Pollination and Fertilisation | ICSE Class 9 Biology Notes

This note covers pollination and fertilisation in flowering plants: why pollination is needed, the pollen grain and embryo sac, self-pollination and cross-pollination with their advantages and disadvantages, flowers pollinated by insects, wind and water, how nature favours cross-pollination, the events between pollination and fertilisation, double fertilisation and triple fusion, post-fertilisation changes, fruits and seeds, and how breeders control pollination.

What is pollination, and why is it needed?

In flowering plants, or angiosperms, the flower is the seat of sexual reproduction. Its male part is the stamen, made of a long, slender stalk, the filament, and a terminal anther, which produces pollen grains containing the male gametes.

The female part is the pistil. It has three parts: the stigma, which serves as a landing platform for pollen grains; the style, the elongated slender part beneath the stigma; and the ovary, the swollen base. The ovary contains ovules, and each ovule has an egg cell, the female gamete.

Both kinds of gametes are non-motile, so they have to be brought together for fertilisation to occur. Pollination is the mechanism that achieves this. Plants depend on external agents, called pollinators, such as wind, water, insects and birds.

Definition: Pollination is the transfer of pollen grains from the anther to the stigma of a flower. It is important for the formation of fruits and seeds.

Experiment: to show that pollination is needed for fruit formation

1. On a garden pea plant, select two young flower buds and three freshly opened flowers.
2. Carefully remove the stamens from one of the buds and from one of the open flowers.
3. Loosely wrap muslin cloth bags around both buds, around the open flower whose stamens were removed, and around one open flower with its stamens intact.
4. Leave the third open flower uncovered.
5. Observe the flowers regularly. When the pods on the uncovered flower are fully developed, remove the bags and record which treatments formed fruits.

Fruits form in all the treatments except the bud from which the stamens were removed. This shows that the transfer of pollen grains from the anther to the stigma is necessary for fruit formation.

What are the pollen grain and the embryo sac?

Pollen grains develop inside the anther, which has four microsporangia, two in each lobe. In each microsporangium, cells of the sporogenous tissue divide by meiosis to form microspore tetrads. The microspores separate and develop into pollen grains, and the microsporangia become pollen sacs packed with pollen grains.

Pollen grains represent the male gametophytes. They are generally spherical, about 25 to 50 micrometres in diameter, with a prominent two-layered wall.

In over 60 per cent of angiosperms, pollen grains are shed at this 2-celled stage. In the rest, the generative cell divides by mitosis into two male gametes before shedding, the 3-celled stage.

The ovule

Ovules arise from the placenta inside the ovary. Each is attached to the placenta by a stalk, the funicle. Its central mass of cells, the nucellus, has abundant reserve food and contains the embryo sac.

One or two protective integuments encircle the nucellus except at the tip, where a small opening, the micropyle, is left. The basal part of the ovule, opposite the micropylar end, is the chalaza.

The embryo sac

A megaspore mother cell in the nucellus divides by meiosis to form four megaspores. In most flowering plants one is functional and the other three degenerate. The nucleus of the functional megaspore divides three times by mitosis, giving 8 free nuclei, and cell walls are then laid down, organising the embryo sac, or female gametophyte.

The egg and the two synergids together form the egg apparatus. A typical mature embryo sac is therefore 8-nucleate but 7-celled: six nuclei are in separate cells, and the two polar nuclei share the large central cell.

An oval embryo sac is drawn with the chalazal end at the top and the micropylar end at the bottom. Three antipodals sit at the chalazal end, and two polar nuclei lie in the large central cell.

At the micropylar end are the egg and the two synergids, with the filiform apparatus labelled at the tips of the synergids.

What are self-pollination and cross-pollination?

Self-pollination is the transfer of pollen grains to the stigma of the same flower or of another flower on the same plant.

Cross-pollination is the transfer of pollen grains from the anther of a flower of one plant to the stigma of a flower of another plant of the same type.

Autogamy, geitonogamy and xenogamy

• Autogamy: pollen moves from the anther to the stigma of the same flower. In a flower that opens, complete autogamy is rather rare, since pollen release and stigma receptivity must coincide and the anthers and stigma must lie close together.
• Geitonogamy: pollen moves to the stigma of another flower of the same plant. It is functionally cross-pollination, because it needs a pollinating agent, but genetically it is similar to autogamy, because the pollen comes from the same plant.
• Xenogamy: pollen moves to the stigma of a flower on a different plant. It is the only type that brings genetically different pollen grains to the stigma.

Chasmogamous and cleistogamous flowers

Some plants, such as Viola (common pansy), Oxalis and Commelina, produce two types of flowers. Chasmogamous flowers open and expose their anthers and stigma. Cleistogamous flowers do not open at all, and their anthers and stigma lie close together.

When the anthers dehisce in the closed bud, pollen grains come in contact with the stigma. Cleistogamous flowers are therefore invariably autogamous, as no cross-pollen can land on the stigma, and they produce assured seed-set even without pollinators.

Flower structure often decides the type. In tomato the stamens cover the stigma, and in wheat the flowers open after pollination, so both are self-pollinated (wheat is mainly self-pollinated, although a little of its pollen is also carried by wind). In papaya, male and female flowers are often borne on different trees, so it is cross-pollinated.

Photograph (a) shows self-pollinated flowers and photograph (b) cross-pollinated flowers. Drawing (c) shows a whole plant bearing a chasmogamous flower on its leafy shoot and cleistogamous flowers near its base, close to the roots.

What are the advantages and disadvantages of self-pollination and cross-pollination?

Self-pollination is reliable but produces little variety. Cross-pollination produces variety but depends on outside help.

Advantages of self-pollination

• Assured seed-set: when the anthers and stigma lie close together, seeds form even without pollinators, as in cleistogamous flowers.
• Less pollen wasted: the flower does not need to produce enormous amounts of pollen to make up for pollen lost on the way.
• Parental characters kept: since the pollen comes from the same plant, the offspring remain similar to the parent, so a desired variety stays pure.

Disadvantages of self-pollination

• Inbreeding depression: continued self-pollination results in inbreeding depression, a decline in the vigour of later generations.
• Less variation: no genetically different pollen reaches the stigma, so genetic diversity falls over generations and the plants are less able to adapt to a changing environment.

Advantages of cross-pollination

• Variation: genetically different pollen reaches the stigma, so the offspring carry new combinations of characters. Variation helps plant species survive and adapt to their environment.
• Healthier offspring: it avoids the inbreeding depression caused by continued self-pollination, and the offspring are usually more vigorous.
• Better varieties: breeders cross chosen parents to combine desirable characters, producing high yielding and disease resistant varieties.

Disadvantages of cross-pollination

• Dependence on agents: pollination fails if the agents are missing. In apple orchards of the lower Himalayan region, falling fruit yield has been associated with climate change and a decline in natural pollinators.
• Chance and waste: in wind and water pollination, pollen reaching the stigma is a chance factor, so flowers produce enormous amounts of pollen compared with the number of ovules.
• Cost of rewards: animal-pollinated flowers must provide rewards such as nectar and pollen to sustain animal visits, and some visitors take the reward without pollinating.
• Unwanted pollen: pollen of the wrong type may land on the stigma, and undesirable characters may be brought in along with desirable ones.

How are flowers adapted for pollination by insects and other animals?

Plants use two abiotic agents, wind and water, and one biotic agent, animals. The majority of flowering plants use animals; only a small proportion use abiotic agents. Pollination by insects is called entomophily, and pollination by birds is called ornithophily.

Common animal pollinators are bees, butterflies, flies, beetles, wasps, ants, moths, birds (sunbirds and humming birds) and bats. Among animals, insects, particularly bees, are the dominant pollinators.

Some primates (lemurs), tree-dwelling rodents and reptiles (the gecko lizard and garden lizard) have also been reported as pollinators in some species.

Features of insect-pollinated flowers

• Most are large, colourful, fragrant and rich in nectar. Animals are attracted to flowers by colour and/or fragrance.
• When the flowers are small, many are clustered into an inflorescence to make them conspicuous.
• Flowers pollinated by flies and beetles secrete foul odours to attract these animals.
• Nectar and pollen grains are the usual floral rewards that sustain animal visits.
• Pollen grains are generally sticky, and may be large or spiny, so they attach to the insect's body. The stigma is also sticky to receive them.

While collecting the reward, the visitor comes in contact with the anthers and its body gets a coating of pollen. When it touches a stigma, it brings about pollination. Sunflower, hibiscus and marigold are pollinated by insects such as bees and butterflies.

The flowers of the coral tree and hibiscus are pollinated by birds such as the Indian white-eye and sunbirds.

Special partnerships

In some species the reward is a safe place to lay eggs. One example is Amorphophallus, which has the tallest flower, about 6 feet in height.

The plant Yucca and a species of moth cannot complete their life cycles without each other. The moth lays its eggs in the locule of the ovary and pollinates the flower, and the larvae come out of the eggs as the seeds start developing.

Many insects may consume pollen or nectar without bringing about pollination. Such floral visitors are called pollen or nectar robbers.

How are flowers adapted for pollination by wind?

Pollination by wind, or anemophily, is the more common of the two kinds of abiotic pollination. It is quite common in grasses, and is seen in wheat, maize and rice.

• Pollen grains are light, small, non-sticky and produced in large numbers, so they can be carried in wind currents.
• The stamens are well exposed, so the pollen is easily dispersed into wind currents.
• The stigma is large, long and often feathery, to trap air-borne pollen grains.
• Each ovary often has a single ovule, and numerous flowers are packed into an inflorescence.
• The flowers are not very colourful and do not produce nectar.

In the corn cob, the parts that wave in the wind to trap pollen grains are the stigmas and styles. Because pollen landing on a stigma is a matter of chance, wind-pollinated flowers produce enormous amounts of pollen compared with the number of ovules.

A plant with a rosette of leaves at its base sends up long, leafless stalks, each ending in a compact, spike-like inflorescence. An enlarged view shows numerous long stamens hanging out on all sides of the inflorescence.

Pollen output and seed formation

The last column divides pollen by seeds, for example 5,00,000 ÷ 200 = 2,500 and 40,000 ÷ 800 = 50. Insect pollination is far more efficient per pollen grain, but wind pollination still works because, with so many grains released, enough reach a stigma.

Insect-pollinated and wind-pollinated flowers compared

How are flowers adapted for pollination by water?

Pollination by water, or hydrophily, is quite rare in flowering plants and is limited to about 30 genera, mostly monocotyledons.

Examples of water-pollinated plants are Vallisneria and Hydrilla, which grow in fresh water, and several marine sea-grasses such as Zostera.

Pollination in Vallisneria

1. The female flower reaches the surface of the water on a long stalk.
2. The male flowers or pollen grains are released on to the surface of the water.
3. Water currents carry them passively.
4. Some of them eventually reach the female flowers and the stigma, bringing about pollination.

On the left, a whole plant with long, ribbon-like leaves; a female flower is carried up on a long, coiled stalk.

On the right, an enlarged female flower sits at the water surface on its stalk, with its stigma labelled and dashed rings around it. Small male flowers float nearby on the surface, and one of them is shown enlarged and labelled as a male flower.

In sea-grasses, the female flowers remain submerged and the pollen grains are released inside the water. The pollen grains of many such species are long and ribbon-like, and are carried passively in the water until some reach a stigma.

In most water-pollinated species, a mucilaginous covering protects the pollen grains from wetting. Like wind-pollinated flowers, water-pollinated flowers are not very colourful and do not produce nectar.

Not all aquatic plants use water for pollination. In most, such as water hyacinth and water lily, the flowers emerge above the water and are pollinated by insects or wind, as in most land plants.

How does nature favour cross-pollination?

Most flowering plants produce hermaphrodite (bisexual) flowers, so pollen grains are likely to reach the stigma of the same flower. Continued self-pollination results in inbreeding depression.

Flowering plants have therefore developed many outbreeding devices that discourage self-pollination and encourage cross-pollination.

Pollen release and stigma receptivity out of step

In some species, pollen is released before the stigma becomes receptive, or the stigma becomes receptive much before the pollen is released. This is called dichogamy. When the anthers mature first it is protandry, and when the stigma matures first it is protogyny.

Anther and stigma at different positions

In other species, the anther and stigma are placed at different positions, so that the pollen cannot come in contact with the stigma of the same flower. This is called herkogamy. Both this device and the previous one prevent autogamy.

Self-incompatibility

Self-incompatibility is a genetic mechanism. It prevents pollen from the same flower, or from other flowers of the same plant, from fertilising the ovules by inhibiting pollen germination or pollen tube growth in the pistil.

Unisexual flowers

If male and female flowers are present on the same plant, as in castor and maize, the plant is monoecious. This prevents autogamy but not geitonogamy.

In several species, such as papaya, male and female flowers are on different plants, so each plant is either male or female. This condition, dioecy, prevents both autogamy and geitonogamy.

What happens between pollination and fertilisation?

Pollination does not guarantee that the right type of pollen arrives. Pollen from other species, or from the same plant in a self-incompatible species, may also land on the stigma. The pistil can recognise whether the pollen is compatible (the right type) or incompatible (the wrong type).

If the pollen is compatible, the pistil accepts it and promotes the events that lead to fertilisation. If it is incompatible, the pistil rejects it by preventing pollen germination on the stigma or pollen tube growth in the style. This recognition is a continuous dialogue between pollen and pistil, mediated by their chemical components.

1. A compatible pollen grain lands on the stigma and is accepted.
2. The pollen grain germinates, producing a pollen tube through one of its germ pores. The contents of the pollen grain move into the tube.
3. The pollen tube grows through the tissues of the stigma and style and reaches the ovary.
4. If the pollen was shed at the 2-celled stage, the generative cell divides to form the two male gametes while the tube grows in the stigma. Pollen shed at the 3-celled stage carries the two male gametes from the beginning.
5. The pollen tube enters an ovule through the micropyle.
6. It then enters one of the synergids through the filiform apparatus, which guides its entry.
7. The pollen tube releases the two male gametes into the cytoplasm of the synergid.

All the events from pollen deposition on the stigma until the pollen tube enters the ovule are together called pollen-pistil interaction. Knowledge of it can help plant breeders obtain desired hybrids, even from incompatible pollinations.

Photograph (a) shows pollen grains germinating on the stigma, and photograph (b) shows pollen tubes growing through the style.

Drawing (c), an L.S. of a pistil, shows one pollen tube running down the style into the ovary and curving round to enter the ovule at the micropyle, where the antipodal, polar nuclei, egg cell and synergid are labelled.

Drawing (d) shows the pollen tube tip, with two male gametes and the vegetative nucleus, reaching the filiform apparatus of a synergid. Drawing (e) shows the male gametes discharged into a synergid, one moving into the egg and the other into the central cell.

Observing pollen germination

Dust some pollen from flowers such as pea, chickpea, Crotalaria, balsam or Vinca on a drop of sugar solution (about 10 per cent) on a glass slide. After about 15 to 30 minutes, pollen tubes can be seen coming out of the pollen grains under the low power lens of a microscope.

What are double fertilisation and triple fusion?

After entering one of the synergids, the pollen tube releases its two male gametes into the synergid's cytoplasm. Each male gamete then fuses with a different part of the embryo sac.

1. One male gamete moves towards the egg cell and fuses with its nucleus. This fusion is syngamy, and it forms a diploid cell, the zygote.
2. The other male gamete moves towards the two polar nuclei in the central cell and fuses with them, forming a triploid primary endosperm nucleus (PEN).
3. Because this second fusion involves three haploid nuclei, it is called triple fusion.
4. After triple fusion, the central cell becomes the primary endosperm cell (PEC), which develops into the endosperm.
5. The zygote develops into the embryo.

Definition: Double fertilisation is the occurrence of two types of fusion, syngamy and triple fusion, in the same embryo sac. It is an event unique to flowering plants.

An elongated embryo sac with the degenerating synergids and the zygote (2n) at one end. The large primary endosperm cell (PEC) fills the middle and holds the primary endosperm nucleus (3n) (PEN), and the degenerating antipodal cells lie at the other end.

Note: Pollination and fertilisation are different events. Pollination only brings pollen to the stigma; fertilisation is the fusion of gametes inside the embryo sac. A flower can be pollinated without being fertilised, for example when the pistil rejects incompatible pollen.

What changes take place in a flower after fertilisation?

Following double fertilisation, the development of the endosperm and embryo, and the maturation of the ovules into seeds and of the ovary into a fruit, are together called post-fertilisation events.

Endosperm

Endosperm development comes before embryo development. The primary endosperm cell divides repeatedly to form a triploid endosperm tissue, whose cells are filled with reserve food for the nutrition of the developing embryo.

In the most common type, the PEN first divides into many free nuclei (free-nuclear endosperm), and cell walls form later. The water of a tender coconut is free-nuclear endosperm, made of thousands of nuclei, and the white kernel around it is cellular endosperm.

The endosperm may be completely used up by the embryo before the seed matures, as in pea, groundnut and beans, or it may persist in the mature seed, as in castor and coconut.

Embryo

The embryo develops at the micropylar end of the embryo sac, where the zygote lies. Most zygotes divide only after some endosperm has formed, which assures nutrition for the developing embryo. The zygote gives rise to the proembryo, and then to the globular, heart-shaped and mature embryo.

The pore left by the micropyle allows oxygen and water to enter the seed during germination. The change of ovules into seeds and of the ovary into a fruit proceeds at the same time.

What are fruits and seeds, and why are they important?

Definition: A seed is the final product of sexual reproduction in angiosperms and is often described as a fertilised ovule. A fruit is the ovary that grows and ripens, usually after fertilisation, with the seeds formed inside it.

A seed typically consists of seed coat(s), cotyledon(s) and an embryo axis. A mature seed may enter a state of inactivity called dormancy, or germinate if moisture, oxygen and a suitable temperature are available.

The wall of the ovary develops into the wall of the fruit, the pericarp. Fruits may be fleshy, as in guava, orange and mango, or dry, as in groundnut and mustard.

True, false and parthenocarpic fruits

Most fruits develop only from the ovary and are called true fruits. In a few species, such as apple, strawberry and cashew, the thalamus also contributes to fruit formation. Such fruits are called false fruits.

In a few species, fruits develop without fertilisation. These are parthenocarpic fruits, and banana is one example. Parthenocarpy can be induced by applying growth hormones, and such fruits are seedless.

An apple is drawn cut lengthwise and crosswise, with the fleshy thalamus labelled around the core and the seeds, endocarp and mesocarp labelled in the centre. Beside it, a cut strawberry shows the thalamus, with small achenes on its surface.

Significance of seeds

• Dependable reproduction: pollination and fertilisation are independent of water, so seed formation is more dependable.
• Dispersal: seeds have better strategies for dispersal to new habitats and help the species colonise other areas.
• Food store: their food reserves nourish young seedlings until they can photosynthesise on their own.
• Protection: the hard seed coat protects the young embryo.
• Variation: as products of sexual reproduction, seeds generate new genetic combinations.
• Agriculture: dry, dormant seeds can be stored as food through the year and used to raise the next crop.

Significance of fruits

• The fruit wall encloses and protects the developing seeds.
• Many fruits have evolved mechanisms for the dispersal of seeds, which takes place by wind, water and animals.
• Bright yellow, orange or red pigments in fruits attract fruit-eating animals that help in seed dispersal.

How do plant breeders control pollination?

A plant breeder is interested in crossing different species, and often genera, to combine desirable characters and produce commercially superior varieties. This artificial hybridisation is one of the major approaches of crop improvement.

In such crosses, only the desired pollen grains must be used, and the stigma must be protected from unwanted pollen. This is achieved by emasculation and bagging.

1. If the female parent bears bisexual flowers, remove the anthers from the flower bud with a pair of forceps before the anthers dehisce. This step is emasculation.
2. Cover the emasculated flower with a bag of suitable size, generally made of butter paper, to prevent contamination of its stigma with unwanted pollen. This is bagging.
3. When the stigma becomes receptive, dust it with mature pollen grains collected from the anthers of the male parent.
4. Rebag the flower and allow the fruit to develop.

If the female parent bears unisexual flowers, emasculation is not needed. The female flower buds are bagged before the flowers open, pollinated with the desired pollen when the stigma becomes receptive, and rebagged.

Pollen grains of a large number of species can be stored for years in liquid nitrogen at −196 °C. Such stored pollen can be used as pollen banks, similar to seed banks, in crop breeding programmes.

Glossary

• Pollination — the transfer of pollen grains from the anther to the stigma of a flower, brought about by agents such as wind, water, insects and birds.
• Geitonogamy — transfer of pollen grains from the anther of one flower to the stigma of another flower on the same plant.
• Xenogamy — transfer of pollen grains to the stigma of a flower on a different plant; the only type that brings genetically different pollen.
• Cleistogamous flower — a flower that does not open at all, so its anthers and stigma lie close together and it is invariably self-pollinated.
• Self-incompatibility — a genetic mechanism that stops pollen from the same flower or plant fertilising the ovules, by inhibiting pollen germination or pollen tube growth.
• Filiform apparatus — special cellular thickenings at the micropylar tip of the synergids that guide the pollen tube into a synergid.
• Embryo sac — the female gametophyte inside the ovule; at maturity it is 8-nucleate and 7-celled.
• Syngamy — the fusion of one male gamete with the nucleus of the egg cell, forming the diploid zygote.
• Triple fusion — the fusion of the second male gamete with the two polar nuclei, forming the triploid primary endosperm nucleus.
• Double fertilisation — the occurrence of both syngamy and triple fusion in the same embryo sac, an event unique to flowering plants.
• Emasculation — the removal of anthers from a bisexual flower bud with forceps, before they dehisce, so that the flower cannot pollinate itself.
• Parthenocarpic fruit — a fruit that develops without fertilisation, such as banana; such fruits are seedless.

Common errors and misconceptions

• Misconception: Pollination and fertilisation are the same process. Correct: Pollination is the transfer of pollen to the stigma. Fertilisation comes later, when a male gamete carried by the pollen tube fuses with the egg inside the embryo sac.
• Misconception: Pollen carried to another flower on the same plant is cross-pollination. Correct: This is geitonogamy. It needs an agent, but genetically it is self-pollination, because the pollen comes from the same plant.
• Misconception: Double fertilisation means that two eggs are fertilised. Correct: One male gamete fuses with the egg (syngamy) and the other fuses with the two polar nuclei (triple fusion), in the same embryo sac.
• Misconception: The endosperm is diploid, like the zygote. Correct: The endosperm is triploid, because the primary endosperm nucleus forms from the fusion of three haploid nuclei.
• Misconception: All aquatic plants are pollinated by water. Correct: Water pollination is limited to about 30 genera. In water hyacinth and water lily, the flowers emerge above the water and are pollinated by insects or wind.
• Misconception: Having separate male and female flowers on the same plant prevents all self-pollination. Correct: Monoecy, as in castor and maize, prevents autogamy but not geitonogamy. Dioecy, as in papaya, prevents both.
• Misconception: Every fruit develops from the ovary after fertilisation. Correct: In apple, strawberry and cashew the thalamus also forms part of the fruit, and parthenocarpic fruits such as banana develop without fertilisation.

Exam-style questions with model answers

Q1. Define pollination and name its two types. [2 marks]

1. Pollination is the transfer of pollen grains from the anther to the stigma of a flower.
2. Its two types are self-pollination, to the stigma of the same flower or of another flower on the same plant, and cross-pollination, to the stigma of a flower on another plant of the same type.

Q2. State three features of wind-pollinated flowers and explain how each helps pollination. [3 marks]

1. Light, non-sticky pollen produced in enormous amounts: it is easily carried by wind currents, and the large number makes up for the pollen lost by chance.
2. Well-exposed stamens: the pollen is easily dispersed into the wind currents.
3. A large, often feathery stigma: it easily traps the air-borne pollen grains as they drift past.

Q3. Give two advantages and two disadvantages of self-pollination. [4 marks]

1. Advantage: seed-set is assured even when pollinators are absent, as in cleistogamous flowers.
2. Advantage: less pollen is wasted, and the characters of the parent plant are maintained in the offspring.
3. Disadvantage: continued self-pollination results in inbreeding depression, so later generations become less vigorous.
4. Disadvantage: no genetically different pollen reaches the stigma, so little new variation arises and the plants are less able to adapt to a changing environment.

Q4. Describe the events that take place from the landing of a compatible pollen grain on the stigma to the formation of the zygote. [5 marks]

1. The pistil recognises the pollen as compatible and accepts it.
2. The pollen grain germinates on the stigma and a pollen tube grows out through one of the germ pores; the contents of the pollen grain move into the tube.
3. The pollen tube grows through the tissues of the stigma and style and reaches the ovary. In pollen shed at the 2-celled stage, the generative cell divides to form two male gametes during this growth; 3-celled pollen carries them from the start.
4. The pollen tube enters the ovule through the micropyle.
5. Guided by the filiform apparatus, it enters one of the synergids and releases the two male gametes into its cytoplasm.
6. One male gamete fuses with the nucleus of the egg cell (syngamy), forming the diploid zygote.
7. The other fuses with the two polar nuclei (triple fusion), forming the triploid primary endosperm nucleus. The two fusions together are called double fertilisation.

Q5. Describe four devices by which flowering plants discourage self-pollination and encourage cross-pollination. Which of them prevents both autogamy and geitonogamy? [5 marks]

1. Pollen release and stigma receptivity are not synchronised: pollen is released before the stigma becomes receptive, or the stigma becomes receptive much before pollen release. This prevents autogamy.
2. The anther and stigma are placed at different positions, so pollen cannot reach the stigma of the same flower. This also prevents autogamy.
3. Self-incompatibility: a genetic mechanism that stops pollen from the same flower or other flowers of the same plant from fertilising the ovules, by inhibiting pollen germination or pollen tube growth in the pistil.
4. Unisexual flowers: in monoecious plants such as castor and maize, male and female flowers on the same plant prevent autogamy but not geitonogamy.
5. Dioecy, as in papaya, where male and female flowers are on different plants, prevents both autogamy and geitonogamy, because no pollen from the same plant can reach the stigma. Self-incompatibility also stops fertilisation by the plant's own pollen, but only after that pollen has landed on the stigma.

Q6. What are emasculation and bagging? Why are they used in artificial hybridisation, and when is emasculation not needed? [4 marks]

1. Emasculation is the removal of the anthers from a bisexual flower bud, with a pair of forceps, before the anthers dehisce.
2. Bagging is covering the emasculated flower with a bag, generally of butter paper, to prevent contamination of its stigma with unwanted pollen.
3. They ensure that only the desired pollen from the male parent is used for pollination. When the stigma becomes receptive, this pollen is dusted on it and the flower is rebagged.
4. Emasculation is not needed when the female parent bears unisexual flowers; the female buds are simply bagged before they open.

Q7. Define a fruit and a seed. State four ways in which seeds are important to flowering plants. [4 marks]

1. A fruit is the ovary that grows and ripens, usually after fertilisation; its wall becomes the pericarp. A seed is a fertilised ovule, the final product of sexual reproduction.
2. Seed formation is dependable, because pollination and fertilisation are independent of water.
3. Seeds can be dispersed to new habitats, helping the species colonise other areas.
4. Their food reserves nourish the young seedling until it can photosynthesise, and the hard seed coat protects the embryo.
5. They generate new genetic combinations, leading to variation.

Key takeaways

• Pollination is the transfer of pollen grains from the anther to the stigma; it is needed because the male and female gametes are non-motile.
• Self-pollination includes autogamy and geitonogamy; cross-pollination, or xenogamy, is the only type that brings genetically different pollen to the stigma.
• Self-pollination assures seed-set without pollinators, but continued self-pollination causes inbreeding depression and reduces variation.
• Insect-pollinated flowers are usually large, colourful, fragrant and rich in nectar, with sticky pollen; wind-pollinated flowers have light pollen, exposed stamens and feathery stigmas.
• Nature favours cross-pollination through out-of-step pollen release and stigma receptivity, separated anthers and stigmas, self-incompatibility and unisexual flowers.
• The pollen tube grows through the stigma and style, enters the ovule through the micropyle and releases two male gametes into a synergid.
• Double fertilisation combines syngamy, which forms the diploid zygote, and triple fusion, which forms the triploid primary endosperm nucleus.
• After fertilisation, ovules become seeds and the ovary becomes the fruit; seeds protect, nourish and spread the next generation.

Test yourself

What is geitonogamy?

Geitonogamy is the transfer of pollen grains from the anther of one flower to the stigma of another flower on the same plant.

Name three plants that are pollinated by water.

Vallisneria and Hydrilla, which grow in fresh water, and marine sea-grasses such as Zostera.

What is the function of the filiform apparatus?

The filiform apparatus, at the micropylar tip of the synergids, guides the pollen tube into one of the synergids.

How many nuclei and how many cells does a typical mature embryo sac have?

It has 8 nuclei but only 7 cells: three antipodals, two synergids, one egg cell and one large central cell holding two polar nuclei.

What is the ploidy of the zygote and of the primary endosperm nucleus?

The zygote is diploid (2n), and the primary endosperm nucleus is triploid (3n), because it forms from three haploid nuclei.

Why is the apple called a false fruit?

In apple the thalamus, and not only the ovary, contributes to the formation of the fruit, so it is a false fruit.

What are pollen or nectar robbers?

They are floral visitors, such as many insects, that consume pollen or nectar from a flower without bringing about its pollination.