Sexual Reproduction in Flowering Plants, revision notes

Overview

Flowering plants reproduce sexually using flowers, structures specialised for producing and bringing together male and female gametes. This chapter covers flower structure, how pollen grains and the embryo sac form, pollination and fertilisation, and how the resulting seeds and fruits develop, are dispersed, and eventually germinate into new plants.

Because each stage depends on the one before it, it helps to trace the whole sequence, flower, pollen and embryo sac, pollination, fertilisation, seed and fruit, dispersal, germination, as a single connected story rather than isolated facts.

Labelling a flower, describing double fertilisation, and comparing self- and cross-pollination are common exam tasks, so precise vocabulary for each floral part and each stage of the process is essential. A common mark-losing habit is switching freely between 'pollen' and 'seed' or between 'ovule' and 'ovary', so checking these terms carefully before an exam is worthwhile.

After a seed forms inside the fruit, it usually needs to be carried away from the parent plant before it can grow successfully, since dispersal reduces competition between parent and offspring for light, water and soil nutrients and lets the species colonise new areas. Fruits and seeds show specific adaptations for wind, water, animal or explosive dispersal that match how far and by what method they travel.

Self-pollination, the transfer of pollen within the same flower or plant, and cross-pollination, the transfer between different plants of the same species, have opposite advantages: self-pollination is reliable even without an external agent but produces less genetic variation, while cross-pollination increases genetic variation but depends on wind or an animal to carry pollen between plants, so many flowers have structures that favour one method over the other.

21.1 Structure of a flower: whorls, parts and functions

A flower is built from four whorls of modified leaves attached to a swollen receptacle at the tip of a flower stalk: the sepals (collectively the calyx) form the outermost whorl and protect the flower bud before it opens; the petals (collectively the corolla) form the next whorl and often attract pollinating agents through colour and scent; the stamens, each made up of an anther and a filament, form the male whorl and produce pollen; and the carpels, each made up of a stigma, style and ovary containing one or more ovules, form the innermost, female whorl.

A flower with all four whorls present is described as complete, while one missing at least one whorl - commonly the petals or the sepals - is incomplete; a flower containing both stamens and carpels is bisexual, while one containing only stamens or only carpels is unisexual. These distinctions matter because a unisexual flower cannot self-pollinate on its own and depends on a separate male or female flower, sometimes on a separate plant altogether, for pollination to occur.

21.2 Formation of a pollen grain and an embryo sac

Inside each anther, a diploid cell called a pollen mother cell (microspore mother cell) divides by meiosis to produce four haploid microspores, and each microspore develops into a pollen grain surrounded by a tough, patterned outer wall. Within the pollen grain, one nucleus divides once more by mitosis to give a tube nucleus, which later directs the growth of the pollen tube, and a generative nucleus, which itself divides to produce two male gametes either before or after pollination.

Inside the ovule within the ovary, a diploid cell called an embryo sac mother cell (megaspore mother cell) also divides by meiosis to produce four haploid megaspores, but only one of these survives while the other three degenerate. The surviving megaspore then divides by mitosis three times without the cell itself dividing, producing eight haploid nuclei arranged as an egg cell flanked by two synergid cells at one end, three antipodal cells at the opposite end, and two polar nuclei that later fuse in the centre to form a single diploid polar nucleus (the embryo sac is therefore usually described as a seven-celled, eight-nucleate structure).

Because both the pollen grain and the embryo sac are produced by meiosis before fertilisation, genetic variation is already introduced into each parent's gametes independently of the variation that fertilisation itself later creates by combining two parents' genes.

21.3 Pollination: self, cross, and the agents that carry pollen

Pollination is the transfer of pollen grains from the anther to the stigma; self-pollination occurs within the same flower or between different flowers on the same plant, while cross-pollination occurs between flowers on different plants of the same species and requires an external agent, usually wind or an insect, to carry the pollen across.

A flower's structure usually signals which agent it relies on, and the two strategies differ sharply enough that a flower's features can be classified reliably from a description alone.

21.3 Fertilisation: the pollen tube and double fertilisation

Self-pollination is reliable even when no external agent is available and preserves a set of genes already suited to the local environment, but it lowers genetic variation among offspring; cross-pollination raises genetic variation and tends to produce hardier offspring, but it depends entirely on an external agent and on a compatible pollen source being present at the right time. Some flowers reduce self-pollination through features such as the anther and stigma of the same flower maturing at different times, which favours receiving pollen from another plant instead.

After a compatible pollen grain lands on the stigma, it absorbs moisture and its tube nucleus directs the growth of a pollen tube down through the tissue of the style towards an ovule in the ovary; the generative nucleus travels inside the growing tube and divides to form two male gametes, if it has not already done so inside the pollen grain.

The pollen tube enters the ovule, usually through a small opening called the micropyle, and releases its two male gametes into the embryo sac; because two separate fusions occur in the same event, this process is called double fertilisation and is unique to flowering plants. One male gamete fuses with the egg cell to form a diploid zygote, which develops into the embryo, while the other fuses with the two polar nuclei (or the single diploid polar nucleus formed by their earlier fusion) to form a triploid nucleus that develops into the endosperm, a nutritive tissue that feeds the growing embryo.

21.4 From ovule to seed, from ovary to fruit

After double fertilisation, the ovule as a whole develops into a seed: the zygote develops into the embryo, made up of a plumule (young shoot), a radicle (young root) and one or two cotyledons (seed leaves); the endosperm develops into a food store; and the outer layers of the ovule harden into a protective seed coat called the testa. At the same time, the ovary wall develops into the fruit, which encloses the seed or seeds and often also helps to disperse them.

Flowering plants fall into two groups based on the number of cotyledons in the seed, and this difference also affects where the seed's food reserve is stored.

21.5 Why seeds matter, how they are dispersed, and how they germinate

A seed lets a plant survive a period unfavourable for growth - such as a dry season or a cold spell - by remaining dormant, with its metabolism slowed almost to a stop, until conditions improve; it also lets the species colonise new ground away from the parent plant, reducing competition between parent and offspring for light, water and soil nutrients.

Because dispersal serves this second purpose, seeds and the fruits around them show adaptations matched to a specific method of travel.

• Wind dispersal - a light seed or fruit with wings or a feathery plume that catches moving air, such as in grasses and in dandelion-type fruits.
• Water dispersal - a fruit with a buoyant, waterproof wall that floats until it reaches a suitable bank, such as a coconut.
• Animal dispersal (external) - a fruit covered in hooks or spines that catch onto fur or feathers and drop off some distance away.
• Animal dispersal (internal) - a fleshy, edible fruit that is eaten whole, with the tough seeds passing unharmed through the animal's gut and being deposited elsewhere in its droppings.
• Explosive dispersal - a dry fruit, such as a pod, that twists as it dries until it splits suddenly and flings the seeds away from the parent plant.

21.5 Conditions for germination

A seed generally needs three external conditions to germinate: water, to soften the testa and activate the enzymes stored inside the seed; oxygen, to allow aerobic respiration that releases the energy the growing embryo needs; and a suitable temperature, at which these enzymes work efficiently. Once these conditions are met, the stored food reserve - in the endosperm or the cotyledons, depending on the seed type - is broken down and transported to the growing radicle and plumule until the seedling can support itself by photosynthesis.

Key concepts to master

• Structure of a flower, The stamen, made up of the anther and filament, is the male reproductive part of a flower, while the carpel, made up of the stigma, style and ovary, is the female part; a single flower may contain one or more of each, and the number and arrangement of these parts is often used to identify a species. Some flowers, described as incomplete, lack one or more of these parts, such as petals or stamens, yet can still reproduce successfully.
• Development of gametes, Pollen grains, each containing a male gamete, form inside the anther, while the embryo sac, containing the female gamete, forms inside the ovule within the ovary; both are produced by meiosis, which creates genetic variation among the gametes even before fertilisation combines two parents' genes.
• Pollination, Pollination is the transfer of pollen grains from the anther to the stigma, either of the same flower (self-pollination) or of a different flower on a different plant of the same species (cross-pollination); the transfer is carried out by an external agent such as wind or an insect, and floral features often reveal which agent a flower relies on. A flower can receive its own pollen and pollen from another plant at the same time, so both types of pollination are not always mutually exclusive events.
• Fertilisation, After pollination, a pollen tube grows down through the style towards the ovule, carrying two male gametes; in double fertilisation, one male gamete fuses with the female gamete to form a diploid zygote and the other fuses with two nuclei in the embryo sac to form a triploid tissue called the endosperm, which stores food for the developing embryo. Because two separate fusions occur from the same pollen tube, double fertilisation is unique to flowering plants and is not seen in animals or in non-flowering plants such as ferns.
• Seeds and fruits, After fertilisation, the ovule develops into a seed containing the embryo, its food store and a protective seed coat, while the ovary wall develops into the fruit, which protects the seed or seeds inside and, in many species, also assists with dispersal.
• Importance of seeds, Seeds allow a plant species to survive conditions unfavourable for growth, such as drought or cold, by remaining dormant until conditions improve, and to be carried away from the parent plant to colonise new areas with less competition for resources.
• Seed dispersal, Seeds and fruits are dispersed by wind, using light weight and structures such as wings or feathery plumes; by water, using a buoyant, waterproof fruit wall; by animals, either through hooks and spines that cling to fur or through a fleshy, edible fruit that is eaten and the seeds later deposited elsewhere; or explosively, when a dry pod twists and splits suddenly, flinging seeds away from the parent plant. A single fruit sometimes combines more than one feature, such as being both lightweight and having a hook, so a real specimen may not fit neatly into only one dispersal category.
• Germination requirements, A seed generally needs three conditions to germinate: water, to soften the seed coat and activate enzymes; oxygen, to allow aerobic respiration that releases the energy needed for growth; and a suitable temperature, at which these enzymes work efficiently; light is not usually required for germination itself, although the seedling needs it soon afterwards for photosynthesis. These three conditions are often tested through a controlled experiment comparing seeds given different combinations of water, air and temperature to show which factor is limiting.
• Self-pollination versus cross-pollination, Self-pollination reliably produces seed even when no pollinating agent is available and preserves a combination of genes already well suited to the environment, but it produces offspring with little genetic variation and can lead to weaker offspring over many generations; cross-pollination increases genetic variation, generally producing hardier offspring, but it depends on an external agent and on the pollen source being compatible. Structural features that reduce self-pollination, such as the anther and stigma maturing at different times, are themselves valid exam answers when asked how a flower favours cross-pollination.

Quick recall checklist

1. Can you define and explain Structure of a flower?
2. Can you define and explain Development of gametes?
3. Can you define and explain Pollination?
4. Can you define and explain Fertilisation?
5. Can you define and explain Seeds and fruits?
6. Can you define and explain Importance of seeds?
7. Can you define and explain Seed dispersal?
8. Can you define and explain Germination requirements?
9. Can you define and explain Self-pollination versus cross-pollination?

Frequently asked questions

More for Sexual Reproduction in Flowering Plants

Source:SRC-DSKP-EN