Reproduction in Organisms
1. Introduction
Reproduction is the biological process by which organisms produce offspring. It ensures the continuity of species and introduces genetic variation.
2. Asexual Reproduction
Involves a single parent. Offspring are genetically identical (clones).
2.1 Modes
Binary fission: Amoeba, Paramoecium (nuclear division followed by cytoplasmic division). Budding: Hydra, Yeast (outgrowth develops into a new individual). Fragmentation: Spirogyra, Planaria (each fragment develops into a complete organism). Spore formation: Rhizopus, Mucor (sporangia produce spores). Regeneration: Starfish, Hydra (lost body parts regrow).
2.2 Advantages
- Rapid multiplication.
- No mate required.
- Preserves successful genotypes.
3. Sexual Reproduction
Involves two parents. Offspring have genetic variation.
3.1 Flower Structure
Androecium: Male reproductive part. Stamens with anthers (produce pollen). Gynoecium: Female reproductive part. Carpel with stigma, style, ovary (contains ovules).
3.2 Pollination
Transfer of pollen grains from anther to stigma.
Self-pollination: Same flower or same plant. Cross-pollination: Different plants. Promotes genetic diversity.
Pollinating agents: Wind (anemophily), insects (entomophily), birds (ornithophily), water (hydrophily).
4. Fertilisation in Plants
4.1 Germination of Pollen
Pollen lands on stigma, absorbs water and nutrients, germinates to produce a pollen tube that grows through the style towards the ovule.
4.2 Double Fertilisation (Unique to Angiosperms)
One male gamete fuses with egg → diploid zygote. Second male gamete fuses with two polar nuclei → triploid primary endosperm nucleus (PEN).
This process is called double fertilisation and results in the formation of the zygote and endosperm.
4.3 Pollination Mechanisms — Expanded
Anemophily (Wind): Pollen light, small, produced in large quantities. Stigma feathery. Examples: Maize, wheat, grasses.
Entomophily (Insects): Flowers large, colourful, scented, with nectar. Pollen sticky. Examples: Sunflower, rose, orchids.
Ornithophily (Birds): Flowers tubular, bright red/yellow, abundant nectar. Example: Bignonia.
Hydrophily (Water): Pollen released on water surface. Example: Vallisneria.
Geitonogamy: Pollination between different flowers of the same plant — genetically equivalent to self-pollination but requires a pollinating agent.
Xenogamy: Pollination between different plants — true cross-pollination, maximum genetic variation.
5. Embryo Development
The zygote develops into an embryo. The PEN develops into endosperm (nutritive tissue).
Monocot endosperm: Persistent (e.g., maize, rice). Dicot endosperm: Absorbed by cotyledons (e.g., pea, bean).
6. Seed Formation
The ovule becomes the seed, the ovary becomes the fruit.
Seed structure: Seed coat (testa), embryo (radicle, plumule, cotyledons), endosperm (if present).
7. Worked Problems
Problem 1: Compare apomixis and polyembryony. Solution: Apomixis is seed formation without fertilisation. Polyembryony is the presence of multiple embryos in a seed.
Problem 2: Explain the significance of double fertilisation. Solution: Double fertilisation ensures that endosperm develops only after fertilisation, conserving resources. It is unique to angiosperms and provides nourishment to the developing embryo.
8. Common Mistakes
'Students often confuse the terms pollination and fertilisation. Pollination is the transfer of pollen to stigma. Fertilisation is the fusion of gametes that occurs after pollination.'
9. ISC Exam Focus
| Topic | Theory Marks | Practical Marks |
|---|---|---|
| Asexual reproduction | 2 | 1 |
| Pollination | 3 | 2 |
| Double fertilisation | 4 | 2 |
| Seed and embryo | 2 | 1 |
10. Self-Test Questions
- Describe the structure of an angiosperm ovule.
- Explain double fertilisation with a labelled diagram.
- Differentiate between self-pollination and cross-pollination.
- What is parthenocarpy? Give one example.
- Describe the development of a dicot embryo from a zygote.
11. Additional ISC Focus Topics
Seed Dormancy and Germination
Seeds may remain dormant even under favourable conditions. Dormancy ensures survival during unfavourable periods. Causes include:
- Hard seed coat impermeable to water/oxygen.
- Presence of chemical inhibitors (abscisic acid).
- Immature embryo.
- Requirement of specific conditions (light, temperature, chilling).
Germination breaks dormancy when conditions become favourable. The radicle emerges first, followed by the plumule.
Apomixis and Polyembryony
Apomixis: Formation of seeds without fertilisation (agamospermy). The embryo develops directly from the diploid cells of the ovule (nucellus or integument). Examples: Citrus, mango, dandelion.
Polyembryony: Presence of more than one embryo in a seed. Can occur by cleavage of the zygote, development of additional embryos from synergids/antipodals, or from nucellar cells (nucellar embryony). Examples: Citrus, groundnut, onion.
'Apomixis and polyembryony are important in agriculture because they produce genetically uniform seeds. Nucellar embryos in citrus are virus-free and used for grafting rootstocks.'
