Sexual Reproduction in Flowering Plants — Notes

The flower is the reproductive organ of flowering plants (angiosperms). Hormonal and structural changes turn a shoot apex into a floral bud, in which the androecium (stamens) and gynoecium (pistils) develop.

A stamen has a filament and an anther. A typical anther is bilobed and dithecous (two thecae per lobe), with four microsporangia at its corners that later become pollen sacs.

Wall of a microsporangium (outside → inside): epidermis, endothecium, middle layers (these three protect and help dehiscence) and tapetum, which nourishes the developing pollen; tapetal cells have dense cytoplasm and often more than one nucleus.

Microsporogenesis: cells of the sporogenous tissue act as microspore (pollen) mother cells (diploid). Each undergoes meiosis to form a microspore tetrad of four haploid cells. As the anther matures and dehydrates, the microspores separate and develop into pollen grains. One meiosis → four pollen grains.

Pollen grains are usually spherical, about 25–50 micrometres in diameter, with a two-layered wall.

• Exine: hard outer layer of sporopollenin, one of the most resistant organic materials; it withstands high temperatures and strong acids and alkalis, and no known enzyme degrades it, so pollen is well preserved as fossils. Germ pores are gaps where sporopollenin is absent.
• Intine: thin, continuous inner layer of cellulose and pectin.

A mature pollen grain contains a large vegetative cell (abundant food reserve, large irregular nucleus) and a small, spindle-shaped generative cell floating in its cytoplasm. In over 60 percent of angiosperms, pollen is shed at this two-celled stage; in the rest, the generative cell divides by mitosis into two male gametes before shedding (three-celled stage).

Pollen and people: pollen of many plants (for example Parthenium, carrot grass) causes allergies such as asthma and bronchitis; pollen is used as a food supplement; viability ranges from about 30 minutes (rice, wheat) to months (some members of the rose, pea and potato families); pollen can be stored for years in liquid nitrogen (about −196 °C) in pollen banks for breeding.

The gynoecium may be monocarpellary (one pistil) or multicarpellary (many); pistils may be syncarpous (fused, as in poppy) or apocarpous (free, as in Michelia). Each pistil has a stigma, a style and an ovary; ovules arise from the placenta in the ovarian cavity.

Ovule (megasporangium): attached to the placenta by the funicle; the junction with the ovule body is the hilum. One or two integuments enclose it except at the micropyle; the opposite, basal end is the chalaza. Inside the integuments is the nucellus, which holds the embryo sac (female gametophyte). An ovule usually has one embryo sac.

Megasporogenesis: a single megaspore mother cell differentiates in the micropylar region of the nucellus and undergoes meiosis to form four megaspores. In most flowering plants one is functional and three degenerate; the embryo sac formed from one megaspore is monosporic.

Embryo sac development: the functional megaspore nucleus divides by mitosis three times, free-nuclear, giving 2, 4 and then 8 nuclei (four at each pole). Six nuclei are walled off into cells; one nucleus from each pole moves to the centre as a polar nucleus. The mature embryo sac is 7-celled and 8-nucleate:

• micropylar end: egg apparatus = egg cell + two synergids (with the filiform apparatus, which guides the pollen tube);
• chalazal end: three antipodal cells;
• centre: large central cell with two polar nuclei.

Pollination is the transfer of pollen grains from an anther to a stigma.

Chasmogamous flowers open and expose anthers and stigma; cleistogamous flowers never open and are always autogamous, giving assured seed set even without pollinators (Viola, Oxalis, Commelina have both kinds).

Wind: light, non-sticky pollen; well-exposed stamens; large, often feathery stigma; usually a single ovule per ovary; many flowers in an inflorescence (maize tassels shed pollen; the silks of the cob are stigmas and styles). Common in grasses.

Water: about 30 genera, mostly monocots: Vallisneria (female flowers reach the surface on long stalks; male flowers or pollen float to them), Hydrilla, seagrasses such as Zostera (long ribbon-like pollen carried under water). Pollen is protected by a mucilaginous covering. Water hyacinth and water lily, though aquatic, are pollinated by insects or wind.

Animals: bees are the dominant biotic agents; others include butterflies, flies, beetles, wasps, moths, sunbirds, hummingbirds and bats. Flowers are large, colourful, fragrant and rich in nectar, or small and clustered; fly- and beetle-pollinated flowers may smell foul. Rewards are nectar and pollen, or a safe place to lay eggs (Amorphophallus). Yucca and its moth depend on each other: the moth lays eggs in the ovary and its larvae eat some seeds. Pollen and nectar robbers take rewards without pollinating.

Continued self-pollination leads to inbreeding depression. Outbreeding devices:

1. Pollen release and stigma receptivity not synchronised.
2. Anther and stigma at different positions.
3. Self-incompatibility: a genetic mechanism that prevents pollen of the same flower or plant from germinating or growing in the pistil.
4. Unisexual flowers: monoecious plants (castor, maize) prevent autogamy but not geitonogamy; dioecious plants (papaya) prevent both.

Pollen–pistil interaction: the pistil recognises compatible pollen through chemical components of the pollen and pistil, and promotes it; incompatible pollen is inhibited at germination or tube growth. Compatible pollen germinates through a germ pore; the pollen tube grows through the stigma and style into the ovary (a two-celled pollen grain's generative cell divides in the tube), enters the ovule through the micropyle, and is guided by the filiform apparatus into a synergid.

Artificial hybridisation: ensures only the desired pollen reaches the stigma.

• Emasculation: removal of anthers from a bisexual flower bud before dehiscence (not needed for female flowers of unisexual plants).
• Bagging: covering the flower with a butter-paper bag to keep out unwanted pollen; when the stigma is receptive, it is dusted with the chosen pollen and rebagged.

The pollen tube releases two male gametes into a synergid.

• Syngamy: male gamete + egg → zygote (2n).
• Triple fusion: male gamete + two polar nuclei → primary endosperm nucleus (3n); the central cell becomes the primary endosperm cell.

Two fusions in one embryo sac = double fertilisation, a feature of flowering plants.

Endosperm develops before the embryo. The primary endosperm cell divides repeatedly to form triploid endosperm with reserve food. Commonly it is first free-nuclear and later becomes cellular: in coconut, the water is free-nuclear endosperm (thousands of nuclei) and the white kernel is cellular endosperm. The endosperm may be consumed completely before seed maturity (non-albuminous seeds: pea, groundnut, beans) or persist (albuminous seeds: wheat, maize, barley, castor, coconut). Persistent nucellus is perisperm (black pepper, beet).

Embryo: the zygote, at the micropylar end, divides after some endosperm forms: proembryo → globular → heart-shaped → mature embryo.

• Dicot: embryonal axis + two cotyledons; epicotyl (above cotyledons) ends in the plumule; hypocotyl (below) ends in the radicle with a root cap.
• Monocot (grass): one cotyledon, the scutellum, lateral; coleoptile encloses the shoot apex and leaf primordia; coleorhiza encloses the radicle and root cap; an epiblast may be present.

The seed is a fertilised ovule. Integuments → seed coat; the micropyle remains as a pore that lets oxygen and water in during germination. Mature seeds dry to about 10–15 percent moisture by mass; metabolism slows and the embryo may become dormant until conditions are favourable.

The ovary → fruit and the ovary wall → pericarp (fleshy, as in guava and orange, or dry, as in groundnut and mustard). In false fruits the thalamus also contributes (apple, strawberry, cashew). Parthenocarpic fruits develop without fertilisation and are seedless (banana); they can be induced with growth hormones.

Advantages of seeds: pollination and fertilisation are free of water; seeds are dispersed widely and colonise new areas; stored food nourishes the seedling; the hard seed coat protects the embryo; seeds of sexual reproduction carry variation; dormancy lets them be stored, which is the basis of agriculture. Some seeds stay viable for very long: an Arctic lupine seed germinated after about 10,000 years, and a date palm seed about 2000 years old has germinated.

Apomixis: formation of seeds without fertilisation, a form of asexual reproduction that mimics sexual reproduction (some members of the sunflower family, grasses).

• A diploid egg forms without reduction division and develops into an embryo without fertilisation; or
• cells of the nucellus (or integuments) divide, protrude into the embryo sac and develop into embryos.

Polyembryony: more than one embryo in a seed, often from nucellar cells, as in many Citrus and mango varieties.

Importance: hybrid seeds give high yields, but progeny of hybrids segregate, so farmers must buy fresh hybrid seed every year. If hybrids were made apomictic, there would be no segregation, and farmers could keep using their own hybrid seed. Scientists are trying to identify the genes for apomixis and transfer them into hybrid varieties.