Sexual Reproduction in Flowering Plants

Written and reviewed by Anjali Sharma, Biology faculty at UniDrill (M.Sc. Botany, Banaras Hindu University · 8 years teaching NCERT Biology). Based on NCERT Biology Class XII, Chapter 1.

What is the water inside a tender coconut?

Rice, wheat, mango and coconut all exist because of something that happens inside a flower and nowhere else in the living world: two separate fusions in one embryo sac. One makes the baby plant. The other makes the food store beside it, and that food store is the white kernel of the coconut, the grain of wheat and the rice you eat. The water is part of the same story, as you will see. This chapter follows the flower from its first cell divisions to a ripe fruit, and the order never changes.

The chapter in one line: flower → pre-fertilisation (male and female gametophytes, pollination) → fertilisation (double fertilisation) → post-fertilisation (endosperm, embryo, seed, fruit).

The flower: where the action is

A flower is the reproductive unit of an angiosperm. Its two reproductive whorls are the androecium, made of stamens (the male part), and the gynoecium, made of pistils (the female part). Each stamen has a filament and a bilobed anther, and each pistil has a stigma, a style and an ovary. Both organs are factories for spores. The anther makes microspores, which become pollen grains, and the ovule makes megaspores, one of which becomes the embryo sac. In each case a diploid mother cell divides by meiosis, and the products are haploid. Remember this early rule: spore formation is meiosis, and gamete formation inside the gametophyte is mitosis.

The male side: anther to pollen grain

A typical anther is bilobed, dithecous (each lobe has two chambers, or theca) and tetrasporangiate, which means it has four microsporangia, one at each corner. A cross section of a young microsporangium shows four wall layers, and their jobs are neatly divided.

In the centre is sporogenous tissue. Each cell of it becomes a pollen mother cell (PMC), and meiosis of the PMC is called microsporogenesis.

The sequence in one line: sporogenous tissue → pollen mother cell (2n) → meiosis → microspore tetrad (n) → pollen grain → two male gametes.

The pollen grain is the male gametophyte. It is generally spherical, 25 to 50 micrometres across, and has a two layered wall.

Inside, the grain has two cells. The vegetative cell is large, rich in food reserves and has a large, irregular nucleus. The generative cell is small, spindle shaped and has dense cytoplasm and a nucleus, and it floats in the cytoplasm of the vegetative cell. In over 60 per cent of angiosperms, pollen is shed at this 2-celled stage. In the rest, the generative cell divides before shedding to give two male gametes, so the pollen is shed 3-celled.

Pollen viability varies a great deal. In rice and wheat the grains stay viable for only about 30 minutes, while in some members of Rosaceae, Leguminosae and Solanaceae they last for months. Pollen can be stored in liquid nitrogen at −196 °C for years, and these pollen banks work like seed banks for crop breeding.

Beyond the textbook. For the exam: exine = sporopollenin, intine = cellulose and pectin, germ pores = where the exine is absent. In reality: sporopollenin is the reason pollen survives as fossils, and its patterns are so distinctive that scientists identify ancient plants and past climates from fossil pollen alone. The same wall that makes pollen tough also makes it a major allergen, and pollen of plants like Parthenium (carrot grass) causes allergies and bronchial problems in many people. Pollen is also sold as a food supplement in tablets and syrups.

The female side: ovule to embryo sac

The gynoecium may have a single pistil (monocarpellary) or several. If there are several pistils and they are fused, the gynoecium is syncarpous (Papaver), and if they are free, it is apocarpous (Michelia). Inside the ovary, the ovule is borne on a cushion called the placenta. The ovule is the megasporangium.

Megasporogenesis. One cell of the nucellus near the micropylar end becomes the megaspore mother cell (MMC). It divides by meiosis to give four megaspores. Usually only one is functional and the other three degenerate. When the embryo sac forms from a single megaspore, the development is called monosporic.

Building the embryo sac. The nucleus of the functional megaspore divides mitotically three times without forming walls (2 → 4 → 8 nuclei), arranged four at each end. Then six of the eight nuclei get walls around them and two of them, one from each end, move to the centre.

Why the embryo sac is 7 celled and 8 nucleate. Six nuclei become separate cells (3 + 3), and the two polar nuclei stay together in one large central cell. So the count is 3 + 3 + 1 = 7 cells, and 3 + 3 + 2 = 8 nuclei.

Pause and predict: an embryo sac has just completed its first two nuclear divisions and has four nuclei. How many cells does it have, and why?

Check your answer: one cell. Wall formation starts only after the third division, so the first rounds of division are free nuclear and the single large cell simply becomes multinucleate.

Pollination: getting pollen to the stigma

Pollination is the transfer of pollen from the anther to the stigma. It is only a transfer, and fertilisation is the later fusion of gametes. By the source of the pollen, there are three kinds.

Flowers can also be sorted by whether they open. Chasmogamous flowers open and expose their anthers and stigma. Cleistogamous flowers never open, so the anthers and stigma lie close together and the flower is invariably autogamous (Viola, Oxalis and Commelina have them). In a closed flower the anthers dehisce within the bud and the pollen falls on the stigma, so a cleistogamous flower gives an assured seed set even when pollinators are absent.

Agents of pollination. Most flowering plants use biotic agents (animals), while a few use abiotic agents (wind and water).

The words "aquatic" and "water pollinated" are not synonyms. Most aquatic plants, such as water hyacinth and water lily, carry their flowers above the water and are pollinated by insects or wind.

The Amorphophallus (the "largest flower", about 6 feet tall) offers a safe egg laying site in the flower. The Yucca and a moth species depend on each other completely: the moth can only lay eggs in the Yucca ovary, and the plant can only be pollinated by that moth. Neither can complete its life cycle without the other, which is called an obligate mutualism.

Beyond the textbook. For the exam: Amorphophallus is described as having the largest flower. In reality: what looks like one giant flower is an inflorescence, a cluster of many tiny flowers. In the Yucca partnership the moth also eats some of the developing seeds, but there are always enough left over for the plant, which is why the arrangement is stable for both.

Outbreeding devices. Continued self-pollination leads to inbreeding depression, so flowering plants have many ways to promote cross-pollination.

Pause and predict: a papaya plant has only male flowers. Can geitonogamy occur on it?

Check your answer: no. Geitonogamy needs a second flower on the same plant that has a stigma, but a male plant has none. This is why dioecy prevents both autogamy and geitonogamy.

Pollen–pistil interaction and artificial hybridisation

Pollen landing on a stigma does not guarantee fertilisation. The pistil recognises the pollen through chemical signals, and it accepts the right pollen and rejects the wrong one. If the pollen is compatible, it germinates through a germ pore, producing a pollen tube that grows through the tissues of the stigma and style and reaches the ovary. It enters the ovule through the micropyle and finally enters one of the synergids through the filiform apparatus, which guides it there.

The route in one line: stigma → style → ovary → micropyle → filiform apparatus → synergid.

Plant breeders use this knowledge for artificial hybridisation. They want only desired pollen to reach the stigma, so they use two steps. Emasculation is the removal of anthers from a bisexual flower bud before they dehisce. Bagging is covering the emasculated flower with a bag of suitable size, usually of butter paper, to prevent contamination from unwanted pollen. When the stigma is receptive, pollen from the chosen male parent is dusted onto it and the flower is bagged again until the fruit forms. If the female parent bears unisexual flowers, emasculation is not needed, and the flowers only need bagging.

Double fertilisation: the event unique to flowering plants

The pollen tube discharges two male gametes into the cytoplasm of the synergid. They move toward the egg and the central cell, and two fusions follow.

The occurrence of both events together is double fertilisation, and it is found only in flowering plants. Triple fusion involves three haploid nuclei, but it is one fusion event. Since everything in a seed ultimately comes from one of these two products, here is the ploidy of each cell in the whole chapter.

Pause and predict: in a plant, the egg cell has 12 chromosomes. How many are there in the zygote, in the PEN, and in a cell of the seed coat?

Check your answer: zygote 24 (12 from the egg + 12 from the male gamete), PEN 36 (12 + 12 + 12), and seed coat 24, because the seed coat is made from the maternal integuments and is diploid.

After fertilisation: endosperm, embryo, seed and fruit

Endosperm comes first. In most flowering plants the endosperm starts developing before the embryo. This ensures a food supply is ready when the embryo starts growing. In the most common pattern, the PEN divides again and again without making walls, and the result is free nuclear endosperm. Walls form later and the tissue becomes cellular endosperm. The two stages are visible in a coconut: the tender coconut water is the free nuclear endosperm, and the white kernel around it is the cellular endosperm.

The embryo. The zygote usually divides only after some endosperm has formed. Embryo development, called embryogeny, starts with a proembryo, then goes to a globular stage, a heart shaped stage and finally a mature embryo.

The sequence in one line: zygote → proembryo → globular → heart shaped → mature embryo.

Seed. The fertilised ovule becomes the seed. It has the seed coat, cotyledons and an embryo axis. The two integuments harden into the seed coat, and the micropyle stays as a small pore that lets in oxygen and water at germination. As the seed matures, it loses water and its metabolism slows, so the embryo may go into dormancy until conditions favour germination.

In some seeds, such as black pepper and beet, a part of the nucellus survives as a thin layer called the perisperm.

Fruit. The ovary becomes the fruit and the ovary wall becomes the pericarp. Fruits that develop only from the ovary are true fruits. In some species the thalamus also contributes, giving a false fruit, such as apple, strawberry and cashew. Some fruits develop without fertilisation and are therefore seedless. This is parthenocarpy (banana), and it can also be induced by growth hormones.

Beyond the textbook. For the exam: in a mature seed the endosperm is either used up (non-albuminous) or retained (albuminous). In reality: endosperm is what people eat most. Wheat, rice and maize get most of their calories from it, and a coconut kernel is endosperm too. The second fusion in double fertilisation is therefore not a curiosity: it builds the food store of the world's staple crops.

Seeds can stay viable for astonishing times. A Lupinus arcticus seed from the Arctic tundra germinated after about 10,000 years, and a date palm seed about 2,000 years old, found near the Dead Sea, also germinated.

Apomixis and polyembryony

Apomixis is the formation of seeds without fertilisation. It is a form of asexual reproduction that mimics sexual reproduction, and it occurs in some members of the Asteraceae and grasses. Seeds can form in two ways. In some species a diploid egg cell forms without reduction division and develops into an embryo without fertilisation. In others, as in many Citrus and Mango varieties, cells of the nucellus surrounding the embryo sac divide, protrude into the sac and develop into embryos. Because such an ovule holds several embryos, this is polyembryony.

Why scientists want apomixis. Hybrid seeds give high yields, but if farmers save and sow their own seeds, the characters segregate in the next generation and the advantage is lost. So hybrid seed has to be bought afresh each year, which is expensive. If a hybrid were apomictic, its seeds would be clones of the parent and would not segregate, and farmers could use the same seed year after year. This is an active area of research.

Beyond the textbook. For the exam: apomixis = seeds without fertilisation; polyembryony = more than one embryo in a seed (orange). In reality: the nucellus is maternal tissue, so a nucellar embryo is genetically identical to the mother plant, not a product of sex. That is why a citrus seed can give a seedling that is a clone of the parent tree, a trait horticulturists value.

Exam traps

60-second revision

Where this chapter connects

• Before this chapter: Morphology of Flowering Plants describes the flower, fruit and seed structures that this chapter explains in terms of function.
• Next: Human Reproduction repeats the same logic in animals: meiosis makes gametes, fertilisation restores diploid, and a nourishing tissue (placenta) feeds the embryo.
• Related: Principles of Inheritance and Variation uses emasculation, bagging and cross-pollination in hybridisation experiments, and Plant Growth and Development explains the growth hormones that induce parthenocarpy.