🌰 Double Fertilization & Seed Development
Double Fertilization & Seed Development
Fertilization (Double Fertilization)
Discovery
- Fertilization in plants was first discovered by Strasburger in 1884 (in Lilium)
- Double fertilization was discovered by S.G. Nawaschin in 1898 — a defining feature of angiosperms
Process
- A pollen grain lands on a compatible stigma
- The pollen tube grows through the style toward the ovule — guided by chemical signals from the synergids
- The pollen tube enters the ovule usually through the micropyle — this route is called porogamy
- Two male gametes (sperm cells) are released inside the embryo sac
Double Fertilization involves two simultaneous fusions:
| Fusion | Process | Product |
|---|---|---|
| Syngamy | One sperm (n) + Egg cell (n) | Zygote (2n) → develops into embryo |
| Triple Fusion | One sperm (n) + 2 polar nuclei (n+n) | Primary Endosperm Nucleus (PEN, 3n) → develops into endosperm |
- This process of two fusions occurring simultaneously is called double fertilization — it is unique to angiosperms (no other plant group exhibits this)
- Discovered by Nawaschin in 1898
Entry Routes of the Pollen Tube:
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Double Fertilization & Seed Development
Fertilization (Double Fertilization)
Discovery
- Fertilization in plants was first discovered by Strasburger in 1884 (in Lilium)
- Double fertilization was discovered by S.G. Nawaschin in 1898 — a defining feature of angiosperms
Process
- A pollen grain lands on a compatible stigma
- The pollen tube grows through the style toward the ovule — guided by chemical signals from the synergids
- The pollen tube enters the ovule usually through the micropyle — this route is called porogamy
- Two male gametes (sperm cells) are released inside the embryo sac
Double Fertilization involves two simultaneous fusions:
| Fusion | Process | Product |
|---|---|---|
| Syngamy | One sperm (n) + Egg cell (n) | Zygote (2n) → develops into embryo |
| Triple Fusion | One sperm (n) + 2 polar nuclei (n+n) | Primary Endosperm Nucleus (PEN, 3n) → develops into endosperm |
- This process of two fusions occurring simultaneously is called double fertilization — it is unique to angiosperms (no other plant group exhibits this)
- Discovered by Nawaschin in 1898
Entry Routes of the Pollen Tube:
| Route | Description |
|---|---|
| Porogamy | Through the micropyle — the most common route |
| Chalazogamy | Through the chalaza (base of ovule) |
| Mesogamy | Through the integuments (side entry) |
IMPORTANT
Double fertilization is the hallmark of angiosperms. It produces both the embryo (from syngamy) and the endosperm (from triple fusion) — ensuring that nutrient storage tissue (endosperm) only develops when an embryo is present, saving the plant's resources.
Post-Fertilization Changes
- After fertilization, the ovule develops into a seed and the ovary develops into a fruit
- The integuments become the seed coat
- The ovary wall becomes the pericarp (fruit wall)
Post-Fertilization Development
Endosperm Development
The primary endosperm nucleus (PEN, 3n) divides repeatedly to form endosperm — a nutritive tissue that nourishes the developing embryo. Endosperm development actually begins before embryo development, ensuring food is ready when the embryo needs it.
Three Types of Endosperm Development:
| Type | Description | Examples |
|---|---|---|
| Nuclear | PEN divides repeatedly by mitosis without cell wall formation initially; cells are formed later | Coconut (liquid endosperm = coconut water is nuclear endosperm!) |
| Cellular | Every nuclear division is immediately followed by cell wall formation | Adoxa, Petunia |
| Helobial | First division produces two unequal chambers; one develops nuclear type, the other cellular type — a combination | Monocots (Helobiae order) |
Embryo Development
After syngamy, the zygote (2n) develops into an embryo through repeated cell divisions:
Zygote → Proembryo → Globular stage → Heart stage → Torpedo stage → Mature embryo
Seed Structure
| Part | Dicot Seed | Monocot Seed |
|---|---|---|
| Seed coat | Testa (outer) + Tegmen (inner) | Fused with fruit wall (pericarp) |
| Endosperm | Mostly absent (consumed during development — food stored in cotyledons instead) | Present (stores starch — the main food reserve) |
| Embryo | Embryonal axis + 2 cotyledons | Embryonal axis + 1 cotyledon (called scutellum) |
| Cotyledons | 2 — store food (as in pea, bean) | 1 — the scutellum absorbs nutrients from endosperm and transfers them to the growing embryo |
| Coleoptile | Absent | Present — protects the plumule (embryonic shoot) as it pushes through the soil |
| Coleorhiza | Absent | Present — protects the radicle (embryonic root) during germination |
Fruit Types
| Type | Description | Examples |
|---|---|---|
| Simple fruit | Develops from a single ovary of a single flower | Mango, tomato, wheat |
| Aggregate fruit | Develops from multiple free carpels of a single flower | Strawberry, raspberry, Annona (custard apple) |
| Composite/Multiple fruit | Develops from the entire inflorescence (many flowers fuse together) | Pineapple (sorosis type), fig (syconus type), mulberry, jackfruit |
Summary Cheat Sheet
| Concept / Topic | Key Details / Explanation |
|---|---|
| Fertilization first discovered by | Strasburger (1884) in Lilium |
| Double fertilization discovered by | S.G. Nawaschin (1898); unique to angiosperms |
| Pollen tube entry — Porogamy | Through micropyle; most common route |
| Pollen tube entry — Chalazogamy | Through the chalaza (base) |
| Pollen tube entry — Mesogamy | Through the integuments (side entry) |
| Syngamy | 1 sperm (n) + egg (n) → Zygote (2n) → develops into embryo |
| Triple fusion | 1 sperm (n) + 2 polar nuclei (n+n) → PEN (3n) → develops into endosperm |
| Double fertilization | = Syngamy + Triple fusion occurring simultaneously |
| Post-fertilization: Ovule | Develops into seed; integuments → seed coat |
| Post-fertilization: Ovary | Develops into fruit; ovary wall → pericarp |
| Endosperm — Nuclear type | PEN divides without cell walls initially; e.g., coconut (coconut water = nuclear endosperm) |
| Endosperm — Cellular type | Every division followed by cell wall formation; e.g., Petunia |
| Endosperm — Helobial type | Two unequal chambers; combination type; e.g., monocots |
| Embryo development stages | Zygote → Proembryo → Globular → Heart → Torpedo → Mature embryo |
| Dicot seed coat | Testa (outer) + Tegmen (inner) |
| Dicot endosperm | Mostly absent (consumed; food stored in 2 cotyledons) |
| Monocot endosperm | Present (stores starch); 1 cotyledon = scutellum |
| Coleoptile | Present in monocots only; protects plumule |
| Coleorhiza | Present in monocots only; protects radicle |
| Simple fruit | From single ovary of single flower (mango, tomato, wheat) |
| Aggregate fruit | From multiple free carpels of single flower (strawberry, raspberry, custard apple) |
| Composite/Multiple fruit | From entire inflorescence (pineapple = sorosis, fig = syconus, jackfruit) |
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