Sexual Reproduction in Flowering Plants: NEET notes

1. The flower and its parts

NCERT §1.1

• For the plant, the flower is the organ of sexual reproduction; for people it also has aesthetic, ornamental, social, religious and cultural value.
• The decision to flower is taken well before a flower is visible. Hormonal and structural changes start first, a floral primordium differentiates, inflorescences form and bear floral buds, and the buds open into flowers.
• Inside the flower two whorls carry reproduction: the androecium, made of stamens, is the male part, and the gynoecium, made of one or more pistils, is the female part.
• Pre-fertilisation events are everything that happens before the gametes fuse: making pollen in the anther, making the embryo sac in the ovule, and carrying pollen to the stigma.

2. Stamen, microsporangium and microsporogenesis

NCERT §1.2.1

• A stamen has a long thin stalk, the filament, and a terminal anther that is usually bilobed. The lower end of the filament is fixed to the thalamus or to a petal.
• A typical angiosperm anther is bilobed and dithecous (each lobe has two thecae, separated by a lengthwise groove). In section it is four-sided with one microsporangium at each corner, two per lobe, so it is tetrasporangiate. The microsporangia mature into pollen sacs.
• In transverse section a microsporangium looks nearly circular and has four wall layers, from outside in: epidermis, endothecium, middle layers and tapetum. The outer three protect it and help the anther dehisce to release pollen.
• The tapetum, the innermost layer, nourishes the developing pollen. Its cells have dense cytoplasm and usually more than one nucleus.
• In a young anther the centre of each microsporangium is filled by sporogenous tissue, a group of compact, uniform cells. Each of these cells can act as a pollen mother cell (PMC), also called a microspore mother cell.
• Microsporogenesis is the making of microspores from a PMC by meiosis. The four haploid microspores of one PMC stay together at first as a microspore tetrad.
• As the anther matures and dries, the tetrads separate and each microspore develops into a pollen grain. Each microsporangium forms thousands of pollen grains, which are set free when the anther dehisces.

3. The pollen grain

NCERT §1.2.1

• A pollen grain is the male gametophyte. Grains are usually spherical and about 25 to 50 micrometres across.
• The wall has two layers. The hard outer exine is made of sporopollenin, among the most resistant organic materials known: it survives high temperature and strong acids and alkalis, and no enzyme is known to break it down.
• The exine has germ pores, gaps where sporopollenin is absent; the pollen tube later grows out through one of them. Sporopollenin is why pollen is so well preserved as fossils, and the exine carries species-typical patterns.
• The inner wall, the intine, is thin and continuous and is made of cellulose and pectin. Inside it the cytoplasm is bounded by the plasma membrane.
• A mature grain has two cells. The vegetative cell is larger, holds plenty of food reserve and a large, irregular nucleus. The generative cell is small, spindle-shaped with dense cytoplasm and a nucleus, and floats inside the vegetative cell.
• In over 60 per cent of angiosperms pollen is shed at this 2-celled stage. In the rest the generative cell divides by mitosis before shedding, so pollen is shed 3-celled: one vegetative cell and two male gametes.
• Pollen of many species triggers allergies and bronchial problems such as asthma and bronchitis. Parthenium (carrot grass), which reached India as a contaminant of imported wheat, has spread everywhere and causes pollen allergy.
• Pollen is rich in nutrients and is sold as tablets and syrups as a food supplement; claims have been made that eating it improves the performance of athletes and race horses.
• Viability varies with species, temperature and humidity: rice and wheat pollen die within about 30 minutes of release, while pollen of some members of Rosaceae, Leguminosae and Solanaceae stays viable for months. Pollen can be kept for years in liquid nitrogen at −196 °C, in pollen banks used by crop breeders.

4. Pistil, ovule and megasporogenesis

NCERT §1.2.2

• The gynoecium may be a single pistil (monocarpellary) or several (multicarpellary). Several pistils may be fused (syncarpous) or free (apocarpous).
• Each pistil has a stigma, the landing platform for pollen; a style, the long slender part below it; and an ovary, the swollen base. The ovary encloses a cavity, the locule, in which the placenta bears the ovules (megasporangia).
• An ovary may hold one ovule (wheat, paddy, mango) or many (papaya, watermelon, orchids).
• The ovule hangs from the placenta by a stalk, the funicle. The point where the body of the ovule joins the funicle is the hilum.
• One or two integuments wrap the ovule except at the tip, where a small opening, the micropyle, is left. The chalaza lies at the opposite, basal end.
• Inside the integuments is the nucellus, a mass of cells with plenty of stored food. The embryo sac (female gametophyte) sits in the nucellus; an ovule usually has one embryo sac, formed from a megaspore.
• Megasporogenesis is the formation of megaspores from the megaspore mother cell (MMC). Usually one MMC differentiates in the micropylar region of the nucellus; it is a large cell with dense cytoplasm and a prominent nucleus.
• The MMC divides by meiosis and gives four haploid megaspores.

5. The embryo sac

NCERT §1.2.2

• In most flowering plants only one of the four megaspores is functional; the other three degenerate. Forming the embryo sac from a single megaspore is called monosporic development.
• The functional megaspore's nucleus divides by mitosis into two nuclei that move to opposite poles (2-nucleate embryo sac). Two more mitotic divisions give the 4-nucleate and then the 8-nucleate stage.
• These three divisions are strictly free-nuclear: the nuclei divide without walls forming right after. Walls are laid down only after the 8-nucleate stage, organising the mature embryo sac.
• Six nuclei are walled off into cells. At the micropylar end, three cells form the egg apparatus: two synergids and one egg cell. At the chalazal end are three antipodal cells.
• The remaining two nuclei, the polar nuclei, lie in the large central cell just below the egg apparatus.
• Synergids carry special thickenings at their micropylar tip, the filiform apparatus, which guides the pollen tube into a synergid.
• So the typical mature angiosperm embryo sac is 8-nucleate but 7-celled: 3 antipodals + 2 synergids + 1 egg + 1 central cell with 2 polar nuclei.

6. Kinds of pollination and its agents

NCERT §1.2.3

• Both gametes of flowering plants are non-motile, so pollination, the transfer of pollen from an anther to a stigma, is needed before fertilisation.
• Autogamy is pollination within the same flower. Complete autogamy is rare in flowers that open, because it needs pollen release and stigma receptivity to coincide and anther and stigma to lie close.
• Viola (common pansy), Oxalis and Commelina make two kinds of flowers: chasmogamous flowers that open and expose anthers and stigma, and cleistogamous flowers that never open. Cleistogamous flowers are always autogamous and set seed even when no pollinator comes.
• Geitonogamy moves pollen to another flower of the same plant. It needs a pollinator, so it works like cross-pollination, but genetically it is the same as autogamy because the pollen comes from the same plant.
• Xenogamy moves pollen to the stigma of a different plant; it is the only kind that brings genetically different pollen to the stigma.
• Abiotic agents are wind and water; biotic agents are animals, and most flowering plants use animals. Wind is the commoner abiotic agent. Wind-pollinated plants have light, non-sticky pollen, well-exposed stamens, large often feathery stigmas, and often one ovule per ovary in crowded inflorescences, as in the maize cob, whose silky threads are the stigmas and styles. Wind pollination is common in grasses.
• Water pollination is rare, limited to about 30 genera, mostly monocots, such as Vallisneria and Hydrilla in fresh water and sea-grasses such as Zostera. In Vallisneria the female flower reaches the surface on a long stalk and pollen floats to it; in sea-grasses the female flowers stay submerged and long, ribbon-like pollen drifts in the water. A mucilage coat keeps such pollen from getting wet. Water hyacinth and water lily, though aquatic, are pollinated by insects or wind.
• Wind- and water-pollinated flowers are not very colourful and do not make nectar.
• Animal pollinators include bees, butterflies, flies, beetles, wasps, ants, moths, birds (sunbirds, humming birds) and bats; among animals, insects, especially bees, dominate. Lemurs, tree-dwelling rodents and lizards (gecko, garden lizard) pollinate some species.
• Insect-pollinated flowers are mostly big, bright, scented and full of nectar; where flowers are small, many are grouped into an inflorescence so they stand out. Flowers served by flies and beetles smell foul. Nectar and pollen are the usual rewards, and the pollen of animal-pollinated flowers is generally sticky.
• Some flowers reward pollinators with a safe place to lay eggs: Amorphophallus, whose flower is about 6 feet tall, and Yucca, where a moth lays eggs in the ovary locule and pollinates the flower, so neither can complete its life cycle without the other. Visitors that take nectar or pollen without pollinating are pollen/nectar robbers.

7. Outbreeding devices and pollen-pistil interaction

NCERT §1.2.3

• Most flowering plants have bisexual flowers, so pollen easily reaches the stigma of the same flower; continued self-pollination leads to inbreeding depression. Plants have evolved devices that discourage selfing and favour cross-pollination.
• Device 1: pollen release and stigma receptivity are not synchronised; either pollen is shed before the stigma is receptive or the stigma is receptive long before pollen is shed. Device 2: anther and stigma sit at different positions. Both prevent autogamy.
• Device 3: self-incompatibility, a genetic mechanism that stops pollen from the same flower or the same plant from fertilising ovules by blocking pollen germination or pollen tube growth in the pistil.
• Device 4: unisexual flowers. Male and female flowers on the same plant (monoecious, as in castor and maize) prevent autogamy but not geitonogamy. Male and female flowers on separate plants (dioecious, as in papaya) prevent both.
• The pistil can recognise pollen as compatible (right type, same species) or incompatible. Compatible pollen is accepted and post-pollination events follow; incompatible pollen is rejected by blocking germination on the stigma or tube growth in the style. This recognition is a chemical dialogue between pollen and pistil components.
• After compatible pollination the grain germinates on the stigma and sends out a pollen tube through one germ pore; the grain's contents move into the tube, which grows through the stigma and style to the ovary.
• If pollen was shed 2-celled, the generative cell divides into two male gametes while the tube grows; 3-celled pollen carries both gametes from the start. The tube enters the ovule through the micropyle and a synergid through the filiform apparatus, which guides its entry.
• Pollen-pistil interaction covers all events from pollen landing on the stigma to the pollen tube entering the ovule. Understanding it helps breeders obtain desired hybrids even from incompatible crosses.
• In artificial hybridisation only the chosen pollen may reach the stigma. For a bisexual female parent, the anthers are removed from the bud with forceps before they dehisce (emasculation), and the flower is covered with a bag, usually of butter paper (bagging). When the stigma is receptive, pollen from the male parent is dusted on it and the flower is rebagged.
• If the female parent has unisexual flowers, emasculation is not needed: the female buds are simply bagged before opening, pollinated when receptive, and rebagged.

8. Double fertilisation

NCERT §1.3

• Inside the synergid the pollen tube releases its two male gametes into the synergid's cytoplasm.
• One male gamete fuses with the egg nucleus: this is syngamy, and it gives a diploid zygote.
• The other male gamete fuses with the two polar nuclei in the central cell, giving a triploid primary endosperm nucleus (PEN). Because three haploid nuclei fuse, this is called triple fusion.
• Two fusions, syngamy and triple fusion, in one embryo sac make up double fertilisation, an event unique to flowering plants.
• After triple fusion the central cell becomes the primary endosperm cell (PEC), which develops into the endosperm; the zygote develops into the embryo.

9. Endosperm and embryo

NCERT §1.4.1, §1.4.2

• Post-fertilisation events are the development of endosperm and embryo, the maturing of ovules into seeds and of the ovary into a fruit.
• Endosperm develops before the embryo. The PEC divides repeatedly to form triploid endosperm tissue whose cells store food for the developing embryo.
• In the commonest type, the PEN undergoes repeated nuclear divisions without walls, giving free-nuclear endosperm; walls form later and it becomes cellular. The number of free nuclei before cellularisation varies widely.
• Tender coconut water is free-nuclear endosperm with thousands of nuclei; the white kernel around it is cellular endosperm.
• Endosperm may be used up completely by the embryo before the seed matures (pea, groundnut, beans) or persist in the mature seed and be used at germination (castor, coconut).
• The zygote sits near the micropyle, so that is where the embryo grows. In most plants the zygote waits to divide until some endosperm exists, an adaptation that guarantees food for the young embryo.
• Early embryogeny is similar in monocots and dicots: the zygote gives a proembryo, then globular, heart-shaped and mature embryo stages.
• A dicot embryo has an embryonal axis and two cotyledons. Above the cotyledons is the epicotyl, ending in the plumule (stem tip); below is the hypocotyl, ending in the radicle (root tip), which is covered by a root cap.
• A monocot embryo has one cotyledon; in grasses it is the scutellum, set to one side of the axis. The radicle and root cap are enclosed in a sheath, the coleorrhiza. Above the scutellum's attachment is the epicotyl, whose shoot apex and a few leaf primordia are enclosed in a hollow foliar sheath, the coleoptile.

10. Seed and fruit

NCERT §1.4.3

• The seed is the final product of sexual reproduction in angiosperms, often described as a fertilised ovule; it forms inside the fruit and has seed coat(s), cotyledon(s) and an embryo axis. Cotyledons are usually thick and swollen with stored food, as in legumes.
• Non-albuminous (ex-albuminous) seeds have no endosperm left because the embryo used it up (pea, groundnut). Albuminous seeds keep part of the endosperm (wheat, maize, barley, castor).
• In a few seeds such as black pepper and beet, part of the nucellus persists; this residual nucellus is the perisperm.
• The integuments harden into tough seed coats. The micropyle stays as a small pore that lets oxygen and water in at germination.
• As the seed matures it dries to 10-15 per cent moisture by mass and its metabolism slows. The embryo may enter dormancy, or it germinates if moisture, oxygen and a suitable temperature are available.
• Ovules becoming seeds and the ovary becoming a fruit happen together. The ovary wall becomes the fruit wall, the pericarp. Fruits may be fleshy (guava, orange, mango) or dry (groundnut, mustard).
• Fruits from the ovary alone are true fruits. In apple, strawberry and cashew the thalamus also forms part of the fruit, so these are false fruits.
• Parthenocarpic fruits develop without fertilisation and are seedless; banana is an example, and parthenocarpy can be induced with growth hormones.
• Seeds make reproduction dependable because pollination and fertilisation do not need water. They disperse to new habitats, feed the seedling until it photosynthesises, protect the embryo with a hard coat, and carry new genetic combinations. Their dryness and dormancy let us store them, which is the basis of agriculture.
• Viability ranges from a few months to hundreds of years. A lupine, Lupinus arcticus, from the Arctic tundra germinated after an estimated 10,000 years of dormancy, and a date palm seed, Phoenix dactylifera, about 2000 years old was found near the Dead Sea at King Herod's palace.
• Some fruits hold thousands of tiny seeds, as in orchids and the parasites Orobanche and Striga, and a tiny Ficus seed grows into a huge tree.

11. Apomixis and polyembryony

NCERT §1.5

• Apomixis means forming seeds without fertilisation; it occurs in some species of Asteraceae and grasses. It is asexual reproduction that imitates the sexual route.
• In some species the egg cell forms without reduction division, so it is diploid, and it develops into an embryo without fertilisation.
• More often, as in many Citrus and mango varieties, some nucellar cells around the embryo sac divide, push into the embryo sac and develop into embryos, so one ovule holds many embryos.
• Polyembryony is the occurrence of more than one embryo in a seed; squeezing an orange seed shows several embryos of different sizes. Nucellar embryos come from the parent's diploid tissue, so they are genetically identical to it: clones.
• Hybrid seed has to be bought every year, because seed saved from a hybrid gives progeny that segregate and lose the hybrid characters, and producing hybrid seed is costly.
• If hybrids were made apomictic, the hybrid progeny would not segregate and farmers could reuse their own seed year after year. This is why laboratories are studying the genetics of apomixis and trying to transfer apomictic genes into hybrid varieties.

Must-know facts

1. A typical anther is bilobed, dithecous and tetrasporangiate: four microsporangia, two per lobe.
2. Microsporangium wall, outside to inside: epidermis, endothecium, middle layers, tapetum; the tapetum nourishes the pollen and its cells are often multinucleate.
3. Exine is sporopollenin, absent only at germ pores; intine is cellulose and pectin.
4. Over 60 per cent of angiosperms shed pollen at the 2-celled stage (vegetative + generative cell).
5. Rice and wheat pollen lose viability within 30 minutes; pollen is stored in liquid nitrogen at −196 °C.
6. Parthenium (carrot grass) came to India with imported wheat and causes pollen allergy.
7. Hilum joins ovule to funicle; micropyle is at the tip; chalaza at the base; nucellus holds the embryo sac.
8. One megaspore functions (monosporic); three free-nuclear mitoses give 8 nuclei; embryo sac is 7-celled, 8-nucleate.
9. Egg apparatus = 2 synergids + 1 egg, at the micropylar end; filiform apparatus of synergids guides the pollen tube.
10. Cleistogamous flowers (Viola, Oxalis, Commelina) are always autogamous.
11. Geitonogamy is functionally cross-pollination but genetically autogamy; only xenogamy brings genetically different pollen.
12. Water pollination: about 30 genera, mostly monocots; Vallisneria, Hydrilla, Zostera. Water hyacinth and water lily are not water-pollinated.
13. Monoecy (castor, maize) prevents autogamy only; dioecy (papaya) prevents autogamy and geitonogamy.
14. Syngamy gives a 2n zygote; triple fusion gives a 3n PEN; together they are double fertilisation.
15. Coconut water is free-nuclear endosperm; the kernel is cellular endosperm.
16. Albuminous: wheat, maize, barley, castor. Non-albuminous: pea, groundnut. Perisperm: black pepper, beet.
17. Mature seeds hold 10-15 per cent moisture; the ovary wall becomes the pericarp.
18. False fruits: apple, strawberry, cashew (thalamus contributes). Parthenocarpy: banana.
19. Lupinus arcticus seed germinated after about 10,000 years; Phoenix dactylifera seed about 2000 years old.
20. Nucellar polyembryony in Citrus and mango; apomixis in some Asteraceae and grasses.

Common traps

Key terms

Dithecous

Of an anther lobe or anther with two thecae per lobe, as in a typical angiosperm anther.

Tapetum

Innermost wall layer of the microsporangium; it feeds the developing pollen and has dense, often multinucleate cells.

Microsporogenesis

Formation of four haploid microspores from a pollen mother cell by meiosis.

Sporopollenin

The highly resistant material of the exine, broken down by no known enzyme.

Germ pore

A gap in the exine without sporopollenin, through which the pollen tube emerges.

Nucellus

Food-rich tissue of the ovule, inside the integuments, that contains the embryo sac.

Monosporic development

Formation of the embryo sac from just one of the four megaspores.

Filiform apparatus

Thickenings at the micropylar tip of the synergids that guide the pollen tube in.

Cleistogamy

Bearing flowers that never open, so pollination is always within the flower.

Geitonogamy

Transfer of pollen to another flower on the same plant.

Xenogamy

Transfer of pollen to the stigma of a flower on a different plant.

Self-incompatibility

A genetic block on germination or tube growth of pollen from the same plant.

Emasculation

Removing anthers from a bisexual bud before they dehisce, in artificial crossing.

Triple fusion

Fusion of one male gamete with the two polar nuclei, giving the 3n primary endosperm nucleus.

Scutellum

The single, laterally placed cotyledon of a grass embryo.

Perisperm

Persistent remains of the nucellus in a seed, as in black pepper and beet.

Parthenocarpy

Development of a fruit without fertilisation, giving a seedless fruit such as banana.

Polyembryony

Presence of more than one embryo in a seed, as in Citrus.