Sexual Reproduction in Flowering Plants — Long Notes

Angiosperms are seed-bearing flowering plants. The flower is a modified shoot that carries out sexual reproduction. Understanding it means dissecting the whorls, the two gametogenesis processes, pollination, fertilisation, and what happens next.

1. The Flower — A Fascinating Organ

• Four whorls (from outer to inner): calyx (sepals), corolla (petals), androecium (stamens), gynoecium (carpels/pistil).
• Only androecium and gynoecium are reproductive; the other two are accessory (protection + attraction).
• Flowers may be bisexual (both whorls, e.g. hibiscus) or unisexual.
• Unisexual + on same plant = monoecious (maize, coconut).
• Unisexual + on different plants = dioecious (papaya, date palm).

2. Pre-Fertilisation: Structures & Events

2.1 Stamen, Microsporangium, and Pollen Grain

Stamen = filament + anther. A typical anther is dithecous (two lobes), each with two microsporangia → tetrasporangiate.

Microsporangium (cross-section): from outside in — epidermis → endothecium → middle layers → tapetum, enclosing sporogenous tissue.

• Tapetum nourishes developing pollen; has dense cytoplasm and typically multiple nuclei.

Microsporogenesis: sporogenous cells act as microspore mother cells (MMCs). Each MMC undergoes meiosis → four haploid microspores arranged as a tetrad.

As anthers mature, microspores separate and become pollen grains. Each pollen grain has:

• Exine — the outer, thick, ornamented wall of sporopollenin — the most resistant known organic material (survives high temperature, acids, alkalis, enzymes). Prominent gaps: germ pores.
• Intine — inner, thin, continuous wall of cellulose + pectin.

Cell content: at maturity, most pollens are 2-celled:

• Vegetative cell — large, abundant reserve food, irregular nucleus.
• Generative cell — small, spindle-shaped, floats in the vegetative cytoplasm. It divides later (either in the pollen or during tube growth) into two male gametes.

Pollen viability varies (30 min in rice/wheat; weeks–months in Rosaceae, Solanaceae, Leguminosae). Pollen banks store viable pollen for years in liquid nitrogen at −196°C.

2.2 The Pistil, Megasporangium (Ovule), and Embryo Sac

Pistil = stigma + style + ovary. A gynoecium may be monocarpellary (single carpel) or multicarpellary (many carpels — apocarpous if free, syncarpous if fused).

Inside the ovary, on the placenta, one or more ovules are attached via a stalk called the funicle. The point where the funicle joins the ovule body is the hilum. Regions of the ovule: integuments (one or two protective layers), micropyle (a small opening at the top through which the pollen tube usually enters), chalaza (basal end), and the nucellus (mass of parenchymatous cells housing the embryo sac).

Megasporogenesis: one nucellar cell differentiates as the megaspore mother cell (MMC) and undergoes meiosis → four haploid megaspores. In most angiosperms, three megaspores degenerate and only one is functional (monosporic development).

Female gametophyte (embryo sac) formation: the functional megaspore's nucleus divides mitotically three times → 8 free nuclei; cellular walls then form → the mature embryo sac has 8 nuclei but 7 cells:

• Egg apparatus at the micropylar end — 1 egg cell flanked by 2 synergids. Synergids have thickenings at their tips called the filiform apparatus, which guide the pollen tube in.
• Central cell — the large middle cell containing two polar nuclei.
• Antipodal cells — 3 cells at the chalazal end.

3. Pollination

Definition: transfer of pollen grains from the anther to the stigma. Types by source:

• Autogamy — same flower. Chasmogamous flowers open (both self and cross possible); cleistogamous flowers never open → assured self-pollination (e.g. Viola, Commelina).
• Geitonogamy — pollen from one flower to another on the same plant. Functionally cross-pollination but genetically self-pollination.
• Xenogamy — pollen from a different plant — the only type that brings genetically different pollen.

3.1 Agents

Abiotic:

• Wind (anemophily): enormous quantities of light, dry, non-sticky pollen; well-exposed stamens; single-ovuled ovaries (grasses, maize, coconut). Feathery stigma to trap pollen.
• Water (hydrophily): rare. Vallisneria — female flower reaches the surface via a long stalk; Zostera — pollen equal in specific gravity to water. Pollen has a mucilaginous covering.

Biotic:

• Insects (bees, butterflies, moths — entomophily): colourful/large/fragrant flowers with nectar or edible pollen. Some obligate mutualisms:
• Yucca & moth (moth lays eggs in ovary; grubs eat some seeds; both cannot reproduce without the other).
• Fig & wasp (similar dependence).
• Also birds (sunbirds — ornithophily), bats (chiropterophily).

3.2 Outbreeding Devices

Continued self-pollination causes inbreeding depression. Plants avoid it by:

1. Dichogamy — anthers and stigmas mature at different times (protandry, protogyny).
2. Herkogamy / heterostyly — anthers and stigma at different heights.
3. Self-incompatibility — genetic mechanism that prevents self-pollen from germinating on the stigma.
4. Dioecy — male and female flowers on different plants.

3.3 Pollen–Pistil Interaction

The pistil "accepts or rejects" pollen. When pollen is compatible:

• Pollen germinates on the stigma → forms a pollen tube from the vegetative cell.
• The generative cell divides (in the tube if not already done) into two male gametes.
• Tube grows through the style, reaches the ovary, then enters the ovule usually via the micropyle (porogamy), then into the embryo sac by passing through a synergid (guided by the filiform apparatus).

4. Double Fertilisation

The pollen tube discharges its two male gametes into the embryo sac. Two independent fusions follow:

1. Syngamy — one male gamete fuses with the egg → zygote (2n) → develops into the embryo.
2. Triple fusion — the other male gamete fuses with the two polar nuclei of the central cell → primary endosperm nucleus (PEN, 3n) → develops into the endosperm.

Because two fusion events occur in the same embryo sac, the phenomenon is called double fertilisation. It is unique to angiosperms.

5. Post-Fertilisation Events

5.1 Endosperm

The PEN divides mitotically (before the zygote) to form endosperm, providing nutrition to the developing embryo. Development can be:

• Free-nuclear (initial nuclear divisions without walls; walls form later) — most common. Coconut water = free-nuclear endosperm; coconut meat = cellular endosperm.
• Cellular — walls form after every nuclear division.
• Helobial — a mix of both.

In endospermic (albuminous) seeds (castor, coconut, maize), endosperm persists and nourishes the seedling. In non-endospermic (ex-albuminous) seeds (pea, groundnut, bean), it is consumed during embryo development and stored in cotyledons.

5.2 Embryo

Starts developing after endosperm formation begins. Stages: proembryo → globular → heart-shaped → mature.

• Dicot embryo: axis has plumule (embryonic shoot) & radicle (embryonic root); two cotyledons attached.
• Monocot embryo: one cotyledon (scutellum), plumule enclosed by coleoptile, radicle enclosed by coleorhiza.

5.3 Seed and Fruit

Old part	Becomes
Ovule	Seed
Integuments	Seed coat (testa + tegmen)
Nucellus	Sometimes perisperm
Ovary wall	Pericarp of fruit
Ovary	Fruit

• True fruit — fruit develops only from the ovary (mango, tomato).
• False fruit — thalamus also contributes (apple, cashew, strawberry).
• Parthenocarpic fruit — develops without fertilisation, hence seedless (banana, seedless grapes). Induced by growth hormones like auxins.

Seed advantages: dormancy allows survival of adverse seasons; hard coat protects; stored food provides a jumpstart on germination.

6. Apomixis and Polyembryony

• Apomixis — production of seeds without meiosis and fertilisation. Common in some Asteraceae and grasses. Preserves hybrid vigour across generations, so hybrid seed can be replanted without loss of quality (very useful in agriculture).
• Polyembryony — more than one embryo per seed. Seen in citrus, mango. Extra embryos may arise from nucellar cells (adventive embryony).

Key take-aways

1. Flowers coordinate microsporogenesis and megasporogenesis to produce haploid gametes.
2. Pollination brings pollen to the stigma; angiosperms use every trick (wind, water, insects, birds, bats) and evolve outbreeding devices to promote variation.
3. Double fertilisation gives us both a diploid zygote (embryo) and a triploid nutritive tissue (endosperm) — a two-for-one deal unique to flowering plants.
4. Downstream, the ovule matures into a seed, the ovary into a fruit; apomixis and parthenocarpy are useful exceptions.