Sexual Reproduction in Flowering Plants

Flower Structure and Reproductive Events

A flower is a modified shoot. Its four whorls are calyx (sepals), corolla (petals), androecium (stamens) and gynoecium (carpels). A stamen includes an anther on a filament. A pistil consists of stigma, style and ovary. Wolffia has exceptionally small flowers, Rafflesia has exceptionally large flowers, and the lotus is India’s national flower.

1. Before fertilisation, flowers form and produce male and female gametophytes. Pollination brings pollen to the stigma.
2. Double fertilisation produces a zygote and the primary endosperm nucleus.
3. After fertilisation, embryo, endosperm, seed and fruit develop.

Anther and Microsporangium

A typical anther is bilobed, with two microsporangia in each lobe. The microsporangia develop into pollen sacs. The anther opens along the line of dehiscence to release pollen.

Structure of the Microsporangium

From outside to inside, the microsporangium wall has epidermis, endothecium, middle layers and tapetum. The outer layers protect it and aid dehiscence. The innermost tapetum nourishes developing pollen.

Microsporogenesis

Sporogenous tissue occupies the centre of a microsporangium. Diploid microspore mother cells there undergo meiosis and produce tetrads of haploid microspores. The tetrads separate and individual microspores mature into pollen grains.

Pollen Grain and Male Gametophyte

Two-Celled Pollen Grain

A two-celled pollen grain contains a large vegetative cell with food reserves and an irregular nucleus, plus a small generative cell with dense cytoplasm. The vegetative cell grows the pollen tube. The generative cell divides to form two male gametes.

Pistil and Ovule

A gynoecium may have one carpel or several. When several carpels are fused the gynoecium is syncarpous, as in Papaver. When free it is apocarpous, as in Michelia. The stigma receives pollen, the style links stigma and ovary, and ovules arise from the placenta inside the ovarian cavity.

Megasporangium (Ovule)

• The funicle attaches the ovule to the placenta. The hilum marks the junction between funicle and ovule.
• The integuments enclose the nucellus and leave a small opening, the micropyle. The chalaza lies opposite the micropyle.
• The nucellus contains nutritive tissue. The embryo sac develops inside it.

Megasporogenesis

A diploid megaspore mother cell differentiates near the micropylar end of the nucellus. Meiosis gives four haploid megaspores. In most flowering plants three degenerate, leaving one functional megaspore.

Development of the Female Gametophyte

Three mitotic divisions of the functional megaspore nucleus give eight nuclei. The mature embryo sac typically has seven cells and eight nuclei. At the micropyle are an egg and two synergids. Three antipodals lie at the chalazal end. The central cell contains two polar nuclei.

Pollination

Pollination Agents

Geitonogamy requires a transfer agent but is genetically equivalent to self-pollination. Xenogamy can bring genetically different pollen. Wind (anemophily) and water (hydrophily) are abiotic agents. Biotic agents include insects (entomophily), birds (ornithophily), bats (chiropterophily), and snails or slugs (malacophily).

Outbreeding Devices

• Unisexuality. Separate male and female flowers prevent autogamy. Plants with sexes on different individuals, such as papaya, also prevent geitonogamy.
• Dichogamy. Anthers release pollen and stigmas become receptive at different times. In protandry anthers mature first, as in sunflower. In protogyny the stigma matures first, as in Gloriosa.
• Herkogamy and heterostyly. Physical separation or different heights of anthers and stigmas reduce self-pollination, as in Primula.
• Pollen prepotency. In some crosses, pollen from another plant may germinate or grow faster than self-pollen, as described in the notes for apple.

Self-Sterility (Self-Incompatibility)

Self-sterility, also called self-incompatibility, is a genetic outbreeding mechanism. The pistil recognises incompatible self-pollen from the same flower or another flower on the same plant. It can prevent the pollen from germinating on the stigma or stop its tube from growing through the pistil. This prevents self-fertilisation.

Pollen–Pistil Interaction

Pollen–pistil interaction starts when pollen lands on the stigma. Chemical signals allow the pistil to accept compatible pollen and reject incompatible pollen. Incompatibility can occur within a species or between species. Compatible grains germinate, and their tubes grow through the style to the ovule. One tube enters a synergid and releases its male gametes. Compatible pollen tubes can compete along the way.

Double Fertilisation

Double fertilisation is characteristic of flowering plants. The pollen tube releases two male gametes into one synergid. Both gametes take part in fusion events in the same embryo sac.

• Syngamy. One haploid male gamete fuses with the haploid egg. The result is a diploid zygote (2n), which develops into the embryo.
• Triple fusion. The other haploid male gamete fuses with the two haploid polar nuclei in the central cell. The result is a triploid primary endosperm nucleus (3n). The central cell becomes the primary endosperm cell and develops into endosperm.

After fertilisation, the ovule matures into a seed and the ovary develops into a fruit.

Endosperm Development

The primary endosperm cell divides repeatedly to form triploid tissue that provides food reserves for the developing embryo. Endosperm development generally begins before embryo development.

• Nuclear type. The first nuclear divisions produce free nuclei. Cell walls can form later.
• Cellular type. A cell wall forms with each nuclear division from the start.
• Helobial type. The first division forms two chambers. Free nuclear divisions can follow within a chamber.

Embryo Development (Embryogeny)

The zygote develops at the micropylar end of the embryo sac. In a typical dicot, it forms a proembryo, then passes through globular and heart-shaped stages before maturing. The mature embryo has an embryonal axis and cotyledons. The plumule gives rise to the shoot and the radicle gives rise to the primary root.

Polyembryony

Polyembryony is the presence of more than one embryo in a single seed. It can accompany apomixis. The notes give Citrus and groundnut as examples.

Parthenocarpy

Parthenocarpy is development of a fruit without fertilisation. Parthenocarpic fruits are generally seedless.

Parthenogenesis

Parthenogenesis is embryo development from an unfertilised egg. Haploid parthenogenesis has been described in Solanum nigrum.

Seed and Fruit Formation

• A seed is a mature ovule. It contains an embryo with cotyledons and a protective seed coat derived from the integuments. The outer and inner coats are called testa and tegmen.
• The micropyle allows entry of water and oxygen during germination. The hilum is the attachment scar. Albuminous seeds retain endosperm, whereas non-albuminous seeds use most or all of it during development. Perisperm is persistent nucellus.
• Dormancy delays germination. Fruits protect developing seeds and aid dispersal.

Apomixis

Apomixis is seed formation without fertilisation. An unreduced egg can develop into an embryo, or diploid nucellar cells can form embryos, as in Citrus and mango.