Sexual Reproduction in Flowering Plants

This is the first chapter of NCERT's Class 12 Biology textbook (pages 3–25 of the 2026-27 reprint) and opens Unit VI, Reproduction. It has five parts — the flower (1.1), pre-fertilisation structures and events (1.2), double fertilisation (1.3), post-fertilisation structures and events (1.4), and apomixis and polyembryony (1.5) — with Figures 1.1–1.15, several observe-it-yourself activities set in the running text, a summary and 18 exercise questions. It is the first science chapter on this site; the conventions used here (labelled schematics, flowcharts for every process, n / 2n / 3n for chromosome sets) are repeated in the science chapters that follow.

• Neighbouring chapters: the next two chapters of this book — Human Reproduction and Reproductive Health — complete Unit VI and will follow on this site. Live geography chapters give the farm and forest setting: Agriculture (Class 10, Chapter 4) and Land Resources and Agriculture (Class 12, India: People and Economy, Chapter 3) on high-yielding and hybrid seeds and the Green Revolution; Biodiversity and Conservation (Class 11, Chapter 14) and Life on Earth (Class 9) on why species and their pollinators matter.
• Prelims — science: UPSC has asked which animals pollinate (2012), what a dichogamous flower is (2002), how seedless tomatoes are produced (1996), which flower part becomes the fruit and which the seed (1998), and which tree has a single co-evolved insect pollinator (2024). Expect statement questions on pollination, double fertilisation, parthenocarpy, apomixis and seed banks.
• GS3 — science, agriculture and environment: hybrid seeds and why farmers must buy them every year; apomixis research; pollen and seed banks for crop breeding; pollinator decline; invasive Parthenium; the debate on GM mustard, whose hybrid system is a modern answer to hand emasculation (Section 5).

• The flower is the seat of sexual reproduction: stamens (androecium, male) make pollen; pistils (gynoecium, female) hold ovules. Fruits and seeds are the end products.
• Anther: 2 lobes × 2 thecae = 4 microsporangia, each with 4 wall layers (epidermis, endothecium, middle layers, tapetum). Microsporogenesis: pollen mother cell (2n) → meiosis → tetrad of microspores (n) → pollen grain (the male gametophyte).
• Pollen wall: exine of sporopollenin (resists heat, acids, alkali; no known enzyme breaks it) with germ pores; intine of cellulose and pectin. Over 60% of angiosperms shed pollen 2-celled.
• Ovule: funicle, hilum, integuments, micropyle, nucellus, chalaza. Megasporogenesis: MMC (2n) → meiosis → 4 megaspores, 1 functional → 3 free-nuclear mitoses → 7-celled, 8-nucleate embryo sac (monosporic).
• Pollination: autogamy, geitonogamy (genetically selfing), xenogamy (only true cross). Agents: wind, water (about 30 genera), animals (most plants). Outbreeding devices: timing, position, self-incompatibility, unisexual flowers (dioecy blocks both kinds of selfing).
• Double fertilisation: syngamy (n + n → 2n zygote) + triple fusion (n + n + n → 3n PEN). Then endosperm (usually first), embryo, seed (from ovule) and fruit (from ovary). Apomixis and polyembryony: seeds without fertilisation — the hope of farmer-saved hybrid seed.

Long before a flower appears, the plant has already ‘decided’ to flower: hormonal and structural changes start the floral primordium, inflorescences form, and they bear buds and then flowers. Inside the flower two reproductive whorls develop — the androecium, a whorl of stamens (male), and the gynoecium, made of one or more pistils (female). These are the two parts where the two key units of sexual reproduction — the male and female gametophytes — develop.

A stamen has a long, slender stalk, the filament, and a terminal, generally two-lobed anther. The base of the filament is attached to the thalamus or to a petal. The number and length of stamens vary greatly between species. A typical angiosperm anther is bilobed, and each lobe has two thecae — it is dithecous; a groove often runs lengthwise between the thecae. In transverse section the anther is four-sided (tetragonal), with four microsporangia at the corners, two in each lobe. The microsporangia develop into pollen sacs, which run the length of the anther and fill with pollen.

In transverse section a microsporangium looks nearly circular and is usually surrounded by four wall layers: the epidermis, endothecium, middle layers and tapetum. The outer three protect and help the anther split open (dehiscence) to release pollen. The innermost layer, the tapetum, nourishes the developing pollen; its cells have dense cytoplasm and generally more than one nucleus. In a young anther the centre of each microsporangium is filled with sporogenous tissue — compact, similar cells.

Microsporogenesis. As the anther develops, each sporogenous cell can give rise to a tetrad, so each is a potential pollen mother cell (PMC) or microspore mother cell. The formation of microspores from a PMC by meiosis is microsporogenesis. The four microspores first sit together as a microspore tetrad; as the anther matures and dries, they separate and develop into pollen grains. Each microsporangium makes several thousand of them, released when the anther dehisces.

The pollen grain is the male gametophyte. Grains are generally spherical, about 25–50 micrometres across, with a prominent two-layered wall. The hard outer exine is made of sporopollenin, one of the most resistant organic materials known: it withstands high temperatures, strong acids and alkali, and no enzyme that degrades it is known — which is why pollen survives so well as fossils. The exine has apertures called germ pores, where sporopollenin is absent, and shows a striking variety of patterns. The inner intine is thin and continuous, made of cellulose and pectin; a plasma membrane surrounds the cytoplasm. A mature grain has two cells: a large vegetative cell with abundant food reserve and a large, irregular nucleus, and a small, spindle-shaped generative cell with dense cytoplasm floating inside it. In over 60 per cent of angiosperms pollen is shed at this 2-celled stage; in the rest the generative cell first divides by mitosis into two male gametes and pollen is shed 3-celled.

Pollen and health. Pollen of many species causes severe allergies and bronchial problems in some people, sometimes leading to chronic respiratory disorders such as asthma and bronchitis. NCERT names Parthenium (carrot grass), which came into India as a contaminant of imported wheat and is now everywhere, as a cause of pollen allergy. On the other side, pollen is rich in nutrients, and pollen tablets and syrups are sold as supplements, with claims that they improve the performance of athletes and race horses.

Viability. Shed pollen must reach a stigma before it loses viability. How long it lasts varies widely and depends partly on temperature and humidity: in rice and wheat, pollen loses viability within 30 minutes of release, while in some members of Rosaceae, Leguminosae and Solanaceae it stays viable for months. Pollen of many species can be stored for years in liquid nitrogen at −196 °C; such stores serve as pollen banks, like seed banks, for crop-breeding programmes — much as animal semen is stored for artificial insemination.

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 slender part beneath it) and an ovary (the swollen base). Inside the ovary is the ovarian cavity (locule) with the placenta, from which the megasporangia — ovules — arise. An ovary may have one ovule (wheat, paddy, mango) or many (papaya, watermelon, orchids).

The ovule. An ovule hangs from the placenta by a stalk, the funicle; the junction of the ovule's body and the funicle is the hilum. One or two protective integuments surround the ovule except at the tip, where a small opening, the micropyle, is left. Opposite the micropylar end is the chalaza, the basal part. Inside the integuments is the nucellus, a mass of cells rich in reserve food, and in the nucellus lies the embryo sac or female gametophyte. An ovule generally has a single embryo sac, formed from one megaspore.

Megasporogenesis. Forming megaspores from the megaspore mother cell (MMC) is megasporogenesis. An ovule generally differentiates a single MMC in the micropylar region of the nucellus — a large cell with dense cytoplasm and a prominent nucleus. It divides by meiosis into four megaspores. In most flowering plants one megaspore is functional and the other three degenerate; only the functional megaspore develops into the embryo sac. Forming the embryo sac from a single megaspore is called monosporic development.

The female gametophyte. The functional megaspore's nucleus divides by mitosis into two nuclei, which move to opposite poles (the 2-nucleate embryo sac). Two more mitoses give the 4-nucleate and then the 8-nucleate stage. These divisions are strictly free nuclear — no cell walls follow them at once. After the 8-nucleate stage, walls are laid down. Six of the eight nuclei get walls and become cells; the remaining two, the polar nuclei, lie below the egg apparatus in a large central cell. At the micropylar end, three cells form the egg apparatus — two synergids and one egg cell; the synergids carry special cellular thickenings at their micropylar tip, the filiform apparatus, which guides the pollen tube into a synergid. At the chalazal end are three antipodals. So a typical mature embryo sac is 8-nucleate but 7-celled.

Male and female gametes are made in the pollen grain and the embryo sac, and both are non-motile. Pollination — the transfer of pollen from an anther to a stigma — brings them together, using external agents. Depending on where the pollen comes from, there are three kinds. Autogamy: pollen reaches the stigma of the same flower. In a flower that opens normally, complete autogamy is rare — it needs pollen release and stigma receptivity to coincide and the anthers and stigma to lie close together. Geitonogamy: pollen reaches another flower of the same plant; it needs an agent, so it is functionally cross-pollination, but genetically it is like autogamy. Xenogamy: pollen reaches a flower of a different plant — the only kind that brings genetically different pollen.

Chasmogamy and cleistogamy. Some plants — Viola (common pansy), Oxalis and Commelina — bear two kinds of flowers: chasmogamous flowers, which open and expose their anthers and stigma like other flowers, and cleistogamous flowers, which never open. In cleistogamous flowers anthers and stigma lie close; the anthers dehisce in the bud and pollen touches the stigma. They are therefore invariably autogamous — no outside pollen can land — and they give assured seed-set even without pollinators.

Wind and water. Because pollen reaching a stigma by wind or water is a matter of chance, such flowers make enormous amounts of pollen compared with the number of ovules. NCERT's familiar example of wind pollination is the corn cob, whose waving threads are styles and stigmas that trap pollen. Water pollination is rare in flowering plants, unlike algae, bryophytes and pteridophytes, where water regularly carries the male gametes — which is believed to limit where some bryophytes and pteridophytes can grow. Most aquatic plants, such as water hyacinth and water lily, actually raise their flowers above water and use insects or wind. In Vallisneria the female flower reaches the surface on a long stalk, and male flowers or pollen released on the surface drift passively to it; in seagrasses the female flowers stay submerged and pollen is released inside the water.

Animals. Flowers of animal-pollinated plants are often adapted to a particular animal species. Animals are drawn by colour and/or fragrance and kept coming by rewards — usually nectar and pollen; while collecting them, the visitor touches the anthers and stigma and its body picks up pollen, which it delivers to the next stigma. In some species the reward is a safe place to lay eggs: NCERT cites the very tall Amorphophallus (about 6 feet), and the plant Yucca and a species of moth that cannot complete their life cycles without each other — the moth lays eggs in the ovary's locule and pollinates the flower, and its larvae hatch as the seeds develop. Insects that take pollen or nectar without pollinating are pollen/nectar robbers.

Most flowering plants have bisexual (hermaphrodite) flowers, so pollen easily reaches the stigma of the same flower, and continued self-pollination leads to inbreeding depression. Plants have evolved several outbreeding devices that discourage selfing and encourage cross-pollination.

Pollen-pistil interaction. Pollination does not guarantee the right pollen. Pollen of other species, or self-pollen in a self-incompatible plant, also lands on the stigma. The pistil can recognise pollen as compatible or incompatible. Compatible pollen is accepted and the events leading to fertilisation are promoted; incompatible pollen is rejected by stopping its germination on the stigma or its tube's growth in the style. This recognition is a continuous dialogue mediated by chemical components of the pollen interacting with those of the pistil, only some of which botanists have identified in recent years.

After compatible pollination the grain germinates on the stigma, sending out a pollen tube through one of its germ pores; 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 the two male gametes as the tube grows; if shed 3-celled, the tube carries the two gametes from the start. In the ovary the tube enters the ovule through the micropyle and then enters one of the synergids through the filiform apparatus, which guides it. All these events — from pollen landing on the stigma until the tube enters the ovule — are together called pollen-pistil interaction. Understanding it helps breeders manipulate the interaction, even in incompatible crosses, to obtain desired hybrids.

Artificial hybridisation. Breeders cross different species and often genera to combine desirable characters into commercially ‘superior’ varieties — a major approach in crop improvement. They must ensure that only the chosen pollen reaches the stigma, and that the stigma is protected from unwanted pollen. This is done by emasculation and bagging.

After entering a synergid, the pollen tube releases its two male gametes into the synergid's cytoplasm. One male gamete moves to the egg cell and fuses with its nucleus — syngamy — giving a diploid cell, the zygote. The other moves to the two polar nuclei in the central cell and fuses with them, giving a triploid primary endosperm nucleus (PEN); because three haploid nuclei fuse, this is triple fusion. Since two kinds of fusion happen in one embryo sac, the event is called double fertilisation, 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.

Post-fertilisation events are the development of the endosperm and embryo, the maturing of ovules into seeds and of the ovary into a fruit. Endosperm development precedes embryo development (NCERT). The primary endosperm cell divides repeatedly to form a triploid endosperm whose cells are filled with reserve food for the developing embryo. In the commonest type, the PEN undergoes successive nuclear divisions to give free nuclei — the free-nuclear endosperm stage; later, walls form and the endosperm becomes cellular. The number of free nuclei before walls form varies greatly. Coconut water from a tender coconut is free-nuclear endosperm made of thousands of nuclei, and the surrounding white kernel is cellular endosperm.

Endosperm may be completely consumed by the developing embryo before the seed matures (pea, groundnut, beans), or it may persist in the mature seed (castor, coconut) and be used during germination.

The embryo develops at the micropylar end of the embryo sac, where the zygote lies. Most zygotes divide only after a certain amount of endosperm has formed — an adaptation that assures nutrition to the embryo. The early stages of embryo development (embryogeny) are similar in monocots and dicots: the zygote gives rise to the proembryo and then to the globular, heart-shaped and mature embryo.

A typical dicot embryo has an embryonal axis and two cotyledons. The part of the axis above the level of the cotyledons is the epicotyl, ending in the plumule (stem tip); the cylindrical part below is the hypocotyl, ending in the radicle (root tip), covered by a root cap. Monocot embryos have only one cotyledon. In the grass family it is called the scutellum and lies to one side (laterally) of the axis. At its lower end the axis has the radicle and root cap enclosed in an undifferentiated sheath, the coleorrhiza. The part of the axis above the scutellum's attachment is the epicotyl, with a shoot apex and a few leaf primordia enclosed in a hollow, leaf-like sheath, the coleoptile.

In angiosperms the seed is the final product of sexual reproduction, often described as a fertilised ovule; seeds form inside fruits. A seed has seed coat(s), cotyledon(s) and an embryo axis. Cotyledons are simple, generally thick and swollen with stored food (as in legumes). Mature seeds are non-albuminous — no endosperm left, all used during embryo development (pea, groundnut) — or albuminous — part of the endosperm retained (wheat, maize, barley, castor). In a few seeds, such as black pepper and beet, remnants of the nucellus persist as the perisperm.

The integuments harden into tough protective seed coats; the micropyle remains as a small pore through which oxygen and water enter during germination. As the seed matures its water content falls to 10–15 per cent moisture by mass, the embryo's metabolism slows, and the embryo may enter dormancy — or, if moisture, oxygen and a suitable temperature are available, germinate.

As ovules become seeds, the ovary becomes a fruit — the two proceed together. The ovary wall becomes the fruit wall, the pericarp. Fruits may be fleshy (guava, orange, mango) or dry (groundnut, mustard), and many have ways of dispersing their seeds. In most plants the other floral parts fall off, but in a few — apple, strawberry, cashew — the thalamus also forms part of the fruit; these are false fruits. Most fruits develop only from the ovary and are true fruits. A few species form fruits without fertilisation — parthenocarpic fruits, such as banana; parthenocarpy can be induced with growth hormones, and such fruits are seedless.

Why seeds matter. Seeds make reproduction independent of water (pollination and fertilisation do not need it), so seed formation is more dependable; they disperse to new habitats; their food reserves nourish seedlings until they can photosynthesise; the hard coat protects the embryo; and, as products of sexual reproduction, they carry new genetic combinations. Seeds are the basis of agriculture: drying and dormancy let them be stored as food all year and as seed for the next season.

NCERT closes the section by pointing to plants' enormous reproductive capacity: each fruit of an orchid holds thousands of tiny seeds, as do the fruits of the parasites Orobanche and Striga, and a single huge Ficus tree grows from a tiny seed and produces billions more.

Seeds are generally products of fertilisation, but a few flowering plants — some species of Asteraceae and grasses — produce seeds without fertilisation. This is apomixis, a form of asexual reproduction that mimics sexual reproduction. It happens in several ways. In some species a diploid egg is formed without reduction division and develops into an embryo without fertilisation. More often, as in many Citrus and mango varieties, some nucellar cells around the embryo sac start dividing, protrude into the sac and develop into embryos, so each ovule contains many embryos. More than one embryo in a seed is polyembryony.

Apomixis and hybrid seed. Hybrid varieties of many food and vegetable crops are widely grown and have tremendously increased productivity. But hybrid seed must be produced every year: if seed collected from a hybrid is sown, the progeny segregate and lose the hybrid's characters. Producing hybrid seed is costly, so it is expensive for farmers. If hybrids were made apomictic, the progeny would not segregate and farmers could keep using their own seed year after year. Because of this importance to the hybrid seed industry, many laboratories are studying the genetics of apomixis and trying to transfer apomictic genes into hybrid varieties.

The chapter puts many questions in italics inside the running text and sets observation activities. Short outlines follow, in page order.

Try each question first, then open the outline. Draw a labelled diagram wherever the question asks for one — it earns marks.

Numbers

One-liners

• Stamen = anther + filament; pistil = stigma + style + ovary.
• The pollen grain is the male gametophyte; the embryo sac is the female gametophyte.
• Sporopollenin in the exine is among the most resistant organic materials known.
• Germ pores are where sporopollenin is absent; the pollen tube comes out there.
• Monosporic: the embryo sac comes from one functional megaspore.
• The filiform apparatus of the synergids guides the pollen tube.
• Only xenogamy brings genetically different pollen.
• Cleistogamous flowers are invariably autogamous.
• Dioecy (papaya) prevents both autogamy and geitonogamy.
• Double fertilisation = syngamy + triple fusion, unique to flowering plants.
• Coconut water = free-nuclear endosperm; white kernel = cellular endosperm.
• Ovule → seed; ovary → fruit; ovary wall → pericarp; integuments → seed coat.
• Apple, strawberry and cashew are false fruits (thalamus contributes).
• Parthenocarpy = seedless fruit without fertilisation; apomixis = seed without fertilisation.
• Polyembryony in Citrus and mango comes from nucellar embryos — clones of the mother.

Traps to avoid

• Microsporogenesis and megasporogenesis both use meiosis; in micro all four products survive, in mega usually one.
• The embryo sac is 8-nucleate but 7-celled — two polar nuclei share the central cell.
• Geitonogamy is functionally cross-pollination but genetically self-pollination.
• Monoecy (castor, maize) prevents autogamy but not geitonogamy.
• Triple fusion is one male gamete + two polar nuclei — not the egg.
• Endosperm is 3n; the seed coat, perisperm and pericarp are 2n maternal tissues.
• Coleoptile covers the shoot; coleorrhiza covers the root.
• Perisperm comes from the nucellus; pericarp from the ovary wall.
• Albuminous seeds keep endosperm (wheat, maize, castor); non-albuminous do not (pea, groundnut). NCERT's ‘non-albuminous or ex-albuminous’ is a slip — both mean the same.
• A false fruit is one where the thalamus contributes — not a seedless fruit.
• Not all aquatic plants are water-pollinated: water hyacinth and water lily use insects or wind.
• NCERT's ‘tallest flower’ of Amorphophallus is an inflorescence.

Prelims — what UPSC asks

• Pollination and pollinators: which of bat, bee and bird are pollinators (2012); the fig tree's single co-evolved insect pollinator (2024); maize as the match for cross-pollination (1991). Learn NCERT's full list of agents and examples, and the wind/water/animal flower features — statement questions are built from them.
• Outbreeding devices by name: 2002 asked for ‘dichogamy’ (androecium and gynoecium maturing at different times) against herkogamy, heterogamy and monogamy. NCERT describes the devices without these names — add dichogamy, protandry, protogyny and herkogamy to your notes.
• Fruit and seed: 1998 matched fruit–ovary and seed–ovule; 1996 asked how seedless tomatoes are made (hormone sprays on flowers — parthenocarpy). Learn the nine ‘what becomes what’ pairs of Section 8.
• Likely next: ploidy of the endosperm and zygote, the 7-celled 8-nucleate embryo sac, sporopollenin, apomixis versus parthenocarpy versus polyembryony, pollen and seed banks, Parthenium as an invasive weed. GM mustard (asked in 2018) sits with the biotechnology chapters but uses this chapter's idea of male sterility for hybrid seed.

Mains — where it appears and how to answer

• No GS Mains question in the bank so far is built on this chapter's core content. Its use in GS3 is as the science behind agriculture answers: why hybrid seed must be bought every year and how apomixis could change that; emasculation, bagging and male sterility in hybrid breeding; seed dormancy, seed and pollen banks for conserving crop diversity; pollinator decline and food security.
• Answer frame for a seeds or crop-biotechnology question: define the biological process in one line (e.g. hybrid vigour and segregation) → the technology (male sterility, apomixis, GM) → benefits (yield, uniformity) → concerns as attributed positions (cost, farmers' seed rights under the PPV&FR Act 2001, biosafety, diversity) → regulation (GEAC) → balanced conclusion.
• A small labelled sketch — the embryo sac, double fertilisation, or a seed — is quick to draw and lifts a science answer.

Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.

Open any question to attempt it in the Question bank — your attempt is saved to your progress.