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Sexual Reproduction in Flowering plants Notes






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Flower - The Fascinating Organs of Angiosperms

  • Flowers are symbolic of beauty, love and tranquility. They form the soul of a garden and convey the message of nature to man. Flowers are objects of aesthetic, ornamental, social, religious and cultural value. Flowers are used on all festive occasions and in marriages and religious ceremonies.
  • Floriculture is the branch of ornamental, horticulture concerned with growing and marketing of flowers and ornamental plants, as well as with flower arrangement.
  • A flower is a modified shoot for sexual reproduction.

Parts of Flower

  • A flower is generally borne on the lateral side of the peduncle at a node in the axil of the leaf-like structure called bract.
  • A flower arising in the axil of a bract is called bracteate and if the bract is absent at the base of a flower the flower is called ebracteate.
  • The stalk of the flower is called pedicel and with pedicel the flower is called pedicellate and if the pedicel is absent the flower is sessile.
  • The pedicel has an upper swollen portion called thalamus (receptacle). It bears four types of floral leaves such as sepals, petals, stamens and carpels. In the group these whorls are called calyx, corolla, androecium and gynoecium.
  • In the flower the male and female reproductive structures, the androecium (whorls of stamens) and gynoecium (whorls of carpels) differentiate and develop.

A diagrammatic representation of L.S. of a flower

Male Reproductive Organ

The male reproductive organ is stamen/Androecium.
It consists of two parts:
  • Long and Slender stalk called filament
  • A terminal, bilobed structure called anther.

(a) A typical stamen;
(b) three–dimensional cut section of an anther

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Anther

A typical Anther is bilobed with each lobe having a theca, that is they are dithecous
Each anther contains 4 microsporangium, that is, two in each lobe.
Note: Later, the two microsporangium of each lobe become fused as a pollen sac, that is, a mature anther has to pollen sac.

Structure of Microsporangium (Pollen sac):
  • A microsporangium is more often circular in outline and is surrounded by four wall layers:
    1. The outermost is the single layer of epidermis.
    2. The second layer is endothecium.
    3. Middle layers of 1-3 layers of cells.
    4. Tapetum, the innermost layer.
  • Tapetum nourishes the developing microspores or pollen grains; while other three wall layers provide protection.


(a) Transverse section of a mature anther
(b) Enlarged view of one microsporangium showing wall layers
(c) A dehisced anther


Microsporogenesis & Formation of Pollen Grains (Male Gametophyte):
  • Every cell of sporogenous tissue is a potential pollen mother cell (PMC) and can give rise to microspore tetrad.
  • The process of formation of haploid microspores from a diploid pollen mother cell through meiosis is called microsporogenesis.
  • Each Pollen mother cell undergoes mitosis to form a cluster of four cells called microspore tetrad.
  • As the anther matures, the microspores dissociate from the tetrad and develop into pollen grains (male gametophyte).
  • Pollen grain is a haploid unicellular body with a single nucleus.

Development of Pollen Grain:
  • The pollen grains represent the male gametophyte.
  • Each pollen grain has a prominent two-layered wall.
  • The outer layer is called exine and made up of sporopollenin.
  • The inner layer is called intine and made up of cellulose and pectin.
  • When pollen grain is mature, its nucleus undergoes mitosis to form a large vegetative cell (tube cell) and a small generative cell which floats in the cytoplasm of vegetative cell.
  • In 60% angiosperms, the pollen grains are shed at this two-celled stage. In certain species the generative cell divides mitotically to form two male gametes and the pollen grains are three-celled during liberation.


(a) Enlarged view of a pollen grain tetrad
(b) stages of a microspore maturing into a pollen grain


Female Reproductive Organ

  • Pistil/gynoecium/carpel represents the female reproductive unit of the plant.
  • It may consist of one carpel that is monocarpellary or more than one couple that is multicarpellary. when the number of carpel is more than one, the gynoecium may be syncarpous (joined/fused) or apocarpous (free).
  • Each carpel has three parts: Ovary- The 0basal, swollen part in which ovules are present.
    Style- The elongated filamentous part.
    Stigma- Distal part which acts as a platform for lending of pollen grains.
  • The ovary encloses one or more cavities called the ovarian cavity or locule inside.
  • Placenta is the tissue seen in the locule that bears the ovule (megasporangium).
  • The number of ovules in the ovary may be one (for example: Mango, rice, wheat) or many ovules (for example: watermelon, banana etc)

(a) A dissected flower of Hibiscus showing pistil (other floral parts have been removed)
((b) Multicarpellary, syncarpous pistil of Papaver
(c) A multicarpellary, apocarpous gynoecium of Michelia
(d) A diagrammatic view of a typical anatropous ovule

Development of Ovule And Female Gametophyte

The Ovule (Megasporangium):
  • The ovule is a small structure attached to the placenta by means of stalk called funicle.
  • The body of the ovule fuses with funicle in the region called helium.
  • The ovule contains a mass of thin walled parenchymatous cells called nucellus.
  • The nucellus is protected by two multicellular coats called integuments except the tip leaving a small opening called micropyle.
  • The basal portion of the nucellus is called chalaza that lies just opposite to the micropyle
  • Cells of nucellus are rich in reserve food material. There is generally a single embryo sac (haploid female gametophyte) located in the nucellus formed from the megaspore mother cell through meiosis.
  • Female gametophyte or embryo sac is embedded in the micropylar region of nucellus.

Megasporogenesis:
  • The process of formation of haploid megaspores from the diploid megaspore mother cell (MMC) is called megasporogenesis.
  • A single megaspore mother cell (MMC) is differentiated in the micropylar region of the nucellus of the ovule. This cell is large and contains dense cytoplasm and prominent nucleus.
  • The MMC undergoes meiosis and forms a cluster of four haploid cells called megaspore tetrad.
  • Of these, soon three degenerate and only one megaspore becomes functional.
  • The functional megaspore enlarges to form embryo sac.
  • The method of embryo sac formation from a single megaspore is termed as monosporic development.
Development of female gametophyte:
  • The functional megaspore forms female gametophyte or embryo sac.
  • The nucleus of functional megaspore divides mitotically to form two nuclei which move to the opposite poles forming 2 nucleate embryo sac.
  • Two successive mitotic divisions in each of these two nuclei results in the formation of a 8-nucleate embryo sac.
  • Cell wall formation starts at the eight-nucleate stage, resulting in the formation of a typical female gametophyte or embryo sac.
  • 3 cells are grouped together at the micropylar end to form egg apparatus consisting of two synergids and one egg cell (female gamete).
  • 3 cells are grouped together at the chalazal end and are called antipodals cells.
  • The remaining two nuclei are called polar nuclei, they move to the centre of embryo sac and fuse to form diploid secondary nucleus (Central cell).
  • Hence, a typical angiosperm embryo sac (female gametophyte) is 8- nucleate and 7-celled.

(a) Parts of the ovule showing a large megaspore mother cell, a dyad and a tetrad of megaspores
(b) 1,2, 4, and 8-nucleate stages of embryo sac and a mature embryo sac
(c) A diagrammatic representation of the mature embryo sac.

Pollination

Pollination is necessary as the pollen grains continue their development only when they fall on a suitable stigmatic surface.
It is the phenomenon of transfer of pollen grains from anther to Stigma of the carpel.
Depending upon the source of pollen grain pollination is a following types:
  • Self pollination
  • Cross pollination

Parts of self-pollination

Autogamy: It is the transfer of pollen grains from anther of a flower to the stigma of the same flower. For example: Pea, Wheat, Rice etc.
Geitonogamy: when pollen from one flower are deposited on the stigma of another flower borne on the same plant. For example: Cucurbits.

Devices favouring self-pollination

  1. Homogamy: It is the condition in which anther and stigma in bisexual flower attain maturity at the same time.
  2. Cleistogamy: In this, flower never open to expose their sex organs and the pollens fall on the stigma of the same flower. Such flowers are called cleistogamous flowers. For example: Commelina, Oxalis, Viola etc.
Note: Chasmogamous flowers are with exposed anthers and stigma.

Cross-pollination (Xenogamy/Allogamy):

Cross pollination can also be defined as the migration of pollen grains from one flower to the stigma of genetically different flowers.
For example: Date palm, Papaya etc.
Outbreeding devices promoting cross-pollination:- Flowering plants have developed many devices to discourage self-pollination and to encourage cross-pollination.
  • In some species pollen release and stigma receptivity are not synchronized. Either the pollen is released before the stigma becomes receptive (protandry) or stigma becomes receptive much before the release of pollen (protogyny).
  • In some other 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.
  • The third device to prevent inbreeding is self-incompatibility. It is a genetic mechanism that prevents the germination of pollen grain on the stigma of the same flower.

Agents for pollination

Abiotic agents:

Anemophily (water pollinated plants): eg. Coconut, Date palm etc exhibit the following characters which favour wind pollination:
  • The pollen grains are dry and unwettable.
  • Wind pollinated flowers bear well exposed stamens.
  • To catch the pollen grains the stigma becomes sticky, hairy, feathery or branched.
  • Anemophilous flowers usually bear a single ovule in each ovary.
Hydrophily (Water pollinated plants): Pollination brought about through the agency of water in plants especially submerged plants is termed hydrophily. For example Hydrilla, Zostera etc.
  • Water helps in movement of male gametes in algae, bryophytes and pteridophytes.
  • The pollen grains are light but covered with wax.
  • Stigma is sticky but unwettable.
  • Scent, colour and nectar absent.

Biotic agents:

Entomophily (Insects pollinated plants): Insects particularly bees are dominating biotic pollinating agents. Entomophilous plants possess the following characteristic features:
  • Majority of insect pollinated flowers are large, colourful, fragrant and rich in nectar. For example Jasmine, Cestrum etc.
  • To catch the pollen grains, the stigmas become sticky.
  • Flowers become more conspicuous by grouping eg. capitulum, corymb, etc.
  • In plants like Papaver, Rosa etc. edible pollen grains are produced.
Ornithophily: The bird pollinated flowers are generally scentless, large in size. They are also beautifully coloured. The pollens are sticky and adheres to the body of the bird. For example Coral tree, Bottle brush etc.
Chiropterophily: The flowers of Durio, Kigella pinnata are pollinated by bats. The bats hold on to the freely exposed, large and relatively tough flower, which open in the evening or night.

Significance of pollination

Male and female gametes in angiosperms are produced in the pollen grain and embryo sac respectively. As both male and female gametes are non-motile, they have to be brought together for fertilization to take place. This aim is achieved by the process of pollination.

Pollen-Pistil Interaction

Recognition Of Compatible pollen:
  • The stigma/pistil has the ability to recognize the right type of pollen i.e. the compatible pollen of the same species.
  • It is the result of interaction between the chemical component of the pollen and those of stigma.
Germination of pollen grains and development of male gametophyte:
  • A compatible pollen grain germinates on the stigma to produce a pollen tube through one of the germ pores.
  • The contents of the pollen grain move into the pollen tube that is the tube nucleus and two male gametes (generative cell).
  • The pollen tube grows through the tissues of the stigma and style and enters the ovule through micropyle.
  • It enters the embryo sac through the filiform apparatus of one of the synergids to liberate the male gametes.
  • The germinated pollen grain with its full grown pollen tube carrying a tube nucleus at its tip and two male gametes is the fully developed male gametophyte of angiosperms.
The events from the deposition of pollen on the stigma till the pollen tube enters the ovule are collectively referred to as pollen-pistil interaction.


Double fertilization

  • The pollen tube releases the two male gametes into the cytoplasm of a synergids.
  • One of the male gametes moves towards the egg cell and fuses with its nucleus, this fusion is called syngamy and it results in the formation of a diploid cell, the zygote that later develops into the embryo.
  • The second male gamete fuses with the secondary nucleus in the central cell to produce a triploid primary endosperm nucleus (PEN), this fusion is called triple fusion as three haploid nuclei are involved in the fusion.
  • Since two fusions, syngamy and triple fusion occur in an embryo sac, the phenomenon is known as double fertilization and it is unique to angiosperms.
  • The central cell with the primary endosperm nucleus is now called primary endosperm cell (PEC) and develops into endosperm.

(a) Pollen grains germinating on the stigma
(b) Pollen tubes growing through the style
(c) L.S. of pistil showing path of pollen tube growth
(d) enlarged view of an egg apparatus showing entry of pollen tube into a synergid
(e) Discharge of male gametes into a synergid and the movements of the sperms, one into the egg and the other into the central cell

Post -fertilization events

All those events which occur in a flower after double fertilization are known as post- fertilization events.
  • Development of endosperm
  • Development of embryo
  • Maturation of ovule into seed
  • Maturation of ovary into fruit

Development of Endosperm

Endosperm development precedes embryo development. There are three methods for this:
  1. Nuclear type
  2. Cellular type
  3. Helobial type
  • The most common is the nuclear time, where triploid primary endosperm cell undergo repeated mitotic division without cytokinesis. At this stage, the development of endosperm is called free nuclear development. For example: coconut water.
  • Subsequently, cell wall formation occurs and the endosperm becomes completely cellular. For example: white part of coconut.
  • The cells of endosperm store food material which are later used by developing embryo.
  • The endosperm which may be completely utilized by the developing embryo before the maturation of seed as in pea, bean, mustard; such seeds are called albuminous or non -endospermic seeds.
  • In some species a portion of endosperm may remain in the mature seeds, for example: maize, coconut, rice; such seeds are called albuminous or endospermic seeds.

Development of embryo

  • The embryo formation stars after a certain amount of endosperm is formed as there is an assured supply of nutrition to the embryo.
  • The zygote divides mitotically to give rise to proembryo first and then subsequently to the globular, then heart -shaped and ultimately becomes horseshoe shaped mature embryo with one or more cotyledon.

(a) Fertilised embryo sac showing zygote and Primary Endosperm Nucleus (PEN);
(b) Stages in embryo development in a dicot [shown in reduced size as compared to (a)]

Differences between dicotyledonous and monocotyledonous embryo
Dicotyledonous Embryo (Seed)
Monocotyledonous Embryo (Seed)
The embryo consists of Two cotyledons and an embryonal axis between them.
The embryo has only one cotyledon (Scutellum) present towards one side of the embryonal axis.
The portion of embryonic Axis above the level of cotyledon is the epicotyl and terminates in the plumule.
The embryonal axis has the epicotyl (plumule) at its upper end and is covered over by a hollow folial structure called coleoptile.

The portion of embryonal axis below the level of attachment of cotyledon is hypocotyl and it terminates in the radical.
The embryonic Axis has the hypocotyl (radical) on the lower end and it is covered by undifferentiated sheet called coleorhiza

For example: Pea, bean.
For example: Rice, wheat, maize


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

Seed

  • A seed is a ripped, fertilised ovule.
  • It consists of a seed coat, cotyledons and embryonal axis.
  • Seed coat is double layered formed by integuments.
  • Micropile is a small opening found on seed coat. It facilitates the entry of water and oxygen into the sea during germination.
  • The cotyledons are generally thick and swollen with food material.
  • As the seed matures, the water content is reduced and the seed becomes dry. The embryo enters a state of inactivity called dormancy.

Advantage of seeds:
In plants:
  1. Seeds have better adaptive strategies for dispersal to new habitat for better survival.
  2. Seeds have reserve food material to nourish the seedling during germination.
  3. The hard seed coat provides protection to young embryo.
  4. Seeds are produced after sexual reproduction, they show genetic recombination leading to variation.

To mankind:
  1. Seeds are stored and used as food throughout the years.
  2. Seeds are also used to raise the crops in the favourable seasons depending on their viability.

Structure of some seeds

Development of fruits

  • Fruits may be defined as the mature form of ripened ovary.
  • The ovary begins to enlarge simultaneously with the development of seed and ultimately become the fruit.
  • The ovary wall becomes the fruit wall called pericarp, which helps in the disposal of seeds.

Types of Fruits:
True fruits: The fruits derived from the ovary of a flower not associated with any non-carpellary part is termed as true fruit, for example- Mango and tomato. The other floral parts degenerate and fall-off.
False fruits: The fruit derived from ovary along with other accessory floral parts like thalamus is called false fruit, for example- Apple cashew nut, strawberry etc.

False fruits of apple and strawberry
Parthenocarpic fruits: Some fruits develop without undergoing fertilization, these are called parthenocarpic fruits, for example- banana and this process of formation of fruit without fertilization in plants is parthenocarpy.
  • Such fruits are seedless.
  • Parthenocarpy can be induced with the help of growth hormones or regulator's like gibberellins and auxins.

Parthenogenesis

  • it is the formation of an embryo directly from the female gamete or egg cell.
  • Haploid parthenogenesis is found in Solanum nigrum (Brinjal).

Polyembryony

Some angiosperms produce more than one embryo in a seed. This is known as polyembryony. It was first noticed by Lewenhoek in seeds of oranges.
Various types of polyembryony are:
  • Cleavage- for example; Nicotiana rustica
  • Embryos from cells of embryo sac other than egg- for example; Argemone mexicana (dry nut).
  • Embryo arises from cells outside embryo sac- for example; Citrus.
  • Embryo from endosperm- for example; Alnus.
  • Other examples are onion groundnut, mango etc.

Note: Nucellus adventive polyembryony is a great significance in horticulture.

Apomixis

  • It is a form of asexual reproduction that mimics sexual reproduction but produces seeds without fertilization.
  • It does not involve formation of zygote through the gametic fusion.
  • It occurs in some species of Asteraceae and grasses.
  • Apomictic seeds are viable seeds produced without pollination or sexual reproduction.
  • These are produced from segments of fruits (Mango stem), male gametic content of pollen (Cyperus) and other vegetative parts.
  • Apomixis is extensively used in the hybrid seed industry. As production of hybrid seeds is costly, so if hybrids are made into apomicts, there is no segregation of character in the hybrid progeny.
  • Then the farmer need not buy hybrid seeds year after year and can use apomictic seeds to raise new crops every year.

Amphimixis

Formation of new individuals through normal process of sexual reproduction by meiotic formation of gametes and their subsequent fusion during fertilization.

Artificial hybridisation:

In this, crossing is made in such a way that only desired pollen grains are utilised for the process of pollination.
The stigma is protected from contamination by unwanted Pollen.
This is achieved by emasculation and bagging. Emasculation: Removal of anthers from floral buds before the anther dehisces using a forcep is called emasculation.
Bagging: When emasculated flowers are covered by bags of suitable size usually made up of butter paper to prevent contamination of its stigma with unwanted pollen is called bagging.
After, pollen grains from desired male gamete are dusted on stigma and then the flowers are rebagged.
It is one of the major approach of the Crop program.

Incompatibility:
  • It is the ability of functional male and female gametes to affect fertilization in particular combinations.
  • It is the interior part of pollen pistil interaction.
  • It is either interspecific or intraspecific.
Self incompatibility (intra- specific): In this, fertilization is prevented between gametes which originate from the same individual and some other individual of the same species.
Significance:
  • It may be used in hybrid seed production.
  • In nature, inbre.eding and outbreeding of plants is regulated by self incompatibility.



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