Plant Reproductive System

Plant Reproductive System

1. Flower Structure & Anther Wall Anatomy

A. Flower as a Modified Shoot

• A modified shoot with condensed internodes functioning as the reproductive organ in Angiosperms.
• 4 Floral Whorls:

  • Calyx (Sepals): Outermost green whorl; protective and photosynthetic.

  • Corolla (Petals): Brightly coloured whorl to attract pollinators.

  ↳ Accessory Whorls: Calyx & Corolla.

  • Androecium (Stamens): Male reproductive organ (Microsporophyll).

  • Gynoecium / Pistil (Carpels): Female reproductive organ (Megasporophyll).

  ↳ Essential Whorls: Androecium & Gynoecium.

B. Microsporophyll (Stamen) & Cross-Section of Anther

• Stamen Anatomy: Long slender stalk called Filament (proximal end attached to thalamus/petal) and terminal bilobed Anther (distal end).
• Typical Angiosperm Anther: Bilobed, Dithecous (two thecae per lobe), Tetragonal, and Tetrasporangiate (contains 4 microsporangia/pollen sacs).
• Anther Wall Layers (Outer to Inner):

  1. Epidermis ($2n$): Single outer protective parenchymatous layer. Has Stomium cells acting as the line of dehiscence.

  2. Endothecium ($2n$): Single layer of elongated cells with fibrous $a$-cellulose thickenings; Hygroscopic (absorbs moisture and loses water to cause dehiscence).

  3. Middle Layers ($2n$): 2 to 3 ephemeral (short-lived) layers; provide nourishment to developing microspores and degenerate.

  4. Tapetum ($3n / 4n / 5n$): Innermost layer with dense cytoplasm and polyploid multi/binucleate cells (due to karyokinesis without cytokinesis).

2. Tapetum Functions, Microsporogenesis & Pollen Grains

A. Vital Functions of Tapetum

1. Provides nourishment to developing microspore mother cells and pollen grains.
2. Forms yellow, sticky lipid-rich coating called Pollen Kit (present in insect-pollinated flowers).
3. Synthesizes Sporopollenin (ubisch bodies) forming exine layer.
4. Secretes enzymes like Callase (dissolves callose wall around microspore tetrad) and pectinases.
5. Produces growth hormones like IAA (Indole-3-acetic acid) for pollen tube elongation.
6. Produces Compatibility Proteins for pollen-pistil interaction.

B. Microsporogenesis & Pollen Development

Process of formation of microspores from Microspore Mother Cell (MMC) inside microsporangium:

$$\text{Sporogenous Cell } (2n) \to \text{Microspore Mother Cell (MMC)} (2n) \xrightarrow{\text{Meiosis I \& II}} \text{Microspore Tetrad} (n) \xrightarrow{\text{Callase Enzyme}} 4 \text{ Free Microspores} (n)$$

• Tetrad Arrangement: Isobilateral in Monocots; Tetrahedral in Dicots.
• Microspore to Pollen Grain Transformation: Microspore undergoes vacuolization $\to$ Asymmetric Mitotic division $\to$ Generates a large Vegetative Cell (food reserve, large irregular nucleus) and a small Generative Cell (spindle-shaped, floats in vegetative cell cytoplasm).
• Shedding Stage: Shed at 2-cell stage in $60\%$ angiosperms. In remaining $40\%$, generative cell divides mitotically into 2 male gametes before shedding (3-cell stage).

3. Pollen Wall Architecture, Viability & Ovule Structure

A. Sporoderm Structure (Exine & Intine)

B. Ovule / Megasporangium Anatomy

• Structure: Attached to placenta by Funicle. Point of attachment of funicle to body of ovule is Hilum.
• Enclosed by protective Integuments leaving a small opening called Micropyle. Opposite basal end is Chalaza.
• Central parenchymatous tissue is Nucellus ($2n$), housing female gametophyte (Embryo sac).
• Types of Ovules:

  • Anatropous: Inverted $180^\circ$ rotation (Most common in $80\%$ angiosperms; micropyle close to funicle).

  • Orthotropous: Straight erect ($0^\circ$; Gymnosperms, Polygonum).

  • Hemianatropous: Curved $90^\circ$ (Ranunculus).

  • Campylotropous: Curved body (Legumes).

  • Circinotropous: $360^\circ$ coiled funicle (Cactus).

4. Megasporogenesis & Embryo Sac Development

A. Megasporogenesis Pathway

Formation of megaspores from Megaspore Mother Cell (MMC) inside nucellus near micropylar region:

$$\text{Single MMC } (2n) \xrightarrow{\text{Meiosis}} \text{Linear Tetrad of 4 Megaspores } (n)$$

• 3 megaspores toward micropyle degenerate! Only 1 functional megaspore toward chalazal end survives.
• Monosporic Development (Polygonum Type): Functional megaspore nucleus undergoes 3 sequential free-nuclear mitotic divisions:

  • $1^{\text{st}}$ Mitosis $\to$ 2-nucleate stage.

  • $2^{\text{nd}}$ Mitosis $\to$ 4-nucleate stage.

  • $3^{\text{rd}}$ Mitosis $\to$ 8-nucleate stage.

B. 7-Celled 8-Nucleate Embryo Sac Organization

Cellular organization of mature female gametophyte after cell wall formation:

• 1. Egg Apparatus (Micropylar end – 3 cells): 1 central Egg Cell ($n$) $+$ 2 Synergids / Helper Cells ($n$).

  • Synergids feature cellular wall thickenings called Filiform Apparatus that guide pollen tube entry into embryo sac chemically.

• 2. Antipodal Cells (Chalazal end – 3 cells): 3 haploid cells that degenerate before or after fertilization.
• 3. Central Cell (Center – 1 cell): Large single cell containing 2 Polar Nuclei ($2n$ secondary nucleus upon fusion).

5. Tetrapoda Vertebrates Overview (Comparative Zoology)

6. Pollination Classification & Abiotic Agents

A. Types of Pollination Based on Source

• 1. Autogamy (Self Pollination): Transfer of pollen grains from anther to stigma of the same flower. No pollinating agent needed.
• 2. Geitonogamy: Transfer of pollen to stigma of another flower of same plant. Functionally cross-pollination (requires agent), but genetically self-pollination!
• 3. Xenogamy (Cross Pollination): Transfer of pollen to stigma of a different plant flower. Brings genetic variation.

B. Conditions Promoting Autogamy & Inbreeding

• Bisexual Flowers & Homogamy: Simultaneous maturation of anther and stigma.
• Cleistogamous Flowers: Flowers that never open (e.g., Viola, Oxalis, Commelina). Assures seed-set even in absence of pollinators. Disadvantage = Inbreeding depression.
• Chasmogamous Flowers: Open flowers with exposed anthers/stigma.

C. Abiotic Pollination (Wind & Water)

• Anemophily (Wind Pollination – Most Common Abiotic): Pollen light, non-sticky, produced in vast quantities. Stamens well-exposed with versatile anthers; stigma large and feathery (e.g., Corn cob tassels, Grasses, Maize).
• Hydrophily (Water Pollination – Rare, ~30 Monocot genera):

  • Epihydrophily (Surface): Vallisneria (Female flower reaches water surface by long stalk; male pollen floats passively).

  • Hypohydrophily (Submerged): Zostera (Sea grass; long ribbon-like pollen released inside water).

  • Note: Aquatic plants like Water Hyacinth (Eichhornia) and Water Lily are pollinated by Insects or Wind, NOT water!

7. Biotic Pollination & Pollen-Pistil Interaction

A. Entomophily (Insect Pollination) & Floral Rewards

• Flowers large, colourful, fragrant, rich in nectar. Foul-smelling flowers (e.g., Rafflesia, Aristolochia) attract flies/beetles.
• Floral Rewards: Nectar, edible pollen, or safe egg-laying sites (e.g., 6-foot tall flower Amorphophallus; Yucca plant and Pronuba moth symbiosis).
• Pollen / Nectar Robbers: Insects consuming nectar/pollen without causing pollination (e.g., Bombus affinis robbing Aquilegia).
• Sexual Deceit: Mediterranean orchid Ophrys mimics female Colpa bee appearance/scent for pseudocopulation.

B. Pollen-Pistil Interaction & Siphonogamy

• Chemical dialogue between pollen and stigma mediated by compatibility proteins. Compatible pollen is accepted to form pollen tube.
• Siphonogamy: Formation of pollen tube to carry non-motile male gametes. Apical growth requires $\text{Ca}^{2+}$, Boron, IAA, and Enzymes. Chemotropic movement guided toward synergids.

8. Double Fertilization & Endosperm Development

A. Mechanism of Double Fertilization

Unique event characteristic of all Angiosperms (discovered by Nawaschin):

• 1. Syngamy (Generative Fertilization):

  $$\mathbf{1^{\text{st}}\text{ Male Gamete } (n) + \text{Egg Cell } (n) \to \text{Zygote } (2n)}$$

• 2. Triple Fusion (Vegetative Fertilization):

  $$\mathbf{2^{\text{nd}}\text{ Male Gamete } (n) + \text{Secondary Nucleus } (2n) \to \text{Primary Endosperm Nucleus (PEN) } (3n)}$$

B. Endosperm Development Types

Endosperm ($3n$) development always precedes embryo development to ensure nutritive support!

• Free Nuclear Endosperm: PEN undergoes repeated free nuclear divisions without cytokinesis (e.g., Coconut Water).
• Cellular Endosperm: Every nuclear division immediately followed by wall formation (e.g., White kernel of Coconut, Petunia).
• Note: Gymnosperm endosperm is haploid ($n$) and forms before fertilization!

9. Embryo, Seed, Fruit & Apomixis

A. Embryogeny (Proembryo $\to$ Heart-Shaped $\to$ Mature)

• Dicot Embryo: Consists of embryonal axis and 2 cotyledons. Axis portion above cotyledon level is Epicotyl (terminates in Plumule/shoot tip); portion below is Hypocotyl (terminates in Radicle/root tip covered with root cap).
• Monocot Embryo: Single shield-shaped cotyledon called Scutellum. Radical protected by Coleorhiza sheath; Plumule protected by Coleoptile sheath.

B. Seed Types & Special Phenomena

• Non-Endospermic / Ex-albuminous Seed: Endosperm completely consumed during embryo development (e.g., Pea, Gram, Groundnut).
• Endospermic / Albuminous Seed: Endosperm retained in mature seed (e.g., Wheat, Maize, Castor, Barley).
• Perisperm: Persistent nucellus in mature seed (e.g., Black Pepper, Beetroot).
• True vs. False vs. Parthenocarpic Fruit:

  • True Fruit: Develops exclusively from ovary (Mango, Guava).

  • False Fruit (Pseudocarp): Thalamus/floral parts contribute to fruit (Apple, Strawberry, Cashew).

  • Parthenocarpic Fruit: Fruit develops without fertilization; seedless (Banana; induced by auxins).

• Apomixis: Form of asexual reproduction that mimics sexual reproduction; seed produced without fertilization (Asteraceae, Grasses).