Fertilisation

Fertilisation in flowering plants is double: two fusions take place inside the embryo sac. This page first covers the pre-fertilisation events that bring pollen there: autogamy, geitonogamy and xenogamy, cleistogamous flowers, pollination by wind, water and animals, outbreeding devices, pollen-pistil interaction and artificial hybridisation. It then explains syngamy and triple fusion. It follows the NCERT Class 12 chapter Sexual Reproduction in Flowering Plants. NEET often asks fertilisation and pollination as match-the-list and statement questions.

1. Pollination

• Male and female gametes are produced in the pollen grain and the embryo sac, respectively.
• Both kinds of gametes are non-motile, so they must be brought together for fertilisation.
• Flowering plants have evolved an amazing array of adaptations for this, and they use external agents.

Pollination: the transfer of pollen grains, shed from the anther, to the stigma of a pistil.

2. Kinds of Pollination

Depending on the source of pollen, pollination is of three types.

2.1 Autogamy

• Autogamy: transfer of pollen from the anther to the stigma of the same flower.
• In a normal flower that opens and exposes its anthers and stigma, complete autogamy is rather rare.
• It needs synchrony between pollen release and stigma receptivity.
• The anthers and the stigma must also lie close to each other.

2.2 Chasmogamous and cleistogamous flowers

• ★ Exam imp Viola (common pansy), Oxalis and Commelina produce two types of flowers.
• Chasmogamous flowers are like the flowers of other species, with exposed anthers and stigma.
• Cleistogamous flowers do not open at all; their anthers and stigma lie close together.
• Their anthers dehisce in the flower bud, and the pollen touches the stigma, so pollination takes place.
• Cleistogamous flowers are therefore invariably autogamous; no cross-pollen can land on the stigma.
• They give assured seed-set even when pollinators are absent.

2.3 Geitonogamy and xenogamy

• Geitonogamy: transfer of pollen from the anther to the stigma of another flower of the same plant.
• ★ Exam imp Geitonogamy is functionally cross-pollination, because it needs a pollinating agent. Genetically, it is similar to autogamy, because the pollen comes from the same plant.
• Xenogamy: transfer of pollen from the anther to the stigma of a different plant.
• Xenogamy is the only type of pollination that brings genetically different pollen grains to the stigma.

3. Agents of Pollination

• Plants use two abiotic agents (wind and water) and one biotic agent (animals).
• The majority of plants use biotic agents; only a small proportion use abiotic agents.
• In wind and water pollination, pollen reaching the stigma is a chance factor.
• To make up for this unpredictability and the loss of pollen, such flowers produce enormous amounts of pollen compared with the number of ovules.

3.1 Pollination by wind

• ★ Exam imp Wind pollination is the more common of the two abiotic types.
• Pollen grains are light and non-sticky, so wind currents can carry them.
• Stamens are well exposed, so pollen is easily dispersed into the wind.
• The stigma is large and often feathery, to trap air-borne pollen.
• Flowers often have a single ovule in each ovary, and numerous flowers are packed into an inflorescence.
• In the corn cob, the 'ears' seen waving are the stigma and style, which wave in the wind to trap pollen.
• Wind pollination is quite common in grasses.

3.2 Pollination by water

• Water pollination is quite rare in flowering plants, limited to about 30 genera, mostly monocotyledons.
• In contrast, water is a regular mode of transport for male gametes in algae, bryophytes and pteridophytes.
• The distribution of some bryophytes and pteridophytes is believed to be limited by their need for water for gamete transport and fertilisation.
• ★ Exam imp Examples: Vallisneria and Hydrilla in fresh water, and marine sea-grasses such as Zostera.
• Not all aquatic plants use water for pollination. In a majority of aquatic plants, such as water hyacinth and water lily, the flowers rise above the water and are pollinated by insects or wind, as in most land plants.

• In most water-pollinated species, pollen is protected from wetting by a mucilaginous covering.
• Both wind- and water-pollinated flowers are not very colourful and do not produce nectar, because they need not attract animals.

3.3 Pollination by animals

• The majority of flowering plants use animals: bees, butterflies, flies, beetles, wasps, ants, moths, birds (sunbirds and humming birds) and bats.
• ★ Exam imp Among animals, insects, particularly bees, are the dominant biotic pollinating agents.
• Larger animals reported as pollinators in some species: some primates (lemurs), arboreal (tree-dwelling) rodents, and reptiles (gecko lizard and garden lizard).
• Flowers of animal-pollinated plants are often adapted to a particular species of animal.
• Most insect-pollinated flowers are large, colourful, fragrant and rich in nectar.
• Small flowers are clustered into an inflorescence to make them conspicuous.
• Animals are attracted by colour and/or fragrance. Flowers pollinated by flies and beetles secrete foul odours.

• To sustain visits, flowers offer rewards; nectar and pollen grains are the usual floral rewards.
• While collecting the reward, the visitor touches the anthers and the stigma. Its body gets a coating of pollen, which is generally sticky in animal-pollinated flowers.
• When it later touches a stigma, the pollen on its body brings about pollination.
• In some species the reward is a safe place to lay eggs, as in Amorphophallus, the tallest flower (about 6 feet high).
• ★ Exam imp 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, the flower is pollinated by the moth, and the larvae hatch as the seeds develop.
• Only visitors that touch both the anthers and the stigma can pollinate. Insects that eat pollen or nectar without pollinating are pollen or nectar robbers.
• Flowers of cucumber, mango, peepal, coriander, papaya, onion, lobia, cotton, tobacco, rose, lemon, eucalyptus and banana are good for watching visitors, at different times over a few days, to see which ones actually touch the anthers and stigma.

4. Outbreeding Devices

• Most flowering plants have hermaphrodite (bisexual) flowers, so pollen is likely to reach the stigma of the same flower.
• ★ Exam imp Continued self-pollination results in inbreeding depression.
• Flowering plants have developed devices that discourage self-pollination and encourage cross-pollination.

5. Pollen-pistil Interaction

• Pollination does not guarantee the right type of pollen: compatible pollen of the same species as the stigma.
• Pollen of the wrong type also lands on the stigma: pollen of other species, or of the same plant if it is self-incompatible.
• The pistil can recognise whether pollen is compatible or incompatible.
• Compatible pollen is accepted, and the pistil promotes post-pollination events that lead to fertilisation.
• Incompatible pollen is rejected: the pistil prevents pollen germination on the stigma or pollen tube growth in the style.
• ★ Exam imp Recognition, acceptance or rejection results from a continuous dialogue between pollen and pistil, mediated by the chemical components of the pollen interacting with those of the pistil.
• Only in recent years have botanists identified some of these pollen and pistil components and their interactions.

5.1 From pollen grain to synergid

1. After compatible pollination, the pollen grain germinates on the stigma and forms a pollen tube through one of the germ pores.
2. The contents of the pollen grain move into the pollen tube.
3. The pollen tube grows through the tissues of the stigma and style and reaches the ovary.
4. In pollen 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 3-celled carries the two male gametes from the beginning.
5. In the ovary, the tube enters the ovule through the micropyle.
6. It then enters one of the synergids through the filiform apparatus, which guides its entry.

• Knowledge of this interaction helps plant breeders manipulate it, even in incompatible pollinations, to obtain desired hybrids.
• Pollen germination can be studied by dusting pollen of pea, chickpea, Crotalaria, balsam or Vinca on a slide with a drop of sugar solution (about 10 per cent). Pollen tubes appear under low power after about 15-30 minutes.

6. Artificial Hybridisation

• A breeder crosses different species, and often genera, to combine desirable characters and produce commercially 'superior' varieties.
• ★ Exam imp Artificial hybridisation is one of the major approaches of crop improvement.
• Only the desired pollen must be used, and the stigma must be protected from unwanted pollen. Emasculation and bagging achieve this.

6.1 Steps when the female parent has bisexual flowers

1. Emasculate: remove the anthers from the flower bud before they dehisce.
2. Bag the emasculated flower with a bag of suitable size, generally of butter paper.
3. When the stigma becomes receptive, dust it with mature pollen collected from the anthers of the male parent.
4. Rebag the flower and allow the fruit to develop.

6.2 When the female parent has unisexual flowers

• No emasculation is needed.
• The female flower buds are bagged before the flowers open.
• When the stigma becomes receptive, it is pollinated with the desired pollen, and the flower is rebagged.

7. Double Fertilisation

1. After entering a synergid, the pollen tube releases the two male gametes into the cytoplasm of the synergid.
2. One male gamete moves to the egg cell and fuses with its nucleus: syngamy. This forms a diploid zygote.
3. The other male gamete moves to the two polar nuclei in the central cell and fuses with them, forming the triploid primary endosperm nucleus (PEN).
4. This second fusion joins three haploid nuclei, so it is called triple fusion.
5. After triple fusion, the central cell becomes the primary endosperm cell (PEC), which develops into the endosperm. The zygote develops into the embryo.

8. Exam Essentials

Pairs to Match

Numbers to Remember

• Kinds of pollination: 3; agents: 2 abiotic + 1 biotic.
• Water pollination: about 30 genera, mostly monocotyledons.
• Amorphophallus flower: about 6 feet high.
• Pollen germination test: about 10 per cent sugar solution; tubes seen after 15-30 minutes.
• Male gametes in a pollen tube: 2; fusions in double fertilisation: 2.
• Triple fusion: 3 haploid nuclei; zygote 2n; primary endosperm nucleus 3n.

Examples to Remember

9. Quick Revision

• Pollination is the transfer of pollen from the anther to the stigma; gametes are non-motile.
• Autogamy (same flower), geitonogamy (same plant) and xenogamy (different plant).
• Cleistogamous flowers of Viola, Oxalis and Commelina never open and are always autogamous.
• Geitonogamy is functionally cross-pollination but genetically like autogamy.
• Agents: wind and water (abiotic), animals (biotic); most plants use animals.
• Wind flowers: light, non-sticky pollen, well-exposed stamens, large often-feathery stigma, often a single ovule, packed flowers.
• Water pollination: about 30 genera; Vallisneria, Hydrilla, Zostera; mucilage protects the pollen.
• Insect flowers: large, colourful, fragrant, nectar-rich; bees dominate; rewards are nectar and pollen.
• Amorphophallus and Yucca offer egg-laying sites; robbers take rewards without pollinating.
• Outbreeding devices: non-synchrony, positions, self-incompatibility, unisexual flowers.
• Pollen-pistil interaction: recognition by chemical dialogue, then the tube grows to the ovule.
• The tube enters through the micropyle and a synergid via the filiform apparatus.
• Artificial hybridisation: emasculation, bagging, dusting, rebagging.
• Syngamy forms the 2n zygote; triple fusion forms the 3n primary endosperm nucleus.
• Double fertilisation is unique to flowering plants.

10. Solved Examples

Common Mistakes to Avoid