Male gametophyte (pollen grain) develops inside the microsporangia (pollen sacs) of the anther.
Female gametophyte (embryo sac) develops inside the nucellus of the ovule, which sits within the ovary of the pistil.

Feature	Microsporogenesis	Megasporogenesis
Site	Microsporangium (anther)	Nucellus of ovule
Mother cell	Microspore mother cell (MMC, 2n)	Megaspore mother cell (MMC, 2n)
Division	Meiosis	Meiosis
Product	4 haploid microspores (tetrad)	4 haploid megaspores (linear tetrad)
Functional cells	All 4 usually functional → pollen grains	Only 1 functional; 3 degenerate

Sporogenous tissue → Pollen Mother Cell (MMC) → Microspore tetrad → Pollen grain → Male gametes.
The sporogenous tissue differentiates into MMCs. Each MMC undergoes meiosis to give a tetrad of four microspores. Each microspore matures into a pollen grain, whose generative cell divides to give two male gametes.

A typical ovule is an ovoid structure attached to the placenta by a stalk called the funicle. The junction of the funicle with the main body is the hilum. The body is protected by one or two integuments that leave a tiny opening at one end — the micropyle. Opposite the micropyle is the chalaza. Inside the integuments is a mass of parenchymatous tissue, the nucellus, which houses the embryo sac (female gametophyte). Refer to Fig 2.2 of Part 2 for the labelled diagram — funicle, hilum, integuments, micropyle, nucellus, embryo sac, chalaza.

In most angiosperms, the MMC in the nucellus undergoes meiosis to produce four megaspores. Three of these degenerate and only one megaspore remains functional. This single surviving megaspore gives rise to the entire female gametophyte (embryo sac) through mitotic divisions. Because the embryo sac develops from just one megaspore, the process is called monosporic development — the commonest type in flowering plants, also called the Polygonum type.

The functional megaspore undergoes three free-nuclear mitotic divisions (2 → 4 → 8 nuclei). Cell walls then organise these 8 nuclei into 7 cells:

• At the micropylar end: an egg apparatus of 3 cells — 1 egg + 2 synergids (synergids have a filiform apparatus).
• At the chalazal end: 3 antipodal cells.
• In the centre: a single large central cell containing two polar nuclei.

Counting: 3 + 3 + 1 = 7 cells, but the central cell has 2 nuclei, so the total nuclei = 3 + 3 + 1 + 1 = 8. Refer to Fig 2.4 of Part 2 for the labelled diagram.

(a) Epipetalous stamens — stamens that are attached to the petals rather than to the thalamus, as in brinjal (Solanum) and Datura.
(b) Syncarpous gynoecium — a gynoecium in which the carpels are fused into a single pistil, as in tomato and papaya. (Opposite: apocarpous — free carpels, as in lotus.)
(c) Actinomorphic flower — a radially symmetrical flower that can be cut into two equal halves by any vertical plane through its centre (e.g., mustard, hibiscus). Opposite: zygomorphic (only one plane of symmetry, e.g., pea).

(a) Hypogynous vs Epigynous

Hypogynous	Epigynous
Ovary sits above other floral parts; ovary is superior	Other parts arise above the ovary; ovary is inferior
Thalamus is conical/convex	Thalamus grows up around the ovary
Examples: mustard, China rose, brinjal	Examples: guava, cucumber, sunflower

(b) Apocarpous vs Syncarpous

Apocarpous	Syncarpous
Carpels are free	Carpels are fused into one structure
Examples: lotus, rose, Michelia	Examples: tomato, papaya, mustard

Cleistogamous flowers are flowers that never open — the bud simply matures, self-pollinates within, sets seed and withers. Examples: Viola, Oxalis, Commelina. The anthers dehisce inside the closed bud and shed pollen directly onto the stigma, forcing autogamy.

Advantages:

• Assured pollination — no pollinator required.
• No loss of pollen to wind/insects; reproductive efficiency is high.
• Seed set is guaranteed even in adverse weather.

Disadvantages:

• No cross-pollination → no genetic recombination between individuals.
• Offspring are genetically uniform, leading to inbreeding depression and low adaptability.

The flower's closed-box design has several evolutionary advantages:

• Protection of delicate reproductive parts from desiccation, UV, predators and mechanical damage — the sepals and petals shield the inner whorls.
• Efficient pollen transfer — bright petals, scent and nectar advertise to pollinators; the tight geometry ensures that a visiting insect picks up and drops pollen on exactly the right spots.
• Cross-pollination is facilitated, promoting genetic diversity.
• Seed and fruit development occur within the protective enclosure of the ovary; the ovary ripens into a fruit that further protects and disperses the seeds.
• Overall, the flower reduces reproductive wastage — fewer gametes are produced than in wind-pollinated gymnosperms, yet success rates are higher.

Pollen banks are facilities that preserve pollen grains under cryogenic conditions (typically in liquid nitrogen at –196 °C), where viability can be maintained for many years.

Importance:

• They store the male germplasm of many plant species and crop varieties, including rare and endangered ones.
• Breeders can cross plants whose flowering times do not coincide — pollen collected in one season can fertilise plants flowering in another.
• They enable intercontinental hybridisation programmes — pollen is easier to ship than a whole plant.
• They conserve biodiversity alongside seed banks.
• In emergencies (fires, epidemics) pollen banks allow extinct varieties to be resurrected as long as their seed lines survive.

Triple fusion is the fusion of one male gamete with the two polar nuclei of the central cell of the embryo sac, producing the Primary Endosperm Nucleus (PEN).

Site: inside the central cell of the embryo sac of the ovule.
Mechanism: After the pollen tube discharges its two male gametes into a synergid, one gamete moves to the egg (syngamy) and the second gamete moves down into the central cell and fuses with both polar nuclei simultaneously.
Nuclei involved: (i) one male gamete (n) + (ii) two polar nuclei (n + n) → PEN (3n), which develops into the triploid endosperm.
Triple fusion along with syngamy constitutes double fertilisation, a characteristic of angiosperms.

Plant breeders produce hybrid seeds that show spectacular hybrid vigour (heterosis). But hybrid vigour is lost from the F2 generation onward, because meiosis and fertilisation segregate the favourable gene combinations. Farmers must therefore buy fresh hybrid seed every planting season — which is expensive.

Apomixis produces seeds without meiosis and without fertilisation, so the embryos are clones of the mother plant. If a hybrid variety could be made apomictic:

• Hybrid vigour is preserved across successive generations — every seed is genetically identical to its parent.
• Farmers can save their own seed instead of buying fresh hybrid seed each season, lowering costs.
• Uniformity of the crop is maintained — simplifying farming practices.
• It speeds up breeding programmes because once a desirable genotype is fixed, apomixis locks it in permanently.

For these reasons, engineering apomixis into staple crops is a major goal of modern plant breeding.