Board exam focus — Sexual Reproduction in Flowering Plants (CBSE & HBSE)

CBSE emphasises double fertilisation, embryo sac structure, and apomixis with application-based questions. HBSE focuses on definitions, stamen and pistil anatomy, and the key terms microsporogenesis, megasporogenesis, and triple fusion. Both boards require labelled diagrams of anther, ovule, and embryo sac.

Stamen, Microsporogenesis and Pollen Grain

The Flower — Site of Sexual Reproduction

The flower is the reproductive unit of angiosperms (flowering plants). It contains both male and female reproductive organs.

• Stamen (androecium) — male reproductive organ
• Pistil/Carpel (gynoecium) — female reproductive organ

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Structure of Stamen

A typical stamen consists of:

1. Filament — long, slender stalk that holds the anther
2. Anther — bilobed structure at the tip; contains pollen sacs (microsporangia)

Anther structure:

• Each anther has two lobes (dithecous)
• Each lobe has two microsporangia (pollen sacs)
• Total: 4 microsporangia per anther
• Microsporangia are connected by connective tissue

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Anther Wall Layers

The wall of the microsporangium has 4 layers (from outside to inside):

Tapetum is the most nutritive layer; its cells are multinucleate/polyploid and provide nutrition to microspores.

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Microsporogenesis

Microsporogenesis is the process of formation of microspores (pollen grains) from the pollen mother cells (PMC / Microspore Mother Cells — MMC).

Steps:

1. The centre of each microsporangium is filled with sporogenous tissue (diploid, 2n)
2. Sporogenous cells develop into pollen mother cells (PMC) (2n)
3. Each PMC undergoes meiosis → tetrad of 4 haploid (n) microspores
4. Each microspore develops into a pollen grain
5. The process of separation of microspores from the tetrad is called microsporogenesis

The four microspores in a tetrad are usually arranged in a tetrahedral pattern.

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Structure of Pollen Grain

A mature pollen grain is a two-celled structure:

Germ pores — thin areas in exine where pollen tube emerges during germination.

Sporopollenin properties:

• Resistant to high temperatures, acids, and alkalis
• Responsible for pollen preservation in fossils (palynology — study of fossil pollen)
• Never degraded by any enzyme known

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Pollen Viability

The ability of pollen to germinate and fertilise:

Pollen can be stored in liquid nitrogen (-196°C) for long periods — used in seed banks.

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Pollen Allergy

• Pollen of certain plants triggers allergic reactions (hay fever) in sensitive individuals
• Examples: Parthenium (congress grass), Amaranthus (pig weed)
• Cause: pollen proteins interact with IgE antibodies on mast cells → histamine release → sneezing, watery eyes
• Carrot grass (Parthenium hysterophorus) — invasive weed, serious pollen allergen in India

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Pollination Types

Diagram Indicator: [Labelled diagram of a bisexual flower showing stamen (filament + anther) with cross-section of anther showing 4 microsporangia and 4 wall layers: epidermis, endothecium, middle layers, tapetum; and a mature pollen grain showing exine, intine, vegetative cell, generative cell, and germ pore]

Pistil, Megasporogenesis and Embryo Sac

Structure of the Pistil (Gynoecium)

The pistil is the female reproductive organ of a flower. It may be:

• Monocarpellary — composed of one carpel (e.g., legumes)
• Polycarpellary — composed of many carpels:
• Apocarpous — carpels free (e.g., lotus, rose)
• Syncarpous — carpels fused (e.g., mustard, tomato)

Parts of a pistil:

1. Stigma — apical part; receives pollen; sticky surface for pollen adhesion
2. Style — elongated middle part; pollen tube grows through it
3. Ovary — basal swollen part; contains one or more ovules (each with an egg cell)

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Structure of the Ovule

The ovule is the structure within the ovary that develops into a seed after fertilisation.

Parts of ovule:

Types of ovules (based on orientation):

• Orthotropous (atropous): body straight, micropyle away from funicle (e.g., Polygonum, Cycas)
• Anatropous: body inverted 180°, micropyle near funicle — most common in angiosperms (e.g., sunflower)
• Hemitropous: body at right angle to funicle (e.g., Ranunculus)
• Campylotropous: body curved (e.g., legumes)

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Megasporogenesis

Megasporogenesis = formation of megaspores from the megaspore mother cell (MMC).

Steps:

1. In the nucellus, one large diploid cell differentiates as the Megaspore Mother Cell (MMC) (2n)
2. MMC undergoes meiosis → 4 haploid (n) megaspores (arranged linearly)
3. 3 megaspores degenerate (functional one is usually the chalazal megaspore — the one farthest from micropyle)
4. The single functional megaspore develops into the female gametophyte (embryo sac)

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Development of the Female Gametophyte (Embryo Sac)

The Polygonum type embryo sac is the most common (70% of angiosperms):

Development (monosporic, 8-nucleate):

1. Functional megaspore nucleus divides mitotically → 2 nuclei
2. Second mitotic division → 4 nuclei
3. Third mitotic division → 8 nuclei
4. Cell walls form → 7-celled, 8-nucleate embryo sac

Structure of mature embryo sac:

Filiform apparatus — finger-like projections on synergids; help guide pollen tube and absorb nutrients from nucellus.

Total: 7 cells, 8 nuclei — the central cell has 2 polar nuclei (so counted as 1 cell with 2 nuclei).

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Key Numbers to Remember

• MMC undergoes meiosis → 4 megaspores → 3 degenerate → 1 functional
• Functional megaspore undergoes 3 mitotic divisions → 8 nuclei → 7 cells
• Embryo sac = 7 cells, 8 nuclei
• Central cell has 2 polar nuclei
• Egg apparatus = egg cell (1) + synergids (2)
• Antipodal cells: 3 (at chalazal end)

Diagram Indicator: [Labelled diagram of (A) L.S. of anatropous ovule showing funicle, hilum, integuments, micropyle, chalaza, nucellus, embryo sac; (B) 7-celled 8-nucleate Polygonum-type embryo sac showing 3 antipodals, central cell with 2 polar nuclei, egg cell, and 2 synergids with filiform apparatus]

Fertilisation, Endosperm, Embryo, Apomixis and Polyembryony

Pollination and Pollen Tube Growth

After pollination, the pollen grain lands on the stigma and germinates:

1. Pollen absorbs water and nutrients from stigma
2. Pollen tube emerges from the germ pore
3. Pollen tube grows through the style (guided by chemotropic substances from synergids)
4. Pollen tube reaches the ovule — enters through the micropyle (porogamy — most common), or through chalaza (chalazogamy) or integuments (mesogamy)
5. Pollen tube enters a synergid (which degenerates) and releases the 2 male gametes

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Double Fertilisation — Unique to Angiosperms

Double fertilisation was discovered by S.G. Nawaschin (1898) in Lilium and Fritillaria.

Process: The pollen tube releases 2 male gametes (n) into the embryo sac.

First fertilisation (Syngamy):

• Male gamete 1 (n) + Egg cell (n) → Zygote (2n)
• This is the true syngamy
• Zygote develops into the embryo

Second fertilisation (Triple Fusion):

• Male gamete 2 (n) + Central cell (with 2 polar nuclei — 2n) → Primary Endosperm Nucleus (PEN) (3n)
• This is called triple fusion (2 polar nuclei + 1 male gamete = 3 nuclei fused)
• PEN develops into endosperm (food tissue for embryo)

Both fertilisation events happening together = Double Fertilisation. It is unique to angiosperms and ensures that endosperm (food reserve) is formed only when an egg is also fertilised.

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Endosperm Development

Endosperm = food storage tissue for developing embryo (3n in most angiosperms).

Most angiosperms have nuclear type endosperm initially.

Endosperm fate:

• In non-endospermic seeds (exalbuminous): endosperm completely consumed by developing embryo; food stored in cotyledons (e.g., pea, groundnut, bean)
• In endospermic seeds (albuminous): endosperm persists in mature seed (e.g., castor, maize, wheat, barley)

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Embryo Development (Embryogeny) — Dicot

Starting from the zygote (2n):

1. Zygote → asymmetric first division → proembryo (apical + basal cells)
2. Basal cell → suspensor (pushes embryo into endosperm)
3. Apical cell → embryo proper
4. Globular stage → heart-shaped stage (cotyledons differentiate) → torpedo stage
5. Mature dicot embryo: 2 cotyledons, plumule (shoot tip), radicle (root tip), hypocotyl (below cotyledons), epicotyl (above cotyledons)

Monocot embryo: 1 cotyledon (scutellum), coleoptile (sheath over plumule), coleorhiza (sheath over radicle).

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Seed and Fruit Formation

• Seed = mature ovule (contains embryo + endosperm + seed coat from integuments)
• Fruit = mature ovary (pericarp = ovary wall)
• True fruit: developed from ovary only (mango, tomato)
• False fruit (pseudocarp): developed from ovary + other floral parts (apple — thalamus; strawberry — receptacle)
• Parthenocarpy: fruit development without fertilisation → seedless fruits (banana, grape varieties)

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Apomixis

Apomixis = production of seeds without fertilisation (asexual seed formation).

Mechanisms:

1. Diplospory: MMC doesn't undergo meiosis → forms embryo sac with 2n cells directly
2. Apospory: Nucellar cells (sporophytic) form embryo sac (bypassing megasporogenesis)
3. Adventive embryony: Embryo develops directly from nucellus/integument cells

Examples: Many species of Asteraceae (dandelion), grasses (Poa) Significance in agriculture: Hybrid seeds can be produced without repeated hybridisation crosses → reduces cost.

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Polyembryony

Polyembryony = presence of more than one embryo in a seed.

Causes:

1. Fertilisation of multiple eggs (if multiple embryo sacs)
2. Adventive embryony (additional embryos from nucellus/integument)
3. Cleavage polyembryony (early embryo splits)

Examples:

• Citrus (lemon, orange) — most common example; nucellus cells form additional embryos
• Mangifera (mango) — polyembryony sometimes occurs
• Groundnut — sometimes

Nucellar embryos are genetically identical to the parent (useful in horticulture for true-to-type propagation).

Diagram Indicator: [Diagram showing (A) double fertilisation with pollen tube releasing 2 male gametes: one fusing with egg cell (zygote) and one fusing with central cell's 2 polar nuclei (PEN = 3n); (B) stages of dicot embryo development from zygote to mature seed with labelled cotyledons, plumule, radicle, suspensor, and endosperm]

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Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Sexual Reproduction in Flowering Plants.