuclear types .
eg -In coconut , the mature fruit contains both cellular
(edible) and the free
nuclear (water)
endosperm.
-Asphodelus.
-Eremurus
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Embryo: Embryo is the young future offspring
which in seed plants develops inside mature
fertilized ovule or seed. It is formed from zygote.
Formation of embryo from zygote is called
EMBRYOGENY.
After fertilization, the zygote of the ovule divides
transversely into two cells – a small terminal/ apical
cell and a large basal cell.
The first division of the zygote produces a
hypobasal cell (BASAL) towards the micropyle and
an epibasal (TERMINAL) towards the chalaza. The
epibasal cell divides repeatedly tp produce a row of
4-8 cells. The terminal cell divides in various planes
to produce a cluster of cells, called the
PROEMBRYO
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. The hypobasal and the remaining cells constitute the
SUSPENSOR. The suspensor pushes the proembryo into
the endosperm to enable the developing embryo to receive
nutrition. A few cells of the proembryo nearest to suspensor
develop into hypocotye and radicle. The other cells give rise
to epicotyl, plumule and embryo, whereas in monocot
plants, one of the two cotyledons get suppressed at an early
stage, leaving only one in the mature embryo. As the
embryo and endosperm develop and mature the
integuements of the ovule become hard ( to provide
protective covering, as seed coat) and lead to the formation
of seed. The seed dormancy and the process of seed
germination occur after this. The single cotylendon is called
scutellum. Extensions of scutellum form coverings,
coleorhiza (around radicle and rootcap) and coleoptile
(around plumule).
Seed and fruit formation
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Seed is the final product of sexual reproduction. It is often
called ripened ovule. Seeds develop inside stored food for
later development of embryo is present either in
cotylendons or endosperm. Where food is present in
cotylendons, the latter are thick. Such seeds are called
EXALBUMINOUS or NON ENDOSPERMIC.
Eg. Pea, Gram, Bean, Groundnut. In exalbuminous seeds,
the endosperm is consumed during growth of the embryo
and transfer of food to cotylendons.
In ALBUMINOUS or ENDOSPERMIC seeds, the whole of
endosperm is not consumed during development of embryo.
Sufficient food is also kept stored in it for future
development of embryo.
Eg. Wheat, Maize, Barley, Castor, Onion, Sunflower etc.
Remants of nucellus persist in some seeds. The residual,
persistent nucellus is called PERISPERM.
Eg. Black Pepper.
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Integuements of ovules harden as tough protective
seed coats. The microphyle remains as a small pore
in the seed coat. This facilitates entry of oxygen and
water into seed during germination. As the seed
matures, its water content is reduced and seeds
become relatively dry. The general metabolic
activity of embryo slow down. The embryo may
enter a state of inactivity called dormancy or if
favourable conditions are available, they germinate.
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As ovules mature in seeds, the ovary develops into fruit i.e.
the transformation of the ovule into seeds and ovary into
fruits proceeds simultaneously. The wall of the ovary
develops into wall of fruit called PERICARB. The fruit may
be fleshy as in Guava. Orange, Mango etc or may be dry as
in groundnut and mustard etc.
In most plants, by the time the fruit develops from the
ovary, other floral parts degenerate and fall off. But, in few
species such Apple, Strawberry, Cashew, etc. The thalamus
also contribute to fruit formation. Such fruits are called
FALSE FRUITS. Most fruits develop only from ovary and
are called true fruit eg. Tomato, Grapes etc.
PARTHENOCARPIC FRUITS: This fruits develops
without fertilization and do not bear seeds.
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Importance of seeds:
DEPENDABLE PROCESS: Seeds formation is connected
with pollination and fertilization that are independent of
water. It is therefore more dependable process.
PERENNATION : Seeds enable the future plant to pass
through unfavourable periods without any harm because of
the:
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Presence of tough covering
Dormant embryo
Dehydrated protoplasm
Inactive enzymes
(c ) FOOD PRESERVE: Seed has sufficient food reserve
that nourishes the germinating embryo and growing
seedling till it becomes photosynthetically independent.
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(d) VARIATION : Being products of sexual
reproduction, seeds have a number of variations due
to genetic recombination's and crossing over. The
variations are helpful in adaptations to varied
environments and struggle for existence.
(e) FOOD : Seeds are major article of human diet.
They are also a source of oils, fibers, spices,
beverages etc. stored seeds functions as an insurance
against drought and vagaries of nature.
(f) AGRICULTURE: Discovery of edible nature
of some seeds, their germination and sowing by
earlier humans gave rise to agriculture. It helped in
development of civilization, progressive
improvement in knowledge, development of science
and technology.
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Seed Viability:
It is the period of time for which the seed retain the ability
to germinate. Environmental conditions which can alter the
viability are humidity and temperature. Genetically seed
viability ranges from a few days (eg. Oxalis), one season
(eg. Birch), 2-5 years (most crop plant) to 100 years (eg.
Trifolium)
Seed Viability has been found out to be more than 1000
years in lotus. 2000 years old seeds of phoenix dactylifera
excavated from king herod’s place near dead sea have been
found viable. Similarly 10,000 years old seeds of Lupinus
arcticus (Lupine) excavated from arctic Tundra not only
germinated but also produced plants that flowered.
Apomixis and Polyembryony
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APOMIXIX (Gk. Apo- without; mixis- marriage)
It is the formation of new individual by asexual
methods which do not involve fusion of gametes or
sex cells. The organism reproducing through
apomixis is called APOMICT. In apomixis, the new
individual are genetically similar to the parent
producing them. They are therefore, clone of there
parents. There are several ways of development of
apomicitic seeds like
I ] Agamospermy: eg. Mango, orange.
II] Parthenogenesis: eg Apple, poa
III] Apogamy
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POLYEMBYONY:- It is the phenomenon of
formation of more than one embryo during the
development of seed. Rao(1965) has found
polyembryony to develop during seed germination
in vandal caused by cleavage of apical promeristem
of single embryo.
Polyembryony was discovered by
Leeuwenhoek(1719) in case of citrus .
Polyembryony is of three types
I] Simple- eg. Poa , Casuarinas, Citrus
II] Cleavage polyembryony- eg. Pinus, orchids
III] Adventitive polyembryony-eg. Citrus, opuntia
,Onion, Mangifera, Trillium,Groundnut.