SEXUAL REPRODUCTION IN FLOWERING PLANTS

Sexual reproduction in flowering plants is a precisely coordinated sequence: formation of male and female gametophytes, pollination, pollen-pistil interaction, double fertilisation, and the formation of seeds and fruits.

A typical flower has four whorls: calyx (sepals), corolla (petals), androecium (stamens) and gynoecium (carpels/pistil). The androecium and gynoecium are the essential reproductive whorls.

Male part: stamen and anther

• A stamen consists of a slender filament bearing an anther.
• A typical angiosperm anther is bilobed and dithecous; each lobe has two microsporangia, so the anther contains four pollen sacs.
• The central sterile tissue is the connective. In young anther, sporogenous tissue forms pollen/microspore mother cells.

Female part: pistil and ovule

• The pistil has stigma, style and ovary. The ovary contains one or more ovules attached to the placenta.
• An ovule has a funicle, hilum, integuments, micropyle, nucellus and embryo sac. The chalazal end lies opposite the micropyle.

Microsporogenesis is the formation of haploid microspores from pollen mother cells by meiosis. The four microspores are first arranged as a tetrad; later they separate and develop into pollen grains.

• Sporopollenin is one of the most resistant organic materials known; exine has germ pores where it is absent.
• At shedding, pollen may be two-celled (vegetative cell + generative cell) or three-celled (vegetative cell + two male gametes).
• The generative cell divides mitotically to form two male gametes; pollen grains remain viable for different durations in different species.

Megasporogenesis is the formation of megaspores from the megaspore mother cell (MMC) by meiosis inside the nucellus. Of the four haploid megaspores, usually only one is functional.

The functional megaspore undergoes three mitotic divisions to form an eight-nucleate, seven-celled embryo sac - the typical monosporic development of most flowering plants.

Pollination is transfer of pollen grains from anther to stigma. It may be self-pollination or cross-pollination.

• Autogamy: pollen reaches stigma of the same flower. Cleistogamous flowers guarantee autogamy.
• Geitonogamy: pollen moves between different flowers of the same plant - genetically self, but functionally cross-pollination.
• Xenogamy: pollen reaches a flower on a different plant of the same species; it brings genetically different pollen.

Pollinating agents and floral adaptations

Wind and water are abiotic agents. Most flowering plants use biotic agents such as insects, birds and bats. Wind-pollinated flowers produce abundant light pollen and have large feathery stigmas; animal-pollinated flowers commonly offer colour, fragrance, nectar and edible pollen.

The pistil recognises compatible pollen. Compatible pollen germinates on the stigma and produces a pollen tube that grows through the style, enters an ovule - usually through the micropyle - and releases two male gametes into a synergid.

In plant breeding, desired pollen is placed on a receptive stigma. To prevent unwanted self-pollination, bisexual flowers are emasculated before anther dehiscence and then bagged. After the stigma becomes receptive, desired pollen is dusted and the flower is rebagged.

One male gamete fuses with the egg cell to form a diploid zygote - syngamy. The other fuses with the two polar nuclei to form the triploid primary endosperm nucleus (PEN) - triple fusion. Together, these two events are double fertilisation.

• The PEN develops into endosperm, which supplies nourishment to the developing embryo.
• The zygote develops into embryo: embryonal axis plus cotyledons. A dicot embryo has two cotyledons; a monocot embryo has one cotyledon called scutellum.
• In monocots, the plumule is protected by coleoptile and the radicle by coleorhiza.

After fertilisation, ovules develop into seeds and the ovary develops into a fruit; the ovary wall becomes the pericarp. Fruits that include thalamus are false fruits, as in apple, strawberry and cashew.

• In albuminous seeds, endosperm persists (wheat, maize, castor, coconut); in non-albuminous seeds it is used up (pea, groundnut, bean).
• Perisperm is persistent nucellus, found in black pepper and beet.
• Apomixis is seed formation without fertilisation. It can preserve hybrid characters across generations.
• Polyembryony means occurrence of more than one embryo in a seed; it is common in Citrus and mango.

• Flowers are reproductive units of angiosperms; male and female gametophytes develop in anther and ovule respectively.
• Microspore mother cells and megaspore mother cells undergo meiosis to form spores.
• The pollen grain is male gametophyte; the embryo sac is female gametophyte and is typically 7-celled, 8-nucleate.
• Pollination is followed by pollen-pistil interaction, pollen-tube growth and fertilisation.
• Double fertilisation produces zygote and primary endosperm nucleus.
• Post-fertilisation changes produce embryo, endosperm, seed and fruit.
• Apomixis and polyembryony are special modes with importance in agriculture and horticulture.