Chapter 1 — Asexual and Sexual Reproduction in Plants
Key Concepts Quick Recap
| Term | Definition |
|---|
| Asexual reproduction | Reproduction without gamete fusion; uniparental; produces clones |
| Vegetative propagation | Asexual reproduction using vegetative parts (root, stem, leaf) |
| Apomixis | Seed formation without fertilisation |
| Parthenogenesis | Development of unfertilised egg into a new individual |
| Pollination | Transfer of pollen from anther to stigma |
| Self-pollination | Transfer of pollen within the same flower (autogamy) or between flowers of the same plant (geitonogamy) |
| Cross-pollination | Transfer of pollen between flowers of different plants (xenogamy) |
| Double fertilisation | One sperm fuses with egg (syngamy); another fuses with polar nuclei (triple fusion) |
| Endosperm | Nutritive tissue formed after triple fusion (3n in angiosperms) |
| Embryogenesis | Development of embryo from zygote |
| Polyembryony | Presence of more than one embryo in a seed |
I. Choose the Best Answer (MCQs)
1. Which of the following is an example of vegetative propagation by leaf? (a) Ginger (b) Bryophyllum (c) Dahlia (d) Banana
Answer(b) Bryophyllum Bryophyllum reproduces vegetatively through adventitious buds (bulbils) that develop on leaf margins and can grow into new plants when they fall to the ground. Ginger = rhizome; Dahlia = tuberous root; Banana = sucker.
2. Apomixis in plants means: (a) Fusion of two gametes (b) Formation of seeds without fertilisation (c) Development of pollen grains (d) Vegetative propagation
Answer(b) Formation of seeds without fertilisation Apomixis is seed production bypassing meiosis and fertilisation. Types: agamospermy (embryo from unfertilised egg), adventive embryony (embryo from nucellus/integument cells as in Citrus), vegetative apomixis (bulbils instead of seeds).
3. Double fertilisation was discovered by: (a) Hofmeister (b) Nawaschin (c) Mendel (d) Strasburger
Answer(b) Nawaschin S.G. Nawaschin (1898) discovered double fertilisation in Fritillaria and Lilium. One sperm (n) + egg (n) → zygote (2n); second sperm (n) + two polar nuclei (n+n) → primary endosperm nucleus (3n).
4. The ploidy of endosperm in a typical angiosperm is: (a) Haploid (b) Diploid (c) Triploid (d) Tetraploid
Answer(c) Triploid (3n) Triple fusion: one male gamete (n) + two polar nuclei (n + n) = Primary Endosperm Nucleus (3n). Endosperm nourishes the developing embryo.
5. Which type of pollination ensures cross-pollination always? (a) Autogamy (b) Geitonogamy (c) Xenogamy (d) Cleistogamy
Answer(c) Xenogamy Xenogamy is cross-pollination between flowers of different plants — always involves genetic exchange. Autogamy = same flower; geitonogamy = same plant (genetically equivalent to self-pollination); cleistogamy = self-pollination in closed flowers.
6. Emasculation in plant breeding refers to: (a) Removal of petals (b) Removal of anthers before pollen release (c) Removal of stigma (d) Artificial pollination
Answer(b) Removal of anthers before pollen release In plant breeding, emasculation removes immature anthers from bisexual flowers to prevent self-pollination, allowing controlled cross-pollination with desired pollen.
7. Polyembryony is commonly seen in: (a) Wheat (b) Citrus (c) Pea (d) Maize
Answer(b) Citrus In Citrus, multiple embryos develop in a seed through adventive embryony — cells of the nucellus or integument develop into extra embryos alongside the zygotic embryo.
II. Short Answer Questions
Q1. Distinguish between autogamy and geitonogamy.
Answer
| Feature | Autogamy | Geitonogamy |
|---|
| Definition | Transfer of pollen from anther to stigma of the same flower | Transfer of pollen between different flowers on the same plant |
| Genetic result | Self-fertilisation | Genetically equivalent to self-fertilisation |
| Pollination agent | Wind, gravity (no agent needed) | Insects, wind |
| Example | Wheat, rice | Maize (wind carries pollen between tassel and silk of same plant) |
Both are forms of self-pollination and do not produce new genetic combinations.
Q2. What is triple fusion? Why is the endosperm triploid?
Answer
Triple fusion is the fusion of the second male gamete (n) with the two polar nuclei (n + n) in the embryo sac.
- Second male gamete (n) + Polar nucleus 1 (n) + Polar nucleus 2 (n) → Primary Endosperm Nucleus (3n)
The endosperm is triploid (3n) because three haploid nuclei fuse. It develops into the endosperm tissue that nourishes the growing embryo. This is unique to angiosperms and occurs simultaneously with syngamy — hence called double fertilisation.
Q3. Explain the significance of double fertilisation.
Answer
Double fertilisation is significant because: 1. Ensures nutrition: Triple fusion produces endosperm only when an egg is also fertilised — ensuring food is made only when an embryo exists. This prevents wastage. 2. Genetic restoration: Syngamy restores the diploid (2n) chromosome number in the zygote. 3. Unique to angiosperms: Distinguishes angiosperms from gymnosperms (which lack triple fusion). 4. Evolutionary advantage: Endosperm is a rich food store that allows angiosperms to produce better-nourished seeds, contributing to their ecological dominance.
Q4. What is apomixis? What are its agricultural applications?
Answer
Apomixis is the production of seeds without fertilisation.
Types:
- Agamospermy: Embryo develops from unfertilised egg (Taraxacum — dandelion).
- Adventive embryony: Embryo from somatic cells (nucellus/integument) — Citrus, Mangifera.
- Vegetative apomixis: Bulbils or vegetative propagules replace seeds (Agave).
Agricultural applications:
- Apomictic seeds preserve the parent genotype — valuable in hybrid seed production (hybrids lose vigour in F2, but apomictic hybrids maintain it).
- Facilitates mass production of superior cultivars without losing hybrid vigour.
- Used in sugarcane, maize, wheat research to fix heterosis permanently.
III. Long Answer Questions
Q1. Describe the process of double fertilisation and embryo development in angiosperms.
Answer
Double Fertilisation:
When a pollen grain germinates on the stigma, the pollen tube grows through the style into the embryo sac. The pollen tube contains two male gametes (n).
Syngamy: Male gamete 1 (n) + Egg cell (n) → Zygote (2n) Triple fusion: Male gamete 2 (n) + Two polar nuclei (n+n) → Primary Endosperm Nucleus / PEN (3n)
These two events occurring simultaneously = Double fertilisation.
Embryo Development (Embryogenesis):
1. Zygote (2n) undergoes mitosis. 2. First division: transverse → Proembryo (two cells: apical and basal). 3. Basal cell → Suspensor (anchors embryo; channels nutrients). 4. Apical cell → Embryo proper. 5. Embryo develops through globular → heart-shaped → torpedo stages. 6. Final embryo (dicot): Plumule (shoot apex) + Radicle (root apex) + Two cotyledons + Hypocotyl/Epicotyl.
The endosperm (from PEN) develops simultaneously to nourish the embryo. In some seeds (e.g. castor) endosperm persists in mature seed; in others (e.g. pea) it is absorbed by cotyledons during development.
Q2. Compare different types of vegetative propagation with examples.
Answer
| Type | Plant Part | Example | Method |
|---|
| Rhizome | Underground stem (horizontal) | Ginger, turmeric | Nodes produce buds |
| Bulb | Modified shoot (fleshy leaves) | Onion, garlic | Axillary buds develop |
| Corm | Short, thick underground stem | Colocasia, Gladiolus | Grows new plants from nodes |
| Tuber | Swollen tip of underground stem | Potato | Eyes (nodes) sprout |
| Tuberous root | Swollen storage root | Dahlia, sweet potato | Adventitious buds |
| Runner/stolon | Horizontal aerial stem | Grass, strawberry | Nodes root in soil |
| Sucker | Lateral shoot from base | Banana, pineapple | Detach and plant |
| Bulbil | Modified axillary bud | Agave, Dioscorea | Falls and grows |
| Leaf | Leaf with adventitious buds | Bryophyllum | Buds develop into plants |
Artificial methods: Cutting, grafting, layering, tissue culture (micropropagation).
Common Mistakes
- Confusing endosperm ploidy: always 3n (triploid) in angiosperms.
- Saying geitonogamy is cross-pollination — it is genetically self-pollination.
- Confusing apomixis (no fertilisation) with parthenogenesis (unfertilised egg develops — still part of apomixis category).
- Forgetting Nawaschin discovered double fertilisation (not Mendel or Strasburger).
Exam Checklist
- Triple fusion formula: n + n + n = 3n (PEN).
- Double fertilisation = syngamy + triple fusion.
- Vegetative propagation table with 3 examples minimum.
- Apomixis types: agamospermy, adventive embryony, vegetative apomixis.
- Emasculation = removal of anthers (not petals).
