NEET Biology Sexual Reproduction in Flowering Plants Notes

This NEET Biology notes page on Sexual Reproduction in Flowering Plants (Chapter 2) covers the structure of the flower, the male and female reproductive units, microsporogenesis and megasporogenesis, the embryo sac, types of pollination and pollinating agents, pollen-pistil interaction, double fertilization, post-fertilization changes, seed and fruit formation, and apomixis with polyembryony.

Key Points to Pay Special Attention To

Terms and Definitions
1. Pedicel
2. Tetrasporangiate tapetum
3. Microspore mother cell (MMC)
4. Translator apparatus
5. Pollinium
6. Pollen kit
7. Syncarpous
8. Apocarpous
9. Filiform apparatus
10. Antipodal
11. Cleistogamy
12. Chasmogamy
13. Geitonogamy

Flow Charts
1. A brief working of female reproductive organ
2. Pollination

Tables
1. Some important flowers
2. Difference between self pollination and cross pollination
3. Difference between dicot and monocot embryo

Sexual Reproduction in Flowering Plants: Overview

Sexual reproduction in flowering plants involves the transformation of diploid sporophytic cells into haploid gametophytic cells by meiosis, and the subsequent fusion of haploid gametes of opposite sex to form a diploid zygote. The zygote then develops into an embryo, which ultimately forms a diploid plant body. In flowering plants, all these steps of sexual reproduction occur within specialized reproductive organs called flowers.

Sexual reproduction in plants includes microsporogenesis, megasporogenesis, pollination, fertilization, embryogenesis and seed formation.

At a Glance

• Flower
• Pre-fertilization: Structures and Events
  • Male Reproductive Units: Stamen, Microsporangium, Microsporogenesis, Pollen grains
  • Female Reproductive Unit: Gynoecium (Pistil), Megasporangium, Megasporogenesis, Embryo sac
  • Pollination: Self Pollination, Cross Pollination, Agents of Pollinations, Pollen-Pistil Interaction
• Double Fertilization
• Post-fertilization Structure and Events: Endosperm, Embryo, Seed
• Apomixis and Polyembryony

Flower

Flowers are the most fascinating organs in plants, found only in angiosperms, that is plants in which seeds are enclosed within fruit. Flowers and their parts are responsible for sexual reproduction in angiosperms, and all flower-bearing plants reproduce sexually. The colour, fragrance and modification in shape of a flower ensure its sexual reproduction. Fruits and seeds are the end products of sexual reproduction.

Flowers are highly modified shoots, bearing nodes and modified floral leaves, which are meant essentially for sexual reproduction. The flower is commonly borne on a short or long stalk called the pedicel. It has an upper swollen region known as receptacle, thalamus or torus.

[Figure: Fig. 2.1 L.S. of flower, labelled sepal, petal, stamen, anther, filament, stigma, style, ovary, ovules, thalamus and pedicel]

Flowers usually have four different kinds of floral members arranged in four whorls attached on the receptacle: calyx, corolla, androecium and gynoecium. Of these, the two lower whorls (calyx and corolla) are sterile and considered non-essential, accessory or helping whorls. The two upper whorls (androecium and gynoecium) are fertile and considered essential or reproductive whorls.

(i) Calyx: It consists of a number of green leafy sepals. It is the outermost whorl. Calyx protects all the inner floral whorls in the bud condition, and it synthesizes food for the plant body when green in colour.
(ii) Corolla: It consists of a number of usually bright coloured petals and is the second whorl in the flower.
(iii) Androecium: This is the male whorl and consists of stamens. Each stamen can be regarded as a modified leaf (microsporophyll) that consists of a filament and an anther. The anther may contain two to four microsporangia that produce a large number of pollen grains.
(iv) Gynoecium or pistil: It is present in the centre of the flower and is the female reproductive whorl. Its unit is called carpel. Each carpel is made up of three parts: ovary, style and stigma. The ovary is the swollen basal part of the carpel that encloses ovules, and each ovule encloses an embryo sac containing an egg. The stigma is the receptive part of the carpel, whereas the style is the tubular structure that joins stigma and ovary.

Thus, androecium and gynoecium are the two innermost whorls of the flower in which sexual reproduction takes place. The anther bears spores in sporangia, and the ovary contains ovules, which bear embryo sacs with egg.

Some Important Flowers

Functions of a Flower

Flowers are modifications of shoots to perform the function of sexual reproduction. The fertile leaves become microsporophylls (stamens) and megasporophylls (carpels), which bear anthers and ovules respectively. The anthers produce pollen grains, and the ovules possess eggs.
(ii) Flowers of most angiosperms are shaped variously to help diverse modes of pollination.
(iii) Flowers provide the site for germination of pollen, development of pollen tube, formation of gametes and fertilization.
(iv) The ovary part of the carpel gets transformed into fruit, and the ovules are transformed into seeds after fertilization.
(v) Some floral parts like calyx and various modifications in ovaries help in the dispersal of fruits and seeds.

Pre-Fertilization: Structures and Events

The process of fertilization starts much before the actual fertilization takes place. Formation of the flower and its component parts, stamen and pistil, is a prerequisite for fertilization. Pre-fertilization includes the male reproductive unit, the female reproductive unit and pollination.

(i) Male Reproductive Unit

The male reproductive unit is studied under the following heads: stamen (structure of anther), microsporangium, microsporogenesis and pollen grains.

Stamen
The stamen is the male reproductive unit of angiosperms. It consists of anther, connective and filament. The long and slender stalk is called the filament, and the terminal structure is the anther.

Typically, the angiospermic anther is bilobed, connected by the connective, and dithecous, that is each lobe has two thecae. The bilobed structure of the anther is tetrasporangiate (four sporangia). In transverse section, it appears as a four-sided tetragonal structure consisting of two microsporangia in each lobe.

Microsporangia develop into pollen sacs filled with pollen grains.

The number, form and length of stamens vary from species to species.

[Figure: Fig. 2.2 (a) A typical stamen, with anther, filament and pollen sacs; (b) three-dimensional cut section of an anther, showing the line of dehiscence and pollen grains]

Microsporangium

A typical microsporangium usually appears circular in outline, with sporogenous tissue at the centre surrounded by four layers of different kinds of cells.

There are two types of tapetum: amoeboid (plasmodial) and secretory (glandular). In amoeboid or plasmodial tapetum, the inner and radial walls break down at an early stage and these cells are free in the microsporangium. In secretory or glandular tapetum, the tapetum remains as such throughout.

Each microsporangium is surrounded by four layers as follows:
(i) Epidermis: The outermost single-layered and protective layer.
(ii) Endothecium: Inner to the epidermis, a single layer of radially elongated cells. Cells possess bands of callose and help in dehiscence of anthers at maturity.
(iii) Middle layers: Three to four layers of thin-walled cells situated just below the endothecium. They are crushed or, where persistent, contain starch and other reserve material to be consumed by the growing microspore mother cells.
(iv) Tapetum: A single-celled innermost layer surrounding the sporogenous tissue, having dense cytoplasm and being multinucleate. Its main function is to provide nutrition to the developing microspore mother cells (MMC) and pollen grains. It also produces enzymes, IAA and food materials. In tapetal cells, nuclear division is not followed by cell division, hence a binucleate or multinucleate condition is achieved.

[Figure: Fig. 2.3 T.S. of young anther; Fig. 2.4 enlarged portion of microsporangium showing epidermis, endothecium, middle layers, tapetum and sporogenous cells; Fig. 2.5 T.S. of a mature dehisced anther showing epidermis, endothecium, middle layers, tapetum, pollen grains and dehisced anther wall]

Labels of Fig. 2.3 (T.S. of young anther): Epidermis, Endothecium, Middle layers, Tapetum, Microsporangium, Connective tissue.

Microsporogenesis

Microsporogenesis is the process of formation of microspores or pollen grains.

Each cell of the sporogenous tissue serves as a microspore mother cell (MMC). These MMCs undergo meiosis and form a microspore tetrad, which is haploid.

As anthers mature, the microspores of the tetrad separate from each other and develop into pollen grains. Each microspore is a unicellular, haploid, oval or rounded structure.

Each microsporangium contains numerous pollen grains, which are released after dehiscence of the anther wall.

Dehiscence of anther is of four types:
(i) Longitudinal dehiscence (for example Datura).
(ii) Transverse dehiscence (for example Ocimum).
(iii) Porous dehiscence (for example Solanum).
(iv) Valvular dehiscence (for example Berberis).

[Figure: Fig. 2.6 Microsporogenesis, showing microspore mother cell, meiosis I, microspore dyad, meiosis II and microspore tetrad]

Connecting concepts: In Drosera, Typha, Elodea and Hydrilla, all four pollen grains of a tetrad do not separate and thus form compound pollen grains. In the family Cyperaceae (Cyperus), out of four pollen in a tetrad, three degenerate and one remains alive, so one meiosis produces one pollen. Sometimes more than four pollen are produced from one microspore mother cell; this is called polyspory, for example Cuscuta. In Calotropis (Asclepiadaceae) and some orchids, all the pollen grains of an anther lobe form a typical structure called pollinium. The pollinium of each anther lobe protrudes to one side and forms a rod-like sterile structure called retinaculum or caudicle. The retinacula of both pollinia are attached to a dotted, dark-coloured gland called corpusculum, which lies on the stigma. Corpusculum and stigma together are called the translator apparatus.

[Figure: pair of pollinia, showing the adhesive bilobed pollinium, the hygroscopic caudicle, the corpusculum and the translator apparatus with stigma]

Pollen Grains

Within each pollen sac there is a fine powdery granular mass of cells called pollen grains. These are the male reproductive bodies of a flower. Generally they are spherical, measuring 20 to 25 micrometres in diameter.

Pollen grains consist of an outer exine and an inner intine. The terms intine and exine were given by Fritzsche (1837).

Exine is a tough, cutinized layer, often with spinous outgrowths but sometimes smooth. It is composed of sporopollenin, which is resistant to physical and biological decomposition, and it protects the pollen from environmental extremes. Germ pores or germ slits are apertures present on the exine layer of the pollen grain. At these pores, sporopollenin is absent.

Exine is differentiated into an inner endexine and an outer ektexine. Ektexine is formed of three layers: (i) inner continuous foot layer, (ii) middle discontinuous baculate layer and (iii) outer discontinuous tectum. The tectum provides a characteristic sculpturing or design over the surface of pollen grains.

Intine is thin, delicate, cutin and pectocellulosic layer lying internal to the exine.

Pollen grains in pine are provided with two distinct wings.

When pollen grains germinate, the intine grows out into a tube called the pollen tube, through a germ pore present on the exine.

A mature pollen grain contains two cells: a large, nucleated vegetative cell, which is irregular in shape with abundant food reserve, and a small, spindle-shaped generative cell.

In most angiospermic species, pollen grains are shed at the two-celled stage, while others shed them at the three-celled stage.

Pollen grains of many species cause severe allergies and respiratory problems, such as asthma and bronchitis. Parthenium, or carrot grass, is a well-known pollen allergen.

Pollen grains are rich in nutrients and are therefore used as food supplements. Athletes and race horses use these as tablets to enhance performance.

The viability period of pollen grains varies from species to species. In cereals (rice and wheat) it is about 30 minutes, while in some plants, for example Rosaceae and Leguminosae, they may remain viable for months.

Viability of pollen grains can be retained for longer duration by keeping them in liquid nitrogen (minus 196 degrees Celsius).

[Figure: Fig. 2.7 Pollen development in flowering plants, showing anther, stomium, conducting strand, generative cell, tube cell, germ pore, exine, intine, pollen tube, tube nucleus, style, ovary and ovule; Fig. 2.8 Pollen grain, showing exine, intine, germ pores, vegetative cell and generative cell]

Terms related to pollen grains
Palynology: Study of pollen grains or microspores.
Geopalynology: Study of fossil pollen grains.
Aeropalynology: Study of pollens found in air or atmosphere.
Iatropalynology: Study of pollens in criminology and medical aspects.
Melittopalynology: Study of pollen grains in honey.

In insect-pollinated flowers, the exine of the pollen grain is covered with a yellowish, viscous and sticky substance called pollenkit. This is perhaps the protective envelope, which also sticks to the body of insects and thus helps in pollination. It is chiefly made up of lipids and carotenoids. In monocots, germ pores are absent and there is one germinal furrow. The development of the male gametophyte from the pollen grain is called microgametogenesis.

Functions of pollenkit:
It helps in insect attraction (entomophily).
It helps in adhering to the body of insects and thus helps in the dispersal of pollen grains.
It protects the pollen grains from UV radiation.
It indirectly takes part in sporophytic incompatibility.

(ii) Female Reproductive Unit

The female reproductive unit is studied under the following heads: gynoecium or pistil, megasporangium or ovule, megasporogenesis and embryo sac.

Gynoecium or Pistil

Gynoecium or pistil is the female reproductive part, and the carpel is the unit of gynoecium. A carpel is considered a modified leaf bearing ovules, or a megasporophyll. The folded carpel bears one or more ovules along the ventral suture.

A gynoecium may consist of one or more carpels. When it consists of more than one carpel, it is called multicarpellary; when it consists of a single carpel, it is called monocarpellary, for example pea and beans.

If there is more than one carpel and they are fused, it is called syncarpous (for example mustard, lemon and lily). If the carpels lie free from each other, it is called apocarpous (for example rose).

A carpel is distinguished into three parts: stigma, style and ovary. The stigma is a small rounded or lobed head that serves as a landing platform for pollen grains. The style is a slender stalk supporting the stigma. The ovary is the swollen basal part, which contains one or more chambers called locules.

The ovary contains one or more small rounded or oval egg-like bodies on its inner side, called ovules (megasporangium). Ovules are borne on a ridge of tissue called the placenta, lying on the inner wall of the ovary. The arrangement of ovules on the placenta is called placentation.

There may be one or more ovules in an ovary.
Ovary with a single ovule: sunflower, mango, paddy and marigold.
Ovary with many ovules: pea, watermelon, mustard and water lily.

[Figure: Fig. 2.9 Gynoecium: (a) syncarpous pentacarpellary pistil of Hibiscus; (b) syncarpous multicarpellary pistil of Papaver; (c) syncarpous multicarpellary gynoecium of Michelia, showing free carpels, stigma, style, ovary and thalamus]

Megasporangium (Ovule)

The integumented nucellus, or megasporangium, is called the ovule. Each ovule is attached to the placenta by a slender stalk known as the funicle. The point of attachment of the body of the ovule with the funicle is known as the hilum. In anatropous ovules, the funicle continues beyond the hilum along the body of the ovule, forming a ridge called the raphe.

The body of the ovule consists of parenchymatous cells called nucellus. The nucellus is surrounded by one or two coverings called integuments. A small gap left in the integuments at one end of the ovule is called the micropyle. The basal part of the ovule is called the chalaza. A large oval cell called the embryo sac, or female gametophyte, is embedded in the nucellus towards the micropylar end.

[Figure: Fig. 2.10 Structure of a typical anatropous ovule before fertilization, labelled chalaza, raphe, nucellus, integuments, antipodal cells, central cell, egg (oosphere), embryo sac, synergids, vascular strand, funiculus, micropyle and secondary nucleus]

Types of ovule in relation to integuments
Bitegmic: Ovule with two integuments, as in polypetalous (Archichlamydeae) dicotyledons and monocotyledons.
Unitegmic: Ovule with a single integument, for example synpetalous or gamopetalous dicotyledons.
Aril: A collar-like outgrowth from the base of the ovule that forms a third integument. Aril is found in litchi and nutmeg.
Caruncle: An outgrowth of the outer integument in the micropylar region. It is common in the ovules of Euphorbiaceae, for example castor (Ricinus).
Ategmic: In some parasites like Loranthus, Viscum and Santalum there is no integument. Such an ovule is called ategmic.

Kinds of ovules, depending upon shape and orientation
Orthotropous or atropous: The micropyle, chalaza and funicle are in a straight line. This is the most primitive type, for example Betel, Piper and Polygonum.
Anatropous: The body of the ovule is completely inverted (turned at 180 degrees), so that the micropyle and hilum lie very close to each other, for example 82% of angiosperm families.
Hemianatropous: The ovule turns at 90 degrees upon the funicle, or the body of the ovule is at right angles to the funicle, for example Ranunculus.
Campylotropous: The ovule is curved more or less at right angles to the funicle, and the micropylar end is bent down slightly, for example members of Leguminosae and Cruciferae, such as Capsella.
Amphitropous: The curvature of the ovule is greater and the embryo sac becomes curved like a horseshoe, for example Lemna, poppy and Alisma.
Circinotropous: The ovule is initially orthotropous but becomes anatropous due to unilateral growth of the funicle. Growth continues until the ovule once again becomes orthotropous. As a result, the funicle completely surrounds the body of the ovule, for example Opuntia (prickly pear).

[Figure: Fig. 2.11 Different forms of the ovule in longitudinal section: (a) orthotropous, (b) anatropous, (c) hemianatropous, (d) campylotropous, (e) amphitropous, (f) circinotropous, with labels for micropyle, chalaza, funicle, integuments, nucellus and embryo sac]

Megasporogenesis

A single MMC with dense cytoplasm and a prominent nucleus differentiates from the nucellus near the micropylar region. This MMC undergoes meiosis to form four haploid cells called megaspores. The process of formation of megaspores from the megaspore mother cell (MMC) is called megasporogenesis.

Female Gametophyte (Embryo Sac)

Out of the four haploid megaspores, either a few degenerate or all take part in embryo sac formation.

Depending upon the number of megaspores taking part in the formation of the female gametophyte, it is classified into three types:
(i) Monosporic type: Only one megaspore undergoes division to form the female gametophyte, while the other three degenerate. All nuclei are genetically identical.
(ii) Bisporic type: Two nuclei take part in the development of the female gametophyte.
(iii) Tetrasporic type: All four nuclei take part in the development of the female gametophyte.

Female gametophyte development in the monosporic type: functional megaspore → (mitosis) → 2 nuclei → (mitosis) → 4 nuclei → (mitosis) → 8 nuclei → walls are laid down → nucleate embryo sac.
These mitotic divisions are not accompanied by immediate wall formation, that is, they are free-nuclear. After the formation of 8 nuclei, walls are laid down, thus forming the female gametophyte or embryo sac.

A mature embryo sac consists of an egg apparatus, a central cell and antipodals.

(i) Egg apparatus
It consists of two synergids and one egg cell lying at the micropylar end. Synergids bear a prominent structure called the filiform apparatus, which is made of finger-like projections. This apparatus is present in the upper part of each synergid.
Synergids guide the path of the pollen tube towards the egg, help in obtaining nourishment from the outer nucellar cells, and also function as shock absorbers during the penetration of the pollen tube into the embryo sac.
The cytoplasm of the egg is inactive, rich in ribosomes, and contains plastids. During fertilization, the first male gamete fuses with the egg nucleus. This process is called syngamy, or true fertilization, and results in a diploid zygote.

(ii) Central cell
Two polar nuclei form the central cell. It is associated with the egg apparatus and antipodals through plasmodesmatal connections. Before fertilization, the nuclei of the central cell fuse and form a diploid nucleus called the secondary nucleus. The secondary nucleus takes part in triple fusion, which leads to the formation of a primary endosperm mother cell. It divides by mitosis and forms a nutritive tissue called endosperm, which provides nutrition to the developing embryo.

(iii) Antipodals
Three cells present at the chalazal end constitute the antipodals. Their protoplasm contains a rich amount of starch, protein, lipids, ascorbic acid and oxidases. They limit the chalazal expression and transfer nutrients and growth hormones to the developing embryo sac or to the developing embryo.

Thus, a typical mature embryo sac of an angiosperm is a seven-celled, eight-nucleate structure: three antipodal cells, three egg apparatus cells and one central cell (with two polar nuclei).

[Figure: Fig. 2.12 Development of embryo sac in stages (A) to (I), showing ovule primordium, archesporial cell, primary parietal cell, functional megaspore, degenerating megaspores, polar nuclei, egg apparatus, secondary nucleus and antipodal cells; Fig. 2.13 Normal or Polygonum type of embryo sac, showing egg apparatus, synergids with filiform apparatus, egg cell, central cell with polar nuclei and antipodals]

Flow Chart 2.1: A brief working of female reproductive organs

• Gynoecium / Pistil (female reproductive organ, consists of one or many carpels)
  • Carpels (unit of gynoecium) consist of:
    • Stigma: pollen receptive organ
    • Style: pollen tube receptive part
    • Ovary: contains one or more Ovules, which contain the Embryo sac (female gametophyte)
      • In the middle of the embryo sac: Egg apparatus, which consists of:
        • Egg: acquires the middle position in the egg apparatus; it is the female gamete or egg nucleus
        • Synergids: occupy the lateral position of the egg cell
      • Towards the chalazal pole of the embryo sac, it consists of:
        • Secondary nucleus: consists of two polar nuclei (in most cases), each coming from a pole, meeting in the centre of the embryo sac and forming a diploid secondary nucleus. It fuses with the second male gamete during triple fusion and forms a triploid primary endosperm mother cell (PEMC).
        • Antipodals: provide nutrition to the embryo sac

Pollination

The process of transfer of pollen grains from an anther to the stigma of the same flower or of a different flower is called pollination. This process of pollination occurs only in gymnosperms and angiosperms. It is facilitated by various factors such as wind, water, insects, birds and animals.

In angiosperms, pollination generally takes place at the two-celled stage (rarely the three-celled stage) of the microspore or pollen, that is the vegetative cell and generative cell.

Chasmogamy and Cleistogamy
Pollination in flowers that open and expose their sex organs is called chasmogamy, and such flowers are called chasmogamous flowers. In chasmogamy, pure autogamy is not possible because pollen from other flowers can also land on the stigma. The majority of angiosperms have chasmogamous flowers, for example Catharanthus and Mirabilis.

Pollination that occurs in closed flowers is called cleistogamy, and such flowers are called cleistogamous flowers. In cleistogamous flowers, the reproductive parts are not exposed. Anthers and stigma lie close to each other. Pure autogamy occurs since there is no chance of cross-pollination. Cleistogamy is the most efficient floral adaptation for promoting self-pollination, for example Viola mirabilis and Oxalis autosella.

[Figure: Fig. 2.14 Cleistogamous flower and chasmogamous flower]

Types of Pollination
Pollination is of two types: self pollination and cross pollination.

Self pollination involves the transfer of pollen grains from the anther to the stigma of the same or a genetically similar flower. It always occurs in crops with bisexual flowers, for example wheat and barley. Self pollination is a rule in cleistogamous flowers. It is of two types: autogamy and geitonogamy.
Autogamy: Pollen from the anthers of a flower is transferred to the stigma of the same flower.
Geitonogamy: Pollen from the anthers of one flower is transferred to the stigma of another flower borne on the same plant. It usually occurs in plants that show a monoecious condition (male and female flowers borne on the same plant). Geitonogamy involves two flowers, but both belong to the same parent plant.
Merits: Pollen grains are not wasted and the purity of the generation is maintained.
Demerits: New and healthier varieties are not formed. It results in weaker progeny, producing weaker seeds and plants.

Cross pollination involves the transfer of pollen grains from the flower of one plant to the stigma of a flower of another plant. It is also called xenogamy. It is performed with the help of an external agency.

[Figure: Fig. 2.15 Self and cross pollination, showing cross pollination (xenogamy) by an insect, self pollination (autogamy) within one flower, and self pollination (geitonogamy) between flowers of the same plant]

Dichogamy, dicliny, herkogamy, heterostyly and self-sterility are the main reasons or adaptations for cross pollination in bisexual flowers.
(i) Dichogamy: Maturation of male and female sex organs occurs at different times in bisexual flowers, for example the Cruciferae family.
(ii) Herkogamy: Male or female sex organs themselves act as a barrier to prevent self pollination, for example Cypripedium and orchids.
(iii) Dicliny or unisexuality: Presence of unisexual flowers in plants prevents self pollination, for example castor, maize and papaya.
(iv) Heterostyly: Presence of styles of different heights in the flowers of many plants prevents self pollination, for example Primula vulgaris and Oxalis.
(v) Self-sterility (self-incompatibility): Pollen from the same flower is incapable of bringing about fertilization, due to the presence of a self-sterile gene, for example tobacco and potato.

Advantages of cross pollination
It increases the adaptability of offspring.
It induces genetic recombination and hence variation in progeny.
Plants produced are more disease resistant.
New and more useful varieties are produced.

Disadvantages of cross pollination
It is a highly wasteful method, because plants must produce a number of devices to help pollen grains use external agencies for transport.
The method is dependent on external agents.
Very good characters of the race get diluted.
Some undesirable characters can enter a race.

In nature, cross pollination is a better method of pollination because it avoids recessive lethal or harmful genes becoming homozygous, it produces healthier plants due to hybrid vigour, and it keeps the variability and hence the adaptability of the race, so that an adverse environment does not destroy the species.

Difference between self pollination and cross pollination

Agents of Pollination

Cross pollination involves external agents for the transfer of pollen grains from one flower to the stigma of another flower. There are two main groups of agents: (i) abiotic agents, like wind and water, and (ii) biotic agents, which include animals such as insects, birds, bats and snails.

(i) Anemophily (pollination by wind)
In anemophily, pollen grains reach the stigma through winds. A large number of pollen grains are formed compared with the number of ovules available for pollination. Pollen grains are small, light, smooth and dry. The stigma is large and feathery for trapping windborne pollen grains. Wind-pollinated flowers have few ovules, or even one ovule in the ovary, because the chances of pollination are rare. Anemophilous flowers are small and inconspicuous, with long and versatile stamens, for example sugarcane, maize, wheat, bamboo, Pinus, papaya, grasses, Typha, date palm, coconut, mulberry and Chenopodium. This type of pollination is mainly observed in Graminae. A tassel of maize gives rise to 20 to 25 million pollen grains.

[Figure: Fig. 2.16 A wind-pollinated plant showing compact inflorescence and well-exposed stamens]

(ii) Hydrophily (pollination by water)
Pollination by water is called hydrophily and is rare in nature. The floral envelope of these plants is highly reduced or absent, thus exposing stamens and stigma. Pollination can occur within water, for example in Zostera and Ceratophyllum, or on the surface of water, for example in Vallisneria.

In Vallisneria (ribbon weed), the female flower reaches the surface of water and assumes a horizontal position due to elongation of its stalk. Male flowers or pollen grains are released on to the surface of water. They are carried passively by water currents, and some eventually reach the stigma.

Both wind- and water-pollinated flowers do not need to attract insects or birds, so they are not very colourful and do not produce nectar. Pollen grains of most water-pollinated species are coated by mucilage that protects them from wetting.

Water is an important medium for fertilization in algae, bryophytes and pteridophytes.

[Figure: Fig. 2.17 Pollination by water in Vallisneria, showing the female flower on the water surface, the male flower and the stigma]

(iii) Biotic agents

Entomophily (pollination by insects) is pollination by insects such as bees, wasps, butterflies and flies. Bees are responsible for over 80% of all pollination done by insects. Plants pollinated by insects are colourful, fragrant and have abundant nectar, which attracts insects.
Apart from searching for nectar, insects also visit flowers to lay eggs. For example, in Amorphophallus a moth deposits its eggs within the ovary of the flower, and the flower in turn gets pollinated by the young moth.
Some insects consume pollen from male organs and nectar from female flowers without facilitating pollination. Such insects are called pollen or nectar robbers.
Connecting concepts: Salvia is an excellent example of insect pollination in which pollination occurs by the lever or turn-pipe mechanism. Other examples are Yucca (by Tegeticula moth), Ophrys speculum (by Colpa aurea, a hairy wasp) and Ficus (by Blastophaga). The flower of the orchid Ophrys resembles in shape, colour and odour the female wasp of Colpa aurea (mimicry). The male wasps pollinate the flowers mistaking them for females (pseudocopulation). Yucca is pollinated by Pronuba (= Tegeticula) yuccasella, which passes its larval stage inside the ripening ovary.

Ornithophily (pollination by birds) is pollination by birds. The most common bird pollinators are sunbird, humming bird, crow, bulbul, parrot and mynah. Flowers are colourful and rich in nectar. The humming bird pollinates while hovering over the flowers and sucking nectar. Birds can derive about half of their body weight of nectar in a single day. The nectar is chiefly made of sugars, which provides a sweet drink to the bird. Examples are Callistemon (bottle brush) and Erythrina (coral tree).

Chiropterophily (pollination by bats) is pollination by bats. Chiropterophilous flowers are dull coloured with a fruity odour, and have abundant nectar and pollen grains. Examples are Kigelia pinnata (sausage tree) and Anthocephalus (kadam tree).

Malacophily is pollination by snails and slugs, for example aroids. Myrmecophily is pollination by ants, for example Rubiaceae.

[Figure: Flow Chart 2.2: Pollination (transfer of pollen grains from anther to stigma) divides into cross pollination (transfer of pollen grains from anther to stigma of different flowers on different plants, requiring an external agency which may be abiotic or biotic) and self pollination (transfer of pollen grains from anthers to stigma of the same flower or another flower borne by the same plant). Self pollination has two types: autogamy (same flower) and geitonogamy (different flowers of the same plant). Abiotic agents: anemophily (by wind), e.g. palm; hydrophily (by water), e.g. Vallisneria. Biotic agents: entomophily (by insects), e.g. Aristolochia, Salvia (bee); ornithophily (by birds), e.g. Bombax, Bauhinia (by humming bird); chiropterophily (by bats); malacophily (by snails and slugs), e.g. aroids; myrmecophily (by ants), e.g. Rubiaceae.]

Pollen-Pistil Interaction

All the events from landing of pollen grains on the stigma until the pollen tube enters the ovule are termed pollen-pistil interaction.

It is not always that the right type of pollen grain falls on the stigma, or, even if the desired pollen interacts with the stigma, it may be incompatible. Pistil and pollen chemical secretions determine the compatibility of pollen grains. If incompatible, either germination of the pollen grain is inhibited or the pollen tube aborts in the style. If compatible, the pollen tube reaches the style and causes fertilization.

In-depth analysis of pollen-pistil interaction can help in the development of hybrids and in overcoming incompatibility.

Emasculation and bagging
Emasculation is the process of removal of the stamens of a flower before they have shed pollen, in order to prevent self pollination and permit cross pollination. Emasculated flowers have to be covered with a bag of suitable size, generally made of butter paper, to prevent contamination of the stigma with unwanted pollen. This process is called bagging. When the bagged stigma attains receptivity, it is dusted with pollen of the desired plant, flowers are re-bagged, and fruits are allowed to develop that contain seeds of the hybrid variety.

[Figure: Fig. 2.18 (a) L.S. of pistil showing the path of pollen tube growth; (b) enlarged view of an egg apparatus showing entry of pollen tube into a synergid; (c) discharge of male gametes into a synergid and movement of sperms, one into the egg and the other into the central cell]

The pollen tube grows through the style towards the ovary. After reaching the ovary, it enters the ovule through the micropyle and enters one of the synergids through the filiform apparatus. If the pollen grain is released at the two-celled stage, the generative cell divides mitotically to produce two male gametes. If the pollen grain is released at the three-celled stage, the two male gametes are already present.

Double Fertilization

The fusion of a male gamete with the female gamete (egg) is called fertilization. The phenomenon of fertilization was reported by Strasburger (1884) in Monotropa.

Fertilization occurs in four steps: (i) germination of pollen on the stigma, (ii) growth of the pollen tube, (iii) pollen tube entry into the embryo sac, and (iv) fusion of gametes.

Pollen grains reach the receptive stigma of the carpel by the act of pollination. After getting attached to the stigma, pollen grains absorb water and swell. After mutual recognition and acceptance of pollen grains, the pollen grain germinates (in vivo) to produce a pollen tube, which grows into the stigma towards the ovarian cavity.

After reaching the ovary, the pollen tube enters the ovule. It may enter the ovule by any one of the following routes:
(i) Porogamy: The pollen tube enters the ovule through the micropyle. This is the most common type, for example in Lily.
(ii) Chalazogamy: The pollen tube enters the ovule from the chalazal region. It is less common, for example in Casuarina, Juglans and Betula. It was first observed by Treub (1891) in Casuarina.
(iii) Mesogamy: The pollen tube enters the ovule through its middle part, that is through the integument (for example Cucurbita and Populus) or through the funicle (for example Pistacia).

[Figure: Fig. 2.19 Various routes of pollen tube entry into the ovule: (a) porogamy, (b) mesogamy, (c) chalazogamy, showing embryo sac, egg apparatus, polar nuclei, antipodals, integuments, funiculus, micropylar pole and chalazal pole]

The pollen tube enters the embryo sac only from the micropylar end, irrespective of its mode of entry into the ovule. The pollen tube either passes between a synergid and the egg cell, or enters one of the synergids through the filiform apparatus. The synergids direct the growth of the pollen tube by secreting chemical substances (chemotropic secretion). The tip of the pollen tube enters one synergid, and the penetrated synergid starts degenerating. After penetration, the tip of the pollen tube enlarges and ruptures, releasing most of its contents, including the two male gametes and the vegetative nucleus, into the synergid.

The two male gametes are released into the embryo sac. One male gamete fuses with the egg to form the diploid zygote (2n). This is called syngamy, or generative fertilization. The other male gamete fuses with the two polar nuclei (or the secondary nucleus) to form the triploid primary endosperm nucleus (3n). This is called triple fusion, or vegetative fertilization. These two acts of fertilization together constitute double fertilization. The process was discovered by S. G. Nawaschin (1898) in Lilium and Fritillaria, and Guignard also contributed. Double fertilization occurs only in angiosperms. The total number of nuclei involved in double fertilization is five: two in syngamy and three in triple fusion.

Significance of double fertilization
The female gametophyte stops its growth at the eight-nucleate stage, and growth continues only after double fertilization.
Syngamy leads to the diploid zygote, which later develops into the embryo. Triple fusion makes the endosperm, which provides nutrition to the developing embryo. Thus, it is necessary for the formation of viable seeds.
The secondary nucleus stops its division before fertilization. Triple fusion induces this dormant nucleus to regain its power of division.
The formation of endosperm is related to the need that arises after fertilization, particularly for the developing embryo.

[Figure: Fig. 2.20 Types of endosperm: nuclear endosperm and cellular endosperm, showing oospore, embryo and endosperm stages]

Post-Fertilization Structures and Events

Post-fertilization events involve the development of endosperm, embryo, ovules and seeds.

Endosperm
Endosperm is the nutritive tissue that provides nourishment to the embryo in seed plants. It also protects the embryo from mechanical injury. Endosperm may be completely consumed by the developing embryo before the seed matures (for example beans and peas), or it may persist in the mature seed and be used up during seed germination (for example coconut).

Endosperm is usually triploid in the majority of plants. The highest ploidy is noted in Arum maculatum. It is 2n in Oenothera, 9n in Peperomia and 5n in Fritillaria.

The fusion of the second male gamete with the central cell in the embryo sac forms the triploid primary endosperm cell. The primary endosperm cell undergoes continuous division without being immediately followed by cell wall formation, leading to a large number of nuclei. This forms free-nuclear endosperm. Later, the free nuclei are followed by cell wall formation and the endosperm becomes cellular.

In gymnosperms, endosperm is formed before fertilization and is always haploid.

On the basis of development, endosperm is of three types: cellular, nuclear and helobial.
Cellular endosperm: lacks a free nuclei stage, for example Datura and Petunia.
Nuclear endosperm: free nuclei are present, for example maize, wheat and rice.
Helobial endosperm: the endosperm is intermediate between cellular and nuclear types. The division of the primary endosperm nucleus is followed by wall formation, and two chambers, micropylar and chalazal, are formed. It is prevalent in monocotyledons, for example Erumurus.

Ruminate endosperm: A mature endosperm with irregularity and unevenness on its surface. Rumination is caused by the activity of the seed coat or by the endosperm itself. It is found in about 32 families of angiosperms, for example Annonaceae, Palmae and Myristicaceae.
Mosaic endosperm: The tissue of endosperm is not homogeneous, but has patches of different colours. This was observed by Webber (1900) in Zea mays, where red and white patches appear irregularly distributed. In Petunia and tomato, the endosperm shows two types of tissue, some consisting of diploid cells and some of triploid cells, which intermix to form a mosaic.
Xenia: The effect of pollen on the endosperm is called xenia. The term was given by Focke (1881), for example maize.
Metaxenia: The effect of pollen on somatic tissue lying outside the endosperm is known as metaxenia. The term was given by Swingle (1928), for example date palm.

Embryo
The zygote formed after fertilization of the egg cell secretes a cellulose wall around itself.

In dicots, after the formation of some endosperm, the zygote elongates and divides into an upper and a lower cell. The lower cell, lying towards the micropyle, divides in one direction into a row of cells called the suspensor. The first cell of the suspensor often enlarges and acts as a haustorium or absorbing organ, while its terminal cell, called the hypophysis cell, divides and gives rise to the apex of the radicle. The upper cell, towards the antipodal end, is called the embryo cell. The embryo cell enlarges and divides repeatedly to form eight cells arranged in two tiers: epibasal (terminal) and hypobasal (near the suspensor). The epibasal tier forms two cotyledons and the plumule in dicots, and one cotyledon in monocots. The hypobasal tier forms the hypocotyl, except its tip.

Dicotyledonous plants have two cotyledons, while monocotyledonous plants have only one cotyledon.

A typical dicotyledonous embryo consists of an embryonal axis and two cotyledons. The portion of the embryonal axis above the cotyledons is the epicotyl, and the portion below is the hypocotyl. The epicotyl terminates in the plumule, and the hypocotyl terminates in the radicle. The plumule gives rise to the shoot, and the radicle gives rise to the root tip.

A mature embryo in monocotyledons has a single cotyledon called the scutellum. At the lower portion of the scutellum, the embryonal axis has the radicle, which bears an apical meristem and a root cap at the lower end. The radicle and root cap are enclosed in the coleorhiza, which is the undifferentiated lower part of the proembryo. The portion of the embryonal axis above the level of the scutellum is the epicotyl. It comprises the shoot apex with some leaf primordia enclosed in a hollow protective structure called the coleoptile.

[Figure: Fig. 2.21 Stages in the development of a dicot embryo: (A) division of zygote into suspensor and embryo cells; (B) formation of suspensor and embryo octant; (C) globular embryo showing regions of radicle, procambium, ground meristem and dermatogen; (D) heart-shaped embryo, with haustorial cell, suspensor cell and root cap]

[Figure: Fig. 2.22 (a) A typical dicot embryo, labelled cotyledons, plumule, shoot apex, hypocotyl, radicle and root cap; (b) L.S. of an embryo of grass, labelled scutellum, coleoptile, coleorhiza, epiblast, plumule, radicle and root cap]

Difference between dicot and monocot embryo

Seed

A seed is a fertilized ovule. It is the final product of fertilization in angiosperms, and it acts as the main propagative unit in plants. Ovules mature into seeds, and simultaneously the ovary develops into a fruit.

Transformation of units of the ovary during seed formation:
Ovary wall becomes fruit wall.
Ovary becomes fruit.
Integuments become seed coats: outer integument becomes testa, and inner integument becomes tegmen.
Ovule becomes seed.

During seed formation, endosperm is consumed in varying amounts by the developing embryo. Depending on this, seeds may be albuminous or non-albuminous.
Albuminous: Seed contains some endosperm that acts as a food storage tissue. For example, dicots such as castor, poppy and custard apple; monocots such as cereals, millets and sugarcane.
Non-albuminous: Seed is devoid of endosperm. For example, dicots such as gram, pea, bean, mustard and mango; monocots such as orchid and Alisma.
Perisperm: In some seeds, remains of nucellus are present, called perisperm, for example black pepper and beet.

As the seed matures, the seed coat hardens, the seed becomes dry and dormant. Under favourable conditions, the period of dormancy ends, oxygen and moisture enter the seed through the micropyle, and seed germination occurs.

[Figure: Fig. 2.23 (a) Structure of some seeds, showing seed coat, micropyle, endosperm, cotyledon, hypocotyl, root axis, shoot apical meristem, scutellum, coleoptile, plumule, radicle, root tip and coleorhiza; (b) false fruits of apple and strawberry, showing thalamus, seed, achene, endocarp and mesocarp]

When floral parts other than the ovary, particularly the thalamus, contribute to fruit formation, such fruits are called false fruits, for example apple, strawberry and cashew. Fruits that develop only from the ovary are called true fruits, for example cucumber, tomato and coconut.

When fruits develop without fertilization, they are called parthenocarpic fruits, for example orange, banana, watermelon and grapes. Parthenocarpy can be induced by the application of growth hormones, and such fruits are seedless.

Advantages of having seeds
Seeds can remain dormant for many years and germinate when favourable conditions return.
Reproductive process is not dependent on water.
Seeds have sufficient food reserves to initiate embryo development and seedling development until photosynthesis begins.
The hard seed coat protects the embryo from environmental extremes.
Since seeds are the product of sexual reproduction, they promote diversity.
Seeds have better adaptive strategies for dispersal to new habitats and help the species to colonize other areas.

Apomixis and Polyembryony

The normal process of the cell cycle involving meiosis and fertilization is called amphimixis.

Apomixis is the production of seeds without fertilization. There are several ways in which apomictic seeds develop. In some species, the diploid egg cell is formed without reduction division and develops into an embryo without fertilization. In some species, the nucellar cells surrounding the embryo sac start dividing, protrude into the embryo sac, and develop into embryos.

Some apomictic plants are citrus, cactus, grasses and Parthenium. Apomixis allows indefinite multiplication of especially favourable biotypes without losing their productivity due to segregation.

Polyembryony is the state of occurrence of more than one embryo in a seed, for example onion and groundnut. It was discovered by A. V. Leeuwenhoek (1719) in citrus. It may be formed by cleavage of the zygote, or by the formation of embryos from cells of the embryo sac other than the egg (usually synergids), or by the development of more than one embryo sac within the same ovule. Polyembryony is practically important because genetically uniform parental-type seedlings are obtained from nucellar embryos. The most important theory of the cause of polyembryony is the neurohormone theory given by Haberland (1921).

Types of polyembryony:
Cleavage polyembryony: Due to cleavage of the zygote or proembryo into two or more embryos, and each split part develops into an embryo. It is common in gymnosperms more than in angiosperms. Erythronium americanum, Nymphaea advena and Crotalaria are angiosperms showing cleavage polyembryony.
Simple polyembryony: Due to the presence of more than one embryo sac, and so more than one oospore or egg, for example Brassica.
Mixed polyembryony: More than one pollen tube enters an ovule and fertilizes synergids (as in Argemone mexicana) and antipodal cells (as in Ulmus americana).
Adventive polyembryony: Diploid nucellus or integument cells form embryos, for example Citrus, Opuntia and Mangifera.

If extra embryos develop from the same embryo sac, it is called true polyembryony. If embryos develop elsewhere, it is called false polyembryony. In Balanophora, an extra embryo develops from the endosperm.

Check Point and Practice Questions from the Chapter

Flowers: True or False
1. Carpel is unit of gynoecium.
2. Calyx is the innermost whorl.
3. Calyx consists of number of green leafy petals.
4. Stigma is the tubular structure which joins style and ovary.
5. Calyx and corolla are essential or reproductive whorls.
6. China rose and sunflower are ornamental flowers.
7. Anther produce pollen grains and ovules possess egg.

Male Reproductive Unit
1. Match the following.
Column I: A. Stamen; B. Microsporangium; C. Tapetum; D. Pollenkit.
Column II: (i) Oily and sticky layer, help in pollination; (ii) Secretes ubisch bodies; (iii) Male reproductive unit; (iv) Surrounded by four layers.
2. Give an example in which pollinium is found.
3. State true or false.
(i) Intine is composed of sporopollenin.
(ii) Cells of endothecium help in dehiscence of anthers at maturity.
(iii) Ocimum shows transverse dehiscence of anther.
(iv) Caudicle and stigma together form translator mechanism.

Female Reproductive Unit
1. ________ is the unit of gynoecium.
2. __________ consists of 3 parts: stigma, _______ and ________.
3. Mango and paddy have ovary with ________ ovules.
4. ________ is the point of attachment of the body of the ovule with the funicle.
5. In _________ ovule, micropyle, chalaza and funicle are in straight line.
6. Ranunculus has ________ ovule, in which the ovule turns at 90 degrees upon the funicle.
7. ________ bears a prominent structure called __________ apparatus, which are finger-like projections.
8. __________ limits the chalazal expression and transfers nutrients and growth hormones to the developing embryo.

Pollination
1. In _______, pollination occurs by lever or turn-pipe mechanism.
2. Autogamy and geitonogamy are two types of ________ pollination.
3. In cleistogamy, flowers do not expose their reproductive organs or parts. (True or false)
4. Self pollination is a rule of chasmogamous flowers. (True or false)
5. Mention any two reasons or adaptations for cross pollination in bisexual flowers.
6. Give two examples each of self pollination and cross pollination.

Pollen-Pistil Interaction
1. Match the following.
Column I: A. Anemophily; B. Hydrophily; C. Chiropterophily; D. Malacophily.
Column II: (i) Vallisneria; (ii) Maize; (iii) Erythrina; (iv) Chrysanthemum.
2. Give one word for the process of removal of the stamens of a flower before they have shed pollen.

Double Fertilization
1. In which plant was double fertilization discovered by Nawaschin?
2. Fill in the blanks: Pollen tube discharges two male gametes into the __________. One male gamete fuses with egg, this event is called _______. It results into _________, which is the first cell of the next generation.
3. Which term is used when the pollen tube enters the ovule through the integument or funiculus?

Embryo and Endosperm
1. Fill in the blanks.
(i) A single cotyledon called scutellum is present in the mature embryo in ______.
(ii) __________ give rise to shoot while _______ give rise to root tip.
(iii) Endosperm is always _______ when it is formed before fertilization in gymnosperms.
2. Give two examples of cellular endosperms.
3. Give two examples each of albuminous and non-albuminous seeds.
4. State true or false.
(i) When thalamus contributes to fruit formation, then it is called false fruits.
(ii) Amphimixis is the state of occurrence of more than one embryo in a seed.