Sexual Reproduction in Flowering Plants

In angiosperms the flower is the organ of sexual reproduction. Hormonal and structural changes make the shoot apex differentiate into a floral primordium; inflorescences then bear floral buds that open into flowers.

Sepals and petals are accessory whorls; the essential whorls are the androecium (stamens, male) and the gynoecium (pistils or carpels, female). A flower with both is bisexual; with one, unisexual. Male and female flowers on the same plant make it monoecious (cucurbits, coconut, maize, castor); on different plants, dioecious (papaya, date palm). A gynoecium is monocarpellary or multicarpellary; several pistils may be fused (syncarpous, Papaver) or free (apocarpous, Michelia).

Long answers are marked in NCERT order: pre-fertilisation structures and events (stamen and pollen, ovule and embryo sac, pollination), double fertilisation, post-fertilisation events (endosperm, embryo, seed, fruit).

A stamen has a slender filament, attached at its proximal end to the thalamus or a petal, and a terminal, generally bilobed anther. Each lobe has two theca, so the anther is dithecous; a longitudinal groove often separates the theca. In transverse section the anther is four-sided with four microsporangia at the corners, two in each lobe; they develop into pollen sacs packed with pollen at maturity.

One microsporangium in T.S. is nearly circular with four wall layers, outside in: epidermis, endothecium, middle layers and tapetum. The outer three protect the sporangium and help the anther dehisce. The tapetum nourishes the developing pollen grains; its cells have dense cytoplasm and generally more than one nucleus, and an anther with a malfunctioning tapetum makes no viable pollen. In a young anther the centre of each microsporangium holds compactly arranged, homogeneous cells, the sporogenous tissue.

Each cell of the sporogenous tissue is a potential pollen mother cell (PMC). Its meiosis forms a microspore tetrad of four haploid microspores: microsporogenesis. As the anther matures and dehydrates the microspores dissociate and develop into pollen grains. Sequence: sporogenous tissue → PMC → microspore tetrad → pollen grain → male gametes.

The pollen grain, the male gametophyte, is generally spherical, 25–50 μm, with a two-layered wall. The hard outer exine is sporopollenin, one of the most resistant organic materials known: it withstands high temperature, strong acids and alkalis, no enzyme degrades it, so pollen fossilises well. Its apertures, the germ pores, lack sporopollenin. The inner intine is thin and continuous, of cellulose and pectin.

A mature grain has a large vegetative cell with abundant food reserve and a large irregular nucleus, and a small spindle-shaped generative cell floating in its cytoplasm. Over 60 per cent of angiosperms shed pollen at this 2-celled stage; in the rest the generative cell divides mitotically first (3-celled).

Box facts: Parthenium (carrot grass) came to India as a contaminant with imported wheat and causes pollen allergy; viability ranges from about 30 minutes (rice, wheat) to months (some Rosaceae, Leguminosae, Solanaceae); pollen stored in liquid nitrogen (−196 °C) forms pollen banks.

Each pistil has a stigma (landing platform for pollen), a slender style and a bulged basal ovary. Inside the ovary is the ovarian cavity or locule, holding the placenta, from which arise the megasporangia, the ovules: one per ovary in wheat, paddy and mango, many in papaya, water melon and orchids.

An ovule is attached to the placenta by a stalk, the funicle; the junction of ovule body and funicle is the hilum. One or two integuments encircle the nucellus except at the tip, where a small opening, the micropyle, is left; opposite it is the chalaza, the basal part. The nucellus, rich in reserve food, fills the integuments, and located in it is the embryo sac (female gametophyte), generally one per ovule, formed from a megaspore by reduction division. In the common anatropous ovule the body is inverted, so the micropyle lies close to the hilum.

Megasporogenesis is the formation of megaspores from the megaspore mother cell. An ovule generally differentiates a single megaspore mother cell (MMC) in the micropylar region of the nucellus. Its meiosis gives four haploid megaspores; in most flowering plants one is functional and three degenerate. An embryo sac formed from a single megaspore is monosporic development, which is why one MMC gives one female gametophyte while one PMC gives four male gametophytes.

The functional megaspore's nucleus divides by mitosis into two nuclei that move to opposite poles; two more divisions give the 4- and 8-nucleate stages. These divisions are strictly free nuclear; walls are laid down only after the 8-nucleate stage. Six nuclei become cells; the remaining two, the polar nuclei, sit below the egg apparatus in the large central cell.

At the micropylar end the egg apparatus has two synergids and one egg cell; the synergids carry cellular thickenings at their micropylar tip, the filiform apparatus, which guides the pollen tube into a synergid. Three antipodals sit at the chalazal end. So the mature embryo sac is 8-nucleate but 7-celled: 3 + 3 + 1, all haploid.

Pollination is the transfer of pollen grains from the anther to the stigma of a pistil, classified by the source of the pollen.

Autogamy: within the same flower. In a normal flower that opens, complete autogamy is rare; it needs synchrony of pollen release and stigma receptivity. Viola, Oxalis and Commelina produce two kinds of flowers: chasmogamous flowers open and expose anthers and stigma; cleistogamous flowers never open, so when the anthers dehisce inside the bud the pollen lands on the stigma. Cleistogamous flowers are invariably autogamous: assured seed-set even without pollinators, but no genetic variation.

Geitonogamy: from one flower to another flower of the same plant; functionally cross-pollination since it needs an agent, but genetically similar to autogamy since the pollen comes from the same plant. Xenogamy: from one plant to a different plant, the only kind that brings genetically different pollen to the stigma.

A single pea plant sets seed because its bisexual flower self-pollinates; a single papaya plant, being dioecious, cannot.

Agents are abiotic (wind, water) or biotic (animals); most plants use biotic agents. With wind and water the landing of pollen on a stigma is a chance factor, so such plants make enormous amounts of pollen relative to the ovules, and their flowers are not colourful and produce no nectar.

Wind pollination, the commoner abiotic type, needs light, non-sticky pollen, well-exposed stamens and a large, often feathery stigma; the flowers often have a single ovule per ovary and are packed in numbers into an inflorescence. The tassels of a corn cob are stigmas and styles waving in the wind to trap pollen.

Water pollination is rare, limited to about 30 genera, mostly monocots: freshwater Vallisneria and Hydrilla, and marine sea-grasses such as Zostera. In Vallisneria the female flower reaches the surface on a long stalk, the male flowers or pollen grains are released on to the surface, and currents carry them passively to the stigma. In sea-grasses female flowers stay submerged and long, ribbon-like pollen travels inside the water. A mucilaginous covering protects the pollen from wetting.

Animal pollination: bees (dominant), butterflies, flies, beetles, wasps, ants, moths, birds and bats. Such flowers are large, colourful, fragrant and nectar-rich; fly- and beetle-pollinated flowers secrete foul odours. Rewards are nectar and pollen, or in Amorphophallus and Yucca a safe place to lay eggs: the moth lays eggs in the locule of the ovary and pollinates the flower, and neither completes its life cycle without the other.

Continued self-pollination causes inbreeding depression, so flowers have devices to discourage it. Four outbreeding devices: (1) pollen release and stigma receptivity are not synchronised; (2) anther and stigma are placed at different positions; (3) self-incompatibility, a genetic mechanism that prevents self-pollen from fertilising the ovules by inhibiting pollen germination or pollen-tube growth in the pistil; (4) unisexual flowers, which on a monoecious plant (castor, maize) prevent autogamy but not geitonogamy, and in a dioecious species (papaya) prevent both.

The pistil recognises compatible pollen and promotes post-pollination events, or rejects incompatible pollen by preventing germination on the stigma or tube growth in the style. After compatible pollination a pollen tube grows out through one germ pore; in 2-celled pollen the generative cell divides into two male gametes during tube growth. The tube grows through stigma and style, enters the ovule through the micropyle and one synergid through the filiform apparatus. All events from pollen deposition to tube entry into the ovule are pollen–pistil interaction.

Artificial hybridisation: if the female parent is bisexual, anthers are removed with forceps from the bud before the anther dehisces (emasculation) and the flower is covered with a butter-paper bag (bagging). When the stigma is receptive, pollen of the chosen male parent is dusted on, the flower is rebagged. A unisexual female flower needs only bagging before it opens.

After entering a synergid the pollen tube releases its two male gametes into its cytoplasm. One fuses with the nucleus of the egg cell: syngamy, forming the diploid zygote. The other moves to the two polar nuclei in the central cell and fuses with them, producing the triploid primary endosperm nucleus (PEN); as three haploid nuclei fuse, this is triple fusion. Two fusions in one embryo sac make the phenomenon double fertilisation, unique to flowering plants.

The central cell is now the primary endosperm cell (PEC) and develops into endosperm; the zygote develops into the embryo; the synergid that received the tube and the antipodals degenerate.

Ploidy, as the examiner wants it: egg (n) + male gamete (n) → zygote 2n; polar nuclei (n + n) + male gamete (n) → PEN 3n. Rice meiocyte 24 chromosomes → gametes and polar nuclei 12 → endosperm 36, embryo 24.

Endosperm and embryo development, the maturing of ovules into seeds and of the ovary into a fruit are together the post-fertilisation events.

Endosperm development precedes embryo development. The primary endosperm cell divides repeatedly into triploid endosperm tissue whose cells are filled with reserve food for the nutrition of the developing embryo. In the most common type the PEN first undergoes successive nuclear divisions without wall formation, the free-nuclear endosperm; later cell walls form and it becomes cellular. Coconut is the example: tender coconut water is free-nuclear endosperm with thousands of nuclei, and the white kernel is cellular endosperm; both are triploid.

The endosperm may be completely consumed by the developing embryo before seed maturation (pea, groundnut, beans) or persist in the mature seed and be used up during germination (castor, coconut). That is the castor-versus-groundnut question: both form endosperm; groundnut's embryo uses it all and stores food in its cotyledons (non-albuminous seed), castor's leaves part of it (albuminous seed).

The embryo develops at the micropylar end of the embryo sac, where the zygote lies. Embryogeny is similar in monocots and dicots: zygote → proembryo → globular → heart-shaped → mature embryo.

A typical dicot embryo has an embryonal axis and two cotyledons. 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.

A monocot embryo has one cotyledon; in the grass family it is the scutellum, placed laterally on the embryonal axis. At the lower end the radicle and root cap are enclosed in an undifferentiated sheath, the coleorrhiza. Above the attachment of the scutellum is the epicotyl, whose shoot apex and leaf primordia are enclosed in a hollow foliar structure, the coleoptile. One-mark functions: the scutellum absorbs food from the endosperm, the coleoptile protects the plumule as it pushes through soil, the coleorrhiza protects the radicle.

The seed, the final product of sexual reproduction, is often described as a fertilised ovule. It typically has seed coat(s), cotyledon(s) and an embryo axis. The integuments harden into protective seed coats, and the micropyle remains as a small pore that lets oxygen and water in during germination.

Non-albuminous seeds have no residual endosperm, since it is completely consumed during embryo development (pea, groundnut). Albuminous seeds retain a part of it (wheat, maize, barley, castor, sunflower). In black pepper and beet remnants of the nucellus persist as the perisperm, which is diploid, unlike the triploid endosperm.

A maturing seed dries to 10–15 per cent moisture by mass, metabolism slows, and the embryo may enter dormancy until moisture, oxygen and a suitable temperature return; dehydration and dormancy make seeds storable, the basis of agriculture.

As ovules mature into seeds the ovary develops into the fruit; the ovary wall becomes the pericarp. In apple, strawberry and cashew the thalamus also contributes: false fruits; fruits from the ovary alone are true fruits. Fruits formed without fertilisation are parthenocarpic and seedless (banana); parthenocarpy can be induced with growth hormones.

A few flowering plants, such as some species of Asteraceae and grasses, produce seeds without fertilisation: apomixis, a form of asexual reproduction that mimics sexual reproduction, so the progeny are genetically identical to the parent.

Apomictic seeds develop in several ways. In some species a diploid egg cell forms 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 it and develop into embryos, so each ovule contains many embryos: polyembryony. A squeezed orange seed shows several embryos, the nucellar ones identical to the mother plant.

Why it matters: seed collected from a hybrid crop segregates and does not maintain the hybrid characters, so costly hybrid seed must be bought every year. If hybrids are made into apomicts there is no segregation and the farmer can keep using seed from his own crop year after year.