The Flower and Pollination
Introduction
The flower is the reproductive organ of angiosperms (flowering plants). ICSE Class 9 covers the structure of flowers, the process of pollination, and the events leading to seed formation.
Structure of a Flower
A typical flower has four whorls arranged on the thalamus (receptacle):
Calyx (Outermost)
- Composed of sepals (usually green)
- Function: Protects the flower in bud stage
Corolla (Second whorl)
- Composed of petals (often colourful and fragrant)
- Function: Attracts pollinators (insects, birds)
Androecium (Male reproductive part)
- Composed of stamens
- Each stamen has:
- Anther: Produces pollen grains (male gametes)
- Filament: Stalk supporting the anther
Gynoecium (Female reproductive part)
- Composed of carpels/pistils
- Each carpel has:
- Stigma: Sticky surface to receive pollen
- Style: Connects stigma to ovary
- Ovary: Contains ovules, each with an egg cell (female gamete)
Types of Flowers
| Basis | Types | Examples |
|---|---|---|
| Presence of parts | Complete (all 4 whorls) vs Incomplete | Hibiscus, Grass |
| Sex | Bisexual (both sex organs) vs Unisexual | China rose, Papaya |
| Symmetry | Actinomorphic (radial) vs Zygomorphic (bilateral) | Mustard, Pea |
Pollination
Definition: Transfer of pollen grains from the anther to the stigma of a flower.
Self-Pollination
Transfer of pollen from anther to stigma of the same flower or another flower on the same plant.
Advantages:
- Does not depend on external agents
- Maintains pure lines
- Ensures reproduction even in isolation
Disadvantages:
- No genetic variation
- Continuous self-pollination leads to weaker offspring
Cross-Pollination
Transfer of pollen from anther of one flower to stigma of a flower on a different plant of the same species.
Advantages:
- Produces genetic variation
- Results in healthier, more vigorous offspring
- Adapts better to changing environments
Disadvantages:
- Depends on external agents
- Wastage of pollen grains
- Less certain than self-pollination
Agents of Cross-Pollination
Wind Pollination (Anemophilous)
- Characteristics: Small, inconspicuous flowers; large feathery stigmas; light, dry pollen; abundant pollen production
- Examples: Grasses, corn, wheat
Water Pollination (Hydrophilous)
- Characteristics: Flowers produced on water surface; pollen floats on water
- Examples: Vallisneria, Hydrilla
Insect Pollination (Entomophilous)
- Characteristics: Large, colourful petals; fragrance and nectar; sticky pollen
- Examples: Rose, sunflower, jasmine
| Agent | Flower Characteristics | Examples |
|---|---|---|
| Wind | Small, dull, feathery stigma, abundant pollen | Grass, wheat |
| Water | On water surface, floating pollen | Vallisneria |
| Insects | Colourful, fragrant, nectar | Rose, Hibiscus |
Fertilisation
- Pollen grain lands on stigma and germinates
- Pollen tube grows through style to reach the ovary
- Male gametes travel through the pollen tube
- One male gamete fuses with the egg cell = zygote (2n)
- The other fuses with polar nuclei = triploid nucleus (3n)
- This is called double fertilisation (unique to angiosperms)
Fruit and Seed Formation
- After fertilisation, the ovary develops into the fruit
- The ovules develop into seeds
- The zygote develops into the embryo
- The triploid nucleus develops into the endosperm (food for the embryo)
Common Mistakes With Fixes
| Mistake | Correction |
|---|---|
| All flowers have both sexes | Some flowers are unisexual (either male or female) |
| Self-pollination is always bad | Self-pollination is useful for maintaining pure lines |
| Pollination and fertilisation are the same | Pollination is pollen transfer; fertilisation is gamete fusion |
| Fruit develops from the ovary wall | The whole ovary becomes the fruit; ovules become seeds |
ICSE Exam Focus
| Topic | Marks (approx.) | Frequency |
|---|---|---|
| Flower structure (labelled diagram) | 4-5 marks | Very common |
| Self vs cross pollination | 3-4 marks | Very common |
| Agents of pollination | 3-4 marks | Common |
| Fertilisation and seed formation | 4-5 marks | Frequently asked |
Self-Test
Q1: Draw a labelled diagram of a typical flower.
Q2: Distinguish between self-pollination and cross-pollination.
Q3: List three characteristics of wind-pollinated flowers.
Q4: What is double fertilisation? Explain briefly.
Q5: After fertilisation, what does the following develop into: (i) Ovary (ii) Ovule (iii) Zygote
