Sexual Reproduction in Flowering Plants — NEET Notes

The flower — a fascinating organ of angiosperms

To a poet, a flower is a thing of beauty. To a biologist, it is a morphological and embryological marvel — and the seat of sexual reproduction in every angiosperm. Long before a flower is visible to the eye, the plant has already decided to flower: hormonal and structural signals trigger the differentiation of the floral primordium, which gives rise to inflorescences, floral buds, and at last the open flower. Within that flower, two whorls do the real reproductive work. The androecium is a whorl of stamens — the male reproductive organ. The gynoecium is the female counterpart, made of one or more pistils. Everything else — calyx, corolla, nectaries, scent glands — exists as an accessory to ensure that pollen from one stamen reaches the stigma of one pistil.

NCERT frames the flower as an evolutionary advertisement, not a decoration. The diversity of size, colour, scent and shape across angiosperms is the product of relentless selection pressure to bring two non-motile gametes together. Once you read the flower in this functional light, every floral peculiarity — feathery stigmas in grasses, foul-smelling spadices in Amorphophallus, cleistogamous closed buds in Viola — begins to make exact biological sense.

Pre-fertilisation — stamen, microsporangium, pollen grain

A typical stamen has two parts: a slender stalk called the filament attached at its base to the thalamus or petal, and a terminal, generally bilobed structure called the anther. The anther of a typical angiosperm is bilobed, with each lobe holding two theca — making it dithecous and tetragonal (four-sided) in transverse section, with one microsporangium at each of the four corners. Each microsporangium develops into a pollen sac, packed lengthwise with thousands of pollen grains, and ruptures along longitudinal grooves at anther dehiscence to release them.

Inside, a microsporangium is bounded by four wall layers from the outside in: the epidermis, the endothecium, the middle layers, and the tapetum. The outer three protect the developing pollen and help the anther dehisce — a NEET trap in itself, since endothecium (not tapetum) carries out dehiscence. The tapetum is the nourishing layer: its cells have dense cytoplasm, often more than one nucleus, and they feed the developing pollen grains until the very end of maturation. Tapetal cells are sometimes binucleate because the nuclei divide without cytokinesis.

At the centre of each young microsporangium sits the sporogenous tissue — a cluster of homogeneous, diploid cells. Each cell is a potential pollen mother cell (PMC), and each PMC undergoes meiosis to produce a tetrad of four haploid microspores. This process — formation of haploid microspores from a diploid PMC by meiosis — is microsporogenesis. As the anther matures and dehydrates, the microspores separate from the tetrad and develop into pollen grains, ready for release at dehiscence.

The pollen grain

A pollen grain is the male gametophyte — small, dispersible, and exquisitely engineered. It is typically spherical, 25–50 µm in diameter, with a two-layered wall. The outer exine is made of sporopollenin, one of the most resistant organic materials known: it withstands high temperature, strong acid and alkali, and is not degraded by any known enzyme. This is exactly why pollen is preserved as fossils across geological time. Sporopollenin is absent at certain spots called germ pores, through which the pollen tube emerges later. The inner wall, the intine, is a thin continuous layer of cellulose and pectin.

When mature, a pollen grain contains two cells — a large vegetative cell with abundant food reserve and an irregular nucleus, and a small spindle-shaped generative cell floating in its cytoplasm. In more than 60 per cent of angiosperms, pollen is shed at this 2-celled stage. In the rest, the generative cell divides mitotically before shedding to produce two male gametes — the 3-celled stage.

Pollen viability is highly variable. In rice and wheat, pollen grains lose viability within 30 minutes of release. In members of Rosaceae, Leguminosae and Solanaceae, viability lasts for months. Pollen of many species can also be stored for years in liquid nitrogen at −196 °C — the basis of pollen banks used in crop breeding. Some pollen, like that of Parthenium (carrot grass), is a notorious allergen and triggers asthma and bronchitis.

Pre-fertilisation — pistil, megasporangium, embryo sac

The female reproductive whorl, the gynoecium, may consist of a single pistil (monocarpellary) or many pistils — fused together (syncarpous, as in Papaver) or free (apocarpous, as in Michelia). Each pistil has three parts: the stigma — the receptive landing platform for pollen; the style — the elongated slender stalk; and the ovary — the basal bulged region containing ovules attached to the placenta. Ovule number ranges from one (wheat, paddy, mango) to many (papaya, watermelon, orchids).

The ovule — the megasporangium

An ovule is a small structure attached to the placenta by a stalk called the funicle. The body of the ovule meets the funicle at the hilum. Each ovule has one or two protective envelopes called integuments that encircle the central mass of cells (the nucellus) except at one tip, where they leave a small opening — the micropyle. Opposite the micropyle lies the chalaza, the basal end of the ovule. Inside the nucellus sits the female gametophyte — the embryo sac.

Megasporogenesis

The ovule differentiates a single megaspore mother cell (MMC) in the micropylar region of the nucellus — a large, dense-cytoplasm cell with a prominent nucleus. The MMC undergoes meiosis to produce four megaspores. This is megasporogenesis. In a majority of flowering plants, three of the four megaspores degenerate and only one — the functional megaspore — develops into the embryo sac. Because the entire 7-celled gametophyte arises from one megaspore, this is called monosporic development.

The functional megaspore enlarges and its nucleus undergoes three successive free-nuclear mitotic divisions. The first division produces 2 nuclei, the second produces 4, the third produces 8 — all without cell wall formation. Only after the 8-nucleate stage do cell walls appear, organising the typical embryo sac. The cell layout is fixed and is a NEET evergreen:

Pollination — types and agents

Because pollen grains and egg cells are both non-motile, pollen must be physically transferred from the anther to the stigma before fertilisation can begin. This transfer is pollination. NCERT classifies pollination two ways — by source of pollen and by agent of transfer — and NEET tests both classifications relentlessly.

By source of pollen

By agent of pollination

Plants exploit two abiotic agents (wind and water) and one biotic agent (animals). The majority use animals. Because abiotic transfer is wasteful and chance-driven, abiotic-pollinated plants produce enormous quantities of pollen relative to the number of ovules.

Outbreeding devices

Most flowering plants produce hermaphrodite flowers, which means self-pollen is always near the stigma. Continued self-pollination leads to inbreeding depression — accumulated genetic defects, loss of vigour, reduced fertility. Evolution has therefore equipped plants with a series of outbreeding devices to discourage autogamy and force cross-pollination. NCERT lists four, in increasing strength:

Pollen-pistil interaction

Pollination delivers pollen to the stigma, but it does not guarantee that the right pollen has arrived. Wrong-species pollen, or self-incompatible pollen from the same plant, also lands routinely. The pistil therefore performs a recognition step: a chemical dialogue between pollen and stigma surface determines whether the pollen is accepted or rejected. Compatible pollen germinates; incompatible pollen is blocked at either germination on the stigma or tube growth through the style.

Following compatible pollination, the pollen grain germinates on the stigma and pushes out a pollen tube through one of the germ pores. The tube grows through the stigma and style, carrying the contents of the pollen grain — vegetative nucleus first, generative cell (or two male gametes) behind it. In 2-celled pollen, the generative cell divides into two male gametes during tube growth. In 3-celled pollen, the gametes are already present at shedding. The pollen tube reaches the ovary, enters the ovule through the micropyle, penetrates one of the synergids through the filiform apparatus, and discharges its two male gametes into the cytoplasm of that synergid. The entire sequence — from pollen landing to pollen tube discharge — is the pollen-pistil interaction.

Artificial hybridisation

Plant breeders exploit the recognition machinery deliberately. To create a hybrid, the breeder must ensure that only chosen pollen reaches the stigma — and unwanted pollen is excluded. Two techniques accomplish this:

Double fertilisation — the defining angiosperm event

Once the pollen tube discharges its two male gametes into a synergid, both gametes participate — neither is wasted. One male gamete fuses with the egg cell. This is syngamy, and it forms the diploid (2n) zygote. The second male gamete fuses with the two polar nuclei in the central cell. Because three haploid nuclei fuse, this is called triple fusion, and the product is the triploid (3n) primary endosperm nucleus (PEN). The central cell now becomes the primary endosperm cell (PEC).

Two fertilisation events inside one embryo sac, both productive, both essential — this is double fertilisation. It is unique to angiosperms. Gymnosperms have only single fertilisation; algae and fungi do not have it at all. Double fertilisation is therefore the molecular signature of the flowering plants and one of the highest-yield single facts in the entire chapter.

Post-fertilisation — endosperm and embryo

The events following double fertilisation are collectively termed post-fertilisation events: endosperm formation, embryo development, ovule-to-seed maturation, and ovary-to-fruit transformation.

Endosperm development

Endosperm forms before the embryo — an adaptation that guarantees the embryo will have food the moment it begins growing. The primary endosperm cell divides repeatedly to build a triploid nutritive tissue rich in reserves. In the commonest pattern, the PEN first undergoes successive free-nuclear divisions — nuclei without cell walls floating in shared cytoplasm. Cell wall formation occurs only later, producing the cellular endosperm. The famous example: coconut water from tender coconut is free-nuclear endosperm (thousands of free nuclei), and the white kernel surrounding it is the cellular endosperm.

Endosperm may either be fully consumed by the developing embryo before seed maturity (pea, groundnut, beans — non-albuminous or ex-albuminous seeds) or it may persist in the mature seed (castor, coconut, wheat, maize, barley — albuminous seeds). In a few seeds like black pepper and beet, the remnants of nucellus also persist, forming a tissue called the perisperm.

Embryo development

The zygote forms at the micropylar end of the embryo sac. Most zygotes stay dormant until a sufficient quantity of endosperm has been laid down — the same adaptation that explains why endosperm forms first. The zygote then divides to form a pro-embryo, which passes through globular and heart-shaped stages before maturing.

A typical dicot embryo has two cotyledons attached to an embryonal axis. The portion of the axis above the cotyledons is the epicotyl, terminating in the plumule (shoot tip). The cylindrical portion below is the hypocotyl, terminating in the radicle (root tip), itself covered by a root cap. A monocot embryo has a single cotyledon; in grasses this cotyledon is shield-shaped and called the scutellum, sitting laterally on the embryonal axis. The radicle and root cap are enclosed in a sheath called the coleorrhiza; the epicotyl ends in the coleoptile, a hollow foliar structure enclosing the shoot apex.

Seed and fruit

In angiosperms, the seed is the final product of sexual reproduction — described, fittingly, as a fertilised ovule. A typical seed contains seed coat(s), cotyledon(s), and an embryonal axis. The integuments harden into protective seed coats; the micropyle persists as a small pore that admits oxygen and water at germination. As maturation proceeds, water content drops to 10–15% by mass, metabolism slows, and the embryo may enter dormancy — or, under favourable conditions, germinate at once.

Seeds give angiosperms unique advantages: they are independent of water, they carry their own food reserve, they spread to new habitats, and as products of sexual reproduction they generate new genetic combinations. Seed longevity varies wildly — some seeds last weeks (cereals), some live for centuries. The oldest verified is a 10,000-year-old Lupinus arcticus from the Arctic Tundra, which germinated and flowered after excavation. A 2,000-year-old date palm (Phoenix dactylifera) from King Herod's palace also revived.

True fruit, false fruit, parthenocarpic fruit

As the ovule matures into the seed, the ovary develops into the fruit — the two transformations proceed simultaneously. The wall of the ovary becomes the wall of the fruit, the pericarp. Most fruits develop from the ovary alone and are called true fruits. In a few species — apple, strawberry, cashew — the thalamus also contributes to fruit formation; such fruits are called false fruits. And in species like banana, fruits develop without fertilisation — these are parthenocarpic fruits, and they are naturally seedless. Parthenocarpy can also be induced artificially using growth hormones.

Apomixis and polyembryony

Not every seed is the product of fertilisation. In some species of Asteraceae and many grasses, seeds form without fertilisation through a phenomenon called apomixis. Apomixis is a form of asexual reproduction that mimics sexual reproduction: a seed is produced, but no syngamy occurs. The mechanism varies — in some species, the diploid egg cell forms without meiosis and develops into the embryo directly; in many Citrus and Mango varieties, nucellar cells around the embryo sac divide, protrude into the sac, and develop into embryos.

This latter mechanism produces multiple embryos in a single seed — a condition called polyembryony. Split open a citrus seed and you can see embryos of different sizes and shapes nested together. Apomictic embryos, since they bypass meiosis and fertilisation, are genetic clones of the parent.

Why apomixis matters for agriculture

Hybrid varieties of crops dramatically boost productivity, but hybrid vigour does not breed true — seeds collected from hybrid plants segregate in the next generation, losing the hybrid characters. Farmers must therefore buy fresh hybrid seed each year, which is expensive. If hybrids could be converted to apomicts, the seeds would breed true indefinitely, and the farmer could keep using harvested seed year after year. This is why active research is going on to transfer apomictic genes into hybrid varieties.