Sexual Reproduction in Flowering Plants - Comprehensive Notes

Overview

Flower = specialised, modified shoot; site of sexual reproduction.
Described as both morphological and embryological marvels.
Complete flower possesses four concentric whorls:
- Calyx → sepals (unit)
- Corolla → petals
- Androecium → stamens (male organ / microsporophylls)
- Gynoecium → carpels or pistils (female organ)
Reproductive sequence:
1. Pre-fertilisation (formation of gametophytes, pollination)
2. Fertilisation (double fertilisation)
3. Post-fertilisation (seed & fruit formation)

Androecium (Stamen / Male Reproductive Organ)

A stamen has two parts:
• Filament (slender stalk; proximal end fixed to thalamus/petal, distal end bears anther)
• Anther (generally dithecous → two lobes; each lobe = two longitudinal pollen-sacs → tetrasporangiate)
The two lobes are connected by parenchymatous connective.
Each pollen sac (microsporangium) houses numerous pollen grains.

Anther wall (four layers from outside to inside)

Epidermis – single protective layer.
Endothecium – hygroscopic; thickened radial walls aid dehiscence.
Middle layers – 1–3 layers; temporary protection, help dehiscence.
Tapetum – innermost nutritive layer; dense cytoplasm, multinucleate (free-nuclear divisions); secretes enzymes & hormones vital for microspore development.

Microsporogenesis

Central sporogenous tissue (homogeneous diploid cells) differentiates into pollen mother cells (PMCs).
Each PMC undergoes meiotic division → tetrad of four haploid microspores.
All four microspores of a tetrad are haploid and often arranged tetrahedrally or linearly.

Pollen Grain (male gametophyte / first cell of male gametophyte)

Diameter: $25 \text{–} 50\,\mu m$ .
Two-layered wall:
• Exine – thick, sporopollenin (chemically inert; allows fossil preservation; no degrading enzyme known). Contains thin, unthickened areas = germ pores (sporopollenin absent).
• Intine – thin, cellulose + pectin.
Study of pollen → Palynology.

Microgametogenesis

Maturation: Haploid nucleus enlarges, mitotically divides →
• Vegetative (tube) cell – large, vacuolated.
• Generative cell – small, spindle-shaped; floats in vegetative cytoplasm.
Pollen typically shed at 2-celled stage in $\approx 60\%$ angiosperms.
In remaining $\approx 40\%$ , generative cell divides inside pollen → 3-celled stage (1 vegetative + 2 male gametes) before release.
Pollen tube arises from vegetative cell through germ pore; carries non-motile male gametes to embryo-sac.

Pollen Viability & Utilisation

Viability = ability to germinate on stigma.
Extremely short-lived (≈ $30$ min) in cereals such as rice & wheat; persists for months in Rosaceae, Leguminosae, Solanaceae.
Cryopreservation: viable for years in liquid N₂ at $-196^{\circ}C$ (pollen banks).
Pollen grains nutrient-rich → tablets & syrups sold as food supplements.
Parthenium (carrot grass) pollen – common allergen; invasive weed introduced via contaminated wheat consignments.

Gynoecium / Pistil (Female Reproductive Organ)

Components:
• Stigma – sticky landing platform for pollen.
• Style – stalk.
• Ovary – encloses ovules.
Carpel number:
• Monocarpellary (single) – eg. pea.
• Multicarpellary: – Syncarpous (fused) eg. Hibiscus, Papaver. – Apocarpous (free) eg. Michelia.
Post-fertilisation conversion: ovule → seed; ovary → true fruit.

Ovule (Megasporangium)

Attachment to placenta via stalk = funicle.
Junction funicle–body = hilum.
Protective integuments (one or two) arise from chalaza (basal region).
Micropyle – opening between integuments at funicular end.
Nucellus – nutritive tissue inside integuments; houses embryo-sac.
Raphe – ridge formed by funicle extension to chalaza.
Ovule numbers: single (wheat, paddy, mango) to many (papaya, watermelon, orchids).
Most common ovule orientation = Anatropous (body inverted; micropyle near funicle).

Megasporogenesis

A sub-micropylar archesporial cell enlarges → Megaspore Mother Cell (MMC, $2n$ ).
MMC undergoes meiosis → linear tetrad of four haploid megaspores.
Usually only one megaspore (chalazal or micropylar, species-specific) remains functional → monosporic development.

Megagametogenesis (Embryo-sac Development)

Functional megaspore nucleus undergoes three consecutive mitoses:
1st → $2$ nuclei (migrate to poles)
2nd → $4$ nuclei
3rd → $8$ nuclei (four at each pole).
Cell organisation (8-nucleate, 7-celled mature embryo-sac):
• Micropylar end: Egg apparatus = 1 egg cell + 2 synergids (with filiform apparatus guiding pollen tube; aid absorption & transport).
• Chalazal end: 3 antipodal cells.
• Central cell: 2 polar nuclei (may fuse → secondary nucleus, $2n$ ).

Pollination

Definition: Transfer of pollen from anther to stigma.

Types (based on genetic relationship)

Autogamy (self) – same flower.
Geitonogamy – different flower on same plant (functionally cross; genetically self). Impossible in dioecious species.
Xenogamy – different plant, same species (true cross-pollination).

Pollinating Agents

Abiotic (~ $\lt 20\%$ flowering spp.): Wind (anemophily) > Water (hydrophily).
Biotic (~majority): insects, birds, bats, etc.

Wind-Pollinated Flower Traits (Anemophilous)

Exposed stamens, versatile anthers.
Large, feathery or brush-like stigmas.
Many small, light, smooth, dry pollen grains.
Numerous flowers per inflorescence; often single ovule per ovary. Eg. grass, corn cob.

Water-Pollinated Traits (Hydrophilous)

Mucilaginous pollen coat prevents wetting.
Flowers unscented, non-coloured, nectarless. Eg. Hydrilla, Vallisneria, Zostera.
Note: Many aquatic plants (water lily, Eichhornia) still use insects/wind.

Insect-Pollinated Traits (Entomophilous)

Bright colours.
Nectar secretion.
Fragrance or putrid odours (for carrion insects).
Rough, sticky pollen surfaces; sometimes aggregated inflorescences (Sunflower).
Pollen/nectar robbers: animals extracting rewards without pollination.

Special Floral Forms

Chasmogamous – flowers open normally at maturity (majority).
Cleistogamous – bisexual flowers never open (Commelina, Viola, Oxalis); obligatorily autogamous; energy efficient (no pollinators required). Plants often bear both flower types.

Out-Breeding / Cross-Pollination Promoting Devices

Unisexuality (dioecy/monoecy) – separate male & female flowers.
Dichogamy – temporal separation of gamete maturity within bisexual flower.
Herkogamy – spatial separation (different positions of stigma & anthers).
Heterostyly – differing style/stamen heights among flowers.
Self-incompatibility – genetic/biochemical blockage of self pollen (self sterility).

Artificial Hybridisation (crop improvement)

Sequence in bisexual flowers:

Selection of parents → 2. Emasculation (remove anthers before dehiscence) → 3. Bagging (isolate female) → 4. Pollen dusting (desired male) → 5. Re-bagging.

Emasculation unnecessary if female parent bears unisexual flowers.

Pollen–Pistil Interaction

Stigma/style tissues biochemically recognise compatible pollen → hydration, germination.
Incompatible pollen rejected.
Pollen tube pathway: stigma → style → ovary → enters ovule via micropyle → synergid via filiform apparatus.

Fertilisation: Double Fertilisation & Triple Fusion

Discovered only in angiosperms.
Inside embryo-sac, two male gametes released:
1. Syngamy: Gamete + egg → zygote ( $2n$ ).
2. Triple fusion: Gamete + two polar nuclei ( $n + n$ ) → Primary Endosperm Nucleus (PEN, $3n$ ).
Both fusions in same embryo-sac = double fertilisation.
Total nuclei participating = $5$ (2 in syngamy + 3 in triple fusion).
Endosperm formation starts before embryogenesis.

Post-Fertilisation Events

Endosperm Development (nutritive tissue, $3n$ )

Free-nuclear (common): PEN divides mitotically without cytokinesis; nuclei migrate peripherally; later cell walls form centripetally (eg. rice, wheat, maize, coconut, sunflower).
Cellular: Every mitosis followed by cytokinesis from start → cellular endosperm (eg. Petunia; not explicitly in transcript).

Embryogenesis (Zygote → Embryo)

Zygote is the biological bridge between generations; embryo = progenitor of new sporophyte.
Zygote undergoes asymmetric division → basal cell (suspensor) + terminal cell (pro-embryo).
Suspensor pushes developing embryo into endosperm for nourishment.
Stages: Pro-embryo → Globular → Heart-shaped → Mature.

Dicot Embryo

Two cotyledons.
Embryonal axis (tigellum) bears:
• Epicotyl (above cotyledonary node) ending in plumule (stem tip).
• Hypocotyl (below cotyledons) ending in radicle; root tip covered by root cap.

Monocot Embryo (e.g., grasses)

Single cotyledon = scutellum (lateral).
Epiblast – rudimentary second cotyledon.
Plumule enclosed in protective coleoptile.
Radicle + root cap enclosed in coleorhiza.
Aleurone layer – outermost protein-rich layer of endosperm, $3n$ .

Seed Types

Final product of sexual reproduction.
Based on endosperm retention:
1. Albuminous (endosperm present) – rice, wheat, maize, barley, castor.
2. Non-albuminous/ex-albuminous (endosperm consumed during embryogenesis) – pea, bean, groundnut, gram, orchid.
Perisperm ( $2n$ ) – persistent nucellus remnants, nutritive; eg. black pepper, beet.
Polyembryony – >1 embryo per seed (Citrus, mango).

Changes After Fertilisation (ploidy retained)

Ovule ( $2n$ ) → Seed ( $2n$ )
Ovary ( $2n$ ) → Fruit ( $2n$ )
Ovary wall ( $2n$ ) → Pericarp / fruit wall ( $2n$ )
Integuments ( $2n$ ) → Seed coat ( $2n$ )
Zygote ( $2n$ ) → Embryo ( $2n$ )
PEN ( $3n$ ) → Endosperm ( $3n$ )

Fruit Types

True fruits: develop solely from ovary (eg. guava, orange, mango, groundnut, mustard).
False fruits (pseudocarps): other floral parts contribute (e.g., thalamus) – apple, strawberry, cashew apple.
Fleshy vs. dry classifications; examples supplied above.

Special Reproductive Phenomena

Apomixis

Asexual production of seeds that mimic sexual cycle.
Embryo arises from diploid nucellar/embryo-sac cell; no meiosis, no fertilisation.
Progeny genetically identical; no character segregation in hybrids.
Found in some Asteraceae, grasses.

Parthenocarpy

Development of fruit from unfertilised flowers → seedless fruits.
Examples: banana, pineapple.

These bullet-point notes encompass all major and minor details, definitions, examples, structures, developmental stages, numerical facts, special terms, and practical or economic implications provided in the transcript on sexual reproduction in flowering plants. They are organised hierarchically to facilitate quick revision for examinations while preserving comprehensive coverage equivalent to the original lecture notes.