Sexual reproduction in flowering plants | ISC Class 12 Biology Notes

This note covers flower structure, pollen and embryo-sac development, pollination, pollen-pistil interaction, artificial hybridisation, double fertilisation, endosperm and embryo development, seeds, fruits, seed dispersal, apomixis, parthenocarpy and polyembryony.

How is a flower organised for sexual reproduction?

Angiosperms are flowering plants. Their flowers contain the reproductive structures that produce male and female gametes, the sex cells that fuse during fertilisation. Sexual reproduction leads to the formation of an embryo, the young plant within a seed.

What are the four floral whorls?

A typical flower has four whorls, or rings of floral organs, on the thalamus, the swollen end of the flower stalk. The stalk is called the pedicel. The outer two whorls are accessory organs; the inner two are reproductive organs.

• Calyx: the outermost whorl, made of sepals. Sepals are generally green and leaf-like and protect the flower in the bud stage.
• Corolla: the whorl of petals. Petals are usually brightly coloured and attract insects for pollination, the transfer of pollen from anther to stigma.
• Androecium: the male reproductive whorl, consisting of stamens. A typical stamen has a slender stalk called the filament and a terminal anther, where pollen develops.
• Gynoecium: the female reproductive part, made of one or more carpels, the ovule-bearing reproductive units. Its stigma receives pollen; its style connects the stigma with the basal, swollen ovary containing ovules.

How do reproductive arrangements differ?

A bisexual flower has both androecium and gynoecium. A unisexual flower has either stamens or carpels. Where carpels are free, the gynoecium is apocarpous, as in lotus and rose. Where they are fused, it is syncarpous, as in mustard and tomato.

An ovule is the structure within the ovary that contains the female gametophyte. A gametophyte is the stage of the life cycle that produces gametes. The pollen grain represents the male gametophyte, while the embryo sac within an ovule represents the female gametophyte.

The ovary and ovule must be distinguished throughout reproduction. Following fertilisation, the ovary develops into the fruit, while ovules develop into seeds. The reproductive events therefore connect floral structure with the formation of both the enclosed young plant and its surrounding protective structures.

How do anthers form microspores and pollen grains?

A typical anther is bilobed and contains four microsporangia, the structures in which microspores develop, with two in each lobe. Microspores are the cells that develop into pollen grains. The microsporangia become pollen sacs extending along the anther.

What surrounds a microsporangium?

A typical microsporangium is generally surrounded by four wall layers. From outside inwards, these are the epidermis, endothecium, middle layers and tapetum. The outer three protect the developing contents and help the anther open to release pollen. This opening is called dehiscence.

The tapetum is the innermost nutritive layer. Its cells nourish developing pollen grains and have dense cytoplasm, the cell contents outside the nucleus. The nucleus contains genetic material; tapetal cells generally contain more than one nucleus. A young microsporangium contains compact, similar cells called sporogenous tissue at its centre.

The transverse section, a cut across the anther, shows four pollen-bearing regions. The enlargement labels epidermis, endothecium, middle layers, tapetum and microspore mother cells. The mature dehisced anther is drawn releasing pollen grains.

See Fig. 1.3 in your NCERT textbook

What happens during microsporogenesis?

Microsporogenesis is the formation of microspores from a pollen mother cell through meiosis. Meiosis is a reduction division that halves the number of sets of chromosomes, the structures carrying genetic material. Diploid means two chromosome sets, written 2n; haploid means one set, written n. Here n denotes one chromosome set.

1. Cells of the sporogenous tissue can function as diploid pollen mother cells, also called microspore mother cells.
2. Each mother cell undergoes meiosis, producing four haploid microspores.
3. The four microspores initially remain together as a microspore tetrad, a group of four cells.
4. As the anther matures and dehydrates, the microspores separate and develop into pollen grains.
5. The mature anther dehisces, releasing the pollen formed within its pollen sacs.

Microspore production and gamete production are different events. Meiosis produces microspores; a later division within the developing male gametophyte produces the two male gametes. A pollen grain should therefore not be labelled as a single male gamete.

What makes a pollen grain suited to its reproductive role?

Pollen grains are generally spherical and about 25 to 50 micrometres in diameter. A micrometre is one-millionth of a metre. The pollen wall has an outer exine and an inner intine, with different composition and properties.

How do the wall and cells differ?

The exine contains sporopollenin, a highly resistant organic material that withstands high temperatures and strong acids and alkali. No enzyme that degrades sporopollenin is so far known. Areas called germ pores lack sporopollenin. The thin, continuous intine contains cellulose and pectin, structural wall materials.

A mature two-celled pollen grain contains a large vegetative cell, with abundant food reserves and a large irregular nucleus, and a smaller generative cell. The generative cell is spindle-shaped, has dense cytoplasm and floats within the vegetative cell's cytoplasm.

In over 60 per cent of angiosperms, pollen is shed at this two-celled stage. In the remaining species, the generative cell divides by mitosis, a division maintaining the chromosome-set number, before shedding. Such pollen contains a vegetative cell and two male gametes.

Why do viability and storage matter?

Pollen viability is the capacity of pollen to remain functional for fertilisation. Its duration is highly variable and depends to some extent on temperature and humidity. In some cereals, such as rice and wheat, pollen loses viability within 30 minutes of release.

In some members of Rosaceae, Leguminoseae and Solanaceae, three flowering-plant families, pollen remains viable for months. Pollen of a large number of species can be stored for years in liquid nitrogen at −196°C, where °C means degrees Celsius. Such pollen banks support crop breeding.

Pollen grains are rich in nutrients and are sold as tablets and syrups. Increased performance in athletes and racehorses has been claimed for pollen consumption. Pollen of many species causes allergies in some people; therefore, nutritional content and claimed benefits must not be confused with universal suitability.

How are the ovule and female gametophyte formed?

The placenta is the tissue inside the ovary from which ovules arise. A stalk called the funicle attaches an ovule to it. The hilum is the junction of the funicle and ovule body. In an anatropous ovule, the ovule is inverted, bringing its opening near the funicle.

What lies inside the ovule?

One or two protective envelopes called integuments surround the ovule's central tissue, the nucellus, except at a small opening called the micropyle. The chalaza is the basal region opposite the micropyle. Nucellar cells contain abundant reserve food.

An ovule generally contains a single embryo sac within the nucellus. Ovules generally differentiate one megaspore mother cell near the micropylar region. This large cell has dense cytoplasm and a prominent nucleus. Megasporogenesis is the formation of megaspores from this mother cell.

The drawing labels the funicle, hilum, micropyle, outer and inner integuments, nucellus and embryo sac. The micropylar and chalazal poles are shown at opposite ends. Keep the embryo sac inside the nucellus.

See Fig. 1.7d in your NCERT textbook

How does the embryo sac develop?

1. The diploid megaspore mother cell undergoes meiosis to produce four haploid megaspores.
2. In a majority of flowering plants, three megaspores degenerate and one remains functional.
3. The functional megaspore nucleus divides by mitosis to produce two nuclei, which move to opposite poles.
4. Two further rounds of nuclear division produce four nuclei and then eight nuclei. These divisions are free-nuclear, meaning that walls do not form immediately after nuclear division.
5. Walls then organise the typical embryo sac into seven cells containing eight nuclei.

Monosporic development means development of an embryo sac from one megaspore. At the micropylar end, the egg apparatus consists of an egg cell and two synergids. Synergids are cells whose micropylar wall thickenings, the filiform apparatus, guide pollen-tube entry.

Three cells called antipodals occupy the chalazal end. The large central cell contains two polar nuclei. Thus, three egg-apparatus cells, three antipodals and one central cell account for seven cells; the central cell's two nuclei make the total eight.

The panels show a megaspore mother cell, a two-cell stage and a megaspore tetrad, followed by two-, four- and eight-nucleate embryo sacs. The mature sac labels egg, synergids, filiform apparatus, polar nuclei, central cell and antipodals.

See Fig. 1.8 in your NCERT textbook

How do the types of pollination differ?

Pollination transfers pollen grains from an anther to a stigma. It brings the male gametophyte to the female reproductive organ but does not itself involve fusion of gametes. The pollen's source determines whether the event is autogamy, geitonogamy or xenogamy.

What are chasmogamy and cleistogamy?

Chasmogamous flowers open and expose their anthers and stigmas. Complete autogamy is rather rare in normal open flowers. It requires synchronised pollen release and stigma receptivity, together with anthers and stigma lying close enough for transfer. Receptivity means readiness of the stigma to receive functional pollen.

Cleistogamous flowers do not open at all. Their anthers and stigma remain close. Pollen released inside the bud reaches the stigma, and cross-pollen cannot enter. They are invariably autogamous and provide assured seed-set even without pollinators, the agents that transfer pollen.

Viola, Oxalis and Commelina produce both chasmogamous and cleistogamous flowers. Cleistogamy illustrates the reproductive assurance possible through self-pollination. Xenogamy, in contrast, introduces genetically different pollen and helps avoid the effects of continued self-pollination.

Note: Geitonogamy is functionally cross-pollination because transfer occurs between flowers through a pollinating agent. Genetically, it resembles autogamy because both flowers belong to the same plant. State both aspects when explaining the term.

How do wind, water and insects bring about pollination?

Pollination agents may be abiotic, meaning non-living, such as wind and water, or biotic, meaning living, such as animals. The majority of plants use biotic agents. Pollen reaching a stigma by wind or water is a chance event, so enormous amounts of pollen compensate for losses.

Which features favour wind pollination?

Wind-pollinated plants have light, non-sticky pollen. They often have exposed stamens and large, often-feathery stigmas. Their flowers often contain one ovule per ovary and occur in compact inflorescences, the arrangements of flowers on a plant. Wind pollination is quite common in grasses.

In corn, the exposed stigma and style catch wind-borne pollen. Exposed reproductive parts assist release and capture, while the light pollen can travel in air currents. Wind- and water-pollinated flowers are not very colourful and do not produce nectar, the sugary floral reward.

How does water pollination occur?

Water pollination is quite rare in flowering plants, being limited to about 30 genera, mostly monocotyledons, plants with one seed leaf. Vallisneria and Hydrilla are freshwater examples; Zostera is a marine seagrass. Aquatic habitat alone does not establish water pollination.

In Vallisneria, a long stalk brings the female flower to the water surface. Male flowers or pollen released onto the surface travel passively in water currents. Some eventually reach the female flower and its stigma.

In seagrasses, female flowers remain submerged. Pollen in many such species is long and ribbon-like and travels inside the water. In most water-pollinated species, a mucilaginous, or jelly-like, covering protects pollen from wetting. Water hyacinth and water lily instead have emergent flowers pollinated by insects or wind.

What attracts insects?

The majority of insect-pollinated flowers are large, colourful, fragrant and nectar-rich. When flowers are small, a number are clustered into a conspicuous inflorescence. Pollen is generally sticky in animal-pollinated flowers. While collecting nectar or pollen, an animal contacts anthers and stigma, carrying pollen between flowers.

Bees are dominant biotic pollinators. Flowers pollinated by flies and beetles secrete foul odours that attract these animals. A visitor that consumes pollen or nectar without pollinating is a pollen or nectar robber; visiting a flower alone does not establish successful pollination.

How do plants discourage self-pollination and recognise pollen?

Outbreeding devices are mechanisms that discourage self-pollination and favour cross-pollination. Continued self-pollination results in inbreeding depression, a reduction in vigour or reproductive performance associated with inbreeding. Several devices act at different stages, from floral arrangement to pollen-tube growth.

Which devices prevent which transfers?

• Different timing: pollen may be released before the stigma is receptive, or the stigma may become receptive much earlier. This lack of synchrony prevents autogamy.
• Different positions: anthers and stigma may be placed so that pollen cannot reach the stigma of the same flower, preventing autogamy.
• Self-incompatibility: a genetic mechanism prevents self-pollen from fertilising ovules by inhibiting pollen germination or pollen-tube growth.
• Monoecy: separate male and female flowers occur on the same plant, as in castor and maize. This prevents autogamy but permits geitonogamy.
• Dioecy: male and female flowers occur on separate plants, as in papaya. This prevents both autogamy and geitonogamy.

What is pollen-pistil interaction?

Pollen-pistil interaction includes pollen recognition and the subsequent promotion or inhibition of its development. Compatible pollen is accepted and can proceed towards fertilisation. Incompatible pollen is rejected through inhibition of germination on the stigma or tube growth through the style.

Recognition depends on chemical interactions between pollen and pistil components. Pollination therefore does not guarantee fertilisation. Even pollen from the same species may fail if it is self-pollen received by a self-incompatible plant.

1. Accepted pollen germinates on the stigma, producing a pollen tube, an outgrowth through a germ pore that carries the male gametes.
2. The tube grows through the stigma and style towards the ovary.
3. In plants shedding two-celled pollen, the generative cell divides during tube growth in the stigma to form two male gametes.
4. The tube enters the ovule through the micropyle and then enters a synergid through the filiform apparatus.

Three-celled pollen already contains two male gametes, so its tube carries them from the beginning. The filiform apparatus guides tube entry; it is not the egg and does not itself take part in either nuclear fusion.

How is artificial hybridisation carried out?

Artificial hybridisation is a controlled cross intended to combine desirable parental characters. A breeder must place the chosen pollen on the chosen stigma and prevent contamination by unwanted pollen. The procedure depends on whether the female parent has bisexual or unisexual flowers.

What are emasculation and bagging?

Emasculation is removal of anthers from a bisexual flower bud before they dehisce, using forceps. Bagging covers the flower with a suitable bag, generally of butter paper, to prevent unwanted pollen from reaching the stigma.

1. Select the bisexual flower bud that will function as the female parent, and remove its anthers before pollen is released.
2. Cover the emasculated flower with a suitable bag to exclude unwanted pollen.
3. When the stigma becomes receptive, dust it with mature pollen collected from the selected male parent.
4. Rebag the pollinated flower and allow the fruit to develop.

For a unisexual female flower, emasculation is unnecessary because there are no anthers to remove. Bag the female bud before it opens, apply the desired pollen when the stigma becomes receptive, and rebag it afterwards.

Why is the sequence important?

Removing anthers after they have released pollen would defeat the purpose of excluding that pollen. Likewise, removing anthers without bagging would leave the stigma exposed to pollen from other flowers. The two operations solve distinct problems and must be explained separately.

Controlled pollination helps breeders bring together chosen parental traits. Knowledge of compatibility and incompatibility is also relevant because a selected pollen grain must be accepted and develop successfully for the cross to lead to fertilisation. Physical placement on the stigma is only the beginning.

What happens during double fertilisation and endosperm formation?

After entering a synergid, the pollen tube releases two male gametes. Double fertilisation comprises two different fusion events within the embryo sac: syngamy and triple fusion. It is unique to flowering plants and produces the beginnings of the embryo and its nutritive tissue.

Which nuclei fuse?

1. One male gamete fuses with the egg nucleus. This fusion is syngamy.
2. Syngamy produces a diploid zygote, the first cell of the new plant, which develops into the embryo.
3. The other male gamete fuses with the two polar nuclei in the central cell. This is triple fusion, involving three haploid nuclei.
4. The product is a triploid primary endosperm nucleus. Triploid means three chromosome sets, written 3n. The central cell becomes the primary endosperm cell and develops into endosperm.

Endosperm is the food-rich tissue that nourishes the developing embryo. Endosperm development precedes embryo development. Most zygotes divide only after a certain amount of endosperm has formed, providing assured nutrition for subsequent embryonic growth.

How do the three endosperm types differ?

In the most common type, repeated divisions of the primary endosperm nucleus produce free nuclei, followed by cellularisation, or formation of cell walls. Tender coconut water is free-nuclear endosperm, while the surrounding white kernel is cellular endosperm. Its cellular condition must be distinguished from cellular development from the outset.

Endosperm may be consumed before seed maturity, as in pea, groundnut and beans, or persist in the mature seed, as in castor and coconut. Double fertilisation thus links embryo formation to formation of the tissue supporting its nutrition.

How do embryos and mature seeds develop?

Embryogeny means embryo development. The embryo forms at the micropylar end, where the zygote lies. Early development is similar in monocotyledons and dicotyledons, plants with one and two cotyledons respectively. Cotyledons are the embryo's seed leaves.

What are the parts of the embryo?

A dicotyledonous embryo passes through proembryo, globular and heart-shaped stages before maturity. The proembryo is the early developing embryo. The mature embryo has two cotyledons and an embryonal axis, the axis bearing the developing shoot and root ends.

The epicotyl is the axis above the cotyledons and ends in the plumule, the embryonic shoot. The hypocotyl is below the cotyledons and ends in the radicle, the embryonic root. A root cap covers the root tip.

In grasses, the single cotyledon is the scutellum, situated to one side of the axis. The coleorrhiza is the sheath around the radicle and root cap. The coleoptile encloses the shoot apex and young leaf structures above the scutellum's attachment.

The dicot drawing labels two cotyledons, plumule, hypocotyl, radicle and root cap. The grass embryo drawing labels the scutellum beside the axis, the coleoptile around the shoot region, and the coleorrhiza around the root region.

See Fig. 1.14 in your NCERT textbook

How do bean and maize seeds differ?

The testa and tegmen are the outer and inner seed-coat layers. In maize, a protein-rich aleurone layer forms the outer covering of the endosperm. The plumule and radicle have the protective sheaths described above.

Albuminous seeds retain some endosperm at maturity, as in wheat, maize, barley and castor. Non-albuminous seeds have consumed it during embryo development, as in pea, groundnut and bean. Occasionally, persistent nucellus remains as perisperm, as in black pepper and beet.

The dicot seed drawings label seed coat, hilum, micropyle, cotyledon, plumule and radicle. The monocot section labels the combined seed coat and fruit wall, aleurone layer, endosperm, scutellum, coleoptile, plumule, radicle and coleorrhiza.

See Figs. 5.14 and 5.15 in your NCERT textbook

As seeds mature, integuments harden into seed coats and the micropyle remains as a pore admitting water and oxygen during germination. Seeds become relatively dry, with 10 to 15 per cent moisture by mass. The embryo may enter dormancy, a state of inactivity, until favourable conditions permit germination.

How do fruits form, and why are seeds and dispersal important?

As ovules mature into seeds, the ovary develops into a fruit. The ovary wall forms the pericarp, or fruit wall. Most fruits develop only from the ovary and are called true fruits. In most plants, other floral parts degenerate and fall off as the fruit develops.

How do true and false fruits differ?

In a few species, the thalamus also contributes to fruit formation. Such fruits are false fruits, as in apple, strawberry and cashew. The term describes the contribution of additional floral tissue, not an absence of reproductive function or nutritional value.

Mango and coconut are drupes, one-seeded fruits with a differentiated pericarp. Both develop from an ovary formed from one carpel, situated above the attachment of the other floral whorls. Their fruit walls contain epicarp, mesocarp and endocarp, the outer, middle and inner regions respectively.

In mango, the epicarp is thin, the mesocarp is fleshy and edible, and the endocarp is stony and hard. Coconut has a fibrous mesocarp. Its hard inner shell is the endocarp; the white kernel and coconut water are endosperm within the seed.

The cut fruit drawings label epicarp, mesocarp, endocarp and seed. Mango shows a broad fleshy middle region around the stone; coconut shows a fibrous surrounding region and a hard inner boundary around the seed.

See Fig. 5.13 in your NCERT textbook

What advantages do seeds provide?

Seed coats protect the embryo, while stored food nourishes the young seedling until it can photosynthesise, or make food using light. Dehydration and dormancy allow seeds to be stored for food and later sowing. Seeds produced sexually also introduce new genetic combinations.

Seed dispersal is movement of seeds away from their site of formation. Adaptations of seeds and fruits help plants reach new habitats and colonise other areas. Dispersal also reduces local competition between seedlings and the parent for resources such as light, water and mineral nutrients.

Fruit formation encloses and protects developing seeds, and many fruits have mechanisms that assist their dispersal. Germination requires adequate moisture, oxygen and a suitable temperature. Dormancy is a possible state of the embryo, not an unavoidable condition in every seed.

How do apomixis, parthenocarpy and polyembryony differ?

The usual sequence links fertilisation, embryo development, seed formation and fruit development. Special reproductive modes alter particular parts of this sequence. Distinguish whether the term concerns seed formation without fertilisation, fruit formation without fertilisation, or the number of embryos within one seed.

What is formed without fertilisation?

Apomixis is seed production without fertilisation. It is a form of asexual reproduction that mimics sexual reproduction and occurs in some species of Asteraceae, a flowering-plant family, and grasses. In some species, a diploid egg forms without reduction division and develops into an embryo without fertilisation.

Parthenocarpy is fruit development without fertilisation. Banana is an example. It can also be induced with growth hormones, and the fruits are seedless. Parthenocarpy concerns the fruit; apomixis concerns seed formation. They are not interchangeable names for the same process.

How can one seed contain several embryos?

Polyembryony is the occurrence of more than one embryo in a seed. In many Citrus and mango varieties, some nucellar cells surrounding the embryo sac begin dividing, project into the sac and develop into embryos. Several embryos can therefore occur within the same seed.

The word polyembryony identifies embryo number, whereas apomixis identifies a mode of reproduction. Nucellar embryo formation connects these ideas, but a definition of polyembryony should not be replaced by a definition of seed formation without fertilisation.

Why is apomixis useful in hybrid seed production?

A hybrid combines parental genetic contributions through a cross. Seeds collected from hybrids can produce offspring in which characters segregate, meaning they separate into different combinations. Such offspring do not maintain the hybrid characters, so hybrid seed ordinarily needs fresh production.

If hybrids are made apomictic, their offspring do not show this segregation. Farmers could retain seed for successive crops without buying newly produced hybrid seed each year. This is a conditional application of apomixis, not a claim that every commercial hybrid already reproduces this way.

Glossary

• Microsporogenesis — Formation of haploid microspores from diploid pollen mother cells through meiosis inside the anther.
• Tapetum — Innermost wall layer of a microsporangium, providing nourishment to developing pollen grains.
• Sporopollenin — Highly resistant organic material forming the exine, absent at the germ pores of pollen.
• Megasporogenesis — Formation of megaspores from a megaspore mother cell through meiotic division within the ovule.
• Monosporic development — Formation of the female gametophyte from a single functional megaspore.
• Filiform apparatus — Special thickenings at the micropylar tips of synergids that guide pollen-tube entry.
• Geitonogamy — Transfer of pollen between different flowers belonging to the same plant.
• Self-incompatibility — Genetic mechanism preventing self-pollen from fertilising ovules through inhibition of germination or tube growth.
• Emasculation — Removal of anthers from a bisexual flower bud before they release pollen.
• Triple fusion — Fusion of one male gamete with two polar nuclei to form the triploid primary endosperm nucleus.
• Albuminous seed — Seed retaining part of its endosperm because the developing embryo has not consumed it completely.
• Perisperm — Residual nucellus that persists within some seeds, including black pepper and beet.
• Apomixis — Production of seeds without fertilisation, a form of asexual reproduction mimicking sexual reproduction.
• Parthenocarpy — Development of a fruit without fertilisation, as occurs in banana.
• Polyembryony — Presence of more than one embryo within a single seed.

Common errors and misconceptions

• Misconception: A pollen grain is a male gamete. Correct: It represents the male gametophyte; its generative cell gives rise to two male gametes.
• Misconception: A typical mature embryo sac has eight cells. Correct: It has seven cells and eight nuclei because its central cell contains two polar nuclei.
• Misconception: Geitonogamy brings pollen from a different plant. Correct: It transfers pollen between different flowers of the same plant and is genetically similar to autogamy.
• Misconception: Every aquatic plant is water-pollinated. Correct: Water hyacinth and water lily have emergent flowers pollinated by insects or wind.
• Misconception: Pollination guarantees fertilisation. Correct: The pistil may reject incompatible pollen by inhibiting its germination or pollen-tube growth.
• Misconception: Double fertilisation means that two male gametes fuse with the egg. Correct: One fuses with the egg; the other fuses with the two polar nuclei.
• Misconception: The white coconut kernel is the fruit wall. Correct: It is cellular endosperm; the fibrous mesocarp and hard endocarp belong to the fruit wall.
• Misconception: Apomixis and parthenocarpy both mean seedless fruit formation. Correct: Apomixis produces seeds without fertilisation; parthenocarpy produces fruits without fertilisation.

Exam-style questions with model answers

Q1. Define microsporogenesis and name the division involved. [2 marks]

1. Microsporogenesis is the formation of microspores from pollen mother cells within the microsporangia of anthers.
2. It involves meiosis, producing four haploid microspores from each diploid pollen mother cell.

Q2. Describe the cellular arrangement of a typical mature, seven-celled, eight-nucleate angiosperm embryo sac. Account for its cell and nuclear totals. [4 marks]

1. The micropylar end contains the egg apparatus, consisting of one egg cell and two synergids, each with a nucleus.
2. The chalazal end contains three antipodal cells, each with a nucleus.
3. One large central cell contains two polar nuclei and occupies the central region of the embryo sac.
4. The three egg-apparatus cells, three antipodals and central cell total seven cells; their nuclei total eight because the central cell has two.

Q3. A plant has separate male and female flowers on the same individual. Pollen moves from a male flower to a female flower on that individual. Name this floral arrangement, identify the pollination type, and explain which form of self-pollination the arrangement prevents. [3 marks]

1. The plant is monoecious because separate male and female flowers occur on the same individual plant.
2. The described transfer is geitonogamy because pollen moves between different flowers of that same plant. Genetically, it is similar to autogamy.
3. The arrangement prevents autogamy, since neither unisexual flower contains both anthers and stigma. It does not prevent the geitonogamy described.

Q4. A breeder chooses a bisexual flower as the female parent and has mature pollen from the desired male parent available. Describe five steps or precautions for controlled pollination, from anther removal to fruit development. [5 marks]

1. Remove the anthers of the chosen female-parent flower while it is still a bud, before they dehisce and release pollen. This is emasculation.
2. Cover the emasculated flower with a suitable bag, generally of butter paper, to prevent its stigma receiving unwanted pollen.
3. Wait until the stigma becomes receptive. Pollen application must be timed to this condition of the female reproductive surface.
4. Dust the receptive stigma with mature pollen collected from the selected male parent, so that the intended pollen is used for the cross.
5. Rebag the flower after pollination to protect it from further unwanted pollen, and allow the fruit to develop.

Q5. A pollen tube releases two haploid male gametes into a typical embryo sac containing a haploid egg and two haploid polar nuclei. Explain the two fusion events, their products and the developmental fate of each product. [5 marks]

1. One male gamete fuses with the egg nucleus. This gametic fusion is called syngamy and takes place within the embryo sac.
2. Syngamy combines two haploid chromosome sets, producing a diploid zygote. The zygote subsequently develops into the embryo of the seed.
3. The other male gamete fuses with the two polar nuclei in the central cell. This event is triple fusion.
4. Three haploid nuclei contribute to triple fusion, producing a triploid primary endosperm nucleus. The resulting primary endosperm cell develops into nutritive endosperm.
5. The occurrence of both syngamy and triple fusion in the same embryo sac is double fertilisation, which is characteristic of flowering plants.

Q6. Compare a mature bean seed with a mature maize seed in cotyledon number, presence of endosperm, and location of the principal food reserve. [3 marks]

1. Bean has a dicotyledonous embryo with two cotyledons. Maize has a monocotyledonous embryo with one shield-shaped cotyledon called the scutellum.
2. Mature bean is non-albuminous because the endosperm has been consumed during development. Mature maize is albuminous and retains a bulky endosperm.
3. The food reserve in bean is stored in the cotyledons, whereas the bulky endosperm is the food-storage tissue in maize.

Q7. Explain four advantages of seed formation, covering embryo protection, seedling nutrition, storage and colonisation of new areas. [4 marks]

1. The hard seed coat protects the young embryo enclosed within the seed.
2. Stored food reserves nourish the young seedling until it becomes capable of carrying out photosynthesis independently.
3. Dehydration and dormancy of mature seeds facilitate storage, allowing seeds to be retained for food or for sowing in a later season.
4. Seed dispersal adaptations help plants reach new habitats and colonise areas away from the place where the seeds formed.

Q8. Define apomixis and parthenocarpy, distinguishing the structure formed without fertilisation in each case. [2 marks]

1. Apomixis is seed formation without fertilisation and is a form of asexual reproduction that mimics sexual reproduction.
2. Parthenocarpy is fruit formation without fertilisation; it produces seedless fruits, as in banana.

Key takeaways

• Meiosis in pollen mother cells produces microspores; subsequent male gametophyte development produces the two male gametes.
• A typical mature embryo sac contains seven cells and eight nuclei, including two polar nuclei in its central cell.
• Autogamy occurs within one flower, geitonogamy between flowers of one plant, and xenogamy between different plants.
• Outbreeding devices discourage self-pollination, while pollen-pistil interaction determines whether received pollen is accepted or rejected.
• Double fertilisation produces a diploid zygote through syngamy and a triploid primary endosperm nucleus through triple fusion.
• Endosperm development precedes embryo development; mature seeds may retain endosperm or consume it during embryonic growth.
• Ovules become seeds, ovaries become fruits, and the ovary wall forms the pericarp surrounding the seeds.
• Apomixis concerns seeds without fertilisation, parthenocarpy concerns fruits without fertilisation, and polyembryony concerns several embryos within one seed.

Test yourself

Which anther wall layer nourishes developing pollen?

The tapetum, the innermost wall layer of a microsporangium, nourishes developing pollen grains.

Why is a typical embryo sac eight-nucleate but seven-celled?

Six cells each contain one nucleus, while the single large central cell contains two polar nuclei.

Why can cleistogamous flowers set seed without pollinators?

They remain closed, and their closely placed anthers and stigma allow released pollen to effect autogamy inside the bud.

Which unisexual floral arrangement prevents both autogamy and geitonogamy?

Dioecy prevents both because male and female flowers occur on different individual plants.

What guides a pollen tube into a synergid?

The filiform apparatus, consisting of special thickenings at the synergid's micropylar tip, guides pollen-tube entry.

How does perisperm differ from pericarp?

Perisperm is persistent nucellus in a seed; pericarp is the fruit wall derived from the ovary wall.

Which maize embryo sheath protects the root region?

The coleorrhiza encloses the radicle and root cap; the coleoptile encloses the shoot region.

Why could apomictic hybrids reduce repeated hybrid-seed purchases?

If hybrids reproduce through apomixis, their offspring do not segregate for hybrid characters, allowing retained seed to be used for successive crops.