Pollination And Fertilization Chapter Notes | Biology Class 9 ICSE PDF Download

Pollination

Pollination is the process by which pollen grains are transferred from the anther of a flower’s stamen (the male reproductive organ) to the stigma of a flower’s carpel (the female reproductive organ). Pollen grains are powdery particles produced by the anthers, each containing nuclei essential for reproduction. This transfer is the first step in the formation of fruits and seeds and can occur in three main ways:

1. Autogamy (Self-Pollination): Pollen from a flower’s anther lands on its own stigma (auto = self, gamy = marriage).
2. Geitonogamy: Pollen from one flower on a plant is transferred to the stigma of another flower on the same plant (geitono = neighboring).
3. Allogamy (Cross-Pollination): Pollen from a flower on one plant is transferred to the stigma of a flower on another plant of the same species (alio = other). This transfer can be facilitated by agents such as wind, insects, or other animals.

Types of Pollination:There are two primary types of pollination:

• Self-Pollination: Occurs when pollen transfers within the same flower or between flowers of the same plant (autogamy or geitonogamy).
• Cross-Pollination: Involves pollen transfer between flowers of different plants of the same species (allogamy).

Pollination must occur between plants of the same species for successful reproduction. In environments like orchards or fields, various plants—such as mango, guava, litchi, neem, wheat, mustard, or pulses—may grow together. Wind and insects like bees or butterflies can carry pollen from multiple plant species. However, only pollen from the same species will successfully germinate on a flower’s stigma, while pollen from other species fails to develop, ensuring species-specific reproduction.

Self-Pollination

Self-pollination occurs when pollen is transferred from the anther to the stigma of the same flower (autogamy) or to the stigma of another flower on the same plant (geitonogamy). This is considered self-pollination because flowers on the same plant share identical genetic characteristics. In rare cases, such as in pansies, some flowers near the ground remain closed (cleistogamy, from "cleisto" meaning closed). In these flowers, the anthers and stigma are positioned close together, mature simultaneously, and ensure self-pollination.

When Can Self-Pollination Occur?
Self-pollination can take place in bisexual flowers, which possess both male and female reproductive organs, or in unisexual flowers where male and female flowers are present on the same plant. For self-pollination to occur, the anther and stigma of a flower (or male and female flowers on the same plant) must mature at the same time to facilitate pollen transfer.

Advantages of self-pollination:

• More reliable in bisexual flowers with simultaneous maturation of stamens and carpels.
• Maintains parental traits consistently.
• Minimal pollen wastage as small quantities suffice.
• Flowers need not be large, showy, or produce scent/nectar, saving energy.

Disadvantages of self-pollination:

• Continuous self-pollination weakens the species over generations.
• Produces less vigorous offspring with poor-quality seeds.
• Cannot eliminate defective traits.
• Limits new variety formation, reducing adaptability.

Cross-Pollination

Cross-pollination involves transferring pollen from an anther of one plant to the stigma of another plant of the same species.

Advantages of cross-pollination:

• Produces healthier offspring.
• Yields abundant and viable seeds.
• Allows creation of new varieties by crossing different varieties or species.

Disadvantages of cross-pollination:

• Uncertain, as it depends on external agents (wind, insects, etc.) which may not be available.
• Requires large pollen production, leading to wastage.
• Energy-intensive due to the need for large, colorful, scented flowers with nectar.

Comparison of self-pollination and cross-pollination:

• Self-pollination: Transfers pollen within the same flower or plant; no external agent needed; anthers and stigma mature simultaneously; can occur in closed flowers; preserves parental traits; no new varieties; offspring less adaptable.
• Cross-pollination: Transfers pollen between different plants; requires external agents; anthers and stigma mature at different times; occurs in open flowers; does not preserve parental traits; produces healthier, adaptable offspring and new varieties.

Nature favors cross-pollination through:

• Unisexuality: Separate male and female flowers or plants (e.g., palms, papaya).
• Dichogamy: Different maturation times for anthers and stigma.
• Protandry: Anthers mature first (e.g., bhindi, sweet pea).
• Protogyny: Stigma matures first (e.g., custard apple, peepal).
• Self-sterility: Pollen from the same flower fails to grow (e.g., sunflower ray florets, orchids).
• Herkogamy: Structural barriers prevent self-pollination (e.g., hood over stigma in pansy, iris).
• Heterostyly: Stigma and anthers at different heights (e.g., primrose, oxalis).

Agents of Cross-Pollination

Common agents: Insects, wind, water, birds, and animals (e.g., squirrels, bats, elephants).
Insect-pollinated (entomophilous) flowers:

• Large and brightly colored to attract insects.
• Emit scent and produce nectar as food for insects.
• Sticky or spiny pollen grains to stick to insects.
• Sticky stigma, often inside the flower.
• Clustered flowers for visibility (e.g., dahlia).

Wind-pollinated (anemophilous) flowers:

• Small, dull-colored, without scent or nectar.
• Long stamens that hang out for wind exposure.
• Large, loosely attached anthers for easy pollen release.
• Abundant, light, dry, and smooth pollen grains.
• Feathery stigmas that hang out to trap pollen (e.g., maize, grass).

Water-pollinated (hydrophilous) flowers:

• Found in aquatic plants.
• Produce large quantities of pollen.
• Pollen grains may float below water surface or male flowers float to meet female flowers (e.g., Vallisneria).
• Bird-pollinated (ornithophilous) flowers: Pollinated by birds (e.g., Bignonia, canna).
• Elephant-pollinated (elephophilous) flowers: Large flowers at ground level, pollinated by elephants’ feet (e.g., Rafflesia).

Artificial pollination:

• Man transfers pollen to stigma, historically practiced in Babylonia with palms.
• Modern practice involves emasculation (removing anthers) and covering flowers to control pollination for breeding new varieties.

Comparison of wind-pollinated and insect-pollinated flowers:

• Wind-pollinated: Unisexual, dull, no scent/nectar; large pollen production; small, light, smooth pollen; long, feathery stigma; protruding stamens (e.g., maize, grass).
• Insect-pollinated: Brightly colored, scented, nectar-producing; less pollen; larger, sticky, spiny pollen; small stigma inside corolla; stamens within corolla (e.g., China rose, salvia).

Some Examples of Pollination

Sweet pea (insect-pollinated):

• Bees land on the corolla, seeking nectar.
• Wing and keel petals are depressed, allowing stamens and stigma to touch the bee’s body.
• Pollen from another flower on the bee transfers to the stigma, enabling cross-pollination.
• Self-pollination possible if cross-pollination fails, as stamens and carpels are enclosed together.

Maize (wind-pollinated):

• Wind shakes loosely attached anthers in the tassel, releasing pollen.
• Pollen falls on feathery stigmas of female flowers.
• Male flowers mature earlier than female flowers, reducing self-pollination chances.

Fertilization

Fertilization is the fusion of male and female gamete nuclei.

In flowering plants:

• Pollen grain is the male gamete.
• Ovule inside the ovary is the female gamete.

Pollen grain structure:

• Has a double wall: outer exine and inner intine.
• Contains a tube nucleus and a generative (male) nucleus before pollination.
• Germinates only on the stigma of the same species; otherwise, it disintegrates.

Ovule structure:

• Located inside the ovary, destined to become a seed.
• Has one or two integuments (protective coverings) with a micropyle opening for pollen tube entry.
• Contains nucellus (food-laden cells) and an embryo sac.

Embryo sac includes:

• Three cells at micropylar end: one egg cell, two synergids.
• Three antipodal cells at the opposite end.
• One large central cell with two polar nuclei.

Germination of pollen grain:

• Pollen germinates on the stigma due to sugar secretions.
• A pollen tube grows from the pollen grain, carrying the tube nucleus and generative nucleus.
• Generative nucleus divides into two sperm nuclei.
• Pollen tube grows through stigma and style, dissolving tissues with enzymes, and enters the ovary via the micropyle.
• Tube nucleus disintegrates after guiding the pollen tube.

Double fertilization:

• Pollen tube enters a synergid and releases two sperm nuclei.
• One sperm nucleus fuses with the egg cell nucleus to form the zygote (embryo).
• The other sperm nucleus fuses with two polar nuclei to form the endosperm nucleus (triple fusion).
• Two fertilizations occur, hence called double fertilization.

Fate of floral parts after fertilization:

• Petals, stamens, style, and stigma wither and usually fall off.
• Calyx may fall off or remain dried (e.g., apple, guava, brinjal).
• Ovary enlarges to form fruit; ovary wall becomes the pericarp (fruit wall, dry or fleshy).
• Ovules become seeds.
• Placenta becomes the seed stalk.
• Outer integument becomes testa; inner integument becomes tegmen.
• Secondary (polar) nucleus forms endosperm.
• Egg cell forms the embryo.
• Synergids and antipodal cells disintegrate.

Fruit contains seeds, which are released when ripe and grow into new plants under suitable conditions.

Points to Remember

• Self-pollination ensures parental traits but leads to weaker generations.
• Cross-pollination produces healthier offspring and new varieties.
• Plants have mechanisms like unisexuality, dichogamy, self-sterility, herkogamy, and heterostyly to favor cross-pollination.
• Insect-pollinated flowers are large, colorful, scented, nectar-producing, with sticky pollen and stigma.
• Wind-pollinated flowers are small, dull, nectarless, with light pollen and feathery stigma.
• Pollen grain germinates on stigma, and the pollen tube reaches the ovule.
• After fertilization, most floral parts fall off, ovary becomes fruit, and ovules become seeds.