Saturday, 28 September 2019

Sexual Reproduction in Flowering Plants

Sexual Reproduction in Flowering Plants

Sexual Reproduction in Flowering Plants

• The process of development of new organisms through the formation and fusion of gametes is called sexual reproduction.
• In angiosperms, the organs specialized to perform sexual reproduction are flowers.
• Flowers are modified condensed reproductive shoots.
• A typical flower has been broad base called thalamus over which four whorls of floral leaves, i.e., sepals (calyx), petals (corolla),
stamens (androecium) and carpels (gynoecium) are borne.
• Stamens and carpels represent male and female reproductive structures of flowers respectively. They are called essential floral organs.
• Sepals and petals are called non-essential floral organs because they have only a supportive role.

STAMEN

• The stamen consists of two parts-filament and anther.
• The filament is long and slender stalk attached proximally to thalamus, petal or tepal. It bears an anther distally.
• The anther is bilobed, knob-like fertile part of the stamen. The two anther lobes separated by a deep groove in the anterior side
and attached to each other by a sterile parenchymatous tissue called connective on the posterior side.
• Each anther lobe has two chambers which posses pollen sacs or microsporangia.
• A bilobed anther is tetrasporangiate.
• A microsporangium or pollen sac is cylindrical sac which appears circular in transverse section.
• It consists of two parts: the outer wall and central homogeneous sporogenous tissue.
• The outer walls four types of layers epidermis, endothecium, 1-3 middle layers, and tapetum.
• The outer three layers protect the young anther and take part in the mechanism of dehiscence in the ripe anther.
• The endothecium is also called fibrous layer due to the presence of fibrous thickenings.
• The tapetal cells enlarge and become filled protoplasmic content as well as nutrients.
• Two types of tapetum are present amoeboid and secretory.

Functions of Tapetum
• It provides nourishment to the developing microspore mother cells and pollen grains either by forming a plasmodium (amoeboid
type) or through diffusion (secretory type).
• It produces lipid-rich Ubisch granules containing sporopollenin for exine formation.
• It secretes enzymes like calls responsible for the degradation of callose wall around pollen tetrad.

Microsporogenesis
• The process of formation of pollen grains through meiosis in pollen mother cells is termed as microsporogenesis.
• Sporogenous tissue with the anther grow and transform into pollen
mother cell which in turn produce tetrads of haploid microspores
or pollen grains.
• The pollen grains of tetrad grow and separate from one another.
• Usually, the arrangement of microspores tetrad is tetrahedral or
isobilateral. However is decussate, linear and T-shaped tetrads are also found.

pollen grain
• The mature anther has two cavities, therefore, it is called dithecous.
• The pollen grain is commonly globular in outline though several other shapes
are found.
• The covering of the pollen grain is called sporocarp consisting of two layers, outer
exine, and inner intine.
• Intine is pesto-cellulosic in nature while exine is made of a highly
resistant fatty substance called sporopollenin. Because of pollen grains are well preserved as microfossils as
sporopollenin is not affected any enzyme high-temperature strong acid or strong alkali.
• The pollen grain is uninucleate in the beginning but at the time of liberation from anther, it is 2-3 celled.

Development of male gametophyte
• Development of male gametophyte is precocious, i.e., it begins inside the microsporangium or pollen sac.
• Young pollen grain has a centrally placed nucleus embedded in dense cytoplasm covered by the plasma membrane.
• It grows in size with the inflow of nutrients and eventually, protoplast divides mitotically to form two unequal cells-small
generative cell and large tube or vegetative cell.
• The generative cell is spindle-shaped to spherical in outline with thin dense cytoplasm surrounding a prominent nucleus.
• The tube cell has a vacuolate cytoplasm rich in the food reserves and cell organelles. Its nucleus is large and irregular.
• Are some species, the generative cell divides into two nonmotile male gametes prior to the dehiscence of anther and release
of the pollen grains.
• Therefore at the time of pollination, the pollen grain is either two-celled (tube cell + generative cell) or 3-celled (tube cell + two
male gametes).
• On the stigma, compatible pollen grain absorbs water and nutrients from the stigmatic secretion through its germ pores.
• The tube or vegetative cells enlarge and comes out of the pollen grain through one of the germ pores to form a pollen tube
covered over by intine.
• The tube nucleus along with generative cell descend to the tip of the pollen tube.
• The generative cells soon divide into two nonmotile male gametes if it is not already divided.
• Each male gamete has a large nucleus surrounded by a thin sheath of cytoplasm and is considered to be one cell.
• The tube nucleus may degenerate completely.
• The pollen grain with pollen tube carrying male gametes represent mature male gametophyte and is 3 celled (1 tube cell and 2
male gametes) and 3 nucleated structure.

PISTIL

• Gynoecium represents the female part of flowers.
• A free unit of gynoecium is called pistil.
• A pistol has three parts-stigma, style, and ovary.
• Stigma is terminal receptive part of the pistil which functions as a landing platform for the pollen grains. The style is the
elongated slender part beneath the stigma that connected stigma with the ovary. The basal bulged part of the pistil is an ovary.
• Inside the ovary lies the ovarian cavity (locule). The placenta is located inside the locule.
• From the placenta, megasporangia arise, commonly called ovules.
• The number of ovules ovary may be one (wheat, paddy, mango) to many (papaya, watermelon, orchids).

Structure of Ovule
• A typical angiosperm ovule is a small structure attached to the placenta by means of a stalk called funicle. The body of the ovule
fuse with the funicle and the point of attachment is called hilum.
• The body of ovule consists of a mass of parenchymatous cells named nucellus.
• The nucellus is surrounded by one (unitegmic ovule, e.g., higher dicots) or two (bitegmic ovule, e.g., monocots and primitive dicots)
or multicellular integuments.
• The integuments leave a narrow passage known as micropyles at one end of the ovule.
• The place of origin of the integuments usually lies at the opposite end of the micropyle, termed as chalaza.
• Embryo sac female gametophyte is present in the micropylar half of nucellus.
• Depending upon the configuration of orientation the body of ovule in relation to funiculus there are six types of ovules in
the angiosperms - orthotropous (erect), anatropous (inverted), hemitropous (half-inverted), campylotropous (body curved),
amphitropous (both body and embryo sac curved), circinotropous (funiculus coiled around the ovule).

Megasporogenesis
• The process of formation haploid megaspores from the diploid megaspore mother cell is called megasporogenesis.
• Generally, a single megaspore mother cell (MMC) differentiates in the micropylar region of the nucellus.
• The megaspore mother cells undergo meiotic division which results in the production of four haploid megaspores.
• In the majority of angiosperms, only one of the megaspores is functional while the other three degenerate.

anatropous ovule
Fig.: Structure of a typical ovule (anatropous ovule) prior to fertilisation.

• The functional megaspores develop into the female gametophyte (embryo sac).
• The formation of embryo sac from a single megaspore is called monosporic development.
• The female gametophyte or embryo sac contains 8 nuclei but 7 cells-3
micropylar, three chalazal and one central.
• The three micropylar cells are known as the egg apparatus. The middle
cells of the egg apparatus are called egg which is larger with a central
vacuole and a nucleus towards the chalazal end while the remaining
two cells are called synergids.
• Each of the synergids bears a filiform apparatus in the micropylar region
which is a mass of finger-like projections of the wall into the cytoplasm.
• The3 chalazal cell of the embryo sac is called antipodal cells.
• The central cells the largest cell of the embryo sac.
• The central cells contain two polar nuclei which often fuse to form a
single diploid secondary or fusion or definitive nuclei.

what is pollination

• The transfer of pollen grain from the anther to stigma is called pollination.

Types of Pollination

Autogamy
• Transfer of pollen grains from the anther to the stigma of the same flower.
Self-pollination
Cross-pollination or Xenogamy
•The plants such as Oxalis, Viola, and Commelina produce two types of flower - chasmogamous flowers with exposed anther and stigma cleistogamous flowers which do not open of all.
• Cleistogamous flowers are invariably autogamous as there is no chance of cross-pollen landing on the stigma.

Agents of Pollination
• Xenogamy or cross-pollination is performed with the help of external agency which may be abiotic (wind, water) or biotic (animals).
• Cross-pollination is named after that agency assists it, e.g., anemophily (wind pollination), hydrophily (water pollination),
entomophily (insect pollination), ornithophily (bird pollination), chiropterophily (bat pollination) and malacophilous (snail pollination).
Cross-pollination flower with different agencies has different characteristic features.

Characteristics of cross-pollinating
Wind pollinated flowers
• Flowers are small and inconspicuous.
• Pollen grains are dry, light and non-sticky.
• Well exposed stamens
• Large often feathery stigma to trap air-borne
pollen grains.
• Single ovule in each ovary and numerous
flowers packed into an inflorescence.
• Common in grasses

Insect pollinated flowers
• The flower is large, colourful, fragrant and rich in
nectar
• The flowers produce odours which may be
pleasant or foul.
• When the flower is small, a number of flowers
are clustered together into an inflorescence to
make them conspicuous.
• Observed in jasmine, Rosa, Magnolia, etc.

Water pollinated flowers
• Flowers are small and inconspicuous.
• Pollen grains are long, ribbon-like and
protected from wetting by a mucilaginous covering.
• Stigma is long, sticky but unwettable
• Observed in Vallisneria, Zostera,
Ceratophyllum.

Cross-Pollination

• Continuous self-pollination results in inbreeding depression. Therefore, angiosperms have developed many devices to
discourage self-pollination and encourage cross-pollination.
• Dichogamy - Pollen release and stigma receptivity are not synchronised in some species.  In protandry, anthers mature earlier
than the stigma of the same flower, e.g., Sunflower; in protogyny stigma mature earlier, e.g., Mirabilis.
• Heterostyled - In some species the anther and stigma are placed at different positions so that the pollen cannot come in contact
with the stigma of the same flower. E.g., Primula, Lythrum.
• Decline - Flowers are unisexual so that self-pollination is not possible. The plant may be monoecious (bearing both male and
female flowers, e.g., Maize) or dioecious (bearing male and female flowers on different plants, e.g., Mulberry, papaya).
• Prepotency - Pollen grains of another flower germinate more rapidly that the pollen grains of the same flower over the stigma,
e.g., apple, grape.
• Herkogamy- the different type of mechanical devices to prevent self-pollination and promote cross-pollination, e.g., Stigma lies inside a
flap in pansy, anthers occur inside corolla pocket in Kalmia.

Pollen Pistil Interaction
• It is a dynamic process that occurs from the time of pollen deposition over the stigma to the time of pollen tube entry into the ovule.
• It is a safety measure to ensure that illegitimate crossing does not occur.
• Pollen grains a number of plants may settle over a stigma.
• Pollen - pistil interaction ensures that only the right pollen belonging to the same species would germinate while others fail to do so.

Artificial Hybridisation
• Artificial hybridisation has been used by plant breeders for the crop improvement programme.
• In artificial hybridisation, it is important to make sure that only the desired pollen grains are used for pollination and the
stigma is protected from contamination.
• This is achieved emasculation and bagging technique.
• Emasculation is the removal of stamens from the floral buds of the female parent (if the female parent bears bisexual flowers)
so that chances of self-pollination are eliminated.
• Bagging is covering of emasculated flower with a bag made of butter paper to prevent contamination of its stigma with
unwanted pollen.
• When the stigma of bagged flower attains receptivity, mature pollen grains collected from the anthers of the male parent
flower are dusted on the stigma and the flowers are rebagged and the fruits are allowed to develop.

FERTILISATION

 In seed plants, i.e., gymnosperms and angiosperms the male gametes
are brought to the egg-contain female gametophyte by a pollen tube (Strasburger, 1884). This phenomenon is called
siphonogamy.
• The pollen tube carrying the male gametes enters the ovule either through its micropyle (porogamy, e.g., lily), chalaza (chalazogamy
e.g., Casuarina) or the sides after piercing through the integuments or funicle (monogamy, e.g., Cucurbita).
• The male gametes move towards the egg cell and fuse
with its nucleus resulting in the formation of a diploid zygote.
• The male gamete moves towards the two polar nuclei
located in the central cells and fuse with them to produce a
triploid primary endosperm nucleus.
• Since two types of fusion take place (syngamy and triple
fusion) an embryo sac, the phenomenon is termed as double
fertilization, an event unique to angiosperms.
• The primary endosperm nucleus develops into endosperm
while the zygote develops into an embryo.

POST-FERTILISATION: STRUCTURES
AND EVENTS

• Following double fertilization, events of endosperm and embryo development, maturation of ovule (s) into the seed (s) and ovary into fruit, are collectively termed as post-fertilization events.

L.S. of an embryo of grass
Fig.: (a) A typical dicot embryo, (b) L.S. of an embryo of grass

• The development of endosperm precedes embryo development since the cells of endosperm are
filled with reserve food materials that are used for the nutrition of developing the embryo.
• In the most common type of endosperm development, the primary endosperm nucleus undergoes
successive nuclear divisions to produce a large number of free nuclei without a cell wall. This stage
of endosperm development is called free nuclear endosperm.
•Cell wall formation occurs subsequently and endosperm becomes cellular.
• The coconut water from tender is coconut free-nuclear endosperm and the surrounding white
kernel is the cellular endosperm.
• The embryo develops micropylar end of the embryo sac where the zygote is situated.
•Early stages of the embryo development are similar in both monocotyledons and dicotyledons.
•The portion of the embryo axis above the level of cotyledons is the epicotyl, which terminates
with the plumule or stem tip. The cylindrical portion below of the level of cotyledons is hypocotyl
that terminates at its lower end radicle or root tip. The root tip is covered by with a root cap.
• Only one cotyledon is present in embryos of monocotyledons. In the grass family, the cotyledon
is called scutellum. Its is the lower end, the embryo axis has the radicle and root cap enclosed in
an undifferentiated sheath called coleorhiza. 
• The seed is the final product of sexual reproduction in angiosperms.
• Seeds are often described as fertilized ovules and are formed inside fruits.
• A seed typically consists of the seed coat(s), cotyledon (s) and an embryo axis.
• Ovules mature seeds, the ovary develops into a fruit.
• The cell wall of the ovary develops into the wall of fruit called pericarp.
• The most plants, by the time the fruit develops from the ovary, other floral parts degenerate and fall off.
• However, in a few species such as apple, cashew, strawberry, etc. the thalamus also contributes to fruit formation. Such fruits
are called false fruits.

• In contrast, true fruits developed only from the ovary.
• In a few species, fruits development without fertilization. Such fruits are called parthenocarpic fruits. Banana is an example of
parthenocarpic fruit.

Albuminous and non-albuminous seed
• Mature seeds may be albuminous or non-albuminous. The Albuminous seeds retain a part of endosperm as it is not completely
used to during embryo development (e.g., wheat, maize, barley, castor, sunflower). Non-albuminous seeds have no residual
endosperm it is completely consumed during embryo development (e.g., pea, groundnut). Occasionally, in some seeds such
as black pepper remnants of nucellus are also persistent. This residual persistent nucellus is the perisperm.

APOMIXIS AND POLYEMBRYONY

•The flowering plants such as some species of Asteraceae and grasses have evolved a mechanism to produce seeds without
fertilization called apomixis.
• There are several methods of apomicts development in seeds, two common methods are recurrent agamospermy and adventive
embryonic.
• Agamospermy is the formation of seed that an embryo form without meiosis and syngamy.

Agamospermy

Non-recurrent agamospermy
• The embryo is haploid and
therefore the seed having it
is nonviable.

Recurrent agamospermy
• All the cells of the embryo sac are diploid as it is formed directly either from a nucellar cell (apropos) or diploid
megaspore mother cell (diaspora).
• The diploid egg, as well as other diploid cells of the embryo sac, can grow into normal embryos.

• Formation of embryo directly from a diploid egg without fertilization is called diploid parthenogenesis, e.g., apple, Poa.
• Adventive embryonic - An embryo develops directly from a diploid cell other than egg like that of nucellus and integument,
e.g., Citrus, Opuntia. Its gives rise to a condition called polyembryony or the phenomenon of having more than one embryo.
In gymnosperms, polyembryony can also occur due to cleavage of the growing embryo. It is called cleavage polyembryony.
The occurrence of polyembryony due to fertilization of more than one egg is called simple polyembryony while the formation of
embryos through sporophytic budding is called adventive polyembryony.
• Polyembrayony is common in onion, groundnut, mango, lemon, orange.

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