Seed and fruit formation, and why they matter
The ovule becomes the seed and the ovary becomes the fruit. Seeds store food, survive dry periods, spread to new places and carry new gene combinations.
What each part becomes after fertilisation
| Before fertilisation | After fertilisation |
|---|---|
| Ovule | Seed |
| Integuments | Tough, protective seed coat(s) |
| Micropyle | A small pore in the seed coat; lets in oxygen and water at germination |
| Zygote | Embryo |
| Primary endosperm cell | Endosperm |
| Nucellus | Usually used up; persists as perisperm in a few seeds |
| Ovary | Fruit |
| Ovary wall | Pericarp (fruit wall) |
Albuminous vs non-albuminous seeds
A perisperm is different from endosperm: it is leftover nucellus, as in black pepper and beet.
Seed maturation, dormancy and viability
- As the seed matures, its water content falls to about 10 to 15% of its mass, and metabolism slows down.
- The embryo may enter dormancy, a resting state. If moisture, oxygen and a suitable temperature are available, it germinates instead.
- Some seeds lose viability within a few months; others stay viable for years.
- A seed of the lupine Lupinus arcticus, dug out of the Arctic tundra, germinated and flowered after an estimated 10,000 years of dormancy.
- A 2000-year-old viable seed of the date palm, Phoenix dactylifera, was found during excavation at King Herod's palace near the Dead Sea.
- Orchid fruits hold thousands of tiny seeds, and so do the parasitic plants Orobanche and Striga, and the fruit of Ficus.
True fruit vs false fruit
In most plants, the other floral parts wither and fall off as the ovary grows into a fruit.
Significance of seeds, fruit formation and seed dispersal
- Pollination and fertilisation do not need water, so seed formation is more dependable.
- Seeds have good strategies for dispersal to new habitats, which helps the species colonise other areas.
- Their stored food nourishes the young seedling until it can make its own food by photosynthesis.
- The hard seed coat protects the young embryo.
- Seeds are products of sexual reproduction, so they carry new genetic combinations (variation).
- The fruit protects the developing seeds, and many fruits have ways to help disperse the seeds when ripe.
- Seeds are the basis of agriculture: dry, dormant seeds can be stored as food all year and sown the next season.
Farmers sun-dry wheat and rice before storing them, because dry seeds stay dormant and keep for months. In Goa, the juicy cashew apple is a false fruit; the cashew nut at its tip holds the seed.
2 marks: “What is a perisperm? How is it different from the endosperm?” Perisperm is persistent nucellus in a seed (black pepper, beet); it is diploid tissue of the parent. Endosperm forms from the PEN after triple fusion and is triploid.
3 marks: “List any three advantages of seeds to flowering plants.” Formation independent of water; dispersal to new habitats; stored food for the seedling; hard coat protects the embryo; new genetic combinations. Any three.
1 marks: “Why is an apple called a false fruit?” Its thalamus also contributes to the fruit, not just the ovary.
False fruits A-S-C the thalamus for help: Apple, Strawberry, Cashew.