Sexual Reproduction in Flowering Plants: Class 12 Biology · Chapter 1

Walk through a mango orchard in February and every branch carries a spray of tiny flowers. By May most of them have dropped, and the few that remain hang as fruit with a single seed inside. Between that flower and that seed lies a carefully staged sequence of events: the sexual life of a flowering plant.

Individual plants die, but a species survives as long as its members leave offspring. Copying itself asexually lets a plant spread fast, yet only sexual reproduction reshuffles genes, so each generation carries combinations that were never tried before. That variety is the raw material for coping with new diseases and changing climates. This chapter follows sexual reproduction in angiosperms, from the first floral bud to a seed that may wait for years before it grows.

1.1 Flower – A Fascinating Organ of Angiosperms

Across India, flowers carry what words cannot: marigold strings over a doorway at Diwali, jasmine woven into a braid, lotus at a temple, white chrysanthemums at a funeral. Growing flowers and ornamental plants for sale is a farming business of its own, floriculture; marigold, rose, tuberose and chrysanthemum are among the flowers India grows commercially.

A botanist sees more in a flower than colour and scent. A flower is a short shoot whose leaves have been reshaped into four whorls on the thalamus Fig. 1.1. The outer two, the sepals (calyx) and petals (corolla), protect the bud and advertise it. The inner two do the reproducing: the stamens (androecium) form the male whorl and the pistils (gynoecium) the female whorl.

So which two parts actually make the units of sexual reproduction? Pollen grains, which carry the male gametes, develop inside the anthers. The embryo sac, which holds the egg, develops inside each ovule in the ovary. The rest of the chapter follows these two structures until their gametes meet.

1.2 Pre-fertilisation: Structures and Events

Pre-fertilisation covers everything that must happen before gametes can fuse: building the male and female gametophytes, then bringing them together. It begins with a switch. Long before any bud is visible, cues such as day length and temperature, relayed by hormones, turn a shoot tip from producing leaves to producing a floral primordium. That primordium grows into a cluster of buds, and within each bud the androecium and gynoecium form.

1.2.1 Stamen, Microsporangium and Pollen Grain

Pull a stamen from a flower and you hold two parts: a slender stalk, the filament, and at its tip the anther, usually two-lobed, where pollen is made. The filament may grow from the thalamus or, in brinjal and datura, from the petals themselves (such stamens are epipetalous). Flowers differ widely in how many stamens they carry and how long these are. Mustard has six, four long and two short, while china rose has dozens whose filaments fuse into a tube around the style.

Look at an anther end-on and it has two lobes. Each lobe holds two microsporangia, so a typical anther has four Fig. 1.2. Botanists sum this up as bilobed, dithecous and tetrasporangiate.11Books word this differently: some call each lobe a theca, others each pair of sacs. The count never changes: two lobes, four microsporangia. A groove often runs down the anther between the halves. A strip of sterile tissue, the connective, joins the lobes and carries a vascular strand up from the filament.

In cross-section a young anther is roughly square, with a microsporangium at each corner. Each sac runs the full length of the anther, and at maturity these four are the pollen sacs.

Structure of a microsporangium

In cross-section a microsporangium is almost circular. From the outside in, its wall has four kinds of layer: the epidermis, the endothecium, a few middle layers and, innermost, the tapetum. Protecting the sac and opening it when the anther is ripe are jobs for the first three. Endothecium cells carry strips of thickening; as the anther dries they shrink unevenly and tear the wall along a weak line, a process called dehiscence.

The tapetum is the nursery layer. Its cells often hold two or more nuclei, because a nucleus can divide by mitosis without the cell dividing after it. Tapetal cells pass food to the developing pollen, release an enzyme that frees young microspores from one another, and secrete the raw material for the hard outer pollen wall. Once the pollen is built, the tapetum breaks down.

At the centre of a young microsporangium lies a compact block of similar cells, the sporogenous tissue. Each of these cells can become a pollen mother cell.

Microsporogenesis

A pollen mother cell (PMC) is diploid. It divides by meiosis into four haploid cells that stay together for a while as a microspore tetrad Fig. 1.3. Botanists call this meiotic route from PMC to microspores microsporogenesis. Because meiosis halves the chromosome number, every cell of a tetrad is haploid (n). Drying of the ripening anther loosens the tetrads, and each freed microspore matures into a pollen grain; a single sac may hold thousands.

The pollen grain

Each pollen grain is a male gametophyte, a tiny haploid structure that will make and deliver the male gametes. Tap a ripe lily or marigold anther over a drop of water on a slide and look under a microscope: grains differ in size, shape, colour and surface pattern. Scanning electron microscopes show them as spiked balls, netted spheres or grooved ovals, patterns so particular that a grain can identify its plant.

Most grains are spheres about 25–50 µm across, smaller than the width of a human hair, wrapped in a two-layered wall. The outer layer, the exine, is the toughest part. It is built from sporopollenin, a polymer so durable that heat, strong acid or strong alkali leaves it intact, and no enzyme is known to digest it. Scientists put this toughness to use: pollen survives in rocks as fossils, and grains trapped in lake mud record which plants grew nearby thousands of years ago.

The exine has gaps. At one or more thin spots, the germ pores, sporopollenin is missing, and a pollen tube will later emerge through one of them. Beneath the exine, an unbroken layer of cellulose and pectin, the intine, lines the grain, and the cytoplasm inside has its own plasma membrane.

Inside, a ripe grain holds two very unequal cells. A small, spindle-shaped generative cell with dense cytoplasm hangs inside a much larger vegetative cell, which carries the food reserves and a big, irregular nucleus. The generative cell will produce the gametes; the vegetative cell will produce the tube. Species differ in timing. Most (over 60 %) release pollen in this two-celled state, while the rest let the generative cell divide by mitosis first and shed three-celled grains holding two male gametes.

Once released, a pollen grain must reach a stigma while it is still alive. How long it stays viable depends on the species and on temperature and humidity. Rice and wheat pollen dies within about half an hour, while pollen of some members of the rose, pea and potato families stays viable for months.

1.2.2 The Pistil, Megasporangium (ovule) and Embryo sac

The gynoecium may be a single carpel (monocarpellary, as in pea) or several (multicarpellary). Several carpels may be fused into one compound pistil (syncarpous, as in lady's finger and tomato) or remain separate (apocarpous, as in lotus and custard apple) Fig. 1.4.

Follow a pollen grain inwards and you meet the three parts of a pistil in order. First comes the stigma, a sticky or feathery receiving surface. Next is the style, a neck that may be short or very long. Last is the ovary, the swollen base. The ovary encloses a cavity, the locule, and on its wall a ridge of tissue, the placenta, bears the ovules. An ovary may hold a single ovule (maize, or each tiny floret of a sunflower head) or hundreds (brinjal, guava, tomato).

The ovule (megasporangium)

Think of an ovule, the flower's megasporangium, as a parcel. At its core is the nucellus, a mass of food-rich cells. Buried in the nucellus is the embryo sac, the female gametophyte; there is normally one per ovule, grown from a single megaspore. Around the nucellus are one or two protective wrappings, the integuments, which leave only a narrow entry channel, the micropyle Fig. 1.5.

The parcel hangs from the placenta on a stalk, the funicle, and the point where stalk meets ovule body is the hilum. The end opposite the micropyle, where integuments and nucellus merge at the base, is the chalaza. Most angiosperm ovules are anatropous: the ovule body has turned right over on its funicle, so the micropyle points back towards the hilum.

Megasporogenesis

One nucellus cell near the micropyle stands out: it is bigger than its neighbours, its cytoplasm is dense and its nucleus is large. This diploid megaspore mother cell (MMC) divides by meiosis into four haploid megaspores, usually lying in a row Fig. 1.6. The formation of megaspores from the MMC is megasporogenesis.

Why must the MMC divide by meiosis? Fertilisation will later fuse two gametes and double the chromosome number. Halving it first keeps the zygote at the same diploid number as its parents, and the crossing over in meiosis adds new combinations of genes.

The female gametophyte (embryo sac)

In most flowering plants three of the four megaspores degenerate, and only one, usually the one nearest the chalaza, survives as the functional megaspore. Because the embryo sac grows from this single megaspore, its development is called monosporic.

Next come three rounds of mitosis, 1 → 2 → 4 → 8 nuclei, and none of them is followed by a cell wall. After the first round the two nuclei drift to opposite ends of the growing sac, and each later round doubles the nuclei at both ends. Such free-nuclear division leaves the young sac as one big cell with eight nuclei; only then do walls form.

Read the mature sac from the chalazal end. Three antipodal cells cluster there. The middle is one large central cell that keeps two nuclei, the polar nuclei, instead of one. At the micropylar end sits the egg apparatus: an egg cell flanked by two synergids, whose micropylar tips carry finger-like wall ingrowths, the filiform apparatus, that help steer an arriving pollen tube. Add them up and the sac has eight nuclei in only seven cells Table 1.2.

1.2.3 Pollination

Pollen grains and embryo sacs form in different organs, and neither the male gametes nor the egg can travel on their own. The bridge is pollination. Since a plant cannot carry its own pollen, it relies on outside carriers: wind, water or animals.

Kinds of pollination

Pollination is named after the source of the pollen Fig. 1.7. In autogamy, pollen lands on the stigma of the very flower that made it. For an open flower this is harder than it sounds: its anthers and stigma must be ready at the same time and close enough to touch. Many open flowers fail one test or the other, so complete autogamy in them is rare.

Violets (Viola) and dayflower (Commelina) hedge their bets with two kinds of flowers on one plant. Alongside ordinary open, chasmogamous flowers they make cleistogamous flowers that stay shut for life. Inside such a closed bud the anthers open against the stigma, so pollination is automatic, and since nothing from outside can get in it is always autogamy. The payoff is guaranteed seed even in a season without pollinators; the price is that all those seeds come from selfing.

In geitonogamy, pollen moves to another flower on the same plant. It needs a carrier, just as cross-pollination does, so from the outside it looks like crossing. Genetically, though, it is selfing, because pollen and egg come from one individual. In xenogamy, pollen comes from a flower on a different plant. Only xenogamy brings genetically different pollen to the stigma, and so only xenogamy produces offspring with new gene combinations.

Agents of pollination

Most flowering plants hire animals as carriers. A minority trust the wind or, rarely, water, and they pay for it in pollen. A carrier that cannot aim drops most grains nowhere near a stigma, so wind- and water-pollinated plants shed vast quantities of pollen for every ovule they need to fertilise.

Wind is the commoner of the two non-living carriers. Picture a flower designed for it. It needs no showy petals. Its stigma should be large and often feathery, to comb grains out of the air, and its stamens should dangle in the open where a breeze can shake them. The pollen must be light, dry and non-sticky. Because each grain is a long shot, such flowers usually hold one ovule each and are packed by the dozen into an inflorescence. Grasses such as wheat, rice, sugarcane and bamboo fit this brief. On a young maize cob, the silky tassel of threads is a bundle of long styles and stigmas, one for each future kernel.

Water pollination comes in two styles Fig. 1.8. On the surface: in Vallisneria, an eelgrass of ponds and slow rivers, a female flower is held at the surface on a long, coiling stalk. Its male flowers break free underwater, bob up and drift on the surface film until a few bump into it. Under the surface: in seagrasses such as Zostera, the flowers stay submerged and release long, thread-like pollen that the current sweeps to the stigmas. Either way the pollen must not get waterlogged, and in most such species a slimy coat of mucilage protects it.

Water pollination is unusual: only about 30 genera use it, most of them monocots, including Hydrilla as well as Vallisneria. Living in water is not enough. Lotus and water hyacinth hold their flowers above the surface and are pollinated by insects, like most land plants. Compare algae, mosses and ferns, whose male gametes must swim to the egg; that need for water is thought to limit where some mosses and ferns can grow.

Flowers pollinated by wind or water are usually small and dull and make no nectar. They have no animal to attract, so spending energy on colour, scent or nectar would be wasted.

Animals do most of the work, and insects, above all bees, lead the field. Butterflies and moths sip nectar through long tongues; flies, beetles, wasps and even ants also carry pollen. Among vertebrates, sunbirds feed at coral-tree and silk-cotton flowers, bats visit large night-opening flowers such as the baobab's, and in a few species lemurs, tree-dwelling rodents or geckos do the job. Often a flower's shape, colour and timing suit one kind of visitor far better than others.

An animal-pollinated flower runs a simple bargain: advertise, reward, and collect a fee in pollen. The advertisement is colour, scent or both, usually on large flowers. Small flowers club together into showy heads, as in sunflower and marigold, and flowers that want flies or beetles advertise with a smell of rotting meat or dung. The reward is usually nectar or pollen. The fee is collected while the visitor feeds: it brushes past the anthers and leaves dusted with sticky pollen, some of which it wipes onto the next stigma it touches.

Outbreeding devices

Inbreeding has a cost. Generation after generation of selfing brings harmful recessive alleles together and produces weaker, less fertile plants, a decline called inbreeding depression. Yet most flowering plants carry anthers and stigma in the same flower, so self pollen is always at hand. Evolution has answered with several outbreeding devices.

• Timing. Pollen is shed before the stigma of the same flower is receptive (protandry), or the stigma is receptive before the pollen is shed (protogyny). This prevents autogamy.
• Position. Anthers and stigma are placed so that a flower's own pollen seldom reaches its stigma (herkogamy). This also prevents autogamy.
• Self-incompatibility. A genetic recognition system lets the pistil reject pollen from the same plant, whether from the same flower or a neighbouring one, by blocking germination or tube growth. Neither autogamy nor geitonogamy then gives seed.
• Unisexual flowers. With male and female flowers on one plant (monoecious, as in maize, castor, cucumber and coconut), autogamy is impossible but geitonogamy is not. With male and female flowers on separate plants (dioecious, as in papaya, date palm and spinach), both autogamy and geitonogamy are ruled out.

Pollen–pistil interaction

A stigma works like a security desk. Grains of every kind arrive: the right species, other species and, in self-incompatible plants, the plant's own. Each grain is checked through an exchange of molecules between pollen and pistil, and only compatible pollen is waved through to germinate and grow a tube. Incompatible pollen is refused, either at the desk, where it cannot germinate on the stigma, or further in, where its tube stops in the style.

Botanists have identified only some of the pollen and pistil molecules in this exchange, and the work continues.

For accepted pollen, a journey follows Fig. 1.10.

1. Germination. The grain absorbs water from the stigma, and a pollen tube bulges out through one germ pore; the grain's cytoplasm and nuclei move into it.
2. Growth. The tube tip forces its way down between the cells of the stigma and style, sometimes for several centimetres, until it enters the ovary. In pollen shed two-celled, the generative cell divides on the way to make the two male gametes; three-celled pollen carries them from the start.
3. Entry. Most tubes enter an ovule through its micropyle and then push into one synergid, steered on this last stretch by the filiform apparatus.

Biologists group everything from the arrival of pollen on the stigma to the tube's entry into the ovule under one name, pollen–pistil interaction. Understanding it helps breeders: by manipulating it, they can sometimes obtain hybrids even from crosses the pistil would normally refuse.

Artificial hybridisation

Breeders often cross two varieties, sometimes even two species or genera, to combine useful traits in one plant. This artificial hybridisation works only if the stigma receives the chosen pollen and nothing else Fig. 1.11. Suppose a breeder wants to cross a high-yielding tomato (the female parent) with a wild relative that resists a leaf virus (the male parent). Tomato flowers are bisexual and can pollinate themselves, so the breeder works in four steps.

1. Emasculate. While the chosen flower is still a bud, remove all its anthers with fine forceps before any has released pollen. This is emasculation.
2. Bag. Slip a small butter-paper bag over the emasculated bud and tie it, so that no pollen carried by insects or wind can reach the stigma. This is bagging.
3. Pollinate. When the stigma becomes receptive, open the bag and dust the stigma with pollen collected from the wild parent's anthers.
4. Rebag and label. Close the bag again, tag the flower with the date and both parents, and let the fruit develop; its seeds are the hybrids.

A female parent with unisexual flowers, such as bottle gourd, needs no emasculation, since its female flowers have no anthers. The female bud is bagged before it opens, pollinated when the stigma is ready and bagged again.

1.3 Double Fertilisation

The pollen tube brings two male gametes, and a flowering plant uses both, something no other plant group does. Once the tube tip bursts inside a synergid, the gametes are released and head for two different targets Fig. 1.12.

• Gamete 1 → egg. Its nucleus joins the egg nucleus (n + n). This fusion of gametes, syngamy, makes the diploid zygote, the founding cell of the embryo.
• Gamete 2 → central cell. Its nucleus joins both polar nuclei (n + n + n). Three nuclei unite, hence the name triple fusion; the result is the triploid primary endosperm nucleus (PEN), and the central cell holding it is now the primary endosperm cell (PEC).

Because one embryo sac hosts both events, the whole process is called double fertilisation. From here the two products part ways: the zygote will become the embryo, while the PEC builds the endosperm that will feed it. The synergids and antipodals, their work finished, degenerate.

1.4 Post-fertilisation: Structures and Events

Once double fertilisation is over, four changes run side by side: the endosperm develops, the embryo develops, each ovule matures into a seed and the ovary matures into a fruit. Together they are the post-fertilisation events.

1.4.1 Endosperm

Why does the endosperm come first? Most zygotes stay undivided until some endosperm exists, so the embryo never starts growing without a food supply.

In most plants the endosperm is built in two phases Fig. 1.13. In the first, the PEN divides again and again but no walls follow, so the central cell fills with free nuclei: this is free-nuclear endosperm. In the second, walls appear around the nuclei, beginning at the edge and closing in, and the tissue becomes cellular endosperm. Species differ widely in how many free nuclei they make before walls arrive. Either way, the finished endosperm is triploid and its cells are packed with food.

A coconut shows both stages at once. The sweet water of a tender green coconut is free-nuclear endosperm, crowded with free nuclei. As the nut matures, walls form around nuclei from the outside inwards, and the white kernel thickens at the expense of the water. The kernel you grate for chutney is cellular endosperm.

Seeds then differ in how much endosperm they keep. A gram or groundnut embryo absorbs all of it before the seed ripens and stores the food in its own cotyledons. Castor, coconut and the cereals keep a large endosperm until germination, when the seedling draws on it.

1.4.2 Embryo

A bean and a grain of wheat look nothing alike, yet their embryos begin in the same way; the differences appear later in embryogeny, the development of the embryo Fig. 1.14. Growth starts at the micropylar end of the sac, where the zygote lies. The zygote divides to form a proembryo carried on a short chain of cells, the suspensor, which pushes it into the endosperm. The proembryo becomes a ball of cells (the globular stage), then a two-lobed heart-shaped embryo whose lobes are the first cotyledons, and finally a mature embryo.

Use the cotyledons as a landmark Fig. 1.15. A dicot embryo's two cotyledons join its central embryonal axis at one level, the cotyledonary node. The stretch of axis above that level is the epicotyl (epi, above), capped by the plumule, the future shoot. The stretch below is the hypocotyl (hypo, below), ending in the radicle, the future root, whose tip is shielded by a root cap.

Monocots have one cotyledon, and the grass embryo adds two protective sheaths. In wheat, rice or maize the single cotyledon is the scutellum, a shield set to one side of the axis and pressed against the endosperm, from which it absorbs food during germination. The root end of the axis (radicle and root cap) is wrapped in the coleorrhiza. The shoot end, the epicotyl above the scutellum with its shoot apex and first leaf primordia, is capped by a hollow, leaf-like coleoptile. It is the pointed sheath you see pushing up first when wheat germinates.

1.4.3 Seed

Everything so far ends in one product, the seed: a ripened, fertilised ovule, packed inside a fruit. Open any seed and you find the same three parts: a protective seed coat, an embryonal axis, and one or two cotyledons, which in beans and gram are thick, swollen food stores.

The figure follows one young seed down three paths Fig. 1.16. If the embryo absorbs every last cell of endosperm before ripening, the seed is non-albuminous (exalbuminous): pea, gram and groundnut are examples. If some endosperm is left over, the seed is albuminous: castor, coconut and cereal grains such as wheat, maize and barley. And in a few seeds a third store survives, leftover nucellus called perisperm, as in black pepper and beet.

Two more changes finish the seed. Its coats harden: the outer integument becomes the testa and the inner one, where present, the tegmen, while the old micropyle survives as a pore that later lets in water and oxygen. And it dries: water falls to about 10–15 % of the seed's mass, and the embryo's metabolism slows almost to nothing. The ripe seed then either rests in dormancy or, given moisture, oxygen and a suitable temperature, germinates.

Meanwhile the ovary around the seeds is turning into a fruit, and its wall into the fruit wall, the pericarp. In a tomato, grape or papaya the pericarp turns soft and juicy; in a pea pod, a cotton boll or the sunflower 'seed' (really a dry one-seeded fruit) it dries out. Either kind may carry hooks, wings, flesh or springs that help scatter the seeds. And since every seed is a fertilised ovule, a fruit can never hold more seeds than its ovary had ovules.

Look inside an apple and you are looking at a false fruit: the crisp flesh is mostly swollen thalamus, and only the papery core around the seeds came from the ovary. A strawberry is similar, its red flesh being thalamus dotted with tiny true fruits. In most plants, by contrast, the other floral parts wither and the fruit comes from the ovary alone: a true fruit.

A banana has no seeds to speak of because its fruit grows without fertilisation, a condition called parthenocarpy. Growers can trigger the same thing in some crops, tomato for instance, by spraying flowers with growth regulators such as auxins; the resulting fruits are seedless.

Why have seeds been so successful? A seed is a travel case: hard-coated and dry, it survives being carried by wind, water or animals to new ground, so a species can colonise new areas. It is a packed lunch: its reserves keep the seedling going until its first leaves can photosynthesise. It is a genetic reshuffle: each seed carries a new mix of its parents' genes. And it does not depend on standing water, because pollination and fertilisation need none, so seeds form reliably even in dry places.

Seeds are also the basis of farming. A dry, dormant seed can be stored, so one harvest feeds people for a year and some of it is kept to sow next season. Farmers know what happens when this fails: after unseasonal rain, grain can sprout while still on the plant, and sprouted grain can neither be stored nor sown.

Seed lifespans range from a few months to centuries, depending on the species and on how dry and cool the seeds are kept; most last a few years.

1.5 Apomixis and Polyembryony

A dandelion seed can grow into a plant identical to its mother without any pollen ever reaching the flower. Some members of the sunflower family (Asteraceae), dandelion among them, and some grasses form seeds with no fertilisation at all. The process is apomixis: asexual reproduction disguised as sexual reproduction, since the end product is still a seed. Contrast parthenocarpy, which skips fertilisation to make a seedless fruit.

Plants reach this result by different routes. One route skips meiosis: an embryo sac forms with diploid nuclei, and its egg develops into an embryo unfertilised. A commoner route, seen in many citrus and mango varieties, uses the nucellus: cells next to the embryo sac begin dividing, bulge into it and develop into embryos of their own Fig. 1.18.

When one seed carries two or more embryos, we call it polyembryony. The extra nucellar embryos are built from the mother's own diploid cells, with no meiosis and no fusion. They are therefore genetically identical to the mother plant and to one another: they are clones. Only the zygotic embryo, if it survives, carries genes from the pollen parent.

Why does apomixis excite plant breeders? Consider a farmer growing hybrid cotton. The hybrid's high yield depends on a particular combination of its two parents' genes, and that combination holds only in the first generation. Seed saved from her crop would give a field of segregating, uneven plants, so she must buy fresh hybrid seed, produced by costly hand-crossing, every season.

If her hybrid were apomictic, its seeds would be clones of the hybrid mother plant, carrying the same gene combination with no segregation. She could then save and resow them year after year. That is why laboratories around the world are working out the genetics of apomixis and trying to move apomixis genes into hybrid crop varieties.