Pollination and Fertilization
ICSE Class 9 · Biology · Chapter 5
What this chapter covers
- Pollination: the first step towards a seed
- Self-pollination
- Cross-pollination
- Agents of pollination
- Flowers pollinated by insects
- Flowers pollinated by wind
- Flowers pollinated by water
- How nature favours cross-pollination
- Events between pollination and fertilisation
- Fertilisation, double fertilisation and triple fusion
- Changes in the flower after fertilisation
- Fruit and seed: definition and significance
Key points
- Pollination is the transfer of pollen from an anther to a stigma; it must happen before fertilisation.
- Self-pollination (autogamy or geitonogamy) uses pollen from the same plant; it is certain and keeps varieties pure, but gives little variation and weakens offspring over many generations.
- Cross-pollination (xenogamy) uses pollen from another plant of the same species; it gives variation and vigour but depends on an outside agent and wastes pollen.
- Homogamy and cleistogamy favour self-pollination.
- Insect-pollinated flowers are large, coloured, scented and have nectar, sticky or spiny pollen and sticky stigmas.
- Wind-pollinated flowers are small and dull, with no scent or nectar, plentiful light dry pollen, exposed anthers and large feathery stigmas.
- Water-pollinated flowers, as in Vallisneria and Zostera, are small and inconspicuous, with mucilage-coated pollen and long unwettable stigmas.
- Nature favours cross-pollination through unisexuality, dichogamy (protandry and protogyny), self-sterility, herkogamy and heterostyly.
- A mature embryo sac has seven cells and eight nuclei: egg and two synergids, central cell with two polar nuclei, and three antipodals.
- The pollen grain germinates on the stigma, the pollen tube grows down the style, the generative cell gives two male gametes, and the tube enters the ovule by the micropyle and the embryo sac through a synergid.
- Syngamy: male gamete + egg gives the diploid zygote, which becomes the embryo.
- Triple fusion: male gamete + two polar nuclei gives the triploid primary endosperm nucleus, which becomes the endosperm.
- Double fertilisation is syngamy and triple fusion together; it occurs only in flowering plants.
- After fertilisation the ovary becomes the fruit, the ovule the seed, the integuments the seed coat; petals and stamens fall.
- A fruit is a ripened ovary that protects and disperses seeds; a seed is a ripened ovule that carries the embryo and continues the species.
Questions answered
Define pollination. How is it different from fertilisation?
Pollination is the transfer of pollen grains from the anther to the stigma. Fertilisation is the fusion of a male gamete with the egg inside the ovule to form a zygote. Pollination happens on the stigma and comes first; fertilisation happens in the embryo sac and comes later.
Distinguish between autogamy and geitonogamy.
In autogamy pollen reaches the stigma of the same flower. In geitonogamy pollen reaches the stigma of another flower on the same plant; it usually needs an agent but is genetically self-pollination.
What is cleistogamy? Why does a cleistogamous flower always self-pollinate?
Cleistogamy is the condition in which a flower never opens. No outside pollen can enter, so only the flower's own pollen can reach its stigma.
Give two advantages and two disadvantages of cross-pollination.
Advantages: the offspring show variation and are more vigorous and disease-resistant; new varieties can arise. Disadvantages: it depends on an outside agent and so is uncertain; much pollen is wasted, or food is spent on petals, scent and nectar.
Why do plant breeders prefer self-pollination for keeping a variety pure?
In self-pollination the pollen and egg come from the same plant, so the offspring receive almost the same characters as the parent and the variety stays unchanged.
Give four features of wind-pollinated flowers, with an example.
Small, dull flowers without scent or nectar; very large amounts of light, dry, smooth pollen; anthers hanging out on long filaments; large feathery stigmas projecting out of the flower. Example: maize.
Why is the pollen of insect-pollinated flowers sticky or spiny, while that of wind-pollinated flowers is smooth and dry?
Sticky or spiny pollen clings to the insect's body and is carried to the next flower. Smooth, dry, light pollen stays separate and floats easily in the air, which is what wind pollination needs.
Describe how pollination takes place in Vallisneria.
Male flowers break free under water, rise and float on the surface. The female flower reaches the surface on a long coiled stalk. Floating male flowers touch the stigma and pollen is transferred at the surface; the stalk then coils and pulls the female flower back under water.
Past board exam questions
ICSE 2019 sample paper · 1 mark
In Vallisneria pollination takes place by the agency of:
- Water
- Wind
- Birds
- Insects
Show answer
Water