🐝 Megasporogenesis & Pollination
Learn embryo sac development and pollination types for CUET Agriculture. Ovule structure, 7-celled embryo sac and pollination agents.
Megasporogenesis (Ovule/Embryo Sac Formation)
Megasporogenesis is the formation of the female gametophyte (embryo sac) inside the ovule — the female side of reproduction.
Ovule Structure
| Part | Description |
|---|---|
| Funicle | The stalk connecting the ovule to the placenta inside the ovary — carries nutrients to the ovule |
| Chalaza | The base of the ovule (opposite to the micropyle) — where the integuments originate |
| Micropyle | A small opening at the top of the ovule — the entry point for the pollen tube in most plants |
| Hilum | The point where the funicle attaches to the ovule body |
| Raphe | A ridge formed by the funicle fused with the ovule body (visible in anatropous ovules) |
| Integuments | Outer coverings (1 or 2 layers) that protect the ovule; later become the seed coat |
| Nucellus | Nutritive tissue inside the integuments that feeds the developing embryo sac; the ovule is also called the megasporangium |
| Embryo sac | The female gametophyte — develops inside the nucellus |
Types of Ovules Based on Orientation
- Orthotropous: Erect, micropyle at top (straight ovule) — found in Piper, Polygonum
- Anatropous: Inverted 180°, the most common type — found in approximately 80% of angiosperms
- Campylotropous: Curved ovule body
- Amphitropous: Both body and embryo sac are curved
- Circinotropous: Ovule rotates during development
NOTE
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Megasporogenesis (Ovule/Embryo Sac Formation)
Megasporogenesis is the formation of the female gametophyte (embryo sac) inside the ovule — the female side of reproduction.
Ovule Structure
| Part | Description |
|---|---|
| Funicle | The stalk connecting the ovule to the placenta inside the ovary — carries nutrients to the ovule |
| Chalaza | The base of the ovule (opposite to the micropyle) — where the integuments originate |
| Micropyle | A small opening at the top of the ovule — the entry point for the pollen tube in most plants |
| Hilum | The point where the funicle attaches to the ovule body |
| Raphe | A ridge formed by the funicle fused with the ovule body (visible in anatropous ovules) |
| Integuments | Outer coverings (1 or 2 layers) that protect the ovule; later become the seed coat |
| Nucellus | Nutritive tissue inside the integuments that feeds the developing embryo sac; the ovule is also called the megasporangium |
| Embryo sac | The female gametophyte — develops inside the nucellus |
Types of Ovules Based on Orientation
- Orthotropous: Erect, micropyle at top (straight ovule) — found in Piper, Polygonum
- Anatropous: Inverted 180°, the most common type — found in approximately 80% of angiosperms
- Campylotropous: Curved ovule body
- Amphitropous: Both body and embryo sac are curved
- Circinotropous: Ovule rotates during development
NOTE
When the funicle, hilum, chalaza, and micropyle are all in a straight line → the ovule is orthotropous. In the much more common anatropous type, the ovule is inverted so the micropyle points toward the funicle.
Process of Megasporogenesis
- Archesporial cell in the nucellus forms the megaspore mother cell (MMC) — this is diploid (2n)
- The MMC undergoes meiosis → produces 4 haploid megaspores arranged in a linear tetrad
- Only one functional megaspore survives — the one closest to the chalaza; the other 3 degenerate
- The functional megaspore undergoes 3 successive mitotic divisions → produces a structure with 7 cells and 8 nuclei
Embryo Sac (Female Gametophyte) — 7 Cells, 8 Nuclei
| Cell Type | Number | Location | Function |
|---|---|---|---|
| Egg cell (oosphere) | 1 | Micropylar end | Fuses with one sperm → zygote (2n) → develops into embryo |
| Synergids | 2 | Micropylar end (flanking the egg) | Guide the pollen tube to the egg; have a specialized filiform apparatus (finger-like projections) |
| Antipodal cells | 3 | Chalazal end | Nutritive function; degenerate after fertilization |
| Central cell | 1 (with 2 polar nuclei) | Center of embryo sac | The 2 polar nuclei fuse with one sperm → Primary Endosperm Nucleus (PEN, 3n) → develops into endosperm |
- Total cells: 7 (1 egg + 2 synergids + 3 antipodals + 1 central cell)
- Total nuclei: 8 (1 egg + 2 synergid nuclei + 3 antipodal nuclei + 2 polar nuclei in central cell)
- This is the Polygonum type embryo sac — the most common pattern, found in ~70% of angiosperms (first described in Polygonum)
- The functional megaspore that develops into the embryo sac is called the female gametophyte
Pollination
Pollination is the transfer of pollen grains from the anther to the stigma of the pistil. This is the crucial step that brings male and female gametes close enough for fertilization to occur.
Types of Pollination
(1) Self-Pollination
Pollen from a flower reaches the stigma of the same flower or another flower on the same plant.
Two types:
| Type | Description | Example |
|---|---|---|
| Autogamy | Pollen reaches the stigma of the same flower — true self-pollination | Pea (Pisum) |
| Geitonogamy | Pollen from one flower reaches another flower on the same plant — genetically equivalent to self-pollination | Maize, corn |
Adaptations for Self-Pollination:
| Adaptation | Description | Example |
|---|---|---|
| Bisexuality | Both male and female parts in the same flower (hermaphrodite) | Pea |
| Homogamy | Anthers and stigma mature at the same time | Rose, potato |
| Cleistogamy | Flowers never open — self-pollination occurs within closed buds, guaranteeing self-fertilization | Commelina, Viola |
| Autogamy | Pollen falls directly on the stigma of the same flower due to proximity | Pea |
(2) Cross-Pollination (Allogamy)
Pollen from one flower reaches the stigma of a flower on a different plant of the same species. Cross-pollination promotes genetic diversity.
Adaptations Promoting Cross-Pollination:
| Mechanism | Description | Example |
|---|---|---|
| Self-incompatibility | Pollen cannot germinate on the stigma of the same plant — a genetic mechanism that rejects "self" pollen | Brassica, pigeon pea (Cajanus) |
| Dicliny (Unisexuality) | Separate male and female flowers (not on the same flower) | Maize (monoecious — both on same plant), papaya (dioecious — on separate plants) |
| Dichogamy | Anthers and stigma mature at different times | Two subtypes below |
| — Protandry | Anthers mature before stigma — pollen is released before the stigma is receptive | Sunflower, Salvia, cotton |
| — Protogyny | Stigma matures before anthers — the stigma is receptive before pollen is available | Ficus, Artocarpum, jackfruit |
| Herkogamy | A physical barrier between anther and stigma prevents self-pollination | Calotropis (pollinia), Caryophyllaceae |
| Heterostyly | Different style lengths in different plants of the same species | Primula (pin flowers with long styles, thrum flowers with short styles) |
Agents of Pollination (Methods of Cross-Pollination)
| Agent | Term | Features | Examples |
|---|---|---|---|
| Wind | Anemophily | Light pollen, produced in large quantity, dry, no nectar or fragrance | Grass, rice, wheat, maize, Typha |
| Water | Hydrophily | Pollen released in or on water; rare among plants | Zostera, Vallisneria, Ceratophyllum |
| Insects | Entomophily | Colorful flowers, nectar and fragrance present, sticky pollen | Salvia, orchids, Yucca |
| Birds | Ornithophily | Large flowers, bright red/orange color, copious nectar | Erythrina, Callistemon, Bombax |
| Bats | Chiropterophily | Large dull flowers, strong smell, open at night | Kigelia, Adansonia (baobab) |
| Snails | Malacophily | Rare | Arisaema, orchids, Lemna |
| Snakes | Ophiophily | Very rare | Some tropical plants |
Water Pollination (Hydrophily) — Two Types:
| Type | Description | Example |
|---|---|---|
| Hypohydrophily | Pollination occurs below water surface (submerged pollination) | Zostera, Ceratophyllum |
| Epihydrophily | Pollination occurs on the water surface (floating pollen or flowers) | Vallisneria |
TIP
Hydrilla and Ceratophyllum are both aquatic plants, but Hydrilla is pollinated by wind (not water) — its flowers emerge above the water surface for wind pollination. This is a common exam trick question!
Summary Cheat Sheet
| Concept / Topic | Key Details / Explanation |
|---|---|
| Megasporogenesis | Formation of female gametophyte (embryo sac) inside the ovule |
| Ovule parts | Funicle (stalk), Chalaza (base), Micropyle (pollen tube entry), Hilum (attachment point), Integuments (→seed coat), Nucellus (nutritive tissue) |
| Most common ovule type | Anatropous (inverted 180°); found in ~80% of angiosperms |
| Orthotropous ovule | Erect; funicle, hilum, chalaza, micropyle in a straight line |
| Megasporogenesis process | MMC (2n) → meiosis → 4 haploid megaspores (linear tetrad) → only 1 functional (closest to chalaza) → 3 mitotic divisions → 7 cells, 8 nuclei |
| Embryo sac (Polygonum type) | 7 cells, 8 nuclei: 1 egg + 2 synergids (micropylar end) + 3 antipodals (chalazal end) + 1 central cell (2 polar nuclei) |
| Egg cell | Fuses with 1 sperm → zygote (2n) → embryo |
| Central cell | 2 polar nuclei fuse with 1 sperm → PEN (3n) → endosperm |
| Synergids | Guide pollen tube to egg; have filiform apparatus |
| Antipodal cells | Nutritive function; degenerate after fertilization |
| Pollination definition | Transfer of pollen from anther to stigma |
| Autogamy | Pollen to stigma of same flower (e.g., pea) |
| Geitonogamy | Pollen to another flower on same plant (e.g., maize) |
| Cleistogamy | Flowers never open; guaranteed self-pollination; e.g., Commelina, Viola |
| Self-incompatibility | Pollen cannot germinate on same plant's stigma; e.g., Brassica, pigeon pea |
| Dichogamy | Anthers and stigma mature at different times |
| Protandry | Anthers mature before stigma (sunflower, cotton) |
| Protogyny | Stigma matures before anthers (Ficus, jackfruit) |
| Anemophily (wind) | Light, dry pollen in large quantity; no nectar; e.g., grass, rice, wheat, maize |
| Entomophily (insects) | Colorful flowers, nectar + fragrance, sticky pollen; e.g., Salvia, orchids |
| Hydrophily (water) | Hypohydrophily (below surface: Zostera) and Epihydrophily (on surface: Vallisneria) |
| Ornithophily (birds) | Large, bright red/orange flowers, copious nectar; e.g., Erythrina, Bombax |
| Chiropterophily (bats) | Large dull flowers, strong smell, open at night; e.g., Kigelia, baobab |
| Hydrilla pollination | By wind (not water) — flowers emerge above surface; common exam trick |
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