🌺 Sexual Reproduction in Flowering Plants

Angiosperms reproduce sexually through specialized floral organs, achieving genetic recombination through targeted pollen delivery, double fertilization, and post-zygotic embryo-seed development.

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1. 🌾 Microsporogenesis and Pollen Grain Ultrastructure

🔬 Microsporangium Concentric Wall Layers and 2-Celled Pollen Grain Ultrastructure

1.1 The Microspore / Pollen Grain

• Exine: Outer hard layer composed of Sporopollenin (most resistant organic material known; resists strong acids, alkalis, high temperature, and enzymatic digestion). Germ pores are apertures where sporopollenin is absent.
• Intine: Inner thin continuous pectocellulosic wall.
• Microgametophyte: 2-celled stage in over 60% of angiosperms (Large Vegetative Cell + Small spindle-shaped Generative Cell).

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2. 🌸 Megasporogenesis and The Monosporic Polygonum Embryo Sac

🌸 Megasporogenesis and 7-Celled, 8-Nucleate Polygonum Embryo Sac Architecture

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3. ⚡ Double Fertilization and Triple Fusion (S.G. Nawaschin, 1898)

⚡ Angiosperm Double Fertilization and Dual Fusion Pathways

Unique defining hallmark of angiosperms:

$$\text{Pollen Tube delivers 2 Male Gametes into Synergid via Filiform Apparatus}$$

1. Syngamy (True Fertilization):$$\text{Male Gamete }\mathbf{(}\mathbf{n}\mathbf{)}\mathbf{+}\text{Egg Cell }\mathbf{(}\mathbf{n}\mathbf{)}⟶\text{Diploid Zygote }\mathbf{(}\mathbf{2}\mathbf{n}\mathbf{)}⟶\text{Embryo}$$
2. Triple Fusion:$$\text{Male Gamete }\mathbf{(}\mathbf{n}\mathbf{)}\mathbf{+}\text{2 Polar Nuclei }\mathbf{(}\mathbf{n}\mathbf{+}\mathbf{n}\mathbf{)}⟶\text{Primary Endosperm Nucleus (PEN, }\mathbf{3}\mathbf{n}\mathbf{)}⟶\text{Endosperm}$$

$$\text{Total Fertilization Events}\mathbf{=}\text{Syngamy}\mathbf{+}\text{Triple Fusion}\mathbf{=}\text{Double Fertilization}\mathbf{(}\mathbf{5}\text{ Nuclei Total Involved}\mathbf{)}$$

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4. 🌱 Endosperm Development and Apomixis

1. Free-Nuclear Endosperm: PEN undergoes repeated free-nuclear divisions without cytokinesis (e.g., tender coconut liquid water is free-nuclear endosperm; surrounding white kernel is cellular endosperm).
2. Apomixis: A form of asexual reproduction that mimics sexual reproduction, producing seeds without fertilization (e.g., Asteraceae and grasses).
3. Polyembryony: Occurrence of more than one embryo in a seed (e.g., Citrus, Mango: nucellar cells protrude into embryo sac and develop into multiple embryos).