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Class 9 Reproduction: How Life Continues: cheat sheet
CBSE Science. How living things make more of their kind: asexual methods, sexual reproduction in flowering plants and humans, and why gametes have half the chromosomes.
Key ideas
- Asexual reproduction needs one parent and gives offspring nearly identical to it; sexual reproduction needs two parents and gives variation.
- Asexual methods: binary fission (Amoeba), multiple fission (Plasmodium), budding (yeast, Hydra), fragmentation (Spirogyra), regeneration (Planaria), spores (Rhizopus).
- Vegetative propagation grows new plants from stems, roots or leaves (potato tubers, rose cuttings, Bryophyllum leaves).
- Pollination carries pollen from the anther to the stigma; fertilisation is the fusion of the male and female gametes in the ovule.
- After fertilisation the ovule becomes the seed and the ovary becomes the fruit.
- Gametes have half the chromosome number, so the zygote gets the full number back.
Remember
| Chromosomes | gamete = body cell ÷ 2; zygote = gamete + gamete |
| Binary fission | cells after k divisions = start × 2ᵏ |
| Germination rate | seeds germinated ÷ seeds sown × 100% |
| Seeds and ovules | one seed for each fertilised ovule |
Watch out
- Mixing up pollination (transfer of pollen) and fertilisation (fusion of gametes).
- Saying the ovary becomes the seed (it becomes the fruit).
- Multiplying by the number of divisions instead of doubling each time.
Worked example
A brinjal has 83 seeds. How many ovules were fertilised, and at least how many pollen grains must have grown pollen tubes?
Working: each seed comes from one fertilised ovule, and each ovule is reached by its own pollen tube
Answer: 83; 83
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